Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session C1: Poster Session I |
Hide Abstracts |
Room: LACC Exhibit Hall 2:00-5:00pm |
|
C1.00001: PHYSICS EDUCATION |
|
C1.00002: Restructuring Introductory Laboratories to Include Investigations at the Nanoscale Kurt Vandervoort, Asif Hyder A series of laboratories are being developed to introduce atomic force microscope applications into introductory courses. The goal is to elucidate fundamental physics concepts at the nanoscale that will complement existing investigations at the macroscale, and to expose students to advanced instrumentation at an early level. Specific modules include: microscopic friction on bacteria cell membranes to complement existing labs on macroscopic friction for pulleys and a mass on an incline; microscopic magnetism exhibited by magnetic stripes on videotape to complement an existing lab on the spatial variation of the magnetic field of a solenoid; microscopic topography of smooth glass, rough glass, diffraction gratings and butterfly wings to complement existing labs on specular and diffuse reflection and interference of light; and microscopic capacitance and electrostatics to complement existing labs on electrolysis and electric field mapping. Funding for this project was provided by the National Science Foundation Nanotechnology Undergraduate Education program, award {\#} 0406533. [Preview Abstract] |
|
C1.00003: An analysis of the effect of activity based instruction on the persistent misconceptions of physics students Emily Reiser, Mark Markes Studies have indicated that student misconceptions negatively impact the effectiveness of physics education. Research has also indicated that activity based instruction (ABI) has greater effectiveness than lecture based instruction (LBI) in many applications. It is natural to define a persistent misconception as the same wrong answer, for a given question, on pre and post evaluation tests. However, in practice, it is not possible to directly examine the impact of ABI on persistent misconceptions defined in this way because students cannot be returned to their pre-instruction state and re-instructed using ABI. In this paper an indirect method of determining the effect of ABI on persistent misconceptions is presented using the FMCE as an evaluation instrument. The unique feature of this analysis is the use of the occasional right to wrong responses as indicators of the change in probability for wrong to different wrong responses. Given this change in probability it is possible to determine the change in probability of wrong to same wrong responses: i.e. persistent misconceptions. The details of this analysis will be presented in the paper. [Preview Abstract] |
|
C1.00004: Physics Education: Getting Out of Our Comfort Zone Peter Muhoro, Ken Cecire, O. Keith Baker Zambia, the Real Africa (as they say), land of the legendary African walking safari, earth's biggest waterfall,~the wild Zambezi River, breath-taking lakes and wetlands, a profusion of birds, abundant wildlife,~and raw, pulsating wilderness, ...all in one friendly country. School attendance has increased substantially since Zambia's independence in 1964. In 2000 some 1.6 million pupils were enrolled in primary schools, representing 78 percent of school-aged children. Only 24 percent of secondary school-aged children were enrolled. Through Hampton University's Center for the Study of Origin and Matter (COSM), a team of high school teachers and two college students held workshops with a theme ``Classical Mechanics'' where high school physics and mathematics teachers attended. The goal of the workshops was to motivate and help the teachers come up with teaching strategies to promote interest in physics and math in the students. [Preview Abstract] |
|
C1.00005: GENERAL I POSTER SESSION |
|
C1.00006: Phase equilibrium predicted by the Soave-Redlich-Kwong equation at low temperatures Kimberly Wadelton, Michael Misovich This study developed vapor-liquid equilibrium relationships for vapor pressure and liquid density predicted by the Soave-Redlich-Kwong cubic equation of state (SRK) in the low temperature limit. Dimensionless variables were defined to simplify calculations. One, reduced density, was defined by dividing the actual density by density in the zero temperature limit. During this process, a useful form of the SRK equation was developed. Vapor pressures and phase densities were calculated numerically using a regula-falsi procedure. The method gave efficient calculations for reduced temperatures from approximately 0.0002 to approximately 0.9999. By manipulating the numerical data for reduced pressure and reduced temperature, functional relationships were suggested and verified analytically. Results of the predictive equations for reduced pressure were compared to the numerical results and differed at worst within one order of magnitude over the temperature range from zero to the critical point. Further modeling of the difference between the predictive equations and the actual SRK results is suggested. [Preview Abstract] |
|
C1.00007: Ferroelectric switching simultaneously studied by the epr and Mertz methods Francisco Rodriguez, Marisela Aparicio, Kennith Ulibarri, Monica Marcial-Armenta, Timothy Usher Ferroelectric materials are characterized by spontaneous electric dipole moments. Memory applications will benefit from a more fundamental understanding. The goal of this experiment is to reconcile the differences in previous ferroelectric experiments: electron paramagnetic resonance (epr) which monitors the bulk of the material and the Mertz technique, which monitors the surface. In this study both methods will be employed simultaneously in order to eliminate any experimental differences. If the differences between the two experimental results persist then, the epr results may be reveling new physics that the Mertz technique is not sensitive to. We will present some early results on the simple ferroelectric crystal KH$_{2}$PO$_{4}$. [Preview Abstract] |
|
C1.00008: Experimental and Computer analyses of Thunder Actuators. Jacob Salazar, Gilberto Camargo, Gevale Ashford, Timothy Usher High displacement, piezoelectric \textbf{THUNDER }actuators have numerous applications such as morphable wing technology, robotics, etc. These applications will be greatly enhanced by robust computer models, based on a clear understanding of the underlying physics and rooted in experimental data. With this in mind the main goal of our study is to compare experimental data of performance with computer models. Five different THUNDER actuator models were tested and simulated. All five models showed percent differences between experiment and simulation ranging from 0.16{\%} to 36{\%}. [Preview Abstract] |
|
C1.00009: ARTIFICIALLY STRUCTURED MATERIALS |
|
C1.00010: Evanescent Photon Induced Surface Plasmons in Multiwalled Carbon Nanotubes Qi Lu, Rahul Rao, Apparao Rao, Pu-Chun Ke Surface plasmons (SPs) are collective excitations of charges running as longitudinal density fluctuations along the surface of a solid. Conventionally SPs have been created through the coupling of evanescent waves via metallic thin films or small metallic particles or nanoshells. Here we show the generation of SPs excited with evanescent photons at the interface between a quartz slide and a MWNT array (2 microns) under the total-internal-reflection condition. The evanescent wave generated at this interface induced plasmons in the MWNT array which in turn excited the fluorescence of the sky blue beads immobilized on the array. The propagation of SPs in the nanotube array, whose thickness is two orders of magnitude greater compared with metallic thin film for SP generation, is attributed to the confinement of the electrons in the outermost tube shells and ballistic transport. Our finding suggests MWNT array can be used as tuneable plasmon waveguide for imaging illumination and biological and nano-optoelectronic sensing. - [Preview Abstract] |
|
C1.00011: Cavity Quantum Electrodynamics with Single Quantum Dots in Microcavities Matthew Rakher, Stefan Strauf, Nick Stolz, Kevin Hennessy, Antonio Badolato, Evelyn Hu, Larry Coldren, Pierre Petroff, Dirk Bouwmeester Several proposals for solid-state cavity quantum electrodynamics rely on a strong interaction between the cavity mode and an embedded single atom or a single quantum dot (QD). In order to achieve a strong light-matter interaction, the cavity must have a small mode volume while maintaining a large quality factor. To this end, InAs/GaAs QDs in GaAs-based microcavities have been investigated using micro-photoluminescence spectroscopy and photon statistics measurements. Individual QDs in these devices have been identified by their photon anti-bunching signatures. Pronounced enhancement of the single QD lifetime has been measured, with some lifetimes limited by the detector resolution (200 ps), corresponding to a Purcell factor of more than 10. Together with active spatial positioning schemes of QDs, these devices are promising avenues to reach the strong coupling regime with individual solid state emitters. [Preview Abstract] |
|
C1.00012: Anisotropic Spin Exchange in Coupled Quantum Dots Kerwin Foster, Layla Hormozi, Dimitrije Stepanenko, Nicholas Bonesteel We study the effect of spin-orbit coupling on the exchange interaction between spins in coupled quantum dots in III-V semiconductors. Our motivatation is recent work showing that spin-orbit induced anisotropic corrections to the isotropic Heisenberg exchange are potentially useful for quantum computation.\footnote{D. Stepanenko and N.E. Bonesteel, PRL {\bf 93}, 140501 (2004).} We show that ferromagnetic direct exchange {\it enhances} the anisotropy of the interaction by reducing the size of the isotropic term --- an important effect if these terms are going to used for quantum computation. If only one orbital is kept per dot (Hund-Mulliken approximation) the effect of ferromagnetic direct exchange is overestimated for large dots.\footnote{G. Burkard, D. Loss, and D.P. DiVincenzo, PRB {\bf 59}, 2070 (1999).} This can be seen, for example, by noting that the calculated isotropic exchange coupling becomes {\it negative} in zero magnetic field for some interdot distances, in violation of the Lieb-Mattis theorem. To reliably estimate the enhancement of the anisotropy, we therefore work within an approximation in which more than one orbital is kept per dot, and show that this new approximation is applicable to a wider range of dot parameters. Apart from the improved reliability of the approximation, adding more orbitals gives new insight into the symmetry of the resulting interaction. [Preview Abstract] |
|
C1.00013: Enhanced transmission through a periodic array of subwavelength apertures controlled by a semiconductor-metal phase transition Eugenii Donev, Jae Yong Suh, Rene Lopez, Leonard Feldman, Richard Haglund We present an experimental study on the tunability of the transmission spectrum through a periodic array of subwavelength apertures patterned on Ag-VO$_{2}$ bilayer film. The nano-structured composite film exhibits the known large optical transmission of a hole array in the metallic layer, but it is further enhanced when the VO$_{2}$ film becomes metallic after undergoing a semiconductor-to-metal phase transition $\sim $ 67 $^{o}$C. This result supports the theoretical explanation that the anomalous transmission is attributed to the resonant coupling of surface plasmon polaritons on both interfaces. The combination of metal and semiconductor-metal transition material gives not only further understanding of the enhanced transmission but also possibility of potential applications based on the transmission tunability. This research was supported by the NSF-NIRT program (DMR0210785) and the U. S. DOE NSET program (DE-FG02-01ER45916). [Preview Abstract] |
|
C1.00014: Detection and Analysis of Quantum Dots from Atomic Force Microscope Images Stephen Poprocki, Leann Erbsen, Susan Lehman On-surface self-assembled quantum dots (QDs) are routinely studied by atomic force microscopy (AFM) to characterize the size and number density of the QDs and to better understand the strain-induced process of self-assembly. However, the analysis of dot height and location is complicated by the difficulty of accurate detection and analysis of the QDs from the AFM image, particularly for samples with a varying background. We have developed a custom program to quickly and accurately locate both dot maxima and dot edges. In addition to eliminating the counting of false dots and the missed counting of real dots, the program measures the height of individual dots from a local background rather than an average background. Results from this new program are compared to those from previous analysis techniques. Sample measurements of QD height and surface density for InAs QDs on GaAs and the uniformity of these dot characteristics across the wafer will also be presented. [Preview Abstract] |
|
C1.00015: Film Formation of Ag Nanoparticles at the Organic-Aqueous Liquid Interface John Vigorita, Jill Sakata, Andrew Dwoskin, Eileen M. Spain A wet-chemical method to make films by spontaneous assembly of passivated Ag nanoparticles at the organic-aqueous liquid interface is presented. The interfacial films exhibit a blue opalescence, or in other cases a silvery color, and are characterized with transmission electron microscopy and UV-visible spectrophotometry. Measurements indicate that nanoparticles in the interfacial film can form superlattices and in some cases nanostructures. [Preview Abstract] |
|
C1.00016: Photoluminescence from Colloidal CdS/CdSe/CdS Quantum Wells Jianfeng Xu, David Battaglia, Xiaogang Peng, Min Xiao We report investigations of colloidal CdS/CdSe/CdS (core/well/shell) quantum wells (QW) by photoluminescence (PL) spectroscopy at temperatures between 77 and 300 K. PL intensity measurements show a transition from a pump-rate-limited regime at low excitation intensity to the range determined by a spontaneous emission lifetime at high excitation limit. The temperature dependences of the peak energy, linewidth and intensity of the PL have all been determined. The PL linewidth is narrower for thicker QWs and becomes broader when the temperature increases due to longitudinal optical (LO) phonon scattering. The LO-phonon scattering strength is around 30 meV, which is independent of the well thickness, and is much smaller than the value for bulk CdSe. The PL dynamic measurements show that the PL lifetimes for the 1-3 monolayer CdSe QWs increase with temperature. Such temperature dependence shows that the radiative recombination is the dominant process, indicating a high quality of the QW structures. [Preview Abstract] |
|
C1.00017: Terahertz Photoresistance in Laterally Modulated 2D Electron Layers Gregory Aizin, Lev Mourokh We study theoretically changes in the resistance of laterally modulated 2D electron layer induced by a terahertz electromagnetic (EM) wave at normal incidence. Electron density modulation is provided by the metal grating gate. Periodic modulation results in the coupling of an external EM wave with plasmon excitations in the layer. Corrections to the dc resistivity of 2D electron layer induced by the terahertz irradiation are found from the solution of classical kinetic equation in the lowest order on the EM wave intensity. We demonstrate that photoresistivity as a function of the frequency of EM wave has resonances at plasmon frequencies and analyze dependence of the height and position of the resonant peak on the electron density and modulation factor. It is shown that the peak height has non-monotonous dependence on the modulation factor, with the latter being determined by the gate voltage and the temperature. The implications of our results for explanation of recent experiments (X.G. Peralta \textit{et al}, Appl. Phys. Lett.,\textbf{ 81}, 1627 (2002)) are discussed. . [Preview Abstract] |
|
C1.00018: Photoluminescence characterization of InAs quantum dots grown on variable stoichiometry InxAlyGa1-x-yAs layers lattice-matched to InP J.G. Mendoza-Alvarez, M.P. Pires, S.M. Landi, A.S. Lopes, P.L. Souza In order to improve the temperature characteristics of InAs quantum dots (QD's) for infrared photodetectos applications, we have grown InAs QD's on In$_{x}$Al$_{y}$Ga$_{1-x-y}$As layers lattice-matched to InP substrates. We have already reported that smaller dot sizes are obtained when the Al concentration increases, and that luminescence emission energies at the long wavelenghts of 2.1 $\mu $m could be obtained. In this work we present results on the photoluminescence (PL) characterization for a set of InAs QD's/ In$_{x}$Al$_{y}$Ga$_{1-x-y}$As for Al concentrations of 0, 5.8, 11, and 16.5{\%}, changing the laser excitation power and the temperature in the range from 15K up to room temperature. From the analysis of the PL spectra we observe that at low temperatures the emission band is composed of two contributions around 0.63 and 0.67 eV which shifts to higher energies as the Al concentration increases. We discuss the nature of these transitions and their behaviour with the laser power and temperature. [Preview Abstract] |
|
C1.00019: The Ordering of Energy in Si-24 Clusters J. Rufinus, K.A. Jackson We present the results of extensive computational works of the ordering of energy of Silicon 24 clusters. The calculations are done in the framework of Density Functional Theory with the exchange functional of Perdew, Burke, and Ernzerhof (PBE). In these calculations we used both the NRLMOL and Gaussian 03 packages. The basis set of 6-31G is used with the Gaussian 03. [Preview Abstract] |
|
C1.00020: Colloidal gold nanorods: from reduction to growth Kyoungweon Park, Mostafa El-Sayed, Mohan Srinivasarao Formation of gold nanorods(NRs) in controlled reduction condition was investigated. Gold NRs were synthesized by seed mediated method where pre-made gold nanospheres were added to a growth solution containing surfactants, reducing agent and compound of gold ion and surfactant. ~ Reduction mechanism was manipulated by changing catalytic activity of seed. Seed of different size and capping agent coverage led to different dispersity of NRs since seed plays a role as catalyst as well as nucleation site. The difference between the redox potentials of gold species and reducing agent($\Delta $E) was controlled by the strength of reducing agent and the stability of the gold compound. As $\Delta $E leading to changing the morphology of resulting gold NRs. The surface of gold NRs with a series of aspect ratio was functionalized by thiolated beta cyclodextrin which binds preferentially to the end of NRs and promotes the orientation of rod-rod pair even without host-guest interaction. [Preview Abstract] |
|
C1.00021: Effects of a Parallel Magnetic Field on Double-Quantum-Well Electron Dynamics Norman Horing, Bing Dong, Hong-Liang Cui We examine electron dynamics in a narrow double-quantum-well system subject to a parallel magnetic field of arbitrary strength. In this, we derive an explicit analytical closed-form solution for the Green's function for Landau-quantized electrons in skipping states of motion between the walls of the thin individual quantum wells, as well as in tunneling states between the two wells; all coupled with in-plane translational motion and hybridized with the zero-field lowest subband energy eigenstates. [Preview Abstract] |
|
C1.00022: QUANTUM INFORMATION, CONCEPTS AND COMPUTATION |
|
C1.00023: Entanglement and Quantum Phase Transition Revisited Yang Min-Fong We show that, for an exactly solvable quantum spin model, a discontinuity in the first derivative of the ground state concurrence appears in the absence of quantum phase transition. It is opposed to the popular belief that the non-analyticity property of entanglement (ground state concurrence) can be used to determine quantum phase transitions. We further point out that the analyticity property of the ground state concurrence in general can be more intricate than that of the ground state energy. Thus there is no one-to-one correspondence between quantum phase transitions and the non-analyticity property of the concurrence. Moreover, we show that the von Neumann entropy, as another measure of entanglement, can not reveal quantum phase transition in the present model. Therefore, in order to link with quantum phase transitions, some other measures of entanglement are needed. [Preview Abstract] |
|
C1.00024: Quantum qubit measurement using a quantum point contact with a quantum Langevin equation approach Bing Dong, Norman Horing, Hong-Liang Cui Quantum measurement of a solid-state qubit by a mesoscopic detector is of fundamental interest and is still a controversial issue in quantum physics. In this work, we employ the theory of open quantum systems, the quantum Heisenberg- Langevin equation approach, to establish a set of quantum Bloch equations for a two-level system (coupled quantum dots) capacitively coupled to a quantum point contact (QPC) in a fully microscopic way. In our derivation, the operators of the qubit and detector are first determined formally by integration of their Heiserberg equations of motion (EOM), exactly to all orders in the tunnel coupling and qubit-detector coupling constants. Next, under the assumption that the timescale of the decay processes is much slower than that of free evolution, we express the time-dependent operators involved in the integrals of these EOM's approximately in terms of their free evolution. Finally, these EOM's are expanded in the powers of the coupling constants up to second order. The resulting Bloch equations allow us to examine qubit relaxation and decoherence in coupled quantum dots induced by measurement processes at arbitrary bias- voltage and temperature. As an illustration, we present analytical and numerical results for the qubit relaxation and decoherence. [Preview Abstract] |
|
C1.00025: Dynamic Nuclear Polarization at high field of the $^{31}$P spins in phosphorus-doped silicon Johan van Tol, Louis-Claude Brunel Phosphorus doped Silicon has been suggested as a prime candidate for the realization of Quantum Computers. Either or both electron or nuclear spins of the shallow donor centers could serve as qubits. Electron and nuclear spins can be manipulated by microwave and radiofrequency pulses respectively, while electric fields can be used to tune spin-spin interaction. The proposals concerning a Si-based approach for quantum computing rely on exchange interaction and relaxation mechanisms that need further characterization. In particular the relaxation times in Si:P are dependent on dopant concentration, strain, temperature, magnetic field, and the $^{29}$Si I= $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ nuclear spins. We present a multifrequency study of the relaxation and dynamic nuclear polarization mechanisms and nuclear relaxation rates in several relevant Si:P samples by cw and pulsed EPR and pulsed ENDOR at 9.6 GHz, 95 GHz, and 240 GHz. Nuclear polarization of the$^{ 31}$P nuclei in excess of 90{\%} was achieved at 3 K at 8.5 Tesla, while the nuclear spin-lattice relaxation time exceeds 30 minutes at these temperatures in natural abundance Si samples. [Preview Abstract] |
|
C1.00026: Realisation of Hardy's Thought Experiment William Irvine, Juan Hodelin, Christoph Simon, Dirk Bouwmeester We present an experimental realisation of Hardy's thought experiment~[Phys. Rev. Lett. {\bf 68}, 2981 (1992)], using photons. The experiment consists of a pair of Mach-Zehnder interferometers that interact through photon bunching at a beam splitter. A striking contradiction is created between the predictions of quantum mechanics and local hidden variable based theories. The contradiction relies on non-maximally entangled position states of two particles. [Preview Abstract] |
|
C1.00027: INSULATORS AND DIELECTRICS |
|
C1.00028: Ferroelectricity in SrTiO$_3$ Ceramics and Thin Films Induced by Pr Doping A. Duran, M. Cruz, J. Heiras, J. Siqueiros We have synthesized samples of Sr$_{1-x}$Pr$_{x}$TiO$_{3}$ with 0$<$x$<$0.15. X-ray diffraction (XRD) and XPS spectroscopy revealed that the Pr ion enters substitutionally in the Sr site with mixed valence in a Pr$^{+3}$/Pr$^{+4}$ = 2:1 ratio. The permittivity measurements show a peak at $\sim $240 $^{0}$C and the polarization measurements display hysteresis loops indicating that the Pr ion induces ferroelectric behavior at room temperature. On the other hand, the ceramic with x=0.025 was used as target to grow thin films by pulsed laser ablation on Pt/TiO$_{2}$/SiO$_{2}$/Si substrates. XRD analysis shows that the deposited sample grows preferentially along the (111) plane. The hysteresis loop was confirmed in thin films for this composition with remnant polarization almost an order of magnitude higher than that of the ceramic with corresponding composition. The results described above suggest two important facts. i) The ferroelectric-paraelectric transition is very probably due to lattice instability as a consequence of the difference in Sr/Pr ion size leading to an off center displacement of the Ti ion, and ii) the enhancement of the remanent polarization in thin films is an indirect evidence that the polarization axis is along the (111) direction in bulk. [Preview Abstract] |
|
C1.00029: Surface wave effect on light scattering from a one-dimensional photonic crystal Aldo Santiago Ramirez Duverger, Raul Garcia Llamas, Jorge Gaspar Armenta We consider attenuated total reflection configuration in order to excite surface waves on one-dimensional photonic crystal-air interface. The scattered reflected light is calculated under resonant condition. We find amplified scattered light when incident light is resonant with the surface wave. [Preview Abstract] |
|
C1.00030: Irreversible electric and magnetic effects in manganese perovskites Empress Etheridge, Feng Chen, Natalia Noginova Microwave absorption and dc charge transport were studied in single crystals of manganese perovskites. Irreversible changes in dc electric conductivity were observed in a broad temperature range. Starting from 140 K and below, similar irreversible changes were observed in microwave field dependent absorption. Irreversibility T-H dependence was obtained. We explain these effects taking into account local lattice and magnetic phase transformation processes. [Preview Abstract] |
|
C1.00031: Comparison of the electronic structures of four crystalline forms of FePO$_{4}$. Ping Tang, N. A. W. Holzwarth The purpose of this study is to investigate the stability of FePO$_{4}$, the fully charged form of the Li ion battery material, LiFePO$_{4}$. The electronic structures of four phases of FePO$_{4}$, having the CrVO$_{4}$,\footnote{J. P. Attfield and co-workers, {\em{J. Solid State Chem.}} {\bf{57}}, 357 (1985)} quartz,\footnote{H. N. Ng and C. Calvo, {\em{Can. J. Chem.}} {\bf{53}}, 2064 (1975)} monoclinic,\footnote{ Y. Song and co-workers, {\em{Inorg. Chem.}} {\bf{41}}, 5778 (2002)} and olivine\footnote{A. S. Andersson and co-workers, {\em{Solid State Ionics}} {\bf{130}}, 41 (2000)} structures are studied using density functional theory with LSDA and GGA exchange-correlation functionals. A ferromagnetic spin configuration is chosen for all of the FePO$_{4}$ calculations. The band structures, density of states, and total energies are compared among all four phases. The results show that the CrVO$_{4}$ phase is metallic while the other phases are insulating. The band gaps of the insulating phases are found to increase with decreasing density of the material. [Preview Abstract] |
|
C1.00032: Diffuse scattering and low-energy phonons in superionic conductor Cu$_{1.8S}$Se Sergey Danilkin, Andreas Hoser, Werner Schweika The neutron diffuse and inelastic scattering were studied in the superionic $\alpha $-phase of copper selenide. In neutron diffraction experiments on Cu$_{1.85}$Se single crystal the diffuse scattering features were observed along [111] direction in vicinity of (400) and (422) reflections. In inelastic neutron scattering measurements performed with time-of-flight spectrometer the elastic and inelastic scattering processes were separated and a strong inelastic scattering was observed also along [111] nearby (400) and (022). This shows that diffuse scattering found in conventional diffraction experiment is mainly inelastic and most probably comes from the low-energy phonons. Such phonons with optic-like behaviour of transverse acoustic modes at $q/q_{m} \quad >$ 0.2-0.4 were found earlier in $\alpha $-Cu$_{1.85}$Se [1]. [1] S.A.~Danilkin, A.N.~Skomorokhov, A.~Hoser, H.~Fuess, V.~Rajevac, N.N.~Bickulova, Crystal structure and lattice dynamics of superionic copper selenide Cu$_{2-\delta }$Se, J. Alloys and Compounds, 2003, v. 361, p. 57-61. [Preview Abstract] |
|
C1.00033: Mössbauer evaluation of aerogel-prepared Fe2(MoO4)3 H.H. Hamdeh, J.C. Ho, R.J. Willey Fe-Mo-O binary oxides are among the catalysts for selective oxidation of, e.g., hydrogen sulfide and hydrocarbons. Their catalytic activities are composition- and structure-dependent. In this work Fe$_{2}$(MoO$_{4})_{3}$ was prepared first as aerogel in an autoclave, from starting materials of ferric acetylacetonate and dioxomolybdenum, then oxidized and annealed. M\"{o}ssbauer measurements were carried out to complement other techniques for structural and magnetic analysis. The data can be well fitted to two subspectra in the ratio of approximately 2:1. The major component remains as a singlet from 300 K down to 25 K, while a well-defined sextet representing a magnetic ordering gradually developed below 200 K for the other. Its hyperfine field, as well as the isomer shift and quadrupole splitting values for both subspectra were obtained. [Preview Abstract] |
|
C1.00034: The role of the symmetry on the luminescent properties of Zr5Ti7O24:Eu3+ Federico Gonzalez, Antonio Muñoz-Flores, Jorge Garcia-Macedo, Manuel Aguilar The study of the luminescent properties of trivalent lanthanides incorporated into several crystalline matrices, is strongly motivated because of their technological applications in optoelectronics devices and flat panel displays. Then, it is important the systematic research of the RE hosted in different kind of matrices with good mechanical and thermal properties and chemically stables. Zirconium titanate Zr$_{5}$Ti$_{7}$O$_{24}$ (ZT) has exhaustively studied in the past years, mainly by their electrical properties, but there are not reports about the lanthanide doped ZT. In order to establish the optical behavior of ZT:Eu$^{3+}$ we prepared this material with molar concentration of Eu$_{2}$O$_{3}$ at 1 mol{\%} using the sol-gel processing. The crystalline structure of the compound was obtained by X-Ray diffraction and corresponds to the space group Pbcn. The time resolved luminescent spectra shown two aggregation types of Eu$^{3+}$ into ZT samples, corresponding with two different exponential decays. We discuss about the origin of the aggregation types of europium and the importance of symmetry on the Eu$^{3+}$ transition lifetimes. . [Preview Abstract] |
|
C1.00035: Time-Resolved Transient Absorption Spectroscopy and Luminescence in Undoped YVO$_{4}$ H.M. Yochum, L. Grigorjeva, D. Millers, V. Pankratov, M.C.S. Yochum, K.T. Stevens Yttrium orthovanadate (YVO$_{4})$ is currently used as a solid-state laser host material and as a polarization component for fiber optic applications but suffers from crystal uniformity and discoloration problems. In an effort to increase our understanding of electron-hole traps and their role on the optical properties of YVO$_{4}$, we have measured transient absorption spectra (1.1 eV - 3.5 eV) and relaxation kinetics following electron-hole pair generation induced by 270 keV, 10 ns electron pulses for probe delays 10 ns--10 microseconds. These experiments were completed on four Czochralski grown YVO$_{4}$ samples with different levels of as-grown absorption in the 380 nm -- 430 nm region. For all samples, we find three transient absorption bands with similar energies. We have also measured the luminescence kinetics induced by ns electron pulses and by light pulses at 355 nm and 420 nm -- 600 nm. [Preview Abstract] |
|
C1.00036: INSTRUMENTATION AND MEASUREMENTS |
|
C1.00037: Hysteresis And Magnetoelastic Domain Wall Behavior of Two Jacob Hoberg, Jason Orris, Gregory Sollenberger, Doug Franklin, Mark Boley We have produced torque sensors from type A-2 and type H-13 tool steels for torque transfer applications in a 0.75 inch outer diameter hollow shaft by magnetically polarizing two adjacent sections with oppositely directed circumferential magnetization. The resultant field signal, found to be linear with applied torque up to 15 N-m, emanated from the domain wall formed between the two regions. Heat treatment resulted in an increase in torque-load sensitivity (field signal in $\mu $G per unit applied shear stress in psi) from 48.2 $\mu $G/psi to 59.2 $\mu $G/psi in the A-2 sample and from 125 $\mu $G/psi to 189 $\mu $G/psi in the H-13 sample, improved linearity of the signals, and a more reliable re-zeroing of the sensors following removal of the applied torque. The axial coercive forces were found to decrease prior and subsequent to heat treatment, with the percent of decrease in correlation to the percent of increase in the sensitivities found above, while the circumferential coercive forces were sufficiently large to guarantee integrity of the magnetically polarized regions comprising the sensor. The width and magnetic intensity of the domain wall in each sensor were also measured using the technique of magnetic force microscopy. [Preview Abstract] |
|
C1.00038: Large-Scale and Small-Scale Torque Sensors From a Magnetoelastic Heat-Treated 14\% Chromium Stainless Steel Christopher Jurs, Jason Orris, Doug Franklin, Mark Boley We have produced a large scale (0.75 inch) and a small scale (0.25 inch) torque sensor from type ESR-420 stainless steel for torque transfer or small scale applications by polarizing two adjacent sections of the shafts with oppositely directed circumferential magnetization. The resultant field signal, found to be linear with applied torque up to 15 N-m, emanated from the domain wall formed between the two regions. A heat treatment consisting of a rapid quench to room temperature from 1038\r{ }C, followed by a slow cool from 871\r{ }C to restore desired magnetic and mechanical properties, was applied to enhance performance. The torque-load sensitivity (field signal in $\mu $G per unit applied shear stress in psi) was found to be linear and as high as 237 $\mu $G/psi, with excellent re- zeroing capability, making it ideal for small-scale applications where weak signals are usually a plaguing problem. Hysteresis properties studied prior and subsequent to the heat treatments show that the axial coercive force remained around 5-6 Oe throughout heat treatment, in correspondence with the large sensitivity values, while the circumferential coercive force remained around 25-27 Oe, which is sufficient to guarantee integrity of the magnetically polarized regions at both scale levels. [Preview Abstract] |
|
C1.00039: The Dependence of Sensitivity, Axial Coercivity, and Domain Wall Height on Chromium Concentration in Polarized Steel Alloy Torque Transducers Ryan Gordon, Jason Orris, Jason Wilson, Doug Franklin, Mark Boley We have discovered that a large enhancement in the sensitivity of magnetically polarized steel torque transducers can be gained by increasing the chromium concentration of the steel to as high as the 12{\%} level (with nickel concentrations under l{\%}), without the sacrifice of any essential mechanical properties. Beyond this optimal concentration level, the enhancement in the sensitivity appears to have saturated and then gradually disappears as additional chromium is added. Among the steel shafts we investigated were four with respective chromium concentrations of 0.15{\%}, 3.5{\%}, 12.0{\%}, and 25.5{\%}, that are correspondingly known as steel types W-1, S-7, D-2, and F-255. Their transducer sensitivities were found as 3.1, 54.2, 97.7, and 19.3 $\mu $G/psi, respectively. Correspondingly, the area of the axial hysteresis curves for the first three of these steels dramatically decreased as the chromium concentration went up to the 12{\%} level, and then gradually increased again as additional chromium was added. Using the technique of magnetic force microscopy, we also determined that the heights of the center domain wall between the two magnetically polarized regions were directly correlated to the observed sensitivities. [Preview Abstract] |
|
C1.00040: Performance of a fast scan mobility based particulate spectrometer Donald Hagen, Philip Whitefield Nanoparticles are emitted from a variety of combustion sources. There is a growing concern, internationally, on the environmental impact of jet engine exhaust emissions in the atmosphere. An accurate assessment requires that the number density and size of the aerosols within engine exhaust and aging plumes be understood and well characterized. Soot particles formed during fuel combustion and emitted metallic particles constitute the solid (nonvolatile) particle fraction present in exhaust plumes. UMR has developed a mobile facility capable of sampling jet and rocket engine exhaust emissions, including both particulate and gas phase species, and has deployed it in numerous ground test and airborne campaigns. Normally emissions measurements are taken with the engine in a stable operating condition. A fast scan particulate spectrometer, Cambustion's DMS500, has recently been added to our facility's instrument suite. It has a fast response, around 200 ms, and covers the size range from 5 to 1000 nm. This fast response allows for reduced engine run times, and therefore lower costs, during measurement campaigns, and for observation of emission transients during changes in engine operating conditions. Here we report on the fast spectrometer's performance in comparison to traditional particulate size and concentration measurement systems, i.e. scanning differential mobility analyzers and condensation nuclei counters. [Preview Abstract] |
|
C1.00041: An Electrically Tunable Barium Strontium Titanate Phase Shifter S.F. Karmanenko, A.A. Semenov, B.A. Kalinikos, A.N. Slavin, G. Srinivasan, J.V. Mantese It is well known that a slot-line fabricated on a dielectric substrate supports the propagation of electromagnetic waves. In order to realize electrical tuning of the slot-line characteristics, nonlinear dielectrics such as barium strontium titanate (BST) could be used as substrates. A prototype BST phase shifter was fabricated and characterized at 47 GHz. The design is based on a multi-slot line patterned on an alumina substrate and a BST film of thickness 0.6 $\mu $m and dielectric permittivity of 1500. The electrode topology on the BSTO film includes three inner copper electrodes of thickness 3 $\mu $m. The width of the inner electrodes is equal to 50 $\mu $m, and the gap between them is also 50 $\mu $m. A part of the outer electrodes had a funnel-shape configuration in order to provide necessary coupling with input/output circuits. Measurements of the phase shift at 47 GHz revealed a differential phase shift of 270 deg. for a bias voltage of 500 V. The total signal attenuation was about 7 dB. -Work supported by a grant from the Delphi Automotive Corporation. [Preview Abstract] |
|
C1.00042: Holographic Visualization of Vibration in a Moist Clarinet Reed Jason Bostron, Mark Sharnoff We have extended the work of Pinard et al (J. Acoust. Soc .Am. 113, 1376 (2003)) [see also Facchinetti et al (ibid, p. 2874)] on dry clarinet reeds to permit the modes of moisture-saturated reeds to be visualized. By means of an artificial embouchure, nitrogen gas at 96{\%} relative humidity was passed into a clarinet whose reed was normally attached at the ligature, but free of other constraint. An image of the reed was focused upon a CCD upon which was incident also a collimated reference wave. Just beyond the clarinet's bell was a loudspeaker which excited the clarinet's air column and reed into vibration. The reed's motion could be analyzed from the fringes visible in single-exposure, time-averaged interferograms. When dry, our reeds exhibited low compliance except in the vicinity of sharp resonances whose frequencies, extending upwards from ca. 2.2 kHz , all lay above the fundamentals of the clarinet's musical voice. By contrast, moist reeds were easily excited at any frequency within our clarinet's playing range, which extended downward to D3 at 147 Hz. At almost any frequency, the vibrations of the moist reed were strong mixtures of the flexing and torsional modes exhibited separately in the resonances of the dry reed. [Preview Abstract] |
|
C1.00043: Determination of the p-n Junction Built-in Voltage using Quantitative Dopant Mapping in the FEG-SEM Payam Kazemian, Colin J. Humphreys, Sjoerd A.M. Mentink Two-dimensional dopant mapping with nanometre-scale spatial resolution is possible in semiconductors using secondary electron (SE) imaging in a Field Emission Gun Scanning Electron Microscope (FEG-SEM). However, existing methods are only qualitative. Here, a quantitative SEM method for measuring dopant concentrations and the built-in voltage across a p-n junction is given. The energy spectrum of SEs emitted from the p-doped side of a p-n junction is shifted in energy with respect to SEs emitted from the n-doped side by eV$_{b}$, where V$_{b}$ is the built-in voltage across the junction. We have measured this energy shift using a FEI XL30s FEG-SEM equipped with a through-the-lens (TTL) SE detector. It is possible to set up an adjustable energy window in this microscope by using the extractor tube potential and the deflector voltage in the TTL detector as the lower and upper edges of the energy window. By varying these voltages, the FEG-SEM can be used to analyse the energies of the secondary electrons emitted from the specimen, and the energy shift between the SEs emitted from the p and n sides of a p-n junction can therefore be measured. This energy shift, essentially eV$_{b}$, across a p-n junction can therefore be obtained using SE imaging in a FEG-SEM, which enables two-dimensional quantitative dopant profiling to be rapidly performed. [Preview Abstract] |
|
C1.00044: A 3-D Finite Element Method Analysis for Microwave Imaging of Complex Permittivity Shuogang Huang, Mark Reeves We develop a new technique to estimate the complex permittivity of dielectric material in microwave frequency by using 3-D Finite Element Method software. Starting with the Perturbation theory, we calculate the electric field pattern in near-field zone of the Scanning Microwave Microscope probe and the post-processing integration converts the field energy to frequency shift. The FEM computation is carried out several times while tip-sample separation varies. We compare numerical simulation results with experimental values both in bulk and thin-film cases. Finally, the sensitivity analysis of the model is discussed. [Preview Abstract] |
|
C1.00045: Atomically resolved studies of reactions at industrial settings - novel design of an ultra high pressure, high temperature scanning tunneling microscope system for probing catalytic conversions David Tang, Miquel Salmeron, Gabor Somorjai In order to observe heterogeneous catalytic reactions that occur well above ambient pressure and temperature, a modified version of the Pan-style STM motor has been designed and constructed in-house. The new design features a much reduced size and a rigid coupling to the sample, and has been tested to show much higher resonant frequency than conventional Beetle-style STM designs, providing the ability to image faster and yielding lower susceptibility to noise. A small flow reactor cell ($\sim $10 mL) has been constructed to house the new STM, whose samples and tips are accessible through a bayonet-sealed access port by the use of a wobble stick and a transfer arm. The reactor cell can be placed inside an UHV system to allow cleaning and characterization of sample before and after experiments, as well as continuous monitoring by mass spectrometry or gas chromatography through a leak valve. The new system also allows \textit{in vacuo} sample and tip exchange without exposing the system to impurities in air. As such, the new ultrahigh pressure scanning tunneling microscope is designed to allow successive STM experiments performed with precise control of temperatures between 300 K and 600 K and pressures between $<$10$^{-9}$ torr and 30 bars. [Preview Abstract] |
|
C1.00046: Integrated Cantilever Loop Probe for Magnetic Resonance Force Microscopy Douglas Lagally, Abdolreza Karbassi, Charles Paulson, Daniel van der Weide The measurement of magnetic fields, especially at the nanoscale, has become an issue of considerable interest. Applications include quantum computing, data storage, and magnetic resonance imaging (MRI). At the interface between conventional atomic force microscopy and MRI lies magnetic resonance force microscopy. Radio frequency (RF) waves excite electrons in a sample, and a magnetic cantilever probe can image a slice of the sample in which the electron resonance is altered by the probe field. We have fabricated a novel probe that incorporates the RF loop onto the cantilever itself, allowing sensitive spatial resolution. While single-electron imaging has recently been accomplished [1], our goal is to produce a more robust system based on readily accessible technology. The probe loop is composed of 250 nm thick Au, patterned to include shielding and coaxial contact pads. The cantilever is etched from high-resistivity single-crystal Si using DRIE. We present proof of theory, along with simulations of magnetic and electrical fields. [1] D. Rugar et al. Nature Vol. 430, p. 329 (2004) [Preview Abstract] |
|
C1.00047: Long-range order of defects revealed by Mossbauer powder diffracometry Jiao Lin, Ryan Monson, Brent Fultz The improvement in our M\"{o}ssbauer powder diffractometer enabled measurement of multiple diffraction patterns at Doppler velocities across all nuclear resonances in a partially-ordered $^{57}$Fe$_{3}$Al. The GCSAXI program was developed to convert detector images to diffraction patterns and then extract the intensities of fundamental and superlattice diffractions. MDS, a M\"{o}ssbauer diffraction simulation program, was developed based on kinematical diffraction theory to calculate M\"{o}ssbauer diffraction intensities from given structures. The MEF program was developed to compare experimental data to calculations and search for best long-range order and short-range order parameters by using a nonlinear fitting alogrithm. The diffraction intensities from the $^{57}$Fe$_{3}$Al sample were analyzed by these three programs to provide data on the long-range order of Fe atoms having different numbers of Al neighbors. It was shown that Fe atoms having three Al atoms as first-nearest neighbors (1nn) are partially simple-cubic ordered, similar to that of Fe atoms with four Al 1nn. The ordered state of Fe atoms with three Al 1nn can be mostly accounted for by homogeneous antisite disorder and antiphase domain boundaries. Further improvement of the performance of our M\"{o}ssbauer powder diffractometeer is under way by exploring the possibility of adopting CdZnTe position sensitive detector. [Preview Abstract] |
|
C1.00048: A novel capillary electrophoresis microchip with amperometric detection using a Prussian blue-modified indium tin oxide electrode Ju-Ho Kim, In-Je Yi, C. J. Kang, Yong-Sang Kim A novel approach to construct a disposable capillary electrophoresis microchip is proposed. The electrocatalytic oxidation of dopamine at a Prussian blue (PB)-modified indium tin oxide (ITO) electrode was described and the amperometric detection of dopamine was then investigated. The PB film on ITO electrode was electrodeposited using FeCl3 and K3Fe(CN)6 mixed solution. Our results indicated that PB film was uniform, smooth, and defect-free. The CE-chip has been tested successfully by detecting dopamine and catechol within a very short time of around 80 sec using an electric field of 60 V/cm. The results also showed that dopamine and catechol mixtures were separated efficiently and rapidly. The microsystems gave a very good reproducibility for peak height and separation time. This microchip is cost effective and adequate for a disposable sensor. [Preview Abstract] |
|
C1.00049: Piezoresistive characteristic of microcantilever-based biosensor according to doping level of polysilicon resistor Kwang-Ho Na, Hyung-Do Kim, Sung-Hwak Park, C.J. Kang, Yong-Sang Kim We investigate about the resolution limit for a surface stress sensitive cantilever that microcantilever-based biosensor with piezoresistor has been fabricated using surface micromachining technique. It is an advantage to obtain as high a gauge factor as possible in order to maximize the sensitivity. This piezoresistive characteristic depends on the doping level and the grain size in the material. From this characterization, the best material composition can be found and then used for further optimization. The LPCVD polysilicon is used for piezoresistor and its thickness is 300nm. The polysilicon were implanted with boron of various and adjustable dose concentration and are encapsulated in LPCVD silicon nitride. We evaluate the characteristics of piezoresistive that doping level with boron. The gauge factor decreases by up to 40{\%} when the doping is increased. The results presented that the optimized piezoresistive readout has much better sensitivity than optical readout. [Preview Abstract] |
|
C1.00050: PHASE TRANSITIONS AND STRONGLY CORRELATED SYSTEMS |
|
C1.00051: Projection operator method CPA to single-particle excitation spectra Yoshiro Kakehashi, Peter Fulde Single-site theories for electron correlations such as the many-body CPA, the dynamical CPA, and the dynamical-mean field theory are useful as a starting point to describe strongly correlated electron systems. Nevertheless, simple and useful treatments of correlations being applicable to the realistic system has not yet been well developed. We propose here the projection operator technique combined with the many-body CPA which allows us to calculate the excitation spectrum directly from the retarded Green function. The basic idea is to introduce an energy dependent Liouville operator for the description of the dynamics of correlated electrons. The self-energy obtained by a renormalized perturbation scheme describes the overall features of excitation spectra. The quasiparticle weight for a half-filled band in infinite dimensions show a simple analytic form $Z=[1-(U/U_{c2})^{2}]/[1+(U/U_{2})^{2}]$ with $U_{c2}=3.705$ and $U_{2}=2.522$ (hypercubic lattice) which reproduces well the result of the numerical renormalization group calculations. The critical Coulomb interaction $U_{c1}$ for a gap formation is 3.69 when the Hartree-Fock wave function is used for the evaluation of the static average, and 3.24 when the correlated wave function is used. The latter agrees with the result of the NRG within 1\% error. [Preview Abstract] |
|
C1.00052: High Resolution NMR $^{15}$N and $^{31}$P NMR Of Antiferroelectric Phase Transition in Ammonium Dihydrogen Arsenate and Ammonium Dihydrogen Phosphate Ozge Gunaydin-Sen, Randall Achey, Riqiang Fu, Naresh Dalal Natural abundance $^{15}$N CPMAS NMR has been used to investigate the paraelectric-antiferroelectric phase transition of NH$_4$H$_2$AsO$_4$ (ADA) (T$_{N}\sim 216$K) and of NH$_4$H$_2 $PO$_4$ (ADP) (148K), with a focus on the role of the NH$_{4}^{+} $ ion. Isotropic chemical shift of $^{15}$N for ADA exhibits an almost linear temperature dependence to within T$_{N}\pm$1K, and then changes discontinuously, followed by another almost linear dependence. The spectra of the paraelectric and antiferroelectric phases coexist around the T$_N$. The sharp anomaly around T$_N$ implies that the NH$_{4}^{+}$ ions undergo a displacive transition, whereas the protons in the O$-$H$\cdots$O bonds undergo an order-disorder transition. The $^{15}$N data thus support a mixed order-disorder-displacive mechanism for this transition. The $^{15}$N data on ADP exhibit somewhat different behavior. $^{31}$P CPMAS measurements will also be presented and discussed in terms of the above model. [Preview Abstract] |
|
C1.00053: Diffuse X-Ray Scattering from the Liquid-Vapor Interfaces of Dilute Ga:Bi and Ga:Tl alloys Dongxu Li, Xu Jiang, Stuart A. Rice, Binhua Lin, Mati Meron X-Ray diffuse scattering and Grazing Incidence Diffraction from the liquid-vapor interfaces of dilute alloys Ga:Bi and Ga:Tl are measured. The contributions from bulk scattering and the diffraction of the segregated monolayer are subtracted, and the resulting intensity is diffuse scattering due to the interfacial roughness. Unlike the pure Ga case, the diffuse scattering intensity is found to follow the capillary type $q^{-2}$ decay up to a larger wavevector $q\sim2nm^{-1}$, and the reduction of wavevector dependent interfacial tension is not observed. The interfacial tensions and the two dimensional compressibility of the segregated monolayer are obtained by fitting the surface scattering intensities to a model with a $q^{-2}$ diffuse scattering intensity and the diffraction from a $q2D$ monolayer. In the Ga:Tl case, the segregated Tl monolayer undergoes a first order phase transition, and the behavior of the diffuse scattering is not effected by the $q2D$ phase transition. [Preview Abstract] |
|
C1.00054: The alpha-Ga (010) Surface Investigated by Room and Low Temperature Scanning Tunneling Microscopy Natalya Pertaya, Kai-Felix Braun, K.-H. Rieder Low temperature STM, room temperature STM and helium atom sccattering (HAS) was used for an investigation of the Ga(010) surface. Here the surface has been imaged for the first time in the reconstructed low temperature phase with scanning tunneling microscopy and the unit cell was determined in detail. The presence of a charge density wave is not expected apriori on the Ga(010) surface, but was identified unambiguously with tunneling spectroscopy. Surprisingly, two domains form well ordered parallel stripes. At room temperature the atomically resolved structure of the 1x1 phase of Ga was imaged for the first time with the normal imaging mode. An extensive helium atom scattering study of the surface phonons complements the investigation of the Ga(010) surface. [Preview Abstract] |
|
C1.00055: Quantum phase transitions in the transverse 1-D Ising model with four-spin interactions O. F. de Alcantara Bonfim, J. Florencio In this work we investigate phase transitions in the transverse Ising model with four-spin interactions, induced by quantum fluctuations. The model is relevant to the physics of poly(vinylidenefluoride-trifluoroethylene)[P(VDF-TrFE)]. We calculate the ground state and the first excited state energies of the system using Lanczos method. Our calculations are performed using rings up to 20 spins. Finite size scaling is applied to the energy gap to obtain the boundary region where a ferromagnetic to paramagnetic transition takes place, as well as the corresponding critical exponents. A new degenerate $<$3,1$>$ phase region is found. The first-order transition boundary between this new phase and the paramagnetic phase is determined by analyzing the behavior of the transverse spin magnetization as the system moves from one region to the other. [Preview Abstract] |
|
C1.00056: X-ray absorption study of the electronic structures of quasi-one-dimensional Co oxides Ca$_3$Co$Y$O$_6$ ($Y$ = Co, Rh, Ir, Mn) J. Okamoto, D.J. Huang, H.-J. Lin, C.T. Chen, A. Fujimori, E.V. Sampathkumaran Crystal structures of Sr$_{3}$NiIrO$_{6}$-type Ca$_{3}$Co$Y$O$_{6}$ ($Y$=Co, Rh, Ir, Mn) are characterized by the chains of alternating CoO$_{6}$ trigonal prisms and $Y$O$_{6}$ octahedra along $c$-axis. Since these chains form a triangular lattice in \textit{ab}-plane and interaction between chains is generally anti-ferromagnetic, magnetic frustration is expected to be observed in these compounds. Ca$_{3}$Co$_{2}$O$_{6}$ shows ferromagnetic-intrachain ordering and anti-ferromagnteic interchain coupling at 24 K and shows magnetic frustration between chains at 10-24 K. Ca$_{3}$CoRhO$_{6}$ also shows the same magnetic frustration at 30-90 K. But this magnetic frustration is not observed in Ca$_{3}$CoIrO$_{6}$ and Ca$_{3}$CoMnO$_{6}$. Direct information about their electronic structures is necessary to understand the mechanism of their interesting magnetic behavior. We have measured Co $2p$ and O $1s$ x-ray absorption spectra (XAS) of Ca$_{3}$Co$Y$O$_{6}$ ($Y$=Co, Rh, Ir, Mn) to study their electronic structures. In the O $1s $XAS spectra large spectral change has been observed in region of 526-528 eV. We will discuss the influence of the trigonal-prism-site Co $3d$ states on the magnetic interaction of Ca$_{3}$Co$Y$O$_{6}$. [Preview Abstract] |
|
C1.00057: Quantum Critical Behavior of Disordered Hardcore Bosons Anand Priyadarshee, Ji-Woo Lee, Shailesh Chandrasekharan, Harold U. Baranger We study the finite temperature and zero temperature critical behavior in hard core bosons with (a) uniform (b) disordered and (c) staggered chemical potential. This model can also be thought of as quantum XY model in a transversed magnetic field. Using the directed loop Monte Carlo algorithm on large square lattices, we calculate the pair susceptibility (Xp) and winding susceptibility (Xw). We find the usual Kosterlitz-Thouless (KT) thermal transition in all three cases. At very low temperatures, however, the phase boundary profile in these cases differ from each other. In the zero temperature analysis, we define the exponent for the scaling of Xp near the critical Mu to be 2beta, and similarly, the exponent for Xw to be Nu. In the uniform case, the exponents are expected to be mean field beta=0.5, Nu=1). Even at Mu of order 98% of the critical Mu and for lattices as large as 96x96, we see deviation from mean field behavior. For the lattices we have explored the curvature in our data suggests that the (beta<0.5, Nu<1) in the uniform case and (beta>=0.5, Nu>=1) in the disordered case. [Preview Abstract] |
|
C1.00058: Ultrasonic dispersion due to the rattling motion in the filled skutterudite La(Os$_{0.5}$Ru$_{0.5})_{4}$Sb$_{12}$ Tatsuya Yanagisawa, W.M. Yuhasz, M.B. Maple, Y. Nemoto, T. Goto The filled skutterudites$ R$M$_{4}$Sb$_{12}$ ($R$: rare-earth, M: transition metal) show a considerable reduction of thermal conductivity, which is caused by the rattling motion of the weakly bonded rare-earth ion in an oversized icosahedral cage of antimony. Ultrasound measurements are useful for observing the rattling motion of the rare-earth ion. Our recent ultrasonic study* demonstrated the ultrasonic dispersion in the elastic constant ($C_{11}-C_{12})$/2 due to the rattling motion of Pr in PrOs$_{4}$Sb$_{12}$, which is the first Pr-based heavy fermion superconductor with $T_{C }$= 1.85 K. In order to examine the rattling motion in a system without 4f-electrons, we have performed ultrasonic measurements on La(Os$_{0.5}$Ru$_{0.5})_{4} $Sb$_{12}$. The longitudinal $C_{11}$ mode shows Debye-type dispersion and, at around 50 K, the ultrasonic echo shows the highest attenuation. The rattling motion of La has thermally activated behavior according to the relaxation data. In contrast, the transverse $C_{44 }$mode shows only a monotonic increase. These results suggest that, like PrOs$_{4}$Sb$_{12}$, the rattling motion in La(Os$_{0.5}$Ru$_{0.5})_{4}$Sb$_{12 }$has $\Gamma _{3}$ symmetry. T. Yanagisawa is supported by a Research Fellowship of the JSPS Young Scientists. The research at UCSD was supported by U.S. DOE., [*Phys. Rev. \textbf{B 69}, 180511 (R) 2004] [Preview Abstract] |
|
C1.00059: Effects of Lattice Anisotropy on Bipolaron Symmetry Jun Zhou, Erica Carlson, David Campbell We study the pairing symmetry of large bipolarons as a function of lattice anisotropy and the binding energy of a small polaron, using variational wavefunctions. We find that an $s$-wave state is stable over a wide range of parameters. However below a threshold binding energy, we find evidence for a singlet to triplet pairing transition. We find that $p$-wave states are stable for sufficient anisotropy, with lobes oriented perpendicular to the easy hopping direction. [Preview Abstract] |
|
C1.00060: Coexistence of `Charge Order' Modulation, Tweed Microstructure and Needle Twins in La1-xCaxMnO3 (x=0.67 and 0.71) James Loudon, Paul Midgley Using transmission electron microscopy, we have observed the coexistence of two non-ferromagnetic phases in La$_{1-x}$Ca$_x$MnO$_3$ ($x = 0.67$ and $0.71$) at 90 K: a modulated `charge ordered' phase and a modulation-free region composed of needle twins. Micron-sized regions containing these needle twins formed below 170~K over 30 seconds in La$_{0.33}$Ca$_{0.67}$MnO$_3$ when cooled at a rate of $\sim 0.2$~K/minute. A tweed microstructure was observed in the interface region between these two phases. We estimate that the needle twins occupied less than 10\% of the volume of the manganites investigated here. [Preview Abstract] |
|
C1.00061: FLUIDS |
|
C1.00062: Static and Dynamic Anomalies in a Repulsive Spherical Ramp Liquid: Theory and Simulation Pradeep Kumar, Sergey V. Buldyrev, Francesco Sciortino, Emanuela Zaccarelli, H.E. Stanley We compare theoretical and simulation results for static and dynamic properties for a model of particles interacting via a spherically symmetric repulsive ramp potential. The model displays anomalies similar to those found in liquid water, namely, expansion upon cooling and an increase of diffusivity upon compression. In particular, we calculate the phase diagram from the simulation and successfully compare it with the phase diagram obtained using the Rogers-Young (RY) closure for the Ornstein-Zernike equation. Both the theoretical and the numerical calculations confirm the presence of a line of isobaric density maxima, and lines of compressibility minima and maxima. Indirect evidence of a liquid-liquid critical point is found. Along constant temperature paths, as the density increases, the dynamics alternates several times between slowing down and speeding up. Finally we confirm that mode coupling theory successfully predicts the behavior of dynamics and the presence of multiple glass phases. [Preview Abstract] |
|
C1.00063: Optical Response of a Nematic Sample Submitted To a Periodic External Electric Field: Role of the Ionic Impurities Laura Olivia Palomares, Juan Adrian Reyes, Giovanni Barbero The influence of the ions dissolved in a nematic liquid crystals on the optical response of a nematic sample submitted to a periodic square wave is investigated. We show that according to the density of ions and of the properties of the dielectric layer deposited on the electrodes to avoid charge injection, different regimes for the optical phase difference are found. The role of the mobility of the ions on the phenomenon is investigated. We evaluate the surface density of adsorbed ions, by assuming a simple expression for the kinetic equation at the limiting surface. We suppose that the diffusion current is negligible with respect to the drift current. In this framework, the electrical problem relevant to the back electric field due to the redistribution of the ions is solved analytically. The adsorption problem is analyzed in the Langmuir approximation of low coverage of adsorbed particles. A possible extension of the model of Langmuir is also proposed. [Preview Abstract] |
|
C1.00064: Spatial Solitons in Chiral Media Carlos Gabriel Avendano, Juan Adrian Reyes We study theoretically the nonlinear propagation of a narrow optical wavepacket through a cholesteric liquid crystal. We derive the equations governing the weakly nonlinear dynamics of an optical field by taking into account the coupling with the liquid crystal. We constructed the solution as the superposition of four narrow wavepackets centered around the linear eigenmodes of the helical structure whose corresponding envelopes A are slowly varying functions of their arguments. We found a system of four coupled equations to describe the resulting vector wavepacket which has some integration constants and that under special conditions reduces to the Nonlinear Schrodinger equation with space-dependent coefficients. We solved this equation both, using a variational approach and performing numerical calculations. We calculated analytically, the soliton spatial scales, the transported power, the nonlinear refraction index and its wavelength dependence showing that this has its maxima at the edges of the reflection band. We also exhibit the existence of some other exact but non self-focused solutions. [Preview Abstract] |
|
C1.00065: Crystallization and micro-structure in high charged colloids Efrain Urrutia-Ba\~nuelos, Helim Aranda-Espiniza, Alvaro Posada-Amarillas, Mart\'In Chavez-Paez Crystallization and micro-structure of the high charged colloids particles was studied by computer simulation, the particles interaction was modeled by a screened Coulomb potential. We used 3000 particles in our simulation cell to let them evolution from an initial random configuration, periodic boundary conditions was imposed to simulate the bulk. The time evolution of the excess entropy shows long-ranged self-ordering and a crystalline transition, the crystalline nucleation depends of the volume fraction as well as the particle charges. The common neighbor analysis (CNA) exhibit the competition of two micro-structure types, icosahedral and bcc. In the equilibrium bcc crystalline order is dominant with relative abundance between 80-90 per cent over the other micro-structures. [Preview Abstract] |
|
C1.00066: Stability of an iodate arsenous-acid reaction front in the presence of Poiseuille flow Robert S. Spangler, Boyd F. Edwards An iodate arsenous acid reaction front propagating against gravity provides an excellent context in which to examine stability. The system tends to be unstable since the heavier fluid component is above the lighter. A front profile whose propagation speed is slowed by curvature tends to stabilize the front. The Navier-Stokes equations predict a parabolic (Poiseuille) velocity profile for a fluid moving between to no-slip boundaries. We extend the existing theory that describes the iodate arsenous acid reaction front to include this Poiseuille flow. [Preview Abstract] |
|
C1.00067: A thermopneumatic-actuated PDMS microfluidic system integrated with micropump amd microvalve Jong-Chul Yoo, Min-Chul Moon, C. J. Kang, Yong-Sang Kim We developed microfluidic devices integrated with microvalves and micropumps which is essential to develop a lab-on-a-chip. The advantage of the proposed device includes low cost fabrication process and the optical transparency using PDMS and ITO glass. Also the proposed micropump has the same fabrication process and substrate with the in-channel structured microvalve. The flow rate of the microvalve is proportional to the channel width, however, the power needed to close the microvalve is around 100 mW which is almost the same regardless of the channel width. The flow rate can be well controlled by ON/OFF switching function of the ITO heater and the closing and the opening times are around 20 sec and 25 sec, respectively. The pumping rate of the micropump increases linearly as the applied pulse voltage to the ITO heater increases. The maximum pump rate of 78 nl/min was obtained at the applied frequency of 6 Hz and duty ratio of 10 {\%}. The characteristics of microfluidic devices integrated with microvalve and micropump will be optimized. [Preview Abstract] |
|
C1.00068: Nonlinear Motion of Optically Torqued Nanorods Keith Bonin, W. Andrew Shelton, Thad Walker We apply light torques to single optically trapped glass nanorods suspended in water. The resulting motion is carefully studied experimentally and consists of two distinct regimes: a linear regime where the rod angle increases linearly with time and a nonlinear regime where the rod angle changes nonlinearly, experiencing accelerations and rapid reversals. We motivate a theoretical model for the motion of such nanorods, which agrees extremely well with the observed motion. In this report, the trapped and torqued nanorods move without influence from surfaces. Such a model system is helpful to understanding the more complex motion that occurs near a surface. Studying such nonlinear motion both free of, and near, a surface is important for understanding nanofluidics and hydrodynamic motion at the nanoscale. As such, we will also present some data on how the motion is different close to a surface. [Preview Abstract] |
|
C1.00069: The sedimentation of a sphere in a vertical cylinder with periodically varying radius Watson L. Vargas, Lyda M. Pineda, Carlos A. Riano The problem of evaluating the forces acting on a rigid body in stagnant or moving fluids is a long standing issue. It has important implications in several engineering applications which involve multiphase flows. We study both experimentally and numerically the motion of a solid sphere settling under gravity through a viscous incompressible Newtonian fluid confined within a vertical cylinder whose radius changes periodically along its length. We explore the response of a settling particle to sudden changes in the geometry of the container which due to hydrodynamic wall effects induces periodic accelerations and decelerations of the particle. Experimental observations are presented for the case where the periodic changes in cylinder radius take place abruptly as well as for the case when the change is gradual. The experimental data are obtained for Reynolds numbers of the particle in the range $1\times10^{-4}$ to 1000. These results are compared to those predicted theoretically by Dorfman {\it et al.}[Phys. Fluids 2003] for a Brownian sphere in a similar geometry. A comparison is made by solving the equation of motion of the sphere --Maxey \& Riley (1983)-- with and without the history integral term. The experimental observations indicate the relevant role played by the Boussinesq--Basset force which gives a more precise prediction of the particle sedimentation velocity. [Preview Abstract] |
|
C1.00070: Continuous separation of human blood components through deterministic lateral displacement John Davis, David Inglis, James Sturm, Robert Austin Using a microfluidic device, the continuous separation of red and white blood cells from their native blood plasma has been demonstrated. The device takes advantage of the asymmetric bifurcation of laminar flow around obstacles. This asymmetry creates a deterministic path through the device which depends on the particle size. All components of a given size follow equivalent migration paths, leading to high resolution, and continuous throughput. One micron diameter fluorescent polystyrene beads were added to a mixture of blood. The blood was sorted into three distinct streams, consisting of the one micron beads, red blood cells and white blood cells, respectfully. [Preview Abstract] |
|
C1.00071: Optical Manipulation and Chaotic Mixing in Micro-droplets: Theory and Experiment Vivek Sharma, Mohan Srinivasarao, Roman Grigoriev, Michael Schatz We use laser-induced thermocapillary forces for precise, non-contact transport, merger and mixing of microdroplets suspended in liquid substrates. The thermal gradients drive flow both inside and outside the droplet. We present a general theoretical framework, to calculate the basic flows and their symmetries and for understanding how complete three-dimensional mixing via chaotic advection requires destruction of all the flow invariants. We present experimental results that test this general theoretical framework, by probing the interior of droplet using fluorescence microscopy and laser light sheet illumination. Our visualization method allows clear discrimination between the cases of complete vs incomplete mixing, as predicted by the theoretical model. The results of this work are expected to apply quite generally to mixing at the microscale in a wide variety of settings, regardless of the specific implementation details for the microfluidic system. [Preview Abstract] |
|
C1.00072: Measurements piezoelectricity and electromechanical effects of ferroelectric liquid crystals Antal Jakli, Clinton Braganza The voltages across a load resistor connected to a commercially available cell filled with room temperature ferroelectric liquid crystal mixtures, CS2003, FA 006 and ZLI 3775, were measure by a lock-in amplifier using period applied air pressure as a reference frequency. Periodic pressure induced electric current (pressure electricity) was measured. The electromechanical effects were studied by varying periodic pressure with no applied voltage and at fixed applied periodic pressure amplitude as a function of applied external voltage. It was observed that the effect of applied pressure is similar to that of applied voltage in that the amount of unwinding of the helix increases with increasing pressure or voltage. The effect is largest when the pressure induces helix unwinding. [Preview Abstract] |
|
C1.00073: MULTIFUNCTIONAL OXIDES [THIN FILMS] |
|
C1.00074: Low Frequency Magnetoelectric Coupling in Bilayers of Lead Zirconate Titanate and Sol-gel Derived Lanthanum Strontium Manganite Wei Yang, Ning Zhang, G. Srinivasan Layered composites of ferromagnetic-piezoelectric oxides show a giant magnetoelectric effect (ME) at low frequencies [1]. This work is on ME coupling in bilayers of lead zirconate titanate (PZT) and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO). Discs of LSMO were made by hot-pressing and sintering of powder obtained by sol-gel techniques. The bilayers were made by bonding LSMO and PZT discs. The ME voltage coefficient $\alpha _{E}=\delta $E/$\delta $H were estimated from the measured induced electric field $\delta $E in the presence of an ac field $\delta $H and a bias field H. Key results are as follows. (i) The maximum $\alpha _{E}$ of 50-60 mV/cm Oe at room temperature is obtained for transverse fields and for LSMO sintered at 1600 K. (ii) $\alpha _{E}$ is found to be temperature independent. (iii) The ME coupling for longitudinal fields is an order of magnitude smaller than for transverse fields. (iv) Theoretical estimates based on a model for bilayers are in qualitative agreement with the data [2]. 1. G. Srinivasan, E. T. Rasmussen, J. Gallegos, R. Srinivasan, Yu. I. Bokhan, and V. M. Laletin, Phys. Rev. B \textbf{64}, 214408 (2001). 2. G. Srinivasan, E. T. Rasmussen, B. J. Levin, and R. Hayes, Phys. Rev.B \textbf{65}, 134402 (2002). \begin{itemize} \item work supported by a grant from the NSF (0302254). \end{itemize} [Preview Abstract] |
|
C1.00075: Growth of ZnO/Zn1-xMgxO multilayer thin films by pulsed laser ablation Toshihiko Maemoto, Nobuyasu Ichiba, Shigehiko Sasa, Masataka Inoue ZnO/Zn$_{1-x}$Mg$_{x}$O multilayer thin films were grown on a-plane Al$_{2}$O$_{3}$ substrates at 400 $^{o}$C by pulsed laser ablation. Multilayer films were grown by stacking alternate layers of ZnO and ZnMgO with equal thickness varying from 2 to 8 nm. These films were characterized by x-ray diffraction, cathodeluminescence, and photoluminescence measurements. ZnO/Zn$_{0.9}$Mg$_{0.1}$O multilayer films showed a clear six-fold symmetry without in-plane rotational domains from pole-figures of X-ray diffraction. Decreasing the thickness of both ZnO and ZnMgO layers affected the band gap energy and optical properties. In particular, the band gap energy increased continuous with decreasing thickness of the ZnO and ZnMgO layers. A ZnO/MgO film was also grown by sequentially stacking ZnO and MgO layers and the resulting band gap energy was about 3.8 eV. [Preview Abstract] |
|
C1.00076: Photoinduced resistivity changes in Bi1-xCaxMnO3 thin films V.N. Smolyaninova, M. Rajeswari, R. Kennedy, M. Overby, S.E. Lofland, L.Z. Chen , R.L. Greene We report charge-ordered Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin films with charge-ordering temperature near room temperature, and observation of large photoinduced resistivity changes in these films associated with melting of the charge ordering by visible light. Films grown under small compressive strain exhibit the largest photoinduced resistivity changes. The lifetime of the photoinduced low-resistance state is on the order of half a minute. These photoinduced resistivity changes in thin films of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ make them very promising for photonic device application. [Preview Abstract] |
|
C1.00077: RuO$_{2}$/Pt Energy Harvesting Cells as Rechargeable Micropower Sources C. Marcel Buford We are studying electrochemical cells that are able to both continuously produce nanoWatts of power and supply microWatt-level bursts for microsensors and autonomous devices.~ The cells are constructed having a hydrous RuO$_{2 }$positive electrode and a Pt negative electrode with characteristic open circuit voltage of $\sim $ 0.4 V derived from the thermodynamic potential difference of the electrodes. Various electrolytes can be used, including H$_{2}$SO$_{4}$ and ionic conducting polymers (e.g. Nafion). When current is drawn from this galvanic cell, the cell is discharged and the voltage is reduced. After discharge, the circuit is opened and the cell returns to its characteristic voltage, appearing to "self-recharge." The phenomena causing the self-recharging of the positive and negative electrodes are known as potential recovery and self-discharge respectively. Previous studies cited in the literature have offered an explanation for the potential recovery of hydrous RuO$_{2}$, however, to date there exists no mechanism for the self-discharge of platinum. We will show that oxygen and low-levels of hydrogen harvested from the environment are electrochemically reduced and oxidized on the hydrous RuO$_{2}$ and Pt respectively recharging the cell. Recharging is a spontaneous process and has been shown to be reversible for up to 700 cycles. We will also show how the performance of the devices can be improved through the use of interdigitated thin film electrodes. [Preview Abstract] |
|
C1.00078: SEMICONDUCTORS |
|
C1.00079: Study of morphological optical and bactericide properties of ZnO-CuO. Enrique Sanchez-Mora, Rutilo Silva-Gonzalez, Patricia Gutierrez-Morales, Estela Gomez-Barojas In this work, we present a comparative study of the morphological and optical properties of ZnO, CuO, and CuO-ZnO thin films deposited on glass and treated thermally at 400 $^{o}$C. The samples were characterized by scanning electron microscopy, IR and UV-Vis spectroscopies. Also, preliminary studies were performed about the effect of these compounds on the growth process of Escherichia coli (E. Coli). The characterization results of these samples show a porous surface morphology, and the presence of surface OH groups. The optical studies of ZnO and CuO samples give energy band gaps (Eg) of 3.45 and 1.93 eV respectively, while the ZnO-CuO absorption edge is shifted toward to the visible region showing two steps, one gives an Eg of 2.60 and the other gives 1.91 eV. The results about bactericidal activity show that the ZnO-CuO thin film is more efficient than the ZnO film. The CuO film does not produce any change on the E-coli bacteria growth. [Preview Abstract] |
|
C1.00080: Drift Instability of Double Quantum Wire Plasmons Coupled to Optical Phonons Vassilios Fessatidis, Norman Horing We examine drift instability of the collective modes of a double quantum wire system with equal and opposite drift velocities in the two wires. In this study we consider the double quantum-wire system embedded in a semi-infinite polar medium, taking account of the role of optical phonons in interaction with the double quantum-wire plasmons and bulk/surface plasmons. The collective mode frequencies are exhibited as functions of $q_{x}$ (wavenumber parallel to the wires) and as functions of $z_{0}$ (distance of first wire from the interface). We find that the region of drift instability of the collective modes is split into two disjoint regions of instability by the optical phonons, with a region of stability between them. [Preview Abstract] |
|
C1.00081: Ground and excited state properties of CaB$_{6}$ determined byLDA and screened--exchange LDA investigations Y.C. Hsue, J.E. Medvedeva, A.J. Freeman CaB$_6$ has recently attracted great interest because it exhibits weak high-temperature ferromagnetism when lightly doped by La and could possibly be useful in room temperature spintronic devices. In pure CaB$_6$, its semiconductor nature has now been established experimentally\footnote{B.K. Cho, et.al., PRB 69, 113202 (2004) and references therein.}. Questions about its theoretical description are centered about the fact that first principles LDA calculations underestimate band gaps and in CaB$_6$ results in a semi-metal with a 0.3 eV overlap at the X point. Here we use the full-potential linearized augmented plane wave method (FLAPW)\footnote{Wimmer, Krakauer, Weinert and Freeman, PRB 24, 864(1981)} within both the LDA and the screened--exchange LDA (sX--LDA) to determine the ground and excited state properties of CaB$_6$. First, we did the geometry optimization with LDA and used this optimized structure in sX--LDA calculations. We find that CaB$_6$ is a semiconductor with a gap of 0.68 eV at X in agreement with the recent experimental results. We also calculated the optical properties with spin--orbit coupling and full matrix elements. Finally, results of similar calculations for Eu and La doped CaB$_6$ will be reported. [Preview Abstract] |
|
C1.00082: Self-Consistent Approach for Calculations of Exciton Binding Energy in Quantum Wells Vladimir Shuvayev, Lev Deych, Alexander Lisyansky, Ilya Ponomarev We develop a new approach to calculations of exciton properties in QW which takes into account an renormalization of QW confining potential by electron-hole Coulomb interaction. For narrow QW this approach gives lower values for exciton binding energies than traditional variational methods, and predicts a modification of absorption spectrum in the spectral region of barrier absorption edge. The method is based on the Hartree-type variational calculations, in which effective electron-hole potential and confining potentials for the motion in the growth direction are calculated in a self-consistent way. The method is flexible enough to take into account such effects as dielectric constant and mass mismatches between well and barrier materials. The former is incorporated by including image charges in a dielectric medium with planar geometry, while the latter is taken into account in a standard way through modification of boundary conditions for electron and hole 1D wave functions along QW growth direction. [Preview Abstract] |
|
C1.00083: Influence of SiGe buffer layer on the optical properties of Ge quantum dots grown on Si substrates by molecular beam epitaxy Yong-Hoon Cho, Sun-Mo Kim, Anisha Gokarna, Ho-Sang Kwack, H. J. Kim, R. Ostroumov, K. L. Wang, Y. H. Kwon, T. W. Kang Influence of a partially relaxed SiGe buffer layer on the structural and optical properties of Ge quantum dots (QDs) was investigated by atomic force microscopy (AFM) and temperature- and excitation power-dependent photoluminescence (PL) measurements. The Ge QD samples with and without a Si$_{0.75}$Ge$_{0.25}$ buffer layer were grown on a Si (100) substrate by a molecular beam epitaxy (MBE), followed by a Si capping layer. A random, bimodal size distribution and a broad emission were observed for the Ge QDs directly grown on Si substrate without a buffer layer (sample A), while a well-aligned, unimodal size distribution and a narrow emission peak were observed for the Ge QDs grown on the relaxed Si$_{0.75}$Ge$_{0.25}$ buffer layer (sample B). The bimodal and unimodal QD size distributions were clearly reflected in the broadening of the PL spectra of samples A and B, respectively. An increase in PL intensity and a change in PL peak energy with increasing excitation power were another important characteristic in this study. We will discuss the influence of the SiGe buffer layer on strain status, band alignment, and optical properties of Ge QDs. [Preview Abstract] |
|
C1.00084: First principles calculations of nanodots formation in high performance thermoelectrics AgPbmSbTem+2 Hirofumi Hazama, Ryoji Asahi, Uichiro Mizutani Recently, AgPb$_{m}$SbTe$_{m+2}$ bulk thermoelectric material with a high figure of merit ZT over 2 was reported.$^{1}$ The excellent ZT mainly due to an enhancement of the Seebeck coefficient and a low thermal conductivity was resulted from simultaneous doping with Ag and Sb at the Pb site in PbTe. We have performed the first principles calculations to clarify effects of the Ag and Sb doping, in particular, on thermoelectric properties. The calculations have been done using the VASP-PAW method$^{2}$ within GGA. The lattice constants and atomic positions were fully optimized for different contents of doping; the obtained lattice constants were quite consistent with experiment. For AgPb$_{30}$SbTe$_{32 }$system, we examined several configurations of Ag and Sb sites, giving the lowest total energy with a short distance between Ag and Sb. This results in preferable nanodots formation between Ag and Sb in AgPb$_{m}$SbTe$_{m+2}$, which were experimentally observed. The detailed calculations of the thermoelectric properties including Seebeck coefficients and thermal conductivities, emphasizing the effects of the nanodots formation, will be reported. $^{1}$K.F. Hsu et al., Science 303, 818 (2004). $^{2}$G. Kresse et al., Comput. Mater. Sci. 6, 15 (1996). [Preview Abstract] |
|
C1.00085: Anomalous effective charge of hypervalent chalcogenide As$_2$Te$_3$: First-principles study Kazuma Nakamura, Shinji Tsuneyuki, Masafuyu Matsui In order to understand the electron-lattice interaction of hypervalent chalcogenide As$_2$Te$_3$, we performed first-principles calculations based on density functional theory. The material exhibits an intermetalic character with a narrow gap $\sim 0.2$ eV, and thus is expected to have a strong electron-lattice coupling. To estimate such couplings we calculated Born effective charge tensors, and found that the resulting charge is $6$ for As and $4$ for Te, which are about three times larger than other chalcogenides, As$_2$Se$_3$ and As$_2$S$_3$. The microscopic mechanism for the anomalous charges is discussed with a tight-binding model. [Preview Abstract] |
|
C1.00086: Optical characterization of TiO2 thin films grown by sol-gel and r.f. sputtering methods for photocatalysis applications J.G. Mendoza-Alvarez, O. Zelaya-Angel, G. Torres-Delgado, R. Castanedo-Perez, S.A. Mayen-Hernandez, J. Marquez-Marin, A. Florido-Cuellar Using the sol-gel method and the r.f. sputtering deposition technique we have grown titanium dioxide (TiO$_{2})$ thin films on Corning glass substrates for photocatalysis applications. For the sol-gel method we have tried different precursors such as acetylacetone, diethanolamine, HCl and HNO$_{3}$, in order to study the influence of the precursor on the structural and optical properties of the TiO$_{2}$ films. For the TiO$_{2}$ films grown by r.f. sputtering we changed the substrate temperature and the post-grown annealing conditions. Using X-ray diffractometry we have determined the presence of both, rutile and anatase crystalline phases in the films, and their correlation to the growth conditions. From the absorption spectra measured by spectrophotometry and photoacoustic spectroscopy we have been able to determine the bandgap energy and their dependence with the growth parameters. We present also results on the photoluminescence spectra for these sets of TiO$_{2}$ thin films and discuss the origin of the different radiative emission bands, and their possible correlation to the bleaching efficiency of these films for methylene blue. [Preview Abstract] |
|
C1.00087: Effects of dopant states on the electronic structures in tungsten doped TiO$_2$ Hao Wang, James Lewis Despite the broad range of applicabilities for TiO$_2$ as a photocatalytic material, only about $3\%$ of the solar spectrum (ultraviolet) can be utilized due to its wide intrinsic band gap. Therefore, considerable efforts have been made to extend the photoresponse of TiO$_2$-based systems further into the visible-light region using dopants. In this work, we explore the electronic structures of tungsten doped TiO$_2$ using an {\it ab-initio} tight-binding method, called F{\small IREBALL}, which is based on density functional theory (DFT) with nonlocal pseudopotentials within the local density approximation (LDA). Our calculations provide an initial glance into electronic properties required for acceptable photocatalysis. [Preview Abstract] |
|
C1.00088: STS and BEES Study of Semiconductor Nanocrystals Jianfei Shao, Sameh Dardona, Alexander Schill, Qusai Darugar, Mostafa El-Sayed, Phillip First Semiconductor nanocrystals CdS/ZnS and CdSe/HgS with core/shell structure are investigated theoretically and experimentally. 8-band envelope function approximation is used to calculate energy levels of these nanocrystals. After binding to the Au(111) surface by organic self-assembled monolayer, dI/dV is measured using STM. The positions of dI/dV peaks are compared with eigen energies of theoretical calculation. This provides a test of envelope function approximation in the scale of several nanometers. In order to investigate the relaxation processes inside nanocrystals, we use our home build UHV low-temperature STM to do ballistic electron emission spectroscopy. [Preview Abstract] |
|
C1.00089: Tunneling Microscopy and Spectroscopy of Semiconductor Nanocrystals Sameh Dardona, Jianfei Shao, Alexander Schill, Qusai Darugar, Mostafa El-Sayed, Phillip N. First Semiconductor nanocrystals of different compounds have been imaged and partially characterized by tunneling based techniques. Our nanocrystals include CdSe, Au and CdS coated with organic molecules. Our initial studies show immobile and isolated single Au, CdSe, and CdS nanocrystals images. Crystals were attached chemically to a conducting substrates using different combinations of ligands and SAM binding layers. Nanocrystal samples have been prepared for Ballistic Electron Emission Spectroscopy (BEES) studies. BEES studies of single nanocrystals are expected to provide us with an understanding of size-dependent electronic structure at the level of single quantum states, and a basic knowledge of the dynamics of electronic excitations within the nanocrystal. [Preview Abstract] |
|
C1.00090: Ab Initio Calculation of Electronic Properties in Si:H Alvaro Posada-Amarillas, Roberto Nunez-Gonzalez In this work we calculate the electronic structure of crystalline silicon with monatomic hydrogen, using the LAPW method within the DFT framework. The density of states, charge density and stable configuration of Si in bond-center (BC) and tetrahedral (Td) sites are calculated. Supercells are used in order to study several hydrogen concentrations, finding a metallic behavior in all cases. Important contributions at the Fermi level are due to hydrogen, and the Td site is the most stable for hydrogen concentrations on the order of 50{\%}. [Preview Abstract] |
|
C1.00091: Light induced electrostatic force microscopy in InN epifilms Wei Sheng Su, C. W. Lu, Yang Fang Chen, E.H. Lin, T.Y. Lin, N.C. Chen, C.F. Shih, K.S. Liu A technique based on electrostatic force microscopy in which light is used to change the charge states of the local region in a solid is introduced and demonstrated. As an illustration, it has been applied to study InN epifilms. Combining with atomic force microscopy, the magnitude of the surface band bending can be obtained for the regions with different surface states. We point out that light induced scanning electrostatic force microscopy is a very useful tool to probe the local electronic transitions of a solid in a sub-micron scale with high sensitivity. [Preview Abstract] |
|
C1.00092: Characterization of Inx Ga1-x As-GaAs heterostructures via electron beam techniques Estela Gomez-Barojas, Rutilo Silva-Gonzalez, Rosa Maria Serrano-Rojas, Miguel Angel Vidal-Borbolla In the case of strained superlattices abrupt heterointerfaces are required because compositional fluctuations at heterointerfaces results in uncertainty in both composition and lattice constant. The aim of this work is to study exsitu the surface morphology, the periodicity and elemental composition of a set of 3 InGaAs-GaAs heterostructures grown on GaAs (100) substrates by a molecular beam epitaxy system. The heterostructures are formed by 10 periods of InGaAs-GaAs epitaxially grown on GaAs substrates with nominal thickness of 500 and 1000 {\AA}, respectively. The techniques used for this purpose are the scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The In content in the heterostructures is determined from corresponding Auger depth profiles. This work has been supported by VIEP-BUAP, Project No. II53G02. [Preview Abstract] |
|
C1.00093: Multiple double donors of magnesium-related impurities in silicon L.T. Ho It is well-known that the group-II magnesium, when diffused into silicon, enters the silicon lattice interstitially and behaves as a helium-like double donor with the ionization energies determined from the excitation spectra measured at liquid helium temperature to be 107.50 meV and 256.47 meV for neutral magnesium donor and singly ionized magnesium donor, respectively. Recently, from the high-resolution FT-IR absorption spectrum of magnesium-doped silicon measured at liquid helium temperature we have observed several additional spectral lines apparently due to different double donors. This clearly indicates that there are multiple double donor centers for magnesium-related impurities in silicon. [Preview Abstract] |
|
C1.00094: Atomic Force Microscopy Study of Viscoelastic Response of Polymer Films near Their Glass Transitions Guanwen Yang, Nanxia Rao, Da-Ming Zhu, Zejie Yin Despite the fact that atomic force microscopy (AFM) has been increasingly used in the study of glass transitions in polymer films, the characteristic behaviors of viscoelastic response measured using AFM and their correlations with the macroscopic mechanic behavior of bulk samples are still to be understood. We have studied viscoelastic response of polymer films near their glass transitions using atomic force microscopy. The response of a polymer film probed by an atomic force microscope is calculated using a Burger's model with four adjustable parameters. It is found that the force distance curve measured with the AFM is strongly affected by the viscosity of the film. The exact shapes of the force distance curves depend on both the temperature and measuring speed. It is also found that the viscoelastic property of the film is strongly affected by the adhesion force between the AFM tip and the polymer film. Several characteristics in a force distance curve near the glass transition will be discussed [Preview Abstract] |
|
C1.00095: Structural Determination of the Low-coverage Phase of Al on Si(001) Surface J. Y. Park, J. H. Seo, C. N. Whang, S. S. Kim, D. S. Choi, K. H. Chae The investigation of group-III metals on Si surfaces has been one of the typical studies in surface science for many years. This system exhibits an abundance of surface structures depending on the coverage and the annealing process. Although various theoretical and experimental studies provide useful information, the growth mechanism and detailed atomic structure model for the adsorption Al on Si(001) have not yet been established and deserve further study. In this study, atomic structure of Al layer on Si(001)-(2$\times $1) surface have been studied by coaxial impact collision ion scattering spectroscopy (CAICISS). When 0.5 ML of Al atoms are adsorbed on Si(001) at room temperature, it is found that Al ad-atoms are dimerized and Al ad-dimers oriented parrallel to the underlying Si dimers at the position of centering T3 site with a height of 1.02 $\AA$ from the first layer of Si(001). The bond length of the Al dimer is 2.67 $\AA$. With increasing Al coverage up to 1 ML, Al ad-dimers still occupied near T3 site and the next favorable site is near HH site. [Preview Abstract] |
|
C1.00096: P-T phase diagram for Rh(III) oxides from first principles Shuping Zhuo, Karl Sohlberg We present a detailed first principles study of the equilibrium lattice structures and the phase diagram for the three observed polymorphs of rhodium(III) oxide. The $P-T$ phase diagram is constructed by first calculating the thermodynamical Gibbs free energies as a function of $P$ and $T$ from the electronic energy, as well as vibrational energy and vibrational entropy contributions in a quasiharmonic approximation. Phase boundaries are then placed between each pair of phases by locating curves of intersection between the corresponding free energy surfaces. The resulting partitioning of phase space is consistent with the published experimental investigations. It is found that Rh$_{2}$O$_{3}$ I is the low-temperature and low-pressure form, Rh$_{2}$O$_{3}$ II is the high-pressure form and Rh$_{2}$O$_{3}$ III is high-temperature form. The calculations predict the existence of triple point for the three phases. [Preview Abstract] |
|
C1.00097: Phasor-like interpretation of tight-binding electronic motion in the single-band approximation Jean-Pierre Gallinar We present a specific interpretation of a previously derived [1] pseudo-spectral method for studying electronic wave-packet evolution within the single-band approximation. As a result of analytical properties of Bessel functions, it is shown that in a homogeneous time-dependent electric field an electron's motion in a one-dimensional nearest-neighbor tight-binding band can be described in terms of a phasor (polygonal) construction in the complex plane. Based upon our polygonal construction, an analogy is established between motion in a constant or in a linearly time-dependent electric field and the optical phenomena of Fraunhofer or Fresnel diffraction, respectively. The first type of diffraction, associated to the figure of a circumference, leads to the usual Bloch oscillation effect. The second one leads to asymptotic localization due to the mathematical properties of the Cornu spiral. Finally, a phasor-like formula for inhomogeneous electric fields, general band structure and dimensionality of electronic motion is presented in terms of a multidimensional integral of appropiate discrete Fourier transforms of the applied external potential. [1] D. Sanjin\'{e}s and J.-P. Gallinar, Phys. Rev. B\textbf{64}, 054301 (2001). [Preview Abstract] |
|
C1.00098: Experimental studies of low-field Landau quantization in two-dimensional GaAs electron systems D.R. Hang, H.D. Yeh, Y.W. Zhang, J.C. Hsiao, H.L. Pang, C.F. Huang We studied effects of low-field Landau quantization in two-dimensional systems by applying a magnetic field $B$ perpendicular to GaAs electron systems. With increasing $B$, Shubnikov-de Haas (SdH) oscillations appear in the longitudinal resistivity \textit{$\rho $}$_{xx}$ before the appearance of the quantum Hall effect (QHE). Universal properties based on the modular symmetry become invalid at low $B$, where we should consider SdH theory, low-field localization, and quantum diffusion model. The crossover from SdH oscillations to the QHE is studied by sweeping $B$ and changing the temperature $T$. By investigating the peak values of the longitudinal resistivity $\rho _{xx}^{pk}$, it is shown that we shall consider the refinement to the theory for the low-field Landau quantization. [Preview Abstract] |
|
C1.00099: Drag and Drift of Excitons Induced by the Current in a Two-Dimensional Electron Gas S. K. Lyo We show theoretically that an electric current in a high-mobility quasi- two-dimensional electron layer induces a significant drift of excitons in an adjacent layer through interlayer Coulomb interaction. The exciton gas is shown to drift with a velocity which can be a significant fraction of the electron drift velocity at low temperatures. The estimated drift length is of the order of micrometers or larger during the typical exciton lifetime for GaAs/Al$_x$Ga$_{1-x}$As double quantum wells. A possible enhancement of the exciton radiative life time due to the drift is discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE under contract DE-AC04-94AL85000. [Preview Abstract] |
|
C1.00100: Phase diagram of a Wigner crystal Keshav Shrivastava We examine the experimental data of absorption of r.f. in the 2- d electron gas in the range from 80 to 400 MHz. We convert all of the frequencies into temperature units by writing h$\nu$=k$_{BT} $. Thus, for example, 80MHz=3.84 mK. The transition temperatures so derived are a few millikelvin(mK). The energy of the state is 2$\nu_{(=-)}$$\mu_{BB}$ (B=$\mu$ H). The pressure is -(dU/dV)$_\sigma$. We devide the field by a constant H$_o$=35Tesla so that the range occurs between 0.5 and 1.0. When the limit of field is extended to infinity 5.76 mK and 13.68 mK become 0.296 and 0.703 which correspond to 1/3 and 2/3. From our theory we identify the spin from the fraction. The H/H$_o$ versus temperature phase diagram in the range from 0.5 mK to 18.5 mK is thus obtained. Because of the use of limit, it corresponds to insulating lattice of electrons which is called the Wigner solid. 1. K. Lai, W. Pan, D. C. Tsui et al PRB 69,125337(2004). 2. K. N. Shrivastava, Phys. Lett. A. 326, 469(2004); A113,435(1986). 3. K. N. Shrivastava, Introduction to quantum Hall effect, Nova Sci. N. Y. (2002) [Preview Abstract] |
|
C1.00101: Coupled Energy-Drift and Force-Balance Equations for High-Field Hot-Carrier Transport Danhong Huang, Paul M. Alsing, Tzveta Apostolova, Dave A. Cardimona Coupled energy-drift and force-balance equations are derived for hot-electron transport under a strong dc electric field. The work done by the frictional force is included into the energy-drift equation for electron relative scattering motion and is found to increase the thermal energy of the electrons. The importance of the hot-electron effect in the energy-drift term under a strong dc field is demonstrated to reduce the field-dependent drift velocity and mobility. The Doppler shift in the energy conservation of scattering electrons interacting with impurities and phonons is found to lead to an anisotropic distribution of electrons in the momentum space along the field direction. Defining a state-independent electron temperature is found to be impossible. [Preview Abstract] |
|
C1.00102: Growth and characterization of AlGaN/GaN heterostructures by plasma-assisted molecular beam epitaxy Ming-Hong Gau, Chia-Ho Hsieh, Chun-Fan Chen, Kung-Yao Chen, Jenn-Kai Tsai, Wan-Tsang Wang, Jih-Chen Chiang, Ikai Lo Transition from 2D to 3D growth GaN epilayer was found in the intermediate Ga-stable regime for optimize N/Ga flux ratio at high temperature (i.e. 788 degree C), and the surface morphology was also sensitive to the N/Ga flux ratio. As the ratio less than transition ratio (i.e. 22.5), the slope of growth rate is 0.389, otherwise the slope is 0.140. In order to enhance the film quality, we selected AlN layer as a buffer to grow the AlGaN/GaN heterostructures under different growth conditions. These samples have been characterized in situ by reflection high-energy electron diffraction (RHEED), and ex situ by field-emission scanning electron microscopy (FESEM) and high resolution X-ray diffraction (DXRD). The electronic properties were studied by van der Pauw Hall measurement as well. [Preview Abstract] |
|
C1.00103: GaN/InGaN/GaN high electron mobility transistor fabricated by RF-MBE Chia Ho Hsieh, Ming Hom Gau, Kuang Yao Chen The GaN/In$_{x}$Ga$_{(1-x)}$N/GaN high electron mobility transistor (HEMT) growth on c-plane sapphire substrate by RF plasma assisted molecular beam epitaxy. The sample were investigated by reflection high-energy diffraction, high resolution x-ray diffraction, field-emission scanning electron microscopy, and Hall effect measurement. Flat interface were monitored by RHEED pattern, as well as smooth morphology determined by FESEM. The effect of the fraction factor x and InGaN thickness on electronic properties were determined from Hall measurement data. The mobility in the designed structure is around 5 cm$^{2}$/Vs at room temperature (300 $^{0} $K), but at low temperature (77 $^{0}$K) the value of mobility is increased to 400 cm$^{2}$/Vs. [Preview Abstract] |
|
C1.00104: Room temperature ferromagnetism in transition metal (Fe, CO) doped TiO$_2$ spin-coated films P. Kharel, P. Talagala, G. Lawes, R. Naik, G.W. Auner, V.M. Naik, R. Suryanarayanan We have prepared transition metal (TM) doped ($\sim$ 5 at \%) TiO$_{2}$ films in both anatase and rutile forms by metalorganic decomposition using a simple spin-coating technique. The appropriate metalorganic precursor solutions were mixed and spin-coated on to sapphire substrates held at room temperature and subjected to pyrolysis in a furnace at 500$^{\circ} $C for 2 minutes to obtain one coat. Films ranging in thickness from 200 to 800 nm were prepared by increasing the number of coats. Further annealing of the films in air at 550$^{\circ}$C yields only anatase form of TM doped TiO$_{2}$ whereas annealing at 750$^{\circ}$C results in rutile form as confirmed by X-ray diffraction and Raman scattering. The films were found to be only weakly ferromagnetic at 5 K and paramagnetic at room temperature. However, when these films were annealed in a vacuum of 2x10$^{-5}$ Torr between 500 to 600$^{\circ}$C, clear signatures of ferromagnetism appeared at room temperature for both anatase and rutile forms. The highest coercivity obtained in our anatase form of Fe doped TiO$_{2}$ sample was 720 Oe at 5K and 450 Oe at 350 K similar to earlier data on laser ablated films. The effect of heat treatment on the magnetic properties and the role played by oxygen defects will be discussed. [Preview Abstract] |
|
C1.00105: Shot Noise Suppression at Non-integer Conductance Plateaus in a Quantum Point Contact Na Young Kim, William Oliver, Yoshihisa Yamamoto, Yoshiro Hirayama We study non-equilibrium differential conductance and current fluctuations in a single quantum point contact. The two-terminal electrical transport properties are measured at 1.5 K as a function of the drain- source voltage and the Schottky split-gate voltage. In differential conductance measurements, conductance plateaus appear at integer multiples of 2e$^{2}$/h when the drain-source voltage is small, and the plateaus evolve to a fractional of 2e$^{2}$/h as the drain-source voltage increases. Our shot noise measurements correspondingly show that the shot noise signal is highly suppressed at both the integer and the non-integer conductance plateaus. This main feature can be understood by the induced electrostatic potential model within a single electron picture. In addition, we observe the 0.7 structure in the differential conductance and the suppressed shot noise around 0.7 (2e$^{2}$/h); however, the previous single-electron model cannot explain the 0.7 structure and the noise suppression, suggesting that this characteristic relates to the electron-electron interactions. [Preview Abstract] |
|
C1.00106: Bandgap engineering of rutile with substitutional nitrogen John Jaffe, Michael Henderson TiO$_{2}$ in both the rutile and anatase structures has been proposed as an absorber of sunlight for photocatalytic processes, including water splitting to produce hydrogen from solar energy. However, the large band gap of TiO$_{2}$, about 3.0 eV, permits the use of only a small part of the solar spectrum. Alloying with other elements, and especially anion doping with nitrogen, sulfur or other anions in place of oxygen, has shown promise for reducing the band gap of the material, permitting more of the solar spectrum to be used so that photocatalysis with TiO$_{2}$ can be made more efficient. However, little is known about the concentration dependence of the bandgap reduction, or about autocompensation of nonisovalent dopants by native defects such as the oxygen vacancy. We have calculated defect formation energies and changes in the DFT bandgap for rutile TiN$_{x}$O$_{2-x}$ for $x$ = 0.042, 0.083, 0.167 and 0.25. The band gap initially decreases with increasing $x$, but the reduction largely saturates at about 0.3 eV for $x \quad \ge $ 0.083. We also report preliminary results on the formation energy and electronic structure of O vacancies in TiN$_{x}$O$_{2-x-\delta }$, and on interactions and complexes of these defects. [Preview Abstract] |
|
C1.00107: Surface States of The Wirtzite Type Nanowires Natalia Malkova, Cun-Zheng Ning We study the surface states at the lateral facets of the wurtzite type nanowires. As particular example we consider nanowires with the same symmetry for all the lateral facets. Our approach is based on the transfer matrix technique within the semi-empirical tight-binding approximation. We calculate the surface states at (1100), (1210) and (0001) surfaces of the semi-infinite crystal first. Then using the cyclic boundary conditions, depending on the radius of the nanowire, we find the quantized spectrum for the surface states in question. We present the result for the surface states of GaN, ZnO, CdS, CdSe and AlN wirtzite nanowires with the (1100) and (1210) lateral surfaces. The dangling bonds are shown to give a set of the states inside the band gap of the semiconductors studied. The orbital character and decay length of the states are calculated. We analyze the symmetry of the quantized surface states and possible optical dipole transitions between them. [Preview Abstract] |
|
C1.00108: Optical Characteristics of Dislocation Free InGaN/GaN Multiple Quantum Wells Embedded in GaN Nanorod Arrays Yong-Hoon Cho, S.M. Yoon, A. Gokarna, Y.P. Sun, H.M. Kim, T.W. Kang, K.L. Wang We investigated the optical properties of dislocation-free InGaN/GaN multiple quantum well (MQW) nanorod arrays (NRAs) by photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. The InGaN MQW NRAs were grown on sapphire substrates by metal-organic hydride vapor phase epitaxy in the absence of a catalyst, resulting in the growth of vertically well-aligned NRAs with homogeneous length and diameter. The temperature and excitation power dependent PL spectra were performed, and a remarkable enhancement in PL efficiency was observed in these dislocation-free InGaN/GaN MQW NRA structures. The origin of the InGaN-related emission was investigated by PL excitation and time-resolved PL experiments. The results suggest that the InGaN-related emission is not much degraded by non-radiative recombination centers and that the emission wavelength is strongly affected by the built-in internal field present in the NRA growth direction. [Preview Abstract] |
|
C1.00109: Heat Capacity and Thermal conductivity of Ge nanocrystals Shang-Fen Ren, Wei Cheng, Peter Y. Yu Thermal conductivity of semiconductor materials has attracted a renewed research interest in recent years especially because the potential applications of nanocrystals and nanowires in thermoelectrics. By using an atomistic model we have investigated phonon properties in Ge and other nanocrystals. In the present work, we have extended our model to compute the heat capacities and thermal conductivities of Ge nanocrystals. The thermal conductivity has been calculated both by the classical Boltzmann approach and by a quantum mechanical approach along the lines of the quantized thermal conductance in 1D systems. The results of the two approaches are compared as a function of nanocrystal size and sample temperature. [Preview Abstract] |
|
C1.00110: SUPERCONDUCTIVITY |
|
C1.00111: Magnetic Flux Trapping Behavior Due to Oxygen Disorder in Y-Ba-Cu-O Melodie Nguyen, Kurt Vandervoort A series of experiments were conducted on Y-Ba-Cu-O single crystals prepared in a variety of oxygen deficient and oxygen disordered states, with transition temperatures that ranged from 20 K to 85 K. Both oxygen deficiency and oxygen disorder along the chain sites strongly affect the superconducting properties. Field cooled and zero-field cooled magnetic transitions were measured using a low-field SQUID magnetometer to ascertain the amount of trapped magnetic flux as a function of the relative degree of oxygen disorder. For all samples measured, the percentage of trapped flux decreased with increased ordering of oxygen. These trends were enhanced for samples with greater oxygen deficiency. The ultimate goal of this project is to link these macroscopic magnetic properties to microscopic surface electronic properties, analyzed through scanning tunneling spectroscopy. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research. [Preview Abstract] |
|
C1.00112: Magnetism in Superconducting Tl2223: a Flux-Trapping study L. Robledo, B. Launspach, K. Prudchenko, J.C. Lee, C. Boekema Muon-spin-rotation ($\mu $SR) data of flux trapping in Tl2223 recorded at 10 K in zero field (ZF) are studied. The Maximum-Entropy (ME) method is used to analyze these time-series data. [1] We search for signals arising from the initial vortex core, critical fields, and other remnants of the initial mixed state at five kOe. For the ZF-$\mu $SR Tl2223 data, we observe four signals of 0 MHz, 1.5 MHz, 9-10 MHz and 24-25 MHz. The 0-MHz originates from the grainboundaries. The 0.7-kOe fields reflect the first critical field. The 1.8-kOe fields correspond to the vortex transition to 3d lines from 2d pancakes. The 110-Oe fields arise from magnetism initially present near the vortex cores [1] before the flux trapping. Thus (antiferro) magnetism appears to exist in the Tl2223 superconductor. Research supported by MARC, REU-NSF, WiSE@SJSU, and the SJSU College of Science. [1] J. Lee \textit{et al}, J Applied Physics 95 (2004) 6906 and \textit{Virtual J Applications of Superconductivity, June 2004 V6 Issue11.} K Prudchenko \textit{et al}, Proc HI Int Conf on Sciences (HI, Jan 2004) \textit {www.jyi.org/volumes/volume10/issue6/articles/prudchenko.html} [Preview Abstract] |
|
C1.00113: Mechanism of vortex pinning by graded triangular arrays of submicrometric defects in a superconducting Nb film T. C. Wu, Lance Horng, J. C. Wu, Jan Kolacek , C. W. Hsiao, T. J. Yang We have investigated the periodic pinning of magnetic flux quanta in the Nb films with graded triangular arrays of submicrometer defects. Arrays of pinning centers with a spacing of about 425nm to 375 nm and a diameter of about 250nm were fabricated by the electron beam lithography. In the mixed states, the minima of magnetoresistance and maxima of critical current as a function of magnetic field appear at certain values of magnetic field corresponding to the flux quanta calculation of the lattice spacing of the homogenous ones (400nm). A comparison of will be discussed graded and homogenous triangular arrays. [Preview Abstract] |
|
C1.00114: Study of the universal behavior of the vortex phase diagram in proton irradiated YBCO single crystals Lucian Undreiu, Lisa Paulius, Christoph Marcenat, Valentina Tobos, Wai Kwok The goal of our work is to gain more insight into the vortex dynamics in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ single crystals with point defects induced by 9 MeV proton irradiation. In order to accomplish a comparative study, different crystals and irradiation doses were used. The results were correlated using both electrical transport and ac specific heat measurements. Special attention was given to investigating the nature of the vortex state below the lower critical point of the first order vortex melting transition. This work was supported by the National Science Foundation through Grant DMR-0072880 and the U. S. Department of Energy, Basic Energy Sciences contract W-31-109-ENG-38. [Preview Abstract] |
|
C1.00115: $^{11}$B NMR Study of Vortex Motion in MgB$_2$ Moohee Lee, Kyuhong Lee, K. H. Kang, B. J. Mean, J. S. Rhee, B. K. Cho We have performed $^{11}$B nuclear magnetic resonance(NMR) measurements to investigate vortex dynamics in the polycrystalline superconductor MgB$_2$. $^{11}$B NMR spectrum, shift, transverse relaxation rate $1/T_2$ were measured down to 4.3 K at ~1.8 T. The spectrum below $T_c$ exhibits a typical local field distribution for a vortex lattice under magnetic field. The peak point of the spectrum shifts toward low magnetic field due to imperfect field penetration. $1/T_2$ data, probing the slow motion of vortices, shows a single peak with a small change of the rate, contrary to the results of nickel borocarbides. Also, the relaxation profile changes from a Lorentzian decay below $T_c$ to Gaussian decay at lower temperature. It strongly suggests that thermal fluctuation of vortices is reduced at low temperature and the vortex motion is much weaker compared with nickel borocarbides. [Preview Abstract] |
|
C1.00116: Boron excess and external pressure effects on NbB2 Richard Falconi, Raul Escamilla, Alejandro Duran, Francisco Morales, Roberto Escudero We report structural and electronic studies on NbB$_{2+x }$with nominal compositions 2.0 $\le $ B/Nb $\le $ 2.6. After refinement the obtained compositions were found that are superconducting for (B/Nb)$_{ref} \quad \ge $ 2.20(2). Structural analyses reveal that the boron excess induces significant changes in the Nb-B bond length, giving rise to an increase in the $c$ axis and in the cell volume. The superconducting transition temperature, T$_{C}$, increases with boron content to a maximum at 9.8 K, for the composition (B/Nb)$_{ref}$ = 2.34(1). Electrical resistance measurements as a function of temperature and pressure show that T$_{C}$ decreases at different rates, depending on boron excess. Samples with high concentration of boron show low dTc/dP rates. These experimental results are discussed in terms of possible Nb vacancies. The changes of T$_{C}$ are correlated with the evolution of the structural parameters and compared with theoretical predictions of band structure. [Preview Abstract] |
|
C1.00117: Grain-boundary doping dependence of irreversible magnetization in $\rm Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}$ : Evidence for pseudogap correlation as a competing order on superconductivity G.C. Kim, M. Cheon, J.H. Lee, Y.C. Kim, D.Y. Jeong High temperature superconductors reveal several unusual behaviors, which can not be explained by conventional metal theories. Among them, the nature of the pseudogap correlation on superconductivity is very important, because it is believed that the pseudogap correlation is associated closely with the origin of high temperature superconductivity. The best way to solve this problem is treating a system where the pseudogap correlation and superconductivity coexist. Here, we report on the nature of a pseudogap correlation through investigating the irreversible magnetization of systems synthesized by annealing underdoped and overdoped polycrystalline $\rm Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}$ at a low temperature in different atmospheres [Preview Abstract] |
|
C1.00118: The Specific Heat of Na$_{0.3}$CoO$_{2}\cdot$1.3H$_{2}$O: Two Superconducting Gaps R.A. Fisher, N. Oeschler, N.E. Phillips, J.E. Gordon, M.L. Foo, R.J. Cava The specific heats (C) of three polycrystalline samples of Na$_{0.3}$CoO$_{2}\cdot $1.3H$_{2}$O were measured in magnetic fields 0 $\le $ B $\le $ 9 T. The first sample was not superconducting, but showed a small antiferromagnetic anomaly at T$_{N}\sim $6 K. Samples two and three showed clean, but broadened, superconducting transitions with T$_{c}$'s of 4.52 and 4.65 K, respectively, and no evidence of magnetic impurities. They have large non-superconducting fractions, and qualitatively different specific heats. In the normal state for both samples $\gamma $ = 16 mJ K$^{-2}$ mol$^{-1}$. For B = 0, both superconducting samples can be fit with two energy gaps (different in magnitude and fraction for each sample), one larger and one smaller than the BCS value. In a plot of C/$\gamma $T vs. T/T$_{c}$ for B = 0 the specific heat of the second sample is strikingly similar to that of MgB$_{2}$, an established two-gap superconductor. The evolution of $\gamma $(B)/$\gamma $ with B in the mixed state for the second sample will be contrasted to that of MgB$_{2}$. [Preview Abstract] |
|
C1.00119: Role of cyclic four-spin exchange in doped two-leg ladders Guillaume Roux, Steve R. White, Didier Poilblanc, Sylvain Capponi, Andreas M. Laeuchli The cyclic four-spin exchange has a dramatic effect in undoped 2-leg ladders since it introduces frustration and biquadratic terms. A quantum phase transition is known to happen for K/J = 0.2, in which K is the magnitude of the cyclic exchange term. Here, we report the first investigations of its role on t-J two-leg ladders using Exact Diagonalisation and DMRG computations. We show that the low energy spectrum is very sensitive to K and discuss the robustness of the magnon-hole pair bound state and the recently discovered resonant mode(at finite doping) by studying the low energy triplet spectrum. We also report the evolution of the pairing energy and the Luther-Emery liquid parameter with K. These results are discussed in connection with experiments on superconducting Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ ladder materials. [Preview Abstract] |
|
C1.00120: The effects of negative dynamic resistance of DC SQUIDs Sing-Lin Wu, Ming-Jye Wang, Cheng-Chung Chi We have fabricated low T$_{C}$ DC SQUIDs using Nb-AlOx-Nb trilayer technology. These SQUIDs are of first-order gradiometer type with direct signal coupling loop of diameter from 10$\mu $m to 20$\mu $m. The Stewart-McCumber parameter is about 0.25 and the current density is about 1000 A/cm$^{2}$. In addition to the expected non-hysteretic and smooth current-voltage (I-V) curves, we have often observed negative dynamic resistance in certain range of I-V curves of some samples. The voltages at which these negative resistances occur shift periodically with the applied magnetic flux. The effect of negative resistance on the noise spectrum will be discussed. We have also studied the I-V curves of dc SQUIDs in the presence of rf fields in the frequency range 2-18GHz. Strong voltage locking at half-integral as well as the usual integral Shapiro step spacing is observed when the applied dc-magnetic flux is close to half-integral numbers of flux quanta. The possible relationship of the half-integer Shapiro steps and the negative resistance will be explored. [Preview Abstract] |
|
C1.00121: Dependence of the intrinsic noise properties of Nb/AlOx/Nb-junctions on the manufacturing geometry Pasi Kivinen, Sorin Paraoanu, Ari Halvari, Paivi Torma We have studied different Nb/AlOx/Nb-junctions and their $1/f$ -noise properties. We compare the results with Al-based junctions and SETs and discuss the role of the sample geometry and the underlying substrate. At certain bias voltages, which is specific for each sample, the Nb-based junctions have an anomalous AC-current, which frequency strongly depends on the applied bias voltage. [Preview Abstract] |
|
C1.00122: Magnetic, Transport, and Optical Properties of LaGa1-xMnxO3 Alloys Natalia Noginova, Feng Chen, Geoffrey Chelule, Vladimir Gavrilenko Manganese doped perovskites are materials promising for data storage, demonstrating optically and field induced memory effects. Optical absorption, electric conductivity, and magnetic resonance of LaGa$_{1-x}$Mn$_{x}$O$_{3 }$crystals with Mn-content $x=$ 0 to 1.0 have been measured at different temperatures. Non-linearity and hysteresis were observed in voltage-current dependencies for all Mn-concentrations studied. Electron energy structure and optical functions of LaGa$_{1-x}$Mn$_{x}$O$_{3 }$are calculated by generalized gradient approximation method within density functional theory using \textit{ab initio} pseudopotentials. Analysis of predicted total (TDOS) and projected densities of states (PDOS) data for higher $x$-values clearly indicate the dominance of 3$d$ antibonding Mn states in the region around Fermi level. Observed significant modifications of optical absorption spectra and transport properties are related to the enhancement of 3$d$(Mn)-2$p$(O) interactions with increase of $x.$ [Preview Abstract] |
|
C1.00123: Local edge modes in doped cuprates with stripes and periodic polarons E. Kaneshita, I. Martin, A.R. Bishop, Z.G. Yu, R.J. McQueeney Several kinds of High-T$_c$ superconductors show inhomogeneous electronic structures: stripe and checkerboard patterns. The heterogeneity in the electronic system gives rise to local pohonon modes. In this study, we calculate the phonon spectra for the system with the inhomogeneous eloctronic structures. we consider the (diagonal and vertical) stripes and periodic polarons within the mean field approximation. The former is the specific electronic structure in underdoped LSCO and the latter is one of the possibilities to explain the checkerboard structure, which is observed in optimal-doped BSCCO and lightly- doped Na-CCOC by STM. In these ground states, we calculate the phonon spectra by means of RPA. We demonstrate the existence of local phonon modes specific to these structures. [Preview Abstract] |
|
C1.00124: Anisotropy breaking and superconductivity in MgB$_{2}$ Sabina Ruiz, Pablo de la Mora When magnesium is replaced by aluminium an extra 3p electron is added to the system, this 3p-electron perturbs the $\sigma $-band structure slightly, while the replacement of magnesium by scandium a 3d electron is added, this 3d electron has a large effect on the electronic structure. With aluminium replacement T$_{c}$ diminishes almost linearly and disappears at $\sim $0.53Al this corresponds to the point where the Fermi level fills up the $\sigma $-bands. The electrical conductivity in the direction of the plane due to the $\sigma $-bands ($\sigma _{a}^{\sigma })$ diminishes with a very similar trend. In this case these bands electrical anisotropy (a-direction vs. c-direction, $\sigma _{a}^{\sigma }$/$\sigma _{c}^{\sigma })$ also diminishes. On the other hand, in ScB$_{2}$ the $\sigma $-bands lose their electrical anisotropy ($\sigma _{a}^{\sigma }$/$\sigma _{c}^{\sigma }\approx $3.9), but the addition of an extra 3d electron does not raise the Fermi level above the $\sigma $-bands. Our results show that at first the anisotropy diminishes with scandium addition until $\sim $0.3Sc and then it remains almost constant, this shows a interesting parallelism with the T$_{c}$-experimental results of Agrestini et al. (2004 J. Phys. and Chem. Sol. 64, 1479), in which T$_{c}$ diminishes with scandium and disappears at 0.3Sc. [Preview Abstract] |
|
C1.00125: Optical properties of YBa2Cu3O7 films deposited on SrTiO3 bi-crystal substrate Mar\'ia A. Navacerrada, Juana V. Acrivos, Hizam Sahibudeen, Jeffrey Kortright, Pannu Nachimuthu Thin films (epitaxially grown by high pressure pure oxygen dc sputtering on bi-crystal substrates with a symmetrical tilt angle of 24$^{o})$ showing a critical temperatures for superconductivity of 90 $\pm $ 1 K, have been investigated at stations 6.3.2/1 ALS near the O K-edge across the grain boundary (GB). The film [001] enhanced scattering, total electron yield and fluorescence were measured with predominantly horizontal plane, linearly polarized incident radiation, between 525 to 535 eV. Previous experiments on single crystals show spectra composed of two main peaks with a relative intensity dependent on orientation. The film a-b axes change their orientation with respect to the beam from one side of GB to the other. Not all the intensity changes observed when moving across the GB can be explained based on previous single crystal results. The geometry of the sample as well as the magnetic order associated with the superconductor are important factors to be considered. [Preview Abstract] |
|
C1.00126: Cu L3,2 and Ba M4,5 electro-optic effects in 50 nm YBa2Cu3O7 (YBCO) films on SrTiO3 (STO) bi-crystals with a 24 DEG ab-grain boundary J.V. Acrivos, H.S. Sahibudeen, M.A. Navacerrada, J.B. Kortright, P. Nachimuthu . Two different films prepared at the Complutense University with a superconducting transition temperature T$_{c}$ = 90 $\pm $ 1 K, and characterized by X-ray diffraction at SSRL were investigated at station 6.3.1 of LBNL-ALS by enhanced YBCO 001 scattering (I$_{s})$, total electron yield (TEY) and fluorescence (F) at the Cu L$_{2,3}$ and Ba M$_{4,5}$ edges. The energy scale was calibrated by the Ti L$_{2,3}$ absorption from the spectrometer mirror, and the line shapes/intensities were checked using BaBr$_{2}$ and CuO standard references. The penetration depths at these energies ensure that the sample bulk dominated the F and I$_{s}$ signals. Comparison of I$_{s}$/I$_{0}$ with TEY/I$_{0}$ and F/I$_{0}$ indicates that the scattered linearly polarized light undergoes a rotation. The Hilbert-Kramers-Kronig analysis of F/I$_{0}$ and I$_{s}$/I$_{0}$ with Lorentzian line shapes of an eV width indicate extra absorption one eV above the L$_{3}$ edge that may be due to excitons. [Preview Abstract] |
|
C1.00127: Nuclear relaxation rate in d+s wave superconductor Keshav Shrivastava The s and d wave type gap symmetries yield distinctively different results for the temperature dependence of the nuclear relaxation rate. We have calculated the nuclear relaxation rate for various amounts of s and d wave admixtures in the gap of the superconductors. It is found that the calculated values of the nuclear relaxation rate for 80\% d wave and 20\% s wave are in good agreement with the experimental measurements in 123, 124 and 247 type superconductors. Thus we find that the order parameter has mixed s+d character in the superconductors. There are several possibilities of symmetries when two gaps are taken into account such as in MgB$_2$. 1. K. N. Shrivastava, Superconductivity: Elementary topics, ISBN 981-02-4451-7, World ci. (2000) [Preview Abstract] |
|
C1.00128: POLYMERIC AND ORGANIC MATERIALS |
|
C1.00129: NEXAFS Determina\-tions of ``Skin'' Orient\-a\-tion of Injection-Molded Thermotropic Liquid Crystalline Copolyesters Robert Bubeck, Lowell Thomas, Stanley Rendon, Wesley Burghardt, Alexander Hexemer, Daniel Fischer Near edge X-ray adsorption fine structure (NEXAFS) spectroscopy is being used to characterize anisotropy in injection-molded plaques fabricated from thermotropic liquid crystalline copolyester polymers (TLCPs). [1] NEXAFS is sensitive to the orientation of the molecular pi orbital of backbone phenyl groups of the top 3 nm of a surface via the variation in intensity of the partial electron yield of Auger electrons of the 1s $\to \quad \pi $* transition of the C=C bonds within the C K edge spectrum. Plaques of various commercial TLC polymers were injection-molded under controlled conditions and comparisons relative to mold and melt temperatures, injection speed, and polymer type were made. Good agreements were obtained between the results via 2-D wide-angle X-ray scattering in transmission and NEXAFS for appropriate processing conditions that resulted in near identical skin and core orientation. [1] R. A. Bubeck et al., \textit{J. Appl. Polym. Sci.}, submitted November, 2004. [Preview Abstract] |
|
C1.00130: Self-assembly in conjugated diblock copolymer in melt Myungae Lee, David Cookson, Binhua Lin, Mati Meron, Tananori Koga, Hengbin Wang, Luping Yu The structural changes of a series of rod-like diblock copolymers with different conjugation lengths of oligo(pheneylene vinylene) and oligothiophene were studied by small-angle x-ray scattering. These rod-rod types of diblock copolymers self-assemble into the different lamellar structures on the nanometer scale, depending upon the conjugation lengths of OPV block. The results demonstrate that the thermal treatment plays a significant role in determining the structures for each copolymer. More specifically, it is shown that the annealing time required for each copolymer to reach the equilibrium state depends on the length of OPV block. The length of OPV block is a key parameter in controlling the long-range feature in OPV-OT diblock copolymers as well. [Preview Abstract] |
|
C1.00131: Thermotropic Side Chain Liquid Crystalline Polypeptides Kathleen Schaefer, Edward Kramer, Patrick Keller, Timothy Deming Synthetic proteins can be produced by cloning the critical genes and expressing them in bacteria, but a simpler route to such materials was developed based on transition metal mediated ring opening polymerization of N-carboxyanhydrides (NCAs). To facilitate thermal processing of polypeptide melts, lysine-based NCAs were synthesized with mesogenic side chains. It was found by circular dichroism and wide angle x-ray scattering that all polymers adoped an $\alpha$-helical backbone conformation with the mesogens oriented parallel to the helical axis. Nematic ordering of the mesogens was observed by polarized optical microscopy, and the solid to nematic transition temperature depended on the length of the flexible spacer connecting the mesogen to the backbone. Chain mobility in the nematic melt allowed fibers to be pulled; WAXS experiments showed a high degree of orientation of both the helix axis and nematic director with the fiber axis, demonstrating that the incorporation of liquid crystalline side chains allows polypeptide-based synthetic materials to be thermally processed. [Preview Abstract] |
|
C1.00132: The Phase Transition Behavior of Side Chain Liquid Crystalline Polymers Containing Sulfone Group Daewon Lee, Jong-Chan Lee, Kookheon Char The phase transition behavior in side chain liquid crystalline polymers (SCLCPs) based on a hydrophilic poly(ethylene oxide) (PEO) main chain and a hydrophobic alkyl side chain containing sulfone groups was investigated by using DSC, POM, synchrotron X- ray scattering, FT-IR and rheological measurements. In the case of poly[oxy(octylsulfonylhexylthiomethyl) ethylene] (8S6EO) containing sulfone groups located at the intermediate position in the side chain, the presence of sulfone groups made it possible to obtain a highly ordered layer structure mainly due to the strong dipole-dipole interactions among sulfone groups. It is also noted that the scattering patterns completely disappeared in the isotropic state. On the other hand, a series of three SCLCPs containing sulfone groups near the hydrophilic backbone, poly[oxy(n-decylsulfonylmethyl) ethylene] (nSEO, n = 14, 16, 18), showed the evident mesophase stability due to its amphiphilic character. Interestingly, it was clearly observed for SEO-series that a broad scattering, related to the correlation hole peak due to the presence of dynamic density inhomogeneity in the disordered state, persisted even above the Ti. We also investigated the effect of length of alkyl side chains on the phase transition behavior of SEO-series, showing the layered structures with a periodic undulation of backbone chains for both 16SEO and 18SEO. [Preview Abstract] |
|
C1.00133: Transition of Hexagonal to Lamellar Packing in a Discotic Liquid Crystal Chenchen Xue, Faysal Ilhan, Shi Ji, Stephen Z .D. Cheng, Michael A. Meador, Mary Ann. B. Meador, R. K. Eby Self-assembled columnar liquid crystal structures were investigated for a discotic mol. (N,N$'$,N$''$-trioctylbemzene-1,3,5-tricarboxamide), candidate for lithium batteries. DSC and WAXD diffraction results revealed that the high-temp. phase transition is between the isotropic melt (I) and a 2D low-ordered hexagonal columnar phase. The low-temp. phase transition was attributed to the transition between a highly ordered, hexagonal columnar phase and the low-ordered hexagonal columnar phase. Between these two phase transitions, there is another recrystallization transition. By annealing at the onset of this transition, it has found the alky tails crystallized and destroyed the hexagonal packing to oblique and finally a lamellar structure. [Preview Abstract] |
|
C1.00134: Control of Anchoring of Nematic Fluids at Polymer Interfaces and Switchable Diffraction Devices Jian Zhou, David Collard, Jung Park, Mohan Srinivasarao In-situ photopolymerization of alkyl acrylate monomers in the presence of a nematic liquid crystal (NLC) provides a cellular matrix of liquid crystalline droplets in which the chemical structure of the encapsulating polymer exerts control over surface anchoring of the NLC. Control can be obtained by variation of the alkyl side chains and through copolymerization of two dissimilar monofunctional acrylates. Copolymerization of two monofunctional acrylates with opposing tendencies of aligning liquid crystal leads to tunability of anchoring behavior over a wide temperature range. We have demonstrated a new electrically switchable diffraction grating based on periodically patterning the anchoring conditions of a nematic fluid within a polymer matrix via a patterned photopolymerization. We used two comonomers with opposite tendency to align the NLC and different reactivity ratio in the copolymerization, which lead to definition of the areas with alternating homeotropic and planar alignment of the NLC through a UV irradiation with a photomask. The photopolymerization-induced diffusion of the monomers accounts for the spatial concentration distribution of these monomers. [Preview Abstract] |
|
C1.00135: Synthesis through Continuous-Wave Plasma of High Quantum Efficiency Light-Emitting Polymers from 1-Naphthaldehyde Arnold Yang, Chun-Chih Chang, Yi-Hsing Chang, Ling-Yu Cheng, Kuo Chu Hwang, Cheng-Hsuan Lai By using continuous-wave (CW) r.f. plasma, light-emitting polymers of high quantum efficiency (35{\%}) were synthesized from 1- naphthaldehyde. The monomer absorbed strongly in the UV but emitted no light, apparently due to the inter-system crossing influenced by the carbonyl groups. Extensive aldehyde cleavage and partial ring opening during plasma polymerization resulted in polymers that fluoresced strongly in the range of 422 -- 455 nm with a lifetime around 35 ns. Relative to the naphthalene chromophore, the plasma polymers illustrated a large red-shift (100 -- 120 nm) in emission but showed no shifting in absorption which peaked at 277 nm. A chain microstructure constructed by isolated naphthalene-type chromophores interconnected by soft alkyl backbones was proposed for the plasma polymers in which vibronic interactions between the excited chromophores and soft alkyl chains were used to explain the large Stokes shift observed. [Preview Abstract] |
|
C1.00136: High Refractive Index Poly(thiophene) for 3-D Organic Photonic Crystals Matthew Graham, Stephen Cheng, Shi Jin, Timothy Bunning Traditional polymers have refractive indices too low to be used in high dimensional photonic crystals (PC). Poly(thiophene) (PT) is predicted to have a high enough refractive index to open a complete photonic band gap (PBG) in the commonly used inverse opal structure for 3-D PC, but experimental values have been significantly less than the prediction. To reconcile this discrepancy, the electro-polymerization of thiophene has been optimized with respect to reactant concentration, additives, and temperature. The resulting PT films had a significantly elevated refractive index, as measured by Variable Angle Spectrographic Ellipsometry, which was sufficient to open a complete PBG. The next step in fabricating an organic 3-D PC is to create high quality opal templates through which the PT could be polymerized. Colloidal crystallization of monodisperse polystyrene spheres was used to fabricate the templates, while a novel nano-mechnical annealing technique was used to perfect the crystal structure. Taken together these developments offer the opportunity to make a completely organic 3-D PC with a complete PBG. [Preview Abstract] |
|
C1.00137: Electrospun liquid silk from the gland of Bombyx mori silk/ Green Fluorescent Proteins (GFP)/ poly(ethylene oxide) Sirina Putthanarat, Woraphon Kataphinan, Ron Eby, Darrell Reneker, Sharon Jones, Rajesh Naik, Barry Farmer We have previously demonstrated that Green Fluorescent Protein (GFP) molecules can be incorporated into silk films and maintain their nonlinear optical properties as well as resist to damage at fluence of 0.1-0.2 J/cm$^{2}$ [1]. In the present study we report the incorporation of GFP into electrsopun fibers of liquid silk from the gland of B. mori. PEO was added to the silk/GFP solution to improve the processability. The silk/GFP/PEO solutions were successfully elctrospun and the morphology of fibers was characterized using optical microscopy, and scanning electron microscopy. The resulting fibers exhibit fluorescent under the UV microscope in the reflection mode (epifluorescence) indicating incorporation of the GFP. The fiber diameters are less than 500 nm. Other characterization techniques are being applied. [1]. S. Putthanarat, et.al., ``Nonlinear Optical Transmission of Silk/Green Fluorescent Protein (GFP) Films,'' Polymer 2004;45:8451. [Preview Abstract] |
|
C1.00138: Phase coherence upon heating in diblock copolymer films Junhan Cho, Kwanwoo Shin, Kwang Soo Cho, Young-Soo Seo, Sushil K. Satija We have performed the neutron reflectivity measurements on the thin films of diblock copolymers that microphase separate upon heating in the bulk. The thin films are confined between two different surfaces, i.e., silicon substrate and air contact. The density profiles of block components were obtained through the film thickness as a function of temperature. The decay lengths of the observed surface ordering and phase coherence transitions upon heating were determined from the profiles. The recently developed compressible Landau analysis has been applied to the copolymer films to interpret the experimental reflectivity results. It was shown that specific interactions and finite compressibility are the two major issues concerning the observed behavior for the copolymers. [Preview Abstract] |
|
C1.00139: Electric Field induced alignment and morphological transitions of triblock copolymers. Akinbode Isaacs-Sodeye, Shujun Chen, Samuel Gido The electric field induced microdomain alignment of poly (styrene-b-ethylene-co-butylene -b-styrene)(SEBS) and poly (styrene-b-isobutylene-b-Styrene)(SIBS) triblock copolymers were investigated using AFM and cross-sectional TEM. For the SIBS samples, evidence of a transition from cylindrical to spherical morphology of the minor polystyrene microdomains was observed. These spherical domains appear to impinge on one another, much like a string of beads, and preferentially align with the direction of the applied electric field. On the other hand the SEBS samples, which had lower molecular weights, showed significant microdomain orientation in the direction of the applied electric field. [Preview Abstract] |
|
C1.00140: Investigation of the Unique Phase Behavior of Amphiphilic Triblock Copolymers (PAA-PMA-PS) in Solvent-Nonsolvent Mixtures Kelly Hales, Honggang Cui, Darrin Pochan, Zhiyun Chen, Qai Ki, Karen Wooley Self-assembling amphiphilic triblock copolymers containing acrylic acid, methyl acrylate, and styrene blocks were investigated under various solution conditions. The block copolymers were dissolved in tetrahydrofuran in the presence of a divalent, organic counterion. Upon the slow addition of water, a variety of unique structures were observed including bulk-like phase separation, spherical micelles, cylindrical micelles, disks, as well as toroidal (ring-like) assemblies. The specific structure formed was dependent on the architecture of the triblock copolymer, the amount of counterion present, as well as the solution conditions. The focus of this work is the basic understanding and characterization of the phase separated structures present in low water content solutions. The understanding of this bulk phase behavior and its effects on the micelles formed at higher water contents will be presented. Cryo-transmission electron microscopy and small, ultra-small angle neutron scattering were used to examine the copolymer solutions \textit{in situ} while negatively stained, cast films were examined via transmission electron microscopy. [Preview Abstract] |
|
C1.00141: Phase behaviour of a diblock copolymer melt under cylindrical confinement Weihua Li, Robert Wickham The phase behaviour of a diblock copolymer melt confined inside an infinitely-long cylindrical pore of radius $R$ is investigated using real-space self-consistent mean-field theory. A short-range, preferential interaction between the walls of the pore and one of the copolymer blocks is assumed. Confinement leads to a rich phase diagram, which is explored in detail for fixed $R=8.5 R_g$, as a function of $\chi N$ and $f$. Disordered, lamellar and cylindrical morphologies are obtained, but with arrangements and symmetries not seen in the bulk, for example: onion-like lamellar layering, arrays of cylinders symmetric under 3-, 4- and 5-fold rotations, and structures intermediate between cylinders and lamellae. Near the pore walls the structural rearrangement is severe, leading to the presence of defects in the cylindrical order. Phase transitions as the pore radius varies have also been observed. [Preview Abstract] |
|
C1.00142: The Influence of Polydispersity on the Thermodynamics of Diblock Copolymers Nathaniel Lynd, Marc Hillmyer The effects of the molecular weight distribution on the thermodynamics of diblock copolymers have been predicted to affect order-disorder transitions (ODT), order-order transitions and the equilibrium morphology adopted.$^{1,2}$ We prepared several sets of Poly[(ethylene-\textit{alt}-propylene)-$b$-(D,L-lactide)] diblock copolymers with controlled molecular weights, compositions and polydispersities (PDIs). Rheology and small angle x-ray scattering were used to evaluate the effects of PDI on the lamellar domain spacing, the ODT, and the resultant morphology. For symmetrical samples, the lamellar domain spacing increased with increasing PDI. The degree of segregation at the ODT (($\chi $N)$_{ODT})$ was dependent upon the volume fraction of the polydisperse component ($f_{PLA})$. Interestingly, for $f_{PLA }$= 0.33 ($\chi $N)$_{ODT}$ decreased with increasing PDI but for $f_{PLA} =$ 0.64 ($\chi $N)$_{ODT}$ increased with increasing PDI. We also demonstrated that an increase in PDI at constant $f_{PLA}$ results in a change in equilibrium morphology. Monte Carlo simulations addressing the effects of fluctuations on the ODT of polydisperse diblock copolymer melts were also performed. .(1) Sides,S.W.; Frederickson, G.H. \textit{J. Chem. Phys.} \textbf{2004}, $121,$4974. (2) Burger,C.; Ruland, W.; Semenov, A.N. \textit{Macromolecules} \textbf{1990}, $23$, 3339. [Preview Abstract] |
|
C1.00143: Phase ordering mechanism of triblock copolymers. A dynamic density functional study Jianfeng Xia, Feng Qiu, Hongdong Zhang, Yuliang Yang Using dynamic density functional theory (DDFT), we studied the morphology and kinetics of microphase separation of linear triblock copolymers. ABC triblock copolymers with equal and unequal interaction energies were taken as examples to investigate the microphase separation kinetics of the linear triblock copolymers. The calculation was carried out in two dimensions and shows that, the equilibrium microphase structures of the linear triblock copolymers are consist with the results obtained by the self-consistent field theory (SCFT) and previous experiments. We found that the ordering process of the triblocks could be divided into two regimes-an initial fast phase separation regime followed by a slow defects annihilation regime-according to the time evolution of the free energy and the global order parameters. Furthermore, we have found that both one-step and two-step microphase separation mechanisms can occur during the disorder to order transition, which depend on the average volume fractions of the different blocks, interaction energies, and topological sequences of the blocks. The results we presented may be guidance to design experiments to explore the kinetics of triblock copolymers and to obtain desired micro-structures in linear triblock copolymer melts. [Preview Abstract] |
|
C1.00144: A TEM, SFM and GISAXS investigation of the ordering behavior of a cylinder forming block copolymer V. Khanna, G.E. Stein, A. Hexemer, E.J. Kramer, X. Li, J. Wang, S.H. Hahn We present a transmission electron microscopy, scanning force microscopy, and grazing incidence small angle X-Ray scattering (GISAXS) investigation of the ordering of a cylinder forming pentablock copolymer, CECEC (C: poly(cyclohexylethylene), E:poly (ethylene), M$_w$ = 60k g/mol f$_E$ = 0.25). 600nm thick films of CECEC are spun cast and annealed in ultra high vacuum at 180 $^{\circ}$C over several days. In the as-cast film the E cylinders are randomly oriented through the depth of the film. Microscopy images and GISAXS patterns of samples that were annealed for 1, 2 and 3 days show cylinders that are predominantly parallel to the plane of the film. After annealing for 7 days, we observe that a substantial fraction of cylinders align perpendicular to the film plane. TEM images suggest that the reorientation is observed to initiate from the air-film interface. This observation is corroborated by angle resolved GISAXS which suggests that the perpendicular orientation is more pronounced at the surface of the film than over the entire thickness. [Preview Abstract] |
|
C1.00145: Graphoepitaxy of Block Copolymer as a Route to Patterning Macroscopic Length Scales with a Single Long-Range Grain Orientation G.E. Stein, H. Cota, A. Hexemer, E.J. Kramer The order of a monolayer of spherical domain poly(styrene-\emph {b}-2 vinyl pyridine) block copolymer melt confined laterally in hexagonal wells is investigated as a function of thermal history and monolayer thickness. The hexagonal geometry is designed to match the hexagonal symmetry of the 2D crystalline and hexatic phases. The wells are 25nm deep, range in width from 7 to $30 \mu$m between parallel edges, and adjacent wells are separated by mesas 2$\mu$m wide. All thermal treatments begin with a disordered film that uniformly covers the wells and mesas. The “best” order is obtained for a monolayer \textbf{t}$^{\star} $-nm thick that is briefly heated to the 2D liquid, followed by a slow cooling through a hexatic intermediate to the 2D crystalline phase. The result is a single crystal with long-range orientational order and a low dislocation density in wells up to 12$\mu$m wide. The close-packed rows align with the well edge, so the orientation across all neighboring wells is correlated. When the monolayer thickness \textbf{t} deviates from \textbf{t}$^{\star}$ by -20\%, a high density of dislocation defects is induced instead of hole formation. [Preview Abstract] |
|
C1.00146: Effect of Chemical Oxidation on the Self-Assembly of Organometallic Block Copolymers Hany Eitouni, Nitash Balsara The thermodynamic interactions in ferrocenyldimethylsilane diblock copolymers were systematically adjusted by oxidation of the ferrocene moiety with silver nitrate and examined using small angle x-ray scattering and depolarized light scattering. The polymers retained microphase separated ordered structures upon oxidation and showed systematic changes in the location of the order-disorder transition as a function of ferrocenium nitrate content. The extent of oxidation can be controlled locally through electrochemical techniques and hence ordered and disordered regions can be maintained within a sample. By controlling the redox properties of the ferrocene moiety in the backbone of the polymer, we have provided a novel method of controlling microstructure and hence bulk properties. [Preview Abstract] |
|
C1.00147: Toroid formation by the co-assembly of charged triblock copolymers with divalent organic counterions Honggang Cui, Kelly Hales, Darrin Pochan, Zhiyun Chen, Qi Kai, Karen Wooley The toroid, or ring, micelle morphology, which is theoretically predicted but rarely observed, has been produced by the self assembly of PAA-b-PMA-b-PS triblock copolymer in combination with an organic counterion and mixed THF/H2O solvent. The toroids consist of a PS core, a sheath of PMA, and the PAA corona. The interaction of positively-charged, multivalent organic salts with the negatively-charged PAA corona plays a decisive role in the formation of rings. The ring formation cannot be explained exclusively by traditional theories that focus on the fusion of high-energy end caps of cylindrical micelles at the expense of a higher micelle curvature penalty. It was also found that final assembled morphologies, including discs, rings, spheres, could be targeted by choosing the correct organic, multivalent counterion characteristic, such as hydrocarbon or polyether chain length between amine groups, the number of amine groups and the molar ratio of amine group to acid group in PAA. Ring morphology is observed in situ with cryoTEM experiments or can be kinetically trapped by either ridding the system of organic solvent or chemically crosslinking the PAA corona. [Preview Abstract] |
|
C1.00148: Solution Characterization of pH-Sensitive Polypeptide Based Block Copolymer Assemblies Kay Gebhardt, Daniel Savin, Manikandan Jayaraman, Jean Frechet A series of amphiphilic block copolymers consisting of poly(1,4- butadiene) and poly(lysine) (PB-b-PLys) were synthesized, and their solution properties were studied using dynamic light scattering and transmission electron microscopy. These materials were found to self-assemble in solution into spherical micelles and vesicles when the side chain amine groups on the lysine units are protected. Upon deprotection, the lysine units become responsive to solution acidity. At high pH, the poly(lysine) chain assumes either an $\alpha$- helical or $\beta$-sheet conformation depending on temperature, while at physiological pH the side chains are protonated, resulting in an expanded coil configuration. These conformational changes result in a swelling of the micellar structure, the magnitude of which depends on the molecular weight and relative composition of the two blocks. For two molecular weights of PB (DP$_n$ = 25, 50), a series of molecular weights for P(Lys) (DP$_n$ = 3, 6, 12, 25, 50) were studied, and the assembly size and morphology were determined as a function of pH, ionic strength and temperature. [Preview Abstract] |
|
C1.00149: Catalysts from Self-Assembled Organometallic Block Copolymers David Durkee, Mark Ellsworth, Nitash Balsara Heterogeneous catalysts were prepared by crosslinking the polyisoprene block of a microphase separated poly(vinyl ferrocenium triflate-\textit{block}-isoprene) copolymer.~ The poly(vinyl ferrocenium triflate) moieties are responsible for catalytic activity while the polyisoprene block provides the support structure.~ The efficacy of the catalyst was analyzed by studying the Michael addition of ethyl-2-oxycyclopentane carboxylate and methyl vinyl ketone.~ The materials were studied via Transmission Electron Microscopy and the reaction rates were monitored by NMR spectroscopy.~ The activities obtained with our self-assembled heterogeneous catalyst were within experimental error of the rate of monomeric ferrocenium triflate as well as other homogeneous catalyst analogues. Our method for synthesizing catalysts is unique because the structures of both the support and active sites are controlled by molecular self-assembly. [Preview Abstract] |
|
C1.00150: Graphoepitaxy of triblock and diblock copolymer blend thin films. K.E. Sohn, G.E. Stein, E.J. Kramer The ordering of a thin film of poly(styrene-b-ethylene-r-butylene-b-styrene) SEBS and poly(styrene-b-ethylene-butylene) blend laterally confined in a long, rectangular silicon oxide well is investigated as a function of annealing time and temperature. [1] The blend is 65wt{\%} triblock (MW=81,000 g/mol) and 35wt{\%} diblock, with the diblock being half the molecular weight of the triblock. In bulk, the blend shows a cylinder to sphere transition at $\sim $140\r{ }C. Thick films exhibit the cylindrical morphology after annealing at 120\r{ }C for several days, but in single layer thin films the PS domains remain spherical even after 7 days annealing. Close packed rows of these spherical domains align along the edges of the wells and form templated grains that grow outwards from these edges as the annealing time increases. [1] R.A. Segalman, H. Yokoyama, and E.J. Kramer, Adv. Mater. \textbf{13}, 1152 (2001). [Preview Abstract] |
|
C1.00151: Morphological Transitions in a Triblock Copolymer and Its Sulfonated Ionomer: Thermal Annealing and Solvent Effects Shujun Chen, Samuel P. Gido The microphase separation behavior of poly(styrene-$b$-(ethylene-\textit{co}-butylene)-$b$-styrene) (SEBS), as well as its sulfonated ionomer (S-SEBS), was studied using TEM and SAXS. The SEBS triblock used in this study has 30 wt {\%} styrene and is expected to form a cylindrical morphology. Upon casting from toluene, a slightly preferential solvent for styrene, however, SEBS revealed a lamellar morphology in TEM, as confirmed by SAXS. After thermal annealing, the morphology changed to core-shell cylinders with EB as the core and styrene as the shell. SAXS confirmed the formation of cylinders in annealed SEBS. The S-SEBS studied has an intermediate sulfonation level of 23 mol {\%}. S-SEBS cast from toluene formed disordered spheres, while S-SEBS cast from THF revealed disordered lamellae. Thermal annealing resulted in little change in the morphology for both samples. Initial examination of S-SEBS samples cast from these two solvents suggested much different viscoelastic properties and quantitative measurements are being conducted using dynamic mechanical analysis. [Preview Abstract] |
|
C1.00152: The Order-to-Disorder Transition of A Symmetric Polystyrene-block-Poly(2-vinyl pyridine) Copolymers with various amounts of Cadnium Chloride Dong Hyun Lee, Du Yeol Ryu, Jin Kon Kim The order-to-disorder transition [ODT] of a symmetric polystyrene-block-poly(2-vinyl pyridine) [PS-P2VP] copolymer with various amounts of cadmium chloride [CdCl$_{2}$] was investigated by rheometry, synchrotron small angle X-ray scattering (SAXS), and transmission electron microscopy. PS-P2VP was synthesized by the sequential anionic polymerization. The CdCl$_{2}$ can coordinate with the nitrogen in P2VP. Even though the amount of CdCl$_{2}$ was very small (less than 0.01 vol {\%}), the T$_{ODT}$ of PS-P2VP coordinated with CdCl$_{2}$ increased drastically. When the volume fraction of CdCl$_{2}$ is just 0.0023 (thus the molar ratio of [CdCl$_{2}$]/[P2VP] = 0.01), the T$_{ODT}$ was increased as large as about 10 $^{o}$C compared with that of neat PS-P2VP. This is because the chain motion of P2VP block is severely restricted by coordination of CdCl$_{2}$ with P2VP. The order-to-disorder transition temperatures for [CdCl$_{2}$] / [2VP] = 0, 0.01, 0.02, 0.03 and 0.04 are 199.2, 209.9, 234.6, 260.2 and 279.8 $^{o}$C, respectively. In addition to the large increase of domain spacing of microstructure was also found in this study even though the amount of CdCl$_{2}$ was very small. [Preview Abstract] |
|
C1.00153: Solvent Annealing Block Copolymer Thin Films of Poly(isoprene-b-lactide) Kevin Cavicchi, Thomas Russell Long-range order is desirable in block copolymer thin films for building nanostructured templates or scaffolds. Exposing films to solvent vapor or ``solvent annealing'' is an effective way to increase chain mobility to obtain large, defect free grains and mediate interactions to achieve specific domain orientations. Results will be presented on solvent annealing thin films of poly(isoprene-b-lactide). By choosing the proper solvent, and controlling the solvent concentration and film thickness, hexagonally packed cylinders oriented parallel and perpendicular to the substrate with good long-range order can be obtained. In addition, efforts to crosslink the polyisoprene and degrade the polylactide to generate a nanoporous film will be discussed. [Preview Abstract] |
|
C1.00154: Phase Behavior of Amphiphilic Block Copolymers in Supercritical Carbon Dioxide William Edmonds, Timothy Lodge, Marc Hillmyer Condensed carbon dioxide represents a promising ``green'' solvent alternative, on the basis of its abundance and modest critical conditions. Amphiphilic block copolymers offer the potential of enhancing the versatility and usefulness of this solvent through the formation of micellar aggregates. Our goal is to understand and define the parameters that control aggregate shape and dimensions in compressible carbon dioxide; these parameters include copolymer volume fraction and solvent density. A series of low molecular weight block copolymers with varying compositions of 1,2-polybutadiene (PBD) and poly(hexafluoropropylene oxide) (PFPO) were synthesized. Using a custom-built apparatus, we characterized the phase behavior of these materials in supercritical carbon dioxide as a function of concentration, temperature, and pressure. The key result indicates that the most soluble amphiphile at a fixed molecular weight of PBD is the one with an intermediate molecular weight of PFPO. These results establish solubility conditions necessary for future experiments to characterize aggregate dimensions and solution viscosity. [Preview Abstract] |
|
C1.00155: Combinatorial Investigation of Crazes in Polymer Nanocomposites Jong-Young Lee, Alfred Crosby Self-assembled block copolymer nanocomposite thin films aligned by external fields or surface energetics have potential applications as photonic crystals as well as magnetic storage media. However, the presence of surface terraces and embedded nanoparticles can initiate defects such as crazes which can alter the materials' intended functions. We combine the copper grid technique and combinatorial approaches to quantify craze growth in these systems. We present results on the effects of film thickness, surface terraces, and nanoparticles on crazing phenomena in polystyrene and model poly(styrene-b-2-vinyl pyridine) copolymers. These results guide the future design of advanced nanocomposite films while providing fundamental insight into the molecular interactions of polymers and inorganic nanoparticles under mechanical strain. [Preview Abstract] |
|
C1.00156: 3-Dimensional Imaging of Evolving Block Copolymer Microstructure using Laser Scanning Confocal Microscopy Wonmok Lee, Jongseung Yoon, Hyunjung Lee, Edwin L. Thomas Block copolymers (BCPs) exhibit various periodic structures through microphase separation with tunable domain shape and size via choice of molecular weight, chain architecture, etc. To extract detailed morphological information of BCPs, microscopic tools such as transmission electron microscopy and atomic force microscopy have been widely used due to the superior imaging capability. However, these methods provide only 2-D info and are not very suitable to following dynamic bulk processes in real time. Structural information over 3-D space is often crucial to elucidate the BCP microstructure in detail and to understand the role of nucleation and defects in various types of phase transitions and ordering processes. Laser scanning confocal microscopy (LSCM) is a unique optical microscopic technique capable of 3-D imaging of various microstructures (resolution limit $>$ 200nm) in non-destructive manner. This length scale can be readily reached with BCPs having 0.5M g/mol molecular weights. We employ LSCM to investigate the 3-D morphology of various BCP self-assemblies including well-ordered cylindrical BCP obtained by directional solidification. We expect LSCM will enable us to explore a variety of unprecedented morphological phenomena related to BCP self-assembly. [Preview Abstract] |
|
C1.00157: Baroplastic Block copolymers Sheldon A. Hewlett, Juan A. Gonzalez Leon, Jeffrey A. Borowitz, Anne M. Mayes Block copolymers with rubbery and glassy components have been observed to have pressure induced miscibility. These microphase-separated materials, termed baroplastics, were able to flow and be processed at temperatures below the Tg of the glassy component by simple compression molding and extrusion. Diblock and triblock copolymers of polystyrene and poly(butyl acrylate) or poly(2-ethyl hexyl acrylate) were synthesized by atom transfer radical polymerization (ATRP) and processed at room temperature into well defined transparent objects. SAXS and SANS measurements demonstrated partial mixing between components as a result of pressure during processing. DSC results also show the presence of distinct domains even after several processing cycles. Their mechanical properties after processing were tested and compared with commercial thermoplastic elastomers. [Preview Abstract] |
|
C1.00158: Phase behavior of cross-linked block copolymers Hyeok Hahn, Enrique Gomez, Jayajit Das, Arup Chakraborty, Nitash Balsara, Mark Ellsworth To study the order-disorder transition in cross-linked copolymer melts we have selectively cross-linked the polyisoprene chains in poly (styrene-block-isoprene) copolymers using high energy radiation. The temperature during irradiation was controlled to allow cross-linking in the ordered or disordered state. We present the phase behavior of block copolymers with either cylindrical or lamellar morphologies before and after the gelation point and compare the results for samples below the gelation point with a mean-field theory based on a coarse grained free energy and the Random Phase Approximation. [Preview Abstract] |
|
C1.00159: The effect of temperature gradient on block copolymer thin film under non-neutral surface condition: simulation and experiment June Huh, Won Ho Jo, Hui Joon Jung, Cheolmin Park Using both computer simulation and experiment, we study the effect of directional moving of temperature gradient on the structure organization in diblock copolymer thin films bound to the surfaces that favor one of the block components. To simulate the time evolution of the morphological development in the block copolymer film under temperature gradient, the dynamical evolution equation based on Cahn-Hilliard-Cook model is numerically integrated on $L\times L\times D$ grid cell. Three different block copolymer morphologies of lamellar, cylindrical and spherical structures confined between non-neutral flat surfaces are initially generated by a rapid quenching from their disordered states and then the temperature gradient ($\nabla _{x}T$ where $T$ is the temperature and $x$ is one of the direction parallel to the surface) is slowly moved along the $x$-direction. We find from both simulation and experiment that in the case of lamellar film $\nabla _{x}T$ always promotes the lamellar layering irrespective of film thickness, remedying all the bumpy lamellar planes to become completely flat. We also find that in the case of cylindrical and spherical morphologies the temperature gradient enhances the spatial symmetry of mesophases. [Preview Abstract] |
|
C1.00160: Compositional and monomer sequence distribution analysis of monodisperse brominated-polystyrenes using interaction chromatography Junwon Han, Chang Yeol Ryu, James J. Semler, Jan Genzer High performance liquid chromatography techniques have been developed for characterizing complex polymers that are often heterogeneous in molecular weight, molecular architecture, and chemical composition. Recently, interaction chromatography (IC) techniques have been developed, which facilitate separation of polymers based on enthalpic ``attraction'' difference among the chemical constituents of the molecule. Here, we use IC for characterizing the composition and monomer sequence distribution in statistical copolymers of poly(styrene-co-4-bromostyrene) (PBr$_{x}$S). The PBr$_{x}$S copolymers were synthesized by brominating monodisperse polystyrenes; the degree of bromination (x) and the sequence distribution have been adjusted by varying the bromination time and the solvent quality, respectively. Both normal-phase (bare silica) and reversed-phase (C18-bonded silica) columns are used at different combinations of solvents and non-solvents to monitor the content of the 4-bromostyrene units in the copolymer and their average monomer sequence distribution. [Preview Abstract] |
|
C1.00161: Templated growth of sub-20nm GaN nanostructures using Block Copolymer Lithography Kasiraman Krishnan, Azar Alizadeh, Christopher Keimel, Seth Taylor, Steven Leboeuf, Suryaprakash Ganti Templated growth of Wide Band Gap nanostructures allows for a precise control over quantum dot size and location, and avoids the nonradiative defects associated with the direct writing techniques. Here we report templated growth of Gallium Nitride nanostructures inside sub 20 nm SiO$_2$ windows using Metal Organic Chemical Vapor deposition (MOCVD). The SiO$_2$ templates were produced by Block copolymer lithography and Reactive Ion Etching (RIE). Polystyrene-b-poly(methyl methacrylate) with cylindrical morphology was oriented perpendicular, by surface treatment with a self-assembled monolayer of a fluorosilane and controlled UV exposure, or by coating with random copolymer brush. The PMMA was selectively removed by RIE and patterns transferred to SiO$_2$. The templates were then used to control both the homo and heteroepitaxial growth of GaN nanopillars with MOCVD. The morphology was characterized by SEM; Cathodoluminescence, Photoluminescence and TEM were further used to examine the defect densities. [Preview Abstract] |
|
C1.00162: Prediction and characterization of biodegradable baroplastics with low temperature processability Nathan Lovell, Ikuo Taniguchi, Anne M. Mayes Interest in biodegradable and biologically-derived materials has prompted substantial research into polyesters like poly(L-lactic acid) (PLA). Although more environmentally benign than conventional thermoplastics, these still require elevated processing temperatures which cause their degradation and preclude their use as matrices for temperature-sensitive pharmaceutics. Here we report on a new class of biodegradable block copolymer, consisting of PLA and one low Tg polyester, that exhibits `baroplastic' behavior. The copolymer components were selected using a compressible blend model to undergo pressure-induced miscibility, allowing their compression molding at temperatures as low as 35$^{o}$C. [Preview Abstract] |
|
C1.00163: Effect of PE on the structural evolution of iPP : analysis of a series of iPP-PE copolymers Kishore Tenneti, Lingyu Li, Chung Tso, Christopher Li The three different morphological forms of isotactic polypropylene (iPP), namely alpha, beta, and gamma, are well established. Alpha is the most common crystal phase while beta is metastable. The gamma form of iPP often exists as the minor component, intermixed with the alpha phase, during crystallization of copolymers at atmospheric pressures. A series of iPP-Polyethylene (PE) copolymers with varying percentages of PE have been investigated. Thermal and x-ray analysis reveal that as the PE content increases from 0 to 5 percent, gamma became the dominant phase. Single crystal morphology of the iPP-PE copolymers have been studied using TEM, SEM and AFM. The signature of alpha phase, the cross-hatch morphology, was observed in all the samples. Gamma single crystals were clearly observed like a ribbon shape and the center of the ribbon consisted of alpha lath. Fractal-like growth was also observed, presumably due to the thermal degradation of the polymer. [Preview Abstract] |
|
C1.00164: Ceramic electrospun nanofibers as selective emitters for thermophotovoltaic energy conversion Woraphon Kataphinan, Vivek Tomer, George Chase, Edward Evans, Rex Ramsier, Daniel Smith, Darrell Reneker Ceramic nanofibers were produced by electrospinning and characterized with electron microscopy, X-ray diffraction, FTIR, and X-ray photoelectron spectroscopy. Rare earth compounds were incorporated in the ceramic nanofibers in order to selectively modify the optical properties of the ceramic electrospun nanofibers. Titania electrospun fibers were studied. X-ray diffraction showed the titania nanofiber annealed at 773 K had the anatase crystal structure, and annealing at 1173 K produced the rutile crystal structure. In order to modify the optical properties, erbia was incorporated into Titania nanofibers. Temperature-dependent emission spectra showed that the erbia-containing nanofibers emit selectively in the near-infrared when convectively heated by hot gas. The electrical power generated by a GaSb photodetector was measured as a function of temperature. Such ceramic nanofibers can be used in thermophotovoltaic energy conversion technology to generate electrical energy from relatively low temperature heat sources, which are presently wasted. [Preview Abstract] |
|
C1.00165: Effect of Organic Modifier and Preparation Method on the Morphology and Crystalline Structure of Poly(vinylideneflouride)-Montmorillonite Nanocomposites Douglas Dillon, Kishore Tenneti, Christopher Li Polymer-layered silicate nanocomposites have proven to be an effective method of improving the physical properties of many different polymers, but PVDF-clay composites have received relatively little attention. In this study, two commercially available montmorillonite-based nanoclays with different organic modifiers were each used to prepare both solution cast and precipitated PVDF-clay composites from dimethylformamide (DMF) solutions. Structure and morphology of the PVDF-clay nanocomposites have been investigated. Depending on which organically modified nanoclay was used, solution cast composites showed either a phase-separated or a partially intercalated morphology, while precipitating the PVDF with water always yielded a completely exfoliated morphology. For solution cast samples, it has also been observed that the nanoclay layers are parallel to the film surface. The presence of the nanoclay in any morphology promoted the crystallization of PVDF's $\beta $ polymorph. [Preview Abstract] |
|
C1.00166: Spherical nanoparticle ordering in block copolymer systems John Papalia, Mary Galvin In recent years, nanoparticles and self-assembled systems have both been areas of extensive research. Our work combines the unique properties of both fields into a single system by investigating the ordering of nanoparticles in block copolymers (BCPs). Balazs et al. put forth an expansive set of theories which predict the behavior of hard nanoparticle inclusions in BCP systems.~ These composites combine the natural drive of the BCPs to self-assemble with the physical properties of the particles, yielding complex systems.~ Our goal is to methodically investigate the theories via model experimental systems. Silica ($<$22 nm) and gold ($\le $15 nm) particles have been incorporated into the styrene phase of a polystyrene-$b$-poly(ethylene-\textit{co}-propylene) BCP. Parameters such as particle size, BCP molecular weight, and surfactant length and coverage are varied in a controlled fashion with their effects on particle ordering studied. Results show surfactant use is almost mandatory to provide the particles with the energetic advantage needed to integrate them into the BCPs. Integrated particles show definitive ordering within discrete BCP domains. Our current and future systems seek to further investigate these results and trends. The combination of these result sets, along with experimentation related to domain size, will aid the design of multi-size particle systems potentially suited for improved photonic devices or nano-scale waveguides. [Preview Abstract] |
|
C1.00167: Pathway Dependent Self-Assembly of Amphiphilic Diblock Copolypeptides Lisa Pakstis, Andrew Nowak, Eric Holowka, Jeffery Thompson, Timothy Deming, Darrin Pochan Diblock copolypeptides consisting of a hydrophilic lysine (K) block and a hydrophobic leucine (L) block were designed to self- assemble due to their amphiphilic nature and the defined secondary structure of the hydrophobic block. In aqueous solution, these amphiphilic copolypeptides assemble into stiff, porous hydrogels at low volume fractions of polymer (vol. fraction polypeptide ( $>$ 0.5 wt \%). However, different self- assembly pathways (e.g. dissolution of the polypeptide into an organic solvent with subsequent addition of water followed by evaporation of the organic component) produce drastically different materials, spanning weak hydrogels, vesicles, twisted fibrils or hexagonal single crystals. Characterization of these disparate materials indicates that assembly is intrinsically controlled on the nanoscale by the interaction of the a–helical hydrophobic block into membranes. Hierarchical, microscale assembly is controlled through the pathway, i.e. kinetics, of assembly. [Preview Abstract] |
|
C1.00168: Disc Micelle Formation of Polystyrene-b-Polymethacrylate-b-Polyacrylic acid Triblock Copolymer Zhibin Li, Zhiyun Chen, Kelly Hales, Honggang Cui, Kai Qi, Karen Wooley, Darrin Pochan The self-assembled structures of polystyrene-b-polymethacrylate-b-polyacrylic acid amphiphilic triblock copolymers were studied in water/THF solvent mixtures with organic diamines as counterions. The system has been investigated by means of transmission electron microscopy (TEM), cryo-TEM, and small angle neutron scattering (SANS). Polymeric discs with diameters of approximately 100 nm are readily formed from this ionic triblock copolymer. By altering the water content of the solvent mixture, the hydrophobic polystyrene chain length, the chemical structure of the diamine counterions, and diamine counterion concentrations, one can predictively access disc or cylindrical micelle structure with the same triblock copolymer. Stacks of discs and the disc-to-rod transition (rods growing out from discs) were observed via direct electron microscopy imaging of these intermediate structures. Different pathways en route to the disc formation, the thermodynamic stability of the disk micelle morphology, and the kinetics of the disk-to-cylinder transition will be discussed. [Preview Abstract] |
|
C1.00169: Langmuir structure of poly (2-vinylpyridine-b-hexyl isocyanate) rod-coil diblock copolymers at the air/water Interface Farhan Ahmad, Kwanwoo Shin, S.H. Han, J.S. Lee We conducted a systematic interfacial study for the complete range (5\%-90\% of rod mole percentage) of an amphiphilic rod-coil system, poly (hexyl isocyanate)-b-(2-vinylpyridine) at the air/water and air/solid interface. We applied Langmuir balance technique, scanning probe microscopy (SPM), transmission electron microscopy (TEM) and X-ray reflectivity for the complete characterization of the monolayer at the interfaces. The phase isotherms showed the well amphiphilic balance for the diblock copolymers, and the formation of stable monolayers. With the increasing rod content, the consistent increase in the monolayer packing density was observed by the phase isotherms and supported by X-ray reflectivity. SPM and TEM characterization showed their interesting surface morphology according to the varying rod mole percentage in the rod-coil system. Rod mole percentage 5\%-15\% showed micellar morphology. Rod mole percentage 23\%-32\% showed distinct and dispersed rods, whereas rod mole percentage 70\%-90\% showed well packed structure similar to lamella phase. We found the tendency of the diblock system to adopt a packed monomolecular structure has increased by the increasing rod content. This lead us to conclude that it is the hexyl-isocyanate (rod part) that governs mostly the interfacial behavior of rod-coil block copolymers. [Preview Abstract] |
|
C1.00170: Coarse Grain Molecular Dynamics Simulations of the Deformation of Polymer Nanocomposites Barry Farmer, Richard Vaia, Kelly Anderson Nanoscopic dispersions of inorganic nanoparticles in a polymer have been shown to enhance many aspects of the material’s performance. Coarse grain molecular dynamics simulations have been used to examine the deformation behavior of these systems. A bead-spring model was used to represent polymer chains and the inorganic layered-silicate phase. The influence of the strength of the interface between the polymer and the layered silicate on the deformation behavior of the nanocomposite has been examined. A weak interface leads to failure between silicate sheets, while for a strong interface, failure occurs in bulk regions of polymer well removed from the silicate. The structure of the deformed material differs from that seen in deformation of the neat polymer. [Preview Abstract] |
|
C1.00171: A Theoretical Study of the Use of Electroosmotic Flow to Extend the Read-Length of DNA Sequencing by End Labeled Free Solution Electrophoresis Laurette McCormick, Gary Slater End Labeled Free Solution Electrophoresis provides a means of separating DNA with free solution capillary electrophoresis, eliminating the need for gels and polymer solutions which increase the run-time and can be difficult to load into a capillary. In free solution electrophoresis, DNA is normally free-draining and all fragments elute at the same time, whereas ELFSE uses an uncharged label molecule attached to each DNA fragment in order to render the electrophoretic mobility size-dependent. We show how an electroosmotic flow could be used to extend the read-length of DNA sequencing with ELFSE. In particular, we demonstrate that the magnitude of the electroosmotic flow must be selected very carefully in order to gain both in speed and in read length. The possibility of having molecules moving in opposite directions is also examined. [Preview Abstract] |
|
C1.00172: Single clay sheets inside electrospun polymer nanofibers Zhaohui Sun, Darrell Reneker Nanofibers were prepared from polymer solution with clay sheets by electrospinning. Plasma etching, as a well controlled process, was used to supply electrically excited gas molecules from a glow discharge. To reveal the structure and arrangement of clay layers in the polymer matrix, plasma etching was used to remove the polymer by controlled gasification to expose the clay sheets due to the difference in reactivity. The shape, flexibility, and orientation of clay sheets were studied by transmission and scanning electron microscopy. Additional quantitative information on size distribution and degree of exfoliation of clay sheets were obtained by analyzing electron micrograph of sample after plasma etching. Samples in various forms including fiber, film and bulk, were thinned by plasma etching. Morphology and dispersion of inorganic fillers were studied by electron microscopy. [Preview Abstract] |
|
C1.00173: Equilibrium configurations of self-assembling polymers: An extended parallel tempering approach Chakravarthy Ayyagari, Dmitry Bedrov, Grant Smith We present a novel algorithm based on tempering a fraction of relaxation limiting interactions to accelerate the process of obtaining uncorrelated equilibrium configurations. A chain of canonically independent replicas with overlapping distributions in energy space can be simulated in parallel and occasionally attempt to exchange the configurations between adjacent replicas. Each replica is simulated in the same ensemble (canonical) but has a different tempering fraction and corresponding relaxation time associated with it. We implemented this algorithm to enhance the equilibration process in self-assembling polymers by tempering the interactions responsible for self-association. Depending on the strength of the relaxation limiting interactions, the efficiency of the algorithm is orders of magnitude higher than that of conventional canonical simulation. [Preview Abstract] |
|
C1.00174: Brush formation at the reactive surface from end-functionalized polymer Ye Zhang, Fang Yin, Dmitry Bedrov, Grant Smith Brush formation at the reactive surface from end-functionalized polymer Ye Zhang, Fang Yin, Dmitry Bedrov, Grant D. Smith Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah, 84112 We use bond-fluctuation Monte Carlo simulation to explore the polymer brush formation by absorbing end-functionalized polymer chains onto reactive surface in melt and good solvent. By examining the dynamically determined brush structure, we find that: (1) penetration of the chains from bulk to the brush significantly reduces upon formation of the brush; (2) brushes become more extended: perpendicular to surface upon formation of the brush; (3) heterogeneous doubly-bonded loops (chains with both end-groups reacted with surface): later formed loop has larger brush height and shorter lateral end spacing than earlier formed ones; (4) adding non-reactive chains into the melt promotes the percentage of doubly-bonded loops in the brush. We also compare the dynamically determined brush structure with equilibrium brush structure by allowing end- groups to move on the surface. Furthermore, we compared the brush structure with singly-bonded brush with half chain length. [Preview Abstract] |
|
C1.00175: Linear Viscoelastic Response of PBX-9501 Binder using Molecular Dynamics Simulations Hemali Davande, Oleg Borodin, Grant Smith Quantum-chemistry based force fields for Estane, bis-dinitropropyl formal (BDNPF) and bis dinitropropyl acetal (BDNPA) plasticizer have been developed, validated and utilized in atomistic molecular dynamics (MD) simulations of a model PBX-9501 binder. The viscoelastic response of unentangled binder melt using MD simulations was studied. These results were then used in prediction of linear viscoelastic response of an entangled melt using theoretical models for viscoelastic response of block copolymers and compared with experiments. [Preview Abstract] |
|
C1.00176: Monte Carlo study of reversibly associated polymers Chun-Chung Chen, Elena E. Dormidontova Monte Carlo simulations are applied to study polymers formed through reversible associations of end-functionalized oligomers. The associations are either through donor-acceptor-type complementary bonding or through two-to-one ligand-metal complexation. For donor-acceptor-type reversible polymers, we study the effects of oligomer length, bond stiffness and concentration on the degree of association and ring-chain equilibrium. For the metallo-supramolecular polymers, the effects of the energy of complexation, cooperativity of the ligand-metal bonds, and the metal concentration on the degree of association and average molecular weight are considered. Criteria for obtaining high molecular weight polymers in a broad range of metal concentrations are suggested based on the simulation and analytical results. [Preview Abstract] |
|
C1.00177: Molecular Dynamics simulations of polymer translocation through a nanoscopic pore Michel G. Gauthier, Gary W. Slater The detection of linear polymers translocating through a nanoscopic pore is an extremely promising idea for the development of new DNA analysis techniques; such systems are already studied experimentally. However, the physics of these macromolecules and the fluid that surround them at the nanoscopic scale is still not well understood. In fact, many theoretical models neglect both excluded volume and hydrodynamic effects. We use Molecular Dynamics simulations with explicit solvent to study the impact of hydrodynamic interactions on the translocation time of a polymer. In particular, we look at the effect of increasing the pore radius when only a small number of fluid particles are located in the pore as the polymer undergoes translocation. We also investigate the relaxation time of the polymer as function of its length and compare it to the translocation time for different initial conditions in order to test the quasi-equilibrium hypothesis of the translocation process. [Preview Abstract] |
|
C1.00178: Universal function for the diffusion coefficient of DNA fragment Jean-Francois Mercier, Gary W. Sltater The separation of DNA fragments by (gel or capillary) electrophoresis has been studied extensively. To characterize the separation achieved by such systems, one needs to understand the impact (and their dependency upon the experimental quantities) of two physical parameters: the electrophoresis mobility $\mu$ and the diffusion coefficient $D$. Three different regimes have been shown to exist for both $\mu$ and $D$: the Ogston regime, the reptation regime and the reptation-with orientation regime (note that separation is only possible for the first two regimes). Both $\mu$ and $D$ are well described by theory for all three regimes. Unfortunatly this results in disjointed scaling regimes and no theory-based general equations can apply to all regimes. Recently, an empirical formula has been proposed that adequately fit the mobility $\mu$ of dsDNA fragments across all three regimes and is compatible with accepted theories. In this work we propose a similar formula for the diffusion coefficent $D$. With those two formulas, one could optimize any separation system quite easily for a wide range of DNA molecular sizes. [Preview Abstract] |
|
C1.00179: Molecular Assembly for Hybrid Electronics Bansal Juhi, Dilip Gersappe The process of forming molecular switches depends critically on the ability to bridge the nanoscale gap between electrodes. The probability that a molecule will bridge this gap depends on a number of factors such as the length of the molecule, its conformational rigidity and the solvent under which the deposition is performed. We use the Rotational Isomeric Scheme to model the rigidity of the molecule and predict conditions under which molecules will bridge the gap between the two electrodes. In our simulations we vary the aspect size of the electrodes, the distance between them and the the size of the molecule. Finally, we use the results of our RIS model to feed into a Monte Carlo simulation that predicts the connectedness of a series of nanowires. The Monte Carlo simulation accounts for the inherenet fluctuations that can occur in the electrode spacing and the aspect ratio of the electrodes. [Preview Abstract] |
|
C1.00180: Short Time Dynamics in Melts Near the Glass Transition Aaron Wilson, John McCoy, Joanne Budzien, Douglas Adolf Molecular dynamics simulations were performed on the diffusion of penetrants in bead-spring chains. When the diffusion coefficient is plotted as a function of packing fraction, (1) results at different temperatures and pressures collapse to a single curve and (2) an apparent glass transition is seen. Short time dynamics are analyzed in terms of the velocity autocorrelation function which is seen to approach the fully caged particle limit. [Preview Abstract] |
|
C1.00181: Molecular Dynamics Simulations of Protein-Polyelectrolyte Multilayer Assembly Venkateswarlu Panchagnula, Junhwan Jeon, Andrey Dobrynin Nanoscale assembly of protein-polyelectrolyte multilayer thin films on solid surfaces has tremendous potential in areas such as biomaterials, drug delivery and fabrication of biosensing devices. The ability to control nanoscale structure and order of these films is essential for surface patterning and templating. We performed molecular dynamics simulations of the protein-polyelectrolyte layer-by-layer assembly on a solid planar surface formed by hexagonally packed particles. Spherical-shaped model protein was constructed by utilizing the charged residues of lysozyme, which was obtained from the Protein Data Bank, to model protein charge distribution. Multilayer build-up was achieved through sequential adsorption of proteins and flexible polyelectrolytes in a layer-by-layer fashion from dilute solutions. The effect of the three dimensional structural rigidity of proteins, charge distribution and hydrophobicity on the film structure was studied. [Preview Abstract] |
|
C1.00182: The Equilibrium Partitioning of SAW Chains into Pores with Heterogeneous Surfaces Jesse Ziebarth, Yongmei Wang The partitioning of a self-avoiding walk polymer chain into a square channel made up by four heterogeneous surfaces has been examined using lattice Monte Carlo simulations in order to better understand the elution pattern observed in liquid chromatography of polymers. The surfaces of the square channel contain 10{\%} attractive sites and 90{\%} non-interacting sites; this composition was chosen to model polar polymers interacting with free silanols which remain on the surface of end-capped silica C$_{18}$ column packings. The attractive interaction, \textit{$\varepsilon $}$_{w}$, between the chains and the attractive surface sites was varied to find the critical condition, the point where the partition coefficient, $K$, varied least with chain length for the system. For a 20 x 20 square channel, the critical condition was found at \textit{$\beta \varepsilon $}$_{w}$ = -1.47, which also coincides with the critical adsorption point of the chains above a single planar surface with the same hetegenouse surface composition. The ``S''-turn elution behavior, where $K$ decreases with increasing polymer chain length up to some chain length, but, then turns and increases with the increasing chain length, was observed before the critical condition point. [Preview Abstract] |
|
C1.00183: A Hybrid Theoretical and Computational Approach to Study Polymer Induced Interaction between Two Parallel Plates Wenhua Jiang, Yongmei Wang, Iwao Teraoka We present a theoretical derivation for the polymer induced force on the two parallel plates based on the osmotic pressures of the bulk and confined solution and the partition coefficient of polymers between the bulk solution and the confined region. The formulation allows us to deduce the polymer induced force for polymers with excluded volume interactions and for a wide range of polymer concentrations by making use of the lattice Monte Carlo simulation results for self-avoiding walk chains. Both the depletion attraction induced by non-adsorptive polymers and the bridging attraction induced by adsorptive polymers are obtained. The approach is significant since it opens up the possibility to study polymer induced force for a variety of other polymeric systems. [Preview Abstract] |
|
C1.00184: The influence of surface chemistry on interfacial moisture and adhesion Emmett O'Brien, Bryan Vogt, Christopher White The effect of surface chemistry and moisture on adhesion was investigated using neutron reflectivity and the shaft-loaded blister test. Four different surface treatments were examined: bare Al$_2$O$_3$, t-butylphosphonic acid, phenyl phosphonic acid, and n-octyltrichlorosilane. Measurements were taken after samples equilibrated in a 100{\%} relative humidity. Previous neutron reflectivity work by Vogt et. al. (submitted to \textit{Macromolecules}) on spincoated films of poly(4-tert-butoxycarbonyl-oxystyrene) has shown that the phenyl and octyl surface treatments reduced the amount of water at the interface, relative to the bare aluminum. The t-butyl treatment did not reduce the interfacial moisture content. These results were correlated to adhesive fracture energy measurement of spin coated PMMA films. The adhesion measurements match the expected trend. The phenyl surface had the largest adhesion strength and a low interfacial moisture content. The t-buty and bare Al$_2$O$_3$ surface had similar moisture contents and adhesion energy. The octyl treated surface had the worst adhesion but the lowest interfacial moisture content. [Preview Abstract] |
|
C1.00185: Ordered structural evolution and relaxation behaviors of a series microphase separated Alexander Jiaokai Jing, Zhihao Shen, Shi Jin, Huabin Wang, Frank W. Harris, Stephen Z. D. Cheng A series of ``hairy-rod'' polyimides, BBPA(n), with multiple alkyl side chains were synthesized. It was found that these polyimides possess a micro-phase separation between the backbones and side chains. This led to the formation of ordered structures in two different length scales, of which both are hexagonal packing: one is attributed to the alkyl side chains on the sub-nanometer scale, and another is for the whole polymer chains on the nanometer scale. The development of the hexagonal structure on the sub-nanometer scale was critically dependent upon the lengths of the alkyl side chains. Three relaxation processes were captured by dynamic mechanical analysis (DM), i.e., segmental motion of the backbones, $\alpha $; the melting of the side chain crystals, $\beta _{1}$, which exits only for the materials with longer side chains (n = 18,16); and the subglass relaxation of side chains, $\beta _{2}$. The peak relaxation temperature of the $\alpha $ process decreases with increasing the length of side chain, while the one of the $\beta _{2}$ process increases. The activation energy of the $\alpha $ relaxation is relatively independent on the length of side chain, whereas, $\beta _{2}$ process shows the increasing of activation energy with increasing the length of side chain. [Preview Abstract] |
|
C1.00186: Soft X-ray Resonant Scattering of Polymers: A Complement to NEXAFS Microscopy Tohru Araki, Shane Harton, Ying Zou, Harald Ade, Gary Mitchell, Jeffrey Stubbs, Jeffrey Kortright SAXS is a method that can provide spatial resolution information of ensemble averages approaching the $\lambda $/2 limit in a backscattering geometry. Using resonant scattering, we can achieve chemical differentiation related to the exquisitely varied NEXAFS of organic materials. It might now offer an important complement to NEXAFS Microscopy for the chemical differentiation of small phases in polymeric materials. To evaluate the utility of resonant scattering, we have acquired data near the C1s, N1s, and O1s absorption edges of a number of model polymer structures with features in the 20-100 nm range, including self-assembled block copolymer thin films and core-shell micro spheres. We have performed the scattering experiments in transmission geometry with polymer samples supported on the silicon nitride membrane to simplify the interpretation and to conduct STXM measurement with the identical samples. The feature size in the phase separated block copolymer thin films from the scattering results is in good agreement with AFM data. Angular scans as a function of photon energy, and photon energy scans at several scattering angles reveal large differences due to the chemical differentiation possible near the absorption edges. High spatial resolution information well below the present resolution limit of STXM is already achievable [Preview Abstract] |
|
C1.00187: Studying Polymer Solutions using Light Scattering Spectroscopy Kiril A. Streletzky, George D.J. Phillies Light scattering spectroscopy provides a non-destructive analytic technique for probing structure and dynamics of polymer solutions. Light scattering spectroscopy in our laboratory combines measurement of the average scattering intensity (static light scattering- SLS) with careful monitoring of the average temporal evolution of intensity fluctuations (dynamic light scattering -DLS) in samples at thermal equilibrium. While SLS yields polymer molecular weight and radius of gyration, DLS yields information on the translational and rotational dynamics of polymers in solution. We also apply optical probe diffusion to measure the transport of dilute mesoscopic probes in polymer solution. If tracer particles are the dominant scatterers in solution, their diffusion provides important information for inferring the physical properties of the polymer. By combining DLS, SLS, and optical probe diffusion, and measuring the low- shear solution viscosity, we studied the neutral, high molecular weight, semiflexible, water-soluble polymer hydroxypropylcellulose (HPC). By combining this wide range of information we obtained important insights into the multimodal dynamics of dilute and concentrated solutions of this multi- application polymer system. [Preview Abstract] |
|
C1.00188: Ballistic Resistance of Polymeric Materials Chad Snyder, Gale Holmes, Kathleen Flynn, Steven Roth, Walter McDonough, Da-Wei Liu Ballistic-resistant body armor has been credited with saving more than 2,500 lives, but new materials are constantly being developed, and there currently exists no method for evaluating armor over time to ensure the continued effectiveness of the protection. We report on progress towards development of a standard test method for reliability of the active polymeric materials that comprise them. [Preview Abstract] |
|
C1.00189: Local mobility of polymer chains with specific interactions Jutta Luettmer-Strathmann The local mobility of polymer chains depends on single-chain properties as well as the local environment of the chain segments. Recent experiments on polymer blends have shown that hydrogen bonding has a large effect on the segmental relaxation of the polymers in the blend; cf. Ref. [1]. In earlier work, we developed a simple lattice model for poly (ethylene oxide) (PEO) in aqueous solutions that accounts for entropic and enthalpic effects of specific interactions. We applied it to the Soret effect of PEO in ethanol/water mixtures and found qualitative agreement between theory and experimental data. In this work, we consider a lattice model for dense polymeric systems with specific interactions and present first results for the effect of hydrogen bonding on the mobility of chain segments. [1] S. H. Zhang, X. Jin, P. C. Painter, and J. Runt, Polymer 45, 3993 (2004). [Preview Abstract] |
|
C1.00190: Shear flow behavior of a dynamically symmetric polymeric bicontinuous microemulsion Ning Zhou, Timothy Lodge, Frank Bates Soft materials with complex internal structure often exhibit fascinating rheological behavior. For example, under flow the poly (ethylethylene) (PEE)/poly(dimethyl siloxane) (PDMS)/PEE-PDMS polymeric bicontinuous microemulsion (B$\mu $E) showed shear-induced macrophase separation.$^{ 1}$ This was tentatively attributed to the extreme dynamical asymmetry of the two homopolymers, i.e., their viscosities differed by three orders of magnitude. To understand the role of the dynamic symmetry of a B$\mu $E when subjected to shear flow, we have developed a new ternary polymer blend system poly(butylene oxide) (PBO)/ poly(ethylenepropylene) (PEP)/PEP-PBO, which is dynamically almost symmetric. We will report on the shear flow behavior of this new B$\mu $E. Reference: [1] Krishnan \textit{et al.} \textit{Phys. Rev. Lett.} 2001, 87, 098301 [Preview Abstract] |
|
C1.00191: Enhanced Mechanical Properties in PVA/SWNT Composite Fibers William Sampson, Joselito Razal, Steve Collins, Ray Baughman, Alan Dalton Composite fibers of polyvinyl alcohol (PVA) and HiPco Single Walled Carbon Nanotubes (SWNT) have been developed at The University of Texas at Dallas that show greatly enhanced mechanical properties, with typical strengths of 1.8GPa and toughness in excess of that of spider silk, making these the toughest known fibers to date. However, the exact interactions leading to the enhanced mechanical properties are not as yet fully understood. We have used a series of Raman and DSC experiments to discover the nature of the strength-enhancing interactions in these composite materials. The results lead to the conclusion that the bulk of the improvements are due to SWNT-nucleated PVA crystallinity, with the SWNTs playing less of a direct role than we originally thought. [Preview Abstract] |
|
C1.00192: Polymer Nanocomposites Made by Solid-State Shear Pulverization: Achievement of Well-Dispersed Nanofiller Sheets, Nanotubes, and Nanoparticles Kosmas G. Kasimatis, Laura M. Dykes, Wesley D. Burghardt, Ramanathan Thillaiyan, L. Catherine Brinson, Rodney Andrews, John M. Torkelson A major stumbling block in the field of nanocomposites concerns the achievement of excellent nanofiller dispersion or exfoliation in various polymers using a scalable, industrially applicable process that mixes polymer and nanofiller directly, without need for solvent or polymerization of monomer. Unfortunately, the production of polymer nanocomposites by twin-screw melt extrusion has met with relatively little success, with positive results limited to a subset of polar polymers such as nylon-6. Here we demonstrate that a novel, continuous process called solid-state shear pulverzation (SSSP) can obtain substantially higher levels of dispersion or exfoliation than melt processing for a range of nanofillers, including clay (silicate) sheets, multiwall carbon nanotubes, and alumina nanaparticles. Characterization has been undertaken by electron microscopy, x-ray scattering, differential scanning calorimetry (crystallization kinetics and physical aging behavior), thermogravimetric analysis, dynamic mechanical analysis, rheometry, and impedance spectroscopy. [Preview Abstract] |
|
C1.00193: Molecular-Level Insulation: An Approach to Controlling Interfacial Charge Transfer Jong Seung Park, Mohan Srinivasarao, Saif A. Haque, James R. Durrant In this presentation, we report a novel approach to the attachment of molecular dyes to nanocrystalline TiO$_{2 }$electrodes. New azo dye rotaxane was synthesized by encapsulating an organic dye inside cyclodextrin, and it is shown to exhibit strong binding to the metal oxide electrode. The encapsulation results in a well-defined spatial separation of the organic dye from the electrode surface, allowing the photogeneration of a long-lived charge separated state. Herein we demonstrate that such cyclodextrin threaded sensitizer dye rotaxanes can be used to functionalise nanocrystalline TiO$_{2}$, resulting in a long-lived photogenerated charge separation. [Preview Abstract] |
|
C1.00194: Diblock copolymer-incorporation onto Polystyrene Colloidal Beads Jong-Man Lee, Kwanwoo Shin Modification of colloidal particles is particularly interesting, since they can be used for colloidal crystal for optical devices, and biological modeling system due to their well-defined periodic structure and biological similarity from spherical nanostructures, like lipoprotein and phases. We provide various applicable functions onto colloidal beads by incorporation of various functionalized diblock copolymers. Interpenetration of the proximal diblock chains into the beads and instant quenching were performed, as developed by Ou-Yang et al.(J. Poly. Sci, 58, 265, 1995), and tightly bounded diblock-bead complexes were accomplished. In this study, various copolymers, PS-based ferrocenylsilane and polyelectrolytes were used, and their modified structures were characterized by scattering and microscopic techniques. These beads were further used for a template for adsorption of biological molecules, where demonstrates various functional activities [Preview Abstract] |
|
C1.00195: Femtosecond Two-Photon Photopolymerization Nayer Eradat, Ian Mitchell Mitchell, Chelsea Bond, Yeon Rim We will report our progress on photo-polymerization of a new material system with electrical conductivity for photonics applications. Two-Photon polymerization is the process of making polymers from monomers using a high-power and ultra-short pulsed laser with the aid of photo-initiators. This laser light is tightly focused through a high numerical aperture microscopy objective to increase resolution to a scale of tens of nanometers. In the summer of 2004, we started a research program at Middle Tennessee State University. Our goal is to fabricate polymer microstructures with applications in photonics. During the last summer, we prepared an experimental setup for two-photon polymerization, and tested it with common material systems in the field. Our experimental setup uses tightly focused femtosecond laser light for a localized initiation of two-photon polymerization, as well as a system of high resolution travel stages for sample movement. [Preview Abstract] |
|
C1.00196: Carbon Nanotube Reinforced Nanocomposite with Controlled CNT Dispersion Lingyu Li, Wenwen Cai, Stephen Kodjie, Kishore Tenneti, Christopher Li Carbon nanotubes (CNTs) are considered an ideal reinforcing fillers in polymer nanocomposites because of their high aspect ratio, nanosize diameter, very low density and excellent physical properties (such as extremely high mechanical strength, high electrical and thermal conductivity). However, in order to achieve homogeneous dispersion of CNTs without damaging their extraordinary properties, non-covalent functionalization is an essential step. Our study of functionalization of CNTs via controlled polymer crystallization method has resulted in the formation of ``nano hybrid shish-kebab'' (NHSK), which is CNT periodically decorated with polymer lamellar crystals. Preliminary results show that the periodicity varies from 20-70nm. By tuning the experimental parameters such as concentration of polymer and crystallization temperature, hybrid polymer spherulite with CNT inside was achieved. This can be considered as CNT reinforced composite with ideal CNT dispersion. Both Nylon 6, 6 and PE were used as matrix materials. Structure and properties of this novel composite were studied using varieties of characterization techniques. [Preview Abstract] |
|
C1.00197: Encapsulation of ZnS:Mn2+ Nanoclusters with PS-PVP Diblock Copolymer Sangcheol Kim, Fr\'ed\'eric S. Diana, Pierre M. Petroff, Edward J. Kramer, Takeshi Otsu, Tomohide Murase ZnS colloidal quantum dots (QDs) were synthesized in a mixture of highly coordinating solvents and doped with Mn$^{2+}$. In this study, we report on the possibility of assembling the block copolymer around the QDs and seek to optimize the electronic properties of the QDs. When the QDs in toluene were mixed with PS-PVP (141 kg/mol, $f_{PVP}$ = 0.059), the inverse micelle formation was induced over 2.5 mg/mL of PS-PVP. The solution was deposited onto a silicon substrate and the distribution of the micelles was observed by SFM. For Mn-doped ZnS nanoparticles, emission in the blue is observed at 404 nm due to sulfur vacancies on the surface and the energy transfer from the band-to-band excitation in ZnS to the Mn$^{2+}$ ion is also facilitated due to the quantum confinement effect, resulting in orange emission (582 nm) associated with the $^{4}$T$_{1}$ to $^{6}$A$_{1}$ [Mn$^{2+}$]-based transition. By adding PS-PVP to a ZnS:Mn/toluene solution, the intensity ratio of blue to orange emission decreases slowly and drops around 0.1 mg/mL. [Preview Abstract] |
|
C1.00198: Optical Properties of Rhodamine 6G Laser Dye and Ag-Nanoparticle Aggregates M.A. Noginov, M. Vondrova, S.M. Williams, M. Bahoura, V.I. Gavrilenko, S.M. Black, A. Sykes, V.P. Drachev, V.M. Shalaev Optical absorption and luminescence spectra of Rhodamine 6G (Rh6G) laser dye of different concentration with a solution of aggregated silver nanoparticles are studied. New emission band located near 610 nm is found at very high concentration of Rh6G and/or in a solution of Rh6G and Ag nanoparticles. Electron energy structure and optical functions of single Rh6G molecules, molecular complexes, and Rh6G molecules adsorbed on Ag(111) surface are studied by generalized gradient approximation method within density functional theory using \textit{ab initio} pseudopotentials. Equilibrium geometries of the systems studied are obtained from both molecular dynamics simulations and X-ray diffraction measurements. Electronic structure of J-type molecular complexes (when two molecules aligned along their dipole moment axes) substantially differs from that of H-type aggregates (with parallel and anti-parallel molecular dipole moments). It is demonstrated that new luminescence line is associated with J-type molecular complexes. Observed modifications of optical properties of Rh6G and Rh6G+Ag complexes are explained in terms of both the changes of electronic structure of the systems and due to the electromagnetic interactions of dipole-dipole and dipole-surface types. [Preview Abstract] |
|
C1.00199: Characterization of Inorganic/Organic Interfaces by Ab Initio Methods Rajeev Pandey, Nicolas Bruque, Roger Lake Self-assembled functional devices using hybrid inorganic/organic interfaces has been an area of recent interest in interdisciplinary science and engineering research. Chemical and biological systems using bottom-up approach hold promise to continue the scaling of nano devices. Our current research is focused on model studies involving chemical/biological self-assembly based on peptides/carbon nanotubes, DNA/carbon nanotubes, quantum dots/carbon nanotubes and assembly based on these interfaces. Some of our recent results using ab initio methods will be presented. [Preview Abstract] |
|
C1.00200: Self-Healing of a Polyurethane-based Polymer Composite Melissa Considine, Erin Dreyer, S. Paul Freese, Paul Ledwith, Joanna Meador An attempt has been made to extend the development of a self-healing polymer system to polyurethane polymers. Self-healing materials can improve reliability and prevent catastrophic failure of critical components that are inaccessible for routine maintenance and inspection. Previous work by others has shown that monomer-filled microcapsules embedded in an epoxy matrix containing dispersed solid catalyst can autonomously heal stress induced cracking. Synthesis of \textit{in-situ} dicyclopentadiene (DCPD) encapsulated in poly(urea-formaldehyde) is embedded in a two-part (rigid) polyurethane matrix containing dispersed Grubb's catalyst. The modified composite is subsequently characterized. Characterization and testing of the as-fabricated polymer composite samples includes optical microscopy, scanning electron microscopy, FTIR spectroscopy, tensile testing and Izod impact testing. Following microcracking, induced toughening of the polymer matrix is anticipated as a result of microcapsule rupture that will release monomer to polymerize upon reaction with the embedded catalyst. [Preview Abstract] |
|
C1.00201: The Nano-mechanical Properties of Polystyrene Thin Films Embedded with Surface Grafted Multiwalled Carbon Nanotubes Arnold Yang, Chih-Chun Hsiao, Tian Shyng Lin, Ling Yu Cheng, Chen-Chi Mar Thin polystyrene (PS) films embedded with well-dispersed PS- grafted multi-walled carbon nanotubes (MWNTs) were successfully prepared via solution casting and the nano-mechanical behavior of the thin films was studied. Percolated network of entangled nanotubes was observed in the nanocomposite films, which resulted strikingly different mechanical properties as compared to the pristine PS films. The nanocomposite films developed crazes upon stretching and the crazes were very short and narrow with a width no more than 2$\mu $m. As the applied strain increased, the crazes increased in number but the widening of crazes was restricted, leading to a very tough mechanical behavior that showed no macroscopic voiding even at large strains beyond 20{\%}. Significantly, no nanotubes were observed inside crazes from SEM or AFM of the plasma etched samples. Instead, they seemed to be excluded from crazes and were found accumulated at the craze boundaries. The observations indicated that craze initiations were largely confined to the defect regions where nanotubes were loose or sparse. [Preview Abstract] |
|
C1.00202: Structural Effects on the Interfacial Strength of Silane Coupling Agent Layers Shigeo Nakamura, Elizabeth Pavlovic, Edward Kramer The structural effects of silane coupling agent layers on crack growth along a model interface between the natural oxide of [100]Si and an epoxy-hardener mixture consisting of the diglycidyl ether of bisphenol A(DGEBA) and triethylenetetramine (TETA) have been investigated. Previous experiments where (3-glycidoxypropyl)trimethoxysilane (GPS) was the coupling agent revealed network structures that were only loosely cross-linked in the center of the layers. The penetration of epoxy into this swellable layer produced a relatively high fracture energy (Gc) and a threshold energy release rate (Gth), immersed in water, that decreased only modestly between 20 and 80 degrees C. Here we compared these results with Gc and Gth from 10nm thick layers of (3-aminopropyl)trimethoxysilane (APS) and APS combined with [3-(Phenylamino)propyl]trimethoxysilane (PAPS) obtained by spin casting the silane coupling agent solutions composed of 90wt{\%} ethanol and 10wt{\%} water on the silicon oxide interface. These layers, which are closer to those used in typical industrial applications, were characterized by FT-IR, scanning force microscopy and X-ray reflection experiments. [Preview Abstract] |
|
C1.00203: Grafting and Loop Formation of Telechelic Polymers at Interfaces Monitored by Fluorescence Labeling Zhenyu Huang, Haining Ji, Jimmy Mays, Mark Dadmun The results from Monte Carlo simulation and mechanical tests have demonstrated that the formation of loops at polymer-polymer interfaces by multiblock copolymers provides an efficient method to improve the strength of biphasic interfaces. In this study, a simple system was generated to model loop formation at interfaces. Telechelic polystyrene terminated with carboxy endgroups was grafted from melt onto a silicon wafer modified with epoxy-terminated silane layers. The free carboxyl end groups in PS layers were labeled with 1-pyrenyldiazomethane (PDAM) and the fluorescence spectra were recorded. The grafting and loop formation of PS on the substrate can thus be monitored. Combined with ellipsometry, contact angle measurements and AFM, the effect of the chain length and concentration of PS on the kinetics of grafting and loop formation were studied. The obtained results provide a method to control the loop formation at interfaces therefore provide an opportunity to further understand the role of the loops in the interfacial properties in multi-component polymer systems. [Preview Abstract] |
|
C1.00204: Nonlinear optical Spectroscopy of Polyimide Surface for homeotropic liquid crystal alignment Masahito Oh-e, Hiroshi Yokoyama, Doseok Kim Surface--specific sum--frequency vibrational spectroscopy and second--harmonic generation were used to study the structures of polyimide (PI) surfaces for homeotropic liquid crystal (LC) alignment and the molecular orientation of LC adsobates on these surfaces. The imide ring was perpendicular to the surface with one of CO bonds protruding out of the surface and the other pointing into the bulk rather than flat on the surface. The ester CO bond in the side chain was sticking out of the surface with a tilt angle of about 40--55$^{\circ}$ from the surface normal, indicating that the rigid side chain core was, more or less, along the surface normal. The part of alkyl chain on the top of the side chain followed the orientation of the side chain core and protruded out of the surface with some gauche defects. The cyano biphenyl LC molecules were adsorbed on the PI preferentially with the terminal cyano group facing the PI surface. [Preview Abstract] |
|
C1.00205: An explicit 3D chain and node mesoscale network model for silica-filled polydimethylsiloxane David Hanson We present a numerical mesoscale model that describes the stress/strain properties of a mesoscale volume element of silica-filled polydimethylsiloxane (PDMS) under tensile or compressive strain. An explicit 3D network, composed of randomly dispersed nodes (filler particles), multiply connected by a large number of chains (PDMS polymers), is strained by an affine transformation. The resulting macroscopic stress is computed by summing the chain tensile forces over free surfaces of the volume element. The non-entropic forces used in the model are computed from atomistic simulations of individual chains for two cases: chains undergoing extension from a natured initial state, or a chain interacting with a hydroxylated silica surface. The forces are computed from changes in the internal energy as a function of the end position of the chain. The distribution of chain lengths of the interparticle connecting segments and free ends are assumed to be Gaussian. The model is validated by comparison of the predicted stress/strain behavior to experimental data. [Preview Abstract] |
|
C1.00206: Stacking of conjugated oligomers and polymers in solution from first-principles Damian Scherlis, Jean-Luc Fattebert, Francois Gygi, Nicola Marzari The elucidation of the intermolecular interactions between conjugated polymers and oligomers in the condensed phase is essential to understand and design organic semiconductors and electrically active polymers. The stacking of charged layers and the spatial arrangement of the chains are determining factors in charge transfer processes and in the electronic and optical properties of these materials. In this work we address these issues from first principles employing different electronic structure techniques in vacuum and in solution. In particular, we use a novel implementation of the Car-Parrinello method at the DFT level, which couples self-consistently the Kohn-Sham equations with the effect of a dielectric continuum. This methodology is contrasted with other well established, quantum chemistry solvation approaches, in the investigation of charged thiophene oligomers and polymers (and other related compounds) in different solvent fields. We assess the role of the dielectric in the stacking of the charged layers, and show how the balance between electrostatics and covalent forces is tilted by the environment, eventually leading to stable or unstable total interactions. [Preview Abstract] |
|
C1.00207: A reassessment of entropic factors influencing molecular weight dependence of surface tension Venkat Minnikanti, Lynden Archer In our previous work we observe that by considering density gradients at a free surface of a polymer, the entropic attraction of an end is significantly increased. Surface tension dependence on molecular weight of a polymer in the past has been shown to predominantly arise due to the molecular weight dependence of the bulk density of polymer and less on entropic reasons of placing a polymer near a surface. In lieu of our previous observations we will reassess the contribution of entropic attraction of the end to the molecular weight dependence of surface tension. Using lattice simulations we will show that entropic factors play a greater role than previously described. Analysis of previous experimental surface tension data within this premise will also be made. [Preview Abstract] |
|
C1.00208: The effect of temperature heterogeneities on the dynamics of reactive ternary systems Christopher Pooley, Anna Balazs We investigate the the role of one of the most important processing variables, namely temperature, on the structural evolution and phase behavior of a complex fluid. Using a lattice Boltzmann algorithm we numerically study the behavior of a partially miscible A/B/C ternary mixture. Within the simulations we explicitly account for fluid flow, phase separation and heat transfer. In particular, we study a system in which the A and B components can reversibly react to produce C. Such reactions involve the liberation and consumption of heat, which alters the local thermodynamic equilibrium near to the interfaces between components. Since the reaction rates themselves are temperature dependent there exists a coupling between compositional and thermal variation within the system, which leads to interesting dynamical behavior. In addition, we investigate how the morphology depends upon laterally applied temperature gradients. With these simulations we take the first steps in capturing the complex compositional and thermal heterogeneities inherently present in chemical reaction vessels. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700