David A. Drabold

Energetics of large fullerenes : balls, tubes, and capsules

First-principles calculations were performed to compare the energies of 29 different fullerene structures, with mass number from 60 to 240, and of eight nonhelical graphite tubes of different radii. A quantity called the planarity, which indicates the completeness of the π-bonding, is the single most important parameter determining the energetics of these structures. Empirical equations were constructed for the energies of nonhelical tubes and for those fullerene structures that may be described as balls or capsules. For a given mass number, bail-shaped fullerenes are energetically favored over capsular (tube-like) fullerenes.

Structure and physical properties of paracrystalline atomistic models of amorphous silicon

We have examined the structure and physical properties of paracrystalline molecular dynamics models of amorphous silicon. Simulations from these models show qualitative agreement with the results of recent mesoscale fluctuation electron microscopy experiments on amorphous silicon and germanium. Such agreement is not found in simulations from continuous random network models. The paracrystalline models consist of topologically crystalline grains which are strongly strained and a disordered matrix between them. We present extensive structural and topological characterization of the medium range order present in the paracrystalline models and examine their physical properties, such as the vibrational density of states, Raman spectra, and electron density of states. We show by direct simulation that the ratio of the transverse acoustic mode to transverse optical mode intensities ITA/ITO in the vibrational density of states and the Raman spectrum can provide a measure of medium range order. In general, we conc...

Band gap engineering in amorphous AlxGa1−xN: Experiment and ab initio calculations

Amorphous alloys of aluminum nitride and gallium nitride deposited at 100 K at compositions ranging from pure AlN to pure GaN with optical band gaps which vary linearly with composition from 3.27 eV (a-GaN) to 5.95 eV (a-AlN) have been synthesized. Ab initio molecular dynamics calculations for these alloys reproduce the band gap versus composition data and give specific information on the electronic localization of the band tail states. There are no midgap states in amorphous AlxGa1−xN alloys. The calculated models have mixed four-fold and three-fold coordination and have no wrong (homopolar nuclear) bonds, demonstrating the strong ionicity in amorphous AlxGa1−xN alloys. It has been found that the valence band tail states are mostly localized on the three-fold coordinated N sites while the conduction band tail states are mostly localized on the three-fold coordinated Ga or Al sites.

Defects, doping, and conduction mechanisms in nitrogen-doped tetrahedral amorphous carbon

First principles methods are used to study N doping of diamondlike amorphous carbon. A structural model containing 216 atoms is introduced, whose properties are in agreement with the available experimental data. The topological and electronic properties for different N doping concentrations are investigated. We find that N occurring in tetrahedral sites or chains of an even number of π bonded sites results in an increase of the Fermi energy, while N incorporation in strained network sites induces structural changes that lead to an increase in the sp2 fraction of the material. The prevalent conduction mechanisms are identified and discussed. While the Fermi energy increases upon N doping, the localization of the conduction-band-tail states limits extended state conduction. These results are compared to the recent experimental reports on N doping of ta-C and we find that the nondoping threefold N incorporation (N30) is energetically most likely, which explains the low doping efficiency seen in experiments.

Observation of light polarization-dependent structural changes in chalcogenide glasses

The atomistic origin of photoinduced vector (polarization-dependent) phenomena in As–Se films is determined by extended x-ray absorption fine structure with in situ exposure to polarized laser light. A vector structural change is observed directly for any material: there is an expansion of the nearest-neighbor distance around the Se atoms, the magnitude of which depends on the direction of light polarization; the effect around As atoms is relatively smaller. The results point to the origin of scalar as well as vector changes in properties, which either persist after the light is removed, or exist only when light is incident on the sample.The atomistic origin of photoinduced vector (polarization-dependent) phenomena in As–Se films is determined by extended x-ray absorption fine structure with in situ exposure to polarized laser light. A vector structural change is observed directly for any material: there is an expansion of the nearest-neighbor distance around the Se atoms, the magnitude of which depends on the direction of light polarization; the effect around As atoms is relatively smaller. The results point to the origin of scalar as well as vector changes in properties, which either persist after the light is removed, or exist only when light is incident on the sample.

Density functional studies of small platinum clusters

We present results of a density functional theory study of the structure and energetics of small Pt clusters. The method of calculation is based on the non-selfconsistent Harris functional version of LDA (as formulated by Sankey and Niklewski, and generalized to include d orbitals in the basis set), which produces excellent results for bulk Pt. We used a dynamical quenching algorithm to obtain minimum-energy structures of clusters for n = 2 - 6. The clusters with n = 4 - 6 are shown to be planar. For we found that there is a variety of low-symmetry geometries that are lower in energy than the icosahedral and cubo-octahedral structures. We also compute the vibrational states of n = 2 - 4, and show that the calculated vibrational frequency and bond energy of the Pt dimer are in good agreement with experiments.

Physical properties of a GeS 2 glass using approximate ab initio molecular dynamics

With the use of ab initio based molecular dynamics simulations we study the structural, dynamical and electronic properties of glassy g-GeS 2 at room temperature. From the radial distribution function we find nearest neighbor distances almost identical to the experimental values and the static structure factor is close to its experimental counterpart. From the Ge-S-Ge bond angle distribution we obtain the correct distribution of corner and edge-sharing GeS 4 tetrahedra. Concerning the dynamical characteristics we find in the mean square displacement of the atoms discontinuous variations corresponding either to the removal of coordination defects around a single particle or to structural rearrangements involving a larger number of atoms. Finally we calculate the vibrational density of states, which exhibits two well separated bands as well as some features characteristic of the amorphous state, and the electronic density of states showing an optical gap of 3.27 eV.

Density dependence of the structural and electronic properties of amorphous GaN

Abstract We have performed quantum molecular-dynamics to simulate amorphous GaN. The network models at different densities are generated in such a way that the charge transfer between ions is included self-consistently throughout the simulation. The pair distribution function indicates that the network models are topologically disordered, and the detailed structural analysis implies the existence of a certain chemical short-range order. An important property is that the network models (64 atoms in a supercell) have no wrong pairs (homopolar bonds) or odd-membered rings in all the densities studied, indicating the strong ionicity in amorphous GaN. The valence band tail states are mostly localized on the threefold coordinated N sites and the conduction band tail states are mostly localized on the threefold coordinated Ga sites. There are no midgap states at any density, and the band gap is 2.6–3.6 eV depending on the density. The possible existence of amorphous GaN is suggested in a small range of density near 90 % of experimental value of wurtzite GaN.

Direct ab-initio molecular dynamic study of ultrafast phase change in Ag-alloyed Ge2Sb2Te5

We employed ab-initio molecular dynamics to directly simulate the effects of Ag alloying (less than 5% Ag concentration) on the phase change properties of Ge2Sb2Te5. The short range order is preserved, whereas a slight improvement in the chemical order is observed. A slight decrease in the fraction of tetrahedral Ge (sp3 bonding) is reflected in the reduction of the optical band gap and in the increased dielectric constant. Simulations of the amorphous to crystalline phase change cycle revealed the fact that the crystallization speed in Ag0.5Ge2Sb2Te5 is not less than that in Ge2Sb2Te5. Moreover, the smaller density difference and the larger energy difference between the two phases of Ag0.5Ge2Sb2Te5 (compared to Ge2Sb2Te5) suggest a smaller residual stress in devices due to phase transition and improved thermal stability for Ag0.5Ge2Sb2Te5. The potential viability of this material suggests the need for a wide exploration of alternative phase change memory materials.

Strain relaxation mechanisms and local structural changes in Si 1 − x Ge x alloys

In this work, we address issues pertinent to the understanding of the structural and electronic properties of