Starting with metal dithiocarbamate complexes, we synthesize colloidal Cu(2)ZnSnS(4) (CZTS) nanocrystals with diameters ranging from 2 to 7 nm. Structural and Raman scattering data confirm that CZTS is obtained rather than other possible material phases. The optical absorption spectra of nanocrystals with diameters less than 3 nm show a shift to higher energy due to quantum confinement.

Copper zinc tin sulfide (Cu 2 ZnSnS 4 or CZTS) is a potential candidate for next generation thin film solar cells because it contains abundant and nontoxic elements and exhibits high light absorption. Thin films of CZTS are typically synthesized by sulfidizing a stack of zinc, copper, and tin films. In addition to CZTS, a variety of binary and ternary metal sulfides can form and distinguishing among phases with similar crystal structure can be difficult. Herein, the authors show that confocal Raman spectroscopy and imaging can distinguish between CZTS and the other binary and ternary sulfides. Specifically, Raman spectroscopy was used to detect and distinguish between CZTS (338 cm À1), Cu 2 SnS 3 (298 cm À1), and Cu 4 SnS 4 (318 cm À1) phases through their characteristic scattering peaks. Confocal Raman spectroscopy was then used to image the distribution of coexisting phases and is demonstrated to be a useful tool for examining the heterogeneity of CZTS films. The authors show that, during sulfidation of a zinc/copper/tin film stack, ternary sulfides of copper and tin, such as Cu 2 SnS 3 form first and are then converted to CZTS. The reason for formation of Cu 2 SnS 3 as an intermediary to CZTS is the strong tendency of copper and tin to form intermetallic alloys upon evaporation. These alloys sulfidize and form copper tin sulfides first, and then eventually convert to CZTS in the presence of zinc. As a consequence, films sulfidized for 8 h at 400 C contain both CZTS and Cu 2 SnS 3 , whereas films sulfidized at 500 C contain nearly phase-pure CZTS. In addition, using Cu Ka radiation, the authors identify three CZTS X-ray diffraction peaks at 37.1 [(202)], 38 [(211)], and 44.9 [(105) and (213)], which are absent in ZnS and very weak in Cu 2 SnS 3. V

The electronic structure, lattice dynamics, and Raman spectra of the kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) were calculated using density functional theory (DFT). Differences in longitudinal and transverse optical (LO-TO) splitting in kesterite, stannite, and PMCA structures can be used to differentiate them. The Γ-point phonon frequencies, which give rise to Raman scattering, exhibit small but measurable shifts, for these three structures. Experimentally measured Raman scattering from CZTS and CZTSe thin films were examined in light of DFT calculations and deconvoluted to explain subtle shifts and asymmetric line shapes often observed in CZTS and CZTSe Raman spectra. Raman spectroscopy in conjunction with ab initio calculations can be used to differentiate between kesterite, stannite, and PMCA structures of CZTS and CZTSe.

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We prepare Ag(2)Se nanocrystals with average diameters between 2.7 and 10.4 nm that exhibit narrow optical absorption features in the near to mid infrared. We demonstrate that these features are broadly tunable due to quantum confinement. They provide the longest wavelength absorption peaks (6.5 μm) yet reported for colloidal nanocrystals.

Fast separation and spatial control of electrons and holes after photogeneration is important in photocatalysis. Ideally, after photogeneration, electrons and holes must be segregated to different parts of the photocatalyst to take part in separate oxidation and reduction reactions. One way to achieve this is by building junctions into the catalyst with built-in chemical potential differences that tend to separate the electron and the hole into two different regions of the catalyst. In this work, we sought to accomplish this by controllably forming junctions between different phases of TiO(2). A synthesis method has been developed to prepare TiO(2)-B core and anatase shell core-shell nanowires. We control the anatase phase surface coverage on the TiO(2)-B core and show that the maximum photocatalytic activity is obtained when the solution containing the reactants can contact both the anatase and TiO(2)-B phases. The photocatalytic activity drops both with bare TiO(2)-B nanowires and with completely anatase covered TiO(2)-B nanowires. In contrast, nanowires partially covered with anatase phase gives the highest photocatalytic activity. The improved photocatalytic activity is attributed to the effective electron-hole separation at the junction between the anatase and TiO(2)-B phases.

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Using density functional theory, we calculated the electronic structure, the lattice dynamics, and the Raman spectra of Cu2ZnSn(S1−xSex)4, (CZTSSe) an emerging photovoltaic material for thin-film solar cells. In particular, we investigated the effects of the local arrangement of S and Se within the unit cell on the electronic properties of these materials. We find that the S-to-Se ratio (e.g., x) and the spatial distribution of the anions in the unit cell can significantly alter the band structure. In particular, the S-to-Se ratio and anion distribution determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. Moreover, we find that x-ray diffraction patterns and phonon dispersion curves are sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends.