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ABSTRACT:Total energy calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) and ultrasoft pseudopotential approximation and an analysis tool of atom-resolved density of states (ADOS) have been used to investigate (1) the energetic profiles for the possible initial dissociative adsorption of XH 4 (XASi and Ge) onto the Si(100)O(2 ϫ 2) surface to evaluate their reactivity and (2) the effect of surface electronic states of Si(100)O(2 ϫ 2) on gaseous molecular precursors XH 4 (XASi and Ge) during initial dissociative adsorption to understand the factors governing their reactivity. Our calculated lower-energy barrier for initial dissociative adsorption of GeH 4 is due to the forming of stronger bond of SiOH between H within GeH 4 and buckled-down Si atom on the Si(100)O(2 ϫ 2) surface accompanying the larger extent of unbuckling of the buckled SiASi dimer on the Si(100)O(2 ϫ 2) surface at the transition state. Our evaluated better reactivity for GeH 4 than SiH 4 (a factor of around 14.6) is slightly larger than observed higher reactivity for GeH 4 than SiH 4 (a factor of between 2 and 5 depending on the incident kinetic energy) employed supersonic molecular bean techniques. Finally, our calculated ADOS indicate that the surface electronic states of buckled SiASi dimer on the Si(100)O(2 ϫ 2) surface energetically favorably participate in the transition state during GeH 4 initial dissociative adsorption to reduce the energy barrier, i.e., enhance its reactivity, in comparison with SiH 4 initial dissociative adsorption onto the Si(100)O(2 ϫ 2) surface under the same reaction conditions.

Molecular dynamics, a computational technique aiming to describe the time-dependent movement of molecules, has been applied to study the sorption kinetics of volatile organic contaminants in soil organic matter. The molecular dynamics simulation results obtain reasonably accurate estimates of diffusion rates and activation energy of the penetration of a volatile organic compound molecule into a model humic substance. The sorption rate of toluene to humic acid decreases with the density of the humic acid matrix and increases with temperature. All the present results indicate that the sorption of toluene into humic acid is mainly diffusion controlled. Finally, the present studies have shown that molecular dynamics of volatile organic compounds in humic substances yields meaningful results, which help in the understanding of diffusion at the molecular level and which facilitate the problem-solving capability for removing the contaminants from the soils.

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Ultrasoft pseudopotential total energy calculation based on density functional theory (DFT) with generalized gradient approximation (GGA) has been used to investigate 1) the energetic profile for the initial dissociative adsorption of XH4 (X = Si and Ge) onto Si(001) and Ge(001) surfaces to evaluate their gas‐surface reactivity in comparison with relevant measured gas‐surface reactivity using supersonic molecular beam techniques, and 2) the effect of different gaseous molecular precursors, i.e. XH4 (X = Si and Ge), and different surfaces, i.e. Si(001) and Ge(001), on their gas‐surface reactivity during initial dissociative adsorption. Our evaluated gas‐surface reactivity for GeH4 is approximately a factor of 18.45 better than that for SiH4 on Si(001)‐(2×2) surface. This calculated result is about three to four times higher than observed gas‐surface reactivity (as much as a factor of 5 depending on the incident kinetic energy) derived from measured gas‐surface reactivity using supersonic molecular beam techniques. We believe that the better evaluated gas‐surface reactivity for GeH4 than SiH4 is due to 1) the forming of a stronger bond of Si‐H between H within GeH4 and buckled‐down Si atom on the Si(001)‐(2×2) surface and 2) the smaller distortion of Ge‐H bond within GeH4 at the transition state. Additionally, our evaluated gas‐surface reactivity for SiH4 on Si(001)‐(2×2) surface is approximately a factor of 21.69 better than SiH4 on Ge(001)‐(2×2) surface. This calculated result is about two times higher than observed gas‐surface reactivity. We attributed this better evaluated gas‐surface reactivity for SiH4 on Si(001)‐(2×2) surface to 1) the smaller distortion of Si‐H bond within SiH4 and 2) the nature of weaker bond of Ge‐H between H within SiH4 and buckled‐down Ge atom on Ge(001)‐(2×2) surface in comparison with that of stronger bond of Si‐H between H within SiH4 and buckled‐down Si atom on Si(001)‐(2×2) surface even though there is the slightly shorter bond length of Ge‐H between H within SiH4 and buckled‐down Ge atom on Ge(001)‐(2×2) surface at the transition state.