Self-assembled shape- and orientation-controlled synthesis of nanoscale Cu3Si triangles, squares, and wires

Controlling shape and orientation is important for the synthesis of functional nanomaterials. In this work, nanoscale Cu3Si triangles, squares, and wires have been grown on Si(111), (100), and (110) substrates, respectively, through a template-free Au-nanoparticle-assisted vapor transport method. The sides of nanotriangles and nanosquares and the growth direction of the nanowires are all along Si <110>, giving rise to long-range ordering of the nanostructures. Au nanoparticles absorb Cu vapor and facilitate the rate-limited diffusion of Si, which is critical for the shape-controlled growth of Cu3Si. This bottom-up approach to synthesize shape- and orientation-controlled Cu3Si nanostructures might be applicable to the tailored growth of other materials.     

Fabrication, structural characterization and photoluminescence of Q-1D semiconductor ZnS hierarchical nanostructures

Quasi-one-dimensional semiconductor ZnS hierarchical nanostructures have been fabricated by thermal evaporation of a mixture of ZnS nanopowders and Sn powders. Sn nanoparticles are located at or close to the tips of the nanowires (or nanoneedles) and served as the catalyst for quasi-one-dimensional ZnS nanostructure growth by a vapour-liquid-solid mechanism. The morphology and microstructure of the ZnS hierarchical nanostructures were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The results show that a large number of ZnS nanoneedles were formed on the outer shells of a long and straight ZnS axial nanowire. The ZnS axial nanowires grow along the [001] direction, and ZnS nanoneedles are aligned over the surface of the ZnS nanowire in the radial direction. The room temperature photoluminescence spectrum exhibits a UV weak emission centred at 337?nm and one blue emission centred at 436?nm from the as-synthesized single-crystalline semiconductor ZnS hierarchical nanostructures.     

Controlled CVD growth of Cu-Sb alloy nanostructures

Sb based alloy nanostructures have attracted much attention due to their many promising applications, e.g. as battery electrodes, thermoelectric materials and magnetic semiconductors. In many cases, these applications require controlled growth of Sb based alloys with desired sizes and shapes to achieve enhanced performance. Here, we report a flexible catalyst-free chemical vapor deposition (CVD) process to prepare Cu-Sb nanostructures with tunable shapes (e.g. nanowires and nanoparticles) by transporting Sb vapor to react with copper foils, which also serve as the substrate. By simply controlling the substrate temperature and distance, various Sb-Cu alloy nanostructures, e.g. Cu(11)Sb(3) nanowires (NWs), Cu(2)Sb nanoparticles (NPs), or pure Sb nanoplates, were obtained. We also found that the growth of Cu(11)Sb(3) NWs in such a catalyst-free CVD process was dependent on the substrate surface roughness. For example, smooth Cu foils could not lead to the growth of Cu(11)Sb(3) nanowires while roughening these smooth Cu foils with rough sand papers could result in the growth of Cu(11)Sb(3) nanowires. The effects of gas flow rate on the size and morphology of the Cu-Sb alloy nanostructures were also investigated. Such a flexible growth strategy could be of practical interest as the growth of some Sb based alloy nanostructures by CVD may not be easy due to the large difference between the condensation temperature of Sb and the other element, e.g. Cu or Co.     

Preparation of α-Fe_2O_3 Nanodisks by Blocking the Growth of (001) Plane

Based on the difference of hydroxy group configuration on different planes of α-Fe2O3 nanoparticles, using the special adsorption and coordination of phosphate on the (001) plane of α-Fe2O3, well-crystallized and well dispersed α-Fe2O3 nanodisks with diameter of 150–200 nm and thickness of 40–80 nm were synthesized via a hydrothermal method. The magnetic properties of synthesized nanodisks were investigated. It was found that the nanodisks possessed a saturation magnetization (Ms) of 0.38 emu/g, a remanent ...     

Raman spectra and photoluminescence properties of In-doped ZnO nanostructures

In-doped ZnO nanostructures with four different morphologies, which are nanotetrapods, nanocombs, nanowires, and nanodisks, have been synthesized on silicon substrates by a simple thermal evaporation method. The XRD patterns show the In-doped ZnO nanostructures are all with the hexagonal wurtzite structure, and a slight difference in lattice parameters had been detected among the samples with various morphologies. The Raman spectra demonstrate that the vibrational mode of 2LA, which is very weak in undoped ZnO, was strongly enhanced with indium ion doping into ZnO structures. The photoluminescence (PL) measurements indicate that the nanodisks have a relative strong ultraviolet (UV) emission than other three kinds of samples.     

Defect transfer from nanoparticles to nanowires

Metal-seeded growth of one-dimensional (1D) semiconductor nanostructures is still a very active field of research, despite the huge progress which has been made in understanding this fundamental phenomenon. Liquid growth promoters allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, and operating pressure. However the transfer of crystallographic information from a catalytic nanoparticle seed to a growing nanowire has not been described in the literature. Here we define the theoretical requirements for transferring defects from nanoparticle seeds to growing semiconductor nanowires and describe why Ag nanoparticles are ideal candidates for this purpose. We detail in this paper the influence of solid Ag growth seeds on the crystal quality of Ge nanowires, synthesized using a supercritical fluid growth process. Significantly, under certain reaction conditions {111} stacking faults in the Ag seeds can be directly transferred to a high percentage of <112>-oriented Ge nanowires, in the form of radial twins in the semiconductor crystals. Defect transfer from nanoparticles to nanowires could open up the possibility of engineering 1D nanostructures with new and tunable physical properties and morphologies.     

Fabrication of single crystalline cadmium nanowires by a facile low temperature vapor phase method

Single-crystalline cadmium nanowires were successfully fabricated by vaporization of cadmium metal powders in a horizontal quartz tube furnace at 250 degrees C. The vaporization was carried out for 30 minutes and yielded nanowires of diameters of 80 to 250 nm and lengths up to several tens of microns. The nanowires were deposited on a Si (111) substrate kept at the lower temperature zone (150-175 degrees C) of the furnace. When the deposition temperature was lower than this, hexagonal nanodisks were produced. The possible mechanism for the formation of the obtained nanostructures is discussed.     

Temperature dependent growth and optical properties of SnO<Subscript>2</Subscript> nanowires and nanobelts

SnO₂ nanowires and nanobelts have been grown by the thermal evaporation of Sn powders. The growth of nanowires and nanobelts has been investigated at different temperatures (750–1000°C). The field emission scanning electron microscopic and transmission electron microscopic studies revealed the growth of nanowires and nano-belts at different growth temperatures. The growth mechanisms of the formation of the nanostructures have also been discussed. X-ray diffraction patterns showed that the nanowires and nanobelts are highly crystalline with tetragonal rutile phase. UV-visible absorption spectrum showed the bulk bandgap value (∼ 3–6 eV) of SnO₂. Photoluminescence spectra demonstrated a Stokes-shifted emission in the wavelength range 558–588 nm. The Raman and Fourier transform infrared spectra revealed the formation of stoichiometric SnO₂ at different growth temperatures.     

碳纳米管原位包覆金属锡纳米线的制备方法及其生长机理

采用直接沉淀法制备出粒径为10nm以下的二氧化锡(SnO2)颗粒,将其作为前躯体采用CVD法原位生长碳纳米管,通过XRD、SEM、TEM等方法观察了该复合物的微观结构,对其生长机理做出了合理的推断。     

Sn-catalyzed growth of MgO nanowires

We reported the fabrication and characterization of MgO nanowires, which were grown by thermal evaporation of the mixture of MgB2 and Sn powders at 800 degrees C through a vapor-liquid-solid (VLS) process. We characterized as-synthesized MgO nanowires using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Sn nanoparticles were located at the tips of the nanowires, serving as catalyst for the growth of MgO nanowires. The produced nanowires were of cubic MgO structures with diameters in the range of 10-170 nm. The PL measurement with a Gaussian fitting exhibited visible light emission bands centered at 403, 576, and 720 nm.     

Effects of processing parameters on ultraviolet emission of In-doped ZnO nanodisks grown by carbothermal reduction

Effects of cooling rate and oxygen partial pressure in flowing Ar on ultraviolet (UV) emission of In-doped ZnO nanodisks grown by carbothermal reduction at 1000 °C were studied. The In doping favored the growth of ZnO nanodisks instead of ZnO nanowires. Air-cooled ZnO nanodisks showed a strong green emission, while furnace cooling in conjunction with introducing O₂, around 1.0%, into flowing Ar during growth significantly enhanced the growth and UV emission of ZnO nanodisks. The causes can be attributed to the reduction of oxygen vacancies and surface defects in ZnO nanodisks. However, higher oxygen partial pressure in flowing Ar resulted in a decrease in the Zn vapor and thus suppressed the growth and UV emission of ZnO nanodisks.     

Selective synthesis and superconductivity of In-Sn intermetallic nanowires sheathed in carbon nanotubes

We demonstrate a simple and reproducible technique to synthesize crystalline and superconducting In-Sn intermetallic nanowires sheathed in carbon nanotubes (CNTs). The method is based on the catalytic reaction of C(2)H(2) over a mixture of both SnO(2) and In(2)O(3) particles. Importantly, tetragonal β-In(3)Sn and hexagonal γ-InSn(4) nanowires with diameters of less than 100?nm are selectively synthesized at different SnO(2) to In(2)O(3) weight ratios. CNTs may serve as cylindrical nanocontainers for continuous growth of liquid-phased In(1-x)Sn(x) nanowires during growth process as well as for their solidification into In-Sn intermetallic nanowires during the cooling process. Microscopic and spectroscopic analyses clearly reveal evidence of a core-shell structure of the CNT-sheathed In-Sn intermetallic nanowires. Magnetization measurements show that the superconducting In-Sn nanowires have a critical magnetic field higher than the value of their bulk intermetallic compounds. Our method can be adopted to the nanofabrication of analogous binary and ternary alloys.     

Large-scale polyol synthesis of single-crystal bismuth nanowires and the role of NaOH in the synthesis process

A modified polyol process is introduced for the production of single-crystal bismuth (Bi) nanowires with uniform diameters along each wire in relatively high yield. The appropriate amount of NaOH in the solution reacts with Bi(3+) to form water-soluble complexing ions BiO(2)(-). The tiny Bi nanoparticles formed at the initial stage could serve as seeds for the subsequent growth of Bi nanostructures in the refluxing process with the aid of PVP. We find that the amount of NaOH determines the reduction rate of BiO(2)(-), which influences the morphologies of the synthesized Bi nanostructures. High reduction rates result in nanowires and nanoparticles, while low reduction rates result in nanoplates.     

Bimetallic Nanostructures as Active Raman Markers: Gold‐Nanoparticle Assembly on 1D and 2D Silver Nanostructure Surfaces

It is demonstrated that bimetallic silver–gold anisotropic nanostructures can be easily assembled from various nanoparticle building blocks with well‐defined geometries by means of electrostatic interactions. One‐dimensional (1D) silver nanowires, two‐dimensional (2D) silver nanoplates, and spherical gold nanoparticles are used as representative building blocks for bottom‐up assembly. The gold nanoparticles are electrostatically bound onto the 1D silver nanowires and the 2D silver nanoplates to give bimetallic nanostructures. The unique feature of the resulting nanostructures is the particle‐to‐particle interaction that subjects absorbed analytes to an enhanced electromagnetic field with strong polarization dependence. The Raman activity of the bimetallic nanostructures is compared with that of the individual nanoparticle blocks by using rhodamine 6G solution as the model analyte. The Raman intensity of the best‐performing silver–gold nanostructure is comparable with the dense array of silver nanowires and silver nanoplates that were prepared by means of the Langmuir–Blodgett technique. An optimized design of a single‐nanostructure substrate for surface‐enhanced Raman spectroscopy (SERS), based on a wet‐assembly technique proposed here, can serve as a compact and low‐cost alternative to fabricated nanoparticle arrays.     

Epitaxial insertion of gold silicide nanodisks during the growth of silicon nanowires

Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.     

Sn-doped hematite nanostructures for photoelectrochemical water splitting

We report on the synthesis and characterization of Sn-doped hematite nanowires and nanocorals as well as their implementation as photoanodes for photoelectrochemical water splitting. The hematite nanowires were prepared on a fluorine-doped tin oxide (FTO) substrate by a hydrothermal method, followed by high temperature sintering in air to incorporate Sn, diffused from the FTO substrate, as a dopant. Sn-doped hematite nanocorals were prepared by the same method, by adding tin(IV) chloride as the Sn precursor. X-ray photoelectron spectroscopy analysis confirms Sn(4+) substitution at Fe(3+) sites in hematite, and Sn-dopant levels increase with sintering temperature. Sn dopant serves as an electron donor and increases the carrier density of hematite nanostructures. The hematite nanowires sintered at 800 °C yielded a pronounced photocurrent density of 1.24 mA/cm(2) at 1.23 V vs RHE, which is the highest value observed for hematite nanowires. In comparison to nanowires, Sn-doped hematite nanocorals exhibit smaller feature sizes and increased surface areas. Significantly, they showed a remarkable photocurrent density of 1.86 mA/cm(2) at 1.23 V vs RHE, which is approximately 1.5 times higher than that of the nanowires. Ultrafast spectroscopy studies revealed that there is significant electron-hole recombination within the first few picoseconds, while Sn doping and the change of surface morphology have no major effect on the ultrafast dynamics of the charge carriers on the picosecond time scales. The enhanced photoactivity in Sn-doped hematite nanostructures should be due to the improved electrical conductivity and increased surface area.     

Low dimensional silver nanostructures: synthesis, growth mechanism, properties and applications

This work presents a review of the recent advances on the low-dimensional (LD) silver nanostructures (e.g., one-dimensional nanorods and nanowires, and two-dimensional nanoplates and nanodisks). First, the methods, either physical or chemical, for the synthesis of silver LD nanostructures are introduced. Then, the use is discussed of advanced experimental techniques (e.g., transmission electron microscope, high-resolution transmission electron microscope, scanning electron microscope, atomic force microscope, ultraviolet-visible and Raman spectra) and theoretical techniques at different time and length scales from quantum mechanics (e.g., ab initio simulation and density function theory) to molecular dynamics method for understanding the principles of governing particle growth, as well as discrete dipolar approximate method for understanding the optical properties of different shapes and sizes of silver LD nanostructures. Subsequently, the functional applications of the LD silver nanostructures in different areas such optical, electronic, and sensing, particularly for those related to surface plasma resonance are summarized based on the recent findings. Finally, some perspectives and comments for future investigation of silver nanostructures are also briefly discussed.     

Self-assembled ferromagnetic cobalt/yttria-stabilized zirconia nanocomposites for ultrahigh density storage applications

We report on a low-cost, innovative approach for synthesizing prepatterned, magnetic nanostructures, the shapes and dimensions of which can be easily tuned to meet requirements for next-generation data storage technology. The magnetic nanostructures consist of self-assembled Co nanodots and nanowires embedded in yttria-stabilized zirconia (YSZ) matrices. The controllable size and aspect ratio of the nanostructures allows the selection of morphologies ranging from nanodots to nanowires. Co nanowires show strong shape anisotropy and large remanence at 300 K. In contrast, Co nanodots display minimal effects of magnetocrystalline anisotropy and superparamagnetic relaxation above the blocking temperature. These prepatterned magnetic nanostructures are very promising candidates for data storage technology with an ultrahigh density of 1 terabit in(-2) or higher.     

Tailoring the photoluminescence of ZnO nanowires using Au nanoparticles

Rational design of hybrid nanostructures through attaching nanowires with nanoparticles is an effective route to enhance the existing functionalities or to explore new ones. We carry out a systematic investigation on the photoluminescence of ZnO nanowire-Au nanoparticle hybrid nanostructures synthesized by attaching Au nanoparticles onto ZnO nanowires. Citrate-stabilized 40?nm Au nanoparticles effectively quench the green emission and enhance the UV emission of the ZnO nanowires, which is consistent with the wavelength-dependent generation of surface plasmon. The UV/green emission intensity ratio could be reversibly and reproducibly tailored by attaching/detaching Au nanoparticles. This enhancement of UV emission diminishes if the Au nanoparticles are coated with a polymer layer. We also find that the orange-red emission of the ZnO nanowires is related to the excess oxygen on the ZnO surface, and it is also tunable via annealing and surface modifications.     

Synthesis of nanowires,nanorods and nanoparticles of ZnO through modulating the ratio of water to methanol by using a mild and simple solution method

ZnO nanowires, nanorods and nanoparticles through modulating the ratio of water to methanol have been synthesized by using a mild and simple solution method. The as-prepared ZnO nanostructures have been characterized by atomic force microscopy and X-ray photoelectron spectroscopy. With the increase of the ratio of water to methanol, the morphology of ZnO nanostructures varied form denser nanowires, to sparse nanowires, to nanorods, and then to nanoparticles. The ratio of water to methanol is supposed to play an important role in the formation of ZnO nanostructures. The mechanism of formation is related to the chemical potential, which is simply proportional to their surface ratio.