Low-temperature synthesis and photoluminescence of ZnO nanostructures by a facile hydrothermal process

We reported a facile route to large-scale ZnO nanostructures by a poly (styrene-alt-maleic acid sodium) (PSMA)-assisted hydrothermal process. Various nanostructures including nanowires, nanobelts and nanorod arrays were fabricated depending on the experimental conditions. The structural studies reveal that all the nanostructures are single crystal with hexagonal phase and preferentially grow along [0 0 0 1]. The organic additive PSMA offers a spatial template for the one-dimensional (1D) growth of ZnO. The photoluminescence (PL) spectra of these nanostructures exhibit coexistence properties of ultraviolet (UV) and green emission. The nanorod arrays and nanobelts exhibit the strongest UV performance and green emission, respectively. We deduce that quantity of surface defects should be responsible for the difference in PL properties of these nanostructures.     

Novel pulsed electron deposition route to ZnO nanowire arrays

Well aligned ZnO nanowire arrays with uniform size are grown on Au-coated indium tin oxide substrates via a novel pulsed electron deposition (PED) technique. These nanowires have single-crystal hexagonal wurtzite structure and are grown along [0 0 0 1]. Au nanoparticles are found at the tip of the nanowires, indicating the growth process follows a typical vapor-liquid-solid mechanism. It is also found that the aligned ZnO nanowire arrays can be grown on Au-coated 6H-SiC and Si substrates, revealing that the PED technique is applicable for growth of ZnO nanowires on some common substrates. All the photoluminescence spectra of the ZnO nanowires reveal strong ultraviolet emission bands, indicating that the high-quality ZnO nanowires can be fabricated via the novel PED technique.     

Diphenylthiocarbazone (dithizone)-assisted solvothermal synthesis and optical properties of one-dimensional CdS nanostructures

One-dimensional (1D) cadmium sulfide (CdS) nanostructures with various aspect ratios were successfully synthesized by a diphenylthiocarbazone (dithizone)-assisted solvothermal method. The results showed that the dithizone-assisted synthesized samples had larger aspect ratio than that prepared in the absence of dithizone, and CdS nanowires with the highest aspect ratio were obtained with an appropriate dithizone amount (0.03 g/50 ml ethylenediamine in the present system). All the 1D CdS nanostructures were in hexagonal wurtzite phase. The as-synthesized large-scale CdS nanowires were in diameters ranging from 70 to 80 nm, length up to 20 μm, and aspect ratios of 250-285. Further characterization indicated that the CdS nanowires were single crystalline with a preferential growth orientation of [0 0 2], c-axis. Two optical absorption peaks were observed at about 488 nm and 502 nm for the CdS nanowire sample with high aspect ratio in the optical absorption spectroscopy, which could be attributed to the nanometer effect of nanowires. It was found that the additive dithizone was a crucial factor in controlling the morphology and optical properties of the 1D CdS nanostructures. The growth mechanism of 1D CdS nanostructure and the effects of dithizone in the present system were discussed.     

Absorption-emission study of hydrothermally grown Al:ZnO nanostructures

The preparation, structural characterization and optical properties of aluminum doped ZnO (Al:ZnO) nanostructures grown under hydrothermal method are reported. One-dimensional (1-D) growth is achieved by the controlled addition of metal nitrate as precursors in the presence of long chain surfactant, poly-ethylene glycol (PEG) at 160 °C for 20 h. The as-synthesized ZnO rods are single crystalline, exhibiting an oriented growth along [001] direction. The Al6 rod has an aspect ratio of 3.2, which can be effectively applied in optoelectronic devices. Comprehensive structural analysis using X-ray diffraction method (XRD) and Energy dispersive X-ray analysis (EDX) indicate that the dopant Al atom occupies Zn sites in ZnO and the elemental composition of Al is consistent with the amount utilized in the hydrothermal synthesis. XRD shows that the Al:ZnO nanostructures from 1 to 9 atomic percent (at.%) has hexagonal wurtzite structure of ZnO. The Al dopant effects on lattice vibration and electronic transitions of the ZnO nanostructures have been investigated by Fourier transform Infrared spectroscopy (FT-IR), Ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) emission recorded at room temperature. The correlation existing between absorption and emission study tell that their characteristic band edge peak of doped ZnO shifts towards higher wavelength side for 3-9 at.% with respect to Al0 thus, exhibiting a red shift phenomenon with decrease in optical bandgap. The observed PL reveals two emission peaks centered at 374 nm and 530 nm. The near band edge (NBE) to defect emission ratio increases with dopant concentration indicating the linear enhancement in crystal quality and declination in zinc vacancies from 3 to 9 at.% of Al.     

Defect mediated photocatalytic activity in shape-controlled ZnO nanostructures

Shape-controlled ZnO nanostructures were synthesized through a facile soft-chemical approach by varying the concentration of OH⁻ ions. X-ray diffraction and Raman spectra reveal the formation of highly crystalline single-phase hexagonal wurtzite nanostructure. It has been observed that the concentration of OH⁻ ions plays an important role in controlling the shape of ZnO nanostructures. TEM micrographs indicate that well-spherical particles of size about 8 nm were formed at lower concentration of OH⁻ ions whereas the higher concentration of OH⁻ ions favor the formation of nanorods of length 30-40 nm. The optical studies confirmed that the band gap and near band edge emission of ZnO nanostructures are strongly dependent on the shape of particles. Furthermore, the decrease in the intensity of green emission as shape of particles changes from sphere to rod indicates the suppressing of defect density, which in turn influences the photocatalytic activity and ferromagnetic-like behavior of ZnO nanostructures.     

Preparation and DSC application of the size-tuned ZnO nanoarrays

ZnO nanoarray was successfully prepared by a simple chemical bath deposition method on the glass slide deposited with a spin-coated ZnO seed crystal layer. With the measurements of SEM, XRD and UV–vis absorption spectra, the properties of the ZnO nanoarray were characterized in detail. The results show that the ZnO nanoarray is the hexagonal wurtzite structure and grow along the [0 0 1] axis. Furthermore, the energy-conversion property of the device was investigated in such ZnO nanoarray constructed dye-sensitized solar cells (DSCs), which has potential applications in the new-concept photovoltaic devices.     

Characterization of ZnO needle-shaped nanostructures grown on NiO catalyst-coated Si substrates

ZnO nanostructures were synthesized over NiO-coated Si substrate by a thermal evaporation of Zn powders in a vertical chemical vapor deposition (CVD) reactor. The ZnO nanostructures had a needle-like morphology and the diameter of the structures decreased linearly from the bottom to the top. The bottom diameters of the ZnO nano-needles normally ranged from 20 to 100 nm and the lengths were in the range of 2–3 μm. The clear lattice fringes in HRTEM image indicated the growth of good quality hexagonal single-crystal ZnO. Field emission (FE) characteristics of the ZnO nano-needles showed that the turn-on field was about 8.87 V/μm with a field enhancement factor of about 1099. The growth mechanism of the ZnO nano-needles was proposed on the basis of experimental data.     

Effects of supply time of Ar gas current on structural properties of Au-catalyzed ZnO nanowires on silicon (1 0 0) grown by vapor–liquid–solid process

ZnO nanowires were grown on Au-coated Si (1 0 0) substrates by the method of vapor–liquid–solid (VLS) growth processing technique. The effects of supply time of Ar gas current on morphology and microstructure of Au-catalyzed ZnO nanowires were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The results showed that the morphologies of ZnO nanostructures strongly depended on the time of flowing Ar gas. When the time of flowing Ar gas was 90 s, ZnO showed nanowires with hexagonal structure. Their diameters and lengths were 160 nm and 20 μm, respectively, on average, and the Raman scattering peak located at 438 cm⁻¹ reached maximum intensity. The results also showed that the ZnO growth could be patterned by controlling the initial position of Au-coated area on the Si substrates.     

Low-temperature synthesis and characterization of ZnO quantum dots

ZnO quantum dots (QDs) were fabricated at low temperature of 200 °C through thermal decomposition method with slight introduction of sodium dodecyl sulfate (SDS). The morphology, structure and optical properties were investigated by the methods of X-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL) and Raman spectrum, respectively. The XRD results showed the as-synthesized ZnO QDs had hexagonal wurtzite structure and the average grain size estimated from Scherrer formula was 7.5 nm which had a good agreement with TEM result. And it is evident that the introduction of SDS can actually decrease the grain size to form ZnO QDs. The Raman results also indicated that the ZnO QDs keep the overall crystal structure of the bulk ZnO. Both spatial confinement within the dot boundaries and phonon localization by defects were the mainly reason for the only few cm⁻¹ redshift of the Raman scatter peaks. The room-temperature photoluminescence reveals that the as-prepared ZnO QDs exhibit an ultraviolet emission at 380 nm and a broad deep level emission band in the range of 420–700 nm in wavelength, which testified the Raman and XRD results that the as-synthesized ZnO QDs had defects. Moreover, the growth mechanism of ZnO QDs was also discussed in the article.     

Synthesis of cadmium oxide doped ZnO nanostructures using electrochemical deposition

Ternary ZnCdO alloy semiconductor nanostructures were grown using electrochemical deposition. Crystalline nanostructures/nanorods with cadmium concentration ranging from 4 to 16 at% in the initial solution were electrodeposited on tin doped indium oxide (ITO) conducting glass substrates at a constant cathodic potential −0.9 V and subsequently annealed in air at 300 °C. X-ray diffraction measurements showed that the nanostructures were of wurtzite structure and possessed a compressive stress along the c-axis direction. The elemental composition of nanostructures was confirmed by energy dispersive spectroscopy (EDS). ZnO nanostructures were found to be highly transparent and had an average transmittance of 85% in the visible range of the spectrum. After the incorporation of Cd content into ZnO the average transmittance decreased and the bandgap tuning was also achieved.     

水热法制备掺Mn的ZnS纳米线及其磁学性能

采用水热法在低温下制备了MnxZn1-xS纳米线.MnxZn1-xS纳米线的形貌和微观结构用透射电子显微镜和X射线衍射仪进行表征.磁性能用振动样品磁强计进行测试.MnxZn1-xS的形貌取决于Mn的含量和在ZnS纳米粒子内外的分布.未掺杂Mn的ZnS纳米线的直径和长度分别为80～200nm和10～20μm.随着Mn含量的增加,MnxZn1-xS纳米线的平均直径逐渐增加,长径比不断减小.X射线衍射结果表明MnxZn1-xS纳米结构结晶性好,为六方纤锌矿结构.在Mn的掺杂量为0.25%时,矫顽力达到最大.随着Mn掺杂量的增加,饱和磁化强度也不断非线性增强.     

High-density arrays of low-defect-concentration zinc oxide nanowire grown on transparent conducting oxide glass substrate by chemical vapor deposition

High-density ZnO nanowire arrays with low defect concentrations were directly grown on transparent conducting oxide glass substrates under catalyst-free and low temperature conditions by chemical vapor deposition (CVD). A possible growth mechanism of the nanowires is studied. The experiments indicate that correct levels of supersaturation and evaporation temperature are beneficial to the growth of ZnO nanowires. Photoluminescence exhibits a weak ultraviolet emission at 380 nm and a strong green emission at 495 nm. While using a double-tube growth system, the visible light emission diminishes and the 380 nm emission is the only emission, suggesting that ZnO nanowires with few defects can be prepared using the present CVD technique at low temperature.     

Room temperature ferromagnetism of Si-doped ZnO thin films prepared by sol–gel method

Pure ZnO and Si-doped ZnO thin films were deposited on quartz substrate by using sol-gel spin coating process. X-ray diffraction analysis shows that all the thin films have hexagonal wurtzite structure and preferred c-axis orientation. Si-doped ZnO films show room temperature ferromagnetism (RTFM) and reach the maximum saturation magnetization value of 1.54 kAm-1 at 3% Siconcentration. RTFM of Si-doped ZnO decreases with the increasing annealing temperature because of the formation of SiO 2 . Photoluminescence measurements suggest that the RTFM in Si-doped ZnO can be attributed to the defect complex related to zinc vacancies V Zn and oxygen interstitials Oi .     

Hydrothermal synthesis and optical properties of Ni doped ZnO hexagonal nanodiscs

Single crystalline Ni-doped ZnO hexagonal nanodiscs are successfully synthesized. Zinc acetate, nickel nitrate, sodium hydroxide and poly (vinyl pyrrolidone) (PVP) were mixed together and transferred to a 100 ml Teflon-lined stainless steel autoclave which kept at 150 °C for 24 h. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanodiscs have edge length 30 nm and thickness of 45 nm. EDX result showed that the amount of Ni in the product is about 12%. Photoluminescence of these doped hexagonal nanodiscs exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by the Ni-doping. The growth mechanism of the doped hexagonal nanodiscs was also discussed.     

Filtered vacuum arc deposition of undoped and doped ZnO thin films: Electrical, optical, and structural properties

Filtered vacuum (cathodic) arc deposition (FVAD, FCVD) of metallic and ceramic thin films at low substrate temperature (50-400 °C) is realized by magnetically directing vacuum arc produced, highly ionized, and energetic plasma beam onto substrates, obtaining high quality coatings at high deposition rates. The plasma beam is magnetically filtered to remove macroparticles that are also produced by the arc. The deposited films are usually characterized by their good optical quality and high adhesion to the substrate. Transparent and electrically conducting (TCO) thin films of ZnO, SnO₂, In₂O₃:Sn (ITO), ZnO:Al (AZO), ZnO:Ga, ZnO:Sb, ZnO:Mg and several types of zinc-stannate oxides (ZnSnO₃, Zn₂SnO₄), which could be used in solar cells, optoelectronic devices, and as gas sensors, have been successfully deposited by FVAD using pure or alloyed zinc cathodes. The oxides are obtained by operating the system with oxygen background at low pressure. Post-deposition treatment has also been applied to improve the properties of TCO films.The deposition rate of FVAD ZnO and ZnO:M thin films, where M is a doping or alloying metal, is in the range of 0.2-15 nm/s. The films are generally nonstoichiometric, polycrystalline n-type semiconductors. In most cases, ZnO films have a wurtzite structure. FVAD of p-type ZnO has also been achieved by Sb doping. The electrical conductivity of as-deposited n-type thin ZnO film is in the range 0.2-6 × 10^− 5 Ω m, carrier electron density is 10²³-2 × 10²⁶ m^− 3, and electron mobility is in the range 10-40 cm²/V s, depending on the deposition parameters: arc current, oxygen pressure, substrate bias, and substrate temperature. As the energy band gap of FVAD ZnO films is ∼ 3.3 eV and its extinction coefficient (k) in the visible and near-IR range is smaller than 0.02, the optical transmission of 500 nm thick ZnO film is ∼ 0.90.     

High-quality vertically aligned ZnO nanorods synthesized by microwave-assisted CBD with ZnO-PVA complex seed layer on Si substrates

We successfully synthesized vertically aligned zinc oxide (ZnO) nanorods on seeded silicon substrates using chemical bath deposition assisted by microwave heating. ZnO nanorods were grown on seed layers of ZnO-polyvinyl alcohol (PVA) nanocomposites spin-coated on p-type Si (1 1 1). The nanorod's diameter was found to be dependent on the annealing temperature of the ZnO-PVA seed layer. We produced ZnO nanorods with diameters in the range of 50-300 nm from five groups of seed layers annealed at 250 °C, 350 °C, 380 °C, 450 °C, and 550 °C. The nanorods were examined with X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy, which revealed hexagonal wurtzite structures perpendicular to the substrate along the z-axis in the direction of (0 0 2). Photoluminescence measurements revealed high UV emission at a high I_UV/I_vis ratio of 175. We also conducted Raman scattering studies on the ZnO nanorods to estimate the lattice vibration modes.     

The evolution behavior of structures and photoluminescence of K-doped ZnO thin films under different annealing temperatures

In this work, 1 at.% K-doped ZnO thin films were prepared by sol-gel method on Si substrates. The evolution behavior of the structures and photoluminescence of these films under different annealing temperatures was deeply studied. The crystal structures and surface morphology of the samples were analyzed by an X-ray diffractometer and an atomic force microscope, respectively. The photoluminescence spectra were used to study the luminescent behavior of the samples. The results showed that the films had a hexagonal wurtzite structure and were preferentially oriented along the c-axis perpendicular to the substrate surface. All the samples showed a strong ultraviolet emission and a weak blue emission. With the increase of annealing temperature, the ZnO grains gradually grew up; at the same time, the blue emission gradually decreased. The sample annealed at 500 °C showed the best crystalline quality and strongest ultraviolet emission. The authors think that the blue emission in these samples is mainly connected with K interstitial defects. When the 1 at.% K-doped ZnO thin film is annealed at high temperatures (≥600 °C), most of K interstitials move into ZnO lattice sites replacing Zn. As a result, the blue emission resulting from K interstitial defects also decreased.     

电气石衬底温度对ZnO薄膜的结晶特征和光学特性的影响

利用超声雾化热解技术 (USP) 在不同温度的电气石和玻璃衬底上生长ZnO纳米片状薄膜。结构研究表明晶体为六方纤锌矿多晶结构。衬底温度越高,Raman特征峰越强,XRD结果给出（002）优势定向越明显,晶体结晶性能越好,晶粒尺寸越大。SEM图像显示片状ZnO晶体沿平行衬底方向叠加形成花状晶柱的微观形貌,沉积温度越高,晶柱宽度越大。UV-Vis表明电气石衬底上ZnO吸收峰强度高于玻璃衬底,最大吸收峰位置发生红移,高温下移动更大。     

Pulsed laser deposition of transparent ZnO/MgO multilayers

We report on the structural, optical and electrical properties of ZnO/MgO multilayers grown by pulsed laser deposition technique. The film thickness of ZnO sublayer (t_ZnO) was found to have great impact on the properties of ZnO/MgO multilayers. Investigations reveal the structural phase transition from wurtzite phase to cubic phase with corresponding decrease in ZnO thickness. The optical transmittance of the multilayers is over 80% in the visible region and there is a gradual shift of absorption edge towards a longer wavelength with corresponding increase in ZnO sublayer thickness. Two absorption bands at around 400 nm and 270 nm were observed in the transmission spectra of ZnO/MgO multilayers for similar ZnO and MgO layer thickness, which has been ascribed to phase separation to hexagonal and cubic phases. The calculated optical band gap E_g shows a widening from 3.51 eV to 6.23 eV with corresponding decrease in ZnO sublayer thickness from 100 nm to 23 nm, which in turn leads to an increase in resistivity in ZnO/MgO multilayers. These results provide important information for the design and modeling of ZnO/MgO optoelectronic devices due to their adjustable bandgap energies.     

Elaboration and characterization of Al doped ZnO nanorod thin films annealed in hydrogen

ZnO thin films doped with Al concentrations of 1.0, 2.0, 3.0, 4.0, 5.0 at% were prepared by a sol-gel spin-coating method on glass substrates and respectively annealed at 550 °C for 2 h in hydrogen and air. The X-ray diffraction and selected-area electron diffraction results confirm that the Al doped ZnO thin films are of wurtzite hexagonal ZnO. The scanning electron microscope results indicate that the Al doped ZnO nanorod thin films can be got by annealing in hydrogen rather than in air. The optical properties reveal that the Al doped ZnO thin films have obviously enhanced transmittance in the visible region. The electrical properties show that the resistivity of 1.0 at% Al doped ZnO thin films has been remarkably reduced from 0.73 Ω m by annealing in air to 3.2 × 10⁻⁵ Ω m by annealing in hydrogen. It is originated that the Al doped ZnO nanorod thin films annealed in hydrogen increased in electron concentration and mobility due to the elimination of adsorbed oxygen species, and multicoordinated hydrogen.