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

Nanocrystals of multicomponent chalcogenides, such as Cu(2)ZnSnS(4) (CZTS), are potential building blocks for low-cost thin-film photovoltaics (PVs). CZTS PV devices with modest efficiencies have been realized through postdeposition annealing at high temperatures in Se vapor. However, little is known about the precise role of Se in the CZTS system. We report the direct solution-phase synthesis and characterization of Cu(2)ZnSn(S(1-x)Se(x))(4) nanocrystals (0 ≤ x ≤ 1) with the aim of probing the role of Se incorporation into CZTS. Our results indicate that increasing the amount of Se increases the lattice parameters, slightly decreases the band gap, and most importantly increases the electrical conductivity of the nanocrystals without a need for annealing.

No abstract

In this paper, the processes associated with the electrodeposition of bismuth telluride (Bi 2 Te 3 ), a thermoelectric material, are reported along with an analysis of the composition and crystallinity of the resulting films. The electrodeposition can be described by the general reaction 3HTeO 2 ϩ ϩ 2Bi 3ϩ ϩ 18e Ϫ ϩ 9H ϩ → Bi 2 Te 3 ϩ 6H 2 O. Cyclic voltammetry studies of Bi, Te, and Bi/Te dissolved in 1 M HNO 3 reveal two different underlying processes depending on the deposition potential. One process involves the reduction of HTeO 2 ϩ to Te 0 and a subsequent interaction between reduced Te 0 and Bi 3ϩ to form Bi 2 Te 3 . A second process at more negative reduction potentials involves reduction of HTeO 2 ϩ to H 2 Te followed by the chemical interaction with Bi 3ϩ . Both processes result in the production of crystalline Bi 2 Te 3 films in the potential range Ϫ0.1 Ͻ E Ͻ Ϫ0.52 V vs. Ag/AgCl ͑3 M NaCl͒ on Pt substrates as determined by powder X-ray diffraction ͑XRD͒. Electron probe microanalyses and XRD reveal that the films are bismuth-rich and less oriented for more negative deposition potentials.Solid-state thermoelectric devices convert thermal energy from a temperature gradient into electrical energy ͑the Seebeck effect͒ or electrical energy into a temperature gradient ͑the Peltier effect͒. Thermoelectric power generators are used most notably in spacecraft power generation systems ͑for example, in Voyager I and II͒ 1,2 and in thermocouples for temperature measurement, while thermoelectric coolers are largely used in charge coupled device ͑CCD͒ cameras, laser diodes, microprocessors, blood analyzers, and portable picnic coolers. 1,2 Thermoelectric coolers ͑also known as Peltier coolers͒ offer several advantages over conventional systems. As solid-state devices, they have no moving parts. They use no ozonedepleting chlorofluorocarbons, potentially offering a more environmentally responsible alternative to conventional refrigeration. Although some large-scale applications have been considered ͑on submarines and surface vessels͒, their efficiency is low compared to conventional refrigerators.Scientific and technological interest in the production of nanostructured thermoelectric materials has been driven by recent theoretical studies, which suggest that quantum confinement of electrons and holes could enhance the efficiency of these materials significantly above that of their bulk values. 3-5 This hypothesis has already been verified for thin multilayers of PbTe/Pb 1Ϫx Eu x Te. 6-11 Larger enhancements are predicted for one-dimensional ͑1-D͒ systems ͑nanowires͒ compared to 2-D systems ͑thin films͒. 12,13 These predictions have stimulated research into the preparation of nanowires of thermoelectric materials.Bismuth telluride (Bi 2 Te 3 ) and its doped derivative compounds are considered to be the best materials to date for near roomtemperature thermoelectric applications. 14,15 The maximum figure of merit ͑ZT͒ occurs for optimized doping levels 16 at approximately 70°C with an effective operating range of Ϫ100 to...

Narrow Bi2Te3 nanowire arrays, where the individual wires are dense and parallel, have been fabricated by electrodeposition. The array–template composites have a high wire density over a large area (see Figure) and are relatively thick, which makes them ideally suited for direct incorporation into existing device structures for thermoelectric or other applications.