We report the first experimental evidence of the cyclic form of ozone, found in three air stable surface reconstructions of MgO (111) annealed above 1450 ± C. The MgO ͑111͒-͑ p 3 3p 3 ͒R30 ± surface consists of equilateral oxygen trimers while the MgO ͑111͒-͑2 3 2͒ and MgO ͑111͒-͑2 p 3 3 2 p 3 ͒R30 ± surfaces are periodic arrangements of trimers and single oxygen atoms. The oxygen trimers appear to be centered over underlying Mg atoms. The structures fit transmission electron diffraction data better than neutral plane faceting based models proposed for the polar MgO (111) surface. [S0031-9007(98)07807-7] PACS numbers: 61.14.Rq, 61.16.Bg, 68.35.BsThe stability of polar oxide surfaces has long been a problematic question in surface science. A bulk terminated polar surface has an infinite surface energy because alternating layers of oppositely charged ions produce a large dipole moment perpendicular to the surface [1]. For the model MgO (111) polar oxide surface theoretical results have pointed to two similar solutions for this problem: microscopic faceting into neutral ͕100͖ planes upon annealing [2-5] and surface reconstructions, which are essentially faceting to neutral planes but on an atomic scale [5][6][7].The microscopic faceting model has long been supported by low energy electron diffraction (LEED) and scanning electron microscopy (SEM) of 1200 ± C annealed MgO (111) surfaces [8,9] which revealed micron-sized triangular facets. These facets were interpreted to be neutral ͕100͖ planes. It has been recently shown, however, that these facets are much shallower vicinal ͕111͖ planes introduced by acid etching in sample preparation [10]

A new model for the Ag͞Si͑111͒-͑3 3 1͒ surface is proposed based on direct phasing of transmission electron diffraction data. The atomic positions in the model are refined using dynamical diffraction calculations. Other existing models were simulated and could not fit the diffraction pattern. The new model is consistent with the experimental results reported in the literature on this surface. [S0031-9007(98)05353-8] PACS numbers: 68.35.Bs, 61.14.Rq, 61.16.Bg, 79.60.Dp 1678 0031-9007͞ 98͞80(8)͞1678(4)$15.00

Investigations of Au on Si͑001͒ have suggested that room-temperature deposition of Au on a clean Si surface results in an interfacial reaction and the formation of a gold-silicide. However, these investigations typically lack direct information about the surface morphology or the exact structure at the interface. Utilizing the capabilities of a surface chemical analysis system attached to a Hitachi UHV H-9000 microscope, a layer plus island growth mode has been observed by high-resolution electron microscopy showing multiply twinned small particles on the surface. The presence of small particles for various coverages has been correlated with the shifts seen in the Si 2 p and Au 4 f binding energies as well as the peak splitting in the Si LVV Auger transition. Our chemical data are consistent with observed shifts in the binding energies of small metal clusters deposited on various substrates, and with the published data for this surface. In addition, the results are consistent with our previous studies of Ag on Si͑001͒, and indicate the growth morphology plays a crucial role in understanding spectroscopic information as well as its correlation to the structure and chemical state of the interface and surface morphology. ͓S0163-1829͑97͒08111-3͔

This paper reviews recent progress in the application of Direct Methods to solve surface structures using surface X-ray or transmission electron diffraction data. The basic ideas of (crystallographic) Direct Methods are presented, as well as the additional problems posed by trying to apply them to surfaces and how they connect to the mathematical theory of projections. Surface crystallography notation is presented, which differs from the widely used LEED notation in that it emphasizes the surface symmetry. This is followed by a description of methods for structure completion and refinement, followed by applications to some experimental systems, both those where the structure was previously known (calibration tests) and a few where it was not, concluding with problems and limitations. Surf. Rev. Lett. 1998.05:1087-1106. Downloaded from www.worldscientific.com by NANYANG TECHNOLOGICAL UNIVERSITY on 08/22/15. For personal use only.

The formation of epitaxial nanotubes ͑nanoarches͒ on the surface of hexagonal BN ͑h-BN͒ during electron irradiation is reported. In addition to implications in terms of understanding fullerene based structures, we suggest that these act as the nucleation sites for cubic BN ͑c-BN͒ growth and may lead to improved film growth. We also report a strong dependence upon the microscope vacuum, which may be critical in understanding irreproducibility in film growth.