Photoresponse of hydrothermally grown lateral ZnO nanowires

In this paper, a simple self-assembled lateral growth of ZnO nanowires (NWs) photodetector has been synthesized by a hydrothermal method at a temperature as low as 85 °C. The ZnO NWs exhibit single-crystalline wurtzite with elongated c-axis and can be selectively lateral self-assembled around the edges of ZnO seeding layer. The current of ZnO NWs is sensitive to the variation of ambient pressures, i.e. 4.47 μA was decreased to 1.48 μA with 5 V-bias as 1.1 × 10^− 6 Torr changed to 760 Torr, accordingly. Moreover, the current-voltage characteristics of ZnO NWs photodetectors can be evidently distinguished by UV illumination (i.e. λ = 325 nm). The photocurrent of ZnO NWs with UV illumination is twice larger than dark current while the voltage biased at 5 V. Consequently, this faster photoresponse convinces that the hydrothermally grown lateral ZnO NWs devices have a fairly good for the fabrication of UV photodetectors.     

ZnO nanowires hydrothermally grown on PET polymer substrates and their characteristics

Zinc oxide nanowires (ZnO NWs) were successfully synthesized on the ITO/PET polymer substrates by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations were carried out to characterize the crystallinity, surface morphologies, and orientations of these NWs, respectively. The influence of NW surface morphologies on the optical and electrical properties of ZnO NWs was studied. The hydrothermally grown ZnO NWs with direct band gap of 3.21 eV emitted ultraviolet photoluminescence of 406 nm at room temperature. Field emission measurements revealed that the threshold electric fields (Eth, current density of 1 mA/cm2) of ZnO NWs/ITO/PET and ZnO NWs/ZnO/ITO/PET are 1.6 and 2.2 V/microm with the enhancement factors, beta values, of 3275 and 4502, respectively. Furthermore, the field emission performance of ZnO NWs deposited on the ITO/PET substrate can be enhanced by illumination with Eth of 1.3 V/microm and displays a maximum emission current density of 18 mA/cm2. The ZnO NWs successfully grown on polymer substrate with high transmittance, low threshold electric field, and high emission current density may be applied to a flexible field emission display in the future.     

Nanoforest of hydrothermally grown hierarchical ZnO nanowires for a high efficiency dye-sensitized solar cell

In this paper, in order to increase the power conversion efficiency we demonstrated the selective growth of "nanoforest" composed of high density, long branched "treelike" multigeneration hierarchical ZnO nanowire photoanodes. The overall light-conversion efficiency of the branched ZnO nanowire DSSCs was almost 5 times higher than the efficiency of DSSCs constructed by upstanding ZnO nanowires. The efficiency increase is due to greatly enhanced surface area for higher dye loading and light harvesting, and also due to reduced charge recombination by providing direct conduction pathways along the crystalline ZnO "nanotree" multi generation branches. We performed a parametric study to determine optimum hierarchical ZnO nanowire photoanodes through the combination of both length-wise growth and branched growth processes. The novel selective hierarchical growth approach represents a low cost, all solution processed hydrothermal method that yields complex hierarchical ZnO nanowire photoanodes by utilizing a simple engineering of seed particles and capping polymer.     

Lasing in hydrothermally grown ZnO nanocrystals

Micro- and nanocrystalline ZnO powders and films have been prepared under mild hydrothermal conditions, and their spectroscopic and lasing properties have been investigated. The polycrystalline ZnO films showed stimulated emission with a lasing threshold of 0.67 MW/cm². Analysis of their lasing behavior suggests that the stimulated emission is generated by individual crystallites, each acting as a microlaser.     

Catalyst-free ZnO nanowires grown on a-plane GaN

ZnO nanowires were grown on a-plane GaN templates by chemical vapor deposition (CVD) without employing a catalyst. The a-plane GaN templates were pre-deposited on an r-plane sapphire substrate by metal-organic CVD. The resulting ZnO nanowires grow in angles off- related to the GaN basal plane. X-ray diffraction (XRD) spectra showed that the ZnO layer was grown with a heteroepitaxial relationship of (110)_ZnO||(110)_GaN. Photoluminescence spectra measured at 17 K exhibited near-band-edge emission at 372 nm with a full width at half maximum of 10 nm. The growth mechanism on a-GaN was the Volmer-Weber (VW) mode and differed from the Stranski-Krastanow (SK) mode observed for growth on c-GaN. This difference results from the higher interfacial free-energy on the a-plane between ZnO and GaN than that on the c-plane orientation.     

Effect of Zn2+ source concentration on hydrothermally grown ZnO nanorods

We studied the effect of Zn2+ source concentration on the structural and optical properties of hydrothermally grown ZnO nanorods. The nanorods were grown on ZnO/p-Si(111) substrate using by a hydrothermal process in various concentrations of reagent at a low temperature (approximately 95 degrees C) and the structural and optical characteristics of ZnO nanorods were subsequently investigated by X-ray diffraction, field-emission scanning electron microscopy, and room temperature photoluminescence. The results demonstrate that the morphology and crystallinity of ZnO nanorods are influenced by the overall concentration of the precursor. The density and diameter of ZnO nanorods with a hexagonal structure are especially sensitivite to concentration of reactants. Furthermore, the structural transition is shown by increasing concentration. At the lowest concentration of Zn2+, the ZnO nanorods grow as single crystals with a low density and variable orientations. On the contrary, at the highest concentration, the nanorods grow as polycrystas due to the supersaturated Zn2+ source.     

Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates

Vertical single-crystal ZnO nanowires with uniform diameter and uniform length were selectively grown on ZnO:Ga/glass templates at 600/spl deg/C by a self-catalyzed vapor-liquid-solid process without any metal catalyst. It was found that the ZnO nanowires are grown preferred oriented in the [002] direction with a small X-ray diffraction full-width half-maximum. Photoluminescence, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements also confirmed good crystal quality of our ZnO nanowires. Field emitters using these ZnO nanowires were also fabricated. It was found that threshold field of the fabricated field emitters was 14 V//spl mu/m. With an applied electric field of 24 V//spl mu/m, it was found that the emission current density was around 0.1 mA/cm/sup 2/.     

Effect of buffer layer thickness on the growth properties of hydrothermally grown ZnO nanorods

We investigated the effect of ZnO buffer layer thickness on the growth of hydrothermally grown ZnO nanorods. A series of ZnO buffer layers with different thicknesses was deposited on a p-Si (111) substrate using a co-sputtering system. After annealing the ZnO buffer layer, ZnO nanorods grown were grown hydrothermally at 95 degrees C. Unlike ZnO nanorods grown on as-deposited ZnO buffer layer, the diameter and length of ZnO nanorods grown on annealed ZnO buffer layers can be controlled. The structural and optical properties of ZnO nanorods grown on annealed ZnO buffer layers were analyzed by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence. The influence of ZnO buffer layer thickness on ZnO nanorods growth is discussed.     

Effects of seed layer patterns on the structural characteristics of hydrothermally grown ZnO nanorods

We investigated the effects of surface pattern size and shape on the characteristics of hydrothermally grown ZnO nanorods. For this purpose, the structural characteristics of ZnO nanorods were examined using X-ray diffraction and scanning electron microscopy. The sputtered ZnO seed layer was patterned using photolithography techniques on a Si substrate. ZnO nanorods with a [0001] texturing structure were successfully grown on selective areas by hydrothermal processes. In our experiments, however, it was observed that the diameter and the texture of the ZnO nanorods were strongly influenced by the size of the surface pattern.     

Structural and optical characterizations of nitrogen-doped ZnO nanowires grown by MOCVD

One dimensional nitrogen-doped ZnO nanowires were deposited on C-plane sapphire using metal organic chemical vapour deposition. Nanowires have been characterized by scanning electron microscopy, transmission electron microscopy, micro-Raman scattering and micro-photoluminescence spectroscopy. The structural analysis has shown a high crystalline quality. In N-doped ZnO nanowires nitrogen incorporation was emphasized by Raman spectral analysis and reduction of nitrogen concentration along the wire, from the bottom to the top was found by local analysis. Low temperature micro-photoluminescence spectra exhibit donor-acceptor pair transitions.     

Effect of annealing and hydrogen plasma treatment on the luminescence of hydrothermally grown bulk ZnO

The optical properties of hydrothermally grown bulk ZnO is investigated by low temperature photoluminescence (PL) spectroscopy. The effects of annealing in an argon atmosphere, as well as the influence of hydrogen plasma exposure, on the PL of as-grown material are studied. The 11 K PL spectrum of the as-grown ZnO shows different excitonic lines in the NBE region: several bound exciton lines are clearly visible. The origin of these bound excitons is discussed, as well as the influence of annealing on these lines. A shift in the deep level emission (DLE) is also observed with an increase in annealing temperature: a red shift is detected when the annealing temperature is increased up to 650 °C, while a subsequent blue shift is observed upon an increase in the annealing temperature. The involvement of Li and Cu in this phenomenon will be discussed and the effect of hydrogen on this DLE will also be studied.     

The effect of metal layers on the morphology and optical properties of hydrothermally grown zinc oxide nanowires

If the silicon industry is to successfully integrate ZnO nanowires (NWs) into existing devices to fully utilise the piezoelectric or optical properties of ZnO NWs, then a detailed understanding of the effect of metal interconnects on the morphology of the NWs during growth needs to be obtained. In this study, ZnO NWs were hydrothermally grown at 90 °C on Au, Ni and a Si substrate control to mimic the typical surfaces of a MetalMUMPs MEMS chip. The growth rate was significantly affected by the metal film below the ZnO seed layer, which was mainly attributed to changes in the roughness and grain size of the seed layer deposited, with the growth rate decreasing with increasing roughness. The growth rate on Si and Au surfaces also increased when isolated from the Ni samples, suggesting that Ni cations released in the solution could also inhibit growth by electrostatically attaching to the NWs surface and acting as a barrier to the incorporation of zinc ions. Furthermore, photoluminescence studies show the addition of metal layers to the substrate reduces the optical quality of the produced ZnO NWs.     

Complete 'melting' of charge order in hydrothermally grown Pr0.57Ca0.41Ba0.02MnO3 nanowires

Nanowires of Pr0.57Ca0.41Ba0.02MnO3 (PCBM) (diameter approximately 80-90 nm and length approximately 3.5 microm) were synthesized by a low reaction temperature hydrothermal method. Single-phase nature of the sample was confirmed by XRD experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. While the bulk PCBM is known to exhibit charge order (CO) below 230 K along with a ferromagnetic transition at 110 K, SQUID measurements on the nanowires of PCBM show that the charge order is completely absent and a ferromagnetic transition occurs at 115 K. However, the magnetization in the nanowires is observed to be less compared to that in the bulk. This observation of the complete 'melting' of the charge order in the PCBM nanowires is particularly significant in view of the observation of only a weakening of the CO in the nanowires of Pr0.5Ca0.5MnO3. Electron paramagnetic resonance experiments were also carried out on the PCBM nanowires using an X-band EPR spectrometer. Characteristic differences were observed in the line width of nanowires when compared with that of the bulk.     

Mechanical properties of microwave hydrothermally synthesized titanate nanowires

In this investigation titanate nanowires were synthesized by a microwave hydrothermal process and their nanomechanical characterization was carried out by a compression experiment via buckling instability using a nanomanipulator inside a scanning electron microscope. Nanowires of diameters 120-150?nm and length tens of microns can be synthesized by keeping a commercial nanoparticle inside a microwave oven at 350?W and 210?°C for 5?h. The nanowire was clamped between two cantilevered AFM tips attached to two opposing stages of the manipulator for nanomechanical characterization. The elasticity coefficients of the titanate nanowires were measured by applying a continuously increasing load and observing the buckling instability of the nanowires. The buckling behavior of a nanowire was analyzed from the series of SEM images of displacement of the cantilever attached to the nanowire due to application of load. The critical loads for different sized titanate nanowires were determined and their corresponding Young's modulus was computed with the Euler pinned-fixed end model. The Young's modulus of these microwave hydrothermal process synthesized titanate nanowires were determined to be approximately in the range 14-17?GPa. This investigation confirms the capability of the nanomanipulator via the buckling technique as a constructive device for measuring the mechanical properties of nanoscale materials.     

Orientation-enhanced growth and optical properties of ZnO nanowires grown on porous silicon substrates

ZnO nanowires have been synthesized on porous silicon substrates with different porosities via the vapour-liquid-solid method. The texture coefficient analysed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. The Raman spectrum reveals the high quality of the ZnO nanowires. From the temperature-dependent photoluminescence spectra, we deduced the activation energies of free and bound excitons.     

Synthesis and microstructure of vertically aligned ZnO nanowires grown by high-pressure-assisted pulsed-laser deposition

The microstructure and growth behavior for vertically aligned Zinc oxide (ZnO) nanowires, synthesized on a ZnO thin film template by pulsed-laser deposition (PLD), is reported. The nanowire growth proceeds without any metal catalyst for nucleation, although an epitaxial ZnO thin film template is necessary in order to achieve uniform alignment. Nanowire growth at argon or oxygen background pressures of 500-mTorr results in nanowire diameters as small as 50–90 nm, with diameters largely determined by growth pressure and temperature. Room temperature photoluminescence show both near-band-edge and deep-level emission. The deep-level emission is believed caused by oxygen vancancies formed during growth.     

Silica-coated ZnO nanowires

ZnO-core/SiO_x shell nanowires were successfully fabricated and their morphology, structure, Raman and photoluminescence properties were examined. Not only the sputter-coated product had an one-dimensional morphology, but the tubular structure of SiO_x shell was also continuous, smooth, and uniform, along the core nanowires. It was found that two fundamental modes (334, 437 cm⁻¹) and 2 s order modes (1106, 1156 cm⁻¹) of hexagonal ZnO appeared in the Raman spectrum of ZnO-core/SiO_x shell nanowires. The photoluminescence (PL) spectrum of the core-shell nanowires were deconvoluted into three Gassian functions, centered at 382, 500, and 758 nm, whether the subsequent thermal annealing was performed or not. The integrated intensities of UV (382 nm) and green (758 nm) emissions were changed by means of the shell-coating and thermal annealing. We have discussed the possible emission mechanisms.     

Tailored interyarn friction in aramid fabrics through morphology control of surface grown ZnO nanowires

Para-aramid fibers, such as Kevlar, have been the most common material used for ballistic applications by providing a variety of energy absorption mechanisms to dissipate a projectile’s momentum. One important mechanism is interyarn friction, which can be controlled through surface treatment of the fibers. It was recently shown that the growth of ZnO nanowires on the surface of the fibers could increase the interyarn friction, producing 10.85 times higher peak load and 22.70 times higher energy absorption compared to neat fabrics. Here, it is demonstrated that variation of the morphology of the nanowires produces a drastic change in the pullout behavior with low aspect ratio nanowires producing the largest increase in pullout force. While weight of the modified fabrics increased only 20% compared to the bare ones, tensile test results show that the growth of ZnO nanowires enhances both tensile strength and elastic modulus of the fabrics by 13% and 10%, respectively. A comprehensive post-test micrograph analysis is performed to reveal possible enhancement mechanisms of modified aramid fabrics with different nanowire morphologies.     

非催化剂法制备ZnO纳米线阵列的近带边高分辨变温光致发光性能研究

采用CVD法在a-面蓝宝石衬底上制备了ZnO籽晶层,然后在籽晶层上用碳热还原法制备了高质量的ZnO纳米线阵列。发现生长后的ZnO纳米线阵列具有良好的近带边发光性能,束缚激子发光峰半峰宽<500!eV。通过变温PL测试,确定3.37737eV的发光峰为自由激子发光,分别通过Vina模型和Vashni模型对其进行拟合,发现在60K以下Vashni模型的拟合相当好,而在60～150K,Vina模型的拟合效果更优。3.29eV处的峰为DAP峰,计算可得样品中的施主浓度为1.8×1018cm-3。     

Size-dependent field-emission characteristics of ZnO nanowires grown by porous anodic aluminum oxide templates assistance

The field-emission electrical properties of hydrothermally synthesized zinc-oxide nanowires grown on SiO₂-Si substrate are reported. Vertically aligned single-crystalline emitters with distinct length L and diameter D are realized by controlling the assisted growth of porous anodic aluminum oxide (AAO) templates. Field-emission measurement revealed that these field emitters exhibited controllable turn-on field E_to and the field enhancement factor β. Sample with feature size of L = 500 nm and D = 80 nm was prepared and then used as a basis for examining the size effect. Lower E_to and higher β were observed consistently for increasing the aspect ratio L/D. The enhanced properties of E_to = 1.48 V/μm and β up to 6100 are achieved for L/D = 53 (L = 1600 nm and D = 30 nm). Optimal characterizing parameters of E_to and β will be reached while pursuing extreme L/D practically. Factors such as the geometric limit of AAO template manufacture and the filling efficiency of ZnO into AAO pores will exert influence on the size-dependent effect.