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A method for measuring refractive index n of nanosize particles in the visible and near-IR spectral ranges is proposed. The method is based on comparing refractive index n colloid of a colloid solution of nanoparticles in several solvents with refractive indices n solvent of corresponding pure solvents and has an accuracy of ±2 × 10 -4 . Upconversion nanosize phosphors (UCNPs) are synthesized in the form of a β-NaYF 4 crystalline matrix doped with Yb 3+ , Er 3+ , and Tm 3+ rare earth ions. UCNPs have a doped core with a diameter of 40 ± 5 nm and undoped shell with a thickness of 3-5 nm. Synthesized nanocrystals possess intense photoluminescence in the blue, green, and red spectral ranges upon excitation by IR radiation with a wavelength of 977 nm. Using a spectroscopic refractometer, the dispersion of the refractive index of β-NaYF 4 /Yb 3+ /Er 3+ /Tm 3+ nanocrystals was measured for the first time in the spectral range of 450-1000 nm with an accuracy of ±2 × 10 -4 .

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GeX 2 ‚dioxane (X ) Cl, Br) complexes insert completely into CBr 4 to afford the sterically crowded cluster compounds (BrCl 2 Ge) 4 C (1) and (Br 3 Ge) 4 C (2) in 80% and 95% yields, respectively. These display physical, spectroscopic, and structural properties that are indicative of highly symmetric molecules with a remarkably strained carbon center. Compounds 1 and 2 react with LiAlH 4 to produce the hydrides (H 3 -Ge) 3 CH (3) and (H 3 Ge) 4 C (4) which are readily identified and characterized by spectroscopic methods and gas-phase electron diffraction. Compound 3 is also conveniently prepared from the LiAlH 4 reduction of (GeBr 3 ) 3 CH (5) which in turn is obtained by insertion of GeBr 2 ‚dioxane into the C-Br bonds of bromoform. Refinement of the diffraction data for 3 confirmed a model of C 3 symmetry, with local C 3V symmetry of the GeH 3 groups, and gave a Ge-C bond length of 1.96 Å. The structure refinement of 4 was based on a model of T symmetry and displayed a rather normal Ge-C bond distance of 1.97 Å, which is substantially shorter than that (2.049 Å) of the strained (Br 3 Ge) 4 C (2) compound. Density functional calculations closely reproduced the observed molecular structures for 3 and 4. The thermal dehydrogenation of 4 on (100) Si surfaces at 500 °C resulted in the growth of a diamond-structured material with an approximate composition of Ge 4 C. Reactions of 4 with (SiH 3 ) 2 on Si yielded heteroepitaxial growth of metastable, monocrystalline (Ge 4 C) x Si y alloy semiconductors that are intended to have band gaps wider than those of pure Si and Si 1-x Ge x alloys and strained superlattices. The covalent cluster species described here not only are of intrinsic molecular interest but also provide a unique route to a new class of semiconductor materials and form a model for local carbon sites in Ge-C crystals and related electronic materials based on the diamond structure.