List of Published PapersB.V. Costa, E.A. Soares Surface Science, 487 (2001) 15-27 In this work we present new results obtained from the application of the fast simulated algorithm (FSA) to the surface structure determination of the Ag(110) and CdTe(110) systems. The influence of a control parameter, the "initial temperature", on the FSA search process was investigated. A scaling behaviour, that measures the efficiency of a search method as a function of the number of parameters to be varied, was obtained for the FSA algorithm, and indicated a favourable linear scaling (N1). A quantitative structure determination of the Ag(111)(root3xroot3)R30o-Sb surface has been performed using low energy electron diffraction. Six possible structural models were tested: Sb overlayers with four different adsorption sites and a substitutional alloy surface layer, with or without a stacking fault relative to the underlying substrate. The results clearly favor the faulted alloy surface model with all outermost layer Sb and Ag atoms occupying hcp hollow sites, in agreement with a recent x-ray diffraction study of this surface phase and both x-ray diffraction and medium energy ion scattering studies of the related Cu(111)(root3xroot3)R30o-Sb surface. Surface structure determination by Low Energy Electron Diffraction (LEED) is based on a comparison between experimentally measured and theoretically calculated intensity versus energy I(V) curves for the diffracted beams. The level of agreement between these, for different structural models, is quantified using a correlation function, the so-called R factor. Minimizing this factor allows one to choose the best structure for which the theoretical simulations are computed. Surface structure determination thus requires an exhaustive search of structural parameter space in order to minimize the R factor. This minimization is usually performed by the use of directed search methods, although they have serious limitations, most notably their inability to distinguish between false and real structures corresponding to local and global R factor minima. In this work we present the implementation of a global search method based on the simulated annealing algorithm, as suggested earlier by Rous, using the Van Hove and Tong standard LEED code and the results of its application to the determination of the structure of the Ag(111) and CdTe(110) surfaces. Two different R factors, RP and R1, have been employed in the structural searches, and the statistical topographies of these two factors were studied. We have also implemented a variation of the simulated annealing algorithm (Fast Simulated Annealing) and applied it to these same two systems. Some preliminary results obtained with this algorithm were used to compare its performance with the oroginal algorithm proposed by Rous. E.A. Soares, L.O. Ladeira e H.D. Pfannes J. Elect. Spec. and Related Phenomena 104 (1999) 99-107. A Bi2Se3 crystal was studied by XPS (X-Ray Photoelectron Spectroscopy) and EELS (Electron Energy Loss Spectroscopy). Al K_alpha radiation and an electron beam energy in the range of 0.1 to 2 keV were used, respectively, to probe a Se-terminated (0001) surface. Samples of the constituent elements (Bi and Se) have also been measured with the same setup. The core level chemical shifts obtained show that a charge transfer occurs in Bi2Se3. The spectra in the valence band region suggest that the density of states in the compound may be obtained by combining the spectra of the constituent elements, that the electronic states in the vicinity of the gap region consist of a mixture of the metal and calcogen p-orbitals and that the band arising from the valence s-orbitals occurs about 12eV below the valence band maximum. The EELS spectra allow to identify the bulk plasmons for the three materials and the Bi5d3 and Bi5d5 interband transitions. Considerations of the energies of the Bi5d5 transitions as measured by XPS and EELS indicate that indicate that the bottom of the conduction band of the compound is 1.2 eV above the Fermi level. The EELS results also shows evidence that the losses occurring at 6.4 eV in the compound and at 5.4(5.5) eV in Bi(Se) have their origins in some surface process. We suggest that they may be associated to a surface plasmon. The Debye temperature and the thermal vibrations of first two surface layers of the CdTe (110) have been studied by low energy electron diffraction (LEED). The full dynamic LEED calculations were performed using the LEEDFIT code that allows one to carry out optimizations of the Debye temperature and vibrational amplitudes of each atom in each surface layer. A quantitative structure determination of the Ag(111) clean surface has been performed using low-energy electron diffraction. Optimisation of the outermost layer spacings and vibrational amplitudes was achieved using two different dynamical scattering computational methods, one using the LEEDFIT code and the second exploiting a Simulated Annealing algorithm implemented on the conventional Van Hove/Tong code. The results obtained show that a bulk-terminated structural model describes the data best and, in particular, the analysis excludes a 2.5% contraction of the outermost layer spacing, which was recently reported on the basis of ion-scattering measurements. The LEED result of no contraction is consistent with prior results for the (111) face of several other metals. The InSb(110) surface structure has been re-examined using the tensor LEED approach. A refinement of the structure as well as the influence of the Debye temperature on the structure determination is presented. |