Showing papers by "Roberto Car published in 1996"
TL;DR: A first-principles investigation of Si 2p core-level shifts at the Si(001)-SiO2 interface finds the shifts are found to be essentially insensitive to second and further neighbors in the structure, confirming the traditional interpretation of the photoemission spectra based on a charge-transfer model.
Abstract: A first-principles investigation of Si 2p core-level shifts at the Si(001)-SiO2 interface is presented. We introduce several relaxed interface models obtained by attaching different crystalline forms of SiO2 to Si(001). These model structures contain the minimal transition region required to accommodate the three intermediate oxidation states of silicon, in accord with photoemission experiments. The bond density mismatch is fixed by saturating all the bonds, as required by electrical measurements, Calculated core shifts are primarily affected by the number of nearest-neighbor oxygen atoms, showing a linear dependence. This result confirms the traditional interpretation of the photoemission spectra based on a charge-transfer model. Core relaxation plays a significant role accounting for more than 50% of the total shifts. The shifts are found to be essentially insensitive to second and further neighbors in the structure. Structural deformations, such as those implied by the distribution of Si-O bond lengths in a-SiO2, yield distributions of core-level shifts that an too small to account for the observed width of the photoemission peaks. In the oxide, we observe a spatial dependence of the Si+4 shifts with distance from the interface plane. We relate this behavior to the dielectric discontinuity at the interface and suggest that this effect explains the shift of the Si+4 with oxide thickness, observed in photoemission experiments.
180 citations
TL;DR: Comparison to all-electron results shows that in both GGA schemes the validity of the pseudopotential approach is as good as in LDA.
Abstract: Pseudopotential calculations within density-functional theory for a few selected solids (Si, GaAs, and Al) are used to assess the validity of two generalized-gradient approximations (GGA's), the one proposed by Becke and Perdew (BP) and the more recent one proposed by Perdew and Wang (PW) in comparison with the currently used local-density approximation (LDA) The GGA's give total energies of atoms and cohesive energies of solids that are closer to experiment than the LDA results Lattice constants are reproduced with the same accuracy as in LDA, while bulk moduli and zone-center phonon frequencies are underestimated with respect to both the LDA and the experimental values Comparison to all-electron results shows that in both GGA schemes the validity of the pseudopotential approach is as good as in LDA The predictions of the two GGA's are similar, the PW functional yielding results marginally, but systematically, closer to experiment The calculated values of the transition pressure in Si between the diamond and the beta-tin structure are 72 kbar (LDA), 164 kbar (BP), and 135 kbar (PW), to be compared with the two available experimental values of 103 and 125 kbar
172 citations
Cray1
TL;DR: In this paper, the results of first-principles molecular dynamics simulations of a surface-induced diamond-to-graphite transition are presented, which provide a microscopic model for the early stages of the graphitization process.
Abstract: GRAPHITIZATION of diamond at ambient pressure was first observed in the 1920s1,2, but the mechanisms responsible for this transformation and, in particular, those underlying the nucleation and growth of graphite in diamond, remain controversial3–5. In addition to their fundamental interest, these processes have technological relevance—for example, for the growth by chemical vapour deposition6 of diamond-like films, which sometimes include graphitic islands7. Here we report the results of first-principles molecular dynamics simulations of a surface-induced diamond-to-graphite transition, which provide a microscopic model for the early stages of the graphitization process. We find that a well defined diamond/graphite interface forms during the transition; the electronic properties of the atoms at this interface suggest that they are highly chemically active sites. In addition to its relevance to graphite inclusion in diamond films, our model should yield insight into the process of selective etching in vapour-deposited carbon films, and possibly also into diamond nucleation.
105 citations
TL;DR: In this article, structural properties of two models for the Si(001)-SiO2 interface were investigated, which derive from attaching tridymite, a crystalline form of SiO2, to Si(1) and then allowing for full relaxation.
Abstract: We present a first‐principles investigation of the structural properties of two models for the Si(001)–SiO2 interface The models derive from attaching tridymite, a crystalline form of SiO2, to Si(001), and then allowing for full relaxation These models do not show electronic states in the silicon gap, as required by electrical experiments They contain the three intermediate oxidation states of silicon, consistent with photoemission experiments We study bond length and bond angle distributions and measures of local strain The strain is localized to a transition region at the interface Strain does not persist in the full oxide
94 citations
TL;DR: Compared to the trydimite models, the longer Si-O bonds found at the interface of the beta-cristobalite derived model suggests that the latter is a higher energy structure.
35 citations
TL;DR: Using a first-principles approach, Si 2p core-level shifts for spherosiloxane H8Si8O12 clusters chemisorbed on Si(001) are calculated, showing agreement with the position and width of the peak at high binding energy in the photoemission spectrum.
Abstract: Using a first-principles approach, we calculate Si 2p core-level shifts for spherosiloxane H8Si8O12 clusters chemisorbed on Si(001). A chemisorption mechanism that preserves the cage structure of the clusters is assumed. The resulting relaxed surface structure yields shifts consistent with the position and width of the peak at high binding energy in the photoemission spectrum. The role of dielectric effects on core-hole relaxation is essential to achieve this agreement. The other experimentally observed peaks at lower binding energies could not be accounted for by the chemisorption model considered here, suggesting that the actual surface structure is more complex.
30 citations
TL;DR: An approach to investigate thermal effects on the photoelectron spectra of small clusters by combining first-principles molecular dynamics and a simplified scheme to account for final-state relaxation effects is introduced and averaged excitation spectra at finite temperature are obtained.
Abstract: We introduce an approach to investigate thermal effects on the photoelectron spectra of small clusters. By combining first-principles molecular dynamics and a simplified scheme to account for final-state relaxation effects, we obtain averaged excitation spectra at finite temperature. We apply our approach to the case of Cu-7(-), in which two isomers are found very close in energy at T = 0 K. At T = 400 K, the isomer of D-5h symmetry transforms into one of C-3v symmetry. This behavior is in accord with the observed photoelectron spectra in which the predominant features can be associated with the C-3v isomer. The averaged spectrum at T = 400 K for this isomer suggests that the splitting observed in the peak at lowest excitation energies is a thermal effect.
22 citations
Cray1
TL;DR: In this paper, a series of fully ab initio molecular dynamic simulations of the diamond C(111)-(2 sx 1) surface, with cells containing from 200 to 300 atoms, was performed, and the transition started at the reconstructed surface layer and rapidly proceeds into the bulk region by highly correlated breaking of z-oriented diamond bonds.
18 citations
TL;DR: In this article, a first-principles approach was used to study Si 2p core-level shifts at water and hydrogen covered Si(001)2×1 surfaces.
Abstract: Using a first‐principles approach, we study Si 2p core‐level shifts at water and hydrogen covered Si(001)2×1 surfaces. After allowing for full relaxation of the surface structures, core‐level shifts are calculated including core‐hole relaxation effects. We find that dissociated water on the Si(001)2×1 surface induces a core‐level shift of 1.1 eV to higher binding energies, in good agreement with experiment. In the case of the hydrogen terminated Si(001)2×1 surface, calculated surface shifts are small (about 0.2 eV) and comparable to shifts of subsurface Si atoms.
13 citations
TL;DR: It is shown that the method can be used in cases in which electrons have to be decoupled from the system and illustrated with results for the formation free energy of the Si vacancy.
Abstract: We illustrate a method for performing first-principles free-energy calculations within the Kohn-Sham scheme. We show that the method can be used in cases in which electrons have to be decoupled from the system and illustrate it with results for the formation free energy of the Si vacancy. The results agree well with the available data from experiments and ab initio calculations. \textcopyright{} 1996 The American Physical Society.
12 citations
TL;DR: In this paper, core-level shifts for a series of molecules are calculated within the local density approximation to density functional theory using a pseudopotential approach, both in the initial-state approximation and including core-hole relaxation effects.
Abstract: Si 2p core-level shifts for a series of molecules are calculated within the local density approximation to density functional theory. Using a pseudopotential approach, the shifts are calculated both in the initial-state approximation and including core-hole relaxation effects. Overall, calculated Si 2p shifts show good agreement with experiment.
TL;DR: In this article, a first-principles molecular dynamics investigation of the atomic motion in Silicon in the presence of an artificially created vacancy at high temperature (T ≥ 1200 K) is presented.
Abstract: We report a first-principles Molecular Dynamics investigation of the atomic motion in Silicon in the presence of an artificially created vacancy at high temperature (T ≥ 1200 K). We observe that atomic diffusion events are affected by strong dynamical correlations. At temperatures close to the melting point we discover characteristic premelting phenomena which involve simultaneous jumps of several atoms and introduce a large amount of disorder in the structure.
TL;DR: In this paper, equilibrium geometries and electronic properties of neutral and anionic Cu-n (n = 2, 10) clusters are determined via first-principles calculations which treat s and d electrons on an equal footing.
Abstract: Equilibrium geometries and electronic properties of neutral and anionic Cu-n (n = 2, 10) clusters are determined via first-principles calculations which treat s and d electrons on an equal footing. We find cluster shapes similar to those reported in the literature for Na-n clusters, but the highly coordinated structures are energetically preferred. Electronic states with atomic s character are strongly hybridized with d states and located mostly at the band edges. Angular decomposition of the electronic wave functions shows that the predictions of the shell model are followed only approximately in Cu-n clusters. Finally we interpret successfully the photoelectron spectrum of Cu-3(-) by accounting for final-state effects.