Synthesis of gallium oxide nanomaterials on source material supply and their growth behavior

High purity and single crystalline beta-Ga2O3 nanomaterials with various morphologies were obtained through the simple thermal evaporation of metal gallium powder on a gold-coated silicon substrate in argon. In this report, the growth behavior of the beta-Ga2O3 nanomaterials as a function of synthesis time and source material supply was delicately surveyed via FESEM and HRTEM. The synthesis time and source material supply affected morphology, growth rate and growth mechanism of the grown nanomaterials. It was confirmed that the growth mechanism of the beta-Ga2O3 nanomaterials was varied in the order of VLS, combination of VLS and VS, and VS, by increasing the synthesis time without regard to the supply of the source material. When the source materials supply was increased, many beta-Ga2O3 nanomaterials with various morphologies, such as sheet, triangle, and belt-like were appeared. It was confirmed that the oxidation reaction of gallium and oxygen for the formation of gallium oxide nanomaterials carried out the precipitation of gallium at the same time due to the supersaturation of the gallium atoms in gold catalyst. The growth and formation mechanism of the beta-Ga2O3 nanomaterials are discussed herein.     

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.     

Growth mechanism and elemental distribution of beta-Ga2O3 crystalline nanowires synthesized by cobalt-assisted chemical vapor deposition

Long beta-Ga2O3 crystalline nanowires are synthesized on patterned silicon substrates using chemical vapor deposition technique. Advanced electron microscopy indicates that the as-grown beta-Ga2O3 nanowires are consisted of poly-crystalline (Co, Ga)O tips and straight crystalline beta-Ga2O3 stems. The catalytic cobalt not only locates at the nanowire tips but diffuses into beta-Ga2O3 nanowire stems several ten nanometers. A solid diffusion growth mechanism is proposed based on the spatial elemental distribution along the beta-Ga2O3 nanowires at nanoscale.     

Soft-template mediated synthesis of GaOOH nanorod-shelled microspheres and thermal conversion to beta-Ga2O3

Micrometer-scale hollow spheres self-assembled by GaOOH nanorods were synthesized under hydrothermal conditions using gallium nitrate and sodium hydroxide as starting materials. The structures and morphologies of the products were studied by X-ray diffraction and scanning electron microscopy. Time-dependent experiments revealed three stages involved in the process of reaction including the initial stage of formation of surfactant vesicles which can be considered as soft templates, followed by the nucleation of GaOOH nanoclusters, and the assembling and growth of nanorods under the modulation of the spherical vesicles. The growth kinetics of the GaOOH nanorods was systematically investigated. Based on the experimental observation, a template-mediated assembling mechanism was proposed. We further demonstrated that the GaOOH nanorods could be converted to gallium oxide (beta-Ga2O3) nanorods by calcination without changing the spherical morphology of the assemblies.     

The effects of gallium droplets on the morphologies and structures of alpha-Fe2O3 nanostructures grown on iron substrates

Alpha-Fe2O3 nanowires and nanobelts were grown by the thermal oxidation of iron substrates with or without gallium droplets in the air. The nanowires and nanobelts show a bicrystal structure with the growth direction uniformly along [110]. The morphological and structural properties of the as-grown alpha-Fe2O3 nanostructures are described and the growth condition dependence of the alpha-Fe2O3 nanostructures is shown. The transformation from nanowires to nanobelts occurs with the increase of growth temperature and addition of gallium. In addition, the growth evolution is investigated with reference to the Fe surface diffusion and supersaturation.     

Cathodoluminescence of rare earth implanted Ga2O3 and GeO2 nanostructures

Rare earth (RE) doped gallium oxide and germanium oxide micro- and nanostructures, mostly nanowires, have been obtained and their morphological and optical properties have been characterized. Undoped oxide micro- and nanostructures were grown by a thermal evaporation method and were subsequently doped with gadolinium or europium ions by ion implantation. No significant changes in the morphologies of the nanostructures were observed after ion implantation and thermal annealing. The luminescence emission properties have been studied with cathodoluminescence (CL) in a scanning electron microscope (SEM). Both β-Ga(2)O(3) and GeO(2) structures implanted with Eu show the characteristic red luminescence peak centered at around 610 nm, due to the (5)D(0)-(7)F(2) Eu(3+) intraionic transition. Sharpening of the luminescence peaks after thermal annealing is observed in Eu implanted β-Ga(2)O(3), which is assigned to the lattice recovery. Gd(3+) as-implanted samples do not show rare earth related luminescence. After annealing, optical activation of Gd(3+) is obtained in both matrices and a sharp ultraviolet peak centered at around 315 nm, associated with the Gd(3+) (6)P(7/2)-(8)S(7/2) intraionic transition, is observed. The influence of the Gd ion implantation and the annealing temperature on the gallium oxide broad intrinsic defect band has been analyzed.     

Formation of novel assembled silver nanostructures from polyglycol solution

This paper described a simple and mild chemical reduction approach to prepare novel silver nanostructures with different morphologies. Dendritic silver nanostructure was obtained by a fast reduction reaction using hydrazine as a reducing agent in aqueous solution of polyglycol, while both the zigzag and linear Ag nanostructures were slowly assembled using polyglycol as a reducing agent. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and field emission scanning electron microscopy (FE-SEM) were used to characterize the obtained silver nanostructures. Fourier transform infrared absorption (FT-IR) spectra were recorded to show that there exists a certain coordination of the oxygen atoms in the polyglycol with Ag⁺ ions in aqueous solution of the AgNO₃/polyglycol. Furthermore, the examination of the morphologies of the products obtained at different stages of the reaction of Ag⁺ ions with polyglycol revealed that such a coordination is of utmost importance for the formation of the silver nanostructures, namely polyglycol provided lots of active sites for the coordination, nucleation, growth and serves as backbones for directing the assembly of the metal particles formed. The formation mechanism of the dendritic silver nanostructure was called a coordination–reduction–nucleation–growth–fractal growth process. The strong surface plasmon absorption bands at 470 nm for the zigzag silver and at 405 nm for the dendritic silver were found.     

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.     

Synthesis and cathodoluminescence of beta-Ga2O3 nanowires with holes

Gallium oxide nanowires were synthesized on Si (001) substrate by chemical vapor deposition, using a Ga/Ga2O3 mixture as a precursor and Au as a catalyst. The structure of the as-synthesized products was examined by X-ray powder diffraction and high-resolution transmission electron microscopy, and found to be monoclinic beta-Ga2O3. The morphologies of the beta-Ga2O3 nanowires were characterized by scanning electron microscopy. The majority of the nanowires contain holes along their length, but a few were also found without holes. The holes are believed to be formed by the reaction of adsorbed Ga droplets on reactive terminating surfaces of the nanowires. For nanowires where these reactive surfaces are not exposed, the reaction of Ga is retarded. Cathodoluminescence (CL) of the nanowires was measured. Three emission bands centered at 376, 454, and 666 nm, respectively, were observed.     

The growth of ultralong and highly blue luminescent gallium oxide nanowires and nanobelts, and direct horizontal nanowire growth on substrates

We report the growth of ultralong β-Ga(2)O(3) nanowires and nanobelts on silicon substrates using a vapor phase transport method. The growth was carried out in a tube furnace, with gallium metal serving as the gallium source. The nanowires and nanobelts can grow to lengths of hundreds of nanometers and even millimeters. Their full lengths have been captured by both scanning electron microscope (SEM) and optical images. X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images have been used to study the crystal structures of these nanowires and nanobelts. Strong blue emission from these ultralong nanostructures can be readily observed by irradiation with an ultraviolet (UV) lamp. Diffuse reflectance spectroscopy measurements gave a band gap of 4.56?eV for these nanostructures. The blue emission shows a band maximum at 470?nm. Interestingly, by annealing the silicon substrates in an oxygen atmosphere to form a thick SiO(2) film, and growing Ga(2)O(3) nanowires over the sputtered gold patterned regions, horizontal Ga(2)O(3) nanowire growth in the non-gold-coated regions can be observed. These horizontal nanowires can grow to as long as over 10?μm in length. Their composition has been confirmed by TEM characterization. This represents one of the first examples of direct horizontal growth of oxide nanowires on substrates.     

Suppressing electrostatic screening in nanostructured electrode arrays

An individual nanostructure provides very high electric field enhancement because the sharp curvature of the nanostructure tip amplifies the local electric field near the apex tip. However any practical nanostructured electrode is comprised of an ensemble (array) of nanostructures. In such systems, mutual electrostatic shielding (or screening) severely limits the maximum achievable electric field enhancement. In this paper, we discuss three approaches for suppression of shielding. These include--(1) reducing anode-to-cathode distance to less than the nanostructure-to-nanostructure spacing, (2) increasing length of selected individual nanostructures within the array, and (3) design of electrodes with multistage amplification. We show that these approaches are effective in alleviating electrostatic shielding and that the enhancement factor of the electrode array (ensemble) can be engineered to match that of the individual (isolated) nanostructure.     

Evaporated nanostructured Y2O3:Eu thin films

Europium-doped yttrium oxide (Y2O3:Eu) is a well-known luminescent material that in recent years has been studied in thin-film form. However, to date there has not been a great effort put into altering the nanostructure of these films. A thin-film deposition technique called glancing angle deposition allows for a high degree of control over the nanostructure of the thin film, resulting in thin films with nanostructure geometries ranging from chevron and post to helix. Glancing-angle deposition was used to make europium-doped yttrium oxide thin films with slanted-post nanostructures. Portions of the films were annealed in air at 850 degrees C for 10 hours following deposition. Scanning electron microscopy was used to characterize the nanostructures of the films, while UV laser excitation was used to characterize the photoluminescence properties of the films. The annealed samples exhibited increased photoluminescent responses compared to unannealed samples; however, the porous nanoscale geometry of the films was unaffected. In order to optimize the photoluminescence properties of the films, both the partial pressure of oxygen during film deposition and the level of europium doping in the source material used were varied. Films fabricated from the source material with a greater amount of europium doping had larger photoluminescent responses, while the optimal partial pressure of oxygen during electron-beam evaporation was found to be less than 1.0 x 10(-4) torr.     

Shape matters: plasmonic nanoparticle shape enhances interaction with dielectric substrate

Numerical analyses of the ultraviolet and visible plasmonic spectra measured from hemispherical gallium nanostructures on dielectric substrates reveal that resonance frequencies are quite sensitive to illumination angle and polarization in a way that depends on nanostructure size, shape, and substrate. Large, polarization-dependent splittings arise from the broken symmetry of hemispherical gallium nanoparticles on sapphire substrates, inducing strong interactions with the substrate that depend sensitively on the angle of illumination and the nanoparticle diameter.     

Surfactant-assisted growth of novel La2(MoO4)3 dendritic nanostructures via facile hydrothermal processes

In this paper, we report for the first time the successful synthesis of novel uniform La2(MoO4)3 dendritic single-crystalline nanostructures via a surfactant-assisted hydrothermal approach. The dendritic nanostructure is composed of trunks with length of several micrometers and plenty of side branches. Both of the trunks and the branches are composed of nanoflakes with thickness of 30-50 nm. The branches are oriented nearly parallel to each other and form an angle of about 45 degrees to the trunk. The polyethylene glycol (PEG) acts as a morphology-directing agent, and by adjusting the experimental parameters, the microstructure of the processed materials could be further controlled in a certain range. The action mechanism of the surfactant has been proposed. This method is a simple and economical route for nanostructure fabrication and might provide a practical reference to the controlled synthesis of other micro-architectures. In addition, the photoluminescence properties of La2(MoO4)3:Eu dendritic nanostructures were studied.     

Three-dimensional hierarchical flowerlike α-Fe2O3 nanostructures: synthesis and ethanol-sensing properties

The α-Fe(2)O(3) hierarchical nanostructures have been successfully synthesized via a simple solvothermal method. The as-prepared samples are loose and porous with flowerlike structure, and the subunits are irregularly shaped nanosheets. The morphology of the α-Fe(2)O(3) structures was observed to be tunable as a function of reaction time. To demonstrate the potential applications, we have fabricated a gas sensor from the as-synthesized hierarchical α-Fe(2)O(3) and investigated it for ethanol detection. Results show that the hierarchical α-Fe(2)O(3) sensor exhibits significantly improved sensor performances in comparison with the compact α-Fe(2)O(3) structures. The enhancement of sensing properties is attributed to the unique porous and well-aligned nanostructure.     

Morphology and magnetic properties of diluted magnetic semiconductor 1D nanostructure Ni(x)Sn(1-x)O(2-delta)

SnO2 and Ni(x)Sn(1-x)O(2-delta) (x = 0.007-0.043) 1D nanostructures are fabricated using a catalyzer assisted chemical vapor deposition (CVD). The morphology of the 1D nanostructure is sensitive to the fabrication conditions. As the Ar flux rate is decreased from 50 sccm to 40 sccm, the 1D nanostructure changes from nanowire to nanobelt. All of the Ni(x)Sn(1-x)O(2-delta) 1D nanostructures exhibit room temperature ferromagnetism (RTFM). With the increasing x, magnetic moment per Ni ion increases at first, reaches a maximum of 3.33 microB in x = 0.025, then decreases. The results of annealing in vacuum and oxidizing atmospheres reveal that oxygen vacancies play a crucial role in introducing ferromagnetism, which implies that the origin of RTFM can be understood by the bound magnetic polaron model (BMP).     

Synthesis of In2O3 nanostructures: from pyramidal monuments, nanotowers to nanopencils

We report on the synthesis of In2O3 nanostructures grown at three different growth temperatures by using the thermal evaporation method. The obtained nanostructural morphologies of In2O3 were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In2O3 nanopencils and pyramidal monument-like structures were reported. The reaction temperature and the difference of temperature between the central heating zones and the product deposition zones, the reaction time, and the surface energies of different growth planes are all responsible for the final crystalline morphologies of the In2O3 nanostructures. A growth mechanism proposed to elucidate the kinetic factors leading the growth of In2O3 nanostructures. The obtained results may not only assist the investigation of new approaches of preparing various nanostructures for potential technical applications and how to modulate the desired morphology, but also give a deeper understanding of the fundamental physical and chemical processes of CVD methods.     

基于碳纳米管模板的氮化镓纳米线束合成

通过金属有机物化学气相沉积方法在碳纳米管模板上生长氮化镓纳米线束.对所生长的纳米结构进行了扫描电镜和X射线能谱分析,结果显示氮化镓纳米晶体可以与碳纳米管形成纳米线束状复合物.纳米线束状复合物直径为100～200 nm,长度为1.5～2.5μm,纳米线的两端呈现尖角状.由于氨气很容易吸附在碳纳米管表面,可知所获得的纳米结构的初始生长机制为碳纳米管的表面氮化.该研究也证明金属有机物化学气相沉积将是用于制造化合物纳米结构材料的一项有效的技术.     

Non-aqueous synthesis of water-dispersible Fe3O4-Ca3(PO4)2 core-shell nanoparticles

The Fe(3)O(4)-Ca(3)(PO(4))(2) core-shell nanoparticles were prepared by one-pot non-aqueous nanoemulsion with the assistance of a biocompatible triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO-PPO-PEO), integrating the magnetic properties of Fe(3)O(4) and the bioactive functions of Ca(3)(PO(4))(2) into single entities. The Fe(3)O(4) nanoparticles were pre-formed first by thermal reduction of Fe(acac)(3) and then the Ca(3)(PO(4))(2) layer was coated by simultaneous deposition of Ca(2+) and PO(4)(3-). The characterization shows that the combination of the two materials into a core-shell nanostructure retains the magnetic properties and the Ca(3)(PO(4))(2) shell forms an hcp phase (a = 7.490 ?, c = 9.534 ?) on the Fe(3)O(4) surface. The magnetic hysteresis curves of the nanoparticles were further elucidated by the Langevin equation, giving an estimation of the effective magnetic dimension of the nanoparticles and reflecting the enhanced susceptibility response as a result of the surface covering. Fourier transform infrared (FTIR) analysis provides the characteristic vibrations of Ca(3)(PO(4))(2) and the presence of the polymer surfactant on the nanoparticle surface. Moreover, the nanoparticles could be directly transferred to water and the aqueous dispersion-collection process of the nanoparticles was demonstrated for application readiness of such core-shell nanostructures in an aqueous medium. Thus, the construction of Fe(3)O(4) and Ca(3)(PO(4))(2) in the core-shell nanostructure has conspicuously led to enhanced performance and multi-functionalities, offering various possible applications of the nanoparticles.     

Nanophotonic switch: gold-in-Ga2O3 peapod nanowires

A novel metal-insulator heterostructure made of twinned Ga2O3 nanowires embedding discrete gold particles along the twin boundary was formed through a reaction between gold, gallium, and silica at 800 degrees C during simple thermal annealing. The Au-in-Ga2O3 peapods spontaneously crystallized under phase separation induced by the formation of twin boundaries. The nanostructures were analyzed by field emission scanning (FESEM) and transmission electron microscopes (FETEM), and their photoresponse was investigated using a double-frequency Nd:YAG laser with a wavelength of 532 nm on a designed single-nanowire device. The surface plasmon resonance (SPR) effects of embedded Au nanoparticles are proposed to be responsible for the remarkable photoresponse of these novel structures.