Semi-transparent,flexible, and electrically conductive silicon mesh by capillarity-driven welding of vapor-liquid-solid-grown nanowires over large areas

Bottom-up synthesis of semiconductor nanowires (NWs) by the vapor-liquid-solid (VLS) mechanism has enabled diverse technological applications for these nanomaterials. Unlike metallic NWs, however, it has been challenging to form large-area interconnected NW networks. Here, we generate centimeter-scale meshes of mechanically and electrically interconnected Si NWs by sequentially growing, collapsing, and joining the NWs using a capillarity-driven welding mechanism. We fabricate meshes from VLS-grown NWs ranging in diameter from 20 to 100 nm and find that the meshes are three-dimensional with a thickness ranging from ~ 1 to ~ 10 microns depending on the NW diameter. Optical extinction measurements reveal that the networks are semi-transparent with a color that depends on the absorption and scattering characteristics of individual NWs. Moreover, active voltage contrast imaging of both centimeter- and micron-scale meshes reveals widespread electrical connectivity. Using a sacrificial layer, we demonstrate that the mesh can be liberated from the growth substrate, yielding a highly flexible and transparent film. Electrical transport measurements both on the growth substrate and on liberated, flexible films reveal electrical conduction across a centimeter scale with a sheet resistance of ~ 160–180 kΩ/square that does not change significantly upon bending. Given the ability to encode complex functionality in semiconductor NWs through the VLS process, we believe these meshes of networked NWs could find application as neuromorphic memory, electrode scaffolds, and bioelectronic interfaces.

     

A liquid phase anion-exchange approach to high-quality all-inorganic halide perovskite micro- and nanowires

All-inorganic halide perovskite nanowires (NWs) are promising materials due to they have broad application prospects in the field of optoelectronics, with mixed-halide perovskite nanowires can change the optoelectronic properties by adjusting the halide ratio. Here, we experimentally investigated the two-process governed anion-exchange reaction in single-crystalline CsPbX₃ micro- and nanowires. The critical parameters affecting the outcome of the reaction are identified as the reaction temperature, reaction time, and precursor concentrations. Upon examining the photoluminescence and morphology of the NWs, high-quality NWs were obtained by optimizing these critical parameters. The bandgap of the NWs can be tuned over the entire visible spectra (430–700 nm). In addition, photodetectors incorporating single NWs were fabricated, which demonstrated excellent responsivity under illumination. Our results expand the validity of liquid-phase anion exchange to the microscale, and lay the basis for liquid-processed optoelectronics and displays.

     

Glycerol and ethylene glycol co-mediated synthesis of uniform multiple crystalline silver nanowires

The large scale synthesis of multiple crystalline silver nanowires (NWs) with uniform diameter were carried out by using glycerol and ethylene glycol (EG) as co-mediated solvents in the presence of poly(vinyl pyrrolidine) (PVP). Experimental results and structural characterizations reveal that Ag NWs are evolved from the multiple crystalline seeds initially generated by the reduction of AgNO₃ with EG and glycerol. Owing to the different reduction ability and viscosity of EG and glycerol, which play an important role for controlling the nucleation at the beginning of reaction, glycerol with high viscosity slows down the migration velocity of Ag⁰ in favor of forming the uniform Ag NWs with small diameter (40 nm) in the presence of PVP molecules selectively adsorbed on the surface of Ag seeds. The yield of the Ag NWs is dependent on the volume ratio of EG and glycerol. In the absence of EG, large amount of Ag nanoparticles (NPs) and few Ag NWs were created. In contrast, Ag nanorods and polyhedral particles are prepared in the case of no glycol added. This paper provides a new approach for the large scale synthesis of Ag NWs with uniform diameter by simply adjusting the solvent components. Furthermore, V-shaped Ag nanostructure was obtained and the possible growth mechanism was discussed.     

A study on rapid growth and piezoelectric effect of ZnO nanowires array

This work presents a rapid and simple synthesis procedure for ZnO nanowires (NWs) array by using the vapor–solid (VS) method. Experimental results indicate that the length and diameter of the grown ZnO NWs are associated with the temperature effect, while the growth density of NWs is strongly related to gas flux during the VS process. Additionally, the synthesized ZnO NWs possess specific crystalline qualities, making them highly promising for piezoelectric device applications. Therefore a piezoelectric type nanogenerator based on the ZnO NWs is also designed in this work, with a high output of piezoelectric current of 0.6 μA cm⁻² obtained as well. Our results further demonstrate the feasibility of applying piezoelectric energy via the rapidly grown ZnO NWs array.     

Study of using aqueous NH3 to synthesize GaN nanowires on Si(1 1 1) by thermal chemical vapor deposition

High-quality GaN nanowires (NWs) and zigzag-shaped NWs were grown on catalyst-free Si(1 1 1) substrate by thermal chemical vapor deposition (TCVD). Gallium (Ga) metal and aqueous NH₃ solution are used as a source of materials. Ga vapor was directly reacts with gaseous NH₃ under controlled nitrogen flow at 1050 °C. Scanning electron microscopy (SEM) images showed that the morphology of GaN displayed various densities of NWs and zigzag NWs depending on the gas flow rate, and increased nitrogen flow rate caused density reduction. The GaN NWs exhibited clear X-ray diffraction analysis (XRD) peaks that corresponded to GaN with hexagonal wurtzite structures. The photoluminescence spectra showed that the ultraviolet band emission of GaN NWs had a strong near band-edge emission (NBE) at 361–367 nm. Yellow band emissions were observed at low and high flow rates due to nitrogen and Ga vacancies, respectively. Moderate N₂ flow resulted in a strong NBE emission and a high optical quality of the NWs. This study shows the possibility of low-cost synthesis of GaN nanostructures on Si wafers using aqueous NH₃ solution.     

Growth of ZnGa2O4 nanowires on a ZnO buffer layer by carbothermal reduction of Ga2O3 powder

The synthesis of pure ZnGa₂O₄ nanowires (NWs) on the ZnO-coated Si substrates could be achieved by carbothermal reduction of Ga₂O₃ powder. The processing parameters such as the weight of Ga₂O₃ powder, the thickness of ZnO buffer layer, and the substrate temperature were explored. The growth of ZnGa₂O₄ NWs followed the vapor-solid process. Surplus ZnO source favored the growth of ZnGa₂O₄ NWs, while higher substrate temperature promoted the growth of Ga₂O₃ nanobelts. The results indicated a window for the growth of abundant and pure ZnGa₂O₄ NWs. The growth mechanism of ZnGa₂O₄ NWs on the ZnO buffer layer via carbothermal reduction of Ga₂O₃ powder was discussed. The ZnGa₂O₄ NWs showed a photoluminescence band centered at 466-475 nm.     

Synthesis and growth mechanism of triangular Mn doped CdS nanowires

Triangular Mn-doped CdS nanowires (NWs) were prepared by thermal evaporation of a mixture of CdS and MnCl₂. The morphologies and detailed structures were characterized by a scanning electron microscope, X-ray diffraction, and transmission electron microscope. The Mn concentration plays an important role in synthesis of the triangular NWs. The morphologies can be varied from hexagonal to triangular by adjusting the amount of MnCl₂ in the reaction mixture. The oriented attachment mechanism is demonstrated to be the most suitable mechanism to explain the growth process of the triangular NWs. The photoluminescence shows the intensity of Mn²⁺ emission peak increases as the molar ratio of MnCl₂increases.     

The solution growth of copper nanowires and nanotubes is driven by screw dislocations

Copper (Cu) nanowires (NWs) are inexpensive conducting nanomaterials intensively explored for transparent conducting electrodes and other applications. However, the mechanism for solution growth of Cu NWs remains elusive so far. Here we show that the one-dimensional anisotropic growth of Cu NWs and nanotubes (NTs) in solution is driven by axial screw dislocations. All three types of evidence for dislocation-driven growth have been conclusively observed using transmission electron microscopy (TEM) techniques: rigorous two-beam TEM analysis that conclusively characterizes the dislocations in the NWs to be pure screw dislocations along <110> direction, twist contour analysis that confirms the presence of Eshelby twist associated with the dislocation, and the observation of spontaneously formed hollow NTs. The reduction-oxidation (redox) electrochemical reaction forming the Cu NWs presents new chemistry for controlling supersaturation to promote dislocation-driven NW growth. Using this understanding to intentionally manipulate the supersaturation, we have further improved the NW growth by using a continuous flow reactor to yield longer Cu NWs under much milder chemical conditions. The rational synthesis of Cu NWs with control over size and geometry will facilitate their applications.     

Synthesis of vertically oriented GaN nanowires on a LiAlO2 substrate via chemical vapor deposition

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on γ-LiAlO₂ (100) substrate coated with a Au layer, via a chemical vapor deposition process at 1000 °C using gallium and ammonia as source materials. The GaN NWs grow along the nonpolar [100] direction with steeply tapering tips, and have triangular cross-sections with widths of 50–100 nm and lengths of up to several microns. The GaN NWs are formed by a vapor-liquid-solid growth mechanism and the tapering tips are attributed to the temperature decrease in the final stage of the synthesis process. The aligned GaN NWs show blue-yellow emission originating from defect levels, residual impurities or surface states of the GaN NWs, and have potential applications in nanotechnology.     

Facile synthesis and growth mechanism of Ni-catalyzed GaAs nanowires on non-crystalline substrates

GaAs nanowires (NWs) have been extensively explored for next generation electronics, photonics and photovoltaics due to their direct bandgap and excellent carrier mobility. Typically, these NWs are grown epitaxially on crystalline substrates, which could limit potential applications requiring high growth yield to be printable or transferable on amorphous and flexible substrates. Here, utilizing Ni as a catalytic seed, we successfully demonstrate the synthesis of highly crystalline, stoichiometric and dense GaAs NWs on amorphous SiO(2) substrates. Notably, the NWs are found to grow via the vapor-solid-solid (VSS) mechanism with non-spherical NiGa catalytic tips and low defect densities while exhibiting a narrow distribution of diameter (21.0 ± 3.9 nm) uniformly along the entire length of the NW (>10 μm). The NWs are then configured into field-effect transistors showing impressive electrical characteristics with I(ON)/I(OFF) > 10(3), which further demonstrates the purity and crystal quality of NWs obtained with this simple synthesis technique, compared to the conventional MBE or MOCVD grown GaAs NWs.     

Morphology-controlled one-dimensional ZnO nanostructures with customized Ga-doping

Enhanced functionality of the nanostructure-based devices can be achieved by customizing the doping, thereby managing the electrical properties of the nanostructures. We have optimized the synthesis condition of the ZnO nanowires (NWs) using hot-walled pulsed laser deposition (HW-PLD) that features the facilitated kinetic energy control of the laser-ablated particles. The electrical properties of the NWs have been managed by doping control while maintaining the NW morphologies. 1, 3, and 5 wt.% Ga concentration in the NWs is evaluated directly with energy dispersive spectrometer (EDS), and the exciton peak shifts are measured with room temperature photoluminescence (PL) to find the correlation between the concentration and the shifts. n-type Ga-doping status has been verified with low temperature PL to find the donor-bound exciton peaks. As for the morphology diversification, we have acquired both zigzag-shaped NWs and nanohorns using the same HW-PLD.     

Ladder-like metal oxide nanowires: Synthesis,electrical transport,and enhanced light absorption properties

Transparent metal oxide nanowires （NWs） have attracted intense research interest in recent years. We report here the synthesis of interesting ladder-like metal oxide NWs, including In2O3, SnO2, ZnO, and Ga2O3, via a facile chemical vapor deposition （CVD） method. Their structural features and growth mechanism are demonstrated in detail by using the ladder-like In2O3 NWs as an example. Single ladder-like NW-based field-effect transistors （FETs） and photodetectors （PDs） of SnO2 were fabricated in order to investigate their electrical transport and light absorption properties. Compared with straight NW-based FETs which operate in an enhancement mode （E-mode）, FETs build on ladder-like NWs operate in a depletion mode （D-mode）. The ladder-like NWs also give higher carrier concentrations than conventional single nanowires. Finite-difference time-domain （FDTD） simulations have been performed on the ladder-like NWs and the results reveal a great enhancement of light absorption with both transverse-electric （TE） and transverse-magnetic （TM） polarization modes, which is in good agreement with the experimental results.     

Rational synthesis of p-type zinc oxide nanowire arrays using simple chemical vapor deposition

We report, for the first time, the synthesis of the high-quality p-type ZnO NWs using a simple chemical vapor deposition method, where phosphorus pentoxide has been used as the dopant source. Single-crystal phosphorus doped ZnO NWs have their growth axis along the 001 direction and form perfect vertical arrays on a-sapphire. P-type doping was confirmed by photoluminescence measurements at various temperatures and by studying the electrical transport in single NWs field-effect transistors. Comparisons of the low-temperature PL of unintentionally doped ZnO (n-type), as-grown phosphorus-doped ZnO, and annealed phosphorus-doped ZnO NWs show clear differences related to the presence of intragap donor and acceptor states. The electrical transport measurements of phosphorus-doped NW FETs indicate a transition from n-type to p-type conduction upon annealing at high temperature, in good agreement with the PL results. The synthesis of p-type ZnO NWs enables novel complementary ZnO NW devices and opens up enormous opportunities for nanoscale electronics, optoelectronics, and medicines.     

One-dimensional Cu-based catalysts with layered Cu–Cu<Subscript>2</Subscript>O–CuO walls for the Rochow reaction

A series of copper catalysts with a core–shell or tubular structure containing various contents of Cu, Cu₂O, and CuO were prepared via controlled oxidation of Cu nanowires (NWs) and used in the synthesis of dimethyldichlorosilane (M2) via the Rochow reaction. The Cu NWs were prepared from copper (II) nitrate using a solution-based reduction method. The samples were characterized by X-ray diffraction, thermogravimetric analysis, temperature-programmed reduction, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy. It was found that the morphology and composition of the catalysts could be tailored by varying the oxidation temperature and time. During the gradual oxidation of Cu NWs, the oxidation reaction initiated on the outer surface and gradually developed into the bulk of the NWs, leading to the formation of catalysts with various structures and layered compositions, e.g., Cu NWs with surface Cu₂O, ternary Cu–Cu₂O–CuO core–shell NWs, binary Cu₂O–CuO nanotubes (NTs), and single CuO NTs. Among these catalysts, ternary Cu–Cu₂O–CuO core–shell NWs exhibited superior M2 selectivity and Si conversion in the Rochow reaction. The enhanced catalytic performance was mainly attributed to improved mass and heat transfer resulting from the peculiar heterostructure and the synergistic effect among layered components. Our work indicated that the catalytic property of Cu-based nanoparticles can be improved by carefully controlling their structures and compositions.

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Ultralong and Mesoporous ZnO and γ-Al_2O_3 Oriented Nanowires Obtained by Template-assisted Hydrothermal Approach

Highly mesoporous Zn O and g-Al2O3nanowires（NWs） are both synthesized by a hydrothermal method using commercially available porous anodic aluminium oxide（AAO） as template. AAO membrane acts as template for Zn O NWs and both as template and precursor for g-Al2O3 NWs. The formation of intermediate phases of porous Zn6Al2（OH）16CO3and boehmite（g-Al OOH） were observed, both occurring during the hydrothermal synthesis of porous Zn O and g-Al2O3 NWs, respectively, and disappearing after annealing at 600 C. This novel template-assisted hydrothermal process leads to the formation of porous Zn O and g-Al2O3NWs（specific surface area of 192 m2 g 1and 263 m2 g 1, respectively）, showing pore sizes around 4 nm in diameter. The influence of the reaction parameters on the nanostructure morphology was also investigated. A Zn O seed layer, deposited on the AAO channels prior to the hydrothermal synthesis, leads to more compact Zn O nanowires（99 m2 g-1） protecting the AAO host from the chemical attack of the precursor solution.     

Optimization of CVD parameters for long ZnO NWs grown on ITO/glass substrate

The optimization of chemical vapour deposition (CVD) parameters for long and vertically aligned (VA) ZnO nanowires (NWs) were investigated. Typical ZnO NWs as a single crystal grown on indium tin oxide (ITO)-coated glass substrate were successfully synthesized. First, the conducted side of ITO–glass substrate was coated with zinc acetate dihydrate to form seed layer of ZnO nanocrystals. Double zone tube furnace connected to vacuum pump was used for ZnO growth process. Zn metal powder was positioned at the first zone at temperature 900 ^° C. The ITO–glass substrate with pre-coated seed layer was then located in the second zone of tube furnace at growth temperature of 550 ^° C. The growth of ZnO NWs was controlled under constant concentration of seed layer, while other parameters such as argon and oxygen flow rates, substrate position, time and oxygen flow rate were varied. The VA ZnO NWs were finally characterized by scanning electron microscopy, X-ray diffractometer and high-resolution transmission electron microscope equipped with energy-dispersive X-ray spectroscopy. The results show that long and VA ZnO NWs were single crystalline with hexagonal wurtzite structure. The ultimate length and average diameter of ZnO NWs were 10 μm and 50–100 nm, respectively. These were achieved under optimized CVD growth parameters. The mechanism of vertical growth model of ZnO NWs is also discussed.     

Quantum confinement and electroluminescence in ultrathin silicon nanowires fabricated by a maskless etching technique

We present a novel approach for the direct synthesis of ultrathin Si nanowires (NWs) exhibiting room temperature light emission. The synthesis is based on a wet etching process assisted by a metal thin film. The thickness-dependent morphology of the metal layer produces uncovered nanometer-size regions which act as precursor sites for NW formation. The process is cheap, fast, maskless and compatible with Si technology. Very dense arrays of long (several micrometers) and small (diameter of 5-9?nm) NWs have been synthesized. An efficient room temperature luminescence, visible with the naked eye, is observed when NWs are optically excited, exhibiting a blue-shift with decreasing NW size in agreement with quantum confinement effects. A prototype device based on Si NWs has been fabricated showing a strong and stable electroluminescence at low voltages. The relevance and the perspectives of the reported results are discussed, opening the route toward novel applications of Si NWs.     

Growth of Silver Nanowires from Controlled Silver Chloride Seeds and Their Application for Fluorescence Enhancement Based on Localized Surface Plasmon Resonance

A “Polyol” method has granted low‐cost and facile process‐controllability for silver‐nanowire (Ag‐NW) synthesis. Although homogenous and heterogeneous nucleation and growth during Ag‐NW synthesis are possible using polyol methods, heterogeneous nucleation and growth of Ag NW guarantees highly selective growth of nanostructures using silver chloride (AgCl) seeds, which provides a stable source of chloride ions (Cl?) and thermodynamic reversibility. In this paper, a microdroplet has been adopted to synthesize uniform AgCl seeds with different diameter that are used for seed‐mediated Ag‐NW synthesis. The concentration of two precursors (AgNO₃ and NaCl) in the droplets is modulated to produce different sizes of AgCl seeds, which determines the diameter and length of Ag NWs. The process of the seed‐mediated growth of Ag NWs has been monitored by observing the peak shift in the time‐resolved UV–vis extinction spectrum. Furthermore, the distinct plasmonic property of Ag NWs for transverse and longitudinal localized‐surface‐plasmon‐resonance (LSPR)‐mediated fluorescence enhancement is utilized. The high aspect ratio and sharp tips work as simple antennas that induce the enhanced fluorescence emission intensity of a fluorophore, which can be applied in the fields of biological tissue imaging and therapy.     

Towards understanding the morphological,magnetic, optical and electrical properties of MnO<Subscript>2</Subscript> nanowires for magneto-and optoelectronic applications

Mixed solutions of manganese sulfate and potassium permanganate were hydrothermally used to synthesis α-MnO₂ nanowires. Energy dispersive analysis of X-ray revealed the formation of stoichiometric MnO₂ and X-ray diffraction revealed the formation of single crystalline α-MnO₂. Examinations using both SEM and TEM yielded nanowires with diameters ranged from 40 to 50 nm and lengths ranged from 5 to 6 µm. The magnetic properties revealed that the α-MnO₂ NWs exhibit ferromagnetic/antiferromagnetic characteristics at liquid nitrogen and room temperatures. This kind of materials may be find usage in electromagnetic shielding applications. Direct optical band gap of 2.6 eV, that was blue shifted from the bulk value, was reported. Photoluminescence examinations displayed a strong emission peak around 395 nm and broad peak around 477 nm. The α-MnO₂ NWs showed a semiconducting behavior where the resistivity decreased with increasing temperature. Two activation energies were reported; the low temperature activation energy was 0.089 eV and the high temperature activation energy was 0.782 eV. Based on the observed high intensity UV emission peak, the obtained α-MnO₂ NWs may find applications in UV light emitting diodes.     

In situ observation of self-assembled Fe13Ge8 nanowires growth on anisotropic Ge (1 1 0) surface

Self-assembled iron germanide nanowires (NWs) were grown by directly depositing Fe onto a Ge (1 1 0) substrate, in an in situ ultra-high vacuum transmission electron microscope from 430 to 500 °C. All observed NWs had a similar length/width aspect ratio (～8:1) at all deposition temperatures, as well as the same elongation orientation with respect to the underlying Ge (1 1 0) substrate. The growth dynamics was investigated by real time observations of NWs growth at elevated temperatures. It is elucidated that the formation of NWs in similar shape at all deposited temperatures is attributed to the similar activation energy barriers in length and width of NWs, which can result in the constant growth rate independent of growth temperatures. Furthermore, the difference in pre-exponential factor along the length and width of growing islands arose due to the anisotropic constraint of the Ge (1 1 0) substrate, leading to the unique elongation of NWs. This growth dynamics suggests the possibility of uniform control of the morphology of self-assembled NWs, as well as other morphologies of bottom-up fabricated devices, at different deposition temperatures.