Optical properties of GaN nanorods grown by molecular-beam epitaxy; dependence on growth time

The growth and optical properties of GaN nanorods grown on Si(111) substrates by rf plasma assisted molecular-beam epitaxy are investigated by means of field emission scanning electron microscopy and photoluminescence measurements as a function of growth time. It is clearly demonstrated that the rate of growth of the nanorod diameter starts to increase after ∼90 min because of the coalescence of neighbouring nanorods. And the optical properties of the samples grown at a high growth rate are dramatically changed due to induced defects. The critical diameter for defect-free GaN nanorods is determined as below ∼140 nm under N-rich conditions.

Shape Selective Growth of CdS One-Dimensional Nanostructures by a Thermal Evaporation Process

CdS one-dimensional nanoforms such as nanowires, nanoribbons, network-like nanowires, pearl necklace type nanowires, helical-like nanowires, and nanowire arrays were formed on Si substrates by a simple thermal evaporation route. The shapes of the one-dimensional CdS nanoforms were controlled by varying the experimental parameters such as temperature and position of the substrates. Formation of the CdS one-dimensional nanoforms was initiated by the Au catalyzed vapor-liquid-solid technique, whereas the vapor-solid process played a crucial role in defining the shapes of the nanoforms. Different optical characterizations such as optical absorbance, photoluminescence, and Raman spectroscopy were adopted to explore the physical and structural quality of these CdS nanoforms.

Diameter-dependent electromechanical properties of GaN nanowires

The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires.

Conversion between hexagonal GaN and beta-Ga(2)O(3) nanowires and their electrical transport properties

We have observed that the hexagonal GaN nanowires grown from a simple chemical vapor deposition method using gallium metal and ammonia gas are usually gallium-doped. By annealing in air, the gallium-doped hexagonal GaN nanowires could be completely converted to beta-Ga(2)O(3) nanowires. Annealing the beta-Ga(2)O(3) nanowires in ammonia could convert them back to undoped hexagonal GaN nanowires. Field effect transistors based on these three kinds of nanowires were fabricated, and their performances were studied. Because of gallium doping, the as-grown GaN nanowires show a weak gating effect. Through the conversion process of GaN nanowires (gallium-doped) --> Ga(2)O(3) nanowires --> GaN nanowires (undoped) via annealing, the final undoped GaN nanowires display different electrical properties than the initial gallium-doped GaN nanowires, show a pronounced n-type gating effect, and can be completely turned off.

Pseudo-two-dimensional structures (HXYH)3n2H6n (XY = GaN, SiC, GeC, SiSi, or GeGe; n = 1-3): density functional characterization of structures and energetics

Hybrid density functional calculations with effective core potential basis sets are performed for monomeric group 13/15 and group 14/14 analogues of cyclohexane, as well as for three different pseudo-two-dimensional structures that can be formed from expanding one and two concentric rings around the central one (trans-fused chairs, a rolling combination of trans- and cis-fused chairs, and cis-fused boats). Varying contributions from torsional strain, angle strain, electrostatics, and nontraditional H-H hydrogen bonding lead to different orderings and magnitudes of motif energies in the various systems: Homoatomic SiSi and GeGe systems prefer the trans-fused chair alternative and heteroatomic systems GaN, SiC, and GeC prefer the rolling chair. Decomposition of structure energies into characteristic fragment contributions indicates that pseudo-one-dimensional rods of poly(imidogallane) are thermodynamically more stable than any of the pseudo-two-dimensional structures.

SiO2 Nanowires Growing on Hexagonally Arranged Circular Patterns Surrounded by TiO2 Films

An effective approach has been demonstrated for the synthesis of novel composite architectures, SiO2 nanowires (NWs) growing on hexagonally arranged circular patterns surrounded by TiO2 films on Si substrate. First, a solution-dipping template strategy is used to create TiO2 films with hexagonally arranged pores on Au-coated Si substrate, resulting in hexagonally arranged circular patterns of catalysts surrounded by TiO2 films. Then the patterned catalysts guide the growth of SiO2 NWs with the original TiO2 films preserved, realizing the composite structures. Such composite architectures combine the photoluminescence (PL) properties of the two components, and also present more favorable PL property, laying a foundation for future advanced nano-optoelectronic devices.

Ferromagnetism in Mn-doped GaN nanowires

Using density functional theory we show that the magnetic coupling of Mn atoms in the nanowires, unlike that in the thin film, is ferromagnetic. This ferromagnetic coupling, brought about due to the confinement of electrons in the radial direction and the curvature of the Mn-doped GaN nanowires' surface, is mediated by N as is evidenced from the overlap between Mn 3d and N 2p states. Calculations of the anisotropic energy further show that the magnetic moment orients preferably along the [1010] direction while the wire axis points along the [0001] direction.

Synthesis of long indium nitride nanowires with uniform diameters in large quantities

Large quantities of indium nitride (InN) nanowires are synthesized by the in situ nitriding of indium oxide (In(2)O(3)) powders in an ammonia (NH(3)) flux. Tens of milligrams of nanowires are obtained in one batch. Every 100 mg of In(2)O(3) starting powder can produce up to 65 mg of InN nanowires under the optimized conditions. The synthesized nanowires grow along the [001] direction with excellent crystallinity. They are of high purity and are 30-50 microm in length with an almost uniform diameter of about 100 nm. Photoluminescence measurements of the nanowires exhibit a strong peak at 707 nm. An optical bandgap of about 1.7 eV is estimated based on the absorption spectrum. The experimental results also demonstrate that the approach of nitriding In(2)O(3) powders in situ is feasible for the synthesis of high-purity InN nanowires in large quantities, with good reproducibility and without catalyst materials. The synthesis of InN nanowires in large quantities would be of benefit to the further study and understanding of their intrinsic properties, as well as being advantageous for their potential application in nanodevices.

Catalytic hydride vapour phase epitaxy growth of GaN nanowires

Catalytic growth of GaN nanowires by hydride vapour phase epitaxy is demonstrated. Nickel-gold was used as a catalyst. Nanowire growth was limited to areas patterned with catalyst. Characterization of the nanowires with transmission electron microscopy, x-ray diffraction, and low temperature photoluminescence shows that the nanowires are stoichiometric 2H-GaN single crystals growing in the [0001] orientation when grown on sapphire, with occasional stacking faults along the c-axis as the only defect type observed in most of the wires. A red shift observed in the photoluminescence was too large to be explained by the minor strain observed alone, and was only marginally affected by temperature, suggesting a superposition of several factors.

Synthesis of MnWO(4) nanofibres by a surfactant-assisted complexation-precipitation approach and control of morphology

Single-crystal MnWO(4) nanofibres with diameters as small as 3-4 nm and length up to 800-1000 nm have been successfully synthesized by a novel surfactant-assisted complexation-precipitation method. X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and electron diffraction (ED) were employed to study the crystal structure and morphologies of the products. Experiments show that the complexation and the surfactant are the crucial factors affecting the morphology of the products. The possible formation process and growth mechanism have been proposed.

Disorder effects in focused-ion-beam-deposited Pt contacts on GaN nanowires

The current-bias (I-V) characteristics at various temperatures, T, of focused-ion-beam (FIB)-deposited Pt contacts on GaN nanowires evolves from low-resistance ohmic (linear I-V) to rectifying as the diameter increases, and both exhibit strongly nonmetallic T-dependence. The small-diameter (66 nm) T-dependent resistance is explained by two-dimensional variable range hopping with a small characteristic energy, ensuring low resistance at 300 K. For large diameters (184 nm), back-to-back Schottky barriers explain the nonlinear I-V at all T values and permit an estimate of doping concentration from the bias-dependent barrier height. Both behaviors can be understood by accounting for the role of FIB-induced amorphization of GaN underneath the contact, as confirmed by cross-sectional transmission electron microscopy.

Synthesis and Optical Properties of Gallium Phosphide Nanotubes

Gallium phosphide nanotubes with zinc blende structure were synthesized for the first time. The as-prepared GaP nanotubes are polycrystalline with diameters of 30-120 nm and occasionally partially filled. The growth has been reasonably proposed to follow vapor-liquid-solid (VLS) mechanism. The integration of the nanotubular structure with the unique intrinsic semiconducting properties of GaP might bring GaP nanotubes some novel optical and electronic properties and applications.

Synthesis and Optical Properties of CdS Nanoribbons

Rapid production of single crystalline CdS nanoribbons with hexagonal wurtzite phase has been achieved by thermal evaporation of CdS powder on Si wafers. The flow rate of the carrier (Ar) gas along with the synthesis temperature plays an important role in defining the size and shape of the CdS nanoribbons. Scanning electron and transmission electron microscopic observations revealed the nanoribbons to have a flat end as well as side surfaces which will make it ideal for optoelectronic devices such as nanolasers and light emitting diodes based on individual nanoribbons. The nanoribbons have widths within 200-400 nm and lengths approximately a few hundred micrometers. Room-temperature photoluminescence measurements show green emission centered at approximately 525 nm which may be ascribed to the near band edge emission. The Raman spectra of the CdS nanoribbons show peaks around 304, 609, 915, and 1220 cm(-1) corresponding to the first-, second-, third-, and fourth-order longitudinal optical phonon modes, respectively.

Carbon nanotubes for biomedical applications

Carbon nanotubes (CNTs) have many unique physical, mechanical, and electronic properties. These distinct properties may be exploited such that they can be used for numerous applications ranging from sensors and actuators to composites. As a result, in a very short duration, CNTs appear to have drawn the attention of both the industry and the academia. However, there are certain challenges that need proper attention before the CNT-based devices can be realized on a large scale in the commercial market. In this paper, we report the use of CNTs for biomedical applications. The paper describes the distinct physical, electronic, and mechanical properties of nanotubes. The basics of synthesis and purification of CNTs are also reviewed. The challenges associated with CNTs, which remain to be fully addressed for their maximum utilization for biomedical applications, are discussed.

Faceted and vertically aligned GaN nanorod arrays fabricated without catalysts or lithography

Monocrystalline, vertically aligned and faceted GaN nanorods with controlled diameter have been synthesized by selective organometallic vapor phase epitaxy (OMVPE) onto GaN exposed at the bottom of pores in silicon dioxide templates patterned by reactive ion etching through self-organized porous anodic alumina films. This process is free of foreign catalysts, and the nanorod diameter control is achieved without the need for low-throughput nanolithographic techniques. The use of conventional OMVPE growth conditions allows for the straightforward adaptation of conventional doping and heterostructure growth as will be necessary for the fabrication of nanorod-based strain-relaxed electrically pumped lasers and light-emitting diodes.

Large-scale synthesis and self-assembly of monodisperse hexagon Cu2S nanoplates

One simple high-temperature solution phase method has been developed to synthesize large-scale, monodisperse, hexgon beta-Cu2S nanoplates. The hexgon nanoplates have edge lengths of 9 +/- 0.5 nm and thicknesses of 4.5 +/- 0.2 nm and self-assemble closely into three-dimensional superlattices. The present results suggest that the simple method might be useful for the synthesis ofmonodispese hexagon nanoplates for many other chalcogenide semiconductors with hexagonal symmetry structure.

Photocatalytic TiO2/glass nanoflake array films

A new approach for the fabrication of oriented TiO2/glass nanoflake arrays has been developed. The ceramic nanoflake array was formed on a glass substrate via a simple, low temperature, and one-step hydrothermally induced phase separation approach without using any templates or additives. The factors affecting the formation of ceramic nanoflakes were examined by various characterization techniques. The results showed that the leaching of the soluble phase from the glass surface through hydrothermal processes resulted in oriented uniform ceramic nanoflake arrays. Electron microscope observations revealed that the nanoflakes formed a continuous porous three-dimensional-network array with a large surface-to-volume ratio. In addition, an anatase TiO2 film was successfully coated onto the nanoflake array by the sol-gel method. The TiO2/glass nanoflake array exhibited high activity for the photocatalytic degradation of acetone and for photoinduced hydrophilic conversion. Such enhancements were attributed to the beneficial effects of the new continuous porous three-dimensional-interconnected nanoflake network and its surface geometrical nanostructure. The present approach provides a convenient route to modify a photocatalytic coating with a porous nano-architectured substrate. This opens extensive new opportunities in the design of semiconductor/ceramic nanostructural array thin films with unusual properties for future optical and electronic applications.

Scanning electron microscopy investigation of Cu–TCNQ micro/nanostructures synthesized via vapor-induced reaction method

Micro/nanostructures based on the metal-organic complex Cu-TCNQ were successfully synthesized by a novel method: vapor-induced reaction. Scanning electron microscopy was used to investigate the morphology on the three different parts of the substrate: the hot reaction area, a transitional reaction area and an induced reaction area. The results show that the morphology of the as-grown structures evolves from microstructures to nanostructures. The formation mechanism of these different structures may be understood from electrochemical principles and the decreasing concentration of TCNQ.

Temperature-dependent growth of germanium oxide and silicon oxide based nanostructures, aligned silicon oxide nanowire assemblies, and silicon oxide microtubes

We demonstrate the temperature-dependent growth of germanium oxide and silicon oxide based composite nanostructures (multiple nanojunctions of Ge nanowires and SiO(x) nanowires, Ge-filled SiO(2) nanotubes, Ge/SiO(2) coaxial nanocables, and a variety of interesting micrometer-sized structures), aligned SiO(x) nanowire assemblies, and SiO(x) microtubes. The structures were characterized by SEM, TEM, energy-dispersive X-ray spectroscopy, and electron diffraction. The combination of laser ablation of a germanium target and thermal evaoporation of silicon monoxide powders resulted in the formation of Ge and SiO(x) species in a carrier gas; the nano/micro-sized structures grow by either a Ge-catalyzed vapor-liquid-solid or a Ge-nanowire-templated vapor-solid process.

Interfacial reactions in confinement: kinetics and temperature dependence of the surface hydrolysis of polystyrene-block-poly(tert-butyl acrylate) thin films

The effect of confinement on the kinetics of the surface hydrolysis of polystyrene-block-poly(tert-butyl acrylate) (PS(n)-b-PtBA(m)) thin films on oxidized silicon substrates in 3 M aqueous hydrochloric acid was systematically investigated. As shown by X-ray photoelectron spectroscopy (XPS) and contact angle measurements, a skin layer of acid-sensitive PtBA is present on the surface of PS(n)-b-PtBA(m) films, consistent with the lower surface tension of PtBA compared to that of PS. The thickness of the skin layer was determined by angle-dependent XPS as approximately 8 nm for PS(690)-b-PtBA(1210). Tapping mode atomic force microscopy showed an increasing surface coverage of swollen poly(acrylic acid)-rich globules with increasing hydrolysis time. Using ex situ Fourier transform infrared spectroscopy, the reaction kinetics was determined quantitatively as a function of temperature, polymer film thickness, thermal pretreatment of the films, and block copolymer composition. The initial stages of the hydrolysis can be described as a pseudo-first-order reaction under all conditions investigated. The corresponding rate constants were found to be 2 orders of magnitude lower than those reported for the hydrolysis of tert-butyl acetate in solution and depended linearly on the fraction of PtBA exposed at the surface. However, the polymer film thickness, thermal pretreatment of the films, block copolymer composition, and local composition did not affect the rate constants. The negative value of the activation entropy (DeltaS(298)++ = -103 J/mol K), determined according to the Arrhenius equation and transition state theory, indicates that the tightness of the transition state is more pronounced in the PS(n)-b-PtBA(m) film compared to reactions in solution. Thus, the spatial constraints due to the incorporation of the reactive ester groups in thin polymer films are responsible for the observed reduced reactivity.

On the thermal annealing conditions for self-synthesis of tungsten carbide nanowires from WC(x) films

The thermal annealing conditions in nitrogen ambient for the self-synthesis of tungsten carbide nanowires from sputter-deposited WC(x) films were investigated. Experimental results show that the temperature window for the growth of nanowires lies in the range of 500-750 °C with the corresponding annealing time interval ranging from 2.5 to 0.25 h. The diameter, length, and density of the grown nanowires are in the range of 10-15 nm, 0.1-0.3 µm, and 210-410 µm(-2), respectively. The degree of carbon depletion in the annealed WC(x) films plays a crucial role in determining both the shape and density of the self-synthesized nanowires. Nanowires synthesized at lower temperatures were seen to be smaller in dimension but higher in density. Material analysis reveals that the phase transition from WC to W(2)C arising from decarburization of the WC(x) film during thermal annealing should be responsible for the self-synthesis of nanowires.

Surfactant-free hydrothermal synthesis of lithium aluminate microbricks and nanorods from aluminium oxide nanoparticles

Beta-LiAlO2 microbricks and rectangular nanorods have been successfully synthesized from Al2O3 nanoparticles by a simple hydrothermal process without any surfactant or template, by simply changing the Li/Al molar ratio.

Novel nanoscale architectures: coated nanotubes and other nanowires

Research has demonstrated that the structure and properties of a nanoscale system are inextricably linked. The advent of nanoscale research in 1991 relied upon nanoscale material production through random formation techniques, such as arc discharge, and the inherent properties and morphology of the system were therefore difficult to control. This article reviews some of the methods and ideas that have developed since the inception of nanotechnology, leading to fine control over the morphology of nanoscale systems and highlighting some interesting nanoscale architecture.