Using state of the art time-dependent density functional theory and multireference second-order perturbation theory, we have accurately calculated (within 0.3 eV) in real space the optical gap of small silicon quantum dots, with diameters up to 25 A. Our results, which support the quantum confinement hypothesis, are in excellent agreement with recent and earlier experimental data on oxygen-free samples and the conclusions of Wilcoxon et al. [Phys. Rev. B 60, 2704 (1999)]. We have found that the diameter of the smallest dot, which could emit photoluminescence in the visible region of the spectrum, is around 22 A. Our work can resolve existing controversies and bridge diverse experimental and theoretical results.

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We report accurate high level calculations of the optical gap and absorption spectrum of small Si nanocrystals, with hydrogen and oxygen at the surface. Our calculations have been performed in the framework of time dependent density functional theory (TDDFT) using the hybrid nonlocal exchange and correlation functional of Becke and Lee, Yang and Parr (B3LYP). The accuracy of these calculations has been verified by the high level multi-reference second order perturbation theory. The effect of oxygen contamination is studied by considering several different bonding configurations of the surface oxygen atoms. We show that for nanocrystals of sizes smaller than 20 angstroms, the widening of the gap due to quantum confinement facilitates the stabilization of Si[double bond, length as m-dash]O double bonds. For this type of bonding, the oxygen related states determine the value of the optical gap and make it significantly lower compared to the corresponding gap of oxygen-free nanocrystals. For diameters larger than 20 angstroms, the double bonds delocalize inside the valence band. We find that for small amounts of oxygen, the size of the optical gap depends strongly on their relative distribution and bonding type, while it is practically insensitive to the exact number of oxygen atoms.

The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H(2)O)(6)](2+) on the "titania/electrolyte solution" interface, the structure of the inner-sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse-reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner-sphere complexes on deposition of the [Co(H(2)O)(6)](2+) ions at the "titania/electrolyte solution" interface. The joint application of semiempirical quantum-mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Co(II) surface concentrations to be determined. It was found that the interface speciation depends on the Co(II) surface concentration. Mononuclear complexes are formed at the compact layer of the "titania/electrolyte solution" interface for low and medium Co(II) surface concentrations. Formation of mono-hydrolyzed Ti(2)O-TiO and the dihydrolyzed TiO-TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H(2)O)(6)](2+) ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H(2)O)(6)](2+) ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner-sphere complexes are formed, in addition to the mononuclear ones, at relatively high Co(II) surface concentrations.

Thin Cu2O films in the thickness range 0.75–230 nm have been prepared on high-quality corning glass, quartz, and Si(100) substrates by radio frequency magnetron sputtering of Cu targets and subsequent oxidation in a furnace under air. Ultraviolet–visible light absorption spectroscopy experiments reveal a blue shift of the energy between the top valence and the first excited conduction sub-bands. The shift increases smoothly as the film thickness decreases. The maximum value observed for the thinnest film is very large, reaching a value of 1.2 eV. Such a shift was not easy to be observed in the past due to the very small Bohr radius of Cu2O. The experimental results, which indicate the presence of intense quantum confinement effects, are well-described by theoretical calculations based on the potential morphing method in the Hartree–Fock approximation.