S. V. Khare

Aluminum nanoscale order in amorphous Al92Sm8 measured by fluctuation electron microscopy

Fluctuation electron microscopy (FEM) measurements and simulations have identified nanoscale aluminum-like medium-range order in rapidly quenched amorphous Al92Sm8 which devitrifies by primary Al crystallization. Al92Sm8 amorphized by plastic deformation shows neither Al nanoscale order, nor primary crystallization. Annealing the rapidly quenched material below the primary crystallization temperature reduces the degree of nanoscale Al order measured by FEM. The FEM measurements suggest that 10–20A diameter regions with Al crystal-like order are associated with primary crystallization in amorphous Al92Sm8, which is consistent with the quenched-in cluster model of primary crystallization.

Energies of steps, kinks, and defects on Ag{100} and Ag{111} using the embedded atom method, and some consequences

Abstract Using the embedded atom method (EAM) we compute the energies of principal steps, kinks, and single-layer clusters of adatoms (islands) and of vacancies for Ag{100} and Ag{111}. The energies are semiquantitatively consistent with experiments. Comparisons are frequently made with estimates based on nearest-neighbor bond counting. On Ag{111} and Pt{111} the ratio of the energies of the two close-packed steps is closer to unity than measured in experiments on Pt. The energies of clusters are essentially proportional to their perimeter, providing an easy way to estimate the binding energy of clusters to step edges. Adatom-vacancy symmetry is a good approximation except for single-site defects. Our calculations of barriers for single-atom diffusion near steps, compared to across terraces, are consistent with the fractal-like fingered growth of islands experimentally observed on {111} but not seen on {100}. Computed spring constants of surface atoms suggest small changes in perpendicular vibration frequencies near step edges.

Quantifying nanoscale order in amorphous materials: simulating fluctuation electron microscopy of amorphous silicon

Fluctuation electron microscopy (FEM) is explicitly sensitive to 3- and 4-body atomic correlation functions in amorphous materials; this is sufficient to establish the existence of structural order on the nanoscale, even when the radial distribution function extracted from diffraction data appears entirely amorphous. However, it remains a formidable challenge to invert the FEM data into a quantitative model of the structure. Here, we quantify the FEM method for a-Si by forward simulating the FEM data from a family of high quality atomistic models. Using a modified WWW method, we construct computational models that contain 10–40 vol% of topologically crystalline grains, 1–3 nm in diameter, in an amorphous matrix and calculate the FEM signal, which consists of the statistical variance V (k) of the dark-field image as a function of scattering vector k. We show that V (k) is a complex function of the size and volume fraction of the ordered regions present in the amorphous matrix. However, the ratio of the variance peaks as a function of k affords the size of the ordered regions; and the magnitude of the variance affords a semi-quantitative measure of the volume fraction. We have also compared models that contain various amounts of strain in the ordered regions. This analysis shows that the amount of strain in realistic models is sufficient to mute variance peaks at high k. We conclude with a comparison between the model results and experimental data.

Determining absolute orientation-dependent step energies: a general theory for the Wulff-construction and for anisotropic two-dimensional island shape fluctuations

We describe an analytical form of the Wulff plot construction procedure and derive a general expression for the surface energy from the three-dimensional equilibrium crystal shape in generalized orthogonal curvilinear coordinates. Particular expressions in Cartesian, spherical polar, and circular cylindrical coordinates are also presented. Corresponding results for a two-dimensional (2D) island on a flat terrace provide relative orientation-dependent step energies within a scale factor λ, the equilibrium chemical potential of the island per unit area. In order to determine λ and, hence obtain absolute step energies, we have developed an exact theoretical approach, applicable to both isotropic and anisotropic 2D island shapes, relating the temporal change in island free energy to thermal fluctuations about the equilibrium shape.

High-pressure studies of Bi2S3.

The high-pressure structural and vibrational properties of Bi2S3 have been probed up to 65 GPa with a combination of experimental and theoretical methods. The ambient-pressure Pnma structure is found to persist up to 50 GPa; further compression leads to structural disorder. Closer inspection of our structural and Raman spectroscopic results reveals notable compressibility changes in specific structural parameters of the Pnma phase beyond 4-6 GPa. By taking the available literature into account, we speculate that a second-order isostructural transition is realized near that pressure, originating probably from a topological modification of the Bi2S3 electronic structure near that pressure. Finally, the Bi(3+) lone-electron pair (LEP) stereochemical activity decreases against pressure increase; an utter vanishing, however, is not expected until 1 Mbar. This persistence of the Bi(3+) LEP activity in Bi2S3 can explain the absence of any structural transitions toward higher crystalline symmetries in the investigated pressure range.

Dislocation-driven surface dynamics on solids

Dislocations are line defects that bound plastically deformed regions in crystalline solids. Dislocations terminating on the surface of materials can strongly influence nanostructural and interfacial stability, mechanical properties, chemical reactions, transport phenomena, and other surface processes. While most theoretical and experimental studies have focused on dislocation motion in bulk solids under applied stress and step formation due to dislocations at surfaces during crystal growth, very little is known about the effects of dislocations on surface dynamics and morphological evolution. Here we investigate the near-equilibrium dynamics of surface-terminated dislocations using low-energy electron microscopy. We observe, in real time, the thermally driven nucleation and shape-preserving growth of spiral steps rotating at constant temperature-dependent angular velocities around cores of dislocations terminating on the (111) surface of TiN in the absence of applied external stress or net mass change. We attribute this phenomenon to point-defect migration from the bulk to the surface along dislocation lines. Our results demonstrate that dislocation-mediated surface roughening can occur even in the absence of deposition or evaporation, and provide fundamental insights into mechanisms controlling nanostructural stability.

Ab initio calculations for properties of MAX phases Ti2TlC, Zr2TlC, and Hf2TlC

Using ab initio calculations we have computed the lattice constants, bulk moduli, and local and total density of states of the MAX phases, Ti2TlC, Zr2TlC, and Hf2TlC in the hexagonal P63∕mmc space group. The results for lattice constants are within 2% of experimental results. The bulk moduli are predicted to be 125, 120, and 131GPa, respectively. These are the lowest values of bulk moduli among all MAX phases studied to date. The electronic density of states shows that all three materials are conducting. These low values of their bulk moduli are attributed to weak metal M (M=Ti,Zr,Hf) bonding with the A element thallium.

Energetics of steps and kinks on Ag and Pt using equivalent crystal theory (ECT)

Abstract Using the equivalent crystal theory (ECT), we have calculated the energies of steps and kinks on vicinal {001} and {111} surfaces of Ag and Pt. We compare with some semiempirical calculations, particularly our recent embedded atom method (EAM) results, and with available experimental data, considering both trends and magnitudes: ECT values are roughly double those from EAM, and may well be more accurate. Like EAM, ECT incorrectly predicts an instability of Ag{110} towards missing row reconstruction.

Evidence from atomistic simulations of fluctuation electron microscopyfor preferred local orientations in amorphous silicon

Simulations from a family of atomistic structural models for unhydrogenated amorphous silicon suggest that fluctuation electron microscopy experiments have observed orientational order of paracrystalline grains in amorphous silicon. This order may consist of correlations in the orientation of nearby paracrystalline grains or anisotropy in the grain shape. This observation makes a natural connection to the known growth modes of microcrystalline silicon and may be useful for other materials systems.

Dynamics of step doubling: simulations for a simple model and comparison with experiment

Abstract To interpret recent experiments on the dynamics of step doubling, we have studied a simple model of this phase transition. With Monte Carlo, we compute the time-dependence of the order parameter in the limit of rapid diffusion across terraces. Analysis of the data shows that the limiting step is the time for adjacent steps to touch each other; subsequent “zipping” together happens rapidly. From this vantage we develop an analytic expression for short times that changes into a phenomenological one for later times. Using data from two physical systems, we compare this function and another based on naive assumptions with a third based on chemical rate theory. For the more recent data, our expression describes the data best. Finally, in the opposite limit in which atoms can only move along step edges, we show characteristic configurations and compute the structure factor.