Density Of Nanowires Research Articles

Growth modulation of Ga2S3 horizontal nanowires and its optical properties

Highly ordered Ga2S3 horizontal nanowires (NWs) on r-plane sapphire were successfully grown by chemical vapor deposition using carbothermal reduction reaction. By adjusting experimental conditions, the density of NWs has increased from 8.4 × 106 to 4.0 × 107 mm−2. The morphology, structure, composition and optical properties of Ga2S3 NWs were characterized through field emission scanning electron microscopy, atomic force microscope, transmission electron microscopy, x-ray diffraction, Raman, x-ray photoelectron spectroscopy and so on. The Au particles at the end of the NWs prove that the growth of the horizontal NWs is controlled by the VLS mechanism. The measurement of nonlinear properties of Ga2S3 nanomaterials indicates one-dimensional Ga2S3 processes excellent nonlinear effect. Our work opens a new way to synthesize Ga2S3 NWs and provides a reference to explore the optical applications of Ga2S3 nanomaterials such as micro tunable laser in the future.

A Mask Based on a Si Epitaxial Layer for the Self-Catalytic Nanowire Growth on GaAs(111)B and GaAs(100) Substrates

GaAs nanowires (NWs) were generated on the surface of GaAs(111)B and GaAs(100) substrates from molecular fluxes by the self-catalytic growth method. A mask for NW growth was fabricated by oxidizing the epitaxial silicon layer that was grown on a substrate surface by the molecular beam epitaxy (MBE) method. Silicon was oxidized in purified air without moving the structures out of the vacuum system of the MBE apparatus. The process of Si/GaAs heterostructure oxidation was investigated using single-wave and spectral ellipsometry. The oxidized silicon surface morphology was studied by the atomic force microscopy methods. The scanning electronic microscopy method was used to examine the samples with NWs. The NW density was about 2.6 × 107 and 3 × 107 cm–2 for (111)B and (100), respectively.

Analytical modeling of orientation effects in random nanowire networks.

Films made from random nanowire arrays are an attractive choice for electronics requiring flexible transparent conductive films. However, thus far there has been no unified theory for predicting their electrical conductivity. In particular, the effects of orientation distribution on network conductivity remain poorly understood. We present a simplified analytical model for random nanowire network electrical conductivity that accurately captures the effects of arbitrary nanowire orientation distributions on conductivity. Our model is an upper bound and converges to the true conductivity as nanowire density grows. The model replaces Monte Carlo sampling with an asymptotically faster computation and in practice can be computed much more quickly than standard computational models. The success of our approximation provides theoretical insight into how nanowire orientation affects electrical conductivity.

Density-dependent of gas-sensing properties of Co3O4 nanowire arrays

Abstract Well-oriented Co3O4 nanowire arrays were synthesized in-situ on Al2O3 substrates via a simple hydrothermal method without seed layers. The phase structure and array morphology of Co3O4 nanowire arrays were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was revealed that the array density of Co3O4 nanowire arrays could be controlled by the concentration of ammonium fluoride. The gas-sensing measurement revealed that the array structure on the surface of gas sensors exerted great impact on their performances. It was found the response value enhanced and then decayed with the increased array density of Co3O4 nanowires. We concluded that the Co3O4 nanowire arrays with moderate array density can possess highly exposed effective surface area to provide more pathways for gas diffusion than other samples. Our studies can provide a significant guidance on array design to fabricate high performance gas sensors. Moreover, the as-designed Co3O4 nanowire arrays have potential application in trimethylamine (TEA) detection.

Маска на основе эпитаксиального слоя Si для самокаталитического роста нитевидных нанокристаллов на подложках GaAs (111)B и (100)

GaAs nanowire (NW) self-catalyzed growth on GaAs (111) B and GaAs (100) substrates was carried out by molecular beam epitaxy. A mask for the self-catalyzed NW growth was created by oxidizing an epitaxial silicon layer grown on the GaAs surface by molecular beam epitaxy (MBE). Silicon oxidation was realized in an atmosphere of purified air under normal conditions without moving the structures out from the vacuum system volume of the molecular beam epitaxy chamber. The oxidation process of a silicon layer was studied using single-wave and spectral ellipsometry and the surface morphology of oxidized silicon was studied by atomic force microscopy. Substrates with NWs were studied by scanning electron microscopy. The NW density was demonstrated to be 2.6•107 cm-2 and 3•107 cm-2 for (111)B and (100), respectively.

Interfacing human induced pluripotent stem cell-derived neurons with designed nanowire arrays as a future platform for medical applications.

Nanostructured substrates such as nanowire arrays form a powerful tool for building next-generation medical devices. So far, human pluripotent stem cell-derived neurons-a revolutionary tool for studying physiological function and modeling neurodegenerative diseases-have not been applied to such innovative substrates, due to the highly demanding nature of stem cell quality control and directed differentiation procedures to generate specialized cell types. Our study closes this gap, by presenting electrophysiologically mature human pluripotent stem cell-derived neurons on a set of nanowires in different patterns and growth densities after only four weeks of maturation-thereof 14 to 16 days on the nanowire arrays. While cell viability is maintained on all nanowire substrates, the settling regime of the cells can be controlled and tuned by the nanowire density from a fakir-like state to a complete nanowire wrapping state. Especially, full electrophysiological integrity of the neurons independent of the settling regime has been revealed by patch clamp experiments showing characteristic action potentials. Based on these results, our protocol has the potential to open new pathways in stem cell research and regenerative medicine utilizing human stem cell-derived neurons on tailor-made nanostructured substrates.

Comparison of ZnO nanowires grown on e-beam evaporated Ag and ZnO seed layers.

In this study, ZnO nanowires with diameters ranging from 50 nm to 500 nm have been synthesized hydrothermally on Ag and ZnO seed layers deposited by electron beam evaporation. ZnO nanowires grown on hetero and homo interfaces have been studied by comparing the growth characteristics of (a) ZnO nanowires on the Ag seed layer and (b) ZnO nanowires grown on the ZnO seed layer, respectively. The surface morphology of the as-evaporated seed layers before the nanowire growth has been investigated. Electron backscatter diffraction (EBSD) has been employed to examine the crystallinity of ZnO nanowires. In addition, the integrity of the Ag–ZnO heterointerface has been investigated using high-resolution transmission electron microscopy (HR-TEM). The length, diameter, density, and alignment of nanowires grown on Ag and ZnO seed layers have been studied as a function of growth time from 0.5 hours to 18 hours and precursor concentration from 5 mM to 18 mM. Furthermore, for both the Ag–ZnO nanowire heterostructure and ZnO–ZnO nanowire homostructure, the role of defects in the optical properties in the wavelength range of 517 nm to 900 nm has been studied using photoluminescence (PL) spectroscopy.

Effect of substrate temperature on GaAs nanowires growth directly on Si (111) substrates by molecular beam epitaxy

Abstract We have demonstrated growth of GaAs nanowires directly on Si (111) substrates by Ga-assisted technique using molecular beam epitaxy. In this work, Ga droplet forming and nanowire growth are performed at the same substrate temperature. Effect of substrate temperature while droplet forming with nanowire growth on the GaAs nanowire structural properties is investigated by Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD). Nucleation of the GaAs nanowires strongly depends on the substrate temperature. The results show that density, length, and diameter of the GaAs nanowires relate to the substrate temperature.

Gram-Scale Synthesis of Pt-Cu Nanowires with Enhanced Electrocatalytic Activity towards Methanol Oxidation Reaction

A facile method to prepare Pt-Cu nanowires (NWs) was introduced. Structural characterization such as high-resolution transmission electron microscope (HR-TEM), selected-area electron diffraction (SAED), EDS element mapping, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS) showed the formation of Pt-Cu alloy, with a width of 4.5 nm on average. The formation process of Pt-Cu NWs was studied; it was found that bromine ion, who has preferential adsorption on Pt (100) face, served as a growth-directing agent; Brij58 not only served as a protector but also played an important role in forming Pt-Cu NWs; the mechanism was proposed. Their electrocatalytic activity towards methanol oxidation was investigated; we found that the current density of Pt-Cu NWs was 295 mA·mg-1 when the ratio of Pt/Cu is 1 : 1, which is 11.5 and 2.35 times higher than that of pure Pt (26 mA·mg-1) and commercial Pt/C (126 mA·mg-1). The high electrocatalytic activity is attributed to the presence of abundant structural defects and surface active sites on the synthesized Pt-Cu NWs.

A facile two-step fabrication of titanium dioxide coated copper oxide nanowires with enhanced photocatalytic performance

Copper oxide (CuxO) nanostructures coated with the titanium dioxide (TiO2) are expected to improve the charge separation efficiency and photoactivity in the photosynthesis and pollutant treatment. In this study, as an alternative fabrication method, CuxO nanowires coated with TiO2 have been produced by thermal oxidation of the copper foil at different temperatures (400–600 °C) and times (2 and 4 h) in ambient conditions and subsequently, hydrolyzed in different molar concentrations (20, 40 and 80 mM) on CuxO nanowires by immersing and then heat treating the mixture of titanium (IV) isopropoxide and isopropyl alcohol. The characteristics of the CuxO/TiO2 nanostructures were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The results showed that the CuO layer was peeled off from the copper foil in the thermal oxidation process which was carried out at a temperature higher than 500 °C and for >2 h. The ideal temperature/time for high nanowire density and aspect ratio of CuO nanowire growth is 450 °C for 2 h. TEM - SAED diffraction pattern of TiO2 coating surface belongs to anatase crystal phase for all prepared samples. Thickness of TiO2 coating increased with increasing concentration but in the highest concentration (80 mM), Ti was not coated on the CuO nanowire and diffuses into the interior. For 20 mM and 40 mM concentration prepared CuxO/TiO2 nanostructures, measured coating thickness was about to 2 nm and 10 nm, respectively.Photocatalytic activity of CuxO/TiO2 catalysts was evaluated by measuring the rate of photodegradation reaction of Reactive Red (RR180) azo dyes under UVA-light. As a result, the CuxO/TiO2 catalyst which was subjected to thermal oxidation at 450 °C for 2 h and hydrolyzed in 40 mM solution shows the highest degradation rate of 35%. It is clearly stated that the degradation rate is highly dependent on nanowire density, nanowire aspect ratio and coated layer thickness.

Analysis of the role of inter-nanowire junctions on current percolation effects in silicon nanonet field-effect transistors

In this paper, we compare the evolution with temperature of the experimental characteristics of Nanonet-based Field-Effect Transistors, with a modelling of carrier transport in the percolating regime. The main electrical parameters of Nanonet-based Field-Effect Transistors that featured different nanowire densities and source-drain distances were extracted from static measurements at different temperatures. The temperature dependence of low field mobility and threshold voltage was explained by the temperature activated behaviour of inter-nanowire junctions, and the activation energy dispersion of individual junctions. A Monte-Carlo simulation of Nanonet FET in the shape of a random percolating network of resistances and thermally activated junctions was used to confirm the influence of activation energy dispersion on low field mobility. The simplest model which was able to capture experimental trends consisted in a bimodal distribution of activation energies, with a subset of non-thermally activated junctions (resistive junctions) while other junctions were thermally activated (energy barriers at the junctions).

MAPbI3 nanowires prepared by a facile vapor transfer method for simple photodetector

MAPbI3 nanowires have become one of the promising materials for optoelectronic devices. Herein, a simple template-free vapor transfer approach to synthesize MAPbI3 nanowires on various substrates for optoelectronic device developed. High crystalline MAPbI3 nanowires were realized via methyl-ammonium iodide (MAI) vapor transfer treatment with PbI2 nanowires. PbI2 nanowires were prepared by a self-assembled procedure where PbI2 film was recrystallized and converted to nanowires structure with the exposure of dimethylacetamide (DMAC) vapor. On top of that, the density of MAPbI3 nanowires could be modulated by simply tuning the thickness of PbI2 film. The as-fabricated MAPbI3 nanowires photodetector exhibits a satisfying responsivity of 0.115 A W−1 @3 V while the rise time is less than 0.63 s and the decay time is less than 0.73 s. The study presents a bright future of MAPbI3 nanowires for optoelectronic application.

Tuning Areal Density and Surface Passivation of ZnO Nanowire Array Enable Efficient PbS QDs Solar Cells with Enhanced Current Density

AbstractColloidal PbS quantum dots (QDs) have provoked a revolution in the field of optoelectronic devices owing to their low‐cost fabrication processing and excellent physical properties. Recently, the fabrication of nanostructured PbS QD photovoltaic (PV) devices based on zinc oxide (ZnO) nanowire array appears as an effective strategy for improving the overall device performance. Despite its potentially strong impact on the device performance, the role of nanowire areal density on photon absorption and exciton dynamics has not yet been studied and still remains unexplored. Here, for the first time, the areal density of ZnO nanowires is tuned through controlling the precursor concentration and its impact on PbS QD PV performance is studied. It is found that the device with optimized ZnO nanowire areal density yields significantly increased power conversion efficiency (PCE) (10.1% vs 8.5% of control nanowire‐based device) due to improved antireflection effect and reduced surface recombination states. To further improve the photovoltaic performance, the ZnO nanowire surface is treated with hydrogen plasma. Transient photovoltage (TPV) measurement reveals that this passivation process noticeably reduces the nonradiative charge recombination yielding a champion device with a remarkable PCE of 10.8%.

Novel fabrication for vertically stacked inverted triangular and diamond-shaped silicon nanowires on (1 0 0) single crystal silicon wafer

The vertical integration of multiple silicon nanowires (multi-SiNWs) has an outstanding ability to enhance drive current, sensitivity and noise immunity by maximizing nanowire density. However, the crystal plane of the SiNW prepared by the present technology for the vertical stacked integration is indeterminate and has plasma damage. In this paper, a novel fabrication method for vertically stacked SiNWs with inverted triangular and diamond-shaped cross-sections on (1 0 0) single crystal silicon wafer is developed and presented, using conventional micromachining processes. The fabrication is based on the crystal plane distribution and anisotropic etching characteristics of (1 0 0) single crystal silicon. An iteration process of self-aligned dry etching and wet etching is first designed to form vertically stacked triangular and diamond-shaped silicon columns. After thermal oxidation thinning and removal of the oxide layer, the vertically stacked triangular and diamond-shaped SiNWs without plasma damage are obtained in the double silicon columns. The vertically integrated approach not only precisely controls the position and shape of the SiNWs but also determines the crystal plane and orientation of the SiNWs. In addition, the triangular and diamond-shaped SiNWs have a larger surface-to-volume ratio than circular and rectangular-shaped SiNWs with the same cross-sectional area and the same length. Therefore, this method can provide a novel, controllable, and economical way to vertically stack more SiNWs with larger surface-to-volume ratio into one chip for high-performance device applications.

Design of AlGaN/AlN Dot‐in‐a‐Wire Heterostructures for Electron‐Pumped UV Emitters

This article describes the fabrication of nitrogen‐polar AlxGa1−xN/AlN (x = 0, 0.1) quantum dot (QD) superlattices (SLs) integrated along GaN nanowires (NWs) for application in electron‐pumped UV sources. The NWs are grown using plasma‐assisted molecular beam epitaxy on n‐type Si(111) wafers using a low‐temperature AlN nucleation layer. Growth conditions are tuned to obtain a high density of noncoalesced NWs. To improve the uniformity of the height along the substrate, the growth begins with a long (≈900 nm) NW base, with a diameter of 30–50 nm. The AlxGa1−xN/AlN active region is 400 nm long (88 periods of QDs), long enough to collect the electron–hole pairs generated by an electron beam with an acceleration voltage ≤5 kV. The spectral response is tuned in the 340–258 nm range by varying the dot/barrier thickness ratio and the Al content in the dots. Internal quantum efficiencies as high as 63% are demonstrated.

Ni, Co-double hydroxide wire structures with controllable voids for electrodes of energy-storage devices

We report the facile synthesis of Ni, Co-double hydroxide wire (NCHW)-based electrodes directly grown on a conductive substrate via a hydrothermal process. Various NCHW nanostructures were grown on Ni foam, and the growth was controlled using different compositions of solvents (ethanol and water). With increasing volume ratio of ethanol to water, the density of the wires decreased, and the spatial voids between the wires increased. The formation of large empty spaces improved the electrochemical performance because the exposure of a large surface area of the structure to the electrolyte resulted in a large number of active sites and facile electrolyte penetration into the structure. The different NCHW structures were ascribed to the pivotal role of the solvent in the urea hydrolysis; the solvent triggered the formation of hydroxides during the hydrothermal synthesis. The electrochemical performance of the NCHW electrodes was investigated via galvanostatic charge/discharge tests, cyclic voltammetry, and electrochemical impedance spectroscopy. The highest specific capacitance was 1694.7 m F/cm2 at 2 mA/cm2, with excellent capacitance retention of 81.5% after 5000 cycles. The superior electrochemical performance of the NCHW electrodes is attributed to the large number of active sites and facile electrolyte diffusion into the structure, due to the well-organized structure with an optimized density of nanowires and large voids between the wires.

Post-deposition annealing effect on the structural, morphological, and photoluminescence properties of β-Ga2O3 nanowires deposited on silicon by glancing angle deposition

β-Ga2O3 nanowire (NW) was fabricated on a Si-substrate using glancing angle deposition (GLAD) technique. To study the structural, morphological, and optical properties of the as-deposited β-Ga2O3 NW, thermal annealing was done at different temperatures ranging from 600 to 1000 °C and the corresponding results were analyzed. XRD analysis shows an increase in crystallinity of the as-deposited sample upon annealing. The average crystallite size was found to be increased from 13.72 to 20.19 nm after annealing at 900 °C. Annealed β-Ga2O3 NW contains more concentration of oxygen due to absorption of O2 molecules. The lattice strain and dislocation density of the as-deposited β-Ga2O3 NW were found to reduce significantly after thermal annealing. The FEG-SEM image confirms the growth of vertically aligned β-Ga2O3 NW. An enhancement in the UV region was observed in the photoluminescence spectra after annealing at 900 °C.

Gallium-assisted growth of InSb nanowire

Indium antimony (InSb) nanowires have been synthesized by chemical vapor deposition and we found that adding gallium as the other evaporation resource can increase the density of nanowires and no doping pollution. For the growth of InSb nanowire, Au film was annealed to form Au nanoparticles as catalysts and explain its catalytic principle. We thought that gallium which coated on the surface of Au nanoparticles assisted nucleation and growth of InSb nanowire in the early stage. The diameter of the InSb nanowires was 60–100nm and 1-5μm in length. The grown nanowires have good crystallinity. We found that the surface of InSb was oxidized, and the main oxide was indium oxide. We discovered the tip morphologies of nanowires are different and discussed the causes of this phenomenon in detail.

Effect of annealing parameters and activation top layer on the growth of copper oxide nanowires

The present work focuses on the study of annealing parameters and activation top layer on the growth of copper oxide nanowires. The copper films of varying thickness (60–800 nm) were synthesized on SiO2/Si substrate by DC magnetron sputtering and the samples annealed in terms of different parameters, especially the optimized annealing temperatures from 300 to 700 °C for 2–8 hrs to understand the growth mechanism and to optimize parameters for the nanowire formation. Furthermore, a thin conductive gold film as an activation top layer enhances the density and aspect ratio of the copper oxide nanowires. The orientation of crystal planes was characterized by XRD and the nanowires growth after annealing was characterized by SEM. The changes in the film were analyzed by the Raman spectroscopy.

Catalytic Growth of Gallium Nitride Nanowires on Wet Chemically Etched Substrates by Chemical Vapor Deposition.

Growth of gallium nitride nanowires on etched sapphire and GaN substrates using binary catalytic alloy were investigated by manipulating the growth time and precursor-to-substrate distance. The variations in behavior at different growth conditions were observed using X-ray diffractometer, Raman spectroscopy, X-ray photoelectron spectroscopy, cathodoluminescence spectroscopy, optical microscopy, atomic force microscopy, and scanning electron microscopy. It was noticed that, in respect of both the substrates, when growth time and/or precursor-to-substrate distance is increased, thickness of the nanowires around the etch pits remains unaltered, but there is variation in the density of nanowires. In addition, formation of gallium nitride microwires within the etch pits was also observed on etched sapphire substrates. Similarly, the thickness and density of the microwires were found to increase with increase in growth time and decrease with increase in precursor-to-substrate distance. The dimensionality scaling of gallium nitride was found to have a positive effect in improving the luminescence property and band gap of the grown nanowires. This method of nanowire growth can be helpful in increasing the probability of multiple reflections in the materials which makes them a suitable candidate for optoelectronic devices.