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The wetting layer formed during the early stages of Ge/Si(001) growth has been found in recent experiments to undergo a roughening process, where the SA surface steps affect the spatial organization of vacancy lines (VLs) by increasing (stretching) or decreasing (squeezing) their average spacing. Using a combination of atomistic simulations and elastic theory of surface defects, we have computed the interaction energy of the SA steps and VLs for each of the observed defect configurations. We find that the repulsive SA-VL interactions lead to an increase in the spacing of the VLs in the “stretch” arrangement, but do not significantly affect the VL spacing in the “squeeze” configuration, providing an explanation for the observed correlations in the wetting layer roughness. [ABSTRACT FROM AUTHOR]

We report on calculations of the formation energies of several [100] and [110] oriented step structures on biaxially stressed Si and Ge (001) surfaces. It is shown that a novel rebonded [100] oriented single-height step is strongly stabilized by compressive strain compared to most well-known step structures. We propose that the side walls of "hut"-shaped quantum dots observed in recent experiments on SiGe/Si films are made up of these steps. Our calculations provide an explanation for the nucleationless growth of shallow mounds, with steps along the [100] and [110] directions in low- and high-misfit films, respectively, and for the stability of the (105) facets under compressive strain. [ABSTRACT FROM AUTHOR]

Using in situ low-energy electron microscopy and density functional theory calculations, we follow the growth of monolayer graphene on Pd(111) via surface segregation of bulk-dissolved carbon. Upon lowering the substrate temperature, nucleation of graphene begins on graphene-free Pd surface and continues to occur during graphene growth. Measurements of graphene growth rates and Pd surface work functions establish that this continued nucleation is due to increasing C adatom concentration on the Pd surface with time. We attribute this anomalous phenomenon to a large barrier for attachment of C adatoms to graphene coupled with a strong binding of the non-graphitic C to the Pd surface. [ABSTRACT FROM AUTHOR]

Based on a dipolar-elastic model for oxygen vacancies on rutile (110), we evaluated analytically the overall energy of a periodic array of two vacancies and extracted the interaction parameters from total-energy density functional theory (DFT) calculations. Our calculations show that the dipole model holds for next-nearest-neighbor vacancies and beyond. The elastic-dipolar interaction vanishes for adjacent vacancies, but they still experience an electrostatic repulsion. The proposed interaction model predicts a vacancy separation distribution that agrees well with that determined in our ultrahigh vacuum scanning tunneling microscopy experiments, and provides a perspective for understanding earlier DFT reports. [ABSTRACT FROM AUTHOR]

Selected-area diffraction establishes that at least six different in-plane orientations of monolayer graphene on Pd(111) can form during graphene growth. From the intensities of low-energy electron microscopy images as a function of incident electron energy, we find that the work functions of the different rotational domains vary by up to 0.15 eV. Density functional theory calculations show that these significant variations result from orientation-dependent charge transfer from Pd to graphene. These findings suggest that graphene electronics will require precise control over the relative orientation of the graphene and metal contacts. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy and density functional theory, we characterize the population of low-coverage Ti atoms on Al(111) as a model surface system for transition metal doped alanate hydrogen storage compounds, such as NaAlH4. When deposited at room temperature, Ti is kinetically trapped in first-layer substitutional sites, avoids nearest-neighbor locations, and preferentially forms next-nearest-neighbor pairs, similar to a structure that has been predicted to dissociate H2 with no energy barrier. The results on this well-defined system suggest the presence of a large population of Ti-pair complexes that may catalyze the dissociative chemisorption of hydrogen in Ti-doped alanate storage materials. [ABSTRACT FROM AUTHOR]

Using in situ high-temperature (700-1000 K) scanning tunneling microscopy (STM), we studied the influence of ethylene on the surface dynamics of oxygen-deficient, rutile-structured TiO2(110). STM images were acquired during annealing the sample as a function of time, oxygen and ethylene pressures, and temperature. With increasing oxygen pressure and/or decreasing temperature, TiO2(110) surface mass increased, consistent with previous results. Interestingly, annealing the sample in ethylene with traces of oxygen also results in the growth of TiO2 at higher rates than those observed during annealing in pure oxygen. Our results indicate that ethylene promotes oxidation of TiO2(110). [ABSTRACT FROM AUTHOR]

Using in situ low-energy electron microscopy and density functional theory, we studied the growth structure and work function of bilayer graphene on Pd(111). Low-energy electron diffraction analysis established that the two graphene layers have multiple rotational orientations relative to each other and the substrate plane. We observed heterogeneous nucleation and simultaneous growth of multiple, faceted layers prior to the completion of second layer. We propose that the faceted shapes are due to the zigzag-terminated edges bounding graphene layers growing under the larger overlying layers. We also found that the work functions of bilayer graphene domains are higher than those of monolayer graphene, and depend sensitively on the orientations of both layers with respect to the substrate. Based on first-principles simulations, we attribute this behavior to oppositely oriented electrostatic dipoles at the graphene/Pd and graphene/graphene interfaces, the strengths of which depend on the orientations of the two graphene layers. [ABSTRACT FROM AUTHOR]