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We have systematically synthesized the alkyl-passivated Si nanoparticles by the solution routes, and have carried out the various spectroscopic studies in order to investigate their detailed electronic structures and optical properties. Alkyl-passivated Si nanoparticles used in this work were synthesized by the oxidation of magnesium silicide with bromine and subsequent surface-termination of the nanoparticles by means of an alkyllithium reagent. Photoluminescence (PL) spectra of thus synthesized alkyl-passivated Si nanoparticles with diameter less than 2 nm exhibit the strong ultraviolet-blue emissions. We have directly investigated their valence-band electronic structures by means of synchrotron-radiation photoemission measurements, and it is concluded that the present PL originates from the electron-hole pair recombination between the modified valence- (HOMO) and conduction-bands (LUMO) due to the intrinsic quantum size effect. These experimental eigenvalues of HOMO-LUMO gaps are quantitatively reproduced by recent quantum Monte Carlo calculations. Moreover, it is found that PL from the alkyl-passivated Si nanoparticles significantly changes when the surface contains contaminants other than alkyl molecules. These results also agree with the predicted ones by quantum Monte Carlo calculations. In addition, it is found that the synchrotron-radiation Si 2p core-level photoemission spectra consist of two components which originate from the inner Si atoms of Si nanoparticles and surface Si atoms bonded to surface-passivants of alkyl molecules (surface component) and that the chemical shifts of these surface components are significantly larger than those of alkyl-terminated bulk Si surfaces. This indicates that the bonding natures between the surface-passivants of alkyl molecules and surface Si atoms are different among the bulk Si surface and Si nanoparticle. From these results, we discuss the detailed electronic, surface chemical, and optical properties of alkyl-passivated Si nanoparticles. |