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The microscopic study of complex systems has reached a high level of accuracy that allows for a deep understanding of their structure, electronic properties, and optical spectra. The theoretical investigation of surfaces is nowadays routinely done within density functional theory, for ground state properties, and, with a larger computational load, within many-body perturbation theory, for excited states properties. In this paper we present and discuss examples of calculations for group IV two-dimensional systems such as a clean silicon surface, a tin-germanium interface, graphene, and graphane, pointing out the importance of a pertinent treatment of many-body effects.

We study the two lowest-energy isomers of the Ge(111)-(2 x 1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the silicon case, the stablest isomer differs from the standard "buckled Pandey chains" reconstruction. This conclusion is supported by accurate total-energy results.

Lerici (La Spezia)

IV Convegno Nazionale sulla Scienza e Tecnologia dei Materiali Ischia Porto (NA) 29 giugno – 2 luglio 2003.

The synthesis of four new complexes based on tridentate bispyrazole ligand by coordination of 4-[bis[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-amino]phenol with different transition metals such as Cu(NO(3))(2), NiCl(2), CoCl(2) and Cu(BF(4))(2) was reported. Their characterization by means of IR. UV-visible and mass spectroscopy was investigated. Their optical pollutant gases recognition capabilities as solid state thin layer on quartz were investigated. Different analytes have been studied such as SO(2), NO(2), CO, CH(4) and NH(3). The coordinated complex layer presents reversible system sensitivity towards SO(2) and NO(2) with good sequences in function with time. No influence on the optical properties was shown in the presence of CO, CH(4) and NH(3).

Electron energy loss spectra of the Si(100) and Si(100):O surfaces are investigated using ab initio methods based on density functional theory. Computed spectra for the clean surface are found to be in good agreement with published experimental data, providing further confirmation that reconstructions of higher order than (2×1) are present on Si(100). The origins of the main spectral structures are analyzed and their relation to the surface bandstructure is illustrated. Oxygen adsorption on the surface dimers is found to cause strong modifications to the calculated spectra, thereby partially explaining the results of surface differential reflectivity experiments.

Despite the simplicity of the system, the description of the excited-state properties of the cleavage surface of diamond requires the use of nonstandard treatment of many-body effects. Using a simple tight-binding model, we show that the typical assumption concerning the identity of quasiparticle and Kohn-Sham wave functions fails in an important part of the Brillouin zone. This simple model allows computation of the qualitatively different quasiparticle wave functions. The optical properties calculated with these wave functions at the Bethe-Salpeter level show important excitonic effects. Moreover the line shape of the reflectance anisotropy spectrum is significantly improved when compared to the measured one.