One step synthesis of vertically aligned ZnO nanowire arrays with tunable length

Length control of ZnO nanowire arrays is a valuable concern for both fundamental research and future device application. In this article, vertically aligned ZnO nanowire arrays were synthesized by a seed layer catalyzed vapor phase transport method in a single experiment cycle. The length of these nanowire arrays exhibits a quasi-continuous evolution. It was found that the type and flow rate of carrier gas have a significant influence on the length modulation of ZnO arrays along the tube. A feasible route to tune the length of ZnO nanowire arrays from several micrometers to nearly 100 μm could be achieved by adjusting proper deposition position and carrier gas.     

Tunable and reversible surface wettability transition of vertically aligned ZnO nanorod arrays

Vertically aligned zinc oxide (ZnO) nanorods (NRs) with different surface morphology were grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrate. To study the effect of surface morphology on wettability, the contact angle (CA) of water was measured. It was demonstrated that the CA of the deposited ZnO NRs varied between 104° and 135° depending upon the surface morphology. The ZnO NRs became super-hydrophilic after ultraviolet (UV) illumination. However, the NR arrays were reconverted to their previous hydrophobic state after low temperature annealing (50 °C) in open atmosphere. Structural effect and preferential adsorption of water molecules on the defective sites of UV illuminated surface was used to explain the transition mechanism. Under the alternations of heat treatment and UV illumination, a reversible transition between hydrophobicity and super-hydrophilicity were observed.     

Catalyst-free highly vertically aligned ZnO nanoneedle arrays grown by plasma-assisted molecular beam epitaxy

This work describes the growth of highly vertically aligned ZnO nanoneedle arrays on wafer-scale catalyst-free c-plane sapphire substrates by plasma-assisted molecular beam epitaxy under high Zn flux conditions. The photoluminescence spectrum of the as-grown samples reveals strong free exciton emissions and donor-bound exciton emissions with an excellent full width at half maximum (FWHM) of 1.4 meV. The field emission of highly vertically aligned ZnO nanoneedle arrays closely follows the Fowler–Nordheim theory. The turn-on electric field was about 5.9 V/µm with a field enhancement factor β of around 793.     

Hybrid solar cells with conducting polymers and vertically aligned silicon nanowire arrays: The effect of silicon conductivity

Organic/inorganic hybrid solar cells, based on vertically aligned n-type silicon nanowires (n-Si NWs) and p-type conducting polymers (PEDOT:PSS), were investigated as a function of Si conductivity. The n-Si NWs were easily prepared from the n-Si wafer by employing a silver nanodot-mediated micro-electrochemical redox reaction. This investigation shows that the photocurrent-to-voltage characteristics of the n-Si NW/PEDOT:PSS cells clearly exhibit a stable rectifying diode behavior. The increase in current density and fill factor using high conductive silicon is attributed to an improved charge transport towards the electrodes achieved by lowering the device's series resistance. Our results also show that the surface area of the nanowire that can form heterojunction domains significantly influences the device performance.     

Synthesis of the vertically aligned carbon hexagonal nanoprism arrays and their application for field emission

A simple approach is demonstrated for effectively growing large-area vertically aligned carbon hexagonal nanoprism arrays on molybdenum substrates by the catalyst-assisted pulsed laser deposition techniques. The carbon hexagonal nanoprisms have uniform shape and length, almost aligned vertically on the substrate, and the average diameters are about 30 nm. The internal angles of the nanoprisms present 60°. The vertically aligned nanorods have also been obtained for a comparison in the presence of catalyst Fe. The sample with vertically aligned carbon hexagonal nanoprism arrays exhibits better field emission behaviors than that with aligned carbon nanorod arrays.     

Emerging of Ag particles on ZnO nanowire arrays for blue-ray hologram storage

Noble-metal/metal-oxide-semiconductor nanostructures as an important material platform have been applied in massive data storage. ZnO exhibits excellent optical modulation ability. However, plasmon induced charge separation effect in Ag/ZnO systems is very weak due to the low chemical activity on surface of the oxide. Herein, we prepare ZnO nanowire arrays via the hydrothermal method, and measure their absorption spectra, photoluminescence spectra and electron paramagnetic resonance, proving the existence of oxygen defects in ZnO. Accordingly, an idea of "electron reverse transfer" is proposed such that blue-ray (403.4 nm) induces reduction of Ag⁺ ions through the excitation of ZnO. Rod-like and spherical silver nanoparticles emerge on the surface and in the gap of ZnO nanowire arrays, respectively, after the visible light stimulus. It is found that nanowire density, oxygen defects and surface roughness are dependent on hydrothermal time. The optimized diffraction efficiency of 0.08% is obtained for reconstructing hologram in the nanocomposite film. This work provides a bright way for construction of ZnO-based optoelectronic integrated devices.     

Sol–gel derived ZnO nanoparticulate films for ultraviolet photodetector (UV) applications

Zinc oxide nanoparticles based UV detector was fabricated on thermally oxidized silicon substrate. ZnO nanoparticle films were deposited using sol–gel route. The seed solution was prepared using two different solvents (methanol and isopropyl alcohol (IPA)). The surface morphology of the prepared films was characterized by FESEM. Structural characterization along with optical measurements was carried out using XRD and UV–vis spectroscopy. For the UV photo-detector, ZnO thin film prepared in IPA is selected based on their structural and optical analysis. The changes in photo-response of ZnO thin film with respect to time was studied under the dark and variable UV intensities. It was observed that the photocurrent increased with a factor of 4.82 under 1.16 mW of UV intensity. It is believe that the synthesized ZnO thin films have potential to use in the ultraviolet photo-detector applications.     

Modulation of field emission properties of vertically aligned ZnO nanorods with aspect ratio and number density

Vertically aligned ZnO nanorod arrays with different aspect ratios were synthesized by hybrid wet chemical route. Modulation of the field emission properties (FE) with aspect ratio of ZnO nanorods was examined. With the increase in the aspect ratio, the emission current density increases from 0.02 to 8 μA/cm² at 7.0 V/μm. Turn-on voltage was seen to decrease from 9.6 to 7 V/μm at a current density of 10 μA/cm² with the increase in aspect ratio in the ZnO films. The interrelation between the FE characteristics (emission thresholds, current density, surface uniformity, etc.) and microstructure of the ZnO nanostructure obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) was discussed. Quality of the ZnO nanorods was also examined by using Raman spectroscopy and Fourier transformed infrared spectroscopy (FTIR). It was found that the observed enhancements of FE characteristics could mainly be attributed to the increase in aspect ratio and associated number density of ZnO nanorods.     

Catalyst-free MOCVD growth of aligned ZnO nanotip arrays on silicon substrate with controlled tip shape

Quasi-aligned zinc oxide nanotip arrays have been grown by MOCVD without using catalyst. The tip shape was controlled systematically by varying the gas flow rate, demonstrating a technique for growing tip arrays of ZnO on silicon. This technology can be large-scale on the wafer level and it has the potential to be directly integrated with clean room silicon technology. The diameter of these ZnO nanowires or nanotips could be controlled by the varying of source flow rate, providing a simple but unique way of fabricating ZnO nanotip arrays for application in field emission and nanogenerators.     

Coupled process of plastics pyrolysis and chemical vapor deposition for controllable synthesis of vertically aligned carbon nanotube arrays

Efficient conversion of waste plastics into advanced materials is of conspicuous environmental, social and economic benefits. A coupled process of plastic pyrolysis and chemical vapor deposition for vertically aligned carbon nanotube (CNT) array growth was proposed. Various kinds of plastics, such as polypropylene, polyethylene, and polyvinyl chloride, were used as carbon sources for the controllable growth of CNT arrays. The relationship between the length of CNT arrays and the growth time was investigated. It was found that the length of aligned CNTs increased with prolonged growth time. CNT arrays with a length of 500 μm were obtained for a 40-min growth and the average growth rate was estimated to be 12 μm/min. The diameter of CNTs in the arrays can be modulated by controlling the growth temperature and the feeding rate of ferrocene. In addition, substrates with larger specific surface area such as ceramic spheres, quartz fibers, and quartz particles, were adopted to support the growth of CNT arrays. Those results provide strong evidence for the feasibility of conversion from waste plastics into CNT arrays via this reported sustainable materials processing.     

Growth and characterization of type-II ZnO/ZnTe core-shell nanowire arrays for solar cell applications

Large area, well-aligned type-II ZnO/ZnTe core-shell nanowire arrays have been fabricated on an a-plane sapphire substrate. The ZnO nanowires were grown in a furnace by chemical vapor deposition with gold as catalyst and then were coated with a ZnTe shell on the ZnO nanowires surface by a metal-organic chemical deposition chamber. The morphology and size distribution of the ZnO/ZnTe core-shell nanowire arrays were studied by scanning electron microscopy (SEM) and the crystal structure was examined by x-ray diffraction (XRD). Transmission measurement was used to study the optical properties of the core-shell nanowires. The results indicated that the ZnO/ZnTe core-shell nanowire arrays have good crystalline quality. In addition, it was found that the nanowire arrays have good light absorption characteristics and these properties make it suitable for making photovoltaic devices.     

ZnO/TiO2 core–shell nanowire arrays for enhanced dye-sensitized solar cell efficiency

We present a new method of synthesizing ZnO/TiO₂ core–shell nanowire (NW) arrays for the fabrication of dye-sensitized solar cells (DSSCs). Vertically aligned ZnO NW arrays were obtained on Si substrates, and modified by a TiO₂ shell in order to solve the recombination problems via a cost-effective spin-coating method. The structure of the ZnO/TiO₂ composite NW arrays was characterized. The experimental results indicate that the TiO₂ shell enhances the performance of the DSSCs, through improving the stability of the ZnO NWs and decreasing the recombination of photogenerated electrons on the NW surface. The highest overall conversion efficiency of the cell reaches about 3.0 %.     

The simulation of temperature dependence of responsivity and response time for 6H-SiC UV photodetector

In this paper the temperature dependence of responsivity and response time for 6H-SiC ultraviolet (UV) photodetector is simulated based on numerical model in the range from 300K to 900K. The simulation results show that the responsivity and the response time of device are less sensitive to temperature and this kind of UV photodetector has excellent temperature stability. Also the effects of device structure and bias voltage on the responsivity and the response time are presented. The thicker the drift region is, the higher the responsivity and the longer the response time are. So the thickness of drift region has to be carefully designed to make trade-off between responsivity and response time.     

Cathodoluminescent and electrical properties of an individual ZnO nanowire with oxygen vacancies

A single ZnO nanowire with intrinsic oxygen vacancies is utilized to fabricate four-contact device with focus ion beam lithography technique. Cathodoluminescent spectra indicate strong near-UV and green emission at both room temperature and low temperatures. Experimental measurement shows the temperature-dependent conductivity of the ZnO nanowire at low temperatures （below 100 K）. The further theoretical analysis confirms that weak localization plays an important role in the electrical transport, which is attributed to the surface states induced by plenty of oxygen vacancies in ZnO nanowire.     

Characteristics of ZnO MSM UV photodetector with Ni contact electrodes on poly propylene carbonate (PPC) plastic substrate

In this study, we report the fabrication of ZnO metal-semiconductor-metal UV photodetector (MSM UV PD) by deposition ZnO thin film on poly propylene carbonate (PPC) plastic substrate using direct current (DC) sputtering technique, and Nickel (Ni) contact as electrodes. The structural, optical and electrical properties of the ZnO thin film were investigated by using atomic force microscopy (AFM), X-Ray diffraction (XRD) measurement, and photoluminescence (PL). The electrical characteristics of the detector were investigated using the current–voltage (I–V) measurements, the dark- and photo-currents were found to be 1.04 and 93.80 μA, respectively. Using forward dark conditions at 5 volt; the barrier height Φ_B was calculated to be 0.675 eV. Under incident wavelength of 385 nm, it was found that the maximum responsivity (R) of the Ni/ZnO/Ni MSM PD was found to be 1.59 A/W.     

Optoelectronic characterisation of an individual ZnO nanowire in contact with a micro-grid template

Optoelectronic characterisation of an individual ZnO nanowire in contact with a micro-grid template has been studied.The low-cost micro-grid template made by photolithography is used to fabricate the ohmic contact metal electrodes.The current increases linearly with the bias,indicating good ohmic contacts between the nanowire and the electrodes.The resistivity of the ZnO nanowire is calculated to be 3.8 ·cm.We investigate the photoresponses of an individual ZnO nanowire under different light illumination using light emitting diodes(λ = 505 nm,460 nm,375 nm) as excitation sources in atmosphere.When individual ZnO nanowire is exposured to different light irradiation,we find that it is extremely sensitive to UV illumination;the conductance is much larger upon UV illumination than that in the dark at room temperature.This phenomenon may be related to the surface oxygen molecule adsorbtion,which indicates their potential application to the optoelectronic switching device.     

Well-aligned ZnO microprism arrays with umbrella-like tips: Low-temperature preparation, structure and UV photoluminescence improvement

Catalyst-free, low-temperature (430 °C), high-density, well-aligned, single-crystalline zinc oxide (ZnO) microprism (ZMP) arrays have been synthesized over the entire c-ZnO/poly-Zn-coated Si substrates by simple thermal evaporation of Zn powder. Specially, the microprisms obtained possess hexagonal umbrella-like tips on which vertical thin ZnO nanowires grow. The growth mechanism of a three-stage thermodynamic process was discussed. Photoluminescence spectra show a strong ultraviolet (UV) emission enhancement of the ZMPs after H⁺ (hydrogen ions) implantation. This kind of special ZnO microstructure may find potential applications in field emission, UV laser emission devices, multifunctional microdevices and highly integrated multichannel nano-optoelectronic devices.     

Layer-by-layer assembly of Ag2S quantum dots-sensitized ZnO/SnO2 core-shell nanowire arrays for enhanced photocatalytic activity

Ag₂S quantum dots-sensitized ZnO/SnO₂ core-shell nanowire arrays were successfully synthesized layer by layer through hydrothermal growth, atomic layer deposition, and successive ionic layer adsorption and reaction process. By introducing the two-layer semiconductors, the bandgap of ZnO component in the arrays was slightly modulated, while the light absorption was obviously improved with an absorptivity higher than 95% in visible and ultraviolet range. In contrast to the ZnO NW arrays, the photocurrent response of the ZnO/SnO₂/Ag₂S NW arrays for the visible light was improved from 0.42 μA to 22.5 μA, and the photodegradation efficiency of methylene blue was increased from 45.24% to 71.61%, and 42.61% to 57.58%, respectively in visible light and ultraviolet light. Band structure analysis indicated that the coating layer brought different staggered gaps and suitable band alignment for efficient photocatalytic performance, which could be extended to design heterogeneous semiconductor nanomaterials for their potential applications.