No abstract

No abstract

The stability of the perfect screw dislocation in silicon has been investigated using both classical potentials and first principles calculations. Although a recent study stated that the stable screw was located both in the 'shuffle' and 'glide' sets of {111} planes (Koizumi et al, 2000, Phil. Mag. A, 80, 609), it is shown that this result depends on the classical potential used, and that the most stable configuration belongs to the 'shuffle' set only, in the centre of one ( 101) hexagon. We also investigated the stability of an sp 2 hybridization in the core of the dislocation, obtained for one metastable configuration in the 'glide' set. The core structures are characterized in several ways, with a description of the three dimensional structure, differential displacement maps, and derivatives of the dis-registry.

We present the results of a calculation of zero-temperature elastic conductance through a finite ''atomic wire'' between Au pads, all supported by a Si͑001͒-͑2ϫ1͒-H surface. The atomic wire consists of a line of dangling bonds which can be fabricated by removing hydrogen atoms by applying voltage pulses to a scanning tunneling microscopy ͑STM͒ tip along one side of a row of H-passivated silicon dimers. Two different line configurations, without and with Peierls distortion, have been considered. We find that the nondistorted line behaves like a single ballistic transmission channel. Conversely, with Peierls distortion present, tunneling occurs through the small resulting energy gap (0.2 eV), leading to inverse decay length of the current of 0.09 Å Ϫ1 . The conductance of the substrate between the pads without the defect line has also been calculated. In this case, tunneling occurs through a much wider energy gap and a larger inverse decay length of 0.41 Å Ϫ1 . These fully three-dimensional atomistic computations represent an application of the electron-scattering quantum-chemistry method which was previously used to calculate the conductance of ''molecular wires'' and of STM junctions with various adsorbates. Compared to molecular wires previously investigated by the same method, the atomic wire studied here exhibits the smallest inverse decay length. ͓S0163-1829͑99͒09123-7͔ PHYSICAL REVIEW B 15 JUNE 1999-II VOLUME 59, NUMBER 24 PRB 59 0163-1829/99/59͑24͒/15910͑7͒/$15.00 15 910

We have carried out an ab initio computational study of SiC nanoparticles with diameters between 1 and 3 nm. Our calculations show that surface composition and termination play a dominant role in determining the optical gaps and thermodynamic stability of these nanoparticles. In particular, we find that the optical gap of cubic SiC dots can be engineered as a function of their size and surface composition to obtain absorption and emission from the UV to the green. Our results suggest that SiC nanoparticles may be used to build new materials for semiconductor-based UV light sources.

Using first principles molecular dynamics simulations, we have determined the threshold displacement energies and the associated created defects in cubic silicon carbide. Contrary to previous studies using classical molecular dynamics, we found values close to the experimental consensus, and also created defects in good agreement with recent works on interstitials stability in silicon carbide. We carefully investigated the limits of this approach. Our work shows that it is possible to calculate displacement energies with first principles accuracy in silicon carbide, and suggests that it may be also the case for other covalent materials.

In order to control and tailor the properties of nanodots, it is essential to separate the effects of quantum confinement from those due to the surface, and to gain insight into the influence of preparation conditions on the dot physical properties. We address these issues for the case of small Ge clusters (1-3 nm), using a combination of empirical and first-principles molecular dynamics techniques. Our results show that over a wide temperature range the diamond structure is more stable than tetragonal, ST12-like structures for clusters containing more than 50 atoms; however, the magnitude of the energy difference between the two geometries is strongly dependent on the surface properties. Based on our structural data, we propose a mechanism which may be responsible for the formation of metastable ST12 clusters in vapor deposition experiments, by cold quenching of amorphous nanoparticles with unsaturated, reconstructed surfaces.In semiconductor nanoparticles quantum confinement leads to an increase of the optical gap compared to the bulk value and thus opens new possibilities for controlling photoluminescence effects, with narrow emission spectra tunable over a wide range of wavelengths [1,2]. These properties make semiconductor dots attractive for many applications including photovoltaics, lasers and infrared dyes. Furthermore, their brightness, low toxicity and the ability to use a single excitation wavelength make them good alternatives to organic dyes for biological labelling, but their low water solubility has limited biological applications. However, recent experiments have shown that using specific coatings, the surface of selected semiconductor nanodots can be tailored to enhance the chemical interaction with a biological sample and the water solubility [3,4].Understanding the influence of surface reconstruction and passivation on the ground state properties of semiconductor nanodots is a key prerequisite not only in designing biological applications, but also for controlling deposition of nanoparticles on surfaces and aggregation of multiple dots into new structures. In order to tailor the properties of nanodots, it is important to separate the effects of quantum confinement from those due to the surface, and to gain insight into the mechanism by which preparation conditions can influence the dot atomic structure and thus its optical properties.Here we address these issues for the case of small Ge dots (1-3 nm), whose atomic structure is the most controversial amongst those of group IV and II-VI semiconductors. While some preparation techniques, including chemical methods [5][6][7][8][9], yield diamond-like Ge dots irrespective of size, several experiments [10-12] suggest a structural transition, as the dot diameter becomes smaller than 4-5 nm. In particular, some experiments [13,14] using vapor deposition techniques indicate a change from a cubic diamond (DIA) to a tetragonal structure, possibly ST12, in contrast to the behavior found for Si [15][16][17] and other II-VI dots [18,19]. In the bulk, the ST...

Electron-beam exposure mechanisms in hydrogen silsesquioxane investigated by vibrational spectroscopy and in situ electron-beam-induced desorption Electron stimulated desorption of anionic fragments from films of pure and electron-irradiated thiophene Thermal desorption spectroscopy study of native and electron irradiated glycine overlayers on graphite (0001)