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We present molecular dynamics simulations directed at understanding self-limiting oxidation of nanoclusters. Atomic oxygen is inserted in an atom-by-atom way in the silicon bonds to form silicon oxide. First, we focus on planar oxidation to calibrate our model and test its capabilities. Then, we present results on oxidation of 50 Å diam silicon spheres. Kinetic causes of self-limitation are investigated by drawing a map of the local stress in the Si/SiO 2 system. We obtain stresses in contrast to in continuum models. For thin oxides, we find in particular tensile pressure in the silicon core and a pressure gradient in the oxide shell. We investigate the effect of pressure gradient on the O 2 transport within the framework of Nerst-Eintein's transport equation. We find that a pressure gradient compatible with experimental estimates yields self-limitation of the oxidation kinetics.

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We have studied the formation energy of the simplest oxygen defects in alpha-quartz, the oxygen vacancy and interstitial, by an ab initio approach based on density functional theory in the local density approximation. We have determined the formation energies and entropies and the migration paths and energies. From our results we can conclude that oxygen diffuses in quartz by an interstitial mechanism: the interstitial has a dumbbell structure; one of the constitutive atoms jumps towards a neighboring oxygen site. The activation energy amounts to 4.7 eV in the intrinsic regime and 2.8 eV in the extrinsic regime.

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The long-standing problem of the oxygen self-diffusion mechanism in silicon dioxide, a prototypical oxide, both in the crystalline and in the amorphous phase, is studied from first principles. We demonstrate that the widely used local-density approximation to density functional theory (DFT) predicts a kinetic behavior of oxygen in strong disagreement with available experiments. Applying a recently developed scheme that combines DFT with quasiparticle energy calculations in the G0W0 approximation considerably improves defect energetics and gives gratifying agreement with experiment.

The activation volume ΔV for chemical diffusion in Ag50Au50 alloy has been determined from the pressure dependence of the rate of decrease in amplitude of composition-modulated thin films. Based on resistivity measurements, it is argued that the technique yields the bulk chemical diffusion coefficient, in close agreement with the experimental results. Eight measurements of the diffusion coefficient were made, four as a function of temperature (237–309 °C) at atmospheric pressure and four as a function of pressure (0–0.85 GPa) at constant temperature (269.5 °C). The samples, 2000–3000-Å-thick multilayers of alternating silver and gold, with periods between 72 and 160 Å, were made with a newly built UHV sputter-deposition apparatus. The bulk chemical interdiffusion activation energy (1.5 eV) and activation volume (0.72 Ω) agree with those expected from other interdiffusion and tracer measurements.