Facilitating epitaxial growth of ZnO films on patterned GaN layers: A solution-concentration-induced successive lateral growth mechanism

The hydrothermal epitaxy of ZnO films on a patterned GaN layer with a honeycomb etching hole array is demonstrated. Through m-planes of the GaN layer exposed on the vertical walls of the etching holes, highly crystalline ZnO films via multiple lateral growth stages can be realized. It is found that higher concentrations of zinc nitrate hexahydrate (ZNH) and hexamethylenetetramine (HMT) in hydrothermal solution yield a larger number of ZnO molecules to speed up ZnO growth during the initial stage of hydrothermal growth, also create secondary crystals and initialize further lateral growth stages to bridge neighboring ZnO prisms after smooth surfaces formed on the m-plane of a ZnO prism. A successive lateral growth mechanism that strongly depends on ZNH and HMT concentrations in the hydrothermal solution is proposed and discussed.     

Seed-assisted growth of epitaxial ZnO nanorod arrays with self-organized periodicity and directional alignment

In this article we report the seed-assisted growth of epitaxial ZnO nanorod (NR) arrays on (0 0 0 1) plane sapphire substrates at low temperatures in aqueous solutions. The self-organized periodic ZnO NR rows with a fairly constant separation were directly grown on bare sapphire surfaces, without the need for any complicated lithography or use of pre-patterned catalysts. The spatial ordering of the ZnO NRs was significantly influenced by the seed growth conditions and by the presence of a self-organized step structure on the annealed sapphire surface. In addition, the effect of the conditions employed to prepare the seeds, including growth parameters and post-growth annealing treatment, on the epitaxial relationship between the ZnO NRs and the sapphire substrate was systematically investigated by X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) observations. Post-growth annealing of the ZnO seeds changed the morphologies and crystallographic alignment of the generated ZnO nanostructures significantly, as a result of the formation of epitaxial spinel ZnAl₂O₄ interlayers, facilitated by zinc cation diffusion and solid state reactions at high temperature.     

Effect of substrates and anions of zinc salts on the morphology of ZnO nanostructures

The ZnO nanostructures were hydrothermally synthesized on glass and Al substrates, respectively, using zinc chloride, zinc nitrate, and zinc acetate as precursor. The as-prepared products were characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). Different ZnO nanostructures were obtained, such as nanorods, nanosheets, flower-like nanostructures and so on. The effects of the substrates and anions of zinc salts on the morphologies of the resulting products have been investigated.     

Characterization and optical property of ZnO nano-, submicro- and microrods synthesized by hydrothermal method on a large-scale

In the present paper, well-dispersed ZnO nano-, submicro- and microrods with hexagonal structure were synthesized by a simple low temperature hydrothermal process from zinc nitrate hexahydrate without using any additional surfactant, organic solvent or catalytic agent. The phase and structural analysis were carried out by X-ray diffraction (XRD), the morphological analysis was carried out by field emission scanning electron microscopy (FESEM) and the optical property was characterized by room-temperature photoluminescence (PL) spectroscopy. The results revealed the high crystal quality of ZnO powder with hexagonal (wurtzite-type) crystal structure and the formation of well-dispersed ZnO nano-, submicro- and microrods with diameters of about 50, 200 and 500 nm, and lengths of 300 nm, 1 μm and 2 μm, respectively, on a large-scale just using the different temperatures. Room-temperature PL spectrum from the ZnO nanorods reveals a strong UV emission peak at about 360 nm and no green emission band at ∼530 nm. The strong UV photoluminescence indicates the good crystallization quality of the ZnO nanorods. Room-temperature PL spectra from the ZnO submicro- and microrods reveal a weak UV emission peak at ∼400 nm and a very strong visible green emission at 530 nm, that is ascribed to the transition between V_oZn_i and valence band.     

Vapour phase transport growth of ZnO layers and nanostructures

We have developed a novel advanced VPT set-up. ZnO layers and nanorods were grown employing a specially designed horizontal vapour transport system with elemental sources at relatively low temperatures without catalysis. We employed 6N elemental Zn carried by nitrogen and 99.995% oxygen as reactants. Sapphire, SiC, bulk ZnO and ZnO epitaxial layers were implemented as substrates for ZnO growth. Growth temperatures ranged from 500 to 900 C. Reactor pressures were from 5 mbar to atmospheric pressure. We employed x-ray diffractometry, optical microscopy, scanning electron microscopy and atomic force microscopy to investigate the obtained ZnO samples and the influence of different growth parameters on the ZnO homo- and heteroepitaxial growth and to optimise the set of growth parameters either for both epitaxial layers and nanostructures. We also show that the quality of the VPT grown ZnO epitaxial layers on sapphire can be even higher (evaluated from FWHM of the XRD rocking curves) than the MBE grown ones used as epiwafers for VPT growth. High quality ZnO layers with extremely narrow FWHM of the (0002) rocking curve of 38″ are fabricated employing our VPT approach.     

Microstructure and optical properties of ZnO nanoparticles prepared by a simple method

In this investigation, ZnO nanoparticles were prepared by a simple and rapid method. This method is based on the short time solid state milling and calcinations of zinc acetate and citric acid powders. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, photoluminescence and UV-vis spectroscopy. It was shown that the calcination temperature significantly affected the particle size and optical properties of the synthesized ZnO nanoparticles. Calculation based on the XRD data shows that the average sizes of ZnO particles are in agreement with those from TEM images and the size of the particles increases on increasing the calcination temperature. Also the band gap of samples decreased from 3.29 to 3.23 eV on increasing the calcination temperature from 350 to 600 °C. Photoluminescence analyses show that many defects such as interstitial zinc, zinc vacancy and oxygen vacancy are responsible for the observed optical properties.     

Low-temperature growth of ZnO nanorods on PET fabrics with two-step hydrothermal method

An effective low-temperature growth method to fabricate hexagonally oriented ZnO nanorod arrays onto PET fabrics is reported. The effect of substrate pre-treatment and C₆H₁₂N₄ concentration on the structure of ZnO nanorod arrays were investigated in details by X-ray diffraction (XRD), FE-SEM and ultraviolet protection factor (UPF). The results show that substrate pre-treatment, C₆H₁₂N₄ concentration indeed have great influence on the growth of ZnO nanorod arrays. It is indispensable to introduce a ZnO seed layer on the substrate and under growth condition of n(C₆H₁₂N₄):n[Zn(NO₃)₂] = 1:1, T = 90 °C, t = 3 h, the well-aligned ZnO nanorod arrays with 40-50 nm in diameter and 300-400 nm in length were achieved on the pre-treated PET fabrics. The ZnO nanorods grown on PET fabrics possessed an ultrahigh ultraviolet protection factor of 480.52 in this study, indicating an excellent protection against ultraviolet radiation in comparison with the untreated PET fabrics.     

Synthesis and optical properties of flower-like ZnO nanorods by thermal evaporation method

Flower-like ZnO nanorods have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si (1 0 0) substrates without any catalyst. The structures, morphologies and optical properties of the products were characterized in detail by using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The synthesized products consisted of large quantities of flower-like ZnO nanostructures in the form of uniform nanorods. The flower-like ZnO nanorods had high purity and well crystallized wurtzite structure, whose high crystalline quality was proved by Raman spectroscopy. The as-synthesized flower-like ZnO nanorods showed a strong ultraviolet emission at 386 nm and a weak and broad yellow-green emission in visible spectrum in its room temperature photoluminescence (PL) spectrum. In addition, the growth mechanism of the flower-like ZnO nanorods was discussed based on the reaction conditions.     

Studies of structural and morphological characteristics of flower-like ZnO thin film and its application as photovoltaic material

In this work a new method has been employed to synthesize nanocrystalline ZnO powder under hydrothermal conditions at 80 °C using aqueous Zn(NO₃)₂·6H₂O solution and diethylamine (DEA) as the starting materials. The ZnO powder prepared by this novel method was characterized by XRD, energy dispersive X-ray spectroscopy (EDX), FTIR and UV–vis techniques. Calculation based on XRD data revealed ZnO particles to be of nanometer size (∼33 nm). The ZnO powder was subsequently used to make its thin film which exhibited flower like morphology when examined by SEM. Thin ZnO films were sensitized with N719 dye, (Bu₄N)₂[Ru(dcbpyH)₂(NCS)₂], and used as photo-anode to construct sandwich type dye-sensitized solar cell (DSSC). With such cells, V_OC = 0.680 V, J_SC = 0.61 mA cm⁻², fill factor = 0.43 and overall conversion efficiency η = 0.23% were achieved on illumination with visible light (80 mW cm⁻²).     

紫外光表面处理对ZnO纳米粒子的晶体结构与光学性质的影响

利用溶胶凝胶法制备了ZnO纳米粒子,用紫外光照射对其进行表面处理.探讨了表面处理对ZnO纳米粒子晶体结构与光学性质的影响.结果显示:在晶体结构方面,紫外光照射会降低ZnO纳米粒子的团聚现象,缩短晶格常数,使其所受应力由压缩应力释放变为伸张应力,增大表面能,使能量最佳化并稳定而导致ZnO纳米粒子形成再构.在光学性质方面,紫外光照射会使ZnO纳米粒子表面产生较多的氧空位,而氧空位又会使其表面容易吸附羟基,使得ZnO纳米粒子变得更加亲水.     

紫外光表面处理对ZnO纳米粒子的晶体结构与光学性质的影响

利用溶胶凝胶法制备了ZnO纳米粒子,用紫外光照射对其进行表面处理。探讨了表面处理对ZnO纳米粒子晶体结构与光学性质的影响。结果显示：在晶体结构方面,紫外光照射会降低ZnO纳米粒子的团聚现象,缩短晶格常数,使其所受应力由压缩应力释放变为伸张应力,增大表面能,为使能量最佳化并稳定而导致ZnO纳米粒子形成再构。在光学性质方面,紫外光照射会使ZnO纳米粒子表面产生较多的氧空位,而氧空位又会使其表面容易吸附羟基,使得ZnO纳米粒子变得更加亲水。     

Preparation and optical properties of single crystal thin films of bismuth substituted iron garnets for magneto-optic applications

Single crystal thin films of Bi substituted iron garnets have been grown by the liquid phase epitaxy technique. The growth conditions and compositions are presented together with the segregation coefficients of Bi and Pb as a function of growth temperature. The Faraday rotation and optical absorption have been measured for wavelengths between 0.7 and 0.46 μm. The superior magneto-optic properties (Faraday rotations of up to ten times larger than that observed for Y₃Fe₅O₁₂) make these epitaxial layers attractive for magneto-optic devices. The influence of Pb incorporation on the optical properties of the films is discussed. Compositions of Bi and Ga(Al) substituted Gd₃Fe₅O₁₂, which exhibit a magnetic compensation temperature near room temperature and high Faraday rotation, have also been prepared. The magnetic switching behaviour as a function of temperature has been determined. It shows that these films are especially suited for thermomagnetic recording using an argon ion laser at a wavelength of 5145 Å.     

Hydrothermal synthesis,characterization and optical absorption property of nanoscale WS2/TiO2 composites

Nanosized tungsten disulfide (WS₂) sensitized titanium dioxide (TiO₂) was successfully prepared via a simple yet facile hydrothermal process. The nanocomposite exhibited a wide and intensive absorption in the visible light region of 400–700 nm, and may have a potential application as a visible photocatalyst. In addition, the sensitization mechanism of the nano-WS₂ was proposed to elaborate the wide visible light absorption of the WS₂/TiO₂ nanocomposites.     

Low temperature photoluminescence and photoacoustic characterization of Zn-doped InxGa1−xAsySb1−y epitaxial layers for photovoltaic applications

In this paper we present results on the characterization of Zn-doped InGaAsSb epitaxial layers to be used in the development of stacked solar cells. Using the liquid phase epitaxy technique we have grown p-type InGaAsSb layers, using Zn as the dopant, and n-type Te-doped GaSb wafers as substrates. A series of Zn-doped InGaAsSb samples were prepared by changing the amount of Zn in the melt in the range: 0.1-0.9 mg to obtain different p-type doping levels, and consequently, different p-n region characteristics. Low temperature photoluminescence spectra (PL) were measured at 15 K using at various excitation powers in the range 80-160 mW. PL spectra show the presence of an exciton-related band emission around 0.642 eV and a band at 0.633 eV which we have related to radiative emission involving Zn-acceptors. Using the photoacoustic technique we measured the interface recombination velocities related to the interface crystalline quality, showing that the layer-substrate interface quality degrades as the Zn concentration in the layers increases.     

氧化锌晶体的水热法生长及性能表征

报道以块状氧化锌陶瓷为培养料,KOH、LiOH和H2O2的混合水溶液为矿化剂体系,采用水热法生长出尺寸为30mm×38mm×8mm的氧化锌晶体。氧化锌晶体 c(0001)和-c(0001)方向的生长速度分别为0.17,0.09mm/day。 c面的颜色为浅绿色,而-c面的颜色为深褐色。在室温下测得 c面的载流子浓度为104cm-3,电阻率为80Ω·cm,迁移率为100cm2/V·s。晶体(0001)面的双晶摇摆曲线的FWHM为45arc-sec。对氧化锌晶体 c面在室温条件下的光致发光谱和吸收光谱进行了测试分析。     

低温外延生长平整ZnO薄膜

在较低温度下实现平整ZnO薄膜的生长有利于ZnO的可控p型掺杂以及获得陡峭异质界面。本文使用分子束外延方法,采用a面蓝宝石为衬底,在450 ℃下生长了一系列ZnO薄膜样品。在富氧生长的条件下,固定氧流量不变,通过调节锌源温度来改变锌束流,以此调控生长速率。样品的生长速率为40~100 nm/h。通过扫描电镜（SEM）表征发现：在高锌束流的生长条件下,样品表面有很多不规则的颗粒;降低锌的供应量后,样品表面逐渐平整。原子力显微镜（AFM）测试结果表明：样品的均方根表面粗糙度（RMS）只有0.238 nm,接近于原子级平整度。这种平整表面的获得得益于较低的生长速率,以及ZnO外延薄膜与a面蓝宝石衬底之间小的晶格失配。     

Effects of the annealing duration of the ZnO buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process

In this study, the effects of the annealing duration of a zinc oxide (ZnO) buffer layer on structural and optical properties of ZnO rods grown by a hydrothermal process are discussed. A ZnO buffer layer was deposited on p-type Si (1 1 1) substrates by the metal organic chemical vapor deposition (MOCVD) method. After that, ZnO rods were grown on the ZnO-buffer/Si (1 1 1) substrate by a hydrothermal process. In order to determine the optimum annealing duration of the buffer layer for the growth of ZnO rods, durations ranging from 0.5 to 30 min were tried. The morphology and crystal structure of the ZnO/ZnO-buffer/Si (1 1 1) were measured by field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). The optical properties were investigated by photoluminescence (PL) measurement.     

Effects of growth duration on the structural and optical properties of ZnO nanorods grown on seed-layer ZnO/polyethylene terephthalate substrates

Well-aligned single crystalline zinc oxide (ZnO) nanorods were successfully grown, by hydrothermal synthesis at a low temperature, on flexible polyethylene terephthalate (PET) substrates with a seed layer. Photoluminescence (PL), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements were used to analyze the optical and structural properties of ZnO nanorods grown for various durations from 0.5 h to 10 h. Regular and well-aligned ZnO nanorods with diameters ranging from 62 nm to 127 nm and lengths from 0.3 μm to 1.65 μm were formed after almost 5 h of growth. The growth rate of ZnO grown on PET substrates is lower than that grown on Si (1 0 0) substrates. Enlarged TEM images show that the tips of the ZnO nanorods grown for 6 h have a round shape, whereas the tips grown for 10 h are sharpened. The crystal properties of ZnO nanorods can be tuned by using the growth duration as a growth condition. The XRD and PL results indicate that the structural and optical properties of the ZnO nanorods are most improved after 5 h and 6 h of growth, respectively.     

Synthesis of morphology-controlled ZnO microstructures via a microwave-assisted hydrothermal method and their gas-sensing property

Controllable ZnO architectures with flower-like and rod-like morphologies were synthesized via a microwave-assisted hydrothermal method. By adjusting the concentration of Zn²⁺ in the aqueous precursors, different morphologies of ZnO microstructures were obtained. The size of ZnO was uniform after ultrasonic treatment. The growth process of ZnO in solution was studied by monitoring the intermediate products, which were extracted at different stages of the reactions: (i) precursor preparation, (ii) microwave irradiation heating, (iii) natural cooling. Studies of the SEM images and XRD data revealed that the formation of ZnO occurred via in situ assembly or dissolution–reprecipitation of zinc hydroxide complexes. The morphology-dependent ethanol sensing performance was observed; the seven-spine ZnO structures exhibit the highest activity.