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The relative energy stability between Si(111)2 × 1 positive and negative buckling is still a puzzle. Both isomers possess comparable energies in Density Functional Theory (DFT) calculations and, hence, should occur with the same probability. This is in contrast to experimental findings, which show that the positive buckling is the observed structure at low and room temperature. Here we investigate the role of the exchange-correlation (xc) approximation in DFT functionals. We find that the particular choice of the xc functional does not change the relative stability between the Si(111)2 × 1 isomers. Moreover, we investigate the effect of donor atoms on ground-state and optical properties of both isomers, highlighting the differences with respect to the undoped case for several possible positions of the phosphorus (P) donor atoms. Considering the most stable position of P, relative stability between the isomers is not strongly affected by doping whereas geometric parameters are different with respect to the no doping case. The optical response bears some resemblance with the undoped case but the intensity of reflectance anisotropy peak is decreased for both isomers. © 2013 Elsevier B.V.

The low-temperature phase of Sn/Ge(1 1 1) 3 × 3 is studied by means of first-principle calculations at the DFT level and in the GW approximation. Experimental STM images taken at 80 K are presented and compared with theoretical predictions. This analysis allows to give support to a geometry in which one Sn adatom is higher than the other two in the surface unit cell and excludes the complementary geometry in which two Sn adatoms are higher than the third. Concerning the very low temperature phase (T < 30 K), we show that from a purely energetic point of view a sqrt3xsqrt3 geometry cannot be excluded, being almost degenerate with the 3 × 3 phase.

The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B-P co-doping results to be easier than simple 13- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcorne. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron-hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the self-energy correction and solving the Bethe-Salpeter equation scheme. Our procedure shows the important role played by the many-body effects. (C) 2006 Elsevier B.V. All rights reserved.

The dependence of the InP(001) surface reconstruction on the chemical potentials of its constituents is explored. Based on first-principles pseudopotential plane-wave calculations the surface phase diagram is constructed. 17 structural models are studied for the phosphorus-rich InP(001) surfaces, with (2×1), p(2×2) and c(2×2) translational symmetry. P-top dimer reconstructions are favoured. Under less P-rich preparation conditions a tendency for disorder is predicted. STM images are also calculated in order to contribute to a solution of the structural puzzle.