Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H11: SPS Undergraduate Research III |
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Sponsoring Units: SPS Chair: Thomas Olsen, American Institute of Physics Room: D222 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H11.00001: Nanodynamics of Ferroelectric Ultrathin Films Ryan Herchig, Qingteng Zhang, Inna Ponomareva An active area of research in nanoscale science is the study of ferroelectric ultrathin films. We will report a first-principles-based study of the nanodynamics in ferroelectric Pb(Zr$_{0.4}$Ti$_{0.6}$)O$_3$ films with thickness 20-192 nm. In our computational experiment we first anneal such films under realistic conditions of partial screening of the surface charge to obtain the ground state nanodomain pattern. After that the films are subjected to $ac$ electric fields with frequencies varying from 0.1 THz to 4.0 THz and close to nanodomain resonance frequency. The domain evolution is then studied as a function of time, electric field frequency, and film thickness in order to quantitatively characterize the laws and parameters associated with it. This allows us to reveal for the first time ever intrinsic high-frequency dynamics of ferroelectric nanostripe domains. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H11.00002: Thermal Conductivity of Random Multilayer Thin Films Anthony Frachioni, B.E. White Jr. Thermoelectric based energy scavenging has tremendous potential for the recovery of waste heat and temperature regulation. Manufactured thermoelectric devices today are limited in efficiency, and therefore widespread use, by high lattice thermal conductivity. In an effort to minimize lattice conductivity with respect to electrical conductivity, opportunities for utilizing the Anderson localization of phonons have been explored. In particular, the thermal conductivity of a model random multilayer thin film with Lennard-Jones bonding has been determined using classical reverse non-equilibrium molecular dynamics as a function of mass induced disorder. Results indicate that the inclusion of random planes in which the atomic mass has been increased by a factor of ten can produce reductions in lattice thermal conductivity by over a factor of one hundred. The dependence of thermal conductivity on the magnitude and nature of this disorder has been measured. Finite size effects have been quantified and a length scale has been determined on which they can be neglected. These results indicate that the pursuit of nanostructured thermoelectric materials in the form of random multilayers may provide a path to efficient and sustainable thermoelectric materials. [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H11.00003: Controlling the microstructure of binary carbide films with elemental substitutions K. Feller, M. Haider, A. Hodges, R. Spreng, E. Posbergh, H. Woodward, S.E. Lofland, J.D. Hettinger, M. Heon, Y. Gogotsi We report on experiments to control the microstructure of textured binary carbide thin films deposited by reactive magnetron sputter deposition. Controlling the microstructure in these materials is important as the microstructure of these films provides a template for the resulting carbide-derived carbon (CDC) film and impacts their performance. Specifically, a combinatorial approach is used to add chromium to TiC films creating a compositional gradient as a function of position. We present a measurement of surface roughness as a function of material composition. The resulting materials, (Ti$_{1-x}$Cr$_{x})$C films, are significantly smoother than their pure TiC counterparts and the resulting CDC's have correlated defects which will improve the performance of the CDC in supercapacitor applications. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H11.00004: Polypeptide Chirality Influences Multilayer Thin Film Growth and Structure Zephra Bell, Dhan Khadka, Donald Haynie Polypeptide multilayer thin films are being developed for a variety of applications.These include coatings for implant devices and systems for drug delivery in thebiomedical sciences, and optical coatings. Subsequent polymer adsorption steps involve polymers of opposite polarity. Here, the polymers were polypeptides. This project compared the consequences of changing polypeptide chirality on film growth and structure. The peptides were poly(L-glutamic acid), its right-handed counterpart, poly(D-glutamic acid), and poly(lysine-tyrosine). The first two are negatively charged at neutral pH, the third one is positively charged. Poly(lysine-tyrosine)/poly(L-glutamic acid) films and poly(lysine-tyrosine)/poly(D-glutamic acid) films werefabricated on 1 mm-thick quartz plates. In one experiment, films were grown to 34layers. The UV absorption spectrum was taken after each layer deposited to determinethe rate of polymer self-assembly. Separately, UV or visible wavelength spectra wereobtained for films stained with a dye cooled/heated in the range 4-65 \r{ }C. In anotherexperiment, a mixture of poly-L-glutamic acid and poly-D-glutamic acid was used as thepolyanion for film buildup. The data show that poly(lysine-tyrosine)/poly(L-glutamicacid) films built up at a higher rate than the corresponding right-handed films. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H11.00005: Thermoelectric Properties and Microstructure of Ca$_{3}$Co$_{4}$O$_{9}$ thin films on SrTiO$_{3}$ and Al$_{2}$O$_{3}$ Substrates T. Paulauskas, Q. Qiao, A. Gulec, R.F. Klie, M. Ozdemir, C. Boyraz, D. Mazumdar, A. Gupta Ca$_{3}$Co$_{4}$O$_{9}$ (CCO), a misfit layered structure exhibiting large Seebeck coefficient at temperatures up to 1000K has attracted increasing attention as a novel high-temperature thermoelectric material. In this work, we investigate CCO thin films grown on SrTiO$_{3}$ (001) and Al$_{2}$O$_{3}$ (0001) using pulsed laser deposition. Quality of the thin films was examined using high-resolution transmission electron microscopy and thermoelectric transport measurements. HRTEM images show incommensurate stacks of CdI$_{2}$-type CoO$_{2}$ layer alternating with rock-salt-type Ca$_{2}$CoO$_{3}$ layer along the c-axis. Perovskite buffer layer about 10nm thick was found present between CCO and SrTiO$_{3}$ accompanied by higher density of stacking faults. The CCO grown on Al$_{2}$O$_{3}$ exhibited numerous misoriented grains and presence of Ca$_{x}$CoO$_{2}$ phase. Seebeck coefficient measurements yield an improvement for both samples compared to the bulk value. We suggest that thermoelectric properties of CCO increase due to additional phonon scattering at the stacking faults as well as at the film surfaces/interfaces. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H11.00006: Antireflective Coatings using Layer-by-Layer Self Assembly of Silica and Titania Nanoparticles Raisa Velasco Castedo, Anitesh Anand Lal, Dan Mazilu It is known that glass substrates (borosilicate glass) reflect about 4{\%} of light at each air/glass interface and thus, they transmit only 92{\%} of light. For some devices like camera lenses, it is important to maximize the amount of transmitted light. Previous research has demonstrated that it is possible to do so by adding antireflective coatings to the substrates. Our research aimed to deposit thin films on glass substrates that would minimize the reflectance of light and thus, maximize its transmittance. The thin films consisted of multiple alternating layers of silica and titania nanoparticles following the theory behind double-quarter periodic systems and were deposited on the substrates via the ISAM (ionically self-assembled monolayers) technique. Several experiments were conducted in order to investigate the factors that affected the quality of the coatings and some of the significant factors observed were the pH and the molarity of the silica, titania and PDDA solutions. A number of factor-level combinations yielded transmittances in excess of 96{\%}, well above the value for uncoated substrates. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H11.00007: Compositional dependence of the narrow band emission from zinc oxide nanowires Bradley Golder, Eric Driscoll, Marian Tzolov Zinc oxide is a versatile platform thanks to the unique combination of optical, semiconducting, and piezoelectric properties of ZnO. The properties can be further diversified by creating microstructures and by varying the Zn/O ratio in the crystallites. We are illustrating this concept for the case of narrow band emission for ZnO nanostrutures grown through chemical vapor transport. The samples were characterized by photoluminescence spectroscopy (pulsed and continuous wave), scanning electron microscopy, and energy dispersive x-ray spectroscopy. Narrow band emission has been observed in the pulsed excitation mode. The narrowing is intensity dependent suggesting a mechanism of stimulated emission. The emission properties were correlated with the degree of oxidation of the ZnO nanocrystallites and with the presence of optically active defects. The influence of different oxidizing agents on the emission properties of the ZnO nanocrystals will be shown. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H11.00008: Dependence of the band gap of highly confined CdSe and PbSe nanocrystals on temperature Aaron Zaubi, J. Bylsma, P. Dey, J. Rejman, S. Witanachchi, P. Mukherjee, D. Karaiskaj, M. Beard We have recorded fluorescence spectra from PbSe and CdSe quantum dots in hexane/toluene respectively between 5K and 300K in order to investigate the temperature dependence of the electronic band gap of these highly confined nanostructures. The band gap for CdSe follows the known blue shift with decreasing temperature. We have measured the temperature dependence of the band gap of PbSe quantum dots for two different diameters below 4 nm and indeed observe a red shift of the band gap with decreasing temperature, which is stronger for the smaller size quantum dots. Such behavior would contradict the expected blue shift of the band gap with decreasing temperature. The origin of this peculiar behavior is not well understood and we are pursuing further theoretical and experimental studies in order to elucidate the mechanism behind it. Using the method of single-nanostructure laser spectroscopy will allow us to observe individual nanostructures while simultaneously removing ensemble averaging effects due to quantum interactions between multiple structures. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H11.00009: Synthesis and characterization of ZnO nanocrystals co-doped with Ce$^{3+}$ and Tb$^{3+}$ Kelly McCutcheon, Christie Larochelle Rare earth doped zinc oxide nanocrystals produce visible emissions under ultraviolet excitation. Using a sol-gel process, we synthesized a series of ZnO nanocrystals doped with Tb$^{3+}$ and Ce$^{3+}$ in silica glass, keeping the ZnO/SiO$_2$ ratio constant at 10/90 and doping with 1\% rare earth by weight, with varying relative concentrations of Tb$^{3+}$ and Ce$^{3+}$. The nanocrystals were characterized using photoexitation and emission spectroscopy, time-resolved photoluminescence, UV/VIS spectroscopy, and transmission electron microscopy. We determined that co-doping with cerium enhanced the visible terbium emissions to a point, with the most effective enhancement occurring at mid-range Ce$^{3+}$ concentrations. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H11.00010: Growth and Morphology of High Mobility Organic Semiconductors Cortney Bougher, Katelyn Goetz, Zhong Li, John Anthony, Oana Jurchescu, Brad Conrad We utilize atomic force microscopy (AFM) to image the growth and morphology of chemically modified, solution-deposited anthradithiophene transistors. We discuss the effects of backbone modifications on crystal structure, film properties, and electrical device performance. These devices display a mobility of 0.001 cm$^2$/Vs to 1 cm$^2$/Vs. Crystal orientation and film structures, such as film thickness, grain size, and growth modes will be discussed. In addition, AFM images are related to diffraction data and conduction channel crystallographic information is extracted. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H11.00011: Effect of the polymer concentration on the ON/OFF states of a TN-LCD: polyvinyl alcohol vs. soy lecithin Romeo de Coss Martinez, Jose Luis Gonzalez Murguia In this work we study the response of a Twisted Nematic Liquid Crystal Display (TN-LCD) by varying both the concentration and the polymer used for the microgroove. We compare the performance of two polymers: polyvinyl alcohol and soy lecithin. In particular, the light transmission for the ON/OFF states is evaluated. The polyvinyl alcohol is a polymer widely used in LCDs while lecithin soy is a natural polymer. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H11.00012: Tunable Schottky diodes fabricated from electrospun crossed SnO$_{2}$/PEDOT-PSSA nanoribbons Katherine Carrasquillo, Nicholas Pinto Hardware in most solid state devices contains at least one interface between a $p$-type and an $n$-type semiconductor. Such hetero-junctions are typically fabricated from all inorganic Si based materials. In the past two decades however, devices fabricated from organic-inorganic semiconductors that are not Si based, or from all organic semiconductors have been the focus of much research. Semiconducting $n$-doped metal oxides are also attractive for use in devices and of particular interest is tin oxide (SnO$_{2})$ as it is stable in air and is optically transparent with a band gap of $\sim $3.4 eV. The $p$-doped conducting polymer PEDOT-PSSA is also stable in air and is widely used in flexible devices. We shall report on the electrospinning technique to fabricate in air Schottky diodes, by simply crossing $n$-doped SnO$_{2}$ and $p$-doped PEDOT-PSSA nanoribbons. The device parameters could be tuned by a back gate bias and by shining UV light. The diode parameters were calculated using the standard thermionic emission model of a Schottky and was tested as a half wave rectifier. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H11.00013: UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics Emilee Sena, Justin Peel, Shreya Nathan, Devin Wesenberg, Marianne Wallis, Thorsteinn Adalsteinsson, Brian McNelis, Richard Barber Organic photovoltaic cells are prepared using an active layer containing a functionalized C60 molecule, [6-6]-phenyl C61 butyric acid octadecyl ester (PCBOD); and a conjugated polymer, poly(3-hexylthiophene) (P3HT). PCBOD functions as an electron acceptor in conjunction with P3HT, the electron donor. Both current-voltage (IV) transport data of solar cells and spectroscopic absorption data of the corresponding active layer are collected at regular time intervals for periods up to several days. IV data show changes in power conversion efficiency which are strongly dependent on device preparation (stoichiometry, annealing, etc.). Ultraviolet and visible light absorption exhibits similar time dependence. Recent results show that annealing the active layer up to 200\r{ }C substantially improves device performance. Further spectroscopic studies, such as Carbon-13 NMR spectroscopy, are ongoing. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H11.00014: Modeling of Quantum Cascade lasers with different waveguide profiles Charles Zhang, Richard Cendejas, Claire Gmachl Quantum Cascade (QC) laser-based sensor systems help us monitor the environment through the detection of trace chemicals that have optical spectra in the mid-infrared. For the laser to become more efficient and usable, the thermal management and the optical and electrical properties of the laser waveguides need to be more closely examined. The performances of QC lasers with different waveguide profiles have so far not been systematically compared and the device optimization for the three design components has not yet been coupled together. Here, we use a finite element solver to calculate the active region peak core temperature, the optical confinement factor and waveguide loss, and the local current density, and compare these for QC lasers with dry- and wet-chemical etch profiles, i.e. with vertical or sloped sidewalls, respectively. Initial results show a preference for wet-etched profiles under thermal conductivity considerations. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H11.00015: Photoluminescence studies of WO$_{3}$ and WO$_{3-x }$ single crystals J. Easley, P. Dey, D. Karaiskaj, S. Deb, T. Ciszek, D. Dessau WO$_{3}$ is an important material to study, not only due to its interesting electronic properties, but also because it has other applications in both electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades [1]. We have studied two WO$_{3}$ single crystals, a transparent and doped WO$_{3-x}$, in an attempt to understand this effect. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not appear to be correlated with temperature. However, polarization studies reveal at least two polarization dependent components of the center. \\[4pt] [1] Satyen K. Deb, Solar energy materials and solar cells \textbf{92}, 245 (2008), and the references therein [Preview Abstract] |
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