Recent advances have been made in thin-film solar cells using CdTe and CuIn(1-x)Ga(x)Se(2) (CIGS) nanoparticles, which have achieved impressive efficiencies. Despite these efficiencies, CdTe and CIGS are not amenable to large-scale production because of the cost and scarcity of Te, In, and Ga. Cu(2)ZnSnS(4) (CZTS), however, is an emerging solar cell material that contains only earth-abundant elements and has a near-optimal direct band gap of 1.45-1.65 eV and a large absorption coefficient. Here we report the direct synthesis of CZTS nanocrystals using the hot-injection method. In-depth characterization indicated that pure stoichiometric CZTS nanocrystals with an average particle size of 12.8 +/- 1.8 nm were formed. Optical measurements showed a band gap of 1.5 eV, which is optimal for a single-junction solar device.

Stoichiometric copper(I) selenide nanoparticles have been synthesized using the hot injection method. The effects of air exposure on the surface composition, crystal structure, and electronic properties were monitored using X-ray photoelectron spectroscopy, X-ray diffraction, and conductivity measurements. The current-voltage response changes from semiconducting to ohmic, and within a week a 3000-fold increase in conductivity is observed under ambient conditions. The enhanced electronic properties can be explained by the oxidation of Cu(+) and Se(2-) on the nanoparticle surface, ultimately leading to a solid-state conversion of the core from monoclinic Cu(2)Se to cubic Cu(1.8)Se. This behavior is a result of the facile solid-state ionic conductivity of cationic Cu within the crystal and the high susceptibility of the nanoparticle surface to oxidation. This regulated transformation is appealing as one could envision using layers of Cu(2)Se nanoparticles as both semiconducting and conducting domains in optoelectronic devices simply by tuning the electronic properties for each layer through controlled oxidation.

We report the synthesis of single-crystalline VO2 nanowires with rectangular cross sections using a vapor transport method. These nanowires have typical diameters of 60 (+/-30) nm and lengths up to >10 mum. Electron microscopy and diffraction measurements show that the VO2 nanowires are single crystalline and exhibit a monoclinic structure. Moreover, they preferentially grow along the [100] direction and are bounded by the (01) and (011) facets. These VO2 nanowires should provide promising materials for fundamental investigations of nanoscale metal-insulator transitions.

Abstract