We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si͑111͒-7ϫ7 surface. As for other trivalent atoms, Tl is found to form a well-defined ͱ3ϫͱ3 surface, indicating the absence of a so-called ''inert pair effect'' considered only for Tl.Another well ordered 1ϫ1 surface at 1.0 monolayer appears to be semiconducting in our angle-resolved photoemission spectra dominated by a unique dispersive surface band near the Fermi level. Our theoretical calculations using density-functional theory show that Tl adatoms occupy the T 4 sites and saturate all the dangling bonds of surface Si atoms to make the surface semiconducting with a band gap of 0.34 eV. The filled surface band observed has been well reproduced in our band calculations.The interaction of group III elements with Si surfaces has been extensively studied mainly because of their technological relevance as dopant materials and the presence of various adsorbate-induced phenomena at semiconductor surfaces. 1 The adsorption of thallium ͑Tl͒ on the Si͑111͒-7ϫ7 surface, in particular, is interesting since Tl has been known to behave quite differently from other group III elements in forming stable ordered surface structures; 2 the only ordered structure formed by Tl has been reported to be a 1ϫ1 surface ͓hereafter denoted as a Tl/Si͑111͒-1ϫ1 surface͔ at 1.0 monolayer͑ML͒ in sharp contrast with a variety of different structures found for other group III elements such as Al and In. 1 The absence of other ordered structures by Tl has been considered to exhibit a so-called ''inert pair effect'' of a Tl atom where the 6s 2 electrons are assumed to be inactive in chemical bonding with Si atoms. Tl, therefore, has been thought to act as a monovalent rather than trivalent atom on the Si͑111͒ surface in a sense chemically close to the alkali metals or novel metals. 2 Because of the large atomic radius and the effectively monovalent character of Tl, the on-top site T 1 directly above the Si first layer of the unreconstructed 1ϫ1 surface has been preferred as the binding site for Tl on the Si͑111͒ surface. 3,4 In addition to the presence of interesting structures such as the superlattice of metallic nanodots at low coverage ϳ0.2 ML 5 and the rotational epitaxy of an incommensurate Tl metallic overlayer, 4 the extremely inert nature of the Tl/Si͑111͒-1ϫ1 surface is another reason why it is so interesting since it may serve as a stable substrate surface to form other atomic structures.In this Brief Report we report results of combined studies for the atomic structures and their electronic properties of the ordered Tl overlayer structures focussed especially on the Tl/Si͑111͒-1ϫ1 surface. We have determined surface bands of the 1ϫ1 irreducible surface Brillouin zone ͑SBZ͒ from our angle-resolved photoemission spectroscopy ͑ARPES͒ measurements. The surface bands and the atomic structure of the Tl/Si͑111͒-1ϫ1 surface have been well reproduced in our theoretical calculations using the density function...

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By employing ab initio total-energy calculations we have studied adsorption and diffusion of Ag atoms on a dimer-reconstructed Si͑001͒ surface. For a single Ag adsorption, the twofold-coordinated cave site above the fourth Si layer atom was found to be the most stable, in agreement with previous works. Inspection of the electronic structures at the cave site revealed that the Ag-Si bonds originate from low-lying 4d electrons and are covalent. Our calculations also exhibit another stable adsorption at the pedestal site that is slightly higher by 0.03 eV in energy than the cave site. Further potential-energy-surface calculations showed that the surface diffusion of a single Ag adatom is unexpectedly highly isotropic and that the energy barrier is 0.5 eV. When more Ag adatoms are adsorbed on the Si surface, the Ag adatoms are expected to form dimers. Actually, we obtained an energy gain of 0.36 eV/dimer through the dimerization. The diffusion of an Ag dimer was also investigated. Surprisingly, we found a very rapid surface-dimer diffusion with an energy barrier of 0.48 eV slightly lower than that of the single Ag adatom. In contrast to the diffusion of the single Ag adatom, the dimer diffusion is anisotropic and preferably occurs along the valley between Si dimer rows by concerted flipflop processes.

Au nanoparticles and Au films for growth of nanowires on the GaAs͑111͒B surface have been studied by a combination of experimental and theoretical techniques. If Au is present in either form, annealing to temperatures relevant for nanowire growth results in the formation of a thin Au wetting layer. Based on density functional theory calculations and experimental data, a structural model is proposed with an Au atom on every third threefold hollow hcp site of the Ga lattice. The authors observe that the stability of Au nanoparticles is governed by the presence of the wetting layer and outdiffusion of Au from the nanoparticles.

Initial nitridation of the Si(100) surface is investigated using photoemission, ion-scattering and ab initio calculations. After dissociation of NO or NH3, nitrogen atoms are found to spontaneously form a stable, highly coordinated N[triple bond]Si(3) species even at room temperature. The majority of the N species is incorporated into the subsurface Si layers occupying an interstitial site, whose atomic structure and unique bonding mechanism is clarified through ab initio calculations. This unusual adsorption behavior elucidates the atomistic mechanism of initial silicon nitride formation on the surface and has important implication on the N-rich layer formation at the SiO(x)N(y)/Si interface.