Alexander Valladares

Ab initio cluster simulation of N-doped tetrahedral amorphous carbon

Abstract The electronic structure of nitrogen-doped tetrahedral carbon clusters, both amorphous and crystalline, with 21, 57 and 59 carbon atoms and various ring topologies, were studied using the self-consistent ab initio density functional theory-local density approximation (DFT-LDA). All clusters were hydrogen saturated. Clusters with n =0,1 and 4 nitrogen atoms were analyzed for each structure using an initial interatomic distance of 0.154 nm as in the bulk. All clusters were energy optimized maintaining tetrahedral symmetry and the position of the outermost atoms in order to simulate the inertia of the bulk. For all the clusters the energy gap increases with one N. For the 21-atom cluster which contains only 6-atom boat-rings the gap remains practically constant as n increases from 1, 6.60 to 4, 6.51 eV, unlike the other clusters. The Fermi energy varies from the top of the valence band to the bottom of the conduction band as the nitrogen concentration increases. In the forced fourfold coordinated environment N is a shallow donor as is P in Si; however, when N becomes essentially threefold coordinated, states appear in the middle of the gap.

Amorphous clusters I. Electronic structure of Si clusters with N, P and As dopants

Amorphous impurity clusters of the type XSi 20 H 28 with X = N, P and As have been studied using the well-known pseudopotential SCF Hartree-Fock method (and the HONDO Program). The local electronic density of states and charge density contours have been obtained. It is found that the covalent nature of the bonding in undoped silicon is altered by the presence of the dopants and both an ionic component and a shielding effect appear when N, P and As are substituted in the center of the amorphous cluster. Also, the local density of states in the neighborhood of a Si atom, nearest neighbor to the center of the cluster, indicates the presence of a new p-state in the band gap. There are quantitative differences in the electronic structure of the clusters as a function of the dopants. These results are analyzed in the light of the local changes and their relevance to the amorphous solid state properties.

A New Approach to the Computer Modeling of Amorphous Nanoporous Structures of Semiconducting and Metallic Materials: A Review

We review our approach to the generation of nanoporous materials, both semiconducting and metallic, which leads to the existence of nanopores within the bulk structure. This method, which we have named as the expanding lattice method, is a novel transferable approach which consists first of constructing crystalline supercells with a large number of atoms and a density close to the real value and then lowering the density by increasing the volume. The resulting supercells are subjected to either ab initio or parameterized—Tersoff-based—molecular dynamics processes at various temperatures, all below the corresponding bulk melting points, followed by geometry relaxations. The resulting samples are essentially amorphous and display pores along some of the “crystallographic” directions without the need of incorporating ad hoc semiconducting atomic structural elements such as graphene-like sheets and/or chain-like patterns (reconstructive simulations) or of reproducing the experimental processes (mimetic simulations). We report radial (pair) distribution functions, nanoporous structures of C and Si, and some computational predictions for their vibrational density of states. We present numerical estimates and discuss possible applications of semiconducting materials for hydrogen storage in potential fuel tanks. Nanopore structures for metallic elements like Al and Au also obtained through the expanding lattice method are reported.

New Approaches to the Computer Simulation of Amorphous Alloys: A Review

In this work we review our new methods to computer generate amorphous atomic topologies of several binary alloys: SiH, SiN, CN; binary systems based on group IV elements like SiC; the GeSe2 chalcogenide; aluminum-based systems: AlN and AlSi, and the CuZr amorphous alloy. We use an ab initio approach based on density functionals and computationally thermally-randomized periodically-continued cells with at least 108 atoms. The computational thermal process to generate the amorphous alloys is the undermelt-quench approach, or one of its variants, that consists in linearly heating the samples to just below their melting (or liquidus) temperatures, and then linearly cooling them afterwards. These processes are carried out from initial crystalline conditions using short and long time steps. We find that a step four-times the default time step is adequate for most of the simulations. Radial distribution functions (partial and total) are calculated and compared whenever possible with experimental results, and the agreement is very good. For some materials we report studies of the effect of the topological disorder on their electronic and vibrational densities of states and on their optical properties.

Superconductivity in Bismuth. A New Look at an Old Problem.

To investigate the relationship between atomic topology, vibrational and electronic properties and superconductivity of bismuth, a 216-atom amorphous structure (a-Bi216) was computer-generated using our undermelt-quench approach. Its pair distribution function compares well with experiment. The calculated electronic and vibrational densities of states (eDOS and vDOS, respectively) show that the amorphous eDOS is about 4 times the crystalline at the Fermi energy, whereas for the vDOS the energy range of the amorphous is roughly the same as the crystalline but the shapes are quite different. A simple BCS estimate of the possible crystalline superconducting transition temperature gives an upper limit of 1.3 mK. The e-ph coupling is more preponderant in a-Bi than in crystalline bismuth (x-Bi) as indicated by the λ obtained via McMillan’s formula, λc = 0.24 and experiment λa = 2.46. Therefore with respect to x-Bi, superconductivity in a-Bi is enhanced by the higher values of λ and of eDOS at the Fermi energy.

The electronic structure of a-SiGe alloys: a cluster simulation

Abstract The electronic structure of 14 amorphous tetrahedral clusters, with 21 semiconducting atoms and 28 hydrogen saturators, of the type a-Si 1− x Ge x are studied using the pseudopotential SCF Hartree–Fock approximation. The Ge concentrations studied are 0, 5, 19, 38, 43, 57, 62, 81, 95 and 100%. For four concentrations (38, 43, 57 and 62%), two types of clusters are studied, Si-like and Ge-like, to investigate the influence of the size of the cluster on the gap. The results obtained indicate a diminishing magnitude of the gap as the Ge concentration increases, and for concentrations between 38 and 62%, a symmetric behavior of the gap is found centered at 50%; the variation of the gap in this region is less than 9%. These results are compared to the experimental ones reported in the literature. The effect of hydrogen on the size of the gap has also been studied both for pure Si and Ge and it is found that the gap increases when four hydrogens are substituted for the central atom in an otherwise unaltered cluster, in agreement with the experiment. The effect of dangling bonds is also reported.

Amorphous clusters: the electronic structure of Ge clusters with B and Al impurities

Abstract Amorphous clusters of the type XSi20H28 with X  B and Al have been studied using the pseudopotential SCF Hartree—Fock and the HONDO program. The local electronic density of states and the charge density were obtained for structures determined by energy optimization of the position of the central impurity, and of the position of the central impurity and the four nearest neighbors, using the AM1 method implemented in the MOPAC program. The structures obtained in this manner are such that the impurities are not located at the center of the clusters; instead both the boron and the aluminum are displaced towards three of the four nearest neighbors, a fact that indicates that there is a tendency for the impurity to remain with the same chemical valence in the cluster as when isolated. The gap energy states introduced by both B and Al are near the valence band and are p-like; the state due to aluminum is closer to the valence band than the one introduced by boron. These results are compared with recent experimental findings.

Amorphous clusters. II : Electronic structure of Ge clusters with N, P and As impurities

The electronic structure of random clusters has been used in the literature to qualitatively understand the properties of amorphous solids. Here impurity clusters of the type XGe20H28 with X = N, P, and As are studied using the pseudopotential SCF Hartree-Fock Method and the HONDO Program. The charge distribution and local density of states are calculated both for pure germanium and for clusters with each of the three impurities mentioned above. It is found that even though the covalent nature of the bonding of pure germanium is still present, its relevance is greatly diminished due to the presence of ionic components and shielding effects caused by the impurities. The charge distribution of the half-filled 57th orbitals clearly shows these effects. The LDOS exhibits the s- and p-components expected for the molecular orbitals of the clusters as well as two types of localized states; one is due to states localized at the bottom of the conduction band whose energies decrease as the mass of the dopants increases, and the other is due to deep states whose energies increase as the mass of the dopants increases. The top of the valence band is practically unaltered by the introduction of the impurities. A comparison is made of these results with experiment, and with those previously reported for amorphous Si clusters.

Computer Modeling of Nanoporous Materials: An ab initio Novel Approach for Silicon and Carbon

Carbon and silicon have been consistently proposed as elements useful in the generation of porous materials. Carbon has been insistently postulated as a promising material to store hydrogen, and crystalline silicogermanate zeolites have recently been synthesized and are being considered in catalytic processes. In the present work we report an approach to generating porous materials, in particular porous carbon and silicon, which leads to the existence of nanopores within the bulk. The method consists in constructing a crystalline diamond-like supercell with 216 atoms with a density (volume) close to the real value, then halving the density by doubling the volume (50% porosity), and subjecting the resulting supercell to an ab initio molecular dynamics process at 300 K for Si, and 1000 K for carbon, followed by geometry relaxation. The resulting samples are essentially amorphous and display pores along some of the “crystallographic” directions. We report their radial distribution functions and the pore structure where prominent.

First principles simulations of antiphase defects on the SP 90° partial dislocation in silicon

We study the structure and energies of formation of antiphase defects on the single period (SP) 90° partial dislocation in silicon using a first principles density functional method. We consider two types of antiphase defect, the type first proposed by Hirsch (1980 J. Microsc. 118 3) wholly inside the dislocation core, and another type that lies partly outside the core. Both types are stable and contain one atom which is threefold coordinated. Each of these atoms has a dangling hybrid which lies in a direction perpendicular to the dislocation line on the slip plane. We obtain values of 1.39 ± 0.03 eV and 1.41 ± 0.03 eV for the average formation energy of single antiphase defects of the inside and outside types, respectively. We have obtained, using a tight binding scheme, bandstructures corresponding to these two types of defect, and we find both of them to be associated with states in the gap and each dangling hybrid to contain one electron.