The structure and electronic properties of the ( √ 13 × √ 13)R13.9 • and (2 √ 3 × 2 √ 3)R30 • ordered phases of C 60 on the Pt(111) surface are investigated using combined dynamic low-energy electron diffraction and density functional theory (DFT) calculations. The two phases have the same local adsorption structure, while they are predicted by DFT calculations to exhibit very different electronic structures due to their different inter-C 60 orientations and distances. This result demonstrates the structural tuning of electronic properties for molecular films or junctions composed of the same materials.

Quantum well (QW) resonances are identified in Ag films on an Fe(100) surface and are used in low energy electron microscopy to monitor film morphology during annealing and growth. We find that Ag films thermally decompose to thicknesses that are stabilized by QW states at the ÿ point. Novel growth morphologies are also observed that highlight the competition between kinetic limitations and the QW state energetics that promote electronic growth. These combined observations help to explain the unusual bifurcation mode of thermal decomposition that was reported previously for this system. DOI: 10.1103/PhysRevLett.93.236104 PACS numbers: 68.55.Jk, 68.37.Nq, 68.55.Ac, 73.21.Fg The properties of ultrathin metal films exhibit remarkable quantum size effects (QSE) due to the discrete quantum well (QW) states that are caused by electron confinement [1]. Considerable early interest in metallic QW states was focused on their role in oscillatory magnetic interlayer coupling [2]. It has also been established recently that QW states can have a dramatic influence on film morphology [3][4][5][6][7][8][9][10][11][12][13] in the same nanoscale thickness regime that magnetic coupling and QSE in other properties, such as work function [14], thermal desorption [15], and interlayer relaxations [8], are observed. In particular, it was found that film morphology can be dominated during growth or annealing by film thicknesses that support strongly bound QW states at the ÿ point, which induce total energy local minima compared to other thicknesses. This ideally occurs with the same periodicity in film thickness that QW states cross the Fermi level. However, a beating of the QW period and the discrete layer spacing is sometimes evident in film morphology [3,11]. The consideration of QW state energetics has therefore accounted for a variety of multiple layer height features in the film morphologies of several systems. This phenomenon, which is often referred to as electronic growth, is envisioned to be a mechanism for self-organization in fabrication.A particularly interesting example of electronic growth was seen in the thermal stability of atomically flat Ag films on the Fe(100) surface [12]. With the exception of N 2 and N 5 monolayer (ML) thick Ag films, which were stable during annealing up to high temperature, an initially uniform N ML thick film was found to transform, or bifurcate, to a combination of N ÿ 1 and N 1 ML thick regions upon slight annealing above room temperature. This unusual behavior was attributed to the shape of the local energy versus thickness landscape defined by QW states in N ÿ 1, N, and N 1 film thicknesses. While the stability of 5 ML thick Ag films was clearly related to the existence of a QW state far below the Fermi level, the energy landscape has negative curvature between QW-induced energy minima [12,13].Thus, for these energetically unfavorable film thicknesses, the system may lower its total energy through bifurcation. The surprising aspect of this result was that an initially uniform N ML thick ...

The temporal evolution of nonequilibrium coverage profiles in the Pb wetting layer on the Si(111) surface is studied using low energy electron microscopy. The initial coverage step profile propagates rapidly at a constant velocity with an unperturbed shape. A model is proposed that attributes this nonclassical equilibration behavior to the diffusion of thermally generated adatoms on top of the wetting layer. This model can account for the observed convectionlike mass transport, as well as its dramatic dependence on Pb coverage. Such anomalous mass transport is believed to facilitate the remarkably efficient self-organization of uniform Pb quantum island height on the Si(111) surface that was observed previously.

Lattice rotations in graphene on Ru(0001) are investigated with low-energy electron microscopy and microlow-energy electron diffraction. The measurements place an upper limit, 250 nm, on the racemic length scale of the recently reported chirality in this system, arising from rotated features within the unit cell. On a longer length scale, small rotations from orientational coincidence with the substrate lattice are found to be present in the vast majority of the graphene layer. The resulting proliferation of small-angle grain boundaries may influence the properties of the graphene layer that is otherwise free of large-angle rotational variants and oriented uniformly with the substrate.

Mass transport in the Pb wetting layer on the Si(111) surface is investigated by observing nonequilibrium coverage profile evolution with low energy electron microscopy and microlow energy electron diffraction. Equilibration of an initial coverage step profile occurs by the exchange of mass between oppositely directed steep coverage gradients that each move with unperturbed shape. The bifurcation of the initial profile, the shape of the profile between the two moving edges, and the time dependence of equilibration are all at odds with expectations for classical diffusion behavior. These observations signal a very unusual coverage dependence of diffusion or may even reveal an exceptional collective superdiffusive mechanism.

The temperature dependence of the step line tension on the Si͑111͒ ͑1 ϫ 1͒ surface is determined from a capillary wave analysis of two-dimensional island edge fluctuations and straight step fluctuations that are observed with low energy electron microscopy. The line tension decreases by nearly 20% with a linear temperature coefficient of −0.14 meV/ Å K between 1145 and 1233 K. Temporal correlations of step fluctuations exhibit the distinctive signature in the wavelength dependence of the relaxation time of a terrace diffusion-limited mechanism for step motion. We also find that the role of desorption in island decay increases dramatically in the temperature range ͑1145-1380 K͒ that island decay is studied. Consequently, we generalize the current quasistatic model of island decay to take account of desorption. The evaluation of the island decay time with this model referenced to the temperature-dependent line tension accurately determines activation energies that are relevant to mass transport and sublimation.

The adsorption and diffusion of CO molecules on a Pt͑111͒ surface have been studied using low-energy electron microscopy ͑LEEM͒. The explicit relationship between LEEM image intensity and CO coverage that is determined during adsorption is used to characterize nonequilibrium CO coverage profiles that are subsequently prepared by laser-induced thermal desorption. Real-time observations of the temporal evolution of these profiles toward equilibrium uniform coverage distributions are analyzed by predictive and inverse solutions of the diffusion equation. These two methods determine consistently the detrimental effect on diffusion of the laser-induced surface damage that is observed directly with LEEM. The inverse method also provides independent information on the coverage dependence of diffusion with high coverage resolution.

The influence of the deposition rate upon the initial growth morphology and magnetism of ultrathin Fe films on the Cu͑100͒ surface has been studied with low-energy electron diffraction, reflection high-energy electron diffraction, surface magneto-optical Kerr effect ͑SMOKE͒, and spin-polarized low-energy electron microscopy ͑SPLEEM͒. Dramatic changes in the first diffraction intensity oscillation during growth at room temperature indicate that simultaneous growth of the first and second atomic layers is converted to more perfect layer-bylayer growth when the deposition rate is reduced below 0.5 ML/min. SPLEEM and SMOKE demonstrate the sensitivity of magnetism in Fe/Cu͑100͒ to details of interface formation during the initial growth in this range of deposition rates. Kinetic and thermodynamic mechanisms that relate to place exchange at the interface are discussed as the source of the observed growth and magnetic behavior.