Vapor-solid Growth Research Articles

Synthesis of SnTe nanoplates with {100} and {111} surfaces.

SnTe is a topological crystalline insulator that possesses spin-polarized, Dirac-dispersive surface states protected by crystal symmetry. Multiple surface states exist on the {100}, {110}, and {111} surfaces of SnTe, with the band structure of surface states depending on the mirror symmetry of a particular surface. Thus, to access surface states selectively, it is critical to control the morphology of SnTe such that only desired crystallographic surfaces are present. Here, we grow SnTe nanostructures using vapor-liquid-solid and vapor-solid growth mechanisms. Previously, SnTe nanowires and nanocrystals have been grown [Saghir et al. Cryst. Growth Des. 2014, 14, 2009-2013; Safdar et al. Cryst. Growth Des. 2014, 14, 2502-2509; Safdar et al. Nano Lett. 2013, 13, 5344-5349; Li et al. Nano Lett. 2013, 13, 5443-5448]. In this report, we demonstrate the synthesis of SnTe nanoplates with lateral dimensions spanning tens of micrometers and thicknesses of a few hundred nanometers. The top and bottom surfaces are either (100) or (111), maximizing topological surface states on these surfaces. Magnetotransport on these SnTe nanoplates shows a high bulk carrier density, consistent with bulk SnTe crystals arising due to defects such as Sn vacancies. In addition, we observe a structural phase transition in these nanoplates from the high-temperature rock salt to a low-temperature rhombohedral structure. For nanoplates with a very high carrier density, we observe a slight upturn in resistance at low temperatures, indicating electron-electron interactions.

Synthesis, photoluminescence and optical constants evaluations of ultralong CdO nanowires prepared by vapor transport method

Ultralong CdO nanowires (NWs) were synthesized on Au coated Si and quartz substrates by using vapor transport method. The synthesis temperature was chosen via thermal gravimetry analysis (TGA) and it was 1150°C. The formation of CdO was confirmed by energy dispersive analysis of X-ray (EDAX) and Fourier transform infrared (FT-IR) spectroscopy analysis. The CdO nanowires was found to grow via a vapor–solid growth mechanism. The diameters of the nanowires were in the range 30–90nm and the lengths were greater than 30μm. The optical constants, the thickness and the surface roughness of the prepared CdO NWs films were determined by spectroscopic ellipsometry measurements. A two layers model was used to fit the calculated data to the experimental ellipsometric spectra. The obtained optical constants were compared with those obtained by other preparation methods. The optical band gap was found to be 2.41eV. An emission peak at ∼550nm was recorded, which should corresponds to the near band-edge emission of CdO.

Macro-scale complexity of nano- to micro-scale architecture of olivine crystals through an iodine vapour transport mechanism

The production of nano- to micro-scale olivine (magnesium and iron silicate) crystals has been achieved at relatively low temperatures through an iodine vapour transport of the metal onto amorphous silicon dioxide. The process occurs down a temperature gradient from 800 to 600 °C yielding high quality crystals with long range crystallinity, highly complex interconnectivity and intricate macroscale architecture. Scanning electron microscopy (SEM) imaging of the substrate before and after the reaction reveals that the amorphous silicon oxide species is mobile, due to the lack of correlation between the silicon oxide layer and the final olivine particles, leading to a vapour–liquid–solid or vapour–solid growth mechanism. This technique demonstrates a facile, low temperature synthetic route towards olivine crystals with nano- to micro-scale dimensions.

Evolution of Hollow TiO2 Nanostructures via the Kirkendall Effect Driven by Cation Exchange with Enhanced Photoelectrochemical Performance

Hollow nanostructures are promising building blocks for electrode scaffolds and catalyst carriers in energy-related systems. In this paper, we report a discovery of hollow TiO2 nanostructure evolution in a vapor-solid deposition system. By introducing TiCl4 vapor pulses to ZnO nanowire templates, we obtained TiO2 tubular nanostructures with well-preserved dimensions and morphology. This process involved the cation exchange reaction between TiCl4 vapor and ZnO solid and the diffusion of reactants and products in their vapor or solid phases, which was likely a manifestation of the Kirkendall effect. The characteristic morphologies and the evolution phenomena of the hollow nanostructures from this vapor-solid system were in a good agreement with the Kirkendall effect discovered in solution systems. Complex hollow TiO2 nanostructures were successfully acquired by replicating various ZnO nanomorphologies, suggesting that this unique cation exchange process could also be a versatile tool for nanostructure replication in vapor-solid growth systems. The evolution of TiO2 nanotubes from ZnO NW scaffolds was seamlessly integrated with TiO2 NR branch growth and thus realized a pure TiO2-phased 3D NW architecture. Because of the significantly enlarged surface area and the trace amount of Zn left in the TiO2 crystals, such 3D TiO2 nanoforests demonstrated enhanced photoelectrochemical performance particularly under AM (air mass) 1.5G illumination, offering a new route for hierarchical functional nanomaterial assembly and application.

Evolution of the faceting, morphology and aspect ratio of gallium oxide nanowires grown by vapor–solid deposition

Gallium oxide nanostructures with high aspect ratio and variable faceting were synthesized by the chemical vapor deposition method via vapor–solid growth mechanism. Systematic investigation of the growth conditions revealed that these nanowires can be produced under the conditions of high temperature and low precursor flow. The nanowires crystalize as the β-phase Ga2O3, which has the monoclinic crystal structure. Preferred growth was along the [010] direction, as corroborated with lattice-resolved imaging and crystal plane models. The high degree of faceting is discussed in terms of the evolution of the nanowire cross section morphology, based on the growth rate of the facet boundaries relative to the nanowire surface planes. The obtained nanowires show intense blue emission, characterized by a broad-band photoluminescence spectrum with a maximum at 430nm and long decay time. This emission arises from the defect-related donor-acceptor pair recombination mechanism, and depends on the nanostructure dimensionality and morphology. The possible influence of controlled nanowire faceting on the observed optical properties is also discussed. Owing to their morphology-dependent optical properties, these nanowires are promising building blocks for electronic and optoelectronic structures and devices.

Processing temperature dependent morphological and optical properties of ZnO nanorods

Zinc Oxide (ZnO) nanorods were synthesized by thermal decomposition of zinc acetate dihydrate at various processing temperatures. The morphology of samples, examined using transmission electron microscopy and field emission scanning electron microscopy, revealed large variations in length and diameter of nanorods. As temperature was increased from 300°C to 450°C, ZnO nanocrystal morphology changed from wire-like to rod-like. This morphology change is a result of competition between nucleation and growth rate in the vapor–solid growth mechanism of nanorods. Photoluminescence spectrum of the nanorods showed both band edge emission as well as native defect related visible emission. Relative intensity of UV and visible emission indicated crystal quality of the nanorods, the ratio increased upto 400°C and deteriorated at higher processing temperature. It is argued that process induced defects dominate at processing temperatures ≤400°C, whereas equilibrium concentration of native defects is high at temperatures >400°C.

Structural and optical properties of SnO2 nanotowers and interconnected nanowires prepared by carbothermal reduction method

Nanotowers and interconnected nanowires of SnO2 have been successfully synthesized using a simple carbothermal reduction method by heating mixtures of SnO2, graphite and Ag powders at 1050°C. Field emission scanning electron microscopy (FESEM) studies revealed that the synthesized nanotowers consist of layer-by-layer stacked SnO2 nanosheets of average thickness 100nm. At higher Ag concentration, interconnected nanowires of SnO2 have been obtained. The formation of SnO2 nanotowers involves vapor–solid growth, while the interconnected SnO2 nanowires were formed by vapor–liquid–solid growth mediated by Ag3Sn liquid nanodroplets. Photoluminescence studies on SnO2 nanotowers revealed intense peaks at 365 and 396nm and a very weak broad band around 545nm. The synthesized SnO2 nanotowers and interconnected SnO2 nanowires have potential applications as functional blocks in future nanodevices.

Facile and controllable synthesis of carbon-encapsulating carbonate apatite nanowires from biomass containing calcium compounds such as CaC2O4 and CaCO3

We report the synthesis of carbon-encapsulating carbonate apatite nanowires through vapor–solid growth by heat-treatment of biomass comprising calcium compounds such as CaC2O4 or CaCO3 at 900 °C using both PH3 and C2H2 as the reactants.

Vapour solid reaction growth of SnO2 nanorods as an anode material for Li ion batteries

Via a vapour–solid reaction growth pathway, phase-segregated SnO2 nanorods were developed in a matrix of CaCl2 salt by reacting CaO particles with a flowing mixture of SnCl4 and Ar gases at elevated temperatures. A half-cell constructed from the as-fabricated SnO2 electrode and a Li foil exhibited a reversible capacity of 435 mA h g−1 after one hundred cycles at a current density of 100 mA g−1.

Synthesis of three-dimensional AlN–Si3N4 branched heterostructures and their photoluminescence properties

Three-dimensional AlN–Si3N4 branched heterostructures are synthesized via extended vapor–liquid–solid and vapor–solid growth of one-dimensional Si3N4 nanostructures and AlN nanocones successively.

The synthesis route and the growth mechanism of aligned GaN nanobelts.

Crystallographically aligned GaN nanobelt arrays were synthesized through a hybrid process of Au-assisted nucleation followed by non-Au-assisted anisotropy vapor-solid growth on a (100) γ-LiAlO2 substrate. To the best of our knowledge, this is the first report on aligned GaN nanobelts.

Cobalt oxide-tungsten oxide nanowire heterostructures: Fabrication and characterization

ABSTRACTNanowire heterostructures comprised of cobalt oxide and tungsten oxide were fabricated in a core/shell configuration. This was achieved by sputter coating tungsten oxide shells on standing cobalt oxide nanowires on a substrate. To ensure the polycrystallinity of tungsten oxide shell, the nanowire heterostructures were subjected to post-sputtering annealing process. The cobalt oxide nanowires for this study were grown employing a thermal method via vapor-solid growth mechanism. The crystal structures, morphologies, dimensions, and phases at various growth stages of nanowire heterostructures were studied using high resolution electron microscopy, energy dispersive spectroscopy, and X-ray diffraction methods. The interfaces of these nanowire heterostructures were also studied and showed variation in the lattice spacing across the heterostructure diameter. Results indicated that the cobalt oxide nanowires survived multiple processing steps and resulted in stable heterostructure configurations. The investigation shows, for the first time, a dry processing route for the formation of such novel nanowire heterostructures.

Growth of regular-shaped β-Ga2O3 nanorods by Ni2+-ion-catalyzed chemical vapor deposition

Regular-shaped monoclinic β-Ga2O3 nanorods with square cross-sections were successfully synthesized and characterized by Ni2+-ion-catalyzed chemical vapor deposition method using CaF2 as a dispersant. The composition, crystal structure, morphology, and optical property were characterized in detail. X-ray diffraction data indicate that the product was a single monoclinic β-Ga2O3 phase with high purity. The regular-shaped nanorods had square cross-sections and an approximate size of 250 nm in diameter and 500–1,000 nm in length. A broad and strong emission band that ranged from 300 to 650 nm was observed with three bands centered at approximately 405 (blue), 467 (dark blue), and 520 nm (green). The growth mechanism of β-Ga2O3 nanorods was consistent with the vapor–solid growth mechanism.

Vapor-Solid Growth of Highly Oriented SnO2 Nanorods for Chemical Sensing Applications

not Available.

Atomic-layer triangular WSe2 sheets: synthesis and layer-dependent photoluminescence property

The exotic band structures and distinctive physical properties of two-dimensional materials have exhibited great potential for fundamental research and technical applications in spintronics, electronics, photonics, optoelectronics, and so forth. Facing the challenge of effective synthesis of WSe2 two-dimensional sheets, for the first time we demonstrate a straightforward catalyst-free vapor–solid (VS) growth method to synthesize ultrathin, even monolayer, WSe2 sheets with high yield, regular shapes and high quality optical properties on sapphire substrates. By detailed layer-number-dependent photoluminescence measurements at a low temperature of 40 K, we find the spin–orbit splitting at the K point of the WSe2 valence band with a fixed energy difference of 0.36 eV independent of layer number and the transition of indirect-to-direct gap when the thickness decreases to monolayer. These results, comparable to those of mechanically peeled WSe2 sheets, further prove the high optical and crystal quality of our WSe2 nanosheets via the VS growth approach. Our efforts may open up new exciting opportunities in future valley-based electronics, optoelectronics and photonics.

Monitoring the Fermi-level position within the bandgap on a single nanowire: A tool for local investigations of doping

The control of the doping in nanowires (NWs) is of fundamental importance for the implementation of NW-based devices. A method is presented to obtain local information about doping by monitoring the Fermi-energy position within the bandgap at the surface along single NWs through spatially resolved x-ray photoemission spectroscopy. The experimental results are complemented by theoretical simulations of the carrier profile, taking into account the presence of electronic surface states and quantifying the impact of carrier depletion at the NW surface. This combined approach allows to determine the effect of the incorporation of Si dopants in GaAs NWs following different growth protocols, such as vapor-liquid-solid axial growth or vapor-solid radial growth, and in the resulting core-shell structures and axial junctions. The method also revelaed the strong dependence of the resulting doping on the morphology of the single NW (orientation, shell thickness). This approach can be easily applied to other nanoscale objects, allowing the direct observation of how doping (or junctions, or adsorbates,…) may locally affect the position of the Fermi level at the surface, which is a crucial factor in several application fields, such as photovoltaic and photocatalysis.

Effects of growth parameters on the yield and morphology of Si3N4 microcoils prepared by chemical vapor deposition

In this study, we provided a reliable chemical vapor deposition (CVD) method to synthesize high-purity Si3N4 microcoils in high yield without the presence of catalyst. The achieved products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscope. The results indicated that the yield and morphology of Si3N4 products were influenced by the synthesis parameters such as reaction temperature, reaction time and gas flow rate. The particular conditions favorable to high yield synthesis of Si3N4 microcoils were obtained through a series of control experiments. Furthermore, the growth of Si3N4 microcoils was supposed to be in accord with vapor-solid (VS) growth process and the different growth rates between the amorphous layer and the crystalline layer were used to explain the formation of the coil geometry.

Synthesis of boron carbide nanoflakes via a bamboo-based carbon thermal reduction method

Boron carbide nanoflakes have been successfully synthesized by a facile and cost-effective bamboo-based carbon thermal reduction method. The majority of the boron carbide products exhibited a flake-like morphology with lateral dimensions of 0.5–50μm in width and more than 50μm in length, while the thickness was less than 150nm. The structural, morphological, and elemental analyses demonstrated that these nanoflakes grew via the fluoride-assisted vapor–liquid–solid combined with vapor–solid growth mechanism. The corresponding growth model was proposed. In addition, the electrical property of individual boron carbide nanoflake was investigated by an in situ two point method inside a transmission electron microscope. The resistivity of boron carbide nanoflakes was measured to be 0.14MΩcm.

ChemInform Abstract: One‐Dimensional ZnO Nanostructures

AbstractReview: 225 refs.

Formation of Germanium Nitride Nanowires on the Surface of Crystalline Germanium

We report on the growth mechanisms of germanium nitride nanowires on the surface of crystalline Ge annealed in hydrazine vapor at different temperatures. In spite of the presence of water (and hence oxygen precursors) in hydrazine, the pure germanium nitride single crystal nanowires were produced in the temperature range of 480–580°C. At temperatures below 520°C, the GeOx clusters were formed first at the Ge surface, followed by the nucleation and growth of nanowires through the Vapor-Liquid-Solid mechanism. The Vapor-Solid growth mechanism was observed at temperatures exceeding 520°C, and Ge3N4 nanobelts were produced instead of nanowires with circular cross-sections. All nanostructures have the alpha germanium nitride structure; however, at the nucleation stage, the presence of beta Ge3N4 phase was also observed in the roots of nanowires.