Impingement Of Atoms Research Articles

Relevance of silicate surface morphology in interstellar H2formation. Insights from quantum chemical calculations

The adsorption of H atoms and their recombination to form an H2 molecule on slab models of the crystalline Mg2SiO4 forsterite (001) and (110) surfaces was studied by means of quantum mechanical calculations based on periodic density functional theory (DFT). Present results are compared with those previously reported for the most stable (010) surface, showing the relevance of the surface morphology and their stability on the H2 formation. Different H chemisorption states were identified, mostly on the outermost O atoms of the surfaces. In agreement with the higher instability of the (001) and (110) surfaces, the calculated adsorption energies are larger than those for the (010) surface. Computed energy barriers for the H hopping on these surfaces are exceedingly high to occur at the very low temperatures of deep space. For the adsorption of two H atoms, the most stable complexes are those in which the H atoms form Mg-H and SiOH surface groups. From these complexes, we did not identify energetically feasible paths for H2 formation through a Langmuir–Hinshelwood mechanism on the (001) surface because the initial states are more stable than the final products. However, on the (110) surface one path was found to be exoergic with very low energy barriers. This differs to that observed for the (010) surface, for which two feasible Langmuir–Hinshelwoodbased channels were identified. H2 formation through the Eley–Rideal mechanism was also simulated, in which an incoming H atom impinges on a pre-adsorbed H atom at the (001) and (110) surfaces in a barrierless way.

Kinetics of axial composition evolution in multi-component alloy nanowires

The axial composition profiles in two-component alloy semiconductor nanowires are theoretically studied based on a comprehensive transient growth model which accounts for both surface diffusion and direct impingement of atoms to catalyst. The composition variation derives from the different growth rates contributed by each component. Our simulations reveal that the component with larger (smaller) diffusivity will segregate near the bottom (top) of the nanowire. In the presence (absence) of direct deposition on nanowire sidewalls, the steady state alloy composition is determined by the ratio of effective diffusion lengths (impingement rates to the catalyst).

Eley-Rideal formation of H 2 involving one of two para-chemisorbed H atoms on a graphite surface

Following the work of Rougeau et al. [N. Rougeau, D. Teillet-Billy, V. Sidis, Chem. Phys. Lett. 431 (2006) 135] we study the title reaction when a gas phase H atom impinges on one of two para-chemisorbed H atoms on a graphite surface. DFT calculations of the PES and QCT dynamics calculations are carried out for the on top approach. We find that the studied reaction is barrier-less. The vibrational energy of the nascent H 2 molecule is centered on the v = 5 level which is significantly lower than what is found for the singly chemisorbed H atom case; also, the energy left in the C atom vibration at the surface is somewhat larger.

Evaporation and impingement effects on drift-induced step instabilities on a Si(001) vicinal face

We theoretically study the effect of evaporation and impingement of atoms on step wandering induced by the drift of adatoms. With a Si(001) vicinal face in mind, the anisotropy in diffusion coefficient is assumed to alternate on consecutive terraces. Without evaporation, steps wander in-phase with step-up drift and grooves perpendicular to the steps appear. The form of the wandering steps is sinusoidal with the width increasing in time as ${t}^{1∕2}$. Evaporation of adatoms suppresses the step wandering and introduces two surface diffusion lengths. When they are longer than the step distance, the step width still increases in proportion to ${t}^{1∕2}$, but with a smaller coefficient than that in the case without evaporation. When one of the surface diffusion lengths is comparable or shorter than the step distance, the saturation of the step width occurs. Impingement of atoms, on the other hand, changes the form of the wandering steps: their front becomes flat and wide and the grooves become steep and narrow. The growth rate of the step width becomes small, but the step width increases with the same exponent $1∕2$.

First-principle molecular-dynamics simulations of the sticking of Ga and N gas-phase atoms on wurtzite GaN surfaces

Abstract GaN is an important material of technological and fundamental interest. The first step to understand the growth of GaN thin films is to study the sticking properties of gas-phase Ga and N atoms on the GaN surface. Using first-principles molecular-dynamics simulations, we find that the Ga gas-phase atom moves very slowly onto the N-terminated GaN(000 1 ) surface, while the N gas-phase atom moves very fast onto the Ga-terminated GaN(000 1 ) surface. The incoming Ga atom is found to recoil at the first descent and then it drops down again toward the atop, H3, or T4 site depending on the initial condition. When the incoming N atom impinges on a Ga adatom or on the bridge site between two Ga adatoms, it is also found to recoil at the first descent. When it drops down again, it traverses to an opening in the Ga adlayer and penetrates down to the 1st N layer beneath. The incoming N atom eventually escapes back to vacuum when it initially impinges on a Ga adatom. When it initially impinges on the bridge site, it can traverse over a large distance and remains on the surface. Our results suggest that the incoming Ga atom is relatively inert, while the incoming N atom is very volatile. Our results also suggest that the sticking coefficient of N be much smaller than that of Ga.

Quantum scattering calculations of energy transfer and isomerization of HCN/HNC in collisions with Ar

Quantum scattering calculations are reported for zero impact parameter collisions of Ar with HCN/HNC for three fixed angles of attack of Ar with respect to the CN axis, in the total energy range 16 000–20 000 cm−1, and using a new Ar–HCN interaction potential based on ab initio data. We find that this interaction potential only weakly couples localized HCN and localized HNC states. As a result, although isomerization is energetically possible in much of the energy range considered, the probability of collision-induced isomerization is found to be small. Detailed analysis of our scattering results shows that “head on” collisions in which the Ar atom impinges on the H end of the molecule are more effective in promoting T→V energy transfer than are “nearly perpendicular” and “tail on” (opposite to the H atom) collisions. Significant energy transfer processes between translation and vibration involve the bending mode ν2 either through pure bend excitation/deexcitation or through smaller ΔE processes in which a larger number of ν2 quanta are exchanged for a smaller number of ν1 or ν3 quanta. Examination of our distributions of inelastic transition probabilities for highly excited states, including a delocalized state, suggests that they mimic a biexponential gap distribution.

Decay of an Island on a Facet via Surface Diffusion

We study the decay process of a small island formed on a facet by numerical integration of a simple model and by theoretical analysis. The relaxation proceeds via surface diffusion without evaporation and impingement of atoms. Steps on the top of the island shrink due to the step tension and a small facet appears. The size of the facet increases as the power of time R f ∼ t ν , where the exponent ν depends on the initial shape and the rate limiting process. In the kinetics-limited case, step bunching occurs from the bottom and the enlargement of the facet stops at some stage. Analytical expressions for the bunch velocity and the island profile are found.

Characterisation of thick film Ti/Al nanolaminates

Titanium/aluminium nanolaminates, with repeat periods between 5 nm and 10 mm and an overall deposit thickness of up to 2 mm, have been produced by physical vapour deposition using both thermal and electron beam evaporation followed by quenching onto a rotating, heated substrate. Deposits were characterised using a range of techniques including energy filtered transmission electron microscopy and conventional electron energy loss spectroscopy. The microstructures of the nanolaminates was shown to vary according to substrate temperature and the relative thickness of the layers. All the samples contained a range of intermetallics both at the layer interfaces and in regions where the layered structure had broken down. Theories have been postulated to account for the presence of the intermetallics, including atomic impingement during deposition and diffusion. Fcc Ti was also observed in cross-sectional TEM specimens.

Surface Treatment Effect on the Grain Size and Surface Roughness of as‐Deposited LPCVD Polysilicon Films

In this work we report on the effect of the substrate treatment on the grain size and the surface roughness of as‐deposited polycrystalline silicon films obtained by low pressure chemical vapor deposition. Deposition of silicon films was performed at 570°C on oxidized silicon substrates which were subjected to different wet surface treatments. We found that the grain size of the as‐deposited polysilicon films was significantly enhanced for films deposited on freshly oxidized silicon wafers, which were loaded immediately for deposition. This was attributed to the impurity‐free surface of the substrate, resulting in a lower nuclei density and subsequently in the development of a larger grain size. The surface roughness of these films, however, was found to be significantly higher than that of films deposited on substrates subjected to a wet surface treatment prior to deposition. The roughness was found to correlate well with the mechanism which predominantly determines the growth of the stable nuclei. When direct impingement of atoms on the nuclei predominates, a wave‐like surface develops while when lateral growth dominates, a smooth surface develops. We found that a smooth surface can be achieved either by subjecting the substrate in a wet surface treatment prior to deposition, which effectively decreases the surface diffusion length of the silicon adatoms and thus increases the density of the initial stable nuclei, or by suitably decreasing the deposition rate at a given temperature in the case of untreated substrates.

Epitaxy on surfaces vicinal to Si(001). II. Growth properties of Si(001) steps.

There is considerable interest in understanding the factors important for epitaxial growth. In particular, one would like to know how aspects of the growth kinetics affect the structures produced. To study epitaxial growth on the Si(001) surface, we have mapped out the potential-energy surfaces seen by a single silicon adatom, as it moves over the Si(001) steps. Standard molecular-dynamics methods were used. The energy surfaces identify the binding sites and give the activation energy for diffusion hops, so that likely pathways for adatom motion can be identified. We find that generally the binding at the step edges is about one-tenth of an eV more than that on the flat surface, so that at low adatom coverage, these steps are relatively weak sinks. Growth, however, does take place more readily at the step edges, because the steps act as heterogeneous nucleation sites for dimers. We have developed simple rate equations to estimate nucleation rates, accommodation coefficients, and step velocities, using the microscopic information given by the energy surfaces as input. The variation of these quantities with temperature and external driving force is explored. In agreement with experimental results, we find that growth takes place most readily at the ends of dimer rows. Furthermore, the predicted anisotropy in the growth properties of the ${\mathit{S}}_{\mathit{A}}$ and ${\mathit{S}}_{\mathit{B}}$ steps is large enough to account for the formation of dimer strings on the flat terraces. We also report on direct simulations of the impingement of atoms from an atomic beam onto the vicinal Si(001) surfaces. These simulations show the formation of kink sites at step edges and the formation of dimer strings on the terraces.

Adsorbate-mediated gas-surface energy transfer: Collision geometry and surface temperature influences

We report here a classical trajectory study of how changes in the angle of incidence and the surface temperature alter the energy transfer occuring when an argon atom impinges on a partially covered tungsten surface. Our calculations suggest that while an increase in the surface temperature has only a negligible effect on the net energy exchange, the dependence on the collision geometry can be more significant.

Quantum theory of atom-surface scattering: Debye-Waller factor

The Debye-Waller factor, introduced historically for X-rays, was used later for electrons, neutrons, and atoms as well. In this process of extension, however, the assumptions on which the Debye-Waller theory rested became more and more questionable until in the case of atoms (whose scattering from surfaces is both strong and slow) serious modifications are necessary. In the present article four models are discussed in order. In Model 1 a fast atom impinges on a surface whose atoms all vibrate deviating from their equilibrium positions by the same vector displacement ϱ. In Model 2 again the impinging atom is fast, but the atoms in the surface vibrate incoherently rather than coherently. It is shown that both Models 1 and 2 yield the conventional Debye-Waller result in the infinite crystal atom mass limit (for Model 2 Einstein oscillators have also to be assumed) and it is also shown how corrections to this result can be built. Turning then to slow impinging atoms, in Model 3 a slow atom impinges on a hard crystal surface, interacting with the rapidly varying potential of the vibrating solid. Model 3 is discussed in detail and it is shown that the Debye-Waller exponent can be written in terms of a time integral of the product of two correlations: the force correlation and the displacement correlation. The result is a dramatic increase of diffraction of relatively heavy atoms (with respect to the conventional theory). Finally, in Model 4 the impinging atom is again slow but the crystal is soft rather than hard. This case is more difficult to treat but a preliminary analysis again indicates a dramatic increase of diffraction since the soft solid adjusts itself to the instantaneous atom position leading to elastic scattering. The experimental implications of the present theory, especially for neon scattering from surfaces, are discussed.

The effect of porous barriers on the molecular composition and total flux of a reactive gas mixture

When a gas mixture that is governed by a pressure dependent equilibrium is passed through a porous barrier, the exit molecular fluxes are functions of the equilibrium constant and the atomic concentrations at the exit face of the barrier rather than of the molecular weights and impingement pressures as has been tacitly assumed for Knudsen flow conditions. It is shown that whether or not molecules of the gas react with each other, the ratio of the total atomic escape flux of each elemental constituent to the total atomic impingement flux of that element for Knudsen flow should be a/l, where l is the barrier thickness and a is the same constant, of the order of the average diameter of barrier pores for each element in the gas mixture. The equilibrium prediction is confirmed for the sodium chloride monomer/dimer equilibrium mixture passed through alumina barriers with pores of the order of 1–10 μ diameter and various thicknesses. Measured dimer fluxes are reduced by the barriers to less than 10−3 times the dimer fluxes in conventional effusion; yet the measured fluxes agree to within about 30% with predictions from the measured monomer fluxes and the monomer/dimer equilibrium constant. The prediction that total atomic flux reductions for different vapors have the same dependence on a/l is confirmed by measurement with sodium chloride vapor and zinc vapor. The influence of porous barriers on the apparent heats of vaporization of monomers and dimers and the possibility of significant influence of surface diffusion on the experimental measurements are discussed.