Vapor-solid Process Research Articles

Growth of aluminum oxide nanorods using sandwich structures composed of Al and SiO x layers

In this work, we report a simple method for growing aluminum oxide nanorods based on low-temperature annealing of a sandwich structure composed of a thin Al film sandwiched between two silicon-rich oxide (SiOx: 0 < x < 2) layers produced by plasma-enhanced chemical vapor deposition. Aluminum oxide nanorods produced using a typical sandwich structure of SiO1.4(20 nm)/Al(2 nm)/SiO1.4(20 nm)/Si substrate exhibited the features of the mean diameter and length of the nanorods of about 0.3 and 1.7 μm, respectively. A possible growth mechanism is discussed on the basis of the vapor–solid process. The nanorod growth is proposed to be mediated by a vapor–solid mechanism in which the dominant vapor-phase source of reactants is Al2O/AlO produced by a phase separation of SiOx.

Synthesis of bamboo‐like SiC whiskers from waste silica fume

A process for the utilization of wasted silica fume is proposed in this work. Silicon carbide (SiC) whiskers several tens of micrometers in length and with a bamboo‐like morphology have been successfully synthesized by a carbothermal reduction process using purified silica fume as the silicon source. The morphology and structure of SiC whiskers were investigated by X‐ray diffraction, Raman spectroscopy, scanning electron microscopy, and high‐resolution transmission electron microscopy. Studies found that the as‐synthesized whiskers were grown as single‐crystalline β‐SiC along the (111) growth direction. The whiskers consisted of hexagonal stems randomly decorated with larger‐diameter knots along their whole length. On the basis of the characterization results, a vapor–solid process was discussed as a possible growth mechanism of the β‐SiC whiskers.

Field emission properties and growth mechanism of In2O3 nanostructures.

Four kinds of nanostructures, nanoneedles, nanohooks, nanorods, and nanotowers of In2O3, have been grown by the vapor transport process with Au catalysts or without any catalysts. The morphology and structure of the prepared nanostructures are determined on the basis of field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). The growth direction of the In2O3 nanoneedles is along the [001], and those of the other three nanostructures are along the [100]. The growth mechanism of the nanoneedles is the vapor-liquid–solid (VLS), and those of the other three nanostructures are the vapor-solid (VS) processes. The field emission properties of four kinds of In2O3 nanostructures have been investigated. Among them, the nanoneedles have the best field emission properties with the lowest turn-on field of 4.9 V/μm and the threshold field of 12 V/μm due to possessing the smallest emitter tip radius and the weakest screening effect.

Field emission characteristics of zinc oxide nanowires synthesized by vapor-solid process

Vertically aligned ZnO nanowire (NW) arrays have been synthesized on silicon substrates by chemical vapor deposition. The growth of ZnO NWs may be dominated by vapor-solid nucleation mechanism. Morphological, structural, optical, and field emission characteristics can be modified by varying the growth time. For growth time that reaches 120 min, the length and diameter of ZnO NWs are 1.5 μm and 350 nm, respectively, and they also show preferential growth orientation along the c-axis. Room-temperature photoluminescence spectra exhibit a sharp UV emission and broad green emission, and the enhanced UV-to-green emission ratio with increasing growth time might originate from the reduced concentration of surface defects. Furthermore, strong alignment and uniform distribution of ZnO NWs can also effectively enhance the antireflection to reach the average reflectance of 5.7% in the visible region. The field emission measurement indicated that the growth time plays an important role in density- and morphology-controlled ZnO NWs, and thus, ZnO NWs are expected to be used in versatile optoelectronic devices.

Temperature-dependent growth mechanism and microstructure of ZnO nanostructures grown from the thermal oxidation of zinc

We report a detailed study on the growth morphologies and microstructure of ZnO nanostructures formed from the oxidation of Zn at different temperatures. ZnO shows bicrystalline nanowire morphology for oxidation below the melting point of Zn, and single-crystalline morphology between the melting and boiling points of Zn, and tetrapod morphology above the boiling point of Zn. The morphological and microstructural variations are attributed to the temperature-dependent oxide growth mechanisms, i.e., the oxidation below the melting point of Zn is dominated by a solid–solid transformation process, a liquid–solid process between the melting and boiling points of Zn, and a vapor–solid process above the boiling point of Zn. The understanding of the oxide growth mechanisms from these results may have practical implications for rational control of the morphology, crystallinity, preferential growth directions, shape and aspect ratio of ZnO nanostructures

Triangular ZnO Nanosheets: Synthesis, Crystallography and Cathodoluminescence

ZnO nanosheets with triangular morphology have been synthesized on an Au-coated silicon substrate through a facile thermal evaporation process. The morphologies and microstructures of the nanosheets were studied by a scanning electron microscope (SEM) and a high-resolution transmission electron microscope (HR-TEM). These studies show that a nanosheet is commonly composed of two parts: a triangular ZnO sheet and an Au nanoparticle attached on its tip-end. Detailed crystallography analyses conclude that the formation of the highly crystalline nanostructures can be assigned to a combination of a vapor-liquid-solid (VLS) process that is believed to be responsible for its initial nucleation and subsequent crystallization along the growth direction, and a vapor-solid (VS) process that is responsible for its further radial growth. The spatially-resolved cathodoluminescence (CL) spectra exhibit a sharp strong near-band-edge (NBE) emission in the ultraviolet range and a negligible green emission.

Morphology evolution, growth mechanism and optical properties of AlN nanostructures

In the present paper, we prepared various kinds of aluminium nitride (AlN) nanostructures utilizing chemical vapor deposition method at atmospheric pressure. Different nanostructures including flower, rod and film were obtained on bare silicon substrates by controlling the growth temperature between 650 and 800 °C. The formation mechanism of these nanostructures is related to vapor–solid process and Ehrlich–Schwoebel barrier. The crystalline phase and morphologies of the as-prepared AlN samples are investigated systematically. Their microstructures are observed by the scanning electron microscope. The X-ray diffraction results demonstrate that the AlN samples exhibit pure phase and grow preferentially along the c-axis. The Raman examination shows there is a strong stress at the interface between the AlN nanostructures and the silicon substrate. The photoluminescence properties indicate that AlN nanostructures possess a broad luminescence band, which can be divided into two subbands by Gaussian fitting, and they are ascribed to nitrogen vacancy as well as the oxygen impurity.

Synthesis and characterization of hafnium carbide microcrystal chains with a carbon-rich shell via CVD

Abstract Novel hafnium carbide (HfC) microcrystal chains, with a periodically changing diameter and a nanoscale sharpening tip at the chain end, have been synthesized via a catalyst-assisted chemical vapor deposition (CVD) process. The as-synthesized chains with many octahedral microcrystals have diameters of between several hundreds of nm and 6 μm and lengths of ∼500 μm. TEM diffraction studies show that the chains are single-crystalline HfC and preferentially grow along a [0 0 1] crystal orientation. TEM/EELS/EDX analysis proves the chains are composed of a HfC core and a thin (several tens of nm to 100 nm) carbon-rich shell with the embedded HfC nanocrystallites (typically below 10 nm) surrounding the core. The growth mechanism model for the chains based on the vapor–liquid–solid process, the vapor–solid process, and the HfC crystal growth characteristics is discussed.

Controllable Low-Temperature Chemical Vapor Deposition Growth and Morphology Dependent Field Emission Property of SnO2 Nanocone Arrays with Different Morphologies

Vertically aligned SnO2 nanocones with different morphologies have been directly grown on fluorine-doped tin oxide (FTO) glass substrates in a large area by heating a mixture of stannous chloride dihydrate (SnCl2·2H2O) and anhydrous zinc chloride (ZnCl2) at 600 °C in air. Control over the SnO2 nanocone arrays with different morphologies is achieved by adjusting the heat treatment time. The SnO2 nanocones are single crystalline with the tetragonal structure. A single-layer SnO2 nanoparticle film is first formed via the vapor-solid (VS) process due to the decentralization function of ZnCl2 vapor, and the SnO2 nanoparticles served as seeds and grew into nanocone arrays via the VS process. The sharp-tipped nanostructure formation may originate from a concentration gradient of reactant in the growth process. The as-obtained whiskerlike nanocone arrays exhibit enhanced field emission properties in comparison with typical nanoconelike structure arrays and other SnO2 nanostructured materials reported previously, and the turn-on field and field-enhancement factor is 1.19 V/μm and 3110, respectively. The experimental result is consistent with the Utsumi's relative figure of merit for pillar-shaped emitters.

Molten-salt-mediated synthesis of SiC nanowires for microwave absorption applications

Silicon carbide (SiC) nanowires were synthesized by a reaction of multiwall carbon nanotubes (MWCNTs) and silicon vapor from molten salt medium at 1250 °C. The phase, morphology, and microstructure of the nanowires were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopy. The results revealed that the nanowires were of single-crystalline β-SiC phase with the growth direction along [111] and had diameters of 20–80 nm and lengths up to several tens of micrometers. The molten salt introduced facilitated the evaporation of Si (vapor) onto MWCNTs (solid) and the growth of SiC nanowires followed the vapor–solid process. The investigation of microwave absorbability indicated that a minimum reflection loss of −17.4 dB at 11.2 GHz could be achieved with 30 wt% SiC nanowires as the filler in the silicone matrix. The attenuation of microwave could be attributed to the dielectric loss and a possible absorption mechanism was also discussed.

Controllable growth of laterally aligned zinc oxide nanorod arrays on a selected surface of the silicon substrate by a catalyst-free vapor solid process – a technique for growing nanocircuits

We report a simple and effective vapor deposition method for directly growing ultra-long, laterally aligned, zinc oxide (ZnO) nanorod arrays only on the side edges of a bare silicon (Si) substrate without using any catalysts and precursors. The growth on the top surface of the substrate is restrained by controlling the flow of source vapor in a tube furnace through the chemical vapor solid process. The optimized growth parameters have been thoroughly investigated and identified. Direct growth of laterally aligned ZnO nanowire arrays on the desired surface of the substrate is successfully achieved. A vapor solid mechanism with source vapor flow rate control has been proposed to explain the synthesis: ZnO nanodots first form on the bare Si substrate side edges due to the local large binding energy and high zinc (Zn) vapor concentration, and then nanorods epitaxially grow from the nanodots. In addition, the lateral, ultra-long ZnO nanorods grown on orthogonal silicon microelectrodes are achieved and could be expected to find important applications in a bottom-up way of fabricating the next generation nanoelectronics.

Luminescent Zn2GeO4 nanorod arrays and nanowires

Large-area Zn2GeO4 nanorod arrays (1.5 × 5 cm) and long Zn2GeO4 nanowires (length up to 1 mm) have been controllably prepared at different areas on the substrate using a simple thermal evaporation method. The formation processes for these nanostructures were investigated carefully. The solid state reaction and vapor-solid process have been found to clearly contribute to the creation of the Zn2GeO4 nanorod arrays and long Zn2GeO4 nanowires, respectively. Photoluminescence (PL) characterization of these nanostructures showed that both the nanorod arrays and nanowires exhibited strong blue-white luminescence. Mn-doped Zn2GeO4 nanorod arrays have also been obtained by doping an appropriate amount of Mn(2+) ions in the products. Zn2GeO4 nanostructures doped with Mn(2+) exhibited bright green luminescence.

Zn1−x Mg x O (0≤x≤0.05) nanowalls grown on catalyst-free sapphire substrates by high-pressure PLD and their photoluminescence properties

We report the growth of Zn1−xMgxO (x=0, 0.02, 0.05 at.%) nanowalls on sapphire substrates without any metal catalysts by a high-pressure pulsed laser deposition (PLD). The influences of the experimental parameters like growth pressure, temperature, and target-substrate distance on the growth of Zn1−xMgxO nanowalls were systemically studied and the growth mechanism was discussed. It was found that large area and uniform Zn1−xMgxO nanowalls with c-orientation can be grown on sapphire substrates when the growth temperature and pressure were 950 °C and 400 Torr at a target-substrate distance of 2 cm. A thin layer assisted vapor-solid (VS) process was proposed for Zn1−xMgxO nanowalls growth. The photoluminescence spectrum shows the bandgap of Zn1−xMgxO nanowall was effectively expanded together with defect-related levels formation in a forbidden gap, which also induced enhancement of visible emission.

Selective formation of thickness-controlled fullerene disks by vapor–solid process

Thin C60 disks were selectively prepared without any solvent using a vapor–solid process. A tube furnace with three separate heating zones was used. Controlling the substrate temperature and the vapor temperature made the selective formation of the thin C60 disks possible. The temperature of the first and second zones for evaporation of C60 was fixed at 650°C, the temperature of the third zone at 500°C, and the temperature of the substrate between 150 and 300°C by water circulation through the sample stage. The thickness of the thin C60 disks decreased as the substrate temperature increased in the range of 150–300°C. This occurred because the growth rate of a crystal increases as the difference between the evaporation temperature and the substrate temperature (crystallization temperature) of the fullerene increases.

Synthesis and characterization of single crystalline GdB44Si2 nanostructures

GdB44Si2 is an excellent thermoelectric material, and nanostructured GdB44Si2 makes it possible to potentially improve its properties further. GdB44Si2 nanowires and nanobelts have been fabricated by chemical vapor deposition and characterized by electron diffraction and high-resolution electron microscopy. These nanostructures are of the YB50 structure and grew in the [010] direction. The nanowires have thickness of less than 100 nm and length of several tens of microns. The nanobelts have thickness of about a few tens of nanometers. Morphological and compositional analyses confirmed that the nanowire growth followed the vapor–liquid–solid mechanism and the nanobelts were formed by a subsequent vapor–solid process of condensation.

Controllable Low Temperature Vapor-Solid Growth and Hexagonal Disk Enhanced Field Emission Property of ZnO Nanorod Arrays and Hexagonal Nanodisk Networks

ZnO nanorod arrays and nanodisk networks were grown directly on Si substrate by thermal evaporation of ZnCl(2) powder and a mixture of ZnCl(2) and InCl(3)·4H(2)O at 450 °C in air, respectively. The ZnO nanorods with the diameters of 0.64 to 0.91 μm and length of about 5.1 μm are single crystalline with the hexagonal structure and grow along the [001] direction. The nanodisk has perfect hexagonal shape, grow mainly along the [0110] directions, and are enclosed by ±(0001) top and bottom surfaces. ZnO nanoparticle films oriented in the [001] direction formed first served as seeds, and grow into nanorod arrays via the vapor-solid (VS) process. However, when InCl(3)·4H(2)O was introduced into the reaction system ZnO thick nanosheet films are first formed because of the local segregation of the doping element of indium. The ZnO thick nanosheet films served as seeds, and grow into nanodisk networks via the V-S process. Photoluminescence and field emission properties of the as-obtained ZnO nanorod arrays and hexagonal nanodisk networks have been studied. It was found that the hexagonal nanodisk networks exhibit strong blue-green emissions originated from defect states and enhanced field emission property.

Defect assisted thermal synthesis of crystalline aluminum borate nanowires

Aluminum borate is attractive in that the material has excellent mechanical properties, chemical inertness, high temperature stability, and a low coefficient of thermal expansion. Moreover, aluminum borate has advantages over a more traditional material, SiC, in that it does not readily oxidize at high temperature and can be produced at lower cost. In this study, we demonstarte a facile route to grow single crystal aluminum borate nanowires directly on bare sapphire surfaces without the need for a catalyst. Our findings point to a growth mechanism in which lattice defects allow B or B2O2 diffusion. The nanowire formation occurs as a means to relieve residual stress that arises due to thermal expansion mismatch between the aluminum borate and alumina phases. Indeed, at a more local scale, this same stress process facilitates diffussion. By adding iron oxide, which has a high diffusion rate in sapphire, one can accelerate this process. The growth mechanism is fundamentally different to the more usual fabrication routes which employ vapor-solid-liquid or vapor-solid growth processes.

Synthesis of silica nanowires by PECVD at low temperature using Zn as a catalyst

Amorphous silica nanowires (NWs) were simply synthesized by annealing Zn film in silicon (Si) and oxygen (O) chemical vapors produced by plasma-enhanced chemical vapor deposition (PECVD) technique with a mixture of SiH4 and N2O gases. The resulting silica NWs were shown to have a silicon-rich oxide (SiO x ) phase in which the oxygen composition (x value) of the SiO x is controlled by varying the mixing ratio of two gases. It was found that well-defined silica (SiO1.34) NWs having a mean diameter of ∼250 nm were formed by annealing Zn film (∼300 nm) at 380 °C for 10 min in Si and O vapors produced using a mixture of SiH4 and N2O achieved with flow rates of 200 sccm and 60 sccm, respectively. They emit light with a peak centered at 550 nm, characteristic of SiO x materials. It is suggested that the NWs are grown via Zn/ZnO-catalyzed vapor–solid process.

Zebra-Striped Fibers in Relation to the H2 Sorption Properties for MgH2 Nanofibers Produced by a Vapor–Solid Process

MgH2 is a promising candidate for solid-state H2 storage applications. To improve the H2 sorption kinetics, highly pure MgH2 nanofibers were synthesized by a vapor–solid process. The fibers showed improved sorption kinetics compared with bulk MgH2, which were investigated by a volumetric pressure–composition–temperature method. To choose a meaningful kinetic model that represented the physical reality and intrinsic kinetic parameters for the unique fiber structure, theoretical modeling of the sorption data and metallographic examinations of the in situ dehydrogenation process and partially hydrogenated samples were carried out. The Johnson–Mehl–Avrami (JMA) model, which is based on the theory of nucleation and growth transformation, was selected for the kinetic mechanism analysis of the hydrogenation/dehydrogenation of the fibers. The theoretical modeling by the JMA model indicated that the phase transformation of Mg to MgH2 (or of MgH2 to Mg) in the individual fibers proceeded one-dimensionally along the...

Field emission properties originated from 2D electronics gas successively tunneling for 1D heterostructures of ZnO nanobelts decorated with In2O3 nanoteeth

ZnO–In2O3 one-dimensional (1D) nanosized heterostructures constructed by ZnO belts and In2O3 tooth-like particles were self-assembled on single crystal silicon substrate using thermal chemical vapor transport and condensation without being aided by any metal catalyst. The morphology, structure, and composition of the as-synthesized 1D heterostructures were analyzed in detail. The widths of the ZnO nanobelts ranged from several tens of nanometers to one micrometer, and the lengths ranged from several tens to one hundred of micrometers. In2O3 tooth-like nanoparticles with sizes of about 50–100 nm were found grown at two edges of ZnO nanobelts. ZnO nanobelts grew along [\( 10\overline{1} 0 \)] direction, whereas In2O3 nanoteeth grew along [\( 31\overline{1} \)], [\( 3\overline{1} 1 \)], [\( \overline{3} \overline{1} 1 \)], and [\( \overline{3} 1\overline{1} \)] directions so as to form rhombus-shaped structures. The growth mechanism of the nanosized heterostructures was discussed on the basis of the vapor–solid process and polar surface effect of ZnO nanobelts. Field emission characteristics of the as-prepared heterostructures were measured and explained by energy band theory of heterostructure in detail. It is important to note that the 2D electronics gas (2DEG) was formed between the ZnO energy band bending down and the interface of the heterostructure. The successive tunneling of 2DEG that took place from ZnO to In2O3 and then from In2O3 to vacuum was the main reason resulting in electronics emission for the nanosized heterostructures in the process of field emission.