For nanowire growth, vapor-solid-solid (vapor-solid) mechanism is actually vapor-quasisolid-solid (vapor-quasiliquid-solid) mechanism

Regulated Dynamics with Two Monolayer Steps in Vapor-Solid-Solid Growth of Nanowires

The growth of ZnTe nanowires and ZnTe-CdTe nanowire heterostructures is studied by in situ transmission electron microscopy. We describe the shape and the change of shape of the solid gold nanoparticle during vapor-solid-solid growth. We show the balance between one monolayer and two monolayer steps, which characterizes the vapor-liquid-solid and vapor-solid-solid growth modes of ZnTe. We discuss the likely role of the mismatch strain and lattice coincidence between gold and ZnTe on the predominance of two monolayer steps during vapor-solid-solid growth and on the subsequent self-regulation of the step dynamics. Finally, the formation of an interface between CdTe and ZnTe is described.

Mechanism of morphology variations in colloidal CuGaS2 nanorods

Cu2-x S nanocrystals can serve as templates and intermediates in the synthesis of a wide range of nanocrystals through seeded growth, cation exchange, and/or catalytic growth. This versatility can facilitate and accelerate the search for environmentally benign nanocrystals of high performance with variable shapes, sizes, and composition. However, expanding the compositional space via Cu2-x S nanocrystals while achieving necessary uniformity requires an improved understanding of the growth mechanisms. Herein we address several unusual and previously unexplained aspects of the growth of CuGaS2 nanorods from Cu2-x S seeds as an example. In particular, we address the origin of the diverse morphologies which manifest from a relatively homogeneous starting mixture. We find that CuGaS2 nanorods start as Cu2-x S/CuGaS2 Janus particles, the majority of which have a {101̄2}/{101̄2} interface that helps to minimize lattice strain. We propose a mechanism that involves concurrent seed growth and cation exchange (CSC), where epitaxial growth of the Cu2-x S seed, rather than the anticipated catalytic or seeded growth of CuGaS2, occurs along with cation exchange that converts growing Cu2-x S to CuGaS2. This mechanism can explain the incorporation of the large number of anions needed to account for the order-of-magnitude volume increase upon CuGaS2 rod growth (which cannot be accounted for by the commonly assumed catalytic growth mechanism) and variations in morphology, including the pervasive tapering and growth direction change. Insights from the CSC growth mechanism also help to explain a previously puzzling phenomenon of regioselective nucleation of CuInSe2 on kinked CuGaS2 nanorods.

A Review of Self-Seeded Germanium Nanowires: Synthesis, Growth Mechanisms and Potential Applications

Ge nanowires are playing a big role in the development of new functional microelectronic modules, such as gate-all-around field-effect transistor devices, on-chip lasers and photodetectors. The widely used three-phase bottom-up growth method utilising a foreign catalyst metal or metalloid is by far the most popular for Ge nanowire growth. However, to fully utilise the potential of Ge nanowires, it is important to explore and understand alternative and functional growth paradigms such as self-seeded nanowire growth, where nanowire growth is usually directed by the in situ-formed catalysts of the growth material, i.e., Ge in this case. Additionally, it is important to understand how the self-seeded nanowires can benefit the device application of nanomaterials as the additional metal seeding can influence electron and phonon transport, and the electronic band structure in the nanomaterials. Here, we review recent advances in the growth and application of self-seeded Ge and Ge-based binary alloy (GeSn) nanowires. Different fabrication methods for growing self-seeded Ge nanowires are delineated and correlated with metal seeded growth. This review also highlights the requirement and advantage of self-seeded growth approach for Ge nanomaterials in the potential applications in energy storage and nanoelectronic devices.

In situ TEM observation of the vapor-solid-solid growth of <001[combining macron]> InAs nanowires

In situ transmission electron microscopy characterization is a powerful method in investigating the growth mechanism of catalyst-induced semiconductor nanowires. By providing direct evidence on the crystal growth at the atomic level, a real-time in situ heating investigation was carried out on Au-catalyzed <001[combining macron]> InAs nanowires. It was found that the Au catalyst maintained itself in the solid form during the nanowire growth, and maintained a fixed epitaxial relationship with its underlying InAs nanowire, indicating the vapor-solid-solid mechanism. Importantly, the growth of <001[combining macron]> InAs nanowires through a layer-by-layer manner at the catalyst/nanowire interface is evident. This study provides direct insights into the vapor-solid-solid growth and clarified the growth mechanism of <001[combining macron]> III-V nanowires, which provides pathways in controlling the growth of <001[combining macron]> semiconductor nanowires.

The Formation Mechanism and Model of the Surface Nanoscale Kirkendall Effect on Au Catalyst Island/GaAs Substrate by Thermal Vapor-Liquid-Solid Method with Two-Step Temperature Mode

The Surface Nanoscale Kirkendall Effect is an important part of the Kirkendall effect, and has special role in the formation of surface nano material configurations. It can also cause faults in interconnection contact systems, yet this kind of effect has not yet been identified and studied in detail. Based on the obtained experimental results, this paper proposes a formation mechanism and model of the mixed-surface nanoscale Kirkendall effect formed by the role of Au metal catalyst islands/strips on a GaAs surface using the thermal Vapor-Liquid-Solid method. The diffusion of Ga, As, O atoms and the absorption of O atoms from a low-vacuum ambient into Au droplets forming surface nanoscale Au/Ga/O clusters leaves behind vacancies and voids; this process results in the nanoscale Kirkendall effect. In addition, the outward diffusion of the surface nanoscale Au/Ga/O clusters leaving behind bare GaAs holes in place of the former Au island forms the surface Kirkendall effect. Consequently, the combination of the two mentioned effects forms a new kind of KE, the so-called Surface Nanoscale Kirkendall Effect. This effect is generated either partly or completely, depending on the technological conditions. Accompanying this effect, the different configurations of nanomaterials have grown in number. The outward diffusion of surface nanoscale Au/Ga/O clusters was caused by the concentration of surface cluster gradients, the weakening of chemical bonds due to the accumulation of vacancies, the porosity, and pit-etching beneath the Au island. The diffusivity of surface Au/Ga/O clusters is numerically estimated. Its values vary from 2 × 10−10 to 10−11 m2/s. Potential applications of the surface nanoscale Kirkendall effect, making use of its advantages, limitations and disadvantages, are also discussed and proposed.

Metal oxide nanostructures: preparation, characterization and functional applications as chemical sensors

Preparation and characterization of different metal oxide (NiO, WO3, ZnO, SnO2 and Nb2O5) nanostructures for chemical sensing are presented. p-Type (NiO) and n-type (WO3, SnO2, ZnO and Nb2O5) metal oxide nanostructures were grown on alumina substrates using evaporation-condensation, thermal oxidation and hydrothermal techniques. Surface morphologies and crystal structures were investigated through scanning electron microscopy and Raman spectroscopy. Furthermore, different batches of sensors have been prepared, and their sensing performances towards carbon monoxide and nitrogen dioxide have been explored. Moreover, metal oxide nanowires have been integrated into an electronic nose and successfully applied to discriminate between drinking and contaminated water.

Surface Hydrogen Enables Subeutectic Vapor-Liquid-Solid Semiconductor Nanowire Growth

Vapor-liquid-solid nanowire growth below the bulk metal-semiconductor eutectic temperature is known for several systems; however, the fundamental processes that govern this behavior are poorly understood. Here, we show that hydrogen atoms adsorbed on the Ge nanowire sidewall enable AuGe catalyst supercooling and control Au transport. Our approach combines in situ infrared spectroscopy to directly and quantitatively determine hydrogen atom coverage with a "regrowth" step that allows catalyst phase to be determined with ex situ electron microscopy. Maintenance of a supercooled catalyst with only hydrogen radical delivery confirms the centrality of sidewall chemistry. This work underscores the importance of the nanowire sidewall and its chemistry on catalyst state, identifies new methods to regulate catalyst composition, and provides synthetic strategies for subeutectic growth in other nanowire systems.

Understanding quantum confinement in nanowires: basics, applications and possible laws

A comprehensive investigation of quantum confinement in nanowires has been carried out. Though applied to silicon nanowires (SiNWs), it is general and applicable to all nanowires. Fundamentals and applications of quantum confinement in nanowires and possible laws obeyed by these nanowires, have been investigated. These laws may serve as backbones of nanowire science and technology. The relationship between energy band gap and nanowire diameter has been studied. This relationship appears to be universal. A thorough review indicates that the first principles results for quantum confinement vary widely. The possible cause of this variation has been examined. Surface passivation and surface reconstruction of nanowires have been elucidated. It has been found that quantum confinement owes its origin to surface strain resulting from surface passivation and surface reconstruction and hence thin nanowires may actually be crystalline-core/amorphous-shell (c-Si/a-Si) nanowires. Experimental data available in the literature corroborate with the suggestion. The study also reveals an intrinsic relationship between quantum confinement and the surface amorphicity of nanowires. It demonstrates that surface amorphicity may be an important tool to investigate the electronic, optoelectronic and sensorial properties of quantum-confined nanowires.

Catalyzed oxidation for nanowire growth

A simple, low-cost and scalable route to substrate-supported nanowire growth is reported based on catalyzed oxidation. The process shares common features with popular catalyzed nanowire growth techniques such as vapor-liquid-solid (VLS), vapor-solid-solid (VSS), or vapor-quasi-solid (VQS) that use catalyst nanoparticles to direct the deposition of reactants from the vapor phase. Catalyzed oxidation for nanowire growth (CONG) utilizes catalyzed anion (e.g. O2) reduction from the vapor phase and metal (e.g. Fe) oxidation from the substrate to produce oxide nanowires (e.g. Fe3O4). The approach represents a new class of nanowire growth methodology that may be applied to a broad range of systems. CONG does not require expensive chemical vapor deposition or physical vapor deposition equipment and can be implemented at intermediate temperatures (400-600 °C) in a standard laboratory furnace. This work also demonstrates a passive approach to catalyst deposition that allows the process to be implemented simply with no lithography or physical vapor deposition steps. This effort validates the general approach by synthesizing MnO, Fe3O4, WO3, MgO, TiO2, ZnO, ReO3, and NiO nanowires via CONG. The process produces single crystalline nanowires that can be grown to high aspect ratio and as high-density nanowire forests. Applications of the as-grown Fe3O4 and ReO3 nanowires for lithium ion battery systems are demonstrated to display high areal energy density and power.

Metal/nanowire contacts, quantum confinement, and their roles in the generation of new, gigantic actions in nanowire transistors

A distinctly new route for the design, modeling and electrical behavior of very short-channel (5-10 nm in channel length) nanowire field-effect transistors (FETs) has been presented. Essential elements of the approach entail a drain current determined by thermionic emission, but not by carrier mobility in the channel of the transistor. A basic understanding of the fundamental physics and the concepts of Schottky-barrier-based design for the proposed route have been described. Quantum confinement in the nanowire channel together with Schottky barrier tailing and temperature-dependent fluctuations of applied biases has been taken into account for the development of the model. Both current-voltage characteristics and transconductance of FETs have been studied. The calculated results are in near-quantitative agreement with the available experiments. Measured data show very diverse (e.g., exponential, linear, saturating, and non-linear non-exponential non-saturating) nanowire transistor characteristics. The model explains these characteristics well and reveals a number of new transistor actions. It highlights the impacts of quantum confinement and Schottky contacts for these new transistor actions. It also quantifies the significant enhancement of the drain-source current and transconductance. With new findings thus achieved, suggestions for the realization of very high-performance, small-diameter (preferably 2 nm), small-Schottky-barrier-height, high-operating temperature, ultra-short-channel-length, nanowire transistors have been made. Optimized design of these transistors has been suggested. And the range (in terms of device and technological parameters) of the proposed model has been elucidated.

Wurtzite CuInS₂ and CuInxGa₁-xS₂ nanoribbons: synthesis, optical and photoelectrical properties

Single crystalline wurtzite ternary and quaternary semiconductor nanoribbons (CuInS(2), CuIn(x)Ga(1-x)S(2)) were synthesized through a solution-based method. The structure and composition of the nanoribbons were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), the corresponding fast Fourier transform (FFT) and nanoscale-resolved elemental mapping. Detailed investigation of the growth mechanism by monitoring the structures and morphologies of the nanoribbons during the growth indicates that Cu(1.75)S nanocrystals are formed first and act as a catalyst for the further growth of the nanoribbons. The high mobility of Cu(+) promotes the generation of Cu(+) vacancies in Cu(1.75)S, which will facilitate the diffusion of Cu, In or Ga species from solution into Cu(1.75)S to reach supersaturated states. The supersaturated species in the Cu(1.75)S catalyst, Cu-In-S and Cu-In-Ga-S species, start to condense and crystallize to form wurtzite CuInS(2) or CuIn(x)Ga(1-x)S(2) phases, firstly resulting in two-sided nanoparticles. Successive crystallizations gradually impel the Cu(1.75)S catalyst head forward and prolong the length of the CuInS(2) or CuIn(x)Ga(1-x)S(2) body, forming heterostructured nanorods and thus nanoribbons. The optical band gaps of CuIn(x)Ga(1-x)S(2) nanoribbons can be continuously adjusted between 1.44 eV and 1.91 eV, depending on the Ga concentration in nanoribbons. The successful preparation of those ternary and quaternary semiconductor nanoribbons provide us an opportunity to study their photovoltaic properties. The primary photoresponsive current measurements demonstrate that wurtzite CuIn(x)Ga(1-x)S(2) nanoribbons are excellent photoactive materials. Furthermore, this facile method could open a new way to synthesize other various nano-structured ternary and quaternary semiconductors, such as CuInSe(2) and CuIn(x)Ga(1-x)Se(2), for applications in solar cells and other fields.

Concentration gradient induced morphology evolution of silica nanostructure growth on photoresist-derived carbon micropatterns

The evolution of silica nanostructure morphology induced by local Si vapor source concentration gradient has been investigated by a smart design of experiments. Silica nanostructure or their assemblies with different morphologies are obtained on photoresist-derived three-dimensional carbon microelectrode array. At a temperature of 1,000°C, rope-, feather-, and octopus-like nanowire assemblies can be obtained along with the Si vapor source concentration gradient flow. While at 950°C, stringlike assemblies, bamboo-like nanostructures with large joints, and hollow structures with smaller sizes can be obtained along with the Si vapor source concentration gradient flow. Both vapor-liquid-solid and vapor-quasiliquid-solid growth mechanisms have been applied to explain the diverse morphologies involving branching, connecting, and batch growth behaviors. The present approach offers a potential method for precise design and controlled synthesis of nanostructures with different features.

Metal organic chemical vapor deposition of phase change Ge1Sb2Te4 nanowires

The self-assembly of Ge(1)Sb(2)Te(4) nanowires (NWs) for phase change memories application was achieved by metal organic chemical vapor deposition, catalyzed by Au nanoislands in a narrow range of temperatures and deposition pressures. In the optimized conditions of 400 °C, 50 mbar, the NWs are Ge(1)Sb(2)Te(4) single hexagonal crystals. Phase change memory switching was reversibly induced by nanosecond current pulses through metal-contacted NWs with threshold voltage of about 1.35 V.

Thermodynamic imbalance, surface energy, and segregation reveal the true origin of nanotube synthesis

Extensive analyses of thermodynamic imbalance, surface energy, and segregation of nanotubes on nanoparticle surfaces are performed. A model for surface energy i developed. In addition, nanotube growth both by vapor-phase and solid-phase mechanisms is described. Segregation of the nanotube species to the periphery of the nanoparticle, the creation of an amorphous shell at this periphery, a droplet created in this shell, and the mediation of this droplet for supersaturation and nucleation of the nanotube species may be the true causes of nanotube growth.

Coexistence of vapor-liquid-solid and vapor-solid-solid growth modes in Pd-assisted InAs nanowires

During the growth of InAs nanowires from Pd catalyst particles on InAs(111)A substrates, two distinct classes of nanowires are observed with smooth or zigzagged sidewalls. It is shown that this is related to a bimodal distribution of the wire-tip diameter: above a critical diameter wires grow with smooth sidewalls, and below with zigzagged morphology. Transmission electron microscopy analysis shows that the catalyst particles at the tip of zigzagged wires are smooth and have a higher aspect ratio than those at the tip of smooth wires. Zigzagged wires grow from liquid particles in the vapor-liquid-solid (VLS) mode whereas the smooth ones grow from solid particles in the vapor-solid-solid (VSS) mode.