Ab-Initio Simulation Laboratory

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The adsorptions of CO molecule on various fcc(111) surfaces (Rh, Ir, Pt, and Cu) have been studied by diffusion quantum Monte Carlo (DMC) calculations, and the results show that the top site is the most stable adsorption site on all the four surfaces, in agreement with experiments. In particular, the site preference including the bridge site for CO/Pt(111) is predicted, i.e., the top site is most preferred followed by the bridge site while the hollow sites are much less favorable, in accordance with the existing experimental observations of the bridge-site adsorption, yet never on the hollow sites. Compared to the DMC results, density functional theory (DFT) calculations with the generalized-gradient approximation (GGA) predict very similar adsorption energies on the top site, but they overestimate those on the bridge and hollow sites. That is, although the nonlocal exchange-correlation contribution is small for the single-coordinated top-site adsorption, it is essential and required to be properly included for a correct description of the higher coordinated bridge- and hollow-sites adsorptions. These altogether explain why the top site adsorption for CO on Rh, Pt, and Cu(111) surfaces was not predicted correctly by the previous standard local or semilocal DFT calculations.

Motivated by the synthesis of the layered structure CoO2via Li atom deintercalation from LixCoO2, herein, we investigated the electronic structure, lattice dynamics, electron–phonon interaction, and superconductivity of monolayer CoO2 using first-principles calculations. This 2D material was predicted to have a ferromagnetic ground state with a metallic band structure and the total magnetization of 0.83μB. Remarkably, the non-spin polarized calculations show that the monolayer CoO2 possesses phonon-mediated superconductivity at 25–28 K owing to its intermediate to strong electron–phonon coupling (EPC). The rather strong EPC in this compound is mainly driven by the acoustic phonons, making this compound one of the highest-temperature superconductors among the existing 2D materials. Moreover, the CoO2 sheets could be synthesized via exfoliation from bulk CoO2 owing to the relatively small interlayer binding energy while maintaining its stability under normal experimental conditions. Compared to its bulk and bilayer counterparts, monolayer CoO2 was found to have highest EPC.

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Self-assembly of atoms or molecules on a crystal surface is considered one of the most promising methods to create molecular devices. Here we report a stepwise self-assembly of C60 molecules into islands with unusual shapes and preferred sizes on a gold–indium-covered Si(111) surface. Specifically, 19-mer islands prefer a non-compact boomerang shape, whereas hexagonal 37-mer islands exhibit extraordinarily enhanced stability and abundance. The stepwise self-assembly is mediated by the moiré interference between an island with its underlying lattice, which essentially maps out the adsorption-energy landscape of a C60 on different positions of the surface with a lateral magnification factor and dictates the probability for the subsequent attachment of C60 to an island’s periphery. Our discovery suggests a new method for exploiting the moiré interference to dynamically assist the self-assembly of particles and provides an unexplored tactic of engineering atomic scale moiré magnifiers to facilitate the growth of monodispersed mesoscopic structures.

The 13-atom metal clusters of fcc elements (Al, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) were studied by density functional theory calculations. The global minima were searched for by the ab initio random structure searching method. In addition to some new lowest-energy structures for Pd13 and Au13, we found that the effective coordination numbers of the lowest-energy clusters would increase with the ratio of the dimer-to-bulk bond length. This correlation, together with the electronic structures of the lowest-energy clusters, divides the 13-atom clusters of these fcc elements into two groups (except for Au13, which prefers a two-dimensional structure due to the relativistic effect). Compact-like clusters that are composed exclusively of triangular motifs are preferred for elements without d-electrons (Al) or with (nearly) filled d-band electrons (Ni, Pd, Cu, Ag). Non-compact clusters composed mainly of square motifs connected by some triangular motifs (Rh, Ir, Pt) are favored for elements with unfilled d-band electrons.

Scanning tunneling microscopy (STM) observations of the close-packed C60 fullerene arrays on Si(111)
R3xR3-Ag surface have revealed the presence of dim C60 molecules which constitute 9–12% of all fullerenes. The dim C60 fullerenes reside  1.6 A lower than the bright (“normal”) C60.While the brightC60 are in continuous rotation, the dim C60 are fixed in one of the single orientations, indicating a more tight bonding to the surface. At room temperature (RT), the dynamic switching from bright to dim C60 and vice versa has been detected. Switching slows down with decreasing temperature and becomes completely frozen at 110 K, which implies that the switching is a thermally driven process. RT deposition of  0.1 monolayer of Ag onto C60 array eliminates completely the dim C60 molecules. Experimental results can be understood if one assumes that formation of the dim C60 is associated with disintegration of Ag trimer on Si(111)R3xR3- Ag surface under a given C60 fullerene.

We have performed calculations of adsorption energetics on the graphene surface using the state-of-the-art diffusion quantum Monte Carlo method. Two types of configurations are considered in this work: the adsorption of a single O, F, or H atom on the graphene surface and the H-saturated graphene system (graphane). The adsorption energies are compared with those obtained from density functional theory with various exchange-correlation functionals. The results indicate that the approximate exchange-correlation functionals significantly overestimate the binding of O and F atoms on graphene, although the preferred adsorption sites are consistent. The energy errors are much less for atomic hydrogen adsorbed on the surface. We also find that a single O or H atom on graphene has a higher energy than in the molecular state, while the adsorption of a single F atom is preferred over the gas phase. In addition, the energetics of graphane is reported. The calculated equilibrium lattice constant turns out to be larger than that of graphene, at variance with a recent experimental suggestion.

The formation process of Al magic clusters on the Si(111)-7 x 7 surface was investigated by means of a variable-temperature scanning tunneling microscope (STM) in situ and was interpreted using density-functional theory (DFT) calculations. At a growth temperature of 450 degrees C, Al atoms hopped among the corner, center, and T4 sites and also across the dimer rows on the Si(111)-7 x 7 surface. At low coverage below 0.08 ML, a single Al atom was adsorbed on the corner or center site. When the coverage was increased to 0.08 ML, Al dimers and trimers appeared, and Al magic clusters were also observed. However, no Al tetramers or pentamers were experimentally confirmed. Careful analysis of STM images suggests that Al trimers could be key precursors for the formation of Al magic clusters, and DFT calculations verified this interpretation. Total-energy calculation results using DFT reveal that this is due to the small energy gain from Al trimer to Al tetramer. These results are important for understanding the atomic structure and the formation mechanism of the magic clusters on the Si(111)-7 x 7 surface.

Adsorption of C-60 onto Si(111)root 3 x root 3-In surface presents a fascinating example of interplay between molecular adsorbate and surface structural defects. It has been found that adsorbing C-60 molecules are trapped by the substitutional Si-defects. In turn, the group of a few adsorbed C-60 can act as a trap for the mobile vacancies of the root 3 x root 3-In reconstruction. Namely, adsorbed C-60 induces a strain in the indium layer, and when a mobile vacancy happens to get into the surface area surrounded by fullerenes, the In atoms between the C-60 and the vacancy shift from the T-4 to the H-3 sites, fixing a vacancy in a given location. (C) 2011 Elsevier B.V. All rights reserved.

Depositing particles randomly on a 1D lattice is expected to result in an equal number of particle pairs separated by even or odd lattice units. Unexpectedly, the even-odd symmetry is broken in the self-selection of distances between indium magic-number clusters on a Si(100)-2 x 1 reconstructed surface. Cluster pairs separated by even units are less abundant because they are linked by silicon atomic chains carrying topological solitons, which induce local strain and create localized electronic states with higher energy. Our findings reveal a unique particle-particle interaction mediated by the presence or absence of topological solitons on alternate lattices.