Vapor Liquid Solid Growth Mechanism Research Articles

Influence of halogen precursors on the growth of InSb nanostructures

The present work highlights the role of halogen compounds in modifying the shape of the InSb nanostructures, while maintaining a high crystalline quality of the nanostructures. One-dimensional (1D) nanowires (NWs) and two-dimensional (2D) nanoplatelets (NPLs) were synthesized by ambient pressure chemical vapor deposition. Our experimental results suggest that at a critical growth temperature of 512 ∘C, InSb NWs grow by the traditional vapor–liquid–solid growth mechanism when gold (Au) nanoparticles are used to initiate growth on an InSb film. The resulting NWs were found to have a cylindrical or tapered shape, were of high crystalline quality, and had stoichiometric composition. In the presence of halogen precursors, a change in morphology was observed and the resulting nanostructures were 2D NPLs and faceted NWs. Using existing models of crystal growth and concepts of volume, surface and edge energies, the experimental results are explained on the basis of chlorine atoms adsorbed on the wide or narrow facets of a nanocrystal, initiating nucleation and facilitating NPL or faceted NW formation. The incorporation of the chlorine atoms add a new degree of freedom to CVD synthesis of nanostructures and the results are promising for the controlled growth of novel 1D and 2D nanostructures for nano-electronic devices.

Vapor–liquid–solid synthesis of Ag2Te using chemical vapor deposition method

Silver telluride, Ag2Te, has been selectively synthesized by conventional chemical vapor deposition (CVD) via the vapor–liquid–solid growth mechanism. The pre-deposited Ag film on the SiO2/Si substrate was chemically reacted with vaporized Te atoms and transformed into liquid-phase Ag–Te during the CVD process. The appropriate supply of Te vapor to the Ag film influenced the stoichiometry of Ag–Te compounds, and the formation of stoichiometric Ag–Te compounds was well-explained by the phase diagram of the Ag–Te system. We found that Ag2Te was grown in the liquid of Ag–Te under the Te-poor condition, while Ag5Te3 and Te were simultaneously solidified under the Te-rich condition. The high-temperature synthesis of Ag2Te showed higher crystallinity and better stoichiometry than the low-temperature synthesis. This study demonstrates that Ag2Te can be selectively synthesized by conventional CVD via delicate control over the phases of the complex Ag–Te system.

High Sodium‐Ion Battery Capacity in Sulfur‐Deficient Tin(II) Sulfide Thin Films with a Microrod Morphology

Sulfur‐deficient SnS thin films for sodium‐ion battery anode application are prepared using aerosol‐assisted chemical vapor deposition. Growth directly onto the metal foil current collector forms sulfur‐deficient SnS microrod structures via a vapor–liquid–solid growth mechanism, with 92 nm average SnS crystallite size and an 800 nm film thickness. The sulfur deficiency is demonstrated with energy‐dispersive X‐ray analysis, powder X‐ray diffraction, and X‐ray absorption near‐edge structure analyses. This sulfur‐deficient SnS material demonstrates a very high capacity in sodium half cells. The first reduction scan at a specific current of 150 mA g−1 shows a capacity of 1084 mAh g−1. At the 50th cycle the specific capacity is 638 mAh g−1 for reduction and 593 mAh g−1 for oxidation. This capacity is demonstrated for tin sulfide itself without the need for a nanostructured carbon support, unlike previous high capacity SnS anodes in the literature. Both the capacity and ex situ characterization experiments indicate a conversion reaction producing tin, followed by alloying with sodium during reduction, and that both of these processes are reversible during oxidation.

New growth mechanism of InAs nanowires array in selective-area growth by MOCVD

Traditional view on the growth mechanism of InAs nanowires (NWs) arrays in selective-area growth (SAG) holds that the growth of InAs NWs is based on the catalyst-free mechanism of faceting growth dominantly. Here, based on analyses and comparisons between the InAs NWs in unselective-area growth (USAG) on bare Si substrates and the InAs NWs arrays in SAG, we put forwards a new point of view that the growth mechanism of InAs NWs arrays in SAG is self-catalyzed vapor–liquid–solid (VLS) mechanism. By designing interrupted growth of InAs NWs in SAG and growth of InAs/GaSb heterostructured NWs in SAG, traces of In droplets were found during SAG of InAs NWs, which directly demonstrate self-catalyzed VLS growth mechanism for the InAs NWs arrays in SAG.

Formation of tubular conduction channel in a SiGe(P)/Si core/shell nanowire heterostructure

Realizing a tubular conduction channel within a one-dimensional core–shell nanowire (NW) enables better understanding of quantum phenomena and exploration of electronic device applications. Herein, we report the growth of a SiGe(P)/Si core/shell NW heterostructure using a chemical vapor deposition coupled with vapor–liquid–solid growth mechanism. The entire NW heterostructure behaves as a p-type semiconductor, which demonstrates that the high-density carriers are confined within the 4 nm-thick Si shell and form a tubular conduction channel. These findings are confirmed by both calculations and the gate-dependent current–voltage (Id–Vg) characteristics. Atomic resolution microscopic analyses suggest a coherent epitaxial core/shell interface where strain is released by forming dislocations along the axial direction of the NW heterostructure. Additional surface passivation achieved via growing a SiGe(P)/Si/SiGe core/multishell NW heterostructure suggests potential strategies to enhance the tubular carrier density, which could be further modified by improving multishell crystallinity and structural design.

Revealing Atomistic Mechanisms of Gold-Catalyzed Germanium Growth Using Molecular Dynamics Simulations

The vapor-liquid-solid (VLS) method is considered a plausible technique for synthesizing germanium (Ge) nanostructures (e.g. nanowires), which have a broad range of applications due to their unique electronic properties and intrinsic compatibility with silicon. However, crystallization failures and material defects are still frequently observed in VLS processes, with insufficient understanding of their underlying mechanisms due to instrumental limitations for high-resolution in-situ characterizations. Employing an accurate interatomic potential well fitted to the gold-germanium (Au-Ge) phase diagram, we performed molecular dynamics simulations for a systematic investigation on the Au-catalyzed growth process of Ge crystals. From the simulations, relationships were established between the overall Ge growth rate and several main synthesis conditions, including substrate crystallographic orientation, temperature and Ge supersaturation in liquid. The dynamical behaviors of Ge atoms near the liquid-solid growing interface were captured, from which the atom surface stability and exchange rate were estimated for quantifying the atomistic details of the growth. These interface properties were further linked to the surface morphologies, to explain the observed orientation-dependent growing modes. This study sheds new lights into the understanding of the VLS growth mechanisms of Ge crystals, and provides scientific guidelines for designing innovative synthesis methods for similar nanomaterials.

Surface Energy‐Mediated Self‐Catalyzed CsPbBr3 Nanowires for Phototransistors

AbstractControllable self‐catalyzed growth of semiconductor nanowires (NWs) is of great importance, particularly to avoid impurities coming from foreign catalysts to deteriorate the NW properties. Although this catalyst‐free NW growth has many obvious advantages, there are very limited works focused on all‐inorganic CsPbBr3 perovskite NWs, which is one of the recent champion materials for electronics and optoelectronics. Here, a direct self‐catalyzed synthesis of freestanding CsPbBr3 NWs via vapor–liquid–solid growth mechanism by chemical vapor deposition is developed. Notably, mainipulation of the substrate surface roughness is the key enabling parameter for the self‐catalyzed NW growth here. It is revealed that the surface energy of substrates, modulated by its surface roughness, is found to effectively mediate the self‐catalytic growth of CsPbBr3 NWs. When configured into photodetectors, the intrinsic p‐type CsPbBr3 NWs exhibit good optoelectronic performance with a photoresponivity of ≈2000 A W−1, a detectivity of ≈2.57 × 1012 Jones, and a fast response down to 362 µs. All these results evidently indicate the technological potential of this self‐catalyzed synthesizing route for other high‐quality all‐inorganic perovskite NWs.

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.

Mg-Doped GaAs Nanowires with Enhanced Surface Alloying for Use as Ohmic Contacts in Nanoelectronic Devices

In this work, we have investigated the structural and electronic properties of Mg-doped GaAs(111) nanowires synthesized through a vapor–liquid–solid growth mechanism. The crystalline structure of these nanowires was measured using synchrotron X-ray diffraction, while their electronic structure was addressed by scanning tunneling spectroscopy. Scanning tunneling microscopy measurements revealed that conducting Ga2Mg/Mg clusters are observed at {110} nanowire lateral surfaces, allowing electrical contacts with reduced Schottky barriers. This suggests that similar alloyed surfaces can be produced with other dopants, enabling the development of distinct Ohmic contacts in these systems. Density functional theory was used to investigate the electronic response of Ga2Mg. While at room temperature, electronic variable-range hopping drives the nanowires into a metallic behavior, quantum confinement is observed at low temperatures.

Near-infrared emission from spatially indirect excitons in type II ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor–liquid–solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron–hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure.

One dimensional Au-ZnO hybrid nanostructures based CO2 detection: Growth mechanism and role of the seed layer on sensing performance

Abstract In the present research, hybrid Au-ZnO one-dimensional (1-D) nanostructures were grown on silicon substrates with an Al-doped ZnO (AZO) seed layer (Ultrasonic Spray Pyrolysis: USP grown) and no seed layer (NSL) using two different catalytic gold films of 2 nm and 4 nm, respectively. Consequently, such 1-D nanostructures growth was associated with the vapor-liquid-solid (VLS) and vapor-solid (VS) processes. Scanning electron microscopy (SEM) imaging analysis confirms that heat treatment triggered Au nanoparticles nucleation with varying diameters. The Au nanoparticles size and underneath seed layer texture strongly affect the morphology and aspect ratio of 1-D ZnO nanostructures. The seed layer (1-D USP) sample resulted in the growth of longer nanowires (NWs) with a high aspect ratio. The NSL sample showed the formation of nanorods (NRs) with a low aspect ratio mainly via VS growth process. X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and photoluminescence (PL) analysis also revealed the differences in the NWs and NRs properties and confirmed VLS and VS growth mechanisms. CO2 gas sensing performance at different concentrations was demonstrated, and NWs with seed layer showed a relatively higher sensing response. In contrast, NSL samples (NRs) exhibited two times faster response. A detailed gas sensing mechanism with different CO2 adsorption modes based on properties of 1-D nanostructures has been discussed. Currently, CO2 sensing and capturing are critical topics in the green transition framework. The present work would be of high significance to the scientific field of NW growth and fulfill the urgent need for CO2 gas sensing.

Facile synthesis of β–Ga2O3 nanowires network for solar-blind ultraviolet photodetector

Gallium oxide (Ga2O3) has become a viable candidate for certain types of high-power devices due to its large energy bandgap of 4.9 eV, which has attracted widespread attention. In particular, Ga2O3 nanowire structures have more unique properties due to its larger specific surface area for the high performance solar-blind ultraviolet (UV) photodetectors. In this work, the ultrafine Ga2O3 nanowire network structure is obtained on the sapphire substrate with an Au catalyst by chemical vapor deposition method at 960 °C for 10 min. We can confirm that the growth of the nanowire follows the vapor–liquid–solid growth mechanism and is a β-type Ga2O3 crystal through the performance test results. A solar-blind UV photodetector based on the nanowires network shows an apparent response to solar-blind UV light and almost no response to 365 nm wavelength. Furthermore, the on–off ratio, light responsivity, and response time are also measured under a 254 nm wavelength UV light irradiation, respectively. This work provides a new preparation method to improve the performance of solar-blind UV photodetector.

Structural and Optical Properties of Self-Catalyzed Axially Heterostructured GaPN/GaP Nanowires Embedded into a Flexible Silicone Membrane.

Controlled growth of heterostructured nanowires and mechanisms of their formation have been actively studied during the last decades due to perspectives of their implementation. Here, we report on the self-catalyzed growth of axially heterostructured GaPN/GaP nanowires on Si(111) by plasma-assisted molecular beam epitaxy. Nanowire composition and structural properties were examined by means of Raman microspectroscopy and transmission electron microscopy. To study the optical properties of the synthesized nanoheterostructures, the nanowire array was embedded into the silicone rubber membrane and further released from the growth substrate. The reported approach allows us to study the nanowire optical properties avoiding the response from the parasitically grown island layer. Photoluminescence and Raman studies reveal different nitrogen content in nanowires and parasitic island layer. The effect is discussed in terms of the difference in vapor solid and vapor liquid solid growth mechanisms. Photoluminescence studies at low temperature (5K) demonstrate the transition to the quasi-direct gap in the nanowires typical for diluted nitrides with low N-content. The bright room temperature photoluminescent response demonstrates the potential application of nanowire/polymer matrix in flexible optoelectronic devices.

Polarization and magneto-optical properties of excitonic emission from wurtzite CdTe/(Cd,Mg)Te core/shell nanowires

Wurtzite CdTe and (Cd,Mn)Te nanowires embedded in (Cd,Mg)Te shells are grown by employing vapour–liquid–solid growth mechanism in a system for molecular beam epitaxy. A combined study involving cathodoluminescence, transmission electron microscopy and micro-photoluminescence is used to correlate optical and structural properties in these structures. Typical features of excitonic emission from individual wurtzite nanowires are highlighted including the emission energy of 1.65 eV, polarization properties and the appearance B-exciton related emission at high excitation densities. Angle dependent magneto-optical study performed on individual (Cd,Mn)Te nanowires reveals heavy-hole-like character of A-excitons typical for wurtzite structure and allows to determine the crystal field splitting, ΔCR. The impact of the strain originating from the lattice mismatched shell is discussed and supported by theoretical calculations.

Review of the Preparation and Structures of Si Nanowires, Ge Quantum Dots and Their Composites

Because the motion of charge carriers in nanowires and quantum dots is restricted within nanoscale in two and three dimensions, respectively, both nanowires and quantum dots exhibit many excellent optoelectronic properties. Particularly, with the advantage of being compatible with Si integrated circuits, Silicon nanowires (SiNWs) and germanium quantum dots (GeQDs) have been extensively studied in the past few decades. In order to explore novel physical properties, the integration of SiNWs and GeQDs has attracted great attention recently. In this paper, recent researches on the preparation methods and structures of SiNWs, GeQDs and their composites are reviewed, respectively. The synthesis of SiNWs with random distribution and ordered arrays by using vapor–liquid–solid growth mechanism and metal-assisted chemical etching technique is firstly summarized. Some special structures of SiNWs are also discussed. Furthermore, the development of some novel structures of GeQDs for further improving their optical properties is reviewed. Finally, the growth mechanism and structure evolution of SiNWs/GeQDs composites are illustrated from the view of theory and experiment. The strain in Ge shell layers and SiNWs, the relationship between Ge growth mode and SiNW diameter, and the distribution of GeQDs on the radial and axial directions of SiNWs are discussed in detail. The research about the growth of SiNWs/GeQDs composite structures is in its early stage, so there are many questions that need to be resolved in future.

Synthesis of ultra-long aluminum nitride nanowires with excellent photoluminescent property by aluminum chloride assisted chemical vapor reaction technique

Abstract Aluminum nitride (AlN) nanowires were performed on iron substrate using aluminum powders as material precursor by anhydrous aluminum chloride (AlCl3) assisted chemical vapor reaction (CVR) method. Yield production of AlN nanowires was significantly enhanced with AlCl3 addition into Al powders due to generating of intermediate gaseous AlCl, which further reacted with N2 to form AlN nanowires. With AlCl3 addition, a thin Fe film on Fe substrate can be transformed into Fe nanoparticles, which served as a catalyst in promoting the formation of AlN nanowires. The length of single-crystalline AlN nanowires was up to hundreds of microns. The formation and growth of AlN nanowires were most likely controlled by vapor–liquid–solid growth mechanism and AlCl3 assisted chemical vapor transport mechanism. The luminescence bands in the range of violet-green luminescence suggest its promising application in light-emitting devices.

Insight of surface treatments for CMOS compatibility of InAs nanowires

A CMOS compatible process is presented in order to grow self-catalyzed InAs nanowires on silicon by molecular beam epitaxy. The crucial step of this process is a new in-situ surface preparation under hydrogen (gas or plasma) during the substrate degassing combined with an in-situ arsenic annealing prior to growth. Morphological and structural characterizations of the InAs nanowires are presented and growth mechanisms are discussed in detail. The major influence of surface termination is exposed both experimentally and theoretically using statistics on ensemble of nanowires and density functional theory (DFT) calculations. The differences observed between Molecular Beam Epitaxy (MBE) and Metal Organic Vapor Phase Epitaxy (MOVPE) growth of InAs nanowires can be explained by these different surfaces terminations. The transition between a vapor solid (VS) and a vapor liquid solid (VLS) growth mechanism is presented. Optimized growth conditions lead to very high aspect ratio nanowires (up to 50 nm in diameter and 3 micron in length) without passing the 410 °C thermal limit, which makes the whole process CMOS compatible. Overall, our results suggest a new method for surface preparation and a possible tuning of the growth mechanism using different surface terminations.

Germanium catalyzed vapor–liquid–solid growth and characterization of amorphous silicon oxide nanotubes: comparison to the growth of its nanowires

One-dimensional (1D) nanostructures were grown with a simple technique using continuous-wave laser vaporization of a Ge target containing 5 at.% Si in high-pressure (up to 0.9 MPa) Ar gas atmosphere. A maximum amount (~ 30% of all products) of 1D nanostructures was obtained at 0.9 MPa and these nanostructures were identified as amorphous silicon oxide (SiOx) nanotubes (NTs) and attached with crystalline Ge-rich NPs with elongated prolate-like or sphere-like shapes at their tips by transmission electron microscopy (TEM), high-angle annular dark-field-scanning TEM, and energy dispersive X-ray line scan spectrometry. As the Ar pressure decreased from 0.9 to 0.03 MPa, the average diameters, wall thicknesses, and lengths of the NTs decreased from 57.9 to 22.9 nm, 13.2 to 6.7 nm, and 2.1 to 0.2 µm, respectively, and the tip NP size decreased from 139.0 to 41.7 nm. There was a strong correlation among the diameters, wall thicknesses, and lengths of the NTs and tip Ge NP sizes, indicating the role of molten Ge NPs as catalyst seeds for the precipitation of SiOx in a vapor–liquid–solid growth mechanism at high temperature. The SiOx precipitation quantities from the seed NPs for the NTs were compared with those of amorphous SiOx nanowires (NWs) at 0.1–0.9 MPa to clarify the growth mechanism of the NTs. We argue that smaller precipitation quantities of SiOx than those for the NWs play a critical role in the formation of cap structures with different sizes and shapes from the molten Ge NPs and the growth of the NTs.

Synthesis and thermoelectric properties of indium telluride nanowires

Thermoelectric materials, along with recent advances in nano-engineering, have attracted much attention due to the potential applications in energy conversion. A simple approach to synthesize In2Te3 nanowires was developed in a thermal chemical vapor deposition furnace by a catalyst-assisted vapor–liquid–solid growth mechanism. Structural characterization of these products was carried out using standard microscopy practices, and the thermoelectric properties of individual as-grown In2Te3 nanowires were measured using a suspended microdevice technique. This approach provides an applicable route to synthesize novel In2Te3 nanowires as well as to identify information about their thermoelectric behavior.

Molecular beam epitaxial growth of high quality Ga-catalyzed GaAs1–xSbx(x > 0.8) nanowires on Si (111) with photoluminescence emission reaching 1.7 μm

The advancement of ternary GaAsSb mismatched alloy system toward the Sb-rich corner of the phase diagram in the nanowire (NW) configuration on silicon remains a challenge. A large lattice mismatch between the silicon substrate and GaAsSb with an Sb-rich composition, along with the low supersaturation and low solubility of Sb in the Ga droplet in the vapor–liquid–solid growth mechanism, causes significant issues during Ga-assisted molecular beam epitaxial growth of these NWs. In this work, we have carried out a systematic study of Sb-rich GaAs1–xSbx NWs grown on Si (111) using variations of the Ga, As, and Sb beam equivalent pressures (BEP) to minimize undesirable parasitic growth and achieve photoemission up to 1.7 μm. Ga-assisted molecular beam epitaxy is the enabling growth technology for the growth of these self-catalyzed GaAs1–xSbx (x > 0.8) NWs. The use of a dual substrate temperature approach along with low As background pressure and a low Ga BEP were found to be the key growth components in achieving a well-faceted NW morphology with a low parasitic layer on the substrate. Energy-dispersive x-ray spectroscopy analysis confirms uniform compositional homogeneity along the NWs, while selected-area electron diffraction patterns in the transmission electron microscope revealed a zinc-blende crystal structure. A peak μ-photoluminescence emission of 1680 nm with a narrow FWHM was obtained at 4 K. Raman spectra at room temperature exhibit only GaSb related LO and TO modes, which attest to the high quality of the NWs grown. This is a promising approach due to the broad scope of applicability to grow other mismatched alloy material systems in a NW configuration.