Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D44: Functional Materials & Devices |
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Sponsoring Units: FIAP Room: Mile High Ballroom 4C |
Monday, March 3, 2014 2:30PM - 2:42PM |
D44.00001: Adsorption of 2,4,6-trinitrotoluene on the ZnO (2$\bar{1}\bar{1}$0) surface: a density functional theory study of the detection mechanism of ZnO nanowire chemiresistors Sufian Alnemrat, Gary Brett, Joseph Hooper We report first-principles calculations of the adsorption of 2,4,6-trinitrotoluene (TNT), a prototypical nitroaromatic explosive, on the ZnO (2$\bar{1}\bar{1}$0) surface. This surface is common among ZnO chemiresistors being considered for trace explosive detection. Recent work has achieved 60 ppb detection of TNT using a ZnO nanowire array, but the physical mechanism of sensing is unclear. Our results indicate that TNT strongly chemisorbs via interactions between the oxygen on the nitro groups and surface zinc, creating surface states within the gap. We present a simple theoretical estimate showing the strong effect of these surface states on the depletion layer of ZnO nanowires. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D44.00002: Organic-inorganic hybrid nanocomposite materials for radiation detection Sunil Sahi, Wei Chen Scintillator is the material that emits light when excited with high energy radiation. Inorganic single crystals and organic (plastic and liquid) scintillator are the most widely used scintillator. Inorganic crystals have higher efficiency and high stopping power but single crystal are difficult to grow and are very expensive. Also, some inorganic scintillators like NaI-Tl are not environmental friendly. On the other hand organic scintillators have poor stopping power because of low Z-value. This limits the application of organic scintillator. Here we have proposed a nanocomposite scintillator by embedding the inorganic nanoparticles into organic polymer. Nanoparticles are synthesized and characterized using XRD and TEM. As synthesized nanoparticles are then embedded in to the polymer matrix to make nanocomposite scintillator and their optical properties have been studied. The nanocomposite scintillators have shown improved luminescence properties as compared to the plastic scintillator. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D44.00003: Structural and spectroscopic properties of rare earth doped crystal-in-glass waveguides as influenced by the initial glass composition Brian Knorr, Adam Stone, Himanshu Jain, Volkmar Dierolf Laser induced crystallization of glasses is a highly spatially selective process which has the potential to produced compact, integrated optics within a glass matrix. Specifically, our interest is in using this technique to create a laser. In order to achieve this goal, preliminary research was performed on single crystal lines ``written'' in Er$_{0.002}$La$_{0.998}$BGeO$_{5}$ glass using a femtosecond pulsed laser. This study revealed promising results including incorporation of the erbium into the crystal and the ability to waveguide with low losses, but also illuminated surprising features concerning the distribution of rare earth (RE) ions within the crystal. To further investigate this phenomenon and its potential consequences for our intended application, additional crystalline waveguides were written inside of a series of glasses with compositions of the form RE$_{x}$La$_{1-x}$BGeO$_{5}$, where RE=Pr, Nd, and Er and x=0.002, 0.010, 0.040, 0.100. and 0.200. These structures were analyzed using micro-Raman and luminescence spectroscopy as well as energy-dispersive x-ray spectroscopy. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D44.00004: Towards Multiple-Bit-Per-Cell Operation In a Single Active Layer-Phase Change Memory Cell Ibrahim Cinar, Vedat Karakas, Onur Dincer, Ozgur Burak Aslan, Aisha Gokce, Barry Stipe, Jordan A. Katine, Gulen Aktas, Ozhan Ozatay High contrast between 0 and 1 logic states in addition to other superior properties of phase change memory (PCM) brought out the possible application of multiple logic levels in a single bit in an effort to boost data storage density. The potential stabilization of resistance levels in between the 0 polycrystalline and 1 amorphous states enables storage of several data in a single device cell (such as 00, 01,10,11 levels). Here we report our investigation of the role of contact geometry and fabrication induced modification of phase change kinetics in stabilizing mixed phase states in an effort to obtain such multiple-bit per cell operation within a single layer PCM material system (Ge$_{\mathrm{2}}$Sb$_{\mathrm{2}}$Te$_{\mathrm{5}})$. The nature of switching dynamics appears highly sensitive to exact programming current distribution and defect density such that a nanoscale square contact with effective current localization at the sharp corners facilitates the formation of stable intermediate phases as compared to a circular one. Resistance maps show that the top contact geometry and engineering of defects can be used as an effective handle to tune the resistance states to optimize memory cells for ultra-high density storage. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D44.00005: Hydrogen Dynamics and Metallic Phase Stabilization in VO$_{2}$ Keith H. Warnick, Bin Wang, Soktrates T. Pantelides Hydrogen doping has been demonstrated to lower the VO$_{2}$ semiconductor-to-metal phase transition below room temperature. We report the results of DFT calculations that show that metallic phase stabilization is due to the lattice distortion caused by interstitial hydrogen attached to oxygen atoms. We show that doping is energetically favored and that there is a fast diffusion in the monoclinic [100] direction that can facilitate atomic hydrogen uptake through surfaces that expose these channels. However, the dissociation of molecular hydrogen on a monoclinic (100) surface has a 1.6 eV activation barrier that impedes hydrogen association or dissociation at the surface without significantly elevated temperatures. These results emphasize the role of lattice distortion in the VO$_{2}$ phase transition and suggest methods to improve the use of hydrogen doping to control the properties of VO$_{2}$. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D44.00006: Quantum Hooke's Law to Classify Pulse Laser Induced Ultrafast Melting Hao Hu, Hepeng Ding, Feng Liu We investigate the ultrafast crystal-to-amorphous phase transition induced by femtosecond pulse laser excitation by exploiting the property of quantum electronic stress (QES) induced by the electron-hole plasma, which follows quantum Hooke's law. We demonstrates that two types of crystal-to-amorphous transitions occur in two distinct material classes: the faster nonthermal process, having a time scale shorter than one picosecond (ps), must occur in materials like ice having an anomalous phase diagram characterized with dT$_{\mathrm{m}}$/dP \textless 0, where T$_{\mathrm{m}}$ is the melting temperature and P is pressure; while the slower thermal process, having a time scale of several ps, occurs preferably in other materials. The nonthermal process is driven by the QES acting like a negative internal pressure, which is generated predominantly by the holes in the electron-hole plasma that increases linearly with hole density. These findings significantly advance our fundamental understanding of physics underlying the ultrafast crystal-to-amorphous phase transitions, enabling quantitative a priori prediction. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D44.00007: \textit{Ab initio} study on ferroelectric instability induced by relativistic effects in PbTe Jinwoong Kim, Seung-Hoon Jhi A recent study [E. S. Bozin \textit{et al.}, Science 330, 1660 (2010)] reported unusual ferroelectric instability in lead chalcogenides at heating, which is contrast to typical ferroelectric transitions that occur at cooling. This study explains the emergence of local dipole formation due to the softening of transverse optical (TO) phonon modes. However, standard first-principles calculations do not support the phonon softening (imaginary frequency). Here, we present that the spin-orbit interaction should be included in the calculations to correctly produce the instability and that, as such, thermal expansion leads to the softening in TO phonon modes. Another controversial finding in experiment that the frequency of TO mode is finite and increases with temperatures can be explained if anharmonic effects are considered together with the spin-orbit interaction. Our study shows that the spin-orbit interaction can be critical for the structural stability and thus affect the thermoelectric or structural phase transition. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D44.00008: Lattice dynamics in perovskite halides CsSnX$_3$ with X=I,Br,Cl Ling-Yi Huang, Walter Lambrecht We investigate the origin of the phase transitions between the cubic, tetragonal and orthorhombic phases of CsSnX$_3$, X=I, Br,Cl, in terms of the imaginary phonon frequencies of the higher symmetry phases at the zone boundaries and the associated rotations and tilts of the SnX$_6$ octahedra. We present first-principles calculations of the phonon band-structure and density of states as well as the predicted infrared spectra. The calculations are done using density functional perturbation theory. In the cubic phase, there are three triply degenerate IR active $T_{1u}$ modes and one silent $T_{2u}$ mode. We find that the calculated modes agree with the experiment when we assign the second and third calculated modes to the experimental first and second modes. Our calculated IR spectra show that the third observed mode in IR absorption is actually the highest LO rather than TO mode and the lowest calculated mode is found to overlap in frequency with a peak in density of phonon states. This indicates the possibility of a strong phonon-phonon interaction and hence short phonon-lifetime or strong broadening which could explain why this mode has not been observed. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D44.00009: Stereochemical activity of lone-pair electrons in ABX$_{3}$ heavy-element halides Young-Moo Byun, Eva Smith, Craig Fennie ABX$_{3}$ halides display many functionalities and properties such as ferroelectricity and superconductivity. ABX$_{3}$~(A $=$ Rb, Cs; B $=$ Sn, Pb; X $=$ F, Cl) form generally in two different structures: sheelite~(CaWO$_{4})$-like and perovskite. The interplay of the stereochemically active lone pair (i.e., second-order Jahn-Teller) B-cation and other structural distortion is the key to determining the stable structure and subsequently the physical properties. It turns out that in these heavy p-block element compounds relativistic effects influence the activity of the lone pair in a nontrivial way. In this talk we will present our first-principles study of structural properties of this family of ABX$_{3}$ halides and how they change with temperature and pressure. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D44.00010: Novel sp3-hybridized framework structure of group 14 elements Manh Cuong Nguyen, Xin Zhao, Cai-Zhuang Wang, Kai-Ming Ho Using genetic algorithm atomic structure prediction method and first-principles calculations, we discovered a novel low-energy metastable structure of group 14 elements in P42/mnm symmetry. The P42/mnm structure is a cage-like distorted sp3-hybridized framework structure with the cage's volume $\sim$ 4\% larger than the average cage's volume of the clathrate type-I structure, indicating P42/mnm structure a good gases or metal atoms encapsulation structure. The band structure calculations show that P42/mnm Si and Ge are semiconducting with energy band gaps close to the optimal values for optoelectronic or photovoltaic applications. The metal atom encapsulation P42/mnm structure of group 14 elements could also be a candidate for rattling-mediated superconducting or ``a phonon glass and an electrical crystal'' thermoelectric materials. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D44.00011: Strain measurements of Ge epilayers on Si by Spectroscopic Ellipsometry A. Ghosh, N. Fernando, A.A. Medina, C.M. Nelson, S. Zollner, S.C. Xu, J. Menendez, J. Kouvetakis Using spectroscopic ellipsometry, we determined the strain of a Ge epilayer grown on a Si (100) substrate. This strain depends on the sample temperature and arises because of the difference in thermal expansion coefficients between Si and Ge. It can be calculated since the thermal expansion coefficients of Si and Ge are known very precisely, if we assume that the Ge epilayer was fully relaxed at the growth temperature, leading to an increase in strain as the temperature decreases. We calculate in-plane tensile strain values of 0.12\% at 300 K or 0.19\% at 77K for our Ge on Si layer, that compares favorably with an in-plane strain of 0.11\% derived from shifts of the Ge lattice reflection at 300 K by x-ray diffraction. This temperature-dependent strain affects the energies of the E1 and E1+Delta1 critical points of the Ge epilayer, which can be measured very precisely using spectroscopic ellipsometry from 77 to 800 K.From the difference in the critical point energies between our Ge epilayers on Si and bulk Ge (up to 20 meV), we can calculate the strain from the known elastic constants and deformation potentials. The strain determined from ellipsometry agrees well with the strain calculated from the temperature-dependent thermal expansion coefficient. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D44.00012: Study of the Electronic Properties of Different Phases of Cu3V-VI4 Based on First-Principle Calculation Tingting Shi, Wanjian Yin, Mowafak Al-Jassim, Yanfa Yan Considering the small energy differences among the Cu3V-VI4 compounds in four different structures, enargite, wurtzite-PMCA, famatinite and zinc-blend-PMCA, a large variety of phases with different band properties may co-exist. This paper systematically studies the trend of the electronic properties of these phases; the band gap will greatly decrease when the phase changes from enargite to wurtzite-PMCA, or from famatinite to zinc-blende-PMCA. For example, the band gap of enargite Cu3PS4 is 2.51 eV, while the famatinite one is 1.72 eV. In addition, the band gap will obviously decrease as we increase the atomic number of group-V or group-VI element for one structure. Due to the wide band range from 0.4 eV to 2.5 eV for all possible Cu3V-VI4 structures, our detailed first-principle study will suggest guidelines for the band gap engineering for potential photovoltaic applications. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D44.00013: A variational polaron-self-interaction corrected total-energy functional for charge excitations in wide-gap insulators Babak Sadigh, Paul Erhart, Daniel Aberg A simple modification of the density-functional theory (DFT) total energy functional is proposed that corrects for the polaron self-interaction error in the semilocal approximations (LDA/GGA) to the exchange-correlation potential. It can accurately reproduce polaron formation in widegap insulating materials. Extensive study of the potential-energy landscapes of self-trapped holes in alkali halides is performed and agreeable comparison with hybrid-DFT and experiment is obtained. The new functional is general, simple to implement and its variational formulation allows for ab-initio molecular-dynamics simulations of polarons in widegap insulators regardless of complexity. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D44.00014: Study of strain-mediated couplings in SrRuO$_3$-CoFe$_2$O$_4$ nanocomposite by Raman spectroscopy Yi-Chun Chen, Chia-Hsien Chien, Yen-Chin Huang, Heng-Jui Liu, Ying-Hao Chu Self-assembled vertical nanostructures have the advantage of high interface-to-volume ratio and can be used to generate new functionalities by the choice of combination of constituents. Recently, we found an interesting behavior of photo-induced magnetization switching in a self-assembled system, CoFe$_2$O$_4$ (CFO) nanopillars embedded in SrRuO$_3$(SRO) matrix. In this study, to further reveal the coupling mechanism of this hetero-structure, we used Raman spectroscopy to investigate their phonons under external stimulus. When an external out-of-plane magnetic field is applied, the CFO A1g phonon (688 cm$^{-1}$) had a red shift due to the negative magnetostriction effect, while the SRO Ag phonon (183 cm$^{-1}$) also had a correspondent red shift. This result indicates the crystal structures of SRO matrix are affected by the deformation of the CFO pillars through the magnetostrictive couplings. Moreover, at the phase transition temperature of SRO (160 K), three phonons (T$_{2u}$, E$_g$, E$_u$) of CFO also had a significant blue shift, which reveals again the strain-mediated coupling. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D44.00015: Selectable trapping or rotation of micro-particles using a plasmonic Archimedes spiral Chen-Bin Huang, Wei-Yi Tsai, Jer-Shing Huang We demonstrate selectable trapping or rotation of dielectric micro-particles by optical near fields generated in a plasmonic Archimedes spiral. Depending on the handedness of circularly polarized excitation, plasmonic near fields can be engineered into either a super-focusing spot for particle trapping or a plasmonic vortex for particle rotation. The optical forces are numerically analyzed. Experimentally, selectable trapping or rotation of single microsphere and sphere cluster are both realized. [Preview Abstract] |
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