Nonequilibrium effects in diffusion of interacting particles on vicinal surfaces

We study the influence of nonequilibrium conditions on the collective diffusion of interacting particles on vicinal surfaces. To this end, we perform Monte Carlo simulations of a lattice-gas model of an ideal stepped surface, where adatoms have nearest-neighbor attractive or repulsive interactions. Applying the Boltzmann–Matano method to spreading density profiles of the adatoms allows the definition of an effective, time-dependent collective diffusion coefficient DtC(θ) for all coverages θ. In the case of diffusion across the steps and strong binding at lower step edges we observe three stages in the behavior of the corresponding Dtxx,C(θ). At early times when the adatoms have not yet crossed the steps, Dtxx,C(θ) is influenced by the presence of steps only weakly. At intermediate times, where the adatoms have crossed several steps, there are sharp peaks at coverages θ1−1∕L, where L is the terrace width. These peaks are due to different rates of relaxation of the density at successive terraces. At late stages of spreading, these peaks vanish and Dtxx,C(θ) crosses over to its equilibrium value, where for strong step edge binding there is a maximum at θ=1∕L. In the case of diffusion in direction along the steps the nonequilibrium effects in Dtyy,C(θ) are much weaker, and are apparent only when diffusion along ledges is strongly suppressed or enhanced.Peer reviewe

Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene

We show that strongly photoexcited graphene monolayers with 35fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative optical conductivity for the first 200fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation

Chemical diffusion of CO in mixed CO+O adlayers and reaction-front propagation in CO oxidation on Pd(100)

Within the framework of a realistic atomistic lattice-gas model, we present the theoretical formulation and simulation procedures for precise analysis of the chemical diffusion flux of highly mobile CO within a nonuniform interacting mixed CO+O adlayer on a Pd(100) surface. The approach applies in both regimes of relatively immobile unequilibrated and fairly mobile near-equilibrated O adlayer distributions. Spatiotemporal behavior in surface reactions is controlled by chemical diffusion in mixed adlayers. Thus, we naturally integrate the above analysis with a previously developed multiscale modeling strategy to describe mesoscale reaction front propagation in CO oxidation on Pd(100). This treatment avoids using a simplified prescription of chemical diffusion and reaction kinetics as in traditional mean-field reaction-diffusion equation approaches

Non-Equilibrium Surface Diffusion Measurements in Systems with Interactions

Diffusion in surface overlayers with adsorbate-adsorbate interactions is described in terms of coverage-dependent diffusion coefficients. The measured phenomenological Arrhenius parameters (activation energy and prefactor) depend on the initial configuration of the system. Since different experimental methods probe the system in different states, the measured diffusion coefficients depend on the method used. Experimental results demonstrating this dependence are presented for O/W(110) -p(2x1) + p(2x2) and Ag/Si(111) -√3 x √3 R30°. They were measured during the evolution of the system to attain a new equilibrium state of different symmetry. In addition, simulations on lattice gas models with interactions, modeling other surface diffusion techniques (Laser-lnduced-Desorption, fluctuation, non-equilibrium kinetics) support the configuration-dependent results

Wetting-layer transformation for Pb nanocrystals grown on Si(111)

doi:10.1063/1.1812593We present the results of in situ x-ray scattering experiments that investigate the growth of Pb nanocrystalline islands on Si(111). It is conclusively shown that the Pb nanocrystals do not reside on top of a Pb wetting layer. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer occupying the region between the islands. As the Pb nanocrystals coalesce at higher coverage we observe increasing disorder that is consistent with misfit strain relaxation. These results have important implications for predicting stable Pb island heights

Interplay between steps and nonequilibrium effects in surface diffusion for a lattice-gas model of O/W(110)

The authors consider the influence of steps and nonequilibrium conditions on surfacediffusion in a strongly interactingsurfaceadsorbate system. This problem is addressed through Monte Carlo simulations of a lattice-gas model of O∕W(110), where steps are described by an additional binding energy EB at the lower step edge positions. Both equilibrium fluctuation and Boltzmann-Matano spreading studies indicate that the role of steps for diffusion across the steps is prominent in the ordered phases at intermediate coverages. The strongest effects are found in the p(2×1) phase, whose periodicity Lp is 2. The collective diffusion then depends on two competing factors: domain growth within the ordered phase, which on a flat surface has two degenerate orientations [p(2×1) and p(1×2)], and the step-induced ordering due to the enhanced binding at the lower step edge position. The latter case favors the p(2×1) phase, in which all adsorption sites right below the step edge are occupied. When these two factors compete, two possible scenarios emerge. First, when the terrace width L does not match the periodicity of the ordered adatom layer (L/Lp is noninteger), the mismatch gives rise to frustration, which eliminates the effect of steps provided that EB is not exceptionally large. Under these circumstances, the collective diffusion coefficient behaves largely as on a flat surface. Second, however, if the terrace width does match the periodicity of the ordered adatom layer (L/Lp is an integer), collective diffusion is strongly affected by steps. In this case, the influence of steps is manifested as the disappearance of the major peak associated with the ordered p(2×1) and p(1×2) structures on a flat surface. This effect is particularly strong for narrow terraces, yet it persists up to about L≈25Lp for small EB and up to about L≈500Lp for EB, which is of the same magnitude as the bare potential of the surface. On real surfaces, similar competition is expected, although the effects are likely to be smaller due to fluctuations in terrace widths. Finally, Boltzmann-Matano spreading simulations indicate that even slight deviations from equilibrium conditions may give rise to transient peaks in the collective diffusion coefficient. These transient structures are due to the interplay between steps and nonequilibrium conditions and emerge at coverages, which do not correspond to the ideal ordered phases.Peer reviewe

Stochastic coarsening model for Pb islands on a Si(111) surface

The coarsening behavior of individual Pb islands on Si(111) surface has been studied by scanning tunneling microscopy. Traditionally island decay follows a smooth power-law dependence on the time until disappearance. In Pb/Si(111), some unstable islands are inactive for a long time but once their decay is triggered they suffer a “sudden death.” Four-layer islands are found to decay rapidly, increasing the area covered by seven-layer islands. All islands, decaying or otherwise, are accompanied by island size fluctuation which involve a large number of perimeter atoms moving collectively as a “quantized” unit. A stochastic model is developed to elucidate the mechanism behind this coarsening behavior of Pb islands. The distinct evolution of the islands with different heights is correctly predicted, and the size fluctuations of islands and the sudden death behavior observed in island coarsening are also recovered. The key ingredients are incorporation of accurate non-Gaussian statistics of the size fluctuations and also accounting for size changes in large quantized bursts

Influence of Quantum Size Effects on Island Coarsening

DOI:10.1103/PhysRevLett.96.10610 http://prl.aps.org/pdf/PRL/v96/i10/e106105Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si 111 - 7 7 . It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., 1=F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.The Advanced Photon Source is supported by the DOE Office of Basic Energy Sciences, Contract No. W-31-109-Eng-38. The -CAT beam line is supported through Ames Laboratory, operated for the U.S. DOE by Iowa State University under Contract No. W-7405-Eng-82. Research funding was supported, in part, by Ames Laboratory (M. C. T.), Canim Scientific Group (E. H. C.), the Missouri University Research Board, the National Science Foundation DMR-0405742, and the Petroleum Research Fund No. 41792AC10 (P. F. M., C. A. J., C. K.), the Natural Sciences and Engineering Research Council (NSERC) of Canada (C. A. J.), the Center for Nanostructured Materials Technology under 21st Century Frontier R&D Programs of the Ministry of Science and Technology (No. 05K1501-02520), Korea (C. K.)