Selective growth of ZnO nanowires on substrates patterned by photolithography and inkjet printing

Zinc oxide nanowires (ZnO NWs) were grown by a two-step growth method, involving the deposition of a patterned ZnO thin seeding layer and the chemical vapor deposition (CVD) of ZnO NWs. Two ways of patterning the seed layer were performed. The seeding solution containing ZnO precursors was deposited by sol–gel/spin-coating technique and patterned by photolithography. In the other case, the seeding solution was directly printed by inkjet printing only on selected portion of the substrate areas. In both cases, crystallization of the seed layer was achieved by thermal annealing in ambient air. Vertically aligned ZnO NWs were then grown by CVD on patterned, seeded substrates. The structure and morphology of ZnO NWs was analyzed by means of X-ray diffraction and field emission scanning electron microscopy measurements, respectively, while the vibrational properties were evaluated through Raman spectroscopy. Results showed that less-defective, vertically aligned, c-axis oriented ZnO NWs were grown on substrates patterned by photolithography while more defective nanostructures were grown on printed seed layer. A feature size of 30 µm was transferred into the patterned seed layer, and a good selectivity in growing ZnO NWs was obtained.     

Synthesis and optical characterization of vertically grown ZnO nanowires in high crystallinity through vapor-liquid-solid growth mechanism

The gas-phase growth and optical characteristics of 1-dimensional ZnO nanostructure have been investigated. The ZnO nanowires (NWs) were grown vertically on Au coated silicon substrates by vapor-liquid-solid (VLS) growth mechanism using chemical vapor deposition (CVD). The ZnO NWs were grown in the crystal direction of [0 0 0 1]. The ZnO NWs exhibit the uniform size of less than 100 nm in diameter and up to 5 μm in length. Photoluminescence (PL) spectrum of ZnO NWs shows the strong band-edge emission at ∼380 nm (∼3.27 eV) without significant deep-level defect emission. The exciton lifetime of ZnO NWs was measured to be approximately 150 ± 10 ps.     

Preparation of silver nanoparticles on zinc oxide nanowires by photocatalytic reduction for use in surface‐enhanced Raman scattering measurements

To increase the sensitivity in surface‐enhanced Raman scattering (SERS) measurements, the high surface area of zinc oxide nanowires (ZnO NWs) was used. ZnO NWs on silicon substrates were prepared and used as substrates for further growth of silver nanoparticles (AgNPs). Ultraviolet (UV) irradiation was used to reduce silver ions to AgNPs on the ZnO wires. With proper growth conditions for both ZnO NWs and AgNPs, the substrates exhibit SERS enhancement factors greater than 10⁶. To understand the influences of the morphologies of the ZnO NWs on the growth of AgNPs, the growing time and temperature were varied. The concentration of silver nitrate and irradiation time of UV radiation were also varied. The resulting AgNPs were probed with para‐nitrothiophenol to quantify the SERS enhancements obtained from the varying conditions. The results indicate that ZnO NWs could be grown at temperatures higher than 490 °C and higher growth temperatures result in smaller diameter of the formed ZnO NWs. Also, the morphologies of ZnO NWs did not significantly alter the SERS signals. The concentration of silver nitrate affects the SERS signals significantly and the optimal concentration was found to be in the range of 10–20 mM. With irradiation times longer than 90 s, the resulting AgNPs showed similar SERS intensities. With optimized conditions, the AgNPs/ZnO substrates are highly suitable for SERS measurements with a typical enhancement factor of higher than 10⁶. Copyright © 2010 John Wiley & Sons, Ltd.     

Tuning the growth of ZnO nanowires

ZnO nanowires (NWs) with different diameters were obtained by controlling the particles of ZnO sub-layer (SL) exploring hydrothermal method; the diameter of the epitaxial NWs could be tuned from 60 to 146 nm when using SL with a thickness of 70 nm. The thickness of the SL would influence the orientation of the NWs. The top agglomerate NWs could be formed on the SL with a thickness of 10 nm, and the NWs with better orientation were obtained using SL with a thickness of 70 nm. Well aligned ZnO NWs grew perpendicular to the completely stress released SL. The diameter of the NWs was also greatly influenced by the solution concentration; thus ultra fine (diameter∼11 nm) ZnO NWs were obtained through adjusting the solution concentration to 0.001 mol/L. Through our research, we also found that the growth rate of the NWs could also be influenced by the different polarity surface of the SL. In other words, the size of the ZnO NWs could be tuned exactly under optimal conditions.     

垂直氧化锌纳米线中的激射现象

通过水热的方法以退火0.5 h的ZnO薄膜作为籽晶,得到垂直的ZnO纳米线.在X射线衍射谱中,除了Si的(400)衍射峰以外,只观察到了ZnO的(002)衍射峰.室温光致发光谱中出现了强的紫外发射峰,同时也伴随着弱的缺陷相关的发射.这些数据表明垂直的ZnO纳米线序列有着较好的晶体质量.同时,通过光泵浦也观察到了ZnO纳...     

Contact welding study of carbon nanotube with ZnO nanowire

Study of proton beam induced welding of multiwall carbon nanotubes (MWCNTs) with ZnO nanowires (NWs) has been carried out by proton (H⁺) beam irradiation. The samples were irradiated by 70-keV proton (H⁺) ion beams at different substrate temperatures. The irradiation-induced defects in CNTs and ZnO NWs were greatly reduced at elevated temperature. The crystalline structure of ZnO NWs and MWCNTs were found to remain stable after the irradiation at 700 K. As a preparation step, a coupling of two parallel ZnO NWs with irradiation has also been demonstrated. The welding mechanisms of MWCNTs and ZnO NWs were also been suggested. These two welding processes between same and distinct nanostructures to form homo- and hetero-junctions have provided an opportunity to mass produce interconnecting one-dimensional structures used for the manufacturing of future nanowire-based electronic circuits and devices.     

微流控技术制备ZnO纳米线阵列及其气敏特性

利用微流控技术在微通道中制备了Zn O纳米线阵列,通过X射线衍射和扫描电子显微镜分别对纳米线的物相和表面形貌进行了表征.结果发现,合成的Zn O纳米线具有良好的c轴择优取向性和结晶度.同时,对Zn O纳米线阵列在丙酮、甲醇和乙醇气体中的气敏特性进行了研究,测试结果表明:在最佳工作温度(475?C)下,纳米线阵列对200 ppm(1 ppm=10-6)丙酮气体的最大灵敏度可达8.26,响应恢复时间分别为9和5 s;通过与传统水热法制备的Zn O纳米线的气敏性能相比较发现,基于微流控技术制备的纳米线阵列具有更高的灵敏度和更快的响应恢复速度.最后,从材料表面氧气分子得失电子的角度对Zn O纳米线气敏机理进行了讨论.     

ZnO纳米线薄膜的合成参数、表面形貌和接触角关系研究

水浴法合成ZnO纳米线薄膜的工艺参数直接影响其表面形貌, 并使其接触角及润湿性能发生变化. 本文仿真分析了轮廓算数平均偏差、偏斜度、峭度、相关长度等特征参数对随机粗糙表面特性的影响规律; 改变生长时间、种子层溶液和生长液的浓度, 批量制备了表面形貌不同的ZnO纳米线薄膜; 提出了取样长度的确定方法, 并基于扫描电镜图像和Matlab图像处理算子对ZnO纳米线薄膜表面形貌的特征参数进行了提取; 将表面形貌高度和水平方向的特征参数引入Wenzel模型, 分析了合成参数、表面形貌特征参数与接触角的影响关系. 结果表明, 合成参数变化时, 选择取样长度5.0 μm为宜; 生长液浓度大于0.125 mol/L时, ZnO纳米线之间发生重结晶, 并呈现疏水性; 改变种子层溶液浓度和生长时间, 均得到超亲水表面. 上述结论可用于不同氧化酶、细胞等在ZnO纳米线薄膜上的有效吸附及相应传感器测试性能的进一步提高. 关键词： ZnO纳米线 水浴合成 表面形貌 接触角     

Solution-phase synthesis of nanomaterials at low temperature

This paper reviews the solution-phase synthesis of nanoparticles via some routes at low temperatures, such as room temperature route, wave-assisted synthesis (γ-irradiation route and sonochemical route), directly heating at low temperatures, and hydrothermal/solvothermal methods. A number of strategies were developed to control the shape, the size, as well as the dispersion of nanostructures. Using diethylamine or n-butylamine as solvent, semiconductor nanorods were yielded. By the hydrothermal treatment of amorphous colloids, Bi₂S₃ nanorods and Se nanowires were obtained. CdS nanowires were prepared in the presence of polyacrylamide. ZnS nanowires were obtained using liquid crystal. The polymer poly (vinyl acetate) tubule acted as both nanoreactor and template for the CdSe nanowire growth. Assisted by the surfactant of sodium dodecyl benzenesulfonate (SDBS), nickel nanobelts were synthesized. In addition, Ag nanowires, Te nanotubes and ZnO nanorod arrays could be prepared without adding any additives or templates. Supported by the National Basic Research Program of China (Grant No. 2005CB623601) and the National Natural Science Foundation of China (Grant No. 20431020)     

Growth by molecular beam epitaxy and electrical characterization of GaAs nanowires

We report on structural and electrical properties of GaAs nanowires (NWs) grown by molecular beam epitaxy (MBE) on GaAs and SiO₂ substrates using Au as growth catalyst. Au–Ga particles are observed on the top of the NWs by transmission electron microscopy (TEM). In most of the observed cases, individual particles contain two Au–Ga compositions, in particular orthorhombic AuGa and β′ hexagonal Au₇Ga₂. The wires grown on GaAs are regularly shaped and tidily oriented on both (1 0 0) and (1 1 1)B substrates. TEM also reveals that the NWs have a wurtzite lattice structure. Electrical transport measurements indicate that nominally undoped NWs are weakly n-type while both Be- and Si-doped wires show p-type behaviour. The effect of the lattice structure on impurity incorporation is briefly discussed.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Growth mechanism studies of ZnO nanowire arrays via hydrothermal method

Well-controlled ZnO nanowire arrays have been synthesized using the hydrothermal method, a low temperature and low cost synthesis method. The process consists of two steps: the ZnO buffer layer deposition on the substrate by spin-coating and the growth of the ZnO nanowire array on the seed layer. We demonstrated that the microstructure and the morphology of the ZnO nanowire arrays can be significantly influenced by the main parameters of the hydrothermal method, such as pH value of the aqueous solution, growth time, and solution temperature during the ZnO nanowire growth. Scanning electron microscopy observations showed that the well oriented and homogeneous ZnO nanowire arrays can be obtained with the optimized synthesis parameters. Both x-ray diffraction spectra and high-resolution transmission electron microscopy (HRTEM) observations revealed a preferred orientation of ZnO nanowires toward the c-axis of the hexagonal Wurtzite structure, and HRTEM images also showed an excellent monocrystallinity of the as-grown ZnO nanowires. For a deposition temperature of 90 °C, two growth stages have been identified during the growth process with the rates of 10 and 3 nm/min, respectively, at the beginning and the end of the nanowire growth. The ZnO nanowires obtained with the optimized growth parameters owning a high aspect ratio about 20. We noticed that the starting temperature of seed layer can seriously influence the nanowire growth morphology; two possible growth mechanisms have been proposed for the seed layer dipped in the solution at room temperature and at a high temperature, respectively.     

A templateless surfactant-free seedless aqueous route to single-crystalline ZnO nanowires synthesis

Zinc oxide (ZnO) nanowires (NWs) have been synthesized using zinc nitrate and hexamethylenetetramine by templateless, surfactant-free and seedless aqueous solution route. The morphology of ZnO NWs was considerably affected by growth time: a longer reaction time results in the formation of ZnO NWs. Structural analysis of the synthesized NWs showed an average diameter of 20–30 nm length of several micrometers and single-crystalline wurtzite hexagonal structure. Photoluminescence studies of ZnO NWs showed a strong green emission peak at 585 nm.     

Synthesis and photoluminescence properties of SnO2/ZnO hierarchical nanostructures

SnO₂/ZnO hierarchical nanostructures were synthesized by a two-step carbon assisted thermal evaporation method. SnO₂ nanowires were synthesized in the first step and were then used as substrates for the following growth of ZnO nanowires in the second step. Sn metal droplets were formed at the surfaces of the SnO₂ nanowires during the second step and were acted as catalyst to facilitate the growth of ZnO nanowires via vapor-liquid-solid mechanism. Room temperature photoluminescence measurements showed that the SnO₂/ZnO hierarchical nanostructures exhibited a strong green emission centered at about 520 nm and a weak emission centered at about 380 nm. The emissions from the SnO₂ were drastically constrained due to screen effect caused by the ZnO layer.     

Gas sensing properties of single crystalline ZnO nanowires grown by thermal evaporation technique

The ZnO NWs were applied as effective material for the fabrication of ethanol (C₂H₅OH) and carbon monoxide (CO) gas sensor. The ZnO NWs were grown by thermal evaporation techniques on non-catalytic Si (100) substrates. The average width and length of ZnO NWs was 60 nm and 20 μm, respectively and they were single crystalline in nature. The maximum response was 51.64 at 300 °C for 1000 ppm of CO gas, while 104.23 at 400 °C for 250 ppm of ethanol gas. The response of ZnO NWs was very high for ethanol compared to the CO, whereas the recovery time for ethanol was very poor compare to CO gas. The response of ZnO NWs was about 25 times higher for ethanol compare to CO, at 400 °C for 100 ppm of each gas. The high response for ethanol is related to electron donating effect of ethanol (10e^?) which was higher than the CO gas (2e^?). The high response of ZnO NWs was attributed to large contacting surface area for electrons, oxygen, target gas molecule, and abundant channels for gas diffusion.     

Functionalized Pd/ZnO Nanowires for Nanosensors

A method for surface doping and functionalization of ZnO nanowires (NWs) with Pd (Pd/ZnO) in a one‐step process is presented. The main advantage of this method is to combine the simultaneous growth, surface doping, and functionalization of NWs by using electrochemical deposition (ECD) at relatively low temperatures (90 °C). Our approach essentially reduces the number of technological steps of nanomaterial synthesis and final nanodevices fabrication with enhanced performances. A series of nanosensor devices is fabricated based on single Pd/ZnO NWs with a radius of about 80 nm using a FIB/SEM system. The influence of Pd nominal composition in Pd/ZnO NW on the H₂ sensing response is studied in detail and a corresponding mechanism is proposed. The results demonstrate an ultra‐high response and selectivity of the synthesized nanosensors to hydrogen gas at room temperature. The optimal concentration of PdCl₂ in the electrolyte to achieve extremely sensitive nanodevices with a gas response (S_H2) ≈ 1.3 × 10⁴ (at 100 ppm H₂ concentration) and relatively high rapidity is 0.75 µM. Theoretical calculations on Pd/ZnO bulk and functionalized surface further validated the experimental hypothesis. Our results demonstrate the importance of noble metal presence on the surface due to doping and functionalization of nanostructures in the fabrication of highly‐sensitive and selective gas nanosensors operating at room temperature with reduced power consumption.     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.     

Synthesis,structural and ellipsometric evaluation of oxygen-deficient and nearly stoichiometric zinc oxide and indium oxide nanowires/nanoparticles

Oxygen-deficient (OD) and nearly stoichiometric (NST) ZnO and In₂O₃ nanowires/nanoparticles were synthesized by chemical vapor deposition on Au-coated silicon substrates. The OD ZnO and OD In₂O₃ nanowires were synthesized at 750 and 950°C, respectively, using Ar flow at ambient pressure. A mixture of flowing Ar and O₂ was used for synthesizing NST ZnO nanowires and NST In₂O₃ nanoparticles. Growth of OD ZnO nanowires and NST In₂O₃ nanoparticles was found to be via a vapor–solid (VS) mechanism and the growth of NST ZnO nanowires was via a vapor–liquid–solid mechanism (VLS). However, it was uncertain whether the growth of OD In₂O₃ nanowires was via a VS or VLS mechanism. The optical constants, thickness and surface roughness of the prepared nanostructured films were determined by spectroscopic ellipsometry measurements. A three-layered model was used to fit the calculated data to the experimental ellipsometric spectra. The refractive index of OD ZnO, NST ZnO nanowires and NST In₂O₃ nanoparticles films displayed normal dispersion behavior. The calculated optical band gap values for OD ZnO, NST ZnO, OD In₂O₃ nanowires and NST In₂O₃ nanoparticles films were 3.03, 3.55, 2.81 and 3.52?eV, respectively.     

Tunable emission and conductivity enhancement by tellurium doping in CdS nanowires for optoelectronic applications

Improvement of the optical and electrical characteristics is essential to get advanced performance from one dimensional (1D) material. Here, we report the first synthesis of a single crystalline Te-doped CdS nanowires (NWs) by a chemical-vapor-deposition (CVD) method. Room temperature photoluminescence (PL) spectra showed that Te concentration plays an important role in tuning emission color from orange to infrared (IR). Decrease in bandgap and PL intensity with increase in Te concentration was observed as compared to undoped CdS NWs. Red and IR emissions were found at 736.5 and 881 nm for doping concentration >6.06%. To our best knowledge, IR emission band has been observed for the first time in CdS NWs. Red-shift of LO phonon mode and its overtone in Raman spectra, and lifetime of red and IR emissions are longer than bandgap of host indicating the doping effect of CdS NWs. Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) of the Te-doped CdS NWs further confirms the presence of Te in the CdS NWs. Output characteristics confirm enhanced output current I_ds with the increase in doping concentration. A possible growth mechanism was proposed. Doping technique offers to develop high-quality, a very stable, effective, and easily-applicable way to enhance the performance of one dimensional optoelectronic devices and solar cell applications.     

Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting

Tungsten trioxide (WO₃) is a technologically important material for photoelectrochemical (PEC) water-splitting for the solar production of hydrogen fuel from water. For PEC water-splitting, high aspect ratio WO₃ nanostructures such as nanowires (NWs) and nanotubes (NTs) are superior to planar WO₃ films because they orthogonalize the directions of light absorption (along the long axis) and charge transport (across the short radius), leading to both efficient light absorption and charge carrier collection. However, PEC water-splitting requires the growth of WO₃ on delicate transparent conducting oxide (TCO) substrates that cannot tolerate high temperature processing. To date, the large-scale, rapid, economical synthesis of high aspect ratio WO₃ nanostructures on these delicate TCO substrates remains a major challenge. Previously, we synthesized WO₃ NW arrays by a rapid, atmospheric and scalable flame vapor deposition (FVD) method, in which a flame oxidizes and evaporates tungsten metal to produce tungsten oxide vapors that condense onto a colder substrate in the form of NWs. Nevertheless, at substrate temperatures low enough to ensure the health of the TCO, the growth of WO₃ NW arrays was non-uniform and sparse due to limitations of the experimental design. Herein, we significantly improve the FVD design to grow uniform and densely packed WO₃ nanostructures on TCO substrates, thereby enabling the application of these WO₃ nanostructures to PEC water-splitting. The morphology of the nanostructures varied from densely packed multi-shell NTs and single-shell NTs to NWs as we increased the substrate temperature in the range 530–700 °C. Importantly, the WO₃ NTs synthesized by FVD had higher areal number density and longer length than state-of-the-art WO₃ NW photoanodes grown by chemical vapor deposition and hydrothermal methods, resulting in stronger light absorption and superior PEC water-splitting performance. Thus, in addition to being scalable, rapid and economical, the FVD method also synthesizes materials of high quality.