D. C. Chrzan

Embedded Binary Eutectic Alloy Nanostructures: A New Class of Phase Change Materials

Phase change materials are essential to a number of technologies ranging from optical data storage to energy storage and transport applications. This widespread interest has given rise to a substantial effort to develop bulk phase change materials well suited for desired applications. Here, we suggest a novel and complementary approach, the use of binary eutectic alloy nanoparticles embedded within a matrix. Using GeSn nanoparticles embedded in silica as an example, we establish that the presence of a nanoparticle/matrix interface enables one to stabilize both nanobicrystal and homogeneous alloy morphologies. Further, the kinetics of switching between the two morphologies can be tuned simply by altering the composition.

Sub-angstrom imaging of dislocation core structures: how well are experiments comparable with theory?

During the past 50 years, transmission electron microscopy (TEM) has evolved from an imaging tool to a quantitative method that approaches the ultimate goal of understanding the atomic structure of materials atom by atom in three dimensions both experimentally and theoretically. Todays TEM abilities are tested in the special case of a Ga-terminated 30° partial dislocation in GaAs:Be where it is shown that a combination of high-resolution phase contrast imaging, scanning TEM (STEM), and local electron energy loss spectroscopy (EELS) allows for a complete analysis of dislocation cores and associated stacking faults. We find that it is already possible to locate atom column positions with picometre precision in directly interpretable images of the projected crystal structure and that chemically different elements can already be identified together with their local electronic structure. In terms of theory, the experimental results can be quantitatively compared with ab initio electronic structure total energy calculations. By combining elasticity theory methods with atomic theory, an equivalent crystal volume can be addressed. Therefore, it is already feasible to merge experiments and theory on a picometre length scale. Whereas current experiments require the utilization of different, specialized instruments, it is foreseeable that the rapid improvement of electron optical elements will soon generate a next generation of microscopes with the ability to image and analyze single atoms in one instrument with deep sub-angstrom spatial resolution and an energy resolution better than 100u2009meV.

Dynamic scaling in a simple one-dimensional model of dislocation activity

We examine a simple one-dimensional (1D) model of dislocation activity, including a stress-activated source and mutually interacting dislocations. We demonstrate, through numerical and analytical steps, that the dislocations emitted from a 1D stress-activated source evolve towards a distribution which is self-similar in time, and we derive the power-law forms and distribution function. We show that the asymptotic distribution is a step function, and the dislocation front moves out linearly in time. The spacing between dislocations in the asymptotic distribution is uniform and increases logarithmically in time. The number of dislocations increases as t/ln(t), and the strain increases as t 2/ln(t).

Superheating and supercooling of Ge nanocrystals embedded in SiO2

Superheating and supercooling of Ge nanocrystals embedded in SiO 2 Q. Xu, 1,2 I.D. Sharp, 1,2 C.W. Yuan, 1,2 D.O. Yi, 1,2 C.Y. Liao, 1,2 A.M. Glaeser, 1,2 A.M. Minor, 4 J.W. Beeman, 1 M.C. Ridgway, 5 P. Kluth, 5 J.W. Ager III, 1 D.C. Chrzan, 1,2 and E.E. Haller 1,2,* Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Materials Science and Engineering, University of California, Berkeley, CA 94720 USA Lawrence Livermore National Laboratory, Livermore, CA 94550, USA National Center for Electron Microscopy , Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia Email: eehaller@lbl.gov Abstract. Free-standing nanocrystals exhibit a size-dependant thermodynamic melting point reduction relative to the bulk melting point that is governed by the surface free energy. The presence of an encapsulating matrix, however, alters the interface free energy of nanocrystals and their thermodynamic melting point can either increase or decrease relative to bulk. Furthermore, kinetic contributions can significantly alter the melting behaviours of embedded nanoscale materials. To study the effect of an encapsulating matrix on the melting behaviour of nanocrystals, we performed in situ electron diffraction measurements on Ge nanocrystals embedded in a silicon dioxide matrix. Ge nanocrystals were formed by multi-energy ion implantation into a 500 nm thick silica thin film on a silicon substrate followed by thermal annealing at 900 °C for 1 h. We present results demonstrating that Ge nanocrystals embedded in SiO 2 exhibit a 470 K melting/solidification hysteresis that is approximately symmetric about the bulk melting point. This unique behaviour, which is thought to be impossible for bulk materials, is well described using a classical thermodynamic model that predicts both kinetic supercooling and kinetic superheating. The presence of the silica matrix suppresses surface pre-melting of nanocrystals. Therefore, heterogeneous nucleation of both the liquid phase and the solid phase are required during the heating and cooling cycle. The magnitude of melting hysteresis is governed primarily by the value of the liquid Ge/solid Ge interface free energy, whereas the relative values of the solid Ge/matrix and liquid Ge/matrix interface free energies govern the position of the hysteresis loop in absolute temperature.

Self-immobilization of superdislocations in L12 alloys: A simple statistical analysis

Abstract A simple statistical analysis of the structure or superdislocations in L12 compounds displaying the yield strength anomaly is developed. This analysis is based on the superkink model of dislocation dynamics, where it is assumed that a superdislocation propagates through the lateral motion of superkinks. The analysis is used to assess the probability that a superdislocation is immobile, and this probability is found to depend exponentially on the nominal length of the superdislocation. The statistical analysis requires knowledge of the global distribution of superkink lengths to obtain quantitative relative immobilization rates; this information is extracted from published experimental work and also from previous simulations. The implications of both superkink distributions are explored. The predictions of the analysis appear consistent with the results from simulations.

Primary creep of Ni3(Al, Ta) in the anomalous flow regime

Abstract The primary creep behavior of single-slip oriented Ni 3 (Al, Ta) has been characterized at low temperatures in the anomalous flow regime. For temperatures ranging from 20 to 200xa0°C, transient creep leading to eventual exhaustion has been measured at all stresses. The decline in the creep rate has been quantitatively shown to occur more quickly than in common metals, as the decline in the creep rate is faster than predicted by the logarithmic creep law. In addition, the temperature dependence of the primary creep behavior is consistent with the flow stress anomaly, as the measured amount of creep strain at a fixed stress decreases with increasing temperature.

Modeling pulsed-laser melting of embedded semiconductor nanoparticles

Pulsed-laser melting (PLM) is commonly used to achieve a fast quench rate in both thin films and nanoparticles. A model for the size evolution during PLM of nanoparticles confined in a transparent matrix, such as those created by ion-beam synthesis, is presented. A self-consistent mean-field rate equations approach that has been used successfully to model ion beam synthesis of germanium nanoparticles in silica is extended to include the PLM process. The PLM model includes classical optical absorption, multiscale heat transport by both analytical and finite difference methods, and melting kinetics for confined nanoparticles. The treatment of nucleation and coarsening behavior developed for the ion beam synthesis model is modified to allow for a non-uniform temperature gradient and for interacting liquid and solid particles with different properties. The model allows prediction of the particle size distribution after PLM under various laser fluences, starting from any particle size distribution including as-implanted or annealed simulated samples. A route for narrowing the size distribution of embedded nanoparticles is suggested, with simulated distribution widths as low as 15% of the average size.

Magnetic structure of {111} stacking faults in nickel

The magnetic structure of {111} stacking faults in nickel is investigated utilizing a fully self‐consistent, layered Korringa–Kohn–Rostoker approach which does not require full three‐dimensional symmetry or the use of finite‐sized slabs. Localized electronic states appear at the faults. The spin polarization is calculated for a twin boundary, an intrinsic fault, an extrinsic fault, and several other stacking sequences. In all cases, the magnetic moment is found to be insensitive to the orientation of the nearest‐neighbor atoms, but instead can be related to the distance to the nearest atom in the direction perpendicular to the fault plane. Very simple empirical expressions for calculating the spin polarization and total energy of any stacking configuration are presented.

Nucleation of melting and solidification in confined high aspect ratio thin films

Classical nucleation theory is used to consider the solidification of a melt confined between two planar surfaces. The critical nucleus shapes and the associated nucleation energy barriers are computed as a function of the thickness of the film and the films relevant bulk and interface energies. The analysis is then repeated for the melting transition, and expressions for the depression and elevation of the melting temperature, relative to the thermodynamic bulk melting temperature of the film material, are found. A nucleus morphology diagram is constructed. This diagram presents the lowest energy morphology of the nuclei, as well as melting points, as a function of the system parameters. Using the nucleus morphology diagram, experimental and system parameters that allow for the desired nucleation behavior can be identified. Furthermore, the nucleus morphology diagram illustrates a region of parameter space where the film is predicted to solidify above its thermodynamic bulk melting temperature, a behavior termed presolidification. The theory is used to predict the temperature at which the nucleation of the solid phase and liquid phase is expected for Ge between two glass substrates. Furthermore, a possible route for controlling the orientation of the film is identified. By controlling the growth temperature, certain orientations may not be able to nucleate, thereby reducing the possible number of orientations within a film.Classical nucleation theory is used to consider the solidification of a melt confined between two planar surfaces. The critical nucleus shapes and the associated nucleation energy barriers are computed as a function of the thickness of the film and the films relevant bulk and interface energies. The analysis is then repeated for the melting transition, and expressions for the depression and elevation of the melting temperature, relative to the thermodynamic bulk melting temperature of the film material, are found. A nucleus morphology diagram is constructed. This diagram presents the lowest energy morphology of the nuclei, as well as melting points, as a function of the system parameters. Using the nucleus morphology diagram, experimental and system parameters that allow for the desired nucleation behavior can be identified. Furthermore, the nucleus morphology diagram illustrates a region of parameter space where the film is predicted to solidify above its thermodynamic bulk melting temperature, a behavi...

Self-consistent mean-field theory of size distribution narrowing during ramped temperature ion beam synthesis

A simple mathematical argument explains a recently identified route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained.