Vapor-liquid-solid Research Articles

Tin versus indium catalyst in the growth of silicon nanowires by plasma enhanced chemical vapor deposition on different substrates

• Silicon nanowires (NWs) grown on various substrates via Vapor–Liquid–Solid method. • Best crystallized SiNWs obtained using tin catalyst on silicon germanium substrate. • Morphology and crystalline nature strongly depend on the junction substrate-catalyst. • Substrate surface energy affects growth by regulating catalyst repartition. The plasma-assisted Vapor-Liquid-Solid technique has been used to grow Silicon nanowires (SiNWs) on different substrates coated with 1 nm of tin and indium, to study the impact of the surface energy of the substrates on the morphology and structure of the NWs. The characteristics of the prepared Si NWs were analyzed by field emission scanning electron microscopy, X-ray diffraction (XRD) and Raman spectroscopy. The morphological characteristics differ in the density, diameter and length of the synthesized SiNWs. The mean diameters of these SiNWs range from 36 to 51 nm at the bottom and from 15 nm to 34 nm at the tip, while their average length varies from 0.2 μm up to 0.54 μm depending on the substrate and catalyst. The presence of strong peaks in all XRD patterns indicates that the as-grown SiNWs are highly crystalline. The XRD and Raman characteristics of the Sn and In catalyzed SiNWs formed on different substrates show that the crystalline grain size is influenced by the substrate nature and catalyst material, which can be related to the substrate surface energy. In particular shorter NWs with improved crystallinity (larger grain size) are obtained when using Sn as a catalyst on an amorphous silicon-germanium (a-SiGe) substrates, while the small grains were obtained on the same substrate when indium was used as a catalyst.

Enhanced electromagnetic microwave absorption of SiC nanowire-reinforced PDC-SiC ceramics catalysed by rare earth

Rare earth elements can modulate the dielectric constant of materials and significantly improve their dielectric properties. Herein, SiCnws/SiC ceramics were prepared through polymer derived ceramics (PDCs) technology with rare earth Sc particles as the catalyst. The Sc particles promote the precipitation of SiC and C from the matrix. Furthermore, the SiCnws, grown via the vapour-liquid-solid (VLS) mechanism, construct the three dimensional (3D) network structure to improve impedance matching and loss characteristics. Remarkably, the SiCnws/SiC ceramics minimum reflection coefficient (RCmin) achieved a value of −33.2 dB at 9.4 GHz with a thickness of 2.75 mm, and the effective absorption bandwidth (EAB) was 4.2 GHz covering the whole X band. When microwaves permeated into the SiCnws/SiC ceramics, those trapped in the 3D network structure underwent a variety of microwave energy dissipation processes, including multiple reflections, scattering, and interface and dipole polarisation. Consequently, SiCnws-reinforced PDC-SiC ceramics catalysed by rare earth emerge as a promising new approach to enhance electromagnetic (EM) wave absorption performance.

Microstructure evolution and growth mechanism of core-shell silicon-based nanowires by thermal evaporation of SiO

Core-shell structured SiC@SiO2 nanowires and Si@SiO2 nanowires were prepared on the surface of carbon/carbon (C/C) composites by a thermal evaporation method using SiO powders as the silicon source and Ni(NO3)2 as the catalyst. The average diameters of SiC@SiO2 nanowires and Si@SiO2 nanowires are about 145 nm, and the core-shell diameter ratios are about 0.41 and 0.53, respectively. The SiO2 shells of such two nanowires resulted from the reaction between SiO and CO and the reaction of SiO itself, respectively, based on the model analysis. The growth of these two nanowires conformed to the vapor—liquid—solid (VLS) mode. In this mode, CO played an important role in the growth of nanowires. There existed a critical partial pressure of CO (pC) determining the microstructure evolution of nanowires into whether SiC@SiO2 or Si@SiO2. The value of pC was calculated to be 4.01×10−15 Pa from the thermodynamic computation. Once the CO partial pressure in the system was greater than the pC, SiO tended to react with CO, causing the formation of SiC@SiO2 nanowires. However, the decomposition of SiO played a predominant role and the products mainly consisted of Si@SiO2 nanowires. This work may be helpful for the regulation of the growth process and the understanding of the growth mechanism of silicon-based nanowires.

Facile growth of oriented SiC nanowires arrays on carbon fiber cloth via CVD

Oriented SiC nanowires (SiCNWs) arrays have garnered much attention owing to their special properties such as outstanding electron emission. Although many preparation methods of oriented SiCNWs arrays have been explored, it is also pivotal to develop a facile and controllable fabrication method. Herein, a chemical vapor deposition (CVD) approach using Fe(NO3)3 as catalyst was utilized to prepare oriented SiCNWs arrays. Morphology and structural analyses demonstrated that the in-situ grown oriented SiCNWs arrays were single crystalline 3C–SiC with smooth surface and a diameter that increased continuously from about 0.2 μm near the carbon fiber to 0.7 μm at the tip, growing toward [111] direction directly. A proper growth mode, Fe–Si solution-assisted vapor–liquid–solid (VLS) mechanism, was proposed to expound the growth of oriented SiCNWs arrays by thermodynamic calculations and tests analyses. Meanwhile, it was proved that the confinement and driving effects of the Fe catalyst led to the formation of the oriented SiCNWs arrays. This method is expected to realize the large-scale preparation of oriented SiCNWs, which will be applied in the fields of toughening materials, field emission, energy storage, and so on.

Bottom‐Up Growth of Graphene Nanospears and Nanoribbons

One dimensional graphene nanostructures are one of the most promising materials for next generation electronics. Here, the chemical vapor depostion growth of graphene nanoribbons (GNRs) and graphene nanospears (GNSs) on a copper surface is reported. The growth of GNRs and GNSs is enabled by a vapor–liquid–solid (VLS) mechanism guided by on-surface propagation of a liquid Cu-Si catalyst particle. The slow lateral growth and the fast VLS vertical growth give rise to spear head-shaped GNSs. In situ observations further confirm that the lateral graphene growth can be completely suppressed and thus GNRs are grown. The synthesized field effect transistor (FET) devices show that the GNRs and GNSs have high carrier mobilities of ≈2000 cm2 V−1 s−1. Both FET and Kelvin probe force microscopy measurements confirm that the Fermi levels of the synthesize GNSs shift downward from the wide part to the tip is strongly p-doped. These findings yield key insights into the growth mechanism of graphene and open a door for achieving a facile and scalable method of synthesizing free standing GNRs and GNSs and their applications, such as the Fermi-level tunable devices.

Facet‐Driven Vapor Phase Growth of Directional Tin‐Doped CdS Nanowires on Sapphire Surface for Photodetectors

Orientation‐controlled large‐area synthesis of nanowires (NWs) is key to their direct integration into circuits and devices for functional exploitation. Herein, the vapor–liquid–solid process yields planar arrays of tin‐doped CdS NWs with precise crystallographic orientation on flat and faceted sapphire surfaces. The nanopatterned catalyst and epitaxial correlation of each surface determine NWs’ exact position, yield, and orientation, while the graphoepitaxial effect steers their growth along the nanochannels. The incorporation of dopants widens the emission spectrum beyond the bandgap, which facilitates enhanced optical transport in NWs. Electron‐beam lithography (EBL) is employed to fabricate photodetector on individual NW, which demonstrates high photoresponsivity and fast response. Graphoepitaxial effect‐based assembly of highly ordered horizontal NW arrays and their facile self‐integration into devices for modern applications, including LEDs, biomedical or photoelectric sensors, photovoltaic cells, and visible span integrated optoelectronic and photonic systems, have promising prospects.

(Invited) Ceramic Nano-Heterostructures By Materials Design: Platforms for Sensing Applications – Opportunities and Challengess

This talk summarizes R&D efforts in the author’s laboratory on the fabrication of oxide nano-heterostructures, exploiting intrinsic material properties, that are highly scalable and do not require use of lithography. One such process creates crystallographically oriented nanofiber arrays of single crystal TiO2 in H2/N2 environment. H2/N2 heat treatment was also used to grow nanofibers on polycrystalline SnO2, showing directional growth on grains with crystal facets. We have also developed a process to create nanofibers of TiO2 on Ti metal/alloys via oxidation under a limited supply of oxygen. In another process, SnO2 nanowires grown from commercial FTO slides using the vapor-liquid-solid (VLS) method were placed in a microwave-assisted hydrothermal chamber where TiO2 nanorods nucleated radially from the SnO2 nanowire cores. We developed yet another interesting nano-structure (nanoislands and/or nanobars) during thermal annealing of an oxide (GDC) on top of another oxide (YSZ) substrate that self-assembles along the softest elastic direction of the substrate. What is common about these structures is that they are fabricated without the use of lithographic techniques and involves simple processes such as gas-phase reactions and stress-driven processes. These nano-heterostructures can be used as platforms for chemical sensing, catalysis, photocatalysis, photovoltaics and biomedical applications. Sensing application presents opportunities and challenges that are presented including an Open access Database Of Resistive type gas Sensors (ODORS) that has been developed and can be used to select suitable sensing materials.

AuPt bimetal‐codecorated ZnO nanowire‐based hydrogen sensors for detecting low concentration hydrogen gas with high response and selectivity

AbstractZnO nanowires decorated with bimetallic AuPt nanoparticles were synthesized using the vapor–liquid–solid method, followed by sputtering and thermal annealing, to fabricate highly sensitive and selective sensors for detecting ppb scale hydrogen gas. The proposed sensor demonstrated a clear pattern enough to recognize, even for 100 ppb hydrogen gas. The sensing responses of the pristine and AuPt bimetal‐codecorated ZnO nanowire‐based sensors were 1.09 and 7.3 for 100 ppb hydrogen gas, respectively. The sensing response of the proposed sensor is seven times as the decoration of AuPt nanoparticles. Furthermore, the proposed sensor exhibited excellent sensing selectivity for various interfering gases including ethanol, acetone, toluene, CO, and H2S. The proposed sensor shows excellent sensing performance because of synergistic effect of decorated AuPt, which play the role of catalysts. Therefore, hydrogen sensors showing superior sensing performance can be fabricated using this nanomaterial, and its advanced sensing performances are analyzed in this study.

Plasma-enhanced chemical vapor deposition for fabrication of yolk-shell SnO2@Void@C nanowires, as an efficient carbon coating technique for improving lithium-ion battery performance

This manuscript describes the implementation of plasma-enhanced chemical vapor deposition (DC-PECVD) and vapor-liquid-solid (VLS) techniques to fabricate a yolk-shell SnO2@Void@C nanowire (NW) structure. SnO2 nanowires have been synthesized on the stainless steel mesh substrate through the VLS method. The PECVD-assisted growth of carbon nanolayer on the SnO2 and SiO2 coated SnO2 NWs has been performed to fabricate SnO2@C core-shell and SnO2@SiO2@C yolk-shell structures, respectively. A consequent silica etching process converted the SnO2@SiO2@C into SnO2@Void@C structure. The electrochemical performance of bare SnO2 NWs, SnO2 NWs @ C, and SnO2 @Void @ C coaxial NWs structures have been investigated in half cell Lithium-ion battery (LIB) coin cells. A noticeable electrochemical enhancement has been observed for the SnO2@Void@C electrode, with a specific capacity of 723 mAh g−1 at 0.2C current density after 100 cycles of charge/discharge, compared to 34 and 455 mAh g−1 for SnO2 NWs and SnO2@C NWs, respectively. This significant improvement can be related to the stable SEI formation on the carbon-coated layer and the electrolyte interface. Besides, the proper void volume created between the SnO2 NWs and the carbon layer provides sufficient space for expanding the SnO2 NWs during the lithiation process. Moreover, the adequate electrical and ionic conductivity of the deposited carbon layer can improve the electrochemical performance of the fabricated anode material. Using PECVD for deposition of the carbon nanolayer benefits from being highly controllable and manageable, as well as its scalability for industrial application. Furthermore, the utilized approach is fast, inexpensive, and low temperature. The reported carbon coating process is proposed as an effective method for protecting the active electrode materials, and as a result, enhancing the performance of lithium-ion batteries.

Integration of silicon nanowire bridges in microtrenches with perpendicular bottom-up growth promoted by surface nanoholes

An integration method of silicon nanowires (SiNWs) bridges in microtrenches was demonstrated combining a local arrangement of catalyst Au nanoparticles by two-step UV lithography, and a vapor–liquid–solid (VLS) bottom-up growth with perpendicularity promotion by surface nanoholes with metal-assisted chemical etching. Au nanoparticles with a diameter of 60 nm were arranged on one-side walls of 10 µm wide microtrenches, with two types of area sizes to evaluate the influence on the yield of SiNWs bridges reaching opposite sidewalls. A four-hour VLS process at 500 °C produced perpendicular SiNWs bridges in the microtrenches, and a higher yield was obtained with a narrow-area arrangement: a 30.7% ratio of densities of SiNWs bridges to Au nanoparticles, and a 2.1/µm2 density in the arrangement area. Fewer SiNWs showed initial oblique growth there, and most of the bridges had a linear morphology. The yield of SiNWs bridges was discussed focusing on depth positions in microtrenches and the depths of surface nanoholes.

Growth of the Serrated GaN Nanowire and its Photoelectrochemical Application

Introducing polyhedral facets into a high surface-to-volume nanowire structure (i.e., serrate-shaped or screw thread-like nanowire) is an effective way for boosting the photoelectrochemical (PEC) activity. However, fabricating such nanowires with serrated surfaces remains a challenge because it usually involves many complex processes, thus limiting mass activity. Here, we demonstrate a strategy for natural growth of the serrated GaN nanowires on a LiGaO2 substrate by using an Au catalyst-assisted vapor-liquid-solid (VLS) method. The specific GaN nanowire grew through an atypical growth mechanism due to the partial deformation of the Au catalyst. The serrated GaN nanowire exhibited a higher photocurrent density of 0.391 mA cm−2 at 1.23 V versus RHE, which was approximately 2.3 times that of the GaN film (0.157 mA cm−2). The high stability of the photoresponse and photocurrent of the serrated nanowire was verified in a wide angle-dependent illumination. This work opens a new way for strengthening the PEC performance of the GaN-based photoanodes by introducing serrate-shaped surfaces on the GaN nanowires.

Highly efficient synthesis of boron nitride nanotubes by catalytic chemical vapor deposition of boron/nickel containing precursors

High-purity and high-yield boron nitride nanotubes with large aspect ratio were prepared by a facile two-step process, including the synthesis of boron/nickel containing precursors by precipitation reactions and subsequent thermally catalytic chemical vapor deposition reactions. The influence of catalyst content and annealing temperature on the phase composition and microstructure of the products were investigated. The results show that it is difficult to exert the catalytic effect of nickel-based catalyst at low temperatures (<1 400 °C). At appropriate temperatures (1 400–1 500 °C), highly crystalline boron nitride nanotubes with a length of more than 50 μm and a diameter of 50 nm are formed. The content of catalyst in the precursor mainly affects the morphology of the boron nitride product. If the content is too low, it is easy to form boron nitride particles; while high catalyst content can easily lead to catalyst aggregation and form a submicron one-dimensional boron nitride with unregular structure. Based on microstructural evolutions, phase changes, and thermodynamic analysis, the vapor-liquid-solid (V-L-S) growth mechanism of the tip growth mode dominates the formation of boron nitride nanotubes has also been verified.

Silica Particle-Mediated Growth of Single Crystal Graphene Ribbons on Cu(111) Foil.

The authors report the growth of micrometer-long single-crystal graphene ribbons (GRs) (tapered when grown above 900°C, but uniform width when grown in the range 850°C to 900°C) using silica particle seeds on single crystal Cu(111) foil. Tapered graphene ribbons grow strictly along the Cu<101> direction on Cu(111) and polycrystalline copper (Cu) foils. Silica particles on both Cu foils form (semi-)molten Cu-Si-O droplets at growth temperatures, then catalyze nucleation and drive the longitudinal growth of graphene ribbons. Longitudinal growth is likely by a vapor-liquid-solid (VLS) mechanism but edge growth (above 900°C) is due to catalytic activation of ethylene (C2 H4 ) and attachment of C atoms or species ("vapor solid" or VS growth) at the edges. It is found, based on the taper angle of the graphene ribbon, that the taper angle is determined by the growth temperature and the growth rates are independent of the particle size. The activation enthalpy (1.73± 0.03eV) for longitudinal ribbon growth on Cu(111) from ethylene is lower than that for VS growth at the edges of the GRs (2.78 ±0.15eV) and for graphene island growth (2.85± 0.07eV) that occurs concurrently.

Thermal synthesis of Ga2O3/SnO2 core–shell nanowires and their structural characterization

Ga2O3 core–SnO2 shell nanowires were synthesized by thermal evaporation of the metallic sources (Ga, Sn) in a horizontal quartz tube furnace at atmospheric pressure and in an oxidizing atmosphere. Au thin layers and 200 ppm Au colloid were applied as growth catalyst of Ga2O3 nanowires in vapor–liquid–solid method. Then the SnO2 shells were grown on the Ga2O3 nanowires cores. The configuration with highly sensitive shell and a relatively low conductivity core should enhance the efficiency of sensors based on these materials. Such narrowing of conductive area where a depletion of the charge carriers may occur, should increase the sensitivity, response and recovery time. The structural and material composition characterization of nanowires was performed by X-ray diffraction, energy-dispersive X-ray and Raman spectroscopy. The lattice parameters and strain of SnO2 shell were calculated. Using the catalyst in the form of a 200 ppm Au colloid compared to Au thin film, Ga2O3/SnO2 nanowires more similar to the core–shell structures were obtained.

Interface Analysis of MOCVD Grown GeTe/Sb2Te3 and Ge-Rich Ge-Sb-Te/Sb2Te3 Core-Shell Nanowires.

Controlling material thickness and element interdiffusion at the interface is crucial for many applications of core-shell nanowires. Herein, we report the thickness-controlled and conformal growth of a Sb2Te3 shell over GeTe and Ge-rich Ge-Sb-Te core nanowires synthesized via metal-organic chemical vapor deposition (MOCVD), catalyzed by the Vapor–Liquid–Solid (VLS) mechanism. The thickness of the Sb2Te3 shell could be adjusted by controlling the growth time without altering the nanowire morphology. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were employed to examine the surface morphology and the structure of the nanowires. The study aims to investigate the interdiffusion, intactness, as well as the oxidation state of the core-shell nanowires. Angle-resolved X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface chemistry of the nanowires. No elemental interdiffusion between the GeTe, Ge-rich Ge-Sb-Te cores, and Sb2Te3 shell of the nanowires was revealed. Chemical bonding between the core and the shell was observed.

In-situ grown CNTs decorated SiCNWs for enhancing electromagnetic wave absorption efficiency

The uniform growth of carbon nanotube (CNT)-decorated SiC nanowires (SiCNWs) to form a three-dimensional (3D) network can significantly enhance the interfacial polarization effect, making it a promising electromagnetic (EM) wave-absorbing material. Nevertheless, the dispersion of CNT-decorated SiCNWs is challenging. In this study, CNT/SiCNWs composites are successfully synthesized by the in situ growth of CNTs at different annealing temperatures under a N2 atmosphere. Specifically, the CNT-decorated SiCNWs form a 3D network by vapor-liquid-solid (VLS) mechanism. The relative complex permittivity of the CNT/SiCNWs composites notably increases with increasing in temperature, adjusting the microstructure and dielectric properties. When the annealing temperature is 900 °C, the minimum reflection coefficient (RCmin) of the CNT/SiCNWs composites decreases from -34.7 to -44 dB with the thickness of the composites increasing from 3.5 to 3.9 mm. The effective absorption bandwidth (EAB) includes 4.2 GHz in the X band of 8.2–12.4 GHz. The results indicate that the CNT/SiCNWs composites exhibit superior EM wave absorption, which is facilitated by the interfacial polarization, dipole polarization, and conduction loss. The 3D network offers multilayer channels for multiple reflections and the scattering energy of the EM waves. Therefore, the CNT/SiCNWs composites are promising high-efficiency microwave-absorbing materials.

Enhancing the incorporation of Sn in vapor–liquid–solid GeSn nanowires by modulation of the droplet composition

We report on the influence of the liquid droplet composition on the Sn incorporation in GeSn nanowires (NWs) grown by the vapor−liquid−solid (VLS) mechanism with different catalysts. The variation of the NW growth rate and morphology with the growth temperature is investigated and 400 °C is identified as the best temperature to grow the longest untapered NWs with a growth rate of 520 nm min−1. When GeSn NWs are grown with pure Au droplets, we observe a core–shell like structure with a low Sn concentration of less than 2% in the NW core regardless of the growth temperature. We then investigate the impact of adding different fractions of Ag, Al, Ga and Si to Au catalyst on the incorporation of Sn. A significant improvement of Sn incorporation up to 9% is obtained using 75:25 Au–Al catalyst, with a high degree of spatial homogeneity across the NW volume. Thermodynamic model based on the energy minimization at the solid–liquid interface is developed, showing a good correlation with the data. These results can be useful for obtaining technologically important GeSn material with a high Sn content and, more generally, for tuning the composition of VLS NWs in other material systems.

A General Synthetic Approach of Organic Lateral Heterostructures for Optical Signal Converters in All-Color Wavelength

A General Synthetic Approach of Organic Lateral Heterostructures for Optical Signal Converters in All-Color Wavelength

Synthesis of ZnO/CNT Nanocomposites for Ultraviolet Sensors

Zinc oxide/carbon nanotube (ZnO/CNTs) nanocomposites are developed on gold (Au)-coated unpolished Si p-type (100) substrates with 2, 4, 6, 8, and 10 nm thicknesses by vapor–liquid–solid method. One set of Au-coated Si substrates are annealed to develop Si–Au samples for better nucleation. XRD, FE-SEM, Raman, and photoluminescence spectroscopic characterizations are used to study structural, morphological, and optical properties on annealed and unannealed catalyst layers with various Au thickness samples. In XRD results, the ZnO/CNT nanocomposites are observed with higher crystallinity and purity of phase. FE-SEM images showed variety of nanostructures with variation in morphologies with respect to Au thickness in annealed and unannealed samples. Clear indication of high defect concentrations and high crystallinity is observed in Raman spectra. It is observed in PL spectra that preferred peak orientation with shift ∼4 nm in the unannealed Au layer and ∼9 nm in annealed Au layer samples exhibited formation of ZnO/CNT nanocomposites. Efficient sensing is observed in the 6-nm thickness Au layer in the unannealed sample. Annealed Au-coated Si samples at 8 and 10 nm thicknesses showed efficient UV sensing with quick response and recovery time.

Synthesis and Characterization of Indium Tin Oxide Nanowires with Surface Modification of Silver Nanoparticles by Electrochemical Method.

In this study, indium tin oxide nanowires (ITO NWs) with high density and crystallinity were synthesized by chemical vapor deposition (CVD) via a vapor–liquid–solid (VLS) route; the NWs were decorated with 1 at% and 3 at% silver nanoparticles on the surface by a unique electrochemical method. The ITO NWs possessed great morphologies with lengths of 5~10 μm and an average diameter of 58.1 nm. Characterization was conducted through transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS) to identify the structure and composition of the ITO NWs. The room temperature photoluminescence (PL) studies show that the ITO NWs were of visible light-emitting properties, and there were a large number of oxygen vacancies on the surface. The successful modification of Ag was confirmed by TEM, XRD and XPS. PL analysis reveals that there was an extra Ag signal at around 1.895 eV, indicating the potential application of Ag-ITO NWs as nanoscale optical materials. Electrical measurements show that more Ag nanoparticles on the surface of ITO NWs contributed to higher resistivity, demonstrating the change in the electron transmission channel of the Ag-ITO NWs. ITO NWs and Ag-ITO NWs are expected to enhance the performance of electronic and optoelectronic devices.