Aligned ZnO Nanowires Research Articles

CATALYST–FREE GROWTH OF WELL–ALIGNED ZnO NANOWIRES ON GRAPHENE/Si SUBSTRATEBY THERMAL EVAPORATION

Vertically well–aligned ZnO nanowire (NW) arrays with high density were directly synthesized on graphene/Si substrate by thermal evaporation of zinc powder without catalysts or additives. The ZnO NWs were characterized by field emission scanning electron microscopy (FE–SEM), high resolution transmission electron microscopy (HRTEM), X–ray diffraction (XRD), photoluminescence (PL), and Raman spectroscopy. The results showed that the obtained ZnO NWs have diameters in the range of 300–350 nm with lengths of several tens micrometers. The prepared ZnO NWs are of a single crystal, which have a hexagonal wurtzite crystal structure with c–axis (002) orientation growth perpendicular to the substrate surface. The NW arrays had a good crystal quality with excellent optical properties, indicating a sharp and strong ultraviolet emission at 380 nm, and a weak visible emission at around 516 nm.

CdTe quantum-dot-modified ZnO nanowire heterostructure

The effect of CdTe quantum-dot (QD) decoration on the photoluminescence (PL) behaviour of ZnO nanowire (NW) array is presented in the present work. Highly crystalline and vertically 40–50 nm diameter range and 1 µm in length aligned ZnO NWs are synthesized using low-cost method. The crystallinity and morphology of the NWs are studied by scanning electron microscopy and X-ray powder diffraction methods.Optical properties of the nanowires are studied using photo-response and PL spectroscopy. CdTe QDs are successfully synthesized on ZnO nanowire surface by dip-coating method. ZnO NWs are sensitized with CdTe QDs characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and PL spectroscopy. The highly quenched PL intensity indicates the charge transfer at interface between CdTe QDs and ZnO NWs and is due to the formation of type-II heterostructure between QDs and NWs. Photo-response behaviour of heterostructure of the film is also been incorporated in the present work.

A facile hydrothermal approach for the density tunable growth of ZnO nanowires and their electrical characterizations

Controlling properties of one-dimensional (1D) semiconducting nanostructures is essential for the advancement of electronic devices. In this work, we present a low-temperature hydrothermal growth process enabling density control of aligned high aspect ratio ZnO nanowires (NWs) on seedless Au surface. A two order of magnitude change in ZnO NW density is demonstrated via careful control of the ammonium hydroxide concentration (NH4OH) in the solution. Based on the experimental observations, we further, hypothesized the growth mechanism leading to the density controlled growth of ZnO NWs. Moreover, the effect of NH4OH on the electrical properties of ZnO NWs, such as doping and field-effect mobility, is thoroughly investigated by fabricating single nanowire field-effect transistors. The electrical study shows the increase of free charge density while decrease of mobility in ZnO NWs with the increase of NH4OH concentration in the growth solution. These findings show that NH4OH can be used for simultaneous tuning of the NW density and electrical properties of the ZnO NWs grown by hydrothermal approach. The present work will guide the engineers and researchers to produce low-temperature density controlled aligned 1D ZnO NWs over wide range of substrates, including plastics, with tunable electrical properties.

Molecular Simulation Study of Piezoelectric Potential Distribution in a ZnO Nanowire under Mechanical Bending

ABSTRACTMolecular dynamics (MD) simulations are carried out to investigate the interfacial sliding dynamics of a ZnO piezoelectric nanogenerator in an atomic force microscope (AFM) setting. The molecular system includes a vertically aligned ZnO nanowire along the [0001] direction and a Pt (111) metal tip sliding over it. We calculate the piezoelectric potential distributions based on the equilibrium molecular configurations using classical ionic charges. Simulation results reveal the very detailed evolution changes of the piezopotential within the nanowire, which are largely contributed from the different internal tensile or compressive strains induced by mechanical bending of the ZnO nanowire. Variations of the normal contact and lateral frictional forces versus the sliding distance of the Pt metal tip are also presented.

ZnO nanowire growth by chemical vapor deposition with spatially controlled density on Zn2GeO4:Mn polycrystalline substrates

Aligned ZnO nanowires were successfully synthesized by CVD growth on polycrystalline Zn2GeO4:Mn substrates. The density of the nanowires was explored as a function of gold catalyst thickness as well as the geometry of the growth system. For the first time we demonstrated that the density of ZnO nanowires can be directly tailored by adjusting the lateral distance between a Zn2GeO4:Mn substrate and source powders, and symmetric growth was observed when source powders were placed both upstream and downstream in two separated zones during the synthesis process in a tube furnace. Electric field modelling was employed to predict a desired nanowire density range for future application of the Zn2GeO4:Mn/ZnO structures to electroluminescent devices.

A rapid responding ultraviolet sensor based on multi-parallel aligned ZnO nanowires field effect transistor

One-dimensional zinc oxide nanowires (ZnO NWs) have been reported to be suitable for UV sensing, but their applications are limited due to restricted surface area which restrains the performance especially in term of response and recovery. In this paper, we fabricate a UV sensor using multi-parallel aligned ZnO NWs based FET in order to overcome the mentioned drawbacks. The sensing characteristics of multi-parallel aligned ZnO NWs based FET toward 0.44mW/cm2 UV illumination has been examined under depletion and enhancement modes, at ambient conditions. The fabricated UV sensor was found to possess the ability to differentiate UV with different intensity, improved sensitivity (in depletion mode), short response and recovery time (< 1s), and with high stability. Our paper reports a simple and effective approach for UV sensor fabrication with high responsivity, sensitivity and reproducibility.

Growth mechanism of seed/catalyst-free zinc oxide nanowire balls using intermittently pumped carrier gas: Synthesis, characterization and applications

We report the growth mechanism of ZnO nanowire balls by thermal evaporation under intermittently pumped carrier gas as a new and simple nucleation method. The structural, morphological, and optical properties of the nanowires were analyzed by field emission scanning electron microscopy, X-ray diffraction, UV–vis spectroscopy, and photoluminescence spectroscopy. Results showed that the proposed method facilitates epitaxial nucleation and growth of high density vertically aligned ZnO nanowires with length of several micrometers and that have a smaller diameter compared to that of ZnO nanowires fabricated by traditional continuous pumping described in the literature under the same growth condition. These findings suggest that the use of intermittently pumped carrier gas is one of effective control parameters that promote the nucleation and growth of nanomaterials using thermal evaporation method.

Enhanced Piezoelectric Behavior of PVDF Nanocomposite by AC Dielectrophoresis Alignment of ZnO Nanowires

In contrast to commercial piezoelectric ceramics, lead-free materials such as ZnO and a polymer matrix are proper candidates for use in ecofriendly applications. In this article, the authors represent a technique using ZnO nanowires with a polyvinylidene fluoride (PVDF) matrix in a piezoelectric polymer composite. By aligning the nanowires in the matrix in a desired direction by AC dielectrophoresis, the piezoelectric behavior was enhanced. The dielectric constant of the composite was improved by increasing the concentration of the ZnO nanowires as well. Specifically, the resulting dielectric constant shows an improvement of 400% with aligned ZnO nanowires by increasing the poling effect compared to that of a randomly oriented nanowire composite without a poling process.

Optimal geometrical design of inertial vibration DC piezoelectric nanogenerators based on obliquely aligned InN nanowire arrays.

Piezoelectric nanogenerators have been investigated to generate electricity from environmental vibrations due to their energy conversion capabilities. In this study, we demonstrate an optimal geometrical design of inertial vibration direct-current piezoelectric nanogenerators based on obliquely aligned InN nanowire (NW) arrays with an optimized oblique angle of ∼58°, and driven by the inertial force of their own weight, using a mechanical shaker without any AC/DC converters. The nanogenerator device manifests potential applications not only as a unique energy harvesting device capable of scavenging energy from weak mechanical vibrations, but also as a sensitive strain sensor. The maximum output power density of the nanogenerator is estimated to be 2.9 nW cm-2, leading to an improvement of about 3-12 times that of vertically aligned ZnO NW DC nanogenerators. Integration of two nanogenerators also exhibits a linear increase in the output power, offering an enormous potential for the creation of self-powered sustainable nanosystems utilizing incessantly natural ambient energy sources.

Bidirectional growth of ZnO nanowires with high optical properties directly on Zn foil

ZnO nanowires (NWs) were directly grown on zinc foils through a two-step process: (i) Zn foils fabrication and (ii) thermal oxidization at temperature 400°C. The dense and orderly aligned ZnO NWs grown on Zn hexagonal-faceted surface presented preferential growth in the [101] orientation and showed bidirectional growth mode with an intersection angle of about 60°. The faceted and bidirectional growth mode of ZnO NWs would be applicable in the fabrication of planar nanodevices. The edge growth mechanism is proposed for the growth of NWs. In photoluminescence (PL) studies, we have observed exciton peak around 368nm with a small PL full width at half maximum (~8meV) and visible emission was strongly suppressed. These characteristics indicated that the ZnO NWs had high optical properties.

Fast growth of well-aligned ZnO nanowire arrays by a microwave heating method and their photocatalytic properties

The fast growth of aligned ZnO nanowire arrays with optimized structure is attractive for electrical and optical devices. In this paper, we report a controllable and rapid growth of ZnO nanowire arrays by a microwave-assisted hydrothermal method. When using different zinc salts as the precursors, the morphology of the samples changes a lot and the length growth rate is several times different. The growth mechanism is also investigated. It is found that the solution near neutral pH value is ideal for fast nanowire growth, in which the length of the nanowires increases linearly with growth time and the growth rate is over ten times faster than that in the traditional hydrothermal method. Therefore, aligned ZnO nanowire arrays can grow up to tens of microns in a few hours, while the density and sizes of these nanowires can be well controlled. The ZnO nanowire arrays used as photocatalysts present good photocatalytic performance to the degradation of methyl orange (MO) due to the large surface area. So this paper provides an effective method to obtain vertically aligned ZnO nanowire arrays for practical applications.

MECHANICAL RESPONSE TO DEEP-CRYOGENIC TREATMENT OF ZnO NANOWIRES GROWN ON BULK ZnO COATING

ZnO nanowires are currently used in many application fields. Thermal stability is often a concern in terms of the mechanical response and, in particular, for the elasticity of the nanowires. Literature works focused, to a certain degree, on the nanowires heating response. Anyhow, no experimental data are nowadays available in literature on the low- and very low-temperature exposures. In the present study, deep-cryogenic treatment was performed on vertically aligned ZnO nanowires produced by metal organic chemical vapor deposition. The critical buckling stress and strain of individual nanowires was not significantly influenced by the cryogenic exposure, while the bulk ZnO halved.

Growth of aligned ZnO nanowires via modified atmospheric pressure chemical vapor deposition

In this work, we report the growth of high-quality aligned ZnO nanowires via a facile atmospheric pressure chemical vapor deposition (CVD) method. The CVD reactor chamber used was more complicated than a conventional one due to the quartz boats loaded with sources (ZnO/C) and substrates being inserted into a semi-open quartz tube, and then placed inside the CVD reactor. The semi-open quartz tube played a very important role in growing the ZnO nanowires, and demonstrated that the transportation properties of Zn and O vapor differ from those in the conventional CVD reactor chamber. Aligned ZnO nanowires were successfully obtained, though they were only found at substrates located upstream. The very high crystalline quality of the obtained ZnO nanowires was demonstrated by high-resolution transmission electron microscopy and room temperature photoluminescence investigations. Such ZnO nanowires with high crystalline quality may provide opportunities for the fabrication of ZnO-based nano-devices in future.

Photovoltaic properties of oriented ZnO nanowires arrays decorated with TiO2 shell layer for dye-sensitized solar cell application

Almost vertically aligned ZnO nanowires have been grown on Silicon substrates via a simple hydrothermal method. In order to improve the photoelectric conversion efficiency for fabricated dye-sensitized solar cells (DSSCs), an easily-operated immersing method was employed to fabricate a TiO2/ZnO nanowires array heterojunction, which has advantage of high aspect ratio, low recombination rate and high absorption of visible light. The structure and surface morphology of the samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. The photovoltaic properties of TiO2/ZnO based DSCCs were measured by considering the power efficiency (η), photocurrent density (J sc), open-circuit voltage (V oc), and fill factor (FF). An efficiency of 0.559% is achieved for the composite cell, increasing 0.426 and 0.185% for the ZnO nanowires cell and TiO2 cell, respectively. The short-circuit current and open-circuit voltage are also enhancing. The improvements are because of high surface are of TiO2 shell layer, as well as fast electron transport and light scattering effect of ZnO nanowires.

Enhanced Field Emission Properties of Aligned ZnO Nanowires

Enhanced Field Emission Properties of Aligned ZnO Nanowires

Low-Cost, Large-Area, Facile, and Rapid Fabrication of Aligned ZnO Nanowire Device Arrays.

Well aligned nanowires of ZnO have been made with an electrospinning technique using zinc acetate precursor solutions. Employment of two connected parallel collector plates with a separating gap of 4 cm resulted in a very high degree of nanowire alignment. By adjusting the process parameters, the deposition density of the wires could be controlled. Field effect transistors were prepared by depositing wires between two gold electrodes on top of a heavily doped Si substrate covered with a 300 nm oxide layer. These devices showed good FET characteristics and photosensitivity under UV-illumination. The method provides a fast and scalable fabrication route for functional nanowire arrays with a high degree of alignment and control over nanowire spacing.

Low switching-threshold-voltage zinc oxide nanowire array resistive random access memory

We have performed the fabrication and characterization of resistive random access memory devices based on Au/ZnO nanowires/Ag capacitor structure. First, vertically aligned ZnO nanowires were grown on Ag electrode layer. The poly 4-vinyl phenol (PVP) was then spin-coated on nanowires as a support and isolation layer. After appropriate etching of PVP in oxygen plasma to expose ZnO nanowire tips, a layer of Au was sputtered onto the tips of ZnO nanowires forming the top electrode. The resulting ZnO nanowire array devices were found to exhibit bipolar resistive switching characteristics. Switching between high resistance state (HRS) and low resistance state (LRS) could be achieved at a very low threshold voltage of 0.3V for a long conducting channel of 500nm, exhibiting a very low electric field, without electrical forming. The resistance ratios of HRS to LRS were higher than two orders of magnitude. The results indicate the application potentials of Au/ZnO nanowires/Ag devices for nonvolatile memory devices.

How Micropatterning and Surface Functionalization Affect the Wetting Behavior of ZnO Nanostructured Surfaces

Smart surfaces with a tunable wetting behavior between superhydrophobic and superhydrophilic states are of great interest for different applications, such as novel self‐cleaning/antifogging layers, or in view of the development of microfluidic devices and chemical/biochemical sensors of new generation. This work shows the tuning of the wetting behavior of hydrothermally grown zinc oxide nanowires (NWs) by combining the dual effect resulting from either surface chemical functionalization of the NWs and their growth on substrates prepatterned with different micro‐sized geometries. The characterization results highlight that vertically aligned ZnO NWs successfully cover all the microfeatures of the patterned substrates and can be successfully functionalized with aminopropyl groups. This chemical functionalization drives the initial transition of as‐prepared, superhydrophilic ZnO NWs toward a superhydrophobic regime. Then, protonation/deprotonation of amine groups, induced by their interaction with acid or basic media, changes the wetting state of ZnO NWs between hydrophilic and hydrophobic regimes. Reversible and successful switching from hydrophobic to hydrophilic states can be achieved under ad hoc conditions, in particular using appropriate micropatterns that do not favor the formation and entrapment of air bubbles.

Enhanced photoelectric performance in self-powered UV detectors based on ZnO nanowires with plasmonic Au nanoparticles scattered electrolyte

Vertically aligned ZnO nanowires (NWs) were grown on a fluorine-doped tin-oxide-coated glass substrate by a hydrothermal method. Au nanoparticles were well dispersed in the mixed solution of ethanol and deionized water. A simple self-powered ultraviolet detector based on solid–liquid heterojunction was fabricated, utilizing ZnO NWs as active photoanode and such prepared mixed solution as electrolyte. The introduction of Au nanoparticles results in considerable improvements in the responsivity and sensitivity of the device compared with the one using deionized water as electrolyte, which is attributed to the enhanced light harvesting by Au nanoparticles.

Dye decorated ZnO-NWs /CdS-NPs heterostructures for efficiency improvement of quantum dots sensitized solar cell

High density vertically aligned ZnO nanowire was coated with CdS nanocrystals of different thicknesses by the RF magnetron sputtering process and applied as photoanode in CdS quantum dot sensitized solar cells. Field emission scanning electron microscopy (FESEM), photoluminescence, and X-Ray diffraction (XRD) were utilized to characterize the samples and study their properties. Results demonstrated that, after dye decoration co-sensitized process, the ZnO/CdS heterostructures showed an overall power conversion efficiency of 2.68%, which is 76.3% improvement over that of pristine ZnO/CdS–QDSSC. Thereby, the QDSSC was assembled with modified ZnO/CdS heterostructures by Dye exhibited high performance.