Vertically-aligned ZnO Nanowires Research Articles

ZnO-NWs/metallic glass nanotube hybrid arrays: Fabrication and material characterization

In this work, we fabricated first-ever nanohybrids ZnO nanowires (ZnO-NWs)/Metallic Nanotube Arrays (MeNTA) structure for the large-scale fabrication of highly dense laterally and vertical aligned ZnO-NWs. To prepare heterogeneous surface MeNTA by using sputter-deposition of different metallic glasses (Zr55Cu30Al10Ni5, Cu47Zr42Al7Ti4, Pd72Cu12Si16, W50Ni25B25 and Ni54B0.3Si36Cr2.7Fe7) over on contact-hole arrays created by a photoresist template with 650 nm height and various diameter (2, 10 and 20 μm). A ZnO seed layer is deposited by sputtering method on MeNTA to provide the nucleation sites to grow the nanowires inside the MeNTA. To perform the material analysis like surface morphology, crystal structural, and optical properties were characterized by using scanning electron microscopy, X-ray diffraction, Raman, XPS and photoluminescence, respectively. Our results confirm that, the nanowires having the hexagonal wurtzite structure and presenting strong UV emissions can be used in optoelectronics, sensors, piezoelectric-nanogenerator, solar cells and battery applications.

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.

The large response current of a vacuum pressure sensor based on a vertically-aligned ZnO nanowires array

A vertically-aligned ZnO nanowires (VA-ZnO-NWs) array was prepared via chemical vapor deposition, which was used to fabricate a vacuum pressure sensor and its sensitive characteristics were measured using a semiconductor parameter tester.

Piezo-Phototronic Enhanced UV Sensing Based on a Nanowire Photodetector Array.

A large array of Schottky UV photodetectors (PDs) based on vertical aligned ZnO nanowires is achieved. By introducing the piezo-phototronic effect, the performance of the PD array is enhanced up to seven times in photoreponsivity, six times in sensitivity, and 2.8 times in detection limit. The UV PD array may have applications in optoelectronic systems, adaptive optical computing, and communication.

Growth And Field Emission Properties Of Vertically-aligned ZnO Nanowire Array On Biaxially Textured Ni-W Substrate By Thermal Evaporation

Vertically well aligned and highly dense ZnO nanowires (NWs) have been grown on biaxially textured Ni substrates by a simple thermal evaporation technique over a large area without using any catalyst. The grown ZnO NWs have crystallized in wurtzite hexagonal structure and have grown along [0001] direction. It is also observed that the degree of vertical alignment of NWs increases with increasing growth temperature. An intense photoluminescence peak at 383 nm with a negligible deep band emission revealed the good crystalline quality of ZnO NWs. Field emission properties of the grown NWs have been examined and a field enhancement factor of 1573 has been obtained, indicating the suitability of grown nanowires for field emission applications. Copyright © 2015 VBRI Press.

Growth kinetics and wettability conversion of vertically-aligned ZnO nanowires synthesized by a hydrothermal method

We report here the first study of the growth kinetics of vertically-aligned ZnO nanowire arrays grown on Al-doped ZnO (AZO) seed layer-coated substrates by a hydrothermal method.

High Density ZnO Nanowire Arrays for Dye-Sensitized Solar Cells Fabrication

Dye-sensitized solar cells (DSSCs) have been extensively studied for the past few years because of their low-cost and high-efficiency. The surface area of the photoanode, which determines the amount of photons absorbed, is one of the most critical factors to improve the efficiency of DSSCs. Although conventional TiO2 nanoparticle-based DSSCs have shown promising results, their performance has been limited to ~11% because of diffusion and back reaction problems. Due to its excellent band gap (3.37 eV) and high exciton binding energy (60 meV) properties, one-dimensional ZnO nanowire arrays are capable of overcoming the limitation of nanoparticle-based DSSCs by improving electron injection and axial electron diffusion. Based on models proposed by our group, the efficiency of DSSCs based on TiO2 core-shell ZnO nanowire arrays (40 µm in lengths, 40 nm in diameter and ~1011 /cm2 of density) can significantly surpass current efficiencies. To reach our objective, we focused on pH value control and using two-step anodized aluminum oxide (AAO) as a template to control the dimension of ZnO nanowire arrays. The synthesis was carried out using both hydrothermal and CVD methods. SEM, TEM, and XRD were applied to characterize these ZnO nanowire arrays. This technique provides a new approach to obtain vertically-aligned ZnO nanowire arrays with both high density and aspect-ratio through a large-scale process. Keywords: Dye sensitized solar cell (DSSC), ZnO Nanowire Arrays, AAO Template, Hydrothermal, CVD

Synthesis of High Density ZnO Nanowire Arrays for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have been extensively studied for the past few years because of their low-cost and high-efficiency. The surface area of the photoanode, which determines the amount of photons absorbed, is one of the most critical factors to improve the efficiency of DSSCs. Although conventional TiO2 nanoparticle-based DSSCs have shown promising results, their performance has been limited to ~11% because of diffusion and back reaction problems. Due to its excellent band gap (3.37 eV) and high exciton binding energy (60 meV) properties, one-dimensional ZnO nanowire arrays are capable of overcoming the limitation of nanoparticle-based DSSCs by improving electron injection and axial electron diffusion. Based on models proposed by our group, the efficiency of DSSCs based on TiO2 core-shell ZnO nanowire arrays (40 µm in lengths, 40 nm in diameter and ~1011 /cm2 of density) can significantly surpass current efficiencies. To reach our objective, we focused on pH value control and using two-step anodized aluminum oxide (AAO) as a template to control the dimension of ZnO nanowire arrays. The synthesis was carried out using both hydrothermal and CVD methods. After that, SEM, TEM, and XRD were applied to characterize these ZnO nanowire arrays. By using this technique, it provides a new approach to obtain vertically-aligned ZnO nanowire arrays with both high density and aspect-ratio through a large-scale process. Keywords: Dye sensitized solar cell (DSSC), ZnO Nanowire Arrays, AAO Template, Hydrothermal, CVD

Growth Control of ZnO Nano-Crystals by Multi-Beam Interference Patterning

We have succeeded in controlling growth density of the ZnO nanowires by introduction of a ZnO buffer layer. Low-density hexagonal cone-shape ZnO cores are formed on the buffer layer, and vertically-aligned ZnO nanowires are grown on the cores. The density of the nanowires was decreased by laser irradiation to the buffer layer before growth of the ZnO nanowires. In addition, periodic growth of ZnO nano-crystals was demonstrated using four beam interference patterning.

Resistive switching in single vertically-aligned ZnO nanowire grown directly on Cu substrate

We report unipolar resistive switching measurements from single vertical ZnO nanowires grown directly on a copper substrate. Electrical measurements using a conductive atomic force microscope show conductive filament formation at 30kV/cm, which is an order of magnitude lower field than that for bulk films and suggest a preferential filament formation likely on the surface of the nanowires. A high resistive ratio of three orders of magnitude was observed between the high and low resistive memory states.

ZnO薄膜的性质对水热生长ZnO纳米线阵列的影响

ZnO nanowire arrays with controlled diameter,density and orientation were prepared on the ZnO film coated substrates via hydrothermal method.The ZnO film were fabricated by atomic layer deposition(ALD) method and annealed at different temperatures.The annealing temperature has strong influences on the grain size,crystalline structure and defect property of the ZnO film.The diameter,density and orientation of ZnO nanowires depended on the features of the ZnO film.Field-emission scanning electron microscopy(SEM),X-ray diffraction(XRD) and photoluminescence(PL) were applied to analyze the ZnO film and ZnO nanowire arrays.The as-prepared vertical aligned ZnO nanowires are highly suitable for use in nanodevices,such as light-emitting diodes and solar cells.

Piezoelectric potential in vertically aligned nanowires for high output nanogenerators

In this work we analyze the coupled piezoelectric and semiconductive behavior of verticallyaligned ZnO nanowires under uniform compression. The screening effect on thepiezoelectric field caused by the free carriers in vertically compressed zinc oxide nanowires(NWs) has been computed by means of both analytical considerations and finite elementcalculations. We predict that, for typical geometries and donor concentrations, the lengthof the NW does not significantly influence the maximum output piezopotential because thepotential mainly drops across the tip, so that relatively short NWs can be sufficient forhigh-efficiency nanogenerators, which is an important result for wet-chemistryfabrication of low-cost, CMOS- or MEMS-compatible nanogenerators. Furthermore,simulations reveal that the dielectric surrounding the NW influences the outputpiezopotential, especially for low donor concentrations. Other parameters such as theapplied force, the sectional area and the donor concentration have been varied inorder to understand their effects on the output voltage of the nanogenerator.

Role of Electro-Deposition Parameters on Preparing Tailored Dimension Vertically-Aligned ZnO Nanowires

A systematic study of the growth of uniform vertically-aligned ZnO nanowires is presented. It is shown how, by careful control over the growth parameters, high quality nanowires of chosen morphology may be synthesized at low temperatures using electro-deposition. The key experimental parameters were the growth temperature, the ZnCl2 concentration in the electrolyte, the applied voltage and the thickness of the catalytic Au film. As a result of optimizing these parameters, ZnO nanowires with tailored dimensions were realized – an essential prerequisite for the use of such nanowires in functional nanoscale devices.

Fabrication of ZnO nanowires and nanorods

In this study, we focused on the fabrication of two types of ZnO nanostructures, nanorods and nanowires, using different techniques. ZnO nanowires were fabricated by cathodically induced sol–gel electrodeposition using an anodic aluminum oxide (AAO) template. ZnO nanowires were approximately 65nm in diameter and 10μm in length. Also, ZnO nanorods were fabricated by a hydrothermal technique using ZnO seed layer coated glass substrate. ZnO nanorods were approximately 30nm in diameter and 1μm in length. Obtained vertical aligned ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and UV–vis spectrophotometry.

ZnO-CdZnS Core-Shell Nanocable Arrays for Highly Efficient Photoelectrochemical Hydrogen Generation

AbstractHigh-density TiO2-CdS and ZnO-CdS core-shell nanocable arrays were synthesized on large-area Ti substrates. The CdS layers were deposited on the pre-grown vertically-aligned TiO2 (rutile) and ZnO nanowire arrays, with a controlled thickness (10~50 nm), using the vapor transport method. The ZnO-CdS nanocables consisted of single-crystalline wurtzite CdS shells whose [001] direction was aligned along the [001] wire axis of the wurtzite ZnO core, which is distinctive from the polycrystalline shell of the TiO2-CdS nanocables. We fabricated the photoelectrochemical cell using the ZnO-CdS photoelectrode exhibits much more efficient hydrogen generation than that using the TiO2-CdS one.

Enhancement of Field-Emission Properties in ZnO Nanowire Array by Post-Annealing in H<SUB>2</SUB> Ambient

We studied the effects of post-annealing in H2 and O2 ambients on field-emission properties of vertically-aligned ZnO nanowire arrays synthesized by carbothermal reduction process. The turn-on electric field was dramatically decreased from 3.78 to 2.37 V/microm after post-annealing in H2 ambient, which was explained by both hydrogen passivation effects of deep levels and surface modification. In other words, we could observe significant decrease of deep level peak in photoluminescence measurements on hydrogen post-annealed ZnO nanowire array. And also hydrogen-related bonds are strongly increased from X-ray photoelectron spectroscopy measurements. These findings suggest that the concentration of conduction electrons increased by hydrogen post-annealing, which results in the enhanced tunneling probability of conduction electrons into the vacuum.

Feasibility study of barium fluoride films as a sacrificial layer for patterning of ZnO nanowire arrays

Vertical aligned ZnO nanowires were grown by metal-organic chemical vapor deposition technique on Si substrate. X-ray diffraction patterns and the high-resolution transmission electron microscopy confirmed the single crystalline growth of the ZnO nanowires along [0 0 0 1] direction. In another experiment, BaF 2 films were deposited on silicon substrate by thermal evaporation. The BaF 2 films show polycrystalline nature along with a granular surface morphology. Based on the solubility of BaF 2 film in cold water, BaF 2 layer was used as a sacrificial layer for the patterning of ZnO nanowire arrays. A sharp edge was obtained between the hexagonal pattern and the surrounding nanowires, which is suitable for micropatterning of ZnO nanowires for electronic devices.

Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition

Vertically aligned ZnO nanowires have been successfully synthesized on c-cut sapphire substrates by a catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD) in Ar and N2 background gases. In NAPLD, the nanoparticles formed in the background gas by laser ablation are used for the growth of the nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which is in turn achieved by varying ablation laser parameters such as the energy and the repetition rate. When single ZnO nanowire synthesized in a N2 background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated emission was clearly observed, indicating high quality of the nanowire.

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.

Synthesis of Vertically-Aligned ZnO Nanowires by Nanoparticle- Assisted Pulsed Laser Deposition and Their Application to Photo-Devices

Vertically aligned ZnO nanowires have been successfully synthesized on annealed c-cut sapphire substrates by a catalyst-free nanoparticle-assisted pulsed-laser deposition (NAPLD) in Ar and N2 background gases. In NAPLD, the nanoparticles formed in the background gas by laser ablation are used as a starting material for the growth of the nanowires. The surface density of the nanowires can be controlled by varying the density of nanoparticles, which are accomplished by changing the energy of the ablation laser, the repetition rate of the laser and so on. When single ZnO nanowire synthesized in a N2 background gas was excited by 355 nm laser-pulse with a pulse-width of 8 ns, stimulated emission was clearly observed, indicating high quality of the nanowire. These nanowires were used as building blocks for an ultraviolet photo-sensor, field emitters and an ultraviolet light emitting diode with a structure of n-ZnO/ZnO nanowire/p-GaN.