Fast UV detection and hydrogen sensing by ZnO nanorod arrays grown on a flexible Kapton tape

ZnO nanorod arrays were grown on a flexible Kapton tape using microwave-assisted chemical bath deposition. High crystalline properties of the produced nanorods were proven by X-ray diffraction patterns and field emission scanning electron microscopy. Additionally, the photoluminescence spectrum showed higher UV peaks compared with visible peaks, which indicates that the ZnO nanorods had high quality and low number of defects. The metal-semiconductor-metal (MSM) configuration was used to fabricate UV and hydrogen gas detectors based on the ZnO nanorods grown on a flexible Kapton tape. Upon exposure to 395 nm UV light, the UV device exhibited fast response and decay times of 37 ms and 44 ms, respectively, at a bias voltage of 30 V. The relative sensitivities of the gas sensor made of the ZnO nanorod arrays, at hydrogen concentration of 2 %, at room temperature, 150 °C and 200 °C, are 0.42, 1.4 and 1.75 respectively.     

N-Type Field-Effect Transistors Using Multiple Mg-Doped ZnO Nanorods

Nanorod field-effect transistors (FETs) that use multiple Mg-doped ZnO nanorods and a SiO₂ gate insulator were fabricated and characterized. The use of multiple nanorods provides higher on-currents without significant degradation in threshold voltage shift and subthreshold slopes. It has been observed that the on-currents of the multiple ZnO nanorod FETs increase approximately linearly with the number of nanorods, with on-currents of ~1 muA per nanorod and little change in off-current (~4times10^-12). The subthreshold slopes and on-off ratios typically improve as the number of nanorods within the device channel is increased, reflecting good uniformity of properties from nanorod to nanorod. It is expected that Mg dopants contribute to high n-type semiconductor characteristics during ZnO nanorod growth. For comparison, nonintentionally doped ZnO nanorod FETs are fabricated, and show low conductivity to compare with Mg-doped ZnO nanorods. In addition, temperature-dependent current-voltage characteristics of single ZnO nanorod FETs indicate that the activation energy of the drain current is very low (0.05-0.16 eV) at gate voltages both above and below threshold     

花状ZnO超细结构的水热自组装

通过简单的水热合成路线,合成出由ZnO纳米棒束组装的花状结构.其组成结构单元ZnO纳米棒沿［001］晶向生长,呈很好的单晶结构.大部分纳米棒直径约为500nm,长约6.0μm.研究结果表明,在无水乙二胺存在的条件下,氨水（28%, v/v）在ZnO花状结构的形成过程中起到了至关重要的作用.调节氨水的含量,组成结构单元ZnO纳米棒可以组装成不同的花状结构.当加入氨水的量使得溶液的pH值达到10时,即可得到由ZnO纳米棒束组装成的花状结构,并简单讨论了这种花状结构的形状结构的形成机理.      

Controllable synthesis and photoluminescence properties of ZnO nanorod and nanopin arrays

Well-aligned ZnO nanorods and nanopins are synthesized on a silicon substrate using a one-step simple thermal evaporation of a mixture of zinc and zinc acetate powder under controlled conditions. A self-assembled ZnO buffer layer was first obtained on the Si substrate. The structure and morphology of the as-synthesized ZnO nanorod and nanopin arrays are characterized using X-ray diffraction, and scanning and transmission electron microscopies, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. The influence of the background atmosphere on the two ZnO nanostructures has been studied. Two different growth mechanisms are mentioned to interpret the formation of ZnO nanorod and nanopin arrays in our work. The room-temperature PL features the ZnO nanorods exhibit only sharp and strong ultraviolet (UV) emission emissions, which confirms the better crystalline and optical quality than the ZnO nanopins.     

Highly sensitive hydrogen detection of catalyst-free ZnO nanorod networks suspended by lithography-assisted growth

We have successfully demonstrated a ZnO nanorod-based 3D nanostructure to show a high sensitivity and very fast response/recovery to hydrogen gas. ZnO nanorods have been synthesized selectively over the pre-defined area at relatively low temperature using a simple self-catalytic solution process assisted by a lithographic method. The conductance of the ZnO nanorod device varies significantly as the concentration of the hydrogen is changed without any additive metal catalyst, revealing a high sensitivity to hydrogen gas. Its superior performance can be explained by the porous structure of its three-dimensional network and the enhanced surface reaction of the hydrogen molecules with the oxygen defects resulting from a high surface-to-volume ratio. It was found that the change of conductance follows a power law depending on the hydrogen concentration. A Langmuir isotherm following an ideal power law and a cross-over behavior of the activation energy with respect to hydrogen concentration were observed. This is a very novel and intriguing phenomenon on nanostructured materials, which suggests competitive surface reactions in ZnO nanorod gas sensors.     

Preparation of nanostructured ZnO nanorods in a hydrothermal-electrochemical process

An array of ZnO nanorods, each nanorod being covered with a shell of porous ZnO was prepared in two steps by hydrothermal-electrochemical processes. The growth of ZnO nanorods was achieved in a zinc nitrate and hexamethylenetetramine aqueous solution on fluorine-doped tin oxide film. The porous ZnO shell was grown from a similar solution in the presence of eosin Y as nanostructuring agent. A dye-sensitized solar cell was assembled using as photoanode an eosin Y sensitized ZnO nanorod/shell layer.     

Fabrication of ZnS/ZnO hierarchical nanostructures by two-step vapor phase method

A simple two-step vapor phase method is presented to fabricate ZnS/ZnO hierarchical nanostructures in bulk quantities. That is ZnS nanobelts were first synthesized and then used as substrate for growth of ZnO nanorod arrays. Investigation results demonstrate that the polar surfaces of ZnS nanobelts could induce a preferred asymmetric growth of ZnO nanorods on the side surfaces. But it is believed that if the local concentration of ZnO was high enough, ZnO nanorods could also grow symmetrically on the top/bottom surface of the ZnS nanobelts. The optical property of the products was also recorded by means of photoluminescence (PL) spectroscopy.     

ZnO纳米棒阵列和薄膜的同步外延生长及其光电化学性能

使用化学气相沉积法在a面蓝宝石衬底上同步外延生长氧化锌(ZnO)竖直纳米棒阵列和薄膜,研究了阵列和薄膜的光电化学性能。结果表明,纳米结构中的竖直单晶纳米棒有六棱柱形和圆柱形,其底部ZnO薄膜使竖直纳米棒互相联通。与ZnO纳米薄膜的比较表明,这种纳米结构具有优异的光电化学性能,其入射光电流效率是ZnO纳米薄膜的2.4倍;光能转化效率是ZnO纳米薄膜的5倍。这种纳米结构优异的光电化学性能,可归因于其高表面积-体积比以及其底部薄膜提供的载流子传输通道。本文分析了这种纳米结构的生长过程,提出了协同生长机理：Au液化吸收气氛中的Zn原子生成合金,合金液滴过饱和后ZnO开始成核,随后在衬底表面生成了ZnO薄膜。同时,还发生了Zn自催化的气-固(VS)生长和Au催化的气-液-固(VLS)生长,分别生成六棱柱纳米棒和圆柱形纳米棒,制备出底部由薄膜连接的竖直纳米棒阵列。     

Controlled growth of ZnO nanomaterials via hydrothermal method: effect of buffer layer

We report a facile solution-based method for the controlled growth of ZnO nanomaterials on an AIN/Si substrate. A ZnO buffer layer was coated on the substrate before growing the ZnO nano-materials. The shape of the ZnO nanomaterials changed from nanosheet to nanorod as the thickness of the ZnO buffer layer increased. Doping of the buffer layer with Ga decreased the average grain size of the ZnO buffer layer, which resulted in the growth of longer and thinner ZnO nanorods on the buffer layer. The UV sensing results of the ZnO nanorod-based device revealed that the aspect ratio of the ZnO nanorods is crucial for enhancing the performance of the device.     

ZnO films and nanorod/shell arrays electrodeposited on PET-ITO electrodes

In this work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. ZnO films with high optical transmittance were prepared from a zinc nitrate solution using a pulsed current technique with a reduced pulse time (3 s). The X-ray diffraction pattern of ZnO film deposited on PET-ITO electrode showed that it has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 0 3]. ZnO nanorods were synthesized on electrochemical seeded substrate in an aqueous solution containing zinc nitrate and hexamethylenetetramine. In order to increase the stability of PET-ITO electrode to electrochemical and chemical stresses during ZnO nanorods deposition the surface of the electrode was treated with a 17 wt% NH₄F aqueous solution. Electrochemical stability of PET-ITO electrode was evaluated in a solution containing nitrate ions and hexamethylenetetramine. ZnO nanorod/shell arrays were fabricated using eosin Y as nanostructuring agent. Photoluminescence spectra of ZnO nanorod and ZnO nanorod/shell arrays prepared on the surface of PET-ITO electrode were discussed comparatively. By employing the 1.5 μm-length ZnO nanorod/shell array covered with a Cu₂O film a photovoltaic device was fabricated on the PET-ITO substrate.     

Density-controlled, solution-based growth of ZnO nanorod arrays via layer-by-layer polymer thin films for enhanced field emission

A simple, scalable, and cost-effective technique for controlling the growth density of ZnO nanorod arrays based on a layer-by-layer polyelectrolyte polymer film is demonstrated. The ZnO nanorods were synthesized using a low temperature (T = 90?°C), solution-based method. The density-control technique utilizes a polymer thin film pre-coated on the substrate to control the mass transport of the reactant to the substrate. The density-controlled arrays were investigated as potential field emission candidates. The field emission results revealed that an emitter density of 7?nanorods?μm(-2) and a tapered nanorod morphology generated a high field enhancement factor of 5884. This novel technique shows promise for applications in flat panel display technology.     

Facile synthesis of zinc hydroxide carbonate flowers on zinc oxide nanorods with attractive luminescent and optochemical performance

A simple synthesis route was designed to fabricate a functional composite, zinc hydroxide carbonate (ZHC) flowers on zinc oxide (ZnO) nanorods. The hydrolysis of hexamethylenetetramine (HMT) can generate various species which are slowly released and gradually change reaction modes in a Zn(NO(3))(2)/HMT solution. As a result, ZnO nanorods and ZHC flowers can be sequentially synthesized and connect very well under constant experimental conditions. The obtained composite has the advantages of both components and exhibits attractive properties. For instance, ZHC flowers on ZnO nanorods exhibit strong blue emission under the excitation of ultraviolet light, and dye-sensitized solar cells with the annealed composite as photoanode achieve much higher conversion efficiency than pure nanorod arrays.     

Electrodeposition of template free hierarchical ZnO nanorod arrays via a chloride medium

We have successfully grown template and buffer free ZnO nanorod films via chloride medium by controlling bath temperature in a simple and cost effective electrochemical deposition method. Thin films of ZnO nano-rods were obtained by applying a potential of ?0.75 V by employing Ag/AgCl reference electrode for 4 h of deposition time. The CV measurements were carried out to determine potential required to deposit ZnO nanorod films whereas chronoamperometry studies were carried out to investigate current and time required to deposit ZnO nanorod films. The formation of ZnO nanorod has been confirmed by scanning electron microscopy (SEM) and Raman spectroscopy. Low angle XRD analysis confirms that ZnO nanorod films have preferred orientation along (101) direction with hexagonal wurtzite crystal structure. The SEM micrographs show nice surface morphology with uniform, dense and highly crystalline hexagonal ZnO nanorods formation. Bath temperature has a little influence on the orientation of nanorods but has a great impact on their aspect ratio. Increase in bath temperature show improvement in crystallinity, increase in diameter and uniform distribution of nanorods. Compositional analysis shows that the amount of oxygen is ~49.35 % and that of Zn is ~50.65 %. The optical band gap values were found to be 3.19 and 3.26 eV for ZnO nanorods prepared at bath temperature 70 and 80 °C respectively. These results indicate that by controlling the bath temperature band gap of ZnO nanorods can be tailored. The obtained results suggest that it is possible to synthesize ZnO nanorod films by a simple, cost effective electrodeposition process which can be useful for opto-electronic devices fabrication.     

A high photocurrent gain in UV photodetector based on Cu doped ZnO nanorods on PEN substrate

Vertical well-aligned Cu-doped ZnO nanorods were successfully synthesized by chemical bath deposition (CBD) method on low cost and flexible polyethylene naphthalate (PEN) substrate. The structural and optical investigations exhibited the high quality of the Cu-doped ZnO nanorods on a flexible PEN substrate. The metal-semiconductor-metal (MSM) configuration was used to fabricate UV photodetector based on the Cu-doped ZnO nanorods grown on PEN substrate. Under a 5 V applied bias, the values of dark current and photocurrent of the Cu-doped ZnO nanorods photodetector were 14.9 µA and 3.27 mA, respectively. Meanwhile, calculated photocurrent gain of the UV photodetector was 219 at 5 V bias voltage. Upon exposure to 365 nm UV light, the UV device exhibited fast response time and recovery time of 0.317 and 0.212 s, respectively, at a bias voltage of 5 V.     

Wettability control of a transparent substrate using ZnO nanorods

This paper presents a simple way of controlling the wettability of a structured surface with ZnO nanorods on a transparent substrate. A combination of ZnO nanostructures and stearic acid was used to create superhydrophobic surfaces with the potential properties of being self-cleaning, waterproof, and antifog. ZnO nanorods were uniformly covered on glass substrates through a simple hydrothermal method with varying growth time which affects the surface morphology. When a substrate is dipped into 10 mM stearic acid in ethanol for 24 h, chemisorption of the stearic acid takes place on the ZnO nanorod surface, after which the hydrophilic ZnO nanorod surfaces are modified into hydrophobic ones. The contact angle of a water droplet on this superhydrophobic ZnO nanorod surface increased from 110 degrees to 150 degrees depending on the growth time (from 3 to 6 h) with a high transparency of above 60%. In addition, the water contact angle can be made to as low as 27 degrees after exposing the substrate to 10-mW/cm2 UV for 1 h.     

Controllable synthesis of vertically aligned ZnO nanorod arrays in aqueous solution

High-density well-aligned ZnO nanorod array was successfully synthesized on a large-area magnetron sputtering deposited Al doped ZnO film-coated Si (AZO/Si) substrate via a convenient solution method. X-ray diffraction and scanning electron microscopy show that the nanorods are well-oriented perpendicular to the substrate. The influences of the reaction temperature, time, on the size and shapes of the as-prepared ZnO nanorods (ZNs) samples have been studied. The length and diameter of the nanorods became bigger when a longer reaction time was used. When the temperature is elevated to 130 degrees C, a new conical ZNs was synthesized. Room-temperature photoluminescence (PL) spectra of all the ZnO products showed a strong ultraviolet (UV) emission. The photoluminescence from free excitons of the ZNs synthesized at higher temperature reflects the high purity and nearly defect free structure of nanorods. The well-aligned feature of the nanorod array is attributed to the nanorods' epitaxial growth from the AZO films.     

CTAB-assisted hydrothermal synthesis of single-crystalline copper-doped ZnO nanorods and investigation of their photoluminescence properties

A simple CTAB-assisted hydrothermal synthesis of undoped and copper-doped ZnO nanorods is reported. The phase and structural analysis carried out by X-ray diffraction, shows the formation of hexagonal wurtzite structure of ZnO. Morphology of the ZnO nanorods was investigated by electron microscopy techniques which showed the formation of well dispersed regular shape ZnO nanorods of 100 ± 10 nm in diameter and 900 ± 100 nm in length. However, size of the copper doped ZnO nanorod slightly increased with increasing copper concentration. Furthermore, the selected area electron diffraction pattern and high resolution transmission electron microscopy reveal that both the undoped and copper doped ZnO nanorods were single crystalline in nature and preferentially grew up along [0001] direction. Optical property was investigated by photoluminescence spectroscopy. The effects of copper doping on the photoluminescence property of ZnO nanorods were investigated.     

Effects of Pt junction on electrical transport of individual ZnO nanorod device fabricated by focused ion beam

The electrical transport of individual ZnO nanorod devices manufactured by focused ion beam (FIB) was investigated by the direct measurement of electrical resistance at electrode junctions of cross-sectioned devices using two nanoprobes. The cathodoluminescence (CL) measurements were also performed to evaluate the crystallinity at the center and edge of the cross-sectioned ZnO nanorods. The electrical transport of the individual ZnO nanorod device depends strongly on the crystallinity of the ZnO nanorod itself and the carbon contents at Pt junctions. The ZnO-Au junction of the device acted as the fastest path for electrical transport.     

微流控技术制备ZnO纳米棒及生物荧光检测性能研究

本研究设计并制备了一种微流控芯片并在其中水热合成了氧化锌(ZnO)纳米棒。利用扫描电子显微镜(SEM)和X射线衍射(XRD)研究了合成条件对ZnO纳米棒的形貌和晶体结构的影响。结果表明, 在微流控芯片中可制备得到致密的ZnO纳米棒, 其直径和长度随加热方式和制备时间的变化而改变。对比研究不同加热方式制备的ZnO纳米棒阵列检测异硫氰酸荧光素标记的羊抗牛IgG的性能, 发现局部加热方式制备的ZnO纳米棒检测荧光素标记蛋白的性能更佳, 在10 pg/mL~1 μg/mL范围内线性良好, 相关系数为0.99209。在此基础上, 用局部加热制备的ZnO纳米棒检测人甲胎蛋白(AFP), 其最低检测限可达1 pg/mL。这些结果表明, 微通道中合成的ZnO纳米棒适用于多通道荧光检测。     

Silver nanoisland induced synthesis of ZnO nanostructures by vapor phase transport

ZnO nanostructures including nanorod and nanotower were synthesized on Ag nanoisland coated Si substrate by thermal evaporation and vapor phase transport at atmospheric pressure. The as-prepared ZnO nanorods and nanotowers were single crystal growing along [0001] direction. The growth of ZnO nanostructures strongly depended on the surface morphology of the nanoisland Ag film deposited by electroless nanoelectrochemistry. The growth mechanism of the ZnO nanostructures was proposed on the basis of experimental data. A strong room-temperature photoluminescence in ZnO nanostructures has been demonstrated. The growth technique would be of particular interest for direct integration in the current silicon-technology-based optoelectronic devices.