Vapor Liquid Solid Mechanism Research Articles

Shape-controlled growth of beta-gallium oxide nanowires and experimental investigation on the origination of electrons

Diameter-controlled growth of monoclinic gallium oxide nanowires was performed by chemical vapor deposition technique. Prior to growth, a thin layer of Au films was fabricated, SEM images indicated that Au could converge nanoparticles to govern the growth of β-Ga2O3 nanowires. The SEM images revealed both diameter and length of β-Ga2O3 nanowires strongly depended on the growth temperature. Nanowires grown at 1100 °C temperature have the minimum diameters and uniform lengths, compared with those grown at 900 °C and 1000 °C. The reasons are that a combination and a competition of vapor-liquid-solid (VLS) and vapor-solid (VS) mechanism occur in the growth process. High temperature would provide enough energy to make Ga and O atoms grow along one-dimensional axial orientation following the VLS mechanism. However, low temperature confines atoms at the nanowires interface and enhance sidewalls coalescence following the VS mechanism. X-ray diffraction (XRD) patterns and refinement indicated that the grown samples were of high crystallinity and had intrinsic oxygen vacancies. Photoluminescence spectra were examined and then followed by Gaussian fitting. Through analysis, the emission peaks were considered to originate from intrinsic oxygen vacancies and Ga-O vacancy pairs, consisting with the XRD refinement results. β-Ga2O3 nanowires was coated on the interdigitated electrode, the response to the ultraviolet light and oxygen was studied.

Diameter-Controlled Growth of Sb2Te3 Nanowires with Au Catalyst by the Vapor–Liquid–Solid Mechanism

Diameter-Controlled Growth of Sb2Te3 Nanowires with Au Catalyst by the Vapor–Liquid–Solid Mechanism

Monte Carlo simulation of planar GaAs nanowire growth

The Monte Carlo model for the planar GaAs nanowire growth via the vapor–liquid-solid mechanism on GaAs substrate using a gallium droplet as a catalyst is realized. The effect of temperature, Ga and As2 deposition rates, substrate orientation and surface properties on the morphology of planar GaAs nanowires is considered. It is shown, that at the initial stages of nanowire growth, a 3D GaAs crystal is formed under a gallium droplet, which sets the nanowire growth direction. The range of temperatures and deposition rates of gallium and arsenic, which ensure the planar nanowire formation on the GaAs(111)A surface, is determined. The investigation of surface properties influence on the nanowire morphology showed that the arsenic diffusion influx into a gallium droplet is of critical importance for the planar GaAs nanowire growth. Ways for achieving a unidirectional growth of planar nanowires on singular and vicinal GaAs surfaces are proposed.

Growth and characterization of germanium telluride nanowires via vapor–liquid–solid mechanism

Phase-change materials (PCMs), which can transition reversibly between crystalline and amorphous phases, have shown great promise for next-generation memory devices due to their nonvolatility, rapid switching periods, and random-access capability. Several groups have investigated phase-change nanowires for memory applications in recent years. The ability to regulate the scale of nanostructures remains one of the most significant obstacles in nanoscience. Herein, we describe the growth and characterization of germanium telluride (GeTe) nanowires, which are essential for phase-change memory devices. GeTe nanowires were produced by combining thermal evaporation and vapor–liquid–solid (VLS) techniques, using 8 nm Au nanoparticles as the metal catalyst. The influence of various growth parameters, including inert gas flow rate, working pressure, growth temperature, growth duration, and growth substrate, was examined. Ar gas flow rate of 30 sccm and working pressure of 75 Torr produced the narrowest GeTe nanowires horizontally grown on a Si substrate. Using scanning electron microscopy, the dimensions, and morphology of GeTe nanowires were analyzed. Transmission electron microscopy and energy-dispersive x-ray spectroscopy were utilized to conduct structural and chemical analyses. Using a SiO2/Si substrate produced GeTe nanowires that were thicker and lengthier. The current–voltage characteristics of GeTe nanowires were investigated, confirming the amorphous nature of GeTe nanowires using conductive atomic force microscopy. In addition, the effects of the VLS mechanism and the Gibbs–Thomson effect were analyzed, which enables the optimization of nanowires for numerous applications, such as memory and reservoir computing.

Metal Oxide Nanowires Grown by a Vapor-Liquid-Solid Growth Mechanism for Resistive Gas-Sensing Applications: An Overview.

Metal oxide nanowires (NWs) with a high surface area, ease of fabrication, and precise control over diameter and chemical composition are among the best candidates for the realization of resistive gas sensors. Among the different techniques used for the synthesis of materials with NW morphology, approaches based on the vapor-liquid-solid (VLS) mechanism are very popular due to the ease of synthesis, low price of starting materials, and possibility of branching. In this review article, we discuss the gas-sensing features of metal oxide NWs grown by the VLS mechanism, with emphasis on the growth conditions and sensing mechanism. The growth and sensing performance of SnO2, ZnO, In2O3, NiO, CuO, and WO3 materials with NW morphology are discussed. The effects of the catalyst type, growth temperature, and other variables on the morphology and gas-sensing performance of NWs are discussed.

Interface catalytic reduction of alumina by nickle for the aluminum nanowire growth: Dynamics observed by in situ TEM

Metal nanowires show promise in a broad range of applications and can be fabricated via a number of methods, such as vapor–liquid–solid process and template-based electrodeposition. However, the synthesis of Al nanowires (NWs) is still challenging from the stable alumina substrate. In this work, the Ni-catalyzed fabrication of Al NWs has been realized using various Al2O3 substrates. The growth dynamics of Al NWs on Ni/Al2O3 was studied using in situ transmission electron microscopy (TEM). The effect of alumina structures, compositions, and growth temperature were investigated. The growth of Al NWs correlates with the Na addition to the alumina support. Since no eutectic mixture of nickel aluminide was formed, a mechanism of Ni-catalyzed reduction of Al2O3 for Al NWs growth has been proposed instead of the vapor–liquid–solid mechanism. The key insights reported here are not restricted to Ni-catalyzed Al NWs growth but can be extended to understanding the dynamic change and catalytic performance of Ni/Al2O3 under working conditions.

Liquid Gallium-Assisted Patterned Growth of β-Ga2O3 Nanowires on Metallic Foils for Photocatalytic Dye Degradation

Herein, we report a facile and effective strategy for patterned growth of one-dimensional (1D) β-Ga2O3 nanowires on metallic substrates, based on the alloying of liquid gallium as a precursor. To demonstrate this method, the specific patterns are painted using gallium on a metallic foil through screen printing, which produces an intermetallic phase by alloying. The subsequent thermal treatment involving the sputtering Au serves as a catalyst to induce the formation of β-Ga2O3 nanowires on the foil. Notably, the gallium in the intermetallic phase maintains the in situ growth of nanowires. Such a growth process depends on the Au sputtering and temperature, which well follows a classical vapor–liquid–solid mechanism. Interestingly, benefiting from the unique physiochemical attributes of liquid gallium, the convenient alloying facilitates the synthesis of the nanowires at a moderately low temperature. This synthetic approach is also universal for a series of metallic substrates including Ag, Cu, Ni, and Co foils, and the production can be scaled up easily. Furthermore, as a model catalyst, the β-Ga2O3 nanowires grown on Ag foil (β-Ga2O3/Ag) display good photocatalytic performance and easy recovery in the degradation of dyes. The work provides an avenue to prepare patterned β-Ga2O3 nanowires based on liquid gallium.

In situ growth of SiC wires on CVI densification of SiC foam

AbstractSilicon carbide (SiC) foam prepared by polymer infiltration and pyrolysis (PIP) process was further densified with β‐SiC by chemical vapor infiltration (CVI) technique. Scanning electron microscopy and high‐resolution transmission electron microscopy images confirmed the presence of highly entangled and branched in situ grown SiC wires of uniform diameter (∼500 nm) over the struts of open‐cell SiC foam. A uniform rate increase in diameter from nanometer to micron range (∼11 μm) was observed with an increase in the CVI reaction period. X‐ray diffraction results showed the formation of highly crystalline β‐SiC structure along the <111> direction with stacking faults. The formation of SiC wires was explained by the vapor–liquid–solid mechanism and evenness of the surface and uniform growth rate of SiC confirmed the homogeneous concentration of gaseous species during CVI reaction. The compressive strength increased with relative density, with maximum values of 5.5 ± 1.26 MPa for ultimate SiC foam (ρ = 400 kg/m3) prepared by hybrid PIP/CVI technique. The thermo‐oxidative stability of the resultant foam was evaluated up to 1650°C under air and shows excellent thermal stability compared to SiC foam prepared by PIP route. The densified SiC foam can find potential applications in the field of hot gas filters, catalyst supports, microwave absorption properties, and heat insulation for high‐temperature applications.

About Some Fundamental Aspects of the Growth Mechanism Vapor-Liquid-Solid Nanowires

This study provides the formation of semiconductor nanowires (NWs) with a singular facet and a curved end surface by the vapor-liquid-solid (VLS) process that is analyzed and explained in details. Given the evidence, it is confirmed that the wettability of a liquid catalyst droplet on a crystal surface and the contact angle between the droplet and crystal play an essential role in the VLS process of NWs development. It is shown that for the VLS mechanism, the formation of NWs depends on the reduction in activation barrier to crystallization caused by the release of surplus-free energy by a spheroidizing drop in the region of the triple junction during the process of lowering surface area. This decreases the necessary supersaturation for the development of NW vertex facets at a fixed growth rate. The source of the extra free energy that drives the catalyst droplet movement during the steady-state development of NWs is the droplet’s outer surface. During the formation of NWs, those angles of inclination of the lateral surface NWs and droplet contact are obtained at which the solid/vapor, solid/liquid, and liquid/vapor interfaces experience the smallest increase in free energy. The wetting hysteresis is demonstrated to occur at the vertex of NWs, and the contact angle of a catalyst droplet may be regarded as an independent and fully-fledged thermodynamic parameter of the system’s state.

LUMINESCENCE OF TWO-DIMENSIONAL ZnO NANOSTRUCTURES: NANOWALLS, NANOSHEETS, NANOCOMBS

Preliminary comparative studies of the photoluminescent properties of two-dimensional ZnO nanostructures with morphology of nanowalls, nanosheets, and nanocombs, fabricated by gas-transport synthesis, have been performed. All structures exhibited near-band-edge (NBE) UV emission of the same order of intensity. Unlike nanocombs, whose spectrum contains a comparatively strong green luminescence band, nanowalls and nanosheets are characterized by a large ratio of the UV and visible components. This distinction is presumably due to the difference in the mechanisms of structure formation: nanowalls and nanosheets are formed according to the vapor–liquid–solid mechanism, whereas nanocombs grow according to the vapor–solid mechanism

Economical Silicon Nanowire Growth via Cooling Controlled Solid–Liquid–Solid Mechanism

AbstractConventional vapor–liquid–solid mechanism of nanowire growth opens up new opportunities of fabricating nanowires with controllable morphologies and aspect ratios. However, gaseous precursors have disadvantages of high material and processing cost, high toxicity, and limited scalability. By contrast, synthesizing nanowires via solid–liquid–solid mechanism could be a facile alternative since the low cost and nontoxic solid precursor is adopted in the process. In this study, the cooling control is found to be very critical for the solid–liquid–solid nanowire growth. Without a sufficient negative vertical temperature gradient, the nucleation and continuous growth of silicon nanowires could not occur. High volume gas flow cooling, fluctuating the heating temperature, decreasing the cooling rate, and applying a heat sink are all efficacious to promote silicon nanowire formation. In addition to the nanowires formed under high gas flow cooling on the silicon wafer sputtered with a nickel thin film, the solid–liquid–solid mechanism‐derived silicon nanowire growth can also be economically achieved by adopting a solution‐based coating of a nickel precursor onto the silicon substrate paired with a programmed slow cooling condition without using any gas, which could be transferred to other eutectic systems for cost‐effective nanomaterial fabrication.

Single‐phase (Hf0.84Ta0.16)C solid solution nanowires growth via catalyst‐assisted chemical vapor deposition

AbstractFabricating one‐dimensional (1D) ceramic nanostructure with superior performance is an important approach to broaden their applications. In this paper, the synthesis of single‐phase (Hf0.84Ta0.16)C solid solution nanowires via catalyst‐assisted chemical vapor deposition was proposed for the first time. The [0 1 1̄] growth direction of (Hf0.84Ta0.16)C solid solution nanowires was discussed in detail and its corresponding growth mechanism was identified to be top‐type vapor–liquid–solid mechanism. More impressively, our work could widen the application ranges of ceramic nanowires and propose the new thinking of fabricating ultrahigh melting point nanostructure.

β-Ga2O3 Nanostructures: Chemical Vapor Deposition Growth Using Thermally Dewetted Au Nanoparticles as Catalyst and Characterization.

β-Ga2O3 nanostructures, including nanowires (NWs), nanosheets (NSHs), and nanorods (NRs), were synthesized using thermally dewetted Au nanoparticles as catalyst in a chemical vapor deposition process. The morphology of the as-grown β-Ga2O3 nanostructures depends strongly on the growth temperature and time. Successful growth of β-Ga2O3 NWs with lengths of 7–25 μm, NSHs, and NRs was achieved. It has been demonstrated that the vapor–liquid–solid mechanism governs the NW growth, and the vapor–solid mechanism occurs in the growth of NSHs and NRs. The X-ray diffraction analysis showed that the as-grown nanostructures were highly pure single-phase β-Ga2O3. The bandgap of the β-Ga2O3 nanostructures was determined to lie in the range of 4.68–4.74 eV. Characteristic Raman peaks were observed with a small blue and red shift, both of 1–3 cm−1, as compared with those from the bulk, indicating the presence of internal strain and defects in the as-grown β-Ga2O3 nanostructures. Strong photoluminescence emission in the UV-blue spectral region was obtained in the β-Ga2O3 nanostructures, regardless of their morphology. The UV (374–377 nm) emission is due to the intrinsic radiative recombination of self-trapped excitons present at the band edge. The strong blue (404–490 nm) emissions, consisting of five bands, are attributed to the presence of the complex defect states in the donor (VO) and acceptor (VGa or VGa–O). These β-Ga2O3 nanostructures are expected to have potential applications in optoelectronic devices such as tunable UV–Vis photodetectors.

Precise strain mapping of nano-twinned axial ZnTe/CdTe hetero-nanowires by scanning nanobeam electron diffraction

The occurrence of strain is inevitable for the growth of lattice mismatched heterostructures. It affects greatly the mechanical, electrical and optical properties of nano-objects. It is also the case for nanowires which are characterized by a high surface to volume ratio. Thus, the knowledge of the strain distribution in nano-objects is critically important for their implementation into devices. This paper presents an experimental data for II-VI semiconductor system. Scanning nanobeam electron diffraction strain mapping technique for hetero-nanowires characterized by a large lattice mismatch (>6% in the case of CdTe/ZnTe) and containing segments with nano-twins has been described. The spatial resolution of about 2 nm is 10 times better than obtained in synchrotron nanobeam systems. The proposed approach allows us to overcome the difficulties related to nanowire thickness variations during the acquisition of the nano-beam electron diffraction data. In addition, the choice of optimal parameters used for the acquisition of nano-beam diffraction data for strain mapping has been discussed. The knowledge of the strain distribution enables, in our particular case, the improvement of the growth model of extremely strained axial nanowires synthetized by vapor-liquid solid growth mechanism. However, our method can be applied for the strain mapping in nanowire heterostructures grown by any other method.

Variation of Surface Nanostructures on (100) PbS Single Crystals during Argon Plasma Treatment

The nanostructuring of the (100) PbS single crystal surface was studied under varying argon plasma treatment conditions. The initial PbS single crystals were grown by high-pressure vertical zone melting, cut into wafer samples, and polished. Subsequently, the PbS single crystals were treated with inductively coupled argon plasma under varying treatment parameters such as ion energy and sputtering time. Plasma treatment with ions at a minimum energy of 25 eV resulted in the formation of nanotips with heights of 30–50 nm. When the ion energy was increased to 75–200 eV, two types of structures formed on the surface: high submicron cones and arrays of nanostructures with various shapes. In particular, the 120 s plasma treatment formed specific cruciform nanostructures with lateral orthogonal elements oriented in four <100> directions. In contrast, plasma treatment with an ion energy of 75 eV for 180 s led to the formation of submicron quasi-spherical lead structures with diameters of 250–600 nm. The nanostructuring mechanisms included a surface micromasking mechanism with lead formation and the vapor–liquid–solid mechanism, with liquid lead droplets acting as self-forming micromasks and growth catalysts depending on the plasma treatment conditions (sputtering time and rate).

Manipulation of the 1T-MoS2 domain in a 2H-MoS2 main phase induced by V-doping via a CVD vapor–liquid–solid mechanism

Controlled synthesis of 1T-MoS2 in the 2H main phase was achieved via one-dimensional gas–liquid–solid (VLS) growth and two-dimensional gas–solid (VS) edge extension.

Phase Change Ge-Rich Ge-Sb-Te/Sb2Te3 Core-Shell Nanowires by Metal Organic Chemical Vapor Deposition.

Ge-rich Ge–Sb–Te compounds are attractive materials for future phase change memories due to their greater crystallization temperature as it provides a wide range of applications. Herein, we report the self-assembled Ge-rich Ge–Sb–Te/Sb2Te3 core-shell nanowires grown by metal-organic chemical vapor deposition. The core Ge-rich Ge–Sb–Te nanowires were self-assembled through the vapor–liquid–solid mechanism, catalyzed by Au nanoparticles on Si (100) and SiO2/Si substrates; conformal overgrowth of the Sb2Te3 shell was subsequently performed at room temperature to realize the core-shell heterostructures. Both Ge-rich Ge–Sb–Te core and Ge-rich Ge–Sb–Te/Sb2Te3 core-shell nanowires were extensively characterized by means of scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman microspectroscopy, and electron energy loss spectroscopy to analyze the surface morphology, crystalline structure, vibrational properties, and elemental composition.

MOCVD Growth of GeTe/Sb2Te3 Core–Shell Nanowires

We report the self-assembly of core–shell GeTe/Sb2Te3 nanowires (NWs) on Si (100), and SiO2/Si substrates by metalorganic chemical vapour deposition, coupled to the vapour–liquid–solid mechanism, catalyzed by Au nanoparticles. Scanning electron microscopy, X-ray diffraction, micro-Raman mapping, high-resolution transmission electron microscopy, and electron energy loss spectroscopy were employed to investigate the morphology, structure, and composition of the obtained core and core–shell NWs. A single crystalline GeTe core and a polycrystalline Sb2Te3 shell formed the NWs, having core and core–shell diameters in the range of 50–130 nm and an average length up to 7 µm.

Suppression of vapor-liquid-solid (VLS) mechanism in the growth of α-Sb2O4 nanobelts by a vapor-deposition approach

In this work we report on the synthesis of novel α-Sb2O4 nanostructures synthesized by a gold catalyzed vapor deposition method in which metallic Sb is used as a precursor. Belts, rods and zigzag morphologies were obtained and characterized by a group of techniques which also provided data on crystalline structure and compositional aspects. Structural characterizations revealed that although the addition of gold nanoparticles plays an important role in the final growth of the nanostructures, no evidence of this element was found in the as-grown samples. In order to clarify the causes of the suppression of the VLS mechanism in these conditions, we carried out experiments in which the syntheses were interrupted in a controlled manner. Our findings revealed that at the early stages of the growth VLS mechanism is suppressed when high levels of Sb supersaturation are reached. A thick crystalline oxide layer rapidly grows at the liquid-gas interface providing preferential sites for the VS growth to take place. Low temperature photoluminescence measurements revealed a strong emission in the visible portion of electromagnetic spectra which can be associated to the presence of oxygen vacancies in these nanostructures.

Synthesis and characterization of single-crystalline TaC whiskers

Tantalum carbide (TaC) whiskers, which have the same chemical composition and crystal structure as a TaC matrix, are believed to be ideal reinforcing skeletons for improving the toughness of monolithic TaC. This is attributed to their high tensile strength and the compatibility of their chemical and thermal properties. In this study, TaC whiskers were synthesized using a typical halide-assisted carbothermal reduction method with Ta2O5, NaCl, Ni(NO3)2·6H2O, and bamboo powders as precursor materials. The results of morphological and compositional analyses confirm that whisker growth occurs via a vapor–liquid–solid mechanism. Transmission electron microscopic analysis results indicate that the synthesized whiskers are single-crystalline with no defects and grow in the [0 0 1] direction. TaC whiskers exhibit a unique core–shell-like morphology and excellent structural stability under ultra-high temperature, which can enhance the structural stability of the TaC matrix when used as reinforcement.