Length Of Nanowires Research Articles

Thermal decomposition of GaAs nanowires

The realization of GaAs nanowire (NW) high-performance quantum devices operated at room temperatures requires that their diameters have to be less than 10 nm. It is shown, that the GaAs NWs with sub 10 nanometers diameters can be fabricated using the thermal decomposition technique. It is demonstrated, that depending on annealing conditions, the NW lengths, as well as shapes, can be modified significantly. The GaAs NWs with bottle-like and diameter-modulated shapes can be obtained. At the first stage of the thermal annealing in the presence of As flux, an increase in NW length was found.

Direct writing of silver nanowire-based ink for flexible transparent capacitive touch pad

Printed electronics is an emerging field involving the fabrication of electronic devices by the patterned deposition of material inks. For many systems, producing stable printable inks is the key challenge. In this work, the formulation of a silver nanowire-based ink for printed transparent electrode applications is described. The nanowire length and rheology of the ink are adjusted for printing, with a single layer printed film having a sheet resistance of approximately 30 Ω/□ and a transmittance of 94% at 550 nm. The number of printed layers and volume per layer are optimized to get maximum transparency with good electrical conduction. A transparent capacitive touch pad, in the form of a 2 × 2 matrix is implemented, using this ink and with PDMS as the dielectric. The touch pad has a high degree of flexibility with a resistance variation less than 2% after 10 000 bending cycles. The formulated nanowire ink can be extended for other flexible and stretchable transparent sensing applications.

Wide spectral photoresponse of template assisted out of plane grown ZnO/NiO composite nanowire photodetector

Zinc oxide (ZnO) one-dimensional nanostructures are extensively used in ultra-violet (UV) detection. To improve the optical sensing capability of ZnO, various nickel oxide (NiO) based p–n junctions have been employed. ZnO/NiO heterojunction based sensing has been limited to UV detection and not been extended to the visible region. In the present work, p-NiO/n-ZnO composite nanowire (NW) heterojunction based UV-visible photodetector is fabricated. A porous anodic aluminum oxide template based electrochemical deposition method is adopted for well separated and vertically aligned growth of composite NWs. The photoresponse is studied in an out of plane contact configuration. The fabricated photodetector shows fast response under UV-visible light with a rise and decay time of tens of ms. The wide spectral photoresponse is analyzed in terms of conduction from defect states of ZnO and interfacial defects during p–n junction formation. Light interaction with heterojunction along the length of the composite NW results in enhanced visible photoresponse of the detector and is further supported by simulation.

Fitting thermal conductivity and optimizing thermoelectric efficiency in [formula omitted] nanowires

We consider a thermoelectric energy generator constituted by a Si∕Ge nanowire of length L. The dependence on composition and temperature of its thermal conductivity is analyzed in view of three series of experimental data obtained at the constant temperatures T=300K, T=400K, and T=500K. The best-fit curve is determined by a nonlinear regression method (NLRM). Then, under the hypothesis of nonlinear constitutive equation for the heat flux, we investigate the thermoelectric efficiency of the system as function of the composition of the nanowire and of the difference of temperature applied to its ends. For each temperature we calculate the value of the composition which realizes the optimal efficiency of the thermoelectric energy conversion. The corresponding value of the thermal conductivity is determined as well.

Does desorption affect the length distributions of nanowires?

State-of-the art models for statistical properties within the nanowire ensembles consider influx of precursors, reflection and surface diffusion of adatoms. These models predict a delay in the nanowire growth start and the evolution toward an asymmetric length distribution. We demonstrate here the effect of desorption of the nanowire material, which has not been considered so far in studies of the nanowire length distributions. We show that at the very beginning of growth the length distribution should be asymmetric due to the slow nucleation of nanowires. At longer times, the length distribution acquires a symmetric Gaussian shape due to the increased weight of desorption. The width of this distribution is larger than Poissonian and increases for higher ratio of desorption over deposition rate. Our model is consistent with the length evolution of organized self-catalyzed GaAs nanowires. We outline that desorption of the nanowire material should be minimized to achieve arrays of highly identical nanowires. These results are relevant for a wide variety of material systems.

Unravelling the strain relaxation processes in silicon nanowire arrays by X-ray diffraction

Investigations performed on silicon nanowires of different lengths by scanning electron microscopy revealed coalescence processes in longer nanowires. Using X-ray diffraction (XRD), it was found that the shape of the pole figure in reciprocal space is ellipsoidal. This is the signature of lattice defects generated by the relaxation of the strain concentrated in the coalescence regions. This observation is strengthened by the deviation of the XRD peaks from Gaussianity and the appearance of the acoustic phonon mode in the Raman spectrum. It implies that bending, torsion and structural defects coexist in the longer nanowires. To separate these effects, a grazing-incidence XRD technique was conceived which allows the nanowire to be scanned along its entire length. Both ω and φ rocking curves were recorded, and their shapes were used to extract the bending and torsion profiles, respectively, along the nanowire length. Dips were found in both profiles of longer nanowires, while they are absent from shorter ones, and these dips correspond to the regions where both bending and torsion relax. The energy dissipated in the nanowires, which tracks the bending and torsion profiles, has been used to estimate the emergent dislocation density in nanowire arrays.

Structure and Electronic Properties of InSb Nanowires Grown in Flexible Polycarbonate Membranes.

A dense array of vertically aligned indium antimonide (InSb) nanowires with high aspect ratio (diameter 150 nm, length 20 m) were grown in the pores of a track-etched polycarbonate membrane via a one-step electrochemical method. There are several reports on InSb nanowire growth in the pores of a mechanically rigid, nano-channel alumina template (NCA), where nanowire growth occurs in the pores of the NCA. This work on InSb nanowire growth in pores of track-etched polycarbonate (PC) membrane sheds light on the various factors that affect nucleation and nanowire growth. The average length and diameter of the as-grown nanowires was about 10 m and 150 nm, respectively. Two possible mechanisms accounting for two different morphologies of the as-grown nanowires are proposed. The polycrystallinity observed in some of the nanowires is explained using the 3D ‘nucleation-coalescence’ mechanism. On the other hand, single crystal nanowires with a high density of twin defects and stacking faults grow epitaxially by a two-dimensional (2D) nucleation/growth mechanism. To assess the electrical quality of the nanowires, two- and four-terminal devices were fabricated using a single InSb nanowire contacted by two Ni electrodes. It was found that, at low bias, the ohmic current is controlled by charge diffusion from the bulk contacts. On the other hand, at high bias, the effects of space charge limited current (SCLC) are evident in the current–voltage behavior, characteristic of transport through structures with reduced electrostatic screening. A cross-over from ohmic to SCLC occurs at about 0.14 V, yielding a free carrier concentration of the order of cm.

(Invited) Effect of Junction Resistance on the Percolation Conductivity of Metal Nanowire Networks for Transparent Conductors

Random networks of one-dimensional (1D) nanoelements, such as carbon nanotubes, graphene nanoribbons, and metal nanowires, have attracted significant research interest recently for next-generation transparent conductors. As a result, these networks can be used in many device applications, such as touch screens, flat panel displays, solar cells, light-emitting diodes (LEDs), and wearable flexible electronics. Among these nanomaterials, metal nanowire networks, such as silver and copper, exhibit high optical transmittance, low sheet resistance, mechanical flexibility, and solution-based fast deposition. These unique properties make metal nanowire networks promising candidates to replace indium tin oxide (ITO), which suffers from brittleness, scarcity, high cost, and slow deposition. At high optical transmittance values required for transparent conductors, the conductivity of metal nanowire networks is governed by percolation transport, which deals with the formation of long-range connectivity in random networks. As a result, Monte Carlo simulations need to be employed to theoretically calculate, predict, and optimize the conductivity of metal nanowire networks [1,2]. Two types of resistance contributions determine the overall conductivity of the nanowire network. The first is the nanowire-nanowire junction resistance and the second is the resistance of the nanowire itself. In most Monte Carlo simulations, it is assumed that the nanowire-nanowire junction resistance is much larger than the nanowire resistance, resulting in a junction resistance-dominated network [2-4]. Although this is the case for carbon nanotube networks, recent experiments have shown that, for metal nanowire networks, the junction resistance can be significantly lowered by post-deposition treatments, such as thermal annealing, plasmonic welding, joule heating, and mechanical pressing. In such cases, the nanowire resistance becomes comparable to the junction resistance and it can no longer be ignored. When the junction resistance becomes much lower than the nanowire resistance, the network becomes nanowire resistance-dominated, which is the lowest resistance limit for a given set of nanowire and device parameters. Despite the significant role the junction resistance plays in determining the percolation conductivity of metal nanowire networks, a systematic study of the effect of nanowire-nanowire junction resistance on the network conductivity as a function of other nanowire and device parameters is currently lacking. In this work, we perform systematic Monte Carlo simulations to study the effect of the nanowire-nanowire junction resistance on the conductivity and percolation critical exponents of metal nanowire networks. We define a parameter called the resistance ratio, which is the ratio of the nanowire-nanowire junction resistance to the nanowire resistance per unit length. We perform Monte Carlo simulations of the nanowire network conductivity by varying the resistance ratio over a span of six orders of magnitude, ranging all the way from a junction resistance-dominated to a nanowire resistance-dominated network. First, we study the effect of the resistance ratio on the nanowire network conductivity at different values of the other nanowire and device parameters, such as nanowire density, nanowire length, device length, device width, nanowire alignment, and nanowire curviness. Next, we study the effect of the resistance ratio on the percolation critical exponents. Approaching the percolation threshold, conductivity exhibits a power law dependence on nanowire and device parameters, which is characterized by percolation critical exponents. We first extract the critical nanowire density, nanowire length, device width, and alignment angle at the percolation threshold by calculating the percolation probability as a function of each variable. Then, we extract the critical exponents as a function of the resistance ratio for nanowire density, nanowire length, device width, nanowire alignment, and nanowire curviness. We find that the resistance ratio plays a crucial role in determining both the conductivity and the percolation critical exponents of metal nanowire networks. This study illustrates how the junction resistance affects the macroscopic conductivity of transparent, conductive metal nanowire networks. It also shows that Monte Carlo simulations are an essential predictive tool for providing insights into percolation transport and optimizing the electronic properties of these networks. These results are not limited to metal nanowire networks, but can be extended to any two-dimensional (2D) or quasi-2D network, film, or nanocomposite consisting of 1D nanoelements.

Using non-continuous gold film as the seed layer for developing zinc oxide nanowire array

This study aims to discuss the grown morphology and current generation of ZnO nanowire array developed from non-continuous Au thin film as the seed and compare the results with the nanowire array from ZnO thin film as the seed layer. The zinc nitrate hexahydrate and hexamethylenetetramine used as the reaction solution and the hydrothermal process with process temperature less than 95°C are applied. It is revealed that ZnO nanowires developed with non-continuous Au film as the seed appear sparser density, present better crystallization direction (002) and higher collimation. The effect of concentration of reaction solution on the diameter, length and distribution of nanowires is reported. The electron back scattering diffraction technique confirms each grown wire is a single crystal from the bottom to top. The ZnO nanowires with piezoelectric property are measured with the conductive atomic force microscope system. The result shows that single ZnO nanowire could generate more than 800 pA current when being bent.

(Invited) Electrodeposition of Au/Cu2o Core-Shell Nanowire Arrays As Photocathode Model Systems for Solar Hydrogen Production

In the past decades, research on the synthesis and characterization of semiconductor nanowires has considerably progressed, especially for applications in the field of energy conversion, such as photoelectrochemical water splitting for hydrogen production. The dimensions, geometry, and areal density of nanowires significantly influence physical processes such as light absorption, electron-hole generation and scattering, as well as recombination of the charge carriers. Among the available fabrication methods, electrodeposition in etched ion-track membranes enables the controlled synthesis of three-dimensional assemblies of functional nanostructures with tailored composition, size and density. We present here the fabrication and characterization of Au/Cu2O core-shell nanowire arrays consisting of a metallic core acting as nanostructured electrical contact, and a well-defined semiconductor shell responsible for light absorption and charge carrier generation and transport. The metallic Au core wires are first fabricated by electrodeposition in etched ion-track membranes. Diameter, length, and number density of the nanowires are adjusted by the membrane parameters. After dissolving the polymer membrane, the Au nanowire arrays are conformally coated by a Cu2O layer by electrodeposition. The thickness of the semiconductor layer is controlled by the deposition time. To increase their chemical stability in aqueous solutions the photoelectrodes are additionally coated with a thin TiO2 film by atomic layer deposition. The photoelectrochemical performance of these Au/Cu2O/TiO2 nanowire-based photoelectrodes will be presented, in particular, as a function of nanowire length, diameter, and areal density. The influence of the nanowire geometry and the core-shell arrangement on the generated photocurrent will be discussed in detail and compared with the photocurrents generated by planar Cu2O/TiO2 films.

Effect of Extrusion Temperature on the Microstructure and Mechanical Properties of SiCnw/2024Al Composite.

In the present research work, the effect of extrusion temperature from 480 to 560 °C on the microstructure and mechanical behavior of the SiCnw/2024Al composite (15 vol.%) has been explored. It has been found that extrusion at higher temperature (above 520 °C) was beneficial for the densification of the composite, while the residual average length and alignment of the SiC nanowires were also increased with the extrusion temperature. Moreover, higher extrusion temperature was helpful for the mechanical strength of the SiCnw/2024Al composite, and the peak-aged SiCnw/2024Al composite extruded at 560 °C revealed the highest strength (709.4 MPa) and elastic modulus (109.8 GPa).

Electrodeposition of equiatomic FeNi and FeCo nanowires: Structural and magnetic properties

In the present work, equiatomic FeNi and FeCo magnetic nanowires (NWs) have been electrodeposited in Anodic Aluminum Oxide (AAO) membranes with 70 nm pore size. The role of the electroactive species Ni2+ and Co2+ on the anomalous codeposition behavior in thin films and nanowires has been discussed and compared. Field Emission Gun-Scanning Electron Microscope (FEG-SEM), X-ray diffraction (XRD) and Vibrating Sample Magnetometer (VSM) were used to evaluate the morphology, composition, crystalline structure and magnetic properties of the materials. SEM-FEG analysis showed very homogeneous NWs lengths for both FeNi and FeCo NWs, Energy dispersive X-ray spectroscopy (EDS) analysis revealed a stable composition along the FeCo NWs and a change in composition along the FeNi NWs. The XRD patterns indicate the predominance of the bcc structure for both alloys. Except for additional peaks of fcc phase found on FeNi alloy, the same crystalline bcc structure is observed after annealing treatment of 300 °C during 20 h. TEM analysis confirmed the ordered character of the bcc FeCo alloy. The hysteresis loops show an easy magnetization direction parallel to the wires due to the predominance of the shape anisotropy. The coercive field (Hc) of FeNi NWs increases with the length of the wires while that of FeCo NWs decreases. The annealing treatment doesn’t affect the magnetic properties of FeNi NWs, whereas a remarkable increase in the coercivity of the FeCo NWs is observed.

Fabry–Perot type resonant modes of exciton luminescence in Cu2O nanowires

Single Cu2O nanowires (NWs) were fabricated by the two-step method we reported previously (Wang et al 2014 RSC Advances 4 37542). Band-edge exciton photoluminescence (PL) was observed from individual NWs at room temperature using excitation at 325 nm. The PL signals were assigned to Fabry–Perot (F-P) type standing waves in a right cylindrical dielectric cavity (resonator) for a representative range of different wire lengths and diameters. We found that the mode spacing of F-P resonances varied inversely as the NW length as expected. For the region of NW diameters from 140 to 200 nm, and NW length between 2–5 mm, E-field simulations by COMSOL Multiphysics finite element analysis indicate that the main F-P mode propagating inside the NW is the HE11 mode. When the diameter exceeds 200 nm, there are at least two F-P type modes supported in the NWs. Our results further the understanding of exciton photoluminescence in Cu2O NWs and demonstrates the existence of enhanced mode frequencies based on the geometry of the optical micro-cavity. We further identify potential applications in exciton-driven optoelectronic devices and light emission enhanced by optical micro-cavities.

Modeling of spatial growth of SnO2 nanowires on silicon substrates by VLS technique

The growth of SnO2 nanowires on silicon substrates using a vapor-liquid-solid (VLS) technique has been theoretically modeled. Since VLS is a catalyst-based method, we have initially modeled the gold droplet using a modified Young’s equation to be more suitable for SnO2 nanowires. Apart from the droplet geometry, the flow profile of the source vapor can also affect the overall formation of nanowires. This effect is more predominant for solid sources of SnO vapor where highly variable density and length of nanowires are observed. To understand this complex phenomenon, we describe a spatial growth model to predict the observed profile of SnO2 nanowires at different locations on silicon substrates. The model is based on gas transportation over the nanosized catalyst droplets, leading to a gradual reduction in both the length and density of the grown wires. This theoretical model is applied to a randomly patterned distribution of gold catalysts, and the results are consistent with the experimental findings. Since gold is used as the growth catalyst for tin oxide nanowires, the parameters of gold droplets atop silicon nanowire and Young’s angle are extracted.

Electrical energy generation from nanowire structures

This work numerically estimate the amount of electric voltage generation from Zing oxide, Barium titanium, Gallium nitride, Lead zirconate titanate (ZnO, BaTiO3, GaN and PZT) nanowire structure respectively and other hetro-structure or doped structure like (ZnMgO, GaAlN) having rectangular and circular cross sections with dimension of length and width (l , b) 600,50 nanometers. The amount of the generated voltage seem enough to power biosensor, nanosensor, or even biomedical device. The effect of nanowire length, width and temperature and the effect of applied force are studied, All theses parameters having profound effect on the amount of the maximum generated voltage. ةيونانلا كلاسلاا بيكارت نم ةيئابرهكلا ةقاطلا ديلوت

Controlled Hydrothermal Synthesis of K<sub>2</sub>Ti<sub>6</sub>O<sub>13</sub> Nanowires and their Photo-Electronic Response

A hydrothermal method using tetrabutyl titanate and KOH as reactants for synthesis of K2Ti6O13 nanowires was developed, obtaining nanowires with a uniform diameter around 10nm. It is shown that the hydrothermal temperature and the KOH concentration have little influence in tuning the growth of K2Ti6O13 nanowires. By changing the time for hydrothermal reaction, the length of K2Ti6O13 nanowires can be controlled from several dozen of nanometers to several hundreds of nanometers. The as-prepared K2Ti6O13 nanowires exhibit a wide and strong absorption band in the ultraviolet range (around 200~300 nm), and stable photocurrent of 0.5μA/cm2, which might suggest potential applications in solar cell and water splitting.

Controlled Growth of Highly Aligned Cu Nanowires by Pulse Electrodeposition in Nanoporous Alumina.

Metallic nanowire networks are emerging as potential replacements for transparent conducting oxide coatings because of their high conductivity, flexibility and relative transparency. However, a cheap, reliable and controlled manufacturing process is required to exploit this and the surface of the copper nanowire needs to be protected if high conductivity is to be retained. In this study a fabrication method for highly aligned and densely packed copper nanowires with controlled length using pulse-electrodeposition and a nanoporous alumina template has been developed. Nanoporous alumina was obtained by anodisation of pure aluminum in oxalic acid using a two-step anodisation process. In order to provide the conductivity at the bottom of the pores, a dendritic structure at the interface was created through the stepwise voltage reduction method with a voltage reduction rate of 15 V/s followed by mild chemical etching. Highly repeatable near 100% filling of copper is achieved. Copper nanowire length was highly controllable from 100 nm to 2 μm with a fixed diameter of 60 ± 5 nm by monitoring current density during the deposition. Such controlled growth of Cu nanowires could lead towards transparent conducting layer applications but the protection of the material against oxidation remains an issue.

In Ukrainian

The purpose of this article is to present theoretical calculations of the optical spectra of periodic silicon nanostructures dependent on their length. For calculations, the structure with silicon nanowires with a constant diameter and period was simulated. The diameter of the nanowires is 80 nm, and the structure period is 100 nm. The dependence of absorption, reflection and transmission spectra on the length of nanowires (500–5000 nm) was investigated. For theoretical studies, the Maxwell equation was solved by the finite difference method in the time domain (FDTD). This method can be applied precisely to general electromagnetic structures, including free-form particles. The advantage of this method is the simplicity and the capability to obtain results for a wide range of wavelengths in single calculation, as well as the capability to specify the properties of materials at any point of the calculation grid, which allows us to consider anisotropic, dispersive and nonlinear environments. At the same time, the FDTD method can be very resource-intensive, especially when simulating complex structures. This method requires from 10 to 30 points per wavelength, and small wavelengths determine the very high sampling frequency. This leads to cumbersome calculations, especially in three dimensions. To simplify calculations, the problem was carried out in two-dimensional form. It is shown that for these parameters of the structure, the reflection coefficient does not depend on the length of the nanowires, although by 30 % it is smaller than that in the solid silicon plate. The transmission coefficient decreases with the increase in the length of the nanowires, although at all calculated wavelengths it remains higher than that in the silicon wafer. It is shown that in the visible region of the spectrum, the absorption coefficient is significantly higher and with the increase of the length of nanowires, an expansion of absorption spectra is observed, indicating an increase in the absorption range of sunlight. It is shown that the use of silicon nanostructures as solar cells is an important and perspective direction of research.

Carrier Recombination in the Base, Interior, and Surface of InAs/InAlAs Core-Shell Nanowires Grown on Silicon.

We report on carrier recombination within self-catalyzed InAs/InAlAs core-shell nanowires (NWs), disentangling recombination rates at the ends, sidewalls, and interior of the NWs. Ultrafast optical pump-probe spectroscopy measurements were performed from 77-293 K on the free-standing, variable-sized NWs grown on lattice-mismatched Si(111) substrates, independently varying NW length and diameter. We found NW carrier recombination in the interior is nontrivial compared to the surface recombination, especially at 293 K. Surface recombination is dominated by carrier recombination at the NW sidewall, while contributions from the highly strained, impure NW base are negligible.

Crystal phase control in self-catalyzed InSb nanowires using basic growth parameter V/III ratio

In this work, the influence of V/III ratio on self-catalyzed InSb nanowire (NW) crystal structure is investigated using metal organic chemical vapor deposition. The effective balance of V/III ratio determines the change of InSb NW crystal structure from zincblende to wurtzite. Particularly, as group-V molar flow varies from 6.4 × 10−5 to 8.0 × 10−5 mol/min, wurtzite segment length of InSb NW increases from 16 ± 9 nm to 89 ± 9 nm, whereas zincblende crystal phase is observed for other molar flows. We interpret the observed phase transition to a lower surface energy of In-Sb alloy droplet than pure In-droplet, which makes triple phase line nucleation energetically favorable. This study gives insight on single growth parameter (V/III ratio) change to the InSb NW crystal control, such a technique is essential to selectively tune the crystal phase of single NW for various applications.