The light-emitting properties of rapid thermally oxidized porous Si are studied by both photoluminescence and cathodoluminescence methods. The structure of the material is examined by transmission electron microscopy, while its oxygen content is determined by x-ray microanalysis. These investigations show that crystalline Si nanostructures remain in the heavily oxidized porous material and account for its ∼750 nm red photo- and cathodoluminescence. The work demonstrates that the previously speculated possible importance of either Si-based amorphous phases or the interesting material, siloxene, in this regard is unrealistic. Furthermore, it is shown that the luminescence properties of silicon oxides are of paramount importance in interpreting the many additional (shorter wavelength) emission bands observed.
thiolate substituents effectively block further secondary interactions or of some more subtle steric or electronic effects remains problematical.Acknowledgment. We gratefully acknowledge support from the National Science Foundation, the National Institutes of Health, the donors of the Petroleum Research Fund, administered by the American Chemical Society, the Herman Frasch Foundation, and Societe Nationale Elf Aquitaine. Funding for the 300-MHz NMR facility was provided by the National Science Foundation.Supplementary Material Available: Tables of bond lengths (Tables SI and S10), bond angles, (Tables S2 and Sll), anisotropic temperature factors (Tables S3 and SI2), and calculated hydrogen atom positions (Tables S4 and S13) for I and III, tables of bond lengths (Table S6), bond angles (Table S7), and anisotropic temperature factors (Table S8)for II, and a summary of the experimental details for the X-ray diffraction studies of I-III (Table S15) (22 pages); tables of observed and calculated structure factors (Tables S5, S9, and S14) for I-III (65 pages). Ordering information is given on any current masthead page.
The nature of the metal-ligand bond in molybdenum(II1) thiocyanate complexes has been investigated. The infrared spectra of a number of transition-metal thiocyanates and isothiocyanates of the types flI(CNS),lnand W(CNS)Jn-have been studied: the feature most characteristic of metal-sulphur bonding is a lower C-S stretching frequency, -700 cm.-l, to be compared with -800 cm.-l when the bonding is to the nitrogen atom. This leads to the conclusion that the [Mo(CNS)J3-complexes are isothiocyanates. Additional evidence in favour of this formulation is provided by X-ray isomorphism and visible and ultraviolet spectra. The role of the water molecules in the hydrated Po(NCS)J3-complexes is discussed and it is concluded that they are not co-ordinated to the metal atom. Magnetic data on a number of tervalent molybdenum complexes are reported. In all cases except K4Mo(CN),,2H,0 the magnetic moments correspond to the presence of three unpaired electrons and hence to six-co-ordination. For the complex cyanide the moment is 1-75 13.14. and the molybdenum atom is considered to be seven-co-ordinate. Finally, factors which are important in deciding whether a metal atom will bond to sulphur or nitrogen in [NCSIare discussed briefly and an explanation is proposed.* " Thiocyanate " and (CNS) arc used without prejudice as to whcther the group is sulphuror nitrogen-bonded to a metal atom.
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