Synthesis of zinc oxide nanosheet thin films and their improved field emission and photoluminescence properties by annealing processing

ZnO nanosheet thin films have been synthesized through a solvothermal route. These obtained nanosheets disperse quasi-vertically and homogenously on the copper substrates and range in thickness from 80 nm to 250 nm. The as-grown nanosheet thin films were then annealed in the oxygen-presented atmosphere. Field emission plots indicate that the value of turn-on field is reduced from 2.86 V/μm to 1.52 V/μm and the corresponding value of threshold field decreases from 7.19 V/μm to 4.45 V/μm after annealing processing. Room temperature photoluminescence spectrum from the sample annealed at 850 °C in almost pure oxygen atmosphere shows only UV emission and a blue shift while the visible light band is unobservable compared with those of the other two samples, indicating that the crystalline quality of the obtained zinc oxide nanosheet thin films is greatly improved through annealing treatment. This solution approach combined with annealing treatment can, therefore, be regarded as a convenient route to fabricate high-quality crystalline ZnO nanomaterials.     

Effect of annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films on sapphire (0 0 0 1) substrates by magnetron sputtering

ZnO thin films were epitaxially grown on sapphire (0 0 0 1) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800 °C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (0 0 2) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800 °C in N₂. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (V_O) and interstitial oxygen (O_i). The biggest ratio of the PL intensity of UV emission to that of visible emission (I_NBE/I_DLE) is observed from ZnO thin films annealed at 800 °C in N₂. Therefore, we suggest that annealing temperature of 800 °C and annealing atmosphere of N₂ are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance.     

Effects of the oxygen partial pressure and annealing atmospheres on the microstructures and optical properties of Cu-doped ZnO films

Cu-doped zinc oxide (ZnO:Cu) films were deposited on p-Si (1 0 0) substrates at 200 °C under various oxygen partial pressures by using radio frequency reactive magnetron sputtering. The properties of the films were characterized by the X-ray diffraction spectroscopy (XRD), energy dispersive spectrometer, X-ray photoelectron spectroscopy (XPS) and fluorescence spectrophotometer with the emphasis on the evolution of microstructures, element composition, valence state of Cu, optical properties. The results indicated that the properties of ZnO:Cu films were significantly affected by oxygen partial pressures. XRD measurements revealed that the sample prepared at the ratio of O₂:Ar of 15:10 sccm had the best crystal quality among all ZnO:Cu films. XPS analysis results suggested that the valence of Cu in the ZnO films was a mixed state of +1 and +2, and the integrated intensity ratio of Cu²⁺ to Cu⁺ increased with the increment of oxygen partial pressure. The photoluminescence measurements at room temperature revealed a violet, two blue and a green emission. We considered that the origin of green emission came from various oxygen defects when the ZnO:Cu films grew in oxygen poor and enriched environment. Furthermore, the influence of annealing atmosphere on the microstructures and optical properties of ZnO:Cu films were discussed.     

非掺杂ZnO薄膜中紫外与绿色发光中心

用直流反应溅射方法在硅衬底上淀积了ZnO薄膜,测量它们的光致发光(PL)光谱,观察到两个发光峰,峰值能量分别为3.18(紫外峰,UV)和2.38eV(绿峰).样品用不同温度分别在氧气、氮气和空气中热处理后,测量了PL光谱中绿峰和紫外峰强度随热处理温度和气氛的变化,同时比较了用FP-LMT方法计算的ZnO中几种本征缺陷的能级位置.根据实验和能级计算的结果,推测出ZnO薄膜中的紫外峰与ZnO带边激子跃迁有关,而绿色发光主要来源于导带底到氧错位缺陷(O_Zn)能级的跃迁,而不是通常认为的氧空 关键词： ZnO薄膜 热处理 光致发光光谱 缺陷能级     

Photoluminescence and absorption in sol-gel-derived ZnO films

Highly c-axis-oriented ZnO films were obtained on corning glass substrate by sol-gel technique. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption in the absorption (UV) spectra are closely related to the post-annealing treatment. The difference between PL peak position and the absorption edge, designated as Stokes shift, is found to decrease with the increase of annealing temperature. The minimum Stokes shift is about 150 meV. The decrease of Stokes shift is attributed to the decrease in carrier concentration in ZnO film with annealing. X-ray diffraction, surface morphology and refractive index results indicate an improvement in crystalline quality with annealing. Annealed films also exhibit a green emission centered at ∼520 nm with activation energy of 0.89 eV. The green emission is attributed to the electron transition from the bottom of the conduction band to the antisite oxygen O_Zn defect levels.     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Effect of Ag doping on the photoluminescence properties of ZnO films

ZnO:Ag films were grown on Si (1 0 0) substrates by ultrasonic spray pyrolysis at various substrate temperatures. The effect of deposition temperature on the structural and the room temperature photoluminescence (RT-PL) properties of ZnO:Ag films was studied. With the deposition temperature rising to 550 °C, the intensity of the near-band edge (NBE) emission at 378 nm decreased and a new emission peak at 399 nm was observed. On the basis of the X-ray diffraction pattern (XRD), the X-ray photoelectron (XPS) spectra of ZnO:Ag films, and the effects of annealing on the PL, we suggest that the 399 nm emission should be attributed to the electron transition from the conduction band to Ag_Zn-related complexes defects radiative centers above the valence band.     

Controlled visible photoluminescence of ZnO thin films prepared by RF magnetron sputtering

ZnO thin films were prepared by RF magnetron sputtering. The photoluminescence dependence on the growth ambient and annealing temperatures and the atmosphere has been studied. Visible photoluminescence with blue, green, orange, and red emission bands has been demonstrated by controlling the preparation conditions. Complete suppression of the visible emission bands was also realized by annealing the O₂-ambient-grown samples in N₂ atmosphere at higher temperatures, which indicated the preparation of ZnO thin films with high optical quality.     

Photoluminescence of thermal-annealed nanocolumnar ZnO thin films grown by electrodeposition

Nanostructured zinc oxide thin films formed by partially oriented hexagonal columns with dimensions of about 100 nm × 300 nm have been prepared by cathodic electrodeposition on conducting glass substrates. After subsequent thermal annealing in air at different temperatures (100-500 °C), structural information on the films was obtained by means of non-resonant Raman spectroscopy. Increasing the annealing temperature leads to a higher degree of crystallinity. The photoluminescence activity of the samples (at low temperature) also improves for increasing annealing temperatures in two ways: increasing the intensity of the near-band edge emission and decreasing the width of the excitonic peak. No emission band in the visible is detected, which attests the high quality of the ZnO nanocolumnar films.     

Effect of MgO on the enhancement of ultraviolet photoluminescence in ZnO

By glycine-nitrate combustion route and followed by 900 °C annealing in air, ZnO-MgO nanocomposite with heterojunction-like structures between ZnO and MgO phase was successfully produced. The ultraviolet photoluminescence band from ZnO is enhanced by the incorporation of MgO, as compared to the pure ZnO synthesized via the similar route. The charge transfer required by electronic equilibrium across the junction creates an electron depletion region in ZnO phase, which greatly changes the electron states of visible emission-related defects, as a result, the band-edge emission is enhanced while the visible emission in ZnO is suppressed. This mechanism may provide an effective way to modify the emission property of nanomaterials.     

Preparation of nanocone ZnO thin film and its aging effect of photoluminescence

In this work, a nanocone ZnO thin film was prepared by electron beam evaporation on a Si (1 0 0) substrate. The structural properties of the film were investigated by X-ray diffraction (XRD), atomic force microscopy and laser Raman scattering, respectively. The aging effect of the nanocone ZnO thin film was studied by photoluminescence spectra. The structural analyses show that the prepared ZnO thin film has a hexagonal wurtzite structure and is preferentially oriented along the c-axis perpendicular to the substrate surface. The photoluminescence spectra show that with the increase of aging time, the green emission of the nanocone ZnO thin film gradually decreases while the ultraviolet emission somewhat increases. The reason for this phenomenon is likely that the green-emission-related oxygen vacancies in the film are gradually filled up. The Raman scattering analyses also suggest that the intensity of the Raman peak related to oxygen vacancies in the nanocone ZnO thin film declines after the film is aged in air for a year. Therefore, the authors think the green emission is mainly connected with oxygen vacancy defects.     

Luminescence mechanism of ZnO thin film investigated by XPS measurement

The effects of annealing environment on the luminescence characteristics of ZnO thin films that were deposited on SiO₂/Si substrates by reactive RF magnetron sputtering were investigated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). An analysis of the O 1s peak of ZnO film revealed that the concentration of oxygen vacancies increased with the annealing temperature from 600 °C to 900 °C under an ambient atmosphere. The PL results demonstrated that the intensity of green light emission at 523 nm also increased with temperature. Under various annealing atmospheres, the analyses of PL indicated that only one emission peak (523 nm) was obtained, indicating that only one class of defect was responsible for the green luminescence. The green light emission was strongest and the concentration of oxygen vacancies was highest when the ZnO film was annealed in ambient atmosphere at 900 °C. The results in this investigation show that the luminescence mechanism of the emission of green light from a ZnO thin film is associated primarily with oxygen vacancies. PACS 81.15.Cd; 81.40.Ef; 78.55.-m; 78.55.Et     

Influence of annealing temperature on the photoluminescence properties of ZnO quantum dots

The properties of ZnO quantum dots (QDs) synthesized by the sol-gel process are reported. The primary focus is on investigating the origin of the visible emission from ZnO QDs by the annealing process. The X-ray diffraction results show that ZnO QDs have hexagonal wurtzite structure and the QD diameter estimated from Debye-Scherrer formula is 8.9 nm, which has a good agreement with the results from transmission electron microscopy images and the theoretical calculation based on the Potential Morphing Method. The room-temperature photoluminescence spectra reveal that the ultraviolet excitation band has a red shift. Meanwhile, the main band of the visible emission shifts to the green luminescence band from the yellow luminescence one with the increase of the annealing temperature. A lot of oxygen atoms enter into Zn vacancies and form oxygen antisites with increasing temperature. That is probably the reason for the change of the visible emission band.     

Epitaxial growth of Sc-doped ZnO films on Si by sol-gel route

The epitaxial growth of doped ZnO films is of great technological importance. Present paper reports a detailed investigation of Sc-doped ZnO films grown on (1 0 0) silicon p-type substrates. The films were deposited by sol-gel technique using zinc acetate dihydrate as precursor, 2-methoxyethanol as solvent and monoethanolamine (MEA) as a stabilizer. Scandium was introduced as dopant in the solution by taking 0.5 wt%¹ of scandium nitrate hexahydrate. The effect of annealing on structural and photoluminescence properties of nano-textured Sc-doped films was investigated in the temperature range of 300-550 °C. Structural investigations were carried out using X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray diffraction study revealed that highly c-axis oriented films with full-width half maximum of 0.21° are obtained at an annealing temperature of 400 °C. The SEM images of ZnO:Sc films have revealed that coalescence of ZnO grains occurs due to annealing. Ostwald ripening was found to be the dominant mass transport mechanism in the coalescence process. A surface roughness of 4.7 nm and packing density of 0.93 were observed for the films annealed at 400 °C. Room temperature photoluminescence (PL) measurements of ZnO:Sc films annealed at 400 °C showed ultraviolet peak at about (382 nm) with FWHM of 141 meV, which are comparable to those found in high-quality ZnO films. The films annealed below or above 400 °C exhibited green emission as well. The presence of green emission has been correlated with the structural changes due to annealing. Reflection high energy electron diffraction pattern confirmed the nearly epitaxial growth of the films.     

Influence of CH3COO on the room temperature photoluminescence of ZnO films prepared by CVD

ZnO films with strong c-axis-preferred orientation have been prepared by a single source chemical vapor deposition technique using zinc acetate as source material at the growth temperature of 230 °C. The strong UV and blue emissions were observed in the photoluminescence spectra of as-grown films. A small quantity of residual zinc acetate was reserved on the surface of as-grown ZnO films and the emission mechanism of blue luminescence was nearly related to the CH₃COO^- of unidentate type. The blue emission disappeared and the green emission appeared after annealing treatment. The green emission is related to the singly ionized oxygen vacancies.     

Influence of post-deposition annealing on the structural,optical and electrical properties of Li and Mg co-doped ZnO thin films deposited by sol–gel technique

Lithium (Li) and magnesium (Mg) co-doped zinc oxide (ZnO) thin films were deposited by sol–gel method using spin coating technique. The films were deposited on glass substrates and annealed at different temperatures. The effects of annealing temperature on the structural, optical and electrical properties of the deposited films were investigated using X-ray diffraction (XRD), Ultraviolet–Visible absorption spectra (UV–VIS), photoluminescence spectra (PL), X-ray photo electron spectroscopy (XPS) and Hall measurements. XRD patterns indicated that the deposited films had a polycrystalline hexagonal wurtzite structure with preferred (0 0 0 2) orientation. All films were found to exhibit a good transparency in the visible range. Analysis of the absorption edge revealed that the optical band gap energies of the films annealed at different temperatures varies between 3.49 eV and 3.69 eV. Room temperature PL spectra of the deposited films annealed at various temperatures consist of a near band edge emission and visible emission due to the electronic defects, which are related to deep level emissions, such as oxide antisite (O_Zn), interstitial zinc (Zn_i), interstitial oxygen (O_i) and zinc vacancy (V_Zn) which are generated during annealing process. The influence of annealing temperature on the chemical state of the dopants in the film was analysed by XPS spectra. Ion beam analysis (Rutherford back scattering) experiments were performed to evaluate the content of Li and Mg in the films. Hall measurements confirmed the p-type nature of the deposited films.     

Single violet luminescence emitted from ZnO films obtained by oxidation of Zn film on quartz glass

The photoluminescence (PL) emission properties of ZnO films obtained on quartz glass substrate by the oxidation of Zn films were studied. The strong single violet emission centering about 413-424 nm was observed in the room temperature PL spectra of the ZnO films. The intensity of violet emission increased and the peak position shift from 424 to 413 nm with increasing oxygen pressures. The violet emission was attributed to the electron transition from the valence band to interstitial zinc (Zn_i) level under low oxygen pressure conditions (50-500 Pa). Under high oxygen pressure conditions (5000-23,000 Pa), both interstitial zinc (Zn_i) and zinc Vacancy (V_Zn) were thought to be responsible for the violet emission.     

A simple growth route towards ZnO thin films and nanorods

Highly orientated ZnO thin films and the self-organized ZnO nanorods can be easily prepared by a simple chemical vapor deposition method using zinc acetate as a source material at the growth temperature of 180 and 320 °C, respectively. The ZnO thin films deposited on Si (100) substrate have good crystallite quality with the thickness of 490 nm after annealing in oxygen at 800 °C. The ZnO nanorods grown along the [0001] direction have average diameter of 40 nm with length up to 700 nm. The growth mechanism for ZnO nanorods can be explained by a vapor-solid (VS) mechanism. Photoluminescence (PL) properties of ZnO thin films and self-organized nanorods were investigated. The luminescence mechanism for green band emission was attributed to oxygen vacancies and the surface states related to oxygen vacancy played a significant role in PL spectra of ZnO nanorods.     

Structural and optical properties of ZnO thin films deposited on quartz glass by pulsed laser deposition

ZnO thin films with typical c-axis (0 0 2) orientation were successfully deposited on quartz glass substrates by pulse laser ablation of Zn target in oxygen atmosphere at a relatively low temperature range of 100-250 °C. The structural and optical properties of the films were studied. In photoluminescence (PL) spectra at room temperature, single ultraviolet emission (without deep-level emission) was obtained from ZnO film deposited at the temperature of 200 °C. This was attributed to its low intrinsic defects.     

Defects, stress and abnormal shift of the (0 0 2) diffraction peak for Li-doped ZnO films

The effect of changes in Li content on the structural property of sol-gel Li-doped ZnO films was investigated in this study. The observed changes of the Li incorporation-induced strain along c-axis are closely related to the different ratios between the concentrations of Li interstitials (Li_i) and Li substituting for Zn (Li_Zn) in the films. According to the observed results from X-ray diffraction (XRD) and photoluminescence measurements, we found that the domination of the dissociative mechanism in the Li-doped ZnO films led to transformation from Li_Zn to Li_i, involving the formation of Zn vacancies (V_Zn). In addition, the interaction between these defects (that is, Li_Zn, Li_i, V_Zn and oxygen vacancy) and the crystal structure may lead to the abnormal shift of the (0 0 2) diffraction peak position determined from XRD measurements.