Exposure to oxygen at 300–340 °C results in triangular etch pits with uniform orientation on the surfaces of atomically thin molybdenum disulfide (MoS2), indicating anisotropic etching terminating on lattice planes. The triangular pits grow laterally with oxidation time. The density of pits scarcely depends on oxidation time, temperature, and MoS2 thickness but varies significantly from sample to sample, indicating that etching is initiated at native defect sites on the basal plane surface rather than activated by substrate effects such as charged impurities or surface roughness. Raman spectroscopy confirms that oxygen treatment produces no molybdenum oxide (MoO3) below 340 °C. However, upon oxidation above 200 °C, the Raman A1g mode upshifts and the linewidth decreases, indicating p-type doping of MoS2. Oxidation at 400 °C results in complete conversion to MoO3.

In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean radius R, the cluster diffusion constant varies as R 21 and R 22. From an analytical Langevin description of the cluster's steplike boundary, we find three cases, R 21 , R 22 , and R 23 , corresponding to the three microscopic surface mass-transport mechanisms of straight steps. We thereby provide a unified treatment of the dynamics of steps and of clusters. For corroboration, we perform Monte Carlo simulations of simple lattice gases and derive atomistic diffusion constants.

The morphology of surfaces of arbitrary orientation in the presence of step and kink Ehrlich-Schwoebel effects (SESE and KESE) during growth is studied within the framework of a model in which steps are continuous lines, and is illustrated by a simple solid-on-solid model. For vicinal surfaces KESE induces an instability often stronger than that from SESE. The possibility of stable kink flow growth is analyzed. Fluctuations can shift the stability threshold. KESE also induces mound formation. [S0031-9007(99)09023-7]

In recent observations of Brownian motion of islands of adsorbed atoms and of vacancies with mean radius R, the cluster diffusion constant D c varies as R Ϫ1 and R Ϫ2 . From an analytical continuum description of the cluster's steplike boundary, we find a single Langevin equation for the motion of the cluster boundary, rather than three special cases. From this we determine D c and the correlation function G sh for fluctuations of the shape around an assumed equilibrium circular shape. In three limiting cases we find the scaling relations D c ϳR Ϫ␣ and, at early elapsed time t, G sh ϳt 1/(1ϩ␣) , where ␣ϭ1, 2, and 3, corresponding to the three generic surface mass-transport mechanisms of straight steps. We thereby provide a unified treatment of the dynamics of steps and of clusters. To check how well the continuum results describe clusters of the size in experiments, we perform Monte Carlo simulations of simple lattice gas models. Further, we estimate atomic diffusion parameters from the available experimental data on diffusion of large clusters.

Using a van der Waals density functional ͑vdW-DF͒ ͓Phys. Rev. Lett. 92, 246401 ͑2004͔͒, we perform ab initio calculations for the adsorption energy of benzene ͑Bz͒ on Cu͑111͒ as a function of lateral position and height. We find that the vdW-DF inclusion of nonlocal correlations ͑responsible for dispersive interactions͒ changes the relative stability of eight binding-position options and increases the binding energy by over an order of magnitude, achieving good agreement with experiment. The admolecules can move almost freely along a honeycomb web of "corridors" passing between fcc and hcp hollow sites via bridge sites. Our diffusion barriers ͑for dilute and two condensed adsorbate phases͒ are consistent with experimental observations. Further vdW-DF calculations suggest that the more compact ͑hexagonal͒ Bz-overlayer phase, with lattice constant a = 6.74 Å, is due to direct Bz-Bz vdW attraction, which extends to ϳ8 Å. We attribute the second, sparser hexagonal Bz phase, with a = 10.24 Å, to indirect electronic interactions mediated by the metallic surface state on Cu͑111͒. To support this claim, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this indirect interaction. Thus, we can account well for benzene self-organization on Cu͑111͒.