Systematic study on experimental conditions for large-scale growth of aligned ZnO nanowires on nitrides

In vapor-liquid-solid (VLS) growth, it is generally believed that nanowires would grow as long as the right catalysts and substrate are supplied as well as the growth temperature is adequate. We show here, however, that oxygen partial pressure plays a key role in determining the quality of the aligned ZnO nanowires. We present a "phase diagram" between the oxygen partial pressure and the growth chamber pressure for synthesizing high quality aligned ZnO nanowires on GaN substrate. This result provides a road map for large-scale, controlled synthesis of ZnO nanowires on nitride semiconductor substrates with the potential to meet the needs of practical applications. The chemical process involved in the growth process is also systematically elaborated based on experimental data received under different conditions.     

Control of the ZnO nanowires nucleation site using microfluidic channels

We report on the growth of uniquely shaped ZnO nanowires with high surface area and patterned over large areas by using a poly(dimethylsiloxane) (PDMS) microfluidic channel technique. The synthesis uses first a patterned seed template fabricated by zinc acetate solution flowing though a microfluidic channel and then growth of ZnO nanowire at the seed using thermal chemical vapor deposition on a silicon substrate. Variations the ZnO nanowire by seed pattern formed within the microfluidic channel were also observed for different substrates and concentrations of the zinc acetate solution. The photocurrent properties of the patterned ZnO nanowires with high surface area, due to their unique shape, were also investigated. These specialized shapes and patterning technique increase the possibility of realizing one-dimensional nanostructure devices such as sensors and optoelectric devices.     

Synthesis and Characterization of ZnO Nanowires

Zinc oxide is a wide bandgap (3.37 eV) semiconductor with a hexagonal wurtzite crystal structure. ZnO prepared in nanowire form may be used as a nanosized ultraviolet light-emitting source. In this study, ZnO nanowires were prepared by vapor-phase transport of Zn vapor onto gold-coated silicon substrates in a tube furnace heated to 900 ?C. Gold serves as a catalyst to capture Zn vapor during nanowire growth. Size control of ZnO nanowires has been achieved by varying the gold film thickness…     

采用高分子自组装ZnO纳米线及其形成机理

介绍了一种能在各种晶面的硅衬底上制备垂直于衬底取向生长的ZnO纳米线阵列的新方法. 该法采用高分子络合和低温氧化烧结反应, 以聚乙烯醇(PVA)高分子材料作为自组装络合载体来控制晶体成核和生长. 首先通过PVA侧链上均匀分布的极性基团羟基(—OH)与锌盐溶液中的Zn²⁺离子发生络合作用, 然后滴加氨水调节络合溶液pH值为8.5±0.1, 使络离子Zn²⁺转变为Zn(OH)₂, 再将硅片浸入此溶液中, 从而在硅衬底表面得到较均匀的Zn(OH)₂纳米点, 随后在125 ℃左右Zn(OH)₂纳米点通过热分解转化为ZnO纳米点, 其后在420 ℃烧结过程中衬底上的ZnO纳米点在PVA高分子网络骨架对其直径的限域下逐渐取向生长成ZnO纳米线, 并且烧结初期PVA碳化形成的碳通过碳热还原ZnO为Zn, 再在氧气氛中氧化为ZnO的方式在纳米线顶端形成了催化活性点, 促进了纳米线顶端ZnO的吸收. 烧结后碳逐渐氧化被完全去除. 采用场发射扫描电镜(FE-SEM)、透射电镜(TEM, HR-TEM)和X射线衍射(XRD)对纳米线的分析结果表明, ZnO纳米线在硅衬底上分布均匀, 具有六方纤锌矿结构, 并且大多沿[0001]方向择优取向生长, 直径为20～80 nm, 长度可从0.5至几微米. 提出了聚合物控制ZnO结晶和形貌的网络骨架限域模型以解释纳米线的生长行为.     

High surface-to-volume ratio ZnO microberets: low temperature synthesis, characterization, and photoluminescence

Novel hollow ZnO microstructures and ZnO microberets (ZMBs) with nanowires grown vertically on both the inner and outer surfaces of beret shells were synthesized on Si(100) substrates by simple thermal evaporation of pure zinc powder without any catalyst or template material at a relative low temperature of 490 degrees C. XRD, SAED, and HRTEM patterns show that the nanowires and shells of ZMBs are single-crystalline wurtzite structures. The growth mechanism of ZMBs is discussed in detail. The formation of these hollow microstructures depends on the optimum starting time of air introduction. It is a good way to grow well-aligned nanowires by using a nanoscale rough ZnO surface to realize a "self-catalyzed" vapor-liquid-solid process. The photoluminescence spectrum reveals a strong green emission related to the high surface-to-volume ratio of ZMBs. These types of special hollow high surface area structural ZMBs may find potential applications in functional architectural composite materials, solar cell photoanodes, and nanooptoelectronic devices.     

Growth of large-area aligned molybdenum nanowires by high temperature chemical vapor deposition: synthesis, growth mechanism, and device application

Large-area aligned Mo nanowires have been grown on stainless steel substrates by high-temperature chemical vapor deposition with the use of Mo metal. The detailed physical and chemical growth processes regarding the formation of the nanowires have been investigated using mass spectroscopy, thermogravimetry, and differential scanning calorimetry analysis, as well as structure analysis by electron microscopy. In reference to Gibbs energy calculation, our study reveals that the growth relies on the decomposition of MoO2 vapors through condensation of its vapor at high substrate temperatures. The aligned growth is a result of competing growth with the nanowires normal to the substrate surface reaching the final growth front. The field emission measurement and the vacuum luminescent tube study show that the Mo nanowires have potential application as electron emitters.     

Density-controlled growth of aligned ZnO nanowires sharing a common contact: a simple, low-cost, and mask-free technique for large-scale applications

An effective, low cost, simple, and mask-free pathway is demonstrated for achieving density control of the aligned ZnO nanowires grown for large-scale applications. By a slight variation of the thickness of the thermally evaporated gold catalyst film, a significant change in the density of aligned ZnO nanowires has been controlled. The growth processes of the nanowires on an Al(0.5)Ga(0.5)N substrate has been studied based on the wetting behavior of gold catalyst with or without source vapor, and the results classify the growth processes into three categories: separated dots initiated growth, continuous layer initiated growth, and scattered particle initiated growth. This study presents an approach for growing aligned nanowire arrays on a ceramic substrate with the simultaneous formation of a continuous conducting electrode at the roots, which is important for device applications, such as field emission.     

Comparative study of ZnO and CuSCN semiconducting nanowire electrodeposition on different substrates

In this study, we are reporting on the electrochemical deposition of two kinds of semiconducting nanowires (ZnO and CuSCN) on different substrates. ZnO and CuSCN are n- and p-type transparent semiconductors whose electrochemical preparation has some similarity, and it is a combination of two steps: an electrochemical reduction with consecutive chemical precipitation. Here, we show that despite the different physicochemical nature of the studied materials, when they are deposited electrochemically, their dimensions depend mainly on the surface state of the used substrate. Thus, depending on the substrate morphology, nanowires with diameters between 50 and 380 nm from both semiconductors could be grown. It is also shown that ZnO and CuSCN nanowires could be successfully grown on glass and plastic substrates as well as on a metallic one. The possibility of growing these transparent semiconductors on flexible substrates opens new perspectives for their use in “invisible” electronic devices.     

One-step electrodeposition of Ag-decorated ZnO nanowires

A new route for synthesizing Ag-decorated ZnO nanowires (NWs) on conductive glass substrates using a one-step electrodeposition technique is described here. The structural, optical, and photoelectrochemical properties of Ag-decorated ZnO nanowires were studied in detail using techniques such X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-visible spectroscopy, photoluminescence, and photoelectrochemical measurements. Both pure and Ag-decorated ZnO nanowires were found to crystallize in the wurtzite structure, irrespective of their Ag contents. Increasing the Ag content from pure ZnO NWs to 3% Ag ZnO NWs decreases the photoluminescence intensity, shifts the optical band gap to the red, and increases the photocurrent up to threefold. This behavior was attributed to the surface plasmon resonance effect induced by the Ag nanoparticles, which inhibits charge recombination and improves charge transport on the ZnO surface.     

Anisotropic wet etched silicon substrates for reoriented and selective growth of ZnO nanowires and enhanced hydrophobicity

Herein we report the fabrication of ZnO nanowires on anisotropic wet etched silicon substrates by selective hydrothermal growth. <100> oriented silicon wafers were first patterned by anisotropic wet etch with a KOH solution, resulting in V-shaped stripes of different periods. Then, a thin layer of gold was deposited and annealed to promote the hydrothermal growth of ZnO nanowires. It was found that the growth rate of ZnO nanowires on <111> surfaces was much higher than that on <100> surfaces. As a first application of such micro- and nanostructured surfaces, we show enhanced wetting properties by measuring the contact angle of water droplets on the samples obtained under different patterning and growth conditions. Our results also demonstrated the possibility of tuning the contact angle of the sample in the range between 115° and 155°, by changing either the pattern of the silicon template or the hydrothermal growth conditions.     

Near-room-temperature production of diameter-tunable ZnO nanorod arrays through natural oxidation of zinc metal

A simple, low-temperature strategy has been developed for the low-cost and large-area fabrication of ZnO nanoarrays on a zinc substrate by the natural oxidation of zinc metal in formamide/water mixtures. The one-step, wet-chemical approach has exhibited well-controlled growth of highly oriented and densely packed ZnO nanoarrays with large-area homogeneity and consisting of nanorods or nanowires with predictable morphologies, such as tunable diameters and identical lengths. The chemical-liquid-deposition process, an analogue to the widely used chemical-vapor-deposition technique, has been used for the near-room-temperature production of ZnO nanoarrays through continuous supply, transport, and thermal decomposition of zinc complexes in a liquid phase.     

Synthesis and characterization of ZnO nanowires by thermal oxidation of Zn thin films at various temperatures

In this research high-quality zinc oxide (ZnO) nanowires have been synthesized by thermal oxidation of metallic Zn thin films. Metallic Zn films with thicknesses of 250 nm have been deposited on a glass substrate by the PVD technique. The deposited zinc thin films were oxidized in air at various temperatures ranging between 450 °C to 650 °C. Surface morphology, structural and optical properties of the ZnO nanowires were examined by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) and photoluminescence (PL) measurements. XRD analysis demonstrated that the ZnO nanowires has a wurtzite structure with orientation of (002), and the nanowires prepared at 600 °C has a better crystalline quality than samples prepared at other temperatures. SEM results indicate that by increasing the oxidation temperature, the dimensions of the ZnO nanowires increase. The optimum temperature for synthesizing high density, ZnO nanowires was determined to be 600 °C. EDX results revealed that only Zn and O are present in the samples, indicating a pure ZnO composition. The PL spectra of as-synthesized nanowires exhibited a strong UV emission and a relatively weak green emission.     

硫化锌与硫化锌/氧化锌异质结纳米线的化学气相沉积法制备与表征

介绍一种利用石墨还原快速制备大量硫化锌纳米线的方法,并分别合成了超晶格型、双轴型、核/壳型的硫化锌/氧化锌异质结纳米线。所合成的硫化锌纳米线存在六方纤锌矿和立方闪锌矿两种晶型,纳米线长度达几十微米,直径在20-50 nm,直径均匀且产量很高。在具有双轴型的硫化锌/氧化锌异质结中,首次发现具有超结构特征的氧化锌。HRTEM分析表明,硫化锌/氧化锌超晶格异质结界面为ZB-ZnS(111)∥ZnO(0001),而核/壳型异质结界面为W-ZnS(0001)∥ZnO(0001),这三个晶面分别为各自晶体的极性面,即所合成的硫化锌/氧化锌异质结中极性面相互平行。对ZnS 和ZnS/ZnO 异质结的生长机制进行了探讨,并对硫化锌纳米线与硫化锌/氧化锌异质结的光学性质进行了分析。     

Property–Performance Control of Multidimensional,Hierarchical, Single‐Crystalline ZnO Nanoarchitectures

Diverse morphologies of multidimensional hierarchical single‐crystalline ZnO nanoarchitectures including nanoflowers, nanobelts, and nanowires are obtained by use of a simple thermal evaporation and vapour‐phase transport deposition technique by placing Au‐coated silicon substrates in different positions inside a furnace at process temperatures as low as 550 °C. The nucleation and growth of ZnO nanostructures are governed by the vapour–solid mechanism, as opposed to the commonly reported vapour–liquid–solid mechanism, when gold is used in the process. The morphological, structural, compositional and optical properties of the synthesized ZnO nanostructures can be effectively tailored by means of the experimental parameters, and these properties are closely related to the local growth temperature and gas‐phase supersaturation at the sample position. In particular, room‐temperature photoluminescence measurements reveal an intense near‐band‐edge ultraviolet emission at about 386 nm for nanobelts and nanoflowers, which suggests that these nanostructures are of sufficient quality for applications in, for example, optoelectronic devices.     

One-dimensional ZnO nanostructure arrays: synthesis and characterization

One-dimensional ZnO nanostructure arrays such as nanowires, nanonails, and nanotrees, have been synthesized by oxygen assisted thermal evaporation of metallic zinc on a quartz substrate over a large area. Morphological evolution of ZnO nanostructures at different time scales and different positions of the substrates have been studied by electron microscopy. A self-catalyzed vapor-liquid-solid (VLS) process is believed to be responsible for the nucleation and subsequently a vapor-solid process is operative for further longitudinal growth. The photoluminescence spectrum showed a weak UV and a broad green emission peak at 3.25 and 2.49 eV, respectively. The latter was attributed to the presence of zinc interstitial defects. Electrical resistivity as a function of temperature showed activated mechanisms to be present. The electrical response of the ZnO nanonail arrays to different gases (CO, NO2, and H2S) indicated that there could be possible application as gas sensors for this material.     

ZnO/CuO hetero-hierarchical nanotrees array: hydrothermal preparation and self-cleaning properties

ZnO/CuO heterohierarchical nanotrees array has been prepared via a simple hydrothermal approach combined with thermal oxidation method on Cu substrates. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometer(XRD) are employed to characterize and analyze the as-synthesized samples. The results demonstrate that the secondary growth of ZnO nanorods enclose with CuO nanowires, leading to the formation of ZnO/CuO heterohierarchical nanotrees array. The hierarchical nanostructures have isotropic crystal symmetry and they have no 6-fold (or 4-fold or 2-fold) symmetry as general epitaxial growth. Enlightened by the similarity with microstructure of lotus, the wettability of ZnO/CuO heterohierarchical nanotrees array has been investigated. It is revealed that as-prepared ZnO/CuO nanotrees array after silanization present remarkable superhydrophobic performance, which is attributed to the trapped air and hierarchical roughness. Furthermore, their wettability could be manipulated by the morphologies of hierarchical ZnO nanorods. At the optimal condition, the greatest static angle of water droplet on the obtained heterohierarchical nanotrees array could reach almost 170°, and this substrate could be used as self-cleaning surface.     

基于ZnO纳米线的圆柱形微纳米跨尺度结构的制备与表征

基于水浴法在光纤纤芯上合成了ZnO纳米线, 得到了圆柱形微纳米跨尺度结构. 将纳米级的随机粗糙表面叠加到微米级的圆柱形基底上, 实现了对圆柱形跨尺度结构表面形貌的仿真分析. 采用扫描电子显微镜(SEM)并结合Matlab图像处理算子对跨尺度结构的表面形貌和ZnO纳米线的几何特征参数进行了表征. 与ZnO纳米线薄膜实际轮廓提取出的特征参数相同, 对均方根粗糙度为39.2 nm、偏斜度为0.1324及峭度为2.7146的圆柱形粗糙表面进行了仿真, 验证了仿真表面与实际轮廓的一致性. 建立了合成工艺参数对ZnO纳米线的长度、直径及长径比等几何特征参数的影响关系, 确定最佳工艺条件为: 种子层溶液Zn²⁺浓度为1.0 mmol/L, 生长液Zn²⁺浓度为0.03 mol/L, 生长时间为1.5 h, 水浴恒温90℃.     

氧化锌纳米线自组装定向生长动力学研究

研究了以极性高分子(如聚丙烯酰胺)长分子链作为自组装网络, 利用高分子软模板控制ZnO纳米点成核和ZnO纳米线定向生长, 从而使ZnO纳米线在半导体硅衬底上自组装生长的过程; 采用差示扫描量热法(DSC)测试了高分子络合-烧结法制备ZnO纳米线的结晶曲线, 对其结晶动力学进行了研究, 推导出结晶动力学方程为: 1－X_t＝exp(－7.475×10^－2t^1.9); 并利用热重(TG)测试结果, 通过热分解反应, 导出了反应动力学方程: dα/dT＝(3.76×10²³/Φ)e^－21340.8/T(1－α) ^2.8, 从而得到了化学反应速度随时间、浓度和温度变化的关系, 并用结果解释了实验现象.     

Length‐Dependent Charge Generation from Vertical Arrays of High‐Aspect‐Ratio ZnO Nanowires

Aqueous chemical growth of zinc oxide nanowires is a flexible and effective approach to obtain dense arrays of vertically oriented nanostructures with high aspect ratio. Herein we present a systematic study of the different synthesis parameters that influence the ZnO seed layer and thus the resulting morphological features of the free‐standing vertically oriented ZnO nanowires. We obtained a homogeneous coverage of transparent conductive substrates with high‐aspect‐ratio nanowire arrays (length/diameter ratio of up to 52). Such nanostructured vertical arrays were examined to assess their electric and piezoelectric properties, and showed an electric charge generation upon mechanical compressive stress. The principle of energy harvesting with these nanostructured ZnO arrays was demonstrated by connecting them to an electronic charge amplifier and storing the generated charge in a series of capacitors. We found that the generated charge and the electrical behavior of the ZnO nanowires are strictly dependent on the nanowire length. We have shown the importance of controlling the morphological properties of such ZnO nanostructures for optimizing a nanogenerator device.     

Comparative structure and optical properties of Ga-, In-, and Sn-doped ZnO nanowires synthesized via thermal evaporation

ZnO nanowires doped with a high concentration Ga, In, and Sn were synthesized via thermal evaporation. The doping content defined as X/(Zn + X) atomic ratio, where X is the doped element, is about 15% for all nanowires. The nanowires consist of single-crystalline wurtzite ZnO crystal, and the average diameter is 80 nm. The growth direction of vertically aligned Ga-doped nanowires is [001], while that of randomly tilted In- and Sn-doped nanowires is [010]. A correlation between the growth direction and the vertical alignment has been suggested. The broaden X-ray diffraction peaks indicate the lattice distortion caused by the doping, and the broadening is most significant in the case of Sn doping. The absorption and photoluminescence of Sn-doped ZnO nanowires shift to the lower energy region than those of In- and Ga-doped nanowires, probably due to the larger charge density of Sn.