Nanowire Mats Research Articles

Optical reflectivity of GaAs nanowire arrays: Experiment and model

In this work, we present a systematic study of the optical reflectivity of GaAs nanowire arrays as a function of nanowire size, morphology, and arrangement on the substrate. To analyze the results, we extend a recently proposed model, which describes the nanowire mat as an ensemble of diffuse optical reflectors, by including the effective medium description and the low or zero-absorption regime. The modified model is very general and well accounts for the behavior of all the samples analyzed in this work, which covers a wide range of common situations: from the array of disordered parallel oriented and small nanowires (1.5–2 μm long and 30–50 nm large) to the randomly oriented wire array with non-uniform distribution of the sizes. The results that we report here for GaAs nanowires can be applied to any other type of semiconductor nanowires and provide useful insights to design more efficient solar cells.

Zn_2GeO_4 and In_2Ge_2O_7 nanowire mats based ultraviolet photodetectors on rigid and flexible substrates

Ternary metal oxide, Zn2GeO4, In2Ge2O7, have potential applications in many research areas. Using a single chemical vapor deposition method, high-quality single crystalline Zn2GeO4 nanowire (NW) mats and In2Ge2O7 NW mats were synthesized on a large scale. Nanowires mats based ultraviolet photodetectors were fabricated on rigid silicon substrates. By simply transferring the nanowire mats to a transparent adhesive PET tape, flexible photodetectors were also fabricated. Both the rigid and flexible photodetectors exhibited excellent photoconductive performance in terms of high sensitivity to the UV light, excellent stability and reproducibility, and fast response and recovery time.

Metal‐Catalyst‐Free Synthesis and Characterization of Single‐Crystalline Silicon Oxynitride Nanowires

Large quantities of single‐crystal silicon oxynitride nanowires with high N concentration have been synthesized directly on silicon substrate at 1200°C without using any metal catalyst. The diameter of these ternary nanowires is ranging from 10 to 180 nm with log‐normal distribution, and the length of these nanowires varies from a few hundreds of micrometers to several millimeters. A vapor‐solid mechanism was proposed to explain the growth of the nanowires. These nanowires are grown to form a disordered mat with an ultrabright white nonspecular appearance. The mat demonstrates highly diffusive reflectivity with the optical reflectivity of around 80% over the whole visible wavelength, which is comparable to the most brilliant white beetle scales found in nature. The whiteness might be resulted from the strong multiscattering of a large fraction of incident light on the disordered nanowire mat. These ultra‐bright white nanowires could form as reflecting surface to meet the stringent requirements of bright‐white light‐emitting‐diode lighting for higher optical efficiency. They can also find applications in diverse fields such as sensors, cosmetics, paints, and tooth whitening.

High Density Germanium Nanowire Growth Directly from Copper Foil by Self-Induced Solid Seeding

Herein, we describe the growth of highly dense germanium nanowire mats directly on copper foil by a self-induced, solid seeded protocol. The existence of Cu3Ge tips on each of the nanowires indicates that growth proceeds via a solid catalyzed route, dependent on the in situ formation of the germanide intermediate. The nanowires show a tight diameter distribution and typically ⟨110⟩ growth directions resulting from similarities in the d spacings between the nanowire and the catalyst seed. The nanowires and substrates were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), and electron backscatter diffraction (EBSD).

Title: Using Alignment and 2D Network Simulations to Study Charge Transport Through Doped ZnO Nanowire Thin Film Electrodes

AbstractFactors affecting charge transport through ZnO nanowire mat films were studied by aligning ZnO nanowires on substrates and coupling experimental measurements with 2D nanowire network simulations. Gallium doped ZnO nanowires were aligned on thermally oxidized silicon wafer by shearing a nanowire dispersion in ethanol. Sheet resistances of nanowire thin films that had current flowing parallel to nanowire alignment direction were compared to thin films that had current flowing perpendicular to nanowire alignment direction. Perpendicular devices showed ∼5 fold greater sheet resistance than parallel devices supporting the hypothesis that aligning nanowires would increase conductivity of ZnO nanowire electrodes. 2‐D nanowire network simulations of thin films showed that the device sheet resistance was dominated by inter‐wire contact resistance. For a given resistivity of ZnO nanowires, the thin film electrodes would have the lowest possible sheet resistance if the inter‐wire contact resistance was one order of magnitude lower than the single nanowire resistance. Simulations suggest that the conductivity of such thin film devices could be further enhanced by using longer nanowires.

Statistical model on the optical properties of silicon nanowire mats

Randomly grown silicon nanowire (SiNW) mats show a high light absorption even for long wavelengths despite the small volume of silicon. We present a statistical model that gives a physical understanding of the mechanisms of the absorption and scattering of light in such SiNW mats. According to this model the two main mechanisms of the effective absorption of light are (i) resonant optical antenna effects of the absorption within the individual SiNWs and (ii) the interaction of the light with several SiNWs in the mat due to strong light scattering. The results of this model are in good agreement with the experimental reflection, transmission, and absorption spectra taken with an integrating sphere.

Hybrid Si Nanowire/Amorphous Silicon FETs for Large-Area Image Sensor Arrays

Silicon nanowire (SiNW) field-effect transistors (FETs) were fabricated from nanowire mats mechanically transferred from a donor growth wafer. Top- and bottom-gate FET structures were fabricated using a doped a-Si:H thin film as the source/drain (s/d) contact. With a graded doping profile for the a-Si:H s/d contacts, the off-current for the hybrid nanowire/thin-film devices was found to decrease by 3 orders of magnitude. Devices with the graded contacts had on/off ratios of ∼10(5), field-effect mobility of ∼50 cm(2)/(V s), and subthreshold swing of 2.5 V/decade. A 2 in. diagonal 160 × 180 pixel image sensor array was fabricated by integrating the SiNW backplane with an a-Si:H p-i-n photodiode.

Supercritical Chemical Fluid Deposition of InP and InAs

Supercritical chemical fluid deposition (SCFD) from [nBu2In(μ-EtBu2)2InnBu2] precursors (E = P or As) dissolved in a homogeneous hexane/CO2 fluid results in the growth of a thin film of InP or InAs with a thick mat of nanowires attached to the surface. InP films were well-adhered to the substrate and the nanowires can be removed without disrupting the film; however, the InAs films were poorly adhered, being dislodged simply by tapping the substrate. In many cases, the InP nanowires are single crystalline, and both the nanowires and the underlying films exhibit band-edge luminescence. Use of supercritical CHF3 as the delivery solvent also leads to good quality InP films and reduced carbon incorporation into the films. The single-source precursor system was responsible for significant amounts of carbon. This work shows that the SCFD method may be generally applicable to the growth of compound semiconductors, with important implications for growth of these materials in confined environments.

Fabrication of Ag nanoparticles dispersed in PVA nanowire mats by microwave irradiation and electro-spinning

In this study, the antimicrobial substance, silver nanoparticles (Ag NPs) loaded in poly (vinyl alcohol) (PVA) nanowire mats were fabricated by conjugation of the electro-spinning method and the microwave-assisted process. The best PVA nanowire mats were fabricated by through control of electro-spinning conditions, which were applied for fabrication of Ag NPs loaded in PVA nanowires. PVA was used not only as a carrier for loading of Ag NPs but also as a reduction agent with which the Ag + ion was already reduced to a large number of Ag NPs by irradiation with a microwave. Ag NPs were synthesized inside the PVA solution depending on the time of microwave irradiation — whether for 60 s or 90 s. Size distribution of Ag NPs was 5–10 nm in diameter for 60 s; and 10–20 nm in diameter for 90 s of irradiation. Presence of Ag NPs acquired through microwave assisted irradiation was confirmed by X-ray diffraction profiles (XRD). Microstructure, particle size distribution, and morphology of both the nanowire mats and the Ag particles were investigated using SEM and TEM techniques. The effect of Ag-NPs on the PVA mechanical property of nano-fibrous mats was investigated according to tensile strength. Antibacterial activity of PVA loaded Ag NPs at different irradiation times was tested on Gram-positive bacteria, Staphylococcus aureus Gram-negative bacteria, and Escherichia coli.

Experimental study of electrical properties of ZnO nanowire random networks for gas sensing and electronic devices

In this paper, we report on studying of ZnO nanowire mats as an electrical nanomaterial with particular interest in their interaction with various gas surroundings for gas sensing characteristics. The ZnO nanowires were synthesized on sapphire substrates using a horizontal tube furnace. The techniques of Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS) were applied to determine the as-grown ZnO nanowires’ morphological and crystal structures, chemical composition and electronic states. Four-terminal current-voltage (I–V) measurements were used to examine the electrical conductance of the ZnO nanowire mats exposed to various testing gases with reference to the vacuum condition. Gas exposure experiments were conducted in a custom-built environmental chamber, which was filled with different testing gases. We observed the current being significantly influenced with ambient CO gas. The I–V behavior of CO gas was also found to be reversible and repeatable after the chamber evacuation, which indicates that the ZnO nanowire mats can be used for gas sensing purposes. A possible interactive model of nanowires and testing gas molecules is proposed to elucidate the sensing selective and sensitive mechanism for gas sensors.

Analytic Model for Diffuse Reflectivity of Silicon Nanowire Mats

Disordered silicon nanowires are diffuse optical reflectors, with reflectivity modified by the nanowire absorption. We present an analytical model which describes the reflectivity, absorption, and transmission of a nanowire mat, across a wide spectral range, and including substrate effects. The model provides the ability to predict the optical properties of other nanowire mat structures, including core/shell heterostructures.

Percolating SnO 2 nanowire network as a stable gas sensor: Direct comparison of long-term performance versus SnO 2 nanoparticle films

A comparative study of the long-term gas-sensing performance of chemiresistors made of: (i) mats of randomly oriented single crystal SnO 2 nanowires and (ii) thin layers of pristine SnO 2 nanoparticles, has been carried out. The sensing elements made of percolating nanowires demonstrate excellent sensitivity and long-term stability toward traces of 2-propanol in air. Different from the nanowire network, the superior initial sensitivity of the nanoparticle layer deteriorates during the first month of the operation and approaches to one observed steadily in the nanowire mats. The better stability of the nanowire mats sensors is explained in framework of reduced propensity of the single crystal nanowires to sinter under real world operation conditions with respect to nanoparticle thin film. At the microscopic level, the letter defines the stability of the percolating paths, analyte delivery and transduction mechanism in nanowire network sensing elements.

Interaction of hybrid nanowire–nanoparticle structures with carbon monoxide

A gas-phase sensor based on a GaN nanowire mat decorated with Au nanoparticles wasstudied both experimentally and theoretically. The sensor is responsive to CO andH2 and could be used to study the water–gas-shift reaction, which involves combining CO andH2O toproduce H2. It was shown that for catalyzing this reaction using support Au nanoparticles, thesequence in which the reactants are exposed to the catalyst surface is critical. Toquantitatively evaluate the sensor response to gas exposure a depletion model wasdeveloped that considered the Au nanoparticle–semiconductor interface as a nano-Schottkybarrier where variation in the depletion region caused changes in the electrical conductivityof the nanowires.

Reflectivity of disordered silicon nanowires

Disordered silicon nanowire mats have high diffuse optical reflectivity, in striking contrast to the low reflectivity of orientationally ordered nanowires. The reflectivity decreases with increasing photon energy across the visible and near infrared spectrum due to absorption in the nanowires. A simple model is used to estimate that the incident photon interacts with up to 20 nanowires before being reflected. A thin coating of hydrogenated amorphous silicon enhances the nanowire absorption.

Gas Sensing With Mats of Gold-Nanoparticle Decorated GaN Nanowires

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We report on the use of mats of gold nanoparticle decorated GaN nanowires as gas sensors. The sensing was by the repeated and reversible measurement of changes in the current-voltage characteristics of the mat of nanowires. The nanowires had diameters of about 200 nm and were many microns long. The mat was grown on a 1-cm diameter sapphire disk and was about 10 <formula formulatype="inline"> <tex>$\mu\hbox{m}$</tex></formula> thick. The selectivity mechanism is attributed to the details of the surface morphology of the gold nanoparticles decorating the surface of the nanowires. The changes in the currents are attributed to a depletion mechanism in the nanowires due to the formation of a Schottky barrier due to the presence of the gold on the inherently n-type GaN. We were able to detect CO, <formula formulatype="inline"><tex>$\hbox{CH}_{4}$</tex> </formula>, <formula formulatype="inline"><tex>$\hbox{CO}_{2}$</tex></formula>, <formula formulatype="inline"><tex>$\hbox{H}_{2}$</tex></formula>, and observed possible evidence of creation of the by-products of the water-gas shift reaction. </para>

A Gradient Microarray Electronic Nose Based on Percolating SnO2 Nanowire Sensing Elements

Fabrication, characterization, and tests of the practical gradient microarray electronic nose with SnO(2) nanowire gas-sensing elements are reported. This novel device has demonstrated an excellent performance as a gas sensor and e-nose system capable of promptly detecting and reliably discriminating between several reducing gases in air at a ppb level of concentration. It has been found that, in addition to the temperature gradient across the nanowire layer, the density and morphological inhomogeneities of nanowire mats define the discriminating power of the electronic nose.

Gas sensing behaviour of mat-like networked tungsten oxide nanowire thin films

Here, we present the results of the resistive response of tungsten trioxidenanowire (mat-like, nanowire networks) and nanoparticle thin films subjected toN2O gas in the temperature range of 373–773 K. The nanowire mats exhibited an order ofmagnitude higher response in the resistivity change compared to that of nanoparticle filmsat temperatures above 523 K. Nanowire mats also exhibited relatively faster adsorption anddesorption times. Impedance spectroscopy studies showed that the gas sensing mechanismfor nanowire mats involves changes in both the nanowire and grain boundary resistances,whereas for nanoparticle films only the grain boundary resistance governs the sensorproperties upon exposure to gases.

Transport properties of hybrid nanoparticle–nanowire systems and their application to gassensing

Experimental data and theoretical modelling of theI–V characteristics of a gas sensor constructed from a mat of Au nanoparticle-coated GaNnanowires are presented. The principal mechanism for the response of the gas sensor tomethane is explained in terms of the formation of a depletion layer within thenanowires due to the presence of the gold nanoparticles. The depth of the depletionlayer is modulated by the potential induced by the physisorption of gas moleculesonto the Au nanoparticles. A statistical model of the temperature-dependentI–V characteristics of bare and Au nanoparticle-decorated mats of GaN nanowires based on Poisson’sequation has been used to determine the depth of the depletion layers of the nanowires. Theroom-temperature carrier concentration for the GaN nanowires was determined to be approximately2.2 × 1017 cm−3. The induced potential due to methane physisorption onto the Au nanoparticlesthat decorate the GaN nanowires was determined to be approximately−37 mV.

Principles and mechanisms of gas sensing by GaN nanowires functionalized with gold nanoparticles

Electrical properties of a chemical sensor constructed from mats of GaN nanowires decorated with gold nanoparticles as a function of exposure to Ar, N2, and methane are presented. The Au nanoparticle decorated nanowires exhibited chemically selective electrical responses. The sensor exhibits a nominal response to Ar and slightly greater response for N2. Upon exposure to methane the conductivity is suppressed by 50% relative to vacuum. The effect is fully reversible and is independent of exposure history. We offer a model by which the change in the current is caused by a change in the depletion depth of the nanowires, the change in the depletion depth being due to an adsorbate induced change in the potential on the gold nanoparticles on the surface of the nanowires.

Gas Sensing with Gold Nanoparticle-Decorated GaN Nanowire Mats

AbstractThe electrical properties of chemical sensors constructed from mats of bare GaN nanowires and GaN nanowires decorated with gold nanoparticles are presented. The sensors were tested in vacuum and at atmospheric pressures of Ar, N2 and methane. The current-voltage (I-V) curves of the sensor constructed with bare GaN nanowires were Ohmic and the device was insensitive to all gases tested. The I-V curves of the sensor constructed from GaN decorated with Au nanoparticles were non-linear and exhibited a drop in conductivity of five orders of magnitude relative to bare GaN nanowire sensors. The Au nanoparticle decorated nanowires also exhibited electrical responses that were chemically selective. The sensor exhibited a nominal response to Ar and a slightly greater response to N2 relative to vacuum. A suppression of the conductivity of the Au-GaN device of 50% was observed upon exposure to methane. Both the drop in conductivity of the Au-GaN nanowire-based sensor, relative to bare GaN nanowires, and the response to methane are explained in terms of the formation of a depletion layer and an increase in the depletion layer width due to physisorption induced surface potentials.