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We report on Raman scattering of GaSe for thicknesses varying from bulk down to bilayer. The Raman spectra using 532 nm excitation show a strong dependence on layer thickness. The two out-of-plane modes of GaSe shift in opposite directions as the thickness increases. However, from pentalayer the Raman spectra of GaSe do not show any further change thus reaching bulk nature in terms of optical properties. We also perform ab initio density functional theory calculations for the geometry, the electronic band structure, and the Raman frequencies for bulk down to one monolayer. Good agreement with the experimental results is found, and we reproduce the trend, driven by the number of layers, in the Raman shifts. Our results present the first systematic approach to a layer dependent Raman study of GaSe and answer contradictions reported in the literature. [ABSTRACT FROM AUTHOR]

With the aid of ab initio, parameter free calculations based on density-functional and many-body perturbation theory, we investigate the electronic band structure and electron affinity of diamond surfaces. We focus on clean, ideal (0 0 1) and (1 1 1) surfaces and on the effect of hydrogen adsorption. Also single sheets of graphane, that is graphene functionalized upon hydrogen, are investigated. At full H-coverage nearly free electron states (NFESs) appear near the conduction band minimum in all the systems under study. At the same time, the electron affinity is strongly reduced becoming negative for the hydrogenated diamond surfaces, and almost zero in graphane. The effects of quasi-particle corrections on the electron affinity and on the NFESs are discussed. [ABSTRACT FROM AUTHOR]