Controlled growth of tetrapod-branched inorganic nanocrystals

Synthesis of Quantum-Sized Cubic ZnS Nanorods by the Oriented Attachment Mechanism

Quantum-sized ZnS nanocrystals with quasi-spherical and rod shapes were synthesized by the aging reaction mixtures containing diethylzinc, sulfur, and amine. Uniform-sized ZnS nanorods with the average dimension of 5 nm x 21 nm, along with a small fraction of 5 nm-sized quasi-spherical nanocrystals, were synthesized by adding diethylzinc to a solution containing sulfur and hexadecylamine at 125 degrees C, followed by aging at 300 degrees C. Subsequent secondary aging of the nanocrystals in oleylamine at 60 degrees C for 24 h produced nearly pure nanorods. Structural characterizations showed that these nanorods had a cubic zinc blende structure, whereas the fabrication of nanorods with this structure has been known to be difficult to achieve via colloidal chemical synthetic routes. High-resolution TEM images and reaction studies demonstrated that these nanorods are formed from the oriented attachment of quasi-spherical nanocrystals. Monodisperse 5 nm-sized quasi-spherical ZnS nanocrystals were separately synthesized by adding diethylzinc to sulfur dissolved in a mixture of hexadecylamine and 1-octadecene at 45 degrees C, followed by aging at 300 degrees C. When oleic acid was substituted for hexadecylamine and all other procedures were unchanged, we obtained 10 nm-sized quasi-spherical ZnS nanocrystals, but with broad particle size distribution. These two different-sized quasi-spherical ZnS nanocrystals showed different proportions of zinc blende and wurtzite crystal structures. The UV absorption spectra and photoluminescence excitation spectra of the 5 nm ZnS quasi-spherical nanocrystals and of the nanorods showed a blue-shift from the bulk band-gap, thus showing a quantum confinement effect. The photoluminescence spectra of the ZnS nanorods and quasi-spherical nanocrystals showed a well-defined excitonic emission feature and size- and shape-dependent quantum confinement effects.

Synthesis and Structural Metastability of CdTe Nanowires

A new organometallic preparation method is described for CdTe nanowires with a high aspect ratio and a predominantly metastable wurtzite phase. The optical and morphological properties of the resulting nanowires were studied, as well as the influence of elevated temperatures on the crystallographic properties. A phase transition from wurtzite to sphalerite was observed at about 500 degrees C. The results show that the wurtzite phase is stabilized by the synthetic method and the surfactants.

In Situ One-Pot Synthesis of 1-Dimensional Transition Metal Oxide Nanocrystals

One-dimensional colloidal metal oxide nanocrystals are of great importance in materials chemistry, but reports on these materials are rare due to lack of well-defined synthetic protocols. In this paper, we present a general and highly effective one-pot synthetic protocol to produce 1-dimensional nanostructures of transition metal oxide (e.g., W(18)O(49), TiO(2), Mn(3)O(4), and V(2)O(5)) through thermally induced crystal growth processes from a mixture of metal chloride and surfactants.

Radial anisotropic growth of rhodium nanoparticles

In this contribution, we report the synthesis of rhodium multipods that result from a homogeneous seeded growth mechanism. Small Rh nanocrystal seeds were synthesized by the reduction of RhCl3 in ethylene glycol in the presence of PVP. These seed particles could be subsequently used, without isolation, to form larger rhodium nanoparticles. A reaction temperature of 190 degrees C led to isotropic cubic Rh particles. Lowering the reaction temperature resulted in more anisotropic growth, which gave Rh cubes with horns at 140 degrees C, and Rh multipods at 90 degrees C. The anisotropic growth occurred in the (111) direction, as determined by high-resolution TEM (HRTEM). Anisotropic growth proceeds via a seeded growth mechanism, and not by oriented attachment.

Selective growth of PbSe on one or both tips of colloidal semiconductor nanorods

PbSe nanocrystals with rock-salt structure are grown on the tips of colloidal CdS and CdSe nanorods. The facets of wurtzite rods provide a substrate with various degrees of reactivity for the growth of PbSe. The presence of dangling Cd bonds may explain subtle differences between nonequivalent facets resulting in the selective nucleation of PbSe only on one of the two tips of each CdS rod. This approach has the potential to facilitate the fabrication of heterostructures with tailored optical and electronic properties.

Amine-Assisted Facetted Etching of CdSe Nanocrystals

The treatment of CdSe nanocrystals (NCs) in a 3-amino-1-propanol (APOL)/water (v/v = 10:1) mixture at 80 degrees C in the presence of O(2) causes them to undergo a slow chemical etching process, as evidenced by spectroscopic and structural investigations. Instead of the continuous blue shift expected from a gradual decrease in NC dimensions, a bottleneck behavior was observed with distinct plateaus in the peak position of photoluminescence (PL) and corresponding maxima in PL quantum yield (i.e., 34 +/-7%). It is presently argued that such etching behavior is a result of two competitive processes taking place on the surface of these CdSe NCs: (i) oxidation of the exposed Se-sites to acidic SeO(x)() entities, which are readily solubilized in the basic APOL/H(2)O mixture, and (ii) coordination of the underlying Cd-sites with both amines and hydroxyl moieties to temporally impede NC dissolution. This is consistent with the HRTEM results, which suggest that the etched NCs adopt pyramidal morphologies with Cd-terminated facets (i.e., (0001) bases and either {011} or {21} sides) and account for the apparent resistance to etching at the plateau regions.

Morphologies and microstructures of nano-sized Cu(2)O particles using a cetyltrimethylammonium template

Hollow polyhedra and cubes of nanostructured Cu(2)O particles have been synthesized by reduction of CuSO(4) with ascorbate acid in the solution phase. The nanostructures were obtained when the cetyltrimethylammonium (CTAB) concentration ranged from 0 to 0.03 M in the presence of NaOH. Structural characterizations, by means of x-ray photoelectron spectroscopy (XPS) for measuring Cu valence states and by electron microscopy for microstructure and chemical analyses, suggest that most Cu(2)O nanoparticles are covered with a thin CuO shell arising possibly from reaction of the adsorbed oxygen on the Cu(2)O particle surface. The blue shift is observed as microstructures of Cu(2)O nanoparticles changed from cubic to hollow in ultraviolet and visible (UV-visible) absorption spectra. Both the Cu(2)O hollow and cubic nanostructures show certain quantum-confined effects. A cationic CTAB template mechanism is proposed to interpret the formation of the Cu(2)O nanoparticles.

Cadmium Amido Alkoxide and Alkoxide Precursors for the Synthesis of Nanocrystalline CdE (E = S, Se, Te)

The synthesis and characterization of a family of alternative precursors for the production of CdE nanoparticles (E = S, Se, and Te) is reported. The reaction of Cd(NR2)2 where NR2 = N(SiMe3)2 with n HOR led to the isolation of the following: n = 1 [Cd(mu-OCH2CMe3)(NR2)(py)]2 (1, py = pyridine), Cd[(mu-OC6H3(Me)(2)-2,6)2Cd(NR2)(py)]2 (2), [Cd(mu-OC6H3(CHMe2)(2)-2,6)(NR2)(py)]2 (3), [Cd(mu-OC6H3(CMe3)(2)-2,6)(NR2)(py)]2 (4), [Cd(mu-OC6H2(NH2)(3)-2,4,6)(NR2)(py)]2 (5), and n = 2 [Cd(mu-OC6H3(Me)(2)-2,6)(OC6H3(Me)(2)-2,6)(py)2]2 (6), and [Cd(mu-OC6H3(CMe3)(2)-2,6)(OC6H3(CMe3)(2)-2,6)(THF)]2 (7). For all but 2, the X-ray crystal structures were solved as discrete dinuclear units bridged by alkoxide ligands and either terminal -NR2 or -OR ligands depending on the stoichiometry of the initial reaction. For 2, a trinuclear species was isolated using four mu-OR and two terminal -NR2 ligands. The coordination of the Cd metal center varied from 3 to 5 where the higher coordination numbers were achieved by binding Lewis basic solvents for the less sterically demanding ligands. These complexes were further characterized in solution by 1H, 13C, and 113Cd NMR along with solid-state 113Cd NMR spectroscopy. The utility of these complexes as "alternative precursors" for the controlled preparation of nanocrystalline CdS, CdSe, and CdTe was explored. To synthesize CdE nanocrystals, select species from this family of compounds were individually heated in a coordinating solvent (trioctylphosphine oxide) and then injected with the appropriate chalcogenide stock solution. Transmission electron spectroscopy and UV-vis spectroscopy were used to characterize the resultant particles.

Molecular engineering of surfaces using self-assembled monolayers

The self-assembly of molecules into structurally organized monolayers (SAMs) uses the flexibility of organic chemistry and coordination chemistry to generate well-defined, synthetic surfaces with known molecular and macroscopic properties. The process of designing monolayers with a specified structure gives a high level of control over the molecular-level composition in the direction perpendicular to a surface; soft lithographic technique gives useful (if lower) resolution in the plane of the surface. Alkanethiolates adsorbed on gold, silver, mercury, palladium and platinum are currently the best-defined systems of SAMs. They provide substrates for a number of applications-from studies of wetting and electron transport to patterns for growing mammalian cells. SAMs have made organic surfaces a central part of surface science. Understanding the principles by which they form, and connecting molecular-level structure with macroscopic properties, opens a wide range of areas to study and exploitation.

Shape and size control of Ag2Se nanocrystals from a single precursor [(Ph3P)3Ag2(SeC{O}Ph)2]

Monodispersed Ag2Se nanocubes and faceted nanocrystals have been synthesized by hexadecylamine (HDA) induced thermolysis of [(PPh3)3Ag2(SeC{O}Ph)2] in a mixture of TOP (tri-n-octyl phosphine) and HDA in the temperature range 95-180 degrees C.

Formation of ZnO hexagonal micro-pyramids: a successful control of the exposed polar surfaces with the assistance of an ionic liquid

Wurtzite ZnO hexagonal micro-pyramids, with all exposed surfaces being polar +/- (0001) and {1011} planes, have been successfully synthesized using ionic liquids as solvents.

On the development of colloidal nanoparticles towards multifunctional structures and their possible use for biological applications

In this Review, we describe the synthesis of high-quality colloidal nanoparticles in organic solvents, the mechanisms by which they can be transferred into aqueous solution, and some of their applications in biology. In particular, we will place emphasis on the creation of multifunctional nanoparticles or nanoparticle assemblies.

Heterostructured Bi2Se3 Nanowires with Periodic Phase Boundaries

Single and multijunction nanowires with diameters of 10-40 nm were synthesized by an electrically assisted sonochemical method in aqueous solution at room temperature. By properly controlling the reaction conditions and reactants, this method opens a new strategy towards the fabrication of one-dimensional superlattices in nanowires under ambient conditions. Such a chemical approach provides a promising and simple way to synthesize high-performance building blocks for device applications.

Cation Exchange Reactions in Ionic Nanocrystals

Cation exchange has been investigated in a wide range of nanocrystals of varying composition, size, and shape. Complete and fully reversible exchange occurs, and the rates of the reactions are much faster than in bulk cation exchange processes. A critical size has been identified below which the shapes of complex nanocrystals evolve toward the equilibrium shape with lowest energy during the exchange reaction. Above the critical size, the anion sublattice remains intact and the basic shapes of the initial nanocrystals are retained throughout the cation exchange. The size-dependent shape change can also be used to infer features of the microscopic mechanism.

Composite mesostructures by nano-confinement

In a physically confined environment, interfacial interactions, symmetry breaking, structural frustration and confinement-induced entropy loss can play dominant roles in determining molecular organization. Here we present a systematic study of the confined assembly of silica-surfactant composite mesostructures within cylindrical nanochannels of varying diameters. Using exactly the same precursors and reaction conditions that form the two-dimensional hexagonal SBA-15 mesostructured thin film, unprecedented silica mesostructures with chiral mesopores such as single- and double-helical geometries spontaneously form inside individual alumina nanochannels. On tightening the degree of confinement, a transition is observed in the mesopore morphology from a coiled cylindrical to a spherical cage-like geometry. Self-consistent field calculations carried out to account for the observed mesostructures accord well with experiment. The mesostructures produced by confined syntheses are useful as templates for fabricating highly ordered mesostructured nanowires and nanowire arrays.

Perfect Orientation Ordered in-Situ One-Dimensional Self-Assembly of Mn-Doped PbSe Nanocrystals

We demonstrate a novel approach for the large-scale, shape-controlled synthesis of one-dimensional (1D) corrugated nanoarrays of Pb(1-x)Mn(x)Se nanocrystals (0.002 < or = x < or = 0.008) through an in-situ self-assembly without using either capping polymer or ionic surfactant. The one-step-prepared 1D nanoarrays exhibit a well-defined morphology, single-crystal orientation, and clean surface without amorphous contamination. The average diameter of the 1D nanoarrays can be controlled and varied from <10 to 80 nm by finely tuning the assembly temperature and the growth time. Four growth models were suggested to explain the in-situ self-assembly processes based on the fundamental building blocks of octahedral nanocrystals by sharing [111] facets.

Self-generation of tiered surfactant superstructures for one-pot synthesis of Co3O4 nanocubes and their close- and non-close-packed organizations

Self-generation of ionic organic capping from nonionic surfactant polyoxyethylene (20) sorbitan trioleate (Tween-85) has been realized for the controlled synthesis of single crystalline Co(3)O(4) quantum dots (3.0-5.7 nm) in cubic morphology from related layered hydroxide precursors at 80-95 degrees C. With chemical modification of hydrophobic functional groups on the surface of Co(3)O(4) nanocubes; furthermore, various nanocube-containing micellar superstructures can be further assembled through hydrophobic interactions between Tween-85 molecules and the surface coating under "one-pot" conditions. In particular, square arrangements, spherical domains, and line-assemblies of the prepared Co(3)O(4) nanocubes and their inter-transformations have been attained for the first time by manipulating intersurfactant-interactions. Hydrolysis of Tween-85 and the resultant tiered surfactant superstructures have been investigated with FTIR/UV-vis/EA/TGA/DTA/XPS methods, and the capping species has been identified to be alkylated oleic carboxylate anions derived from Tween-85. Pronounced quantum confinement effects have been observed with the prepared Co(3)O(4) nanocubes, and the optical band gap energies determined are 3.95 and 2.13 eV, respectively, for O(2-)--> Co(2+) and O(2-)--> Co(3+) charge-transfer processes.

Synthesis of square gadolinium-oxide nanoplates

In this paper, we report a colloidal synthesis of high-quality, square, plate-shaped Gd2O3 nanocrystals. These nanoplates are single crystalline with a thickness of the Gd2O3 unit-cell edge length, approaching the lower limit of crystal growth.

Single-step synthesis and stabilization of metal nanoparticles in aqueous pluronic block copolymer solutions at ambient temperature

A single-step synthesis of gold nanoparticles with an average diameter of approximately 10 nm from hydrogen tetrachloroaureate(III) hydrate (HAuCl4.3H2O) has been achieved in air-saturated aqueous solutions that contain poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers but not any other reducing agent. These amphiphilic block copolymers act as both reductants and colloidal stabilizers and prove very efficient in both functions. The formation of gold nanoparticles is controlled by the overall molecular weight and relative block length of the block copolymer. The synthesis procedure reported here is environmentally benign and economic, as it involves the minimum possible number of components: it uses water as the solvent, it uses commercially available polymers, it proceeds fast to completion, and it results in a "ready-to-use" product.

Seeded high yield synthesis of short Au nanorods in aqueous solution

Short gold nanorods of average lengths ranging between 20 and 100 nm (with corresponding aspect ratios of 2 and 4) were synthesized in excellent yield (approximately 97%). These nanorods were characterized by dark-field microscopy, UV-visible spectrophotometry, and transmission electron microscopy. Temporal evolution of rod shape had also been followed by UV-visible spectrophotometry and transmission electron microscopy and indicates that the nanorods briefly increase in length, then increase slightly in width, as they grow. The effect of the synthetic parameters on the rod dimension and yield was explored to find out suitable conditions to produce short nanorods; short nanorods have both plasmon bands in the visible region of the spectrum, which is a valuable property for sensor applications.

Selective growth of metal tips onto semiconductor quantum rods and tetrapods

We show the anisotropic selective growth of gold tips onto semiconductor (cadmium selenide) nanorods and tetrapods by a simple reaction. The size of the gold tips can be controlled by the concentration of the starting materials. The new nanostructures display modified optical properties caused by the strong coupling between the gold and semiconductor parts. The gold tips show increased conductivity as well as selective chemical affinity for forming self-assembled chains of rods. Such gold-tipped nanostructures provide natural contact points for self-assembly and for electrical devices and can solve the difficult problem of contacting colloidal nanorods and tetrapods to the external world.

Room Temperature, High-Yield Synthesis of Multiple Shapes of Gold Nanoparticles in Aqueous Solution

A seed-mediated growth method was used to control the morphology and dimensions of Au nanocrystals by the manipulation of the experimental parameters in aqueous solution at room temperature. This chemical route produces various structural architectures with rod-, rectangle-, hexagon-, cube-, triangle-, and starlike profiles and branched (such as bi-, tri-, tetra-, and multipod) Au nanocrystals of various dimensions in high yield in the presence of a single surfactant, cetyltrimethylammonium bromide.

Colloidal nanocrystal heterostructures with linear and branched topology

The development of colloidal quantum dots has led to practical applications of quantum confinement, such as in solution-processed solar cells, lasers and as biological labels. Further scientific and technological advances should be achievable if these colloidal quantum systems could be electronically coupled in a general way. For example, this was the case when it became possible to couple solid-state embedded quantum dots into quantum dot molecules. Similarly, the preparation of nanowires with linear alternating compositions--another form of coupled quantum dots--has led to the rapid development of single-nanowire light-emitting diodes and single-electron transistors. Current strategies to connect colloidal quantum dots use organic coupling agents, which suffer from limited control over coupling parameters and over the geometry and complexity of assemblies. Here we demonstrate a general approach for fabricating inorganically coupled colloidal quantum dots and rods, connected epitaxially at branched and linear junctions within single nanocrystals. We achieve control over branching and composition throughout the growth of nanocrystal heterostructures to independently tune the properties of each component and the nature of their interactions. Distinct dots and rods are coupled through potential barriers of tuneable height and width, and arranged in three-dimensional space at well-defined angles and distances. Such control allows investigation of potential applications ranging from quantum information processing to artificial photosynthesis.

Synthesis of branched 'nanotrees' by controlled seeding of multiple branching events

The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years. Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials. Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour-liquid-solid growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees.

Cubatic phase for tetrapods

We investigate the phase behavior of tetrapods, hard nonconvex bodies formed by four rods connected under tetrahedral angles. We predict that, depending on the relative lengths of the rods these particles can form a uniaxial nematic phase, and more surprisingly they can exhibit a cubatic phase, a special case of the biaxial nematic phase. These predictions may be experimentally testable, as experimental realizations of tetrapods have recently become available.

Shape Evolution of Single-Crystalline Iron Oxide Nanocrystals

Shape- and size-controlled synthesis of single-crystalline maghemite (gamma-Fe2O3) nanocrystals are performed by utilizing a solution-based one-step thermolysis method. Modulating the growth parameters, such as the type and amount of capping ligands as well as the growth time, is shown to have a significant effect on the overall shape and size of the obtained nanocrystals and on the ripening process itself. The resulting shapes of the novel structures are diverse, including slightly faceted spheres, diamonds, prisms, and hexagons, all of which are in fact truncated dodecahedron structures with different degrees of truncation along the {111}, {110}, or {100} faces. Spherical nanocrystals are easily assembled into the three-dimensional superlattices, demonstrating the uniformity of these nanocrystals. The size-dependent magnetic properties are examined, and large hexagon-shaped gamma-Fe2O3 nanocrystals are shown to be ferrimagnetic at room temperature.

Nanocable-Aligned ZnS Tetrapod Nanocrystals

An effective method for synthesizing ZnS-core/carbon-sheath nanocables and nanocable-aligned ZnS tetrapod nanocrystals has been developed. Nanocable heterostructures and nanocable-linked ZnS tetrapods were synthesized in a controllable way. The tetrapods are single crystalline cubic ZnS with triangular-prism branches stretching out in four {111} directions. The tetrapods were aligned together with ZnS-C nanocables along the [100] direction.

Cadmium Selenide Quantum Wires and the Transition from 3D to 2D Confinement

Soluble CdSe quantum wires are prepared by the solution-liquid-solid mechanism, using monodisperse bismith nanoparticles to catalyze wire growth. The quantum wires have micrometer lengths, diameters in the range of 5-20 nm, and diameter distributions of +/-10-20%. Spectroscopically determined wire band gaps compare closely to those calculated by the semiemipirical pseudopotential method, confirming 2D quantum confinement. The diameter dependence of the quantum wire band gaps is compared to that of CdSe quantum dots and rods. Quantum rod band gaps are shown to be delimited by the band gaps of dots and wires of like diameter, for short and long rods, respectively. The experimental data suggest that a length of ca. 30 nm is required for the third dimension of quantum confinement to fully vanish in CdSe rods. That length is about six times the bulk CdSe exciton Bohr radius.

Monopod, Bipod, Tripod, and Tetrapod Gold Nanocrystals

Multipod gold nanocrystals have been synthesized via a simple solution-phase chemical reduction method at room temperature. Two types of structurally different particles are observed, including the regular tripod, which lies on the substrate with its {111} plane, and the cross-like tetrapod that sits on its {100} plane. Both structures have pods of 10 nm sizes and grow along 110 directions. Variously shaped particles, including tadpole-like or teardrop-like monopods, 90 degrees L-shaped, 180 degrees I-shaped, and 120 degrees V-shaped bipods, T-shaped, Y-shaped, and regular triangular tripods, and cross-like tetrapods, have been produced. Due to the high conductivity of metal, these branched structures are quite promising considering their applications as interconnections in the "bottom-up" self-assembly approach toward future nanocircuits and nanodevices, as well as in other related fields.

Direct Growth of Shape-Controlled Nanocrystals on Nanotubes via Biological Recognition

The new biological approach was examined to fabricate shape-controlled Ag nanocrystals grown directly on surfaces, inspired by nature that various shapes of nanocrystals are produced accurately and reproducibly in biological systems. Here we demonstrate the direct growth of hexagon-shaped Ag nanocrystals on sequenced peptide-coated nanotubes via biological recognition. When the peptide, Asn-Pro-Ser-Ser-Leu-Phe-Arg-Tyr-Leu-Pro-Ser-Asp, recognizing and effecting the Ag nanocrystal growth on the (111) face, was sequenced and incorporated onto template nanotube surfaces, the biomineralization of Ag ions on the nanotubes led the isotropic hexagon-shaped Ag nanocrystal coating under pH control of the growth solution. Multiple Ag nanocrystal shapes were observed when the peptide mineralized Ag ions without the template nanotubes, and therefore the template nanotube has a significant influence on regulating the majority of Ag nanocrystals into the hexagonal shape. This biological approach, using specific peptide sequences on surfaces to control nanocrystal shapes, may be developed as a simple and economical method to produce building blocks with desired physical properties for new generation of electronics, sensors, and optical devices.

Shape Effects on Electronic States of Nanocrystals

High-performance supercomputing and high-fidelity atomistic methods are used to study the shape effects on the single-particle electronic states of nanocrystals. We found that the shape can be used as an efficient way to control the electronic structures of the nanocrystals. Changing the shape is more flexible and provides more variety of electronic states than simply changing the size of the system. The special features of the electronic states achieved by different shapes of the nanocrystals can be used in various device applications. Simple rules are summarized to predict the electronic structure shape effects on similar nanocrystals.

Synthesis of CdS and ZnS nanowires using single-source molecular precursors

Single-source molecular precursors were used to synthesize II-VI compound semiconductor nanowires for the first time. Cadmium sulfide and zinc sulfide nanowires were prepared using cadmium diethyldithiocarbamate, Cd(S2CNEt2)2, and zinc diethyldithiocarbamate, Zn(S2CNEt2)2, respectively, as precursors in a gold nanocluster-catalyzed vapor-liquid-solid growth process. High-resolution transmission electron microscopy studies show that the CdS and ZnS nanowires are single-crystal wurtzite structures with stoichiometric compositions. In addition, photoluminescence measurements demonstrate that these nanowires exhibit high-quality optical properties. The applicability of our approach to the synthesis of other compound and alloy semiconductors nanowires as well as nanowire heterostructures of these materials is discussed.