Ab-Initio Simulation Laboratory

Experimentally, self-organized Ag planar clusters have been observed on the periodic template found on the Pb quantum islands, which are grown on the Si(111) surface. These planar clusters register a remarkable abundance variation at some specific atomic numbers and possess enhanced stability. They are thus denoted as two-dimensional magic Ag nanoclusters (or nanopucks). In this work, detailed calculations based on ab initio density functional theory are made to illuminate how the size and shape effects related to electronic confinement influence the sequence of these two-dimensional Ag nanostructures. The simulation results demonstrate that the evolution of a sequence of planar magic Ag clusters is strongly correlated with their electronic structures. Meanwhile, the role of substrate in the formation of magic Ag clusters is also examined. The symmetry and size of the periodic pattern on the substrate have helped to build up the distinguishable geometric structures in experiment. Further analysis of the related electronic and geometrical properties of these clusters not only explains the occurrence and sequence of the magic numbers but also helps to elucidate the mechanism of their formation.

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We present first-principles calculations that provide a detailed diffusion picture of an adsorbed Si atom on the Si(111)-(7x7) surface. Several diffusion paths for the adsorbed Si atom are established by mapping out the total energy as a function of its positions on the surface. For diffusion between the faulted and unfaulted halves, the energy barriers range from 0.96 to 1.21 eV, while remarkable low-energy barriers from 0.3 to 0.7 eV are discovered within the faulted and unfaulted regions.

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.

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We demonstrate a direct surface structural tool with high resolution of approximately 1 angstrom in all directions by direct Fourier transformation of measured Kikuchi patterns using a multiple-energy phase-summing method. In this method, with an integral over continuous energy spectra in each direction, both the forward- and backward-scattering oscillations are selected for Fourier transformation by varying the energy range and size of the grid. High-fidelity and artifact-free three-dimensional images of Ag atoms for (100) and (111) single-crystal surfaces are obtained.

A multiple-energy phase-summing method is applied to diffuse low-energy electron diffraction intensity spectra to obtain three-dimensional images of surface atoms. In this demonstration, calculated DLEED intensity spectra from a multiple scattering method are directly inverted to produce high-fidelity 3D images of near-neighbor atoms measured from an adatom. No prior knowledge of adsorption site, bond length, bond angle, or type of atom is needed. The images are essentially free from artifacts and have a spatial resolution of approximately 1 angstrom when viewed along any cut-plane. These results demonstrate that holographic DLEED has the potential of being an accurate and direct structural tool for low-density disordered adsorption systems.

A simple oscillatory intensity variation in medium-energy electron diffraction found recently [Abukawa ei al., Phys. Rev. Lett. 82, 335 (1999)] was termed correlated thermal diffuse scattering (CTDS). The potential of CTDS as a direct surface structural tool has been fully explored for the Si(001)2 X 1 surface at 300 K in a very-grazing-incidence condition. Nearly 2 pi solid-angle, three-dimensional (3D) CTDS patterns were measured for an energy range of 500-2000 eV. The 3D Patterson functions obtained by Fourier inversion of the measured CTDS patterns clearly revealed the building blocks of the Si(001)2 X 1 surface, i.e., the bond orientations and lengths of the buckled Si dimers, within an accuracy of 1 degrees and 0.1 Angstrom, respectively.

We report the first atomically resolved images of ordered Au trimers on Si(111)root 3X root 3R30 degrees-Au using wave-front reconstruction of scanned-energy glancing-angle Kikuchi electron spectra. Each Au image has a resolution (full width at half magnitude) of less than 1 Angstrom. The images indicate that Au trimers art ordered and nonrotated within the surface plane and with respect to the second-layer Si plane providing direct evidence of the conjugate honeycomb-chained-trimer model for the Au-root 3 system.

More than 50 symmetry-inequivalent emitter-scatterer (E-S) pairs were observed for the Si(111)-(7 x 7) surface using Kikchi electron holography (KEH) with various incident/detection configurations. For different configurations, the E-S pairs in the backscattering direction are preferentially enhanced. Thus, one can obtain detailed structural information by changing the incident/detection direction, which will be helpful in sorting out the correct surface structure model.

We demonstrate that two-dimensionally resolved diffuse low-energy electron diffraction intensities can be measured with sufficient accuracy and at multiple energies to allow direct inversion for a low coverage (5%) disordered K/Ni(100) surface. The data inversion reveals three-dimensional coordinates of atoms with atom images whose full width at half maximum is less than 1 angstrom in all spatial directions. By varying the angle of incidence, first layer and second layer near-neighbor Ni atoms are separately imaged. This is the first demonstration of multiple-energy internal-source electron holography using measured elastically backscattered electrons.

A Patterson-like scheme is proposed for direct inversion of the conventional low-energy electron diffraction (LEED) intensity versus energy I(E) curves, which is in contrast with the previously suggested holographic scheme. Using the Si(111)-(7 X 7) and Si(113)-(3 X 2) surfaces as examples, high quality three-dimensional images, with a resolution better than 0.5 Angstrom, of both surface atoms and bulk atoms are obtained from the direct Patterson inversion of LEED-I(E) curves with the integral-energy phase-summing method.

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