Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U26: Amorphous and Nanocrystalline Materials: Theory and Experiment |
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Sponsoring Units: DCOMP DMP Chair: David Drabold, Ohio University Room: LACC 501B |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U26.00001: Theory of hydrogen related meta-stability in disordered silicon Blair Tuttle Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability will be discussed for amorphous and ppoly-crystalline silicon. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U26.00002: Nucleation and growth simulation of Si nanocrystals in Si-rich oxide Mihail Petkov, L. Douglas Bell Ion implantation profile in SiO2 layers on Si substrate calculated by TRIM was used as initial conditions for a diffusion-driven nucleation and growth model. Nucleation was initiated at randomly chosen seeds satisfying the Poisson distribution, which were used as centers in a Voronoi tessellation of space. The volume fraction of implanted Si ions in each Voronoi polyhedron was calculated based on the polygon volume and the fluence. The nanocrystal growth was considered to occur at the nucleation centers by diffusing of Si ions, which become localized at the seeds. Zero net flux across each of the surfaces of the Voronoi polyhedra was assumed for the diffusing Si species. The calculations were by with periodic boundary conditions in the directions normal to the implantation. We report size and depth distribution of the Si nanocrystals formed under the above conditions. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U26.00003: Studies of Thermally Annealed Graphitic Amorphous Carbon Resulting in a Decrease of Quasi-Stone-Wales Defects and Increase in Bandgap J.R. Dennison, T.E. Doyle, Jodie Corbridge, Sterling Smith, Neal Nickles We used a novel vibrational dynamics model for planar disordered materials (the embedded ring approach) to determine the structural evolution of thermally annealed graphitic amorphous carbon (g-C). The vibrational model assumes that constituent atoms of a material are arranged in n-membered planar rings embedded in the effective medium, a continuous random network of atoms. Standard structural models of g-C-a ubiquitous form of disordered carbon present in the production of diamond films, fullerenes, graphenes, nanotubes, and graphite-suppose that g-C is composed primarily of a structural distribution of such carbon rings with 4 to 8 atoms. We have calculated the in-plane normal modes and frequencies for embedded carbon rings and used these frequencies to fit Raman spectra of g-C annealed to temperatures ranging from 22 \r{ }C to 1050 \r{ }C. From the relative intensities of the different frequency peaks, our procedures provide quantitative ring statistics for the structure of g-C. In particular, we have found that unannealed g-C can have many 5- and 7-membered rings, but that the fraction of 6-membered rings increases with annealing temperature consistent with the known result that g-C evolves to nanocrystalline graphite under high T annealing. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U26.00004: Magnetic Rare Earth (Gd) Doped Amorphous Carbon Li Zeng, Erik Helgren, Frances Hellman Previous studies on rare earth (RE) doped amorphous silicon ($a-$RE$_{x}$Si$_{1-x}$, RE=Gd, Tb) have shown remarkable physics for compositions near the three-dimensional metal-insulator transition: many orders of magnitude negative magnetoresistance (MR) at low temperatures, and a high onset temperature (T*) where the effects of magnetic dopants ``turns on.'' Both MR and T* are significantly reduced by substituting Ge for Si, an effect we suggest is due to the reduced band gap and consequently larger dielectric constant and larger electron screening of Ge. This suggestion is supported by a systematic decrease in T* and MR with increasing x and by data on ternary alloys (with non-magnetic Y additions). To test this theory, we have prepared samples of $a-$RE$_{x}$C. Amorphous C has the unique feature of a band gap which can be tuned by varying the sp$^{2}$/sp$^{3 }$bonding ratio. As anticipated, $a-$RE$_{x}$C$_{1-x}$ shows even larger negative MR at low temperatures and higher characteristic temperature T*. Chemical and structural properties were studied by RBS, TEM, Raman spectroscopy. The temperature and magnetic field dependence of conductivity and magnetic properties and comparisons to previous work will be discussed. Thanks to the NSF for support. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U26.00005: Probing the Intermediate Range Order in Novel Rare Earth Phosphate Glasses Using Neutron Diffraction Mustafa Rajabali, Kanishka Marasinghe, Richard Brow, Nathaniel Wyckoff, Chris Benmore, Joan Siewenie, Qiang Mei, Robert Hart Neutron diffraction has been used to study the atomic structure and especially the coordination environment of rare earth ions for (x)R$_{2}$O$_{3}$ (1-x)P$_{2}$O$_{5 }$,where R is praseodymium or neodymium and x ranges between 0.05 and 0.28. Such information can help further developing these exciting materials for potential optical and magnetic applications. In the case of neodymium containing samples, the method of isotopic substitution was used to measure the first order difference function involving only neodymium correlations. Merits of this technique as applied to rare earth phosphate glasses as well as the dependence of the atomic structure on the R/P ratio will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U26.00006: Velocity of sound and elastic properties of lanthanum gallo-germanate glasses Luu-Gen Hwa, Way C. Chao The velocity of sound of a group of lanthanum gallo-germanate glasses is obtained by the ultrasonic pulse-echo measurements, at room temperature. Both longitudinal and transverse velocities of these glasses are composition dependence. The experimental results are used to obtain the elastic constants. The correlation of elastic stiffness, the cross-link density, and the fractal bond connectivity of these glasses are discussed. The derived experimental values of Young's modulus, bulk modulus, shear modulus and Poisson's ratio for our glasses are compared with those theoretically calculated values in terms of the Makishima-Mackenzie model. A possible existence of both tetrahedral (four-fold coordination) and octahedral (six-fold coordination) of Ga and Ge in the structure of these glasses is discussed. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U26.00007: Crystallization Behavior of Chemically Prepared Nanoparticles of Amorphous Fe-B* Erik Pearson, Adrian Gorea, Thomas Ekiert, Karl Unruh The crystallization behavior of amorphous Fe-B nanoparticles prepared by reducing an aqueous solution of Fe$^{+2}$ ions with NaBH$_{4}$ has been studied by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and x-ray diffraction (XRD) measurements. At a heating rate of 10 \r{ }C/min the DSC measurements show a sharp and well defined exothermic peak at a temperature of about 475 \r{ }C and a Kissinger analysis of the shift in the position of this peak as a function of the heating rate yields an activation energy of about 3.6 eV/at. The VSM measurements also exhibit a sharp increase in the magnetization at a temperature 475 \r{ }C (at a heating rate of 10 \r{ }C/min). X-ray diffraction measurements on samples heated to temperatures slightly above 475 \r{ }C verify that the observed DSC and VSM signals correspond to the transformation from the as-prepared amorphous structure to a crystalline structure. * This work has been supported by AFRL DARPA METAMATERIALS contract no. F33615-01-2-2166, ARO DEPSCOR grant no. W911NF-04-1-0264, and the Undergraduate Research Program at the University of Delaware. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U26.00008: {\textit In situ} Raman Scattering Studies of High-Pressure Stability and Transformations in the Matrix of a Nanostructured Glass-Ceramic Composite Kristina Lipinska-Kalita, Stephen Gramsch, Patricia Kalita, Russell Hemley High-pressure Raman scattering studies were performed on a glass-based composite with nanometer-sized gallium oxide aggregates embedded in a potassium-silicate host glass. The aim of our studies was to advance the understanding of pressure-driven structural transformations in the glass matrix of the composite. Throughout the studied pressure range the Raman spectra confirmed that the glass matrix undergoes a range of structural transformations comparable to that reported previously for a pure SiO$_{2}$ glass. Compression from ambient up to 10.8 GPa was completely reversible on decompression to ambient pressure. At higher pressures the Raman spectra demonstrated a breakdown of the intermediate-range order in the glass matrix and a permanent reduction in SiO$_{4}$ ring statistics toward smaller than six-ring configurations and a coordination change of the silicon atom. The overall spectral profile at the end of the decompression cycle indicated the occurrence of permanent reconstructive structural changes in the glass matrix. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U26.00009: Absence of Dipole Glass Transition for Randomly Dilute Classical Ising Dipoles Clare Yu, Joseph Snider Randomly dilute dipoles with long range dipolar interactions appear in a variety of solid insulating materials. Based on theoretical studies of spin glasses with long range interactions, one would expect such dilute dipolar systems to undergo a spin glass-like transition as the temperature decreases. However, there has been no experimental evidence for such a transition in very dilute systems. One example where such a transition has not been definitively observed is two level systems that dominate the physics of glasses at low temperatures. Another is LiHo$_x$Y$_{1-x}$F$_4$ with $x=4.5\%$. We have investigated the absence of a phase transition in dilute dipolar glasses. Using Wang-Landau Monte Carlo simulations, we show that at low concentrations $x$, dipoles randomly placed on a cubic lattice with dipolar interactions do not undergo a phase transition as the temperature decreases. We define a characteristic ``glass'' temperature $T_g$ as the temperature where the distribution $P(q,T)$ is flattest. $q$ is the overlap order parameter. We find that in the thermodynamic limit $T_g$ goes to zero as $1/\sqrt{N}$ where $N$ is the number of dipoles. The entropy per particle at low temperatures is larger for lower concentrations ($x=4.5\%$) than for higher concentrations ($x=20\%$). [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U26.00010: Discontinuous molecular dynamics study of the diffusion of fluids in dynamic random media Kamakshi Jagannathan, Arun Yethiraj The static and dynamic properties of dimeric hard sphere fluids in random media are studied using discontinuous molecular dynamics. The media is composed of a random collection of hard spheres that are dynamic in the sense that they are connected by a string to their respective initial positions, and can move in the spherical volume defined by the length of the string, $l$. The fluid diffusion coefficient is calculated as a function of $l$ for different volume fractions of the fluid and media. In the $l\rightarrow 0$ and $l\rightarrow \infty$ limits, the system reproduces the limits of a fluid in a static media and in a hard sphere liquid, respectively. Much of the phenomenology of glass forming materials is reproduced by this model. For example, this model mimics experimental studies for impurity diffusion in glass forming materials. The diffusion behavior changes from a power law behavior (in $l$) above a critical $l$ to an Arrhenius behavior below this critical $l$. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U26.00011: Glass Relaxation and the Dielectric Constant Of Moist, Porous Rock Arthur H Thompson The low-frequency, dielectric constants of moist, porous rocks are important properties related to hydrocarbon saturation and mineral exploration. Numerous theoretical and experimental studies have attempted to explain the unusually large values of the dielectric constant at frequencies below 1000 Hz. This study tests the hypothesis that a disordered arrangement of dynamically-coupled, electrochemical dipoles in the pore fluids explains the dielectric constant. The dynamic coupling leads to relaxation times on the order of tens of minutes and divergent dielectric constants as frequency approaches zero. Temperature-dependent relaxation measurements test and support the hypothesis. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U26.00012: Terrace Selection at an Icosahedral Quasicrystal Surface Baris Unal, Cynthia Jenks, Thomas A. Lograsso, Amy Ross, Patricia A. Thiel Quasicrystals are aperiodic, but well-ordered, intermetallics. Using scanning tunneling microscopy, we investigate the effects of annealing temperature on the structure of a fivefold surface of icosahedral Al--Pd--Mn. After annealing at 900-915 K shallow void-rich terminations are created although the density of the voids are nearly zero after annealing at 925-950 K. The terminations that are consumed by voids have a distinctive atomic local configuration, very similar to ``rings'' identified in the model of Papadopolos and Kasner [1]. During the coalescence and the growth of the voids, a different termination becomes exposed. We suggest that the shallow steps associated with the voids, and the rings, signal a surface that is at an intermediate stage of structural equilibration. These exposed terraces give us a new insight into the structure of quasicrystal. [1] Papadopolos, Z., et al., Phys. Rev. B 66, 184207 (2002) [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U26.00013: Structure and Phase Separation in Ultrathin Ag/Cu Amorphous Alloy System Hao Chen, Jim Zuo The structure of disordered metallic alloys is an important but unsolved problem. Previous studies on Ag-Cu system showed that relatively homogeneous solid solutions formed at liquid nitrogen temperature decompose into separate phases or evolve into crystalline structure at a higher temperature. In this research project, we prepared ultra-thin Ag-Cu films on amorphous carbon support by HV magnetron sputtering with both targets. With high energy Ag and Cu atoms bombarding on the carbon substrate, they are forced to form amorphous alloy or nano-crystalline thin film at room temperature. We have investigated the structure of ultra-thin Ag-Cu films by examining their pair distribution function (PDF) using electron diffraction and observed phase separation process directly in STEM images. In the STEM Z-contrast images, since the contrast is directly related to the atomic number (Z) of the components, we can see clearly the phase separation process. Experimental results show that the sample morphology evolutions are different in samples with different thickness, and the phase separation depends on various Ag/Cu atomic ratios. In Ag$_{50}$Cu$_{50}$ sample, early stage phase separation is associated with increasing Cu crystallite size, indicates that Cu diffuse out of Ag-Cu solid solution phase. [Preview Abstract] |
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U26.00014: Amorphization of Aluminum Nanoparticles Sindee Simon, Juan Sun The melting behavior of aluminum nanoparticles with an oxide passivation layer is examined using a differential scanning calorimetry (DSC). Both broad and narrow size-distributed particles are studied, and the weight-average particle radius ranges from 8 nm to 50 nm. With decreasing particle size, the melting response moves towards lower temperatures, as predicted by Gibbs-Thomson equation. The latent heat of fusion also decreases and is significantly smaller than that predicted by the surface tension; the heat of fusion is only 20 percent of the bulk value at our smallest particle size. An analysis suggests that a passivated aluminum nanoparticle of 6 nm radius will become amorphous and have no heat of fusion due to the presence of defects induced by the particle's small size. At the onset of amorphization, we calculate that in this system, approximately one defect will exist for every seven atoms. [Preview Abstract] |
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