Study of ZnO-coated SnO2 nanostructures synthesized by a two-step process

We investigated the influence of the ZnO coating on the properties of one-dimensional (1D) nanostructures of SnO₂. We have employed X-ray diffraction, scanning electron microscope, transmission electron microscope and photoluminescence (PL) spectroscopy to characterize both as-synthesized and ZnO-coated products. We observed that deposition process of ZnO by using an atomic layer deposition technique resulted in the SnO₂ core/ZnO shell structure. The photoluminescence of the ZnO-coated products exhibited broad bands in the UV and green region, suggesting a possible contribution of the emission from the ZnO outlayers.     

Ion beam synthesis and carrier dynamics of ZnO nanoparticles embedded in a SiO2 matrix

Zinc oxide (ZnO) nanostructures have been synthesized by the implantation of ZnO molecular ions into SiO₂ followed by high temperature thermal annealing. 35 keV ZnO^? ions were implanted to a fluence of 5×10¹⁶ ions/cm² into SiO₂ at room temperature (RT). The implanted sample was annealed in an oxygen environment to allow the growth of ZnO precipitates. In the as-implanted sample, Zn nanoparticles up to 4.5 nm in diameter were observed and were distributed throughout the implanted depth in the SiO₂. The highest concentration of Zn from the implantation was at a depth of 25 nm. During annealing, Zn diffused into the substrate and combined with oxygen to form ZnO. ZnO nanostructures thus formed had diameters up to 8 nm, embedded in SiO₂. Donor-bound exciton (D, X), acceptor-bound exciton (A, X), and donor–acceptor-pair (DAP) transitions were observed in low temperature photoluminescence (PL) measurements on an annealed sample. RT-PL measurement showed band-edge emission in the ultraviolet region with a full width at half maximum of 121 meV. Time-resolved PL measurements performed at 4 K revealed an excitonic lifetime of 160 ps.     

Effect of zinc nitrate concentration on the structural and the optical properties of ZnO nanostructures

ZnO nanorods and nanodisks were formed on indium-tin-oxide-coated glass substrates by using an electrochemical deposition method. Scanning electron microscopy images showed that the ZnO nanorods were transformed into nanodisks with increasing Zn(NO₃)₂ concentration. X-ray diffraction patterns showed that the ZnO nanostructures had wurzite structures. The full widths at half maxima of the near band-edge emission peak of photoluminescence spectra at 300 K for ZnO nanorods were small, indicative of the high quality of the nanorods. These results indicate that the structural and the optical properties of ZnO nanostructures vary by changing Zn(NO₃)₂ concentration.     

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.     

Fabrication and characterization of ZnO micro and nanostructures prepared by thermal evaporation

A simple method of thermal evaporation to fabricate micro and nanostructures of zinc oxide was presented. ZnO micro and nanostructures, prepared under different quantity of O₂, were characterized by techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and analytical transmission electron Microscope. The SEM images indicated that the products prepared under the condition of sufficient O₂ were needle-like microrods and the samples synthesized under the condition of deficient O₂ were nanorods and nanowires with very high aspect ratio. The results of XRD and Raman shifts revealed that the ZnO micro and nanostructures synthesized under different quantity of O₂ were both single crystalline with the hexagonal wurtzite structure. The HRTEM images indicated that the ZnO nanowire prepared under the condition of deficient O₂ was single crystalline and grown along the direction of [0 0 1]. Photoluminescence measurement was carried out and it showed that the spectra of ZnO micro and nanostructures prepared under different quantity of O₂ exhibited similar emission features. In addition, the growth mechanism of ZnO micro and nanostructures was preliminarily discussed.     

Controlling the orientation of ZnO nanorod arrays using TiO<Subscript>2</Subscript> thin film templates dip-coated by sol–gel

The oriented ZnO nanorod arrays have been synthesized on a silicon wafer that coated with TiO₂ films by aqueous chemical method. The morphologies, phase structure and the photoluminescence (PL) properties of the as-obtained product were investigated by field-emission scanning electron microscopy (FE-SEM), X-ray diffractometer (XRD), transmission electron microscope (TEM) and PL spectrum. The nanorods were about 100 nm in diameter and more than 1 μm in length, which possessed wurtzite structure with a c axis growth direction. The room-temperature PL measurement of the nanorod arrays showed strong ultraviolet emission. The effect of the crystal structure and the thickness of TiO₂ films on the morphologies of ZnO nanostructures were investigated. It was found that the rutile TiO₂ films were appropriate to the oriented growth of ZnO nanorod arrays in comparison with anatase TiO₂ films. Moreover, flakelike ZnO nanostructures were obtained with increasing the thickness of anatase TiO₂ films.     

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.     

Synthesis of bamboo-leaf-shaped ZnO nanostructures by oxidation of Zn/SiO2 composite films deposited with radio frequency magnetron co-sputtering

Bamboo-leaf-shaped ZnO nanostructures were synthesized by oxidation of metal Zn/SiO₂ matrix composite thin films deposited on Si(1 1 1) substrates with radio frequency magnetron co-sputtering. The synthesized bamboo-leaf-shaped ZnO are single crystalline in nature with widths ranging from 30 to 60 nm and lengths of up to 5-10 μm, room temperature photoluminescence spectrum of the nanostructures shows a strong and sharp UV emission band at 372 nm and a weak and broad green emission band at about 520 nm which indicates relatively excellent crystallization and optical quality of the ZnO nanostructures synthesized by this novel method.     

Significant Fowler–Nordheim tunneling across ZnO – Nanorod based nanojunctions for nanoelectronic device applications

We demonstrate significant Fowler–Nordheim (FN) tunneling across Al/Al₂O₃/ZnO metal–insulator–semiconductor (MIS) and Ag/ZnO metal–semiconductor (MS) nanojunctions. The transport properties of ZnO nanostructures in the form of urchins and randomly distributed nanorods were investigated in terms of various conduction mechanism. The minimum voltage necessary for triggering Fowler–Nordheim (FN) tunneling, under forward biasing, was ～1.2 V and ～3.4 V; respectively, below which only direct tunneling and thermionic emission events were evident. Mediated through Al₂O₃ layer, the FN tunneling was more prominent across MIS junction than MS one. The weak FN tunneling across MS junction was owing to interfacial charge transfer process through the atomic scale gapping between adjacent nanostructures. The extent of such type of tunneling is found to be nanostructure morphology dependent and largely rely on the free electrons donated by the native donor defects in the crystal structure of ZnO. The significant FN tunneling across the MIS and MS junctions has a direct relevance in designing nanoscale field emission devices/components working at low voltage with high throughputs.     

40% Efficiency enhancement in solar cells using ZnO nanorods as shell prepared via novel hydrothermal synthesis

Herein, rod-like ZnO nanostructures were synthesized via a novel hydrothermal route using Zn(OAc)₂, ethylenediamine and hydrazine as a new set of starting reagents. The as-synthesized products were characterized by techniques including XRD, EDS, SEM, XPS, Pl and FTIR. The prepared ZnO nanostructures were utilized as shell on TiO₂ film in DSSCs. Effect of precursor type, morphology and thickness of ZnO shell (number of electrophoresis cycle) on solar cells efficiency were well studied. Our results showed that ethylenediamine has crucial effect on morphology of synthesized ZnO nanostructures and using ZnO nanostructures leads to an increase in DSSCs efficiency compared to bare TiO₂ from 4.66 to 7.13% (~40% improvement). Moreover, highest amount of solar cell efficiency (7.13%) was obtained by using ZnO nanorods with two cycle of electrophoresis for deposition.     

Preparation and properties of ZnO nanostructures by electrochemical anodization method

ZnO thin films with diverse nanostructures, including nanodot, nanowire and nanoflower, have been fabricated on zinc foils by a simple and rapid electrochemical anodization method. The ZnO thin films reveal very strong visible emission that is ascribed to the transition between V_OZn_i and valence band. Under the dc or ac electric field, the electroinduced surface wettability conversion from the superhydrophobic to hydrophilic state was observed and the generation of surface defective sites on ZnO films under electric field was used to explain the transition mechanism. This work provides a simple and rapid method for synthesizing different ZnO nanostructures in large scale, and electric field can be used to modulate the wettability of ZnO nanostructures.     

Hierarchical ZnO/Bi2O3 nanostructures: synthesis, characterization, and electron-beam modification

Two types of ZnO/Bi₂O₃ nanonecklace heterostructures were fabricated using the vapor-phase transport (VPT) method for the first time. These hierarchical structures were well characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) attached. The growth mechanism of the novel structures were proposed based on these characterizations. Electron-beam irradiation was found to be a powerful and controllable tool in further tailoring such ZnO/Bi₂O₃ nanonecklace heterostructures. In addition, photoluminescence (PL) emission from the hierarchical nanostructures showed enhancement comparing to the pure Bi₂O₃ powder.     

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

ZnO and Al-doped ZnO(ZAO) thin films have been prepared on glass substrates by direct current (dc) magnetron sputtering from 99.99% pure Zn metallic and ZnO:3 wt%Al₂O₃ ceramic targets, the effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. It shows that the surface morphologies of ZAO films exhibit difference from that of ZnO films, while their preferential crystalline growth orientation revealed by X-ray diffraction remains always the (0 0 2). The optical transmittance and photoluminescence (PL) spectra of both ZnO and ZAO films are obviously influenced by the substrate temperature. All films exhibit a transmittance higher than 86% in the visible region, while the optical transmittance of ZAO films is slightly smaller than that of ZnO films. More significantly, Al-doping leads to a larger optical band gap (E_g) of the films. It is found from the PL measurement that near-band-edge (NBE) emission and deep-level (DL) emission are observed in pure ZnO thin films. However, when Al was doped into thin films, the DL emission of the thin films is depressed. As the substrate temperature increases, the peak of NBE emission has a blueshift to region of higher photon energy, which shows a trend similar to the E_g in optical transmittance measurement.     

Optical properties of ZnO nanoparticles synthesized by varying the sodium hydroxide to zinc acetate molar ratios using a Sol-Gel process

Material property dependence on the OH⁻/Zn²⁺ molar ratio of the precursor was investigated by varying the amount of NaOH during synthesis of ZnO. It was necessary to control the water content and temperature of the mixture to ensure the reproducibility. It was observed that the structural properties, particle size, photoluminescence intensity and wavelength of maximum intensity were influenced by the molar ratio of the precursor. The XRD spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO, indicating a single phase. UV measurements show the absorption that comes from the ZnO nanoparticles in visible region. The absorption edge of these ZnO nanoparticles are shifted to higher energies and the determined band gap energies are blue shifted as the OH⁻/Zn² molar ration increases, due to the quantum confinement effects. The photoluminescence characterization of the ZnO nanostructures exhibited a broad emission band centred at green (600 nm) region for all molar ratios except for OH⁻/Zn²⁺ = 1.7 where a second blue emission around 468 nm was also observed. The photoluminescence properties of ZnO nanoparticles were largely determined by the size and surface properties of the nanoparticles.     

Effect of zinc sources on the morphology of ZnO nanostructures and their photoluminescence properties

The morphology and photoluminescence properties of ZnO nanostructures synthesized from deferent zinc sources by a vapor deposition process were investigated. The zinc sources involved pure zinc, ZnO, and ZnCO₃ powders, respectively. It was found that the zinc sources have a strong effect on the morphology of the ZnO nanostructures. For the pure zinc and ZnO sources, uniform ZnO nanowires and tetrapods are obtained, respectively. However, in the case of the ZnCO₃ source, the products are nanowire–tetrapod combined nanostructures, in which ZnO nanowires grow from the ends of tetrapod arms. The morphology differences of these products may be mainly concerned with the yield and constituents of the corresponding zinc vapor. Photoluminescence measurements show that the nanowires have a relatively stronger near-band UV emission than the other products. The strongest green-light emission from the tetrapods implies that more defects exist in the tetrapods. An evident peak at 430 nm is found in the spectrum of the nanowire–tetrapod combined nanostructures, which may be caused by oxygen-depletion interface traps. PACS 73.61.Tm; 81.10.Bk; 78.55.Et     

Zinc oxide nanostructured layers for gas sensing applications

Various kinds of zinc oxide (ZnO) nanostructures, such as columns, pencils, hexagonal pyramids, hexagonal hierarchical structures, as well as smooth and rough films, were grown by pulsed laser deposition using KrF and ArF excimer lasers, without use of any catalyst. ZnO films were deposited at substrate temperatures from 500 to 700°C and oxygen background pressures of 1, 5, 50, and 100 Pa. Quite different morphologies of the deposited films were observed using scanning electron microscopy when different laser wavelengths (248 or 193 nm) were used to ablate the bulk ZnO target. Photoluminescence studies were performed at different temperatures (down to 7 K). The gas sensing properties of the different nanostructures were tested against low concentrations of NO₂. The variation in the photoluminescence emission of the films when exposed to NO₂ was used as transduction mechanism to reveal the presence of the gas. The nanostructured films with higher surface-to-volume ratio and higher total surface available for gas adsorption presented higher responses, detecting NO₂ concentrations down to 3 ppm at room temperature.     

Effect of bi-layer ratio in ZnO/Al2O3 multilayers on microstructure and functional properties of ZnO nanocrystals embedded in Al2O3 matrix

Zinc oxide (ZnO) nanocrystals (NCs) embedded in alumina (Al₂O₃) matrix were produced via rapid thermal annealing (RTA) of pulsed laser deposited ZnO/Al₂O₃ multilayered nanostructures. The effect of the thickness ratio (R) between Al₂O₃ and ZnO in one bi-layer on the microstructure and functional properties of NCs has been investigated. Grazing incidence small angle X-ray scattering confirmed the formation of nanocrystals after RTA. Grazing incidence wide angle X-ray scattering studies revealed that ZnO NCs have a high crystalline quality with (100) as preferred orientation. Tensile strain of NCs decreases with increasing R and is correlated to the distribution of NCs. From Raman analysis, it is noticed that the phonon frequency of the E₂ mode, related to the ZnO wurtzite phase, in NCs is shifted towards that of bulk ZnO with increasing R. Photoluminescence studies revealed that the near edge peak position shifts from 382 nm to 371 nm as the ratio R changes from 1.5 to 4 and is attributed to the strain effect. The intensity of emission in the yellow–green region due to defects decreases significantly with increasing R. Current–voltage (I–V) characteristics of Al/ZnO NCs embedded in Al₂O₃/n-Si (100)/Al have shown a hysteresis behavior. The increasing width of the hysteresis with increasing R revealed that the origin of the hysteresis might be due to the existence of polar surface charges on well-separated NCs. The high-resistance and low-resistance states in I–V hysteresis curves seem to be governed by Fowler–Nordheim tunneling and Schottky emission mechanisms, respectively.     

Zinc oxide nanostructures for applications as ethanol sensors and dye-sensitized solar cells

ZnO nanostructures were prepared by thermal oxidation technique for applying as ethanol sensors and dye-sensitized solar cells. To improve sensitivity of the sensor based on ZnO nanostructures, gold doping was performed in ZnO nanostructures. Gold-doped with 0%, 5%, and 10% by weight were investigated. The improvement of sensor sensitivity toward ethanol due to gold doping was observed at entire operating temperature and ethanol concentration. The sensitivity up to 145 was obtained for 10% Au-doped ZnO sensor. This can be explained by an increase of the quantity of oxygen ion due to catalytic effect of gold. Also, it was found that oxygen ion species at the surface of the Au-doped ZnO sensor remained O²⁻ as pure ZnO sensor. For dye-sensitized solar cell application, the dye-sensitized solar cell structure based on ZnO as a photoelectrode was FTO/ZnO/Eosin-Y/electrolyte/Pt counter electrode. ZnO with different morphologies of nanobelt, nano-tetrapod, and powder were investigated. It was found that DSSCs with ZnO powder showed higher photocurrent, photovoltage and overall energy conversion efficiencies than that of ZnO nanobelt and ZnO nano-tetrapod. The best results of DSSCs were the short circuit current (J_sc) of 1.25 mA/cm², the open circuit voltage (V_oc) of 0.45 V, the fill factor (FF) of 0.65 and the overall energy conversion efficiency (η) of 0.68%.     

Burstein–Moss shift and room temperature near-band-edge luminescence in lithium-doped zinc oxide

Nanopowders of pure and lithium-doped semiconducting ZnO (Zn_1−x Li _x O, where x= 0, 0.01, 0.03, 0.06, 0.09 and 0.15 in atomic percent (at.%)) are prepared by PEG-assisted low-temperature hydrothermal method. The average crystallite size is calculated using Debye–Scherrer formula and corrected for strain-induced broadening by Williamson–Hall (W–H) plot. The peak shift in XRD and the lattice constant of ZnO as a function of unit cell composition are predicted by Vegard’s law. The evolution of ZnO nanostructures from rod-shaped to particle nature is observed from TEM images and the influence of dopant on the morphology is investigated. The optical absorption measurement marks an indication that the incorporation of lithium ion into the lattice of ZnO widens the optical band gap energy from ∼2.60 to ∼3.20 eV. The near band edge (NBE) emission peak centered at ∼3.10 eV is considered to be the dominant emission peak in the PL spectra. Blue emission peak is not observed in doped ZnO, thus promoting defect-free nanoparticles. The Burstein–Moss shift serves as a qualitative tool to analyze the widening of the optical band gap and to study the shape of the NBE luminescence in doped ZnO nanopowders. FT-IR spectra are used to identify the strong metal–oxide (Zn–O) interaction.     

Ag掺杂p型ZnO薄膜及其光电性能研究

采用超声喷雾热分解法在石英衬底上以醋酸锌水溶液为前驱体,以硝酸银水溶液为Ag掺杂源生长了Ag掺杂ZnO(ZnO：Ag)薄膜.研究了衬底温度对所得ZnO：Ag薄膜的晶体结构、电学和光学性质的影响规律.所得ZnO：Ag薄膜结构良好,在室温光致发光谱中检测到很强的近带边紫外发光峰,透射光谱中观测到非常陡峭的紫外吸收截止边和较高的可见光区透过率,表明薄膜具有较高的晶体质量与较好的光学特性.霍尔效应测试表明,在500℃下获得了p型导电的ZnO：Ag薄膜,载流子浓度为5.30×10¹⁵cm关键词： ZnO：Ag薄膜 p型掺杂 超声喷雾热分解 霍尔效应