Adatom density

The essential influence of surface roughening is also present in this model. Grand canonical Monte Carlo calculations were used to generate adatom populations at various temperatures up to Chemical potentials corresponding to those in the bulk LJ crystal were used, and these produced adatom densities that increased with temperature and roughly approximated the values observed in Ising model simulations below T. ... 

A plot of the adatom density versus T is shown in Fig. 4. An anomalous increase in the density is observed at high temperatures. The dashed line represents the adatom population that would be predicted if there were no lateral interactions. However, the LJ potential between adatoms tends to stabilize them at the higher coverages, and it is this effect that causes the deviation from Arrhenius behavior at high temperatures. A similar temperature dependence is observed in the rate of mass transport on some metal surfaces (8,9), and it is possible that it is caused by the enhanced population of the superlayer at high temperatures. 

Adatoms, density versus temperature, 222 Adsorbates, metal clusters, 28-29 A1 clusters, 25-27,29 Al,... 

Figure 2.3. The projected adatom density of states in the Newns-Anderson model in two limiting cases top) when the band width is larger and bottom) when the band width is smaller than the hopping...

Let S and 5f(t) be the total surface area and the area free from the adsorbate. Introducing the adatom density ni(t) on free area and the fractional coverage g of the free area yields... 

The experimental methods used to measure surface diffusion are manifold, and we restrict ourselves to a description of a few of the most relevant ones. Amongst the microscopy techniques, the field emission microscope (FEM) has been employed to monitor diffusion indirectly by the adatom density fluctuations it induces... 

Estimates of the equilibrium adatom density at a given temperature can be derived from the adatom creation energies of the system of interest. These energies can of course be calculated, as the values above for vacancies. However, precise experimental values can be derived from thermal desorption spectroscopy (TDS), as is illustrated for the example Ni/W(110) [61]. 

Film morphology can also be modified because fast particle bombardment can damage the substrate surface and create local sites with more dangling bonds. These are relatively reactive with adatoms, having extra bonds available, and can serve as preferential nucleation sites. At the same time, disruption of clusters increases the critical nucleus size making nucleation more difficult. These two effects do not cancel each other out because early and late nucleation are subject to different constraints. The first nuclei to form do so in the absence of sinks for adatoms. Therefore the adatom density can increase until the adatom supersaturation favors nucleation enough to counteract the disruption of clusters by fast particles. At the same time, the creation of favored nucleation sites also contributes to favoring nucleation. On balance, fast particle bombardment may accelerate the formation of the early nuclei. 

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