Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y31: Amorphous Solids, Glasses & Liquids II |
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Sponsoring Units: DCMP Chair: Horacio Castillo, Ohio University Room: C145 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y31.00001: Synthesis, Homogenization and Molecular Structure of Chalcogenide glasses Siddhesh Bhosle, Kapila Gunasekera, Punit Boolchand, Ping Chen Over the years, bulk glasses have been synthesized by reacting starting materials in evacuated (10$^{-5}$ Torr to 10$^{-7}$Torr) quartz tubings for various periods at suitable elevated temperatures. The lack of a non-invasive structural probe to track spatial heterogeneity of samples during synthesis has been an impediment to tune synthesis conditions, and obtain a homogeneous product. We have developed a Raman profiling technique to understand the homogenization kinetics of Ge$_x$Se$_{100-x}$ melts, and find dry samples (2 gram size) take 7 days of reaction at 950$^{\circ}$C to homogenize on a scale of 50 microns, while wet ones homogenize quicker ($\sim$3 days), but possess physical properties measurably different from their dry counterparts. Rotating sample tubes during synthesis assists in homogenization of samples incrementally but not dramatically. A score of compositions were homogenized across the 10\% $<$ x $<$ 33.3\% range, and calorimetric, Raman scattering, and molar volume data accumulated. These data provide clear evidence for three distinct regimes of behavior as a function of Ge content, which are identified with the three elastic phases discussed earlier.\footnote{P.Boolchand et al. J.Non-Cryst. Solids 293, 348 (2001).} [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y31.00002: Elastic phases in Ge$_x$Sb$_x$Se$_{100-2x}$ ternary glasses K. Gunasekera, P. Boolchand, M. Micoulaut The rigidity and stress phase transitions in titled ternary glasses are examined in Raman scattering, modulated DSC and molar volume measurements, and found to occur at x$_c$(1) = 14.9\% (rigidity) and x$_c$(2) = 17.5\% (stress). Raman scattering provides evidence of the structural motifs populated in these networks. Using Size Increasing Cluster Agglomeration, Rigidity theory and the decoded structural motifs, we have calculated the rigidity and stress transitions in the first step of agglomeration to occur at x$_c$(1)$^t$ = 15.2\% and x$_c$(2)$^t$ = 17.5\% respectively, in reasonable accord with experiments. Theory predicts and experiments confirm that these transitions will coalesce if edge-sharing Ge- tetrahedral motifs were absent in the structure, a circumstance that prevails in the Ge-deficient Ge$_7$Sb$_x$Se$_{93-x}$ ternary, where a narrow IP is reported.\footnote{B.J. Madhu et al. Eur. Phys. J. B 71,21 (2009).} These results underscore the central role played by topology in determining the elastic phases of network glasses. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y31.00003: Short Range Order Signature in Crystalline and Amorphous GeSbTe Xanes Spectra Jean-Yves Raty, C\'eline Otjacques, Rengin Pekoz, Christophe Bichara, Vince Lordi A new implementation of XANES spectra calculations within DFT and PAW potentials is used to compute the XANES spectra of various amorphous and crystalline GeSbTe structures. A clear correlation between the local order, either tetrahedral or distorted octahedral, and the shape of the XANES signal is observed. These calculations provide a new interpretation of past XANES measurements, relating essentially the phase change mechanism to a moderate modification of the local environment of the Ge atoms. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y31.00004: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y31.00005: Photocrystallization in a-Se films with and without As-Se buffer layers B.A. Weinstein, R.E. Tallman, G.P. Lindberg, J.A. Rowlands, A. Reznik, M. Kubota, K. Tanioka Photo-induced crystallization is studied for temperatures of 250 - 435 K in amorphous Se (a-Se) HARP* imaging targets using Raman spectroscopy to detect the appearance and growth-rate of trigonal Se in these thin-film structures. We observe striking differences between HARP films in which a thin buffer layer of As-Se alloy is, or is not, deposited prior to growth of the principal a-Se photoconductive layer. Films containing an As-Se buffer appear to be much more stable; no photocrystallization is found within the temperature range studied, even after 3.5 hours of laser exposure (0.17W/mm$^{2}$ at 647nm). Whereas for films with no As-Se buffer, photocrystallization readily occurs in two temperature regimes below and above the a-Se glass transition (Tg $\sim $310K), and there is a regime in the neighborhood of Tg where photocrystallization is absent. We discuss these results in terms of a polymerization model under the competing effects of shear-strain at the substrate and As-cross-linking in the buffer layer, which, respectively, tend to promote and inhibit crystallization in the thick Se over-layer. *\textit{High-gain Avalanche Rushing Photodetector}. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y31.00006: Diffraction studies of short- and intermediate-range order of phosphorus-selenium glasses David Price, Aleksei Bytchkov, Mariana Miloshova, Eugene Bychkov, Shinji Kohara, Louis Hennet We present state-of-the-art neutron and X-ray diffraction data that provide a definitive picture of the short- and intermediate-range structure of P-Se glasses spanning both glass regions. Specific goals were (1) to obtain detailed information about the development with increasing of intermediate-range order on the length scale around 10 {\AA}, based on the behavior of the first sharp diffraction peak; and (2) to obtain a reliable statistical picture of the short-range order, using the information about types and concentrations of local structural units provided by recent NMR measurements to interpret the trends observed as the P concentration is varied. Particular attention is given to the fine structure of the first peak in the pair distribution function and to a feature in the structure factor at 7.5 {\AA}$^{-1}$, highlighted by Sergi et al. as a signature of molecular units. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y31.00007: Structure investigation of ultra-small CdSe nanoparticles using the atomic PDF Ahmad S. Masadeh, Simon J.L. Billinge, Emil S. Bozin, James R. McBride, Sandra J. Rosenthal The size-dependent structure of CdSe nanoparticles, with diameter ranging from 1.5 to 3.6 nm, has been studied using the atomic pair distribution function (PDF) method. The samples are prepared by the methods of Peng \textit{et al} [1], with modifications. The structure of the smallest stable size, ($\sim $1.5 nm), have been found to posses locally distorted wurtzite structure, with no clear evidence of a heavily disordered surface region [2]. The PDF data of the smallest particle show an extra structural peak appears around r = 3.5 A indicates there is structure modification happened in this sample. This peak start appearing the nanoparticles PDF data gradually as nanoparticle size decreases. The structural parameters are reported quantitatively. We measure a size-dependent strain on the Cd-Se bond which reaches 1.0{\%} at the smallest particle size [3]. The size of the well-ordered core extracted directly from the data agrees with the size determined from other methods. \\[0pt] [1] Peng, et al, \textit{JACS}., 120, 5343-5344 (1998). [2] Gilbert et al, \textit{Science}, 305, 651-654 (2004). [3] Masadeh et al. \textit{ PRB} \textbf{76}, 115413 (2007). [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y31.00008: Evidence of an Intermediate Phase in bulk alloy oxide glass sysem S. Chakraborty, P. Boolchand Reversibility windows have been observed in modified oxides (alkali-silicates and -germanates) and identified with Intermediate Phases(IPs).\footnote{V. Rompicharla J. Phys.: Condens. Matter 20, 202101 (2008).} Here we find preliminary evidence of an IP in a ternary oxide glass, (B$_2$O3)$_5$(TeO$_2$)$_{95-x}$(V2O5)$_x$, which is composed of network formers. Bulk glasses are synthesized across the 18\% $<$ x $<$ 35\% composition range, and examined in Raman scattering, modulated DSC and molar volume experiments. Glass transition temperatures T$_g$(x) steadily decrease with V$_2$O$_5$ content x, and reveal the enthalpy of relaxation at T$_g$ to show a global minimum in the 24\% $<$ x $<$ 27\% range, the reversibility window (IP). Molar volumes reveal a minimum in this window. Raman scattering reveals a Boson mode, and at least six other vibrational bands in the 100 cm$^{-1}$ $<$ $\nu$ $<$ 1700 cm$^{-1}$ range. Compositional trends in vibrational mode strengths and frequency are established. These results will be presented in relation to glass structure evolution with vanadia content and the underlying elastic phases. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y31.00009: Glasses, Stress, Attenuation and Thermal Conductivity Jiansheng Wu, Clare Yu A wide variety of amorphous materials exhibit similar behavior in their thermal properties. Examples include universal features in the specific heat,thermal conductivity, and ultrasonic attenuation. Recent experiments from the Parpia group at Cornell find that high stress silicon nitride thin film resonators exhibit a remarkably high Q factor, exceeding that of amorphous SiO$_2$ by 2 to 3 orders of magnitude over a broad range of temperatures, and even exceeding that of single crystal silicon at room temperature. We present a model of why the stress reduces the attenuation. The basic assumption is that high stress increases the potential barriers of the excitations of defects that produce the loss, thus reducing the effective density of lossy fluctuators. We predict that high stress could lead to high thermal conductivity and low dielectric loss, making high stress SiN an excellent candidate as a substrate for integrated circuits. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y31.00010: Highly stable glasses as a general phenomenon: Physical vapor depositions of four different trisnaphthylbenzene isomers Kevin Dawson, Lei Zhu, Laura Kopff, Robert J. McMahon, Lian Yu, M.D. Ediger Glasses of each of the trisnaphthylbenzene (TNB) isomers, a low molecular weight glass forming family of four isomers, were created by physical vapor deposition. These glasses were analyzed using differential scanning calorimetry and wide angle x-ray scattering, and then compared to glasses prepared by quenching each melt. All four isomers produced stable glasses (increased onset temperature, large enthalpy overshoot, and excess x-ray scattering) when vapor-deposited at 0.85 T$_{g}$ and at low deposition rates. This result is surprising as one of the TNB isomers readily crystallizes when cooled as a liquid. When coupled with previous experiments, these results show that stable glasses are not just produced by a small set of good glass forming molecules but seem to be a general phenomenon. Thus physical vapor deposition can be used as a general route to create unusual glasses for future scientific exploration and technological uses. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y31.00011: In-situ characterization of vapor-deposited glasses of toluene by differential AC chip nanocalorimetry Mathias Ahrenberg, Katie Whitaker, Heiko Huth, Mark D. Ediger, Christoph Schick We use ac nanocalorimetry to investigate extraordinarily stable glasses of toluene prepared by vapor deposition. For that purpose we've built a vapor deposition chamber that allows in-situ characterization of vapor-deposited organic glasses down to liquid nitrogen temperature. With highly sensitive nanocalorimeters in a differential setup, we are able to measure ng-samples over a frequency range from 0.1 Hz up to 8 kHz. The device was used to investigate the transformation of as-deposited stable toluene glasses into ordinary glasses. For films about 100 nm thick, the transformation was studied as a function of time at constant temperature above the common glass transition and as function of temperature at constant heating rate. The stability of the thin films was investigated as a function of substrate temperature and deposition rate. [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y31.00012: Thermal Evolution of Defects and Hydrogenated Surfaces in nc-Si:H Kristin Kiriluk, Don Williamson, David Bobela, Arun Madan, Feng Zhu, P. Craig Taylor Photovoltaics research has created a push for new materials and nanotechnology is a primary focus. The most familiar of the nanomaterials is hydrogenated nanocrystalline silicon (nc-Si:H). nc-Si:H has less light-induced degradation than a-Si:H and is cheaper to make than crystalline silicon. X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and electron spin resonance (ESR) experiments explored the crystallite size, orientation and defect density on nc-Si:H samples with varying crystalline volume fraction (CVF). Samples with CVF $\sim $ 50{\%} show preferential [220] crystallite orientation, whose microstructure changes with thermal annealing. Modeling of SAXS data for as-grown material shows that the crystallite surfaces are 20{\%} to 40{\%} hydrogenated. After high temperature annealing, hydrogen leaves these surfaces and the ESR signal increases by about 10 times. We discuss these results and then speculate on the relationship between hydrogen, defects, and microstructure. [Preview Abstract] |
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