The ability to engineer the band gap energy of semiconductor nanocrystals has led to the development of nanomaterials with many new exciting properties and applications. Band gap engineering has thus proven to be an effective tool in the design of new nanocrystal-based semiconductor devices. As reported in numerous publications over the last three decades, tuning the size of nanocrystalline semiconductors is one way of adjusting the band gap energy. On the other hand, research on band gap engineering via control of nanocrystal composition, which is achieved by adjusting the constituent stoichiometries of alloyed semiconductors, is still in its infancy. In this Account, we summarize recent research on colloidal alloyed semiconductor nanocrystals that exhibit novel composition-tunable properties. Alloying of two semiconductors at the nanometer scale produces materials that display properties distinct not only from the properties of their bulk counterparts but also from those of their parent semiconductors. As a result, alloyed nanocrystals possess additional properties that are composition-dependent aside from the properties that emerge due to quantum confinement effects. For example, although the size-dependent emission wavelength of the widely studied CdSe nanocrystals can be continuously tuned to cover almost the entire visible spectrum, the near-infrared (NIR) region is far outside its spectral range. By contrast, certain alloy compositions of nanocrystalline CdSe(x)Te(1-x), an alloy of CdSe and CdTe, can efficiently emit light in the NIR spectral window. These NIR-emitting nanocrystals are potentially useful in several biomedical applications. In addition, highly stable nanocrystals formed by alloying CdSe with ZnSe (i.e., Zn(x)Cd(1-x)Se) emit blue light with excellent efficiency, a property seldom achieved by the parent binary systems. As a result, these materials can be used in short-wavelength optoelectronic devices. In the future, we foresee new discoveries related to these interesting nanoalloys. In particular, colloidal semiconductor nanoalloys that exhibit composition-dependent magnetic properties have yet to be reported. Further studies of the alloying mechanism are also needed to develop improved synthetic strategies for the preparation of these alloyed nanomaterials.

Semiconductor nanostructures that can effectively serve as light-responsive photocatalysts have been of considerable interest over the past decade. This is because their use in light-induced photocatalysis can potentially address some of the most serious environmental and energy-related concerns facing the world today. One important application is photocatalytic hydrogen production from water under solar radiation. It is regarded as a clean and sustainable approach to hydrogen fuel generation because it makes use of renewable resources (i.e., sunlight and water), does not involve fossil fuel consumption, and does not result in environmental pollution or greenhouse gas emission. Another notable application is the photocatalytic degradation of nonbiodegradable dyes, which offers an effective way of ridding industrial wastewater of toxic organic pollutants prior to its release into the environment. Metal oxide semiconductors (e.g., TiO2) are the most widely studied class of semiconductor photocatalysts. Their nanostructured forms have been reported to efficiently generate hydrogen from water and effectively degrade organic dyes under ultraviolet-light irradiation. However, the wide band gap characteristic of most metal oxides precludes absorption of light in the visible region, which makes up a considerable portion of the solar radiation spectrum. Meanwhile, nanostructures of cadmium chalcogenide semiconductors (e.g., CdS), with their relatively narrow band gap that can be easily adjusted through size control and alloying, have displayed immense potential as visible-light-responsive photocatalysts, but the intrinsic toxicity of cadmium poses potential risks to human health and the environment. In developing new nanostructured semiconductors for light-driven photocatalysis, it is important to choose a semiconducting material that has a high absorption coefficient over a wide spectral range and is safe for use in real-world settings. Among the most promising candidates are the multinary chalcogenide semiconductors (MCSs), which include the ternary I-III-VI2 semiconductors (e.g., AgGaS2, CuInS2, and CuInSe2) and the quaternary I2-II-IV-VI4 semiconductors (e.g., Cu2ZnGeS4, Cu2ZnSnS4, and Ag2ZnSnS4). These inorganic compounds consist of environmentally benign elemental components, exhibit excellent light-harvesting properties, and possess band gap energies that are well-suited for solar photon absorption. Moreover, the band structures of these materials can be conveniently modified through alloying to boost their ability to harvest visible photons. In this Account, we provide a summary of recent research on the use of ternary I-III-VI2 and quaternary I2-II-IV-VI4 semiconductor nanostructures for light-induced photocatalytic applications, with focus on hydrogen production and organic dye degradation. We include a review of the solution-based methods that have been employed to prepare multinary chalcogenide semiconductor nanostructures of varying compositions, sizes, shapes, and crystal structures, which are factors that a...

The ability of nanoscopic materials to self-organize into large-scale assembly structures that exhibit unique collective properties has opened up new and exciting opportunities in the field of nanotechnology. Although earlier work on nanoscale self-assembly has focused on colloidal spherical nanocrystals as building blocks, there has been significant interest in recent years in the self-assembly of colloidal nanocrystals having well-defined facets or anisotropic shapes. In this review, particular attention is drawn to anisotropic one-dimensional (1D) nanocrystals, notably nanorods and nanowires, which can be arranged into a multitude of higher-order assembly structures. Different strategies have been developed to realize self-assembly of colloidal 1D nanocrystals and these are highlighted in the first part of this review. Self-assembly can take place (1) on substrates through evaporation control, external field facilitation and template use; (2) at interfaces, such as the liquid-liquid and the gas-liquid interface; and (3) in solutions via chemical bonding, depletion attraction forces and linker-mediated interactions. The choice of a self-assembly approach is pivotal to achieving the desired assembly configuration with properties that can be exploited for functional device applications. In the subsequent sections, the various assembly structures that have been created through 1D nanocrystal self-assembly are presented. These organized structures are broadly categorized into non-close-packed and close-packed configurations, and are further classified based on the different types of 1D nanocrystal alignment (side-by-side and end-to-end), orientation (horizontal and vertical) and ordering (nematic and smectic), and depending on the dimensionality of the structure (2D and 3D). The conditions under which different types of arrangements are achieved are also discussed.

Highly emissive and air-stable AgInS2-ZnS quantum dots (ZAIS QDs) with quantum yields of up to 20% have been successfully synthesized directly in aqueous media in the presence of polyacrylic acid (PAA) and mercaptoacetic acid (MAA) as stabilizing and reactivity-controlling agents. The as-prepared water-dispersible ZAIS QDs are around 3 nm in size, possess the tetragonal chalcopyrite crystal structure, and exhibit long fluorescence lifetimes (>100 ns). In addition, these ZAIS QDs are found to exhibit excellent optical and colloidal stability in physiologically relevant pH values as well as very low cytotoxicity, which render them particularly suitable for biological applications. Their potential use in biological labelling of baculoviral vectors is demonstrated.

Cu 2 ZnSnS 4 (CZTS), a quarternary chalcogenide p-type semiconductor, is currently receiving considerable attention as absorber materials for low-cost photovoltaics due to its high absorption coefficient, optimal band gap, and naturally abundant and nontoxic elemental components.[1] The growing technological interest in this material has motivated the study of the nature of its crystal structures.[2] The groundstate crystal structure of CZTS is the kesterite form (space group I4 ), which is a tetragonal superstructure derived from the binary II-VI cubic zinc-blende (ZB) lattice. Several other ZB-related structural modifications of CZTS have been considered, and these include the stannite structure (space group I4 2m). This structure type differs from the kesterite form only in the ordering of Cu + and Zn 2 + ions. In a recent theoretical study, Chen et al. have shown that there are two low-energy structural configurations of CZTS that are not based on the ZB unit cell.[2f] Instead, these two predicted structures are derivatives of the binary II-VI hexagonal wurtzite (WZ) structure and are conveniently referred to as WZ-kesterite and WZ-stannite, owing to their structural relationship with the ZB-derived kesterite and stannite polytypes, respectively. The WZ-kesterite form is a monoclinic (pseudo-orthorhombic) superstructure of the WZ unit cell and has the space group Pc. The WZ-stannite phase, on the other hand, is described by an orthorhombic supercell with space group Pmn2 1 . In bulk form, WZ-type superstructures have long been reported for a number of quarternary chalcogenides such as Cu 2 MGeS 4 (in which M = Mn, Zn, Cd).[3] Bulk CZTS with a WZ-derived phase, however, has yet to be synthesized.In nanocrystalline form, CZTS materials have been colloidally prepared by means of the hot-injection synthetic strategy involving the reaction of the Cu, Zn, and Sn precursors with elemental sulfur in oleylamine at high-temperature conditions.[4] The nanocrystals produced by this method are quite polydisperse in shape and size, and adopt the thermodynamically more stable ZB-derived tetragonal phase. Very recently, Lu et al. have employed the hot-injection technique and used dodecanethiol as the sulfur source in preparing CZTS nanoprisms and nanoplates that are 20-50 nm in size.[5] X-ray diffraction (XRD) measurements revealed that these nanocrystals possess a WZ-related crystal structure. Their proposed structure is based on the hexagonal WZZnS unit cell described by the space group P6 3 mc, in which the metal cations are randomly distributed in the cation sites (i.e., cation-disordered). However, the possibility that their WZ-type CZTS exhibits the theoretically predicted lower-energy cation-ordered WZ-kesterite and WZ-stannite structures has not been considered.Herein, we provide a facile noninjection synthetic route to preparing monodisperse anisotropic CZTS nanocrystals that adopt a WZ-type crystal structure. The noninjection or "heating up" approach to colloidal nanocrystals is better in terms of synthetic reprodu...

In fabricating materials at the nanometer scale, nanotechnologists typically employ two general strategies: bottom-up and top-down. While the bottom-up approach constructs nanomaterials from basic building blocks like atoms or molecules, the top-down approach produces nanostructures by deconstructing larger materials with the use of lithographic tools (i.e., physical top-down) or through chemical-based processes (i.e., chemical top-down). This tutorial review summarizes the various top-down nanofabrication methods, with great emphasis on the chemical routes that can generate nanoporous materials and ordered arrays of nanostructures with three-dimensional features. The chemical top-down routes that are discussed in detail include (1) templated etching, (2) selective dealloying, (3) anisotropic dissolution, and (4) thermal decomposition. These emerging nanofabrication tools open up new avenues in the creation of functional nanostructures with a wide array of promising applications.

We have synthesized a series of lanthanide dithiocarbamate precursors for the synthesis of lanthanide sulfide materials and nanoparticles. Three dithiocarbamate complexes with europium, [Eu(S 2 CNRR′) 3 L], where L ) 1,10-phenanthroline, and R ) methyl, R′ ) ethyl (1), R ) R′ ) n Propyl (2), and R ) R′ ) i Butyl (3), as well as the lanthanide complexes, [Ln(S 2 CNR 2 ) 3 L], where R ) ethyl, L ) 1, 10-phenanthroline, and Ln ) Nd (4), Sm (5), Gd (6), Ho (7), and Er (8), were synthesized and characterized by single-crystal X-ray diffraction, infrared, NMR, and UV-visible spectroscopy. We have used thermal analysis coupled with GC-MS and X-ray powder diffraction to determine the mechanism of decomposition. With R ) Et, smaller Ln ions give lower precursor decomposition temperatures, consistent with the higher lattice energies of the product Ln sulfides. Because they are monomeric, and water-and air-stable, these compounds should be ideal precursors for forming LnS as nanoparticles and bulk materials.

DC and ac magnetization, resistivity, specific heat, and neutron diffraction data reveal that stoichiometric LaOFeP is metallic and non-superconducting above. Neutron diffraction data at room temperature and K T 10 = are well described by the stoichiometric, tetragonal ZrCuSiAs structure and show no signs of structural distortions or long range magnetic ordering, to an estimated detectability limit of 0.07 μ B /Fe. We propose a model, based on the shape of the iron-pnictide tetrahedron, that explains the differences between LaOFeP and LaOFeAs, the parent compound of the recently discovered high-Tc oxyarsenides, which, in contrast, shows both structural and spin density wave (SDW) transitions.