Anisotropy in Nucleation and Growth of Two-Dimensional Islands during Homoepitaxy on Hex Reconstructed Au(100)

We present results of a comprehensive scanning tunneling microscopy study of the nucleation and growth of Au islands on Au(100). It is shown that the reconstruction of the substrate produces strong anisotropic effects. Rate equation analysis of the experimental flux and temperature dependence of the island density suggests: (i) a critical size of i=3 for T=315−380 K, but i\u3e3 above 400 K; and (ii) strongly anisotropic diffusion, preferentially parallel to the reconstruction rows (activation energy ∼0.2 eV). We comment on energetic and kinetic aspects of the observed island shape anisotropy

Effects of Small Island Mobility on Growth in Molecular Beam Epitaxy

The effects of mobility of small islands on island growth in molecular beam epitaxy are studied. It is shown that small island mobility affects both the scaling and morphology of islands during growth. Three microscopic models are considered, in which the critical island sizes are $i^*=1,2$ and 3 (such that islands of size $s \le i^*$ are mobile while islands of size $s \ge i^{\ast}+1$ are immobile). As i^* increases, islands become more compact, while the exponent $\gamma$ which relates the island density to deposition rate increases. The morphological changes are quantified by using fractal analysis. It is shown that the fractal dimensions are rather insensitive to changes in i^*. However, the prefactors provide a quantitative measure of the changing morphologies.Comment: 25 pages of text, RevTeX, additional 8 figures in 10 PS files. Replaced version include some minor changes, notation, etc. To be published in: Phys. Rev B, Vol. 55 (1997

CO adsorption and oxidation on bimetallic Pt/Ru(0001) surfaces \u2013 a combined STM and TPD/TPR study

The results of a comparative STM and TPD/TPR study on the adsorption and oxidation of CO on pure Ru(0001) and on structurally and compositionally well-defined, bimetallic Pt/Ru(0001) surfaces are presented. The defect structure and local surface composition of the bimetallic substrates, which are produced by epitaxial growth of monolayer Pt islands or by Pt deposition and subsequent surface alloying, are characterized by high-resolution STM images with chemical contrast. TPD and TPR experiments for CO adsorption/oxidation on these surfaces show a distinct lowering of the CO adsorption energy on the bimetallic surfaces with respect to the pure Ru(0001) and Pt(lll) surfaces, with the onset of CO desorption already at 230 K. The reduction in adsorption energy is attributed to an electronic modification of the deposit metal due to interaction with the Ru(0001) substrate. The bimetallic surface alloy catalyzes CO oxidation under UHV conditions. The reduced temperature for CO2 desorption as compared to Pt(lll) indicates an even lower barrier than on the latter surface, whereas the pure Ru(0001) surface is inert under these conditions. The results are discussed with respect to the superior CO tolerance of bimetallic PtRu catalysts in low-temperature fuel cells as compared to monometallic Pt catalysts, leading to a mechanistic explanation of that phenomenon distinctly different from previous ideas. (C) 1998 Elsevier Science B.V. All rights reserved

Island-size distributions in submonolayer epitaxial growth: Influence of the mobility of small clusters

We examine the influence of dimer mobility on the size distribution of two-dimensional islands formed by irreversible nucleation and growth during deposition. We first characterize the transition in scaling of the mean island density with increasing dimer mobility, from the classic form described by Venables [Philos. Mag. 27, 697 (1973)] to the modified form for "rapid" mobility described by Villain et al. [J. Phys. (France) I 2, 2107 (1992)]. The corresponding transition in the asymptotic scaling function describing the shape of the island-size distribution is then also characterized. In addition, we contrast the mean-field form of the scaling function for rapid dimer mobility with that for zero mobility. Analysis of experimental data for Au/Au(100), Fe/Fe(100), Cu/Cu(100), and Pt/Pt(111) homoepitaxy reveals no clear evidence for a regime of modified island density scaling due to rapid dimer mobility. However, for Fe/Fe(100) below 400 K, we argue that mobility of small clusters significantly influences the shape of the island-size distribution, even before it affects the mean island density.This article is from Physical Review B 53 (1996): 4099, doi: 10.1103/PhysRevB.53.4099.</p

Anisotropy in Nucleation and Growth of Two-Dimensional Islands during Homoepitaxy on "Hex" Reconstructed Au(100)

We present results of a comprehensive scanning tunneling microscopy study of the nucleation and growth of Au islands on Au(100). It is shown that the reconstruction of the substrate produces strong anisotropic effects. Rate equation analysis of the experimental flux and temperature dependence of the island density suggests: (i) a critical size of i=3 for T=315−380 K, but i>3 above 400 K; and (ii) strongly anisotropic diffusion, preferentially parallel to the reconstruction rows (activation energy ∼0.2 eV). We comment on energetic and kinetic aspects of the observed island shape anisotropy.This article is published as Günther, S., E. Kopatzki, M. C. Bartelt, J. W. Evans, and R. J. Behm. "Anisotropy in nucleation and growth of two-dimensional islands during homoepitaxy on" hex" reconstructed Au (100)." Physical review letters 73, no. 4 (1994): 553, doi:10.1103/PhysRevLett.73.553. Posted with permission.</p