Effects of annealing temperature on morphologies and optical properties of ZnO nanostructures

The effects of annealing temperature on the morphologies and optical properties of ZnO nanostructures synthesized by sol–gel method were investigated in detail. The SEM results showed that uniform ZnO nanorods formed at 900 C. The PL results showed an ultraviolet emission peak and a relatively broad visible light emission peak for all ZnO nanostructures sintered at different temperature. The increase of the crystal size and decrease of tensile stress resulted in the UV emission peak shifted from 386 to 389 nm when annealing temperature rose from 850 to 1000 C. The growth mechanism of the ZnO nanorods is discussed.     

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.     

Effect of S- and Sn-doping to the optical properties of ZnO nanobelts

In this study, the optical properties of S- and Sn-doped ZnO nanobelts, grown by thermal evaporation, were investigated. The sulfur and tin contents in the nanobelts were about 12% and 8% (atomic), respectively. The average widths of the S- and Sn-doped ZnO nanobelts were 73 and 121 nm, respectively. Room temperature photoluminescence (PL) spectroscopy exhibits significantly different optical properties for the two types of nanobelts. The PL result of the S-doped ZnO nanobelts shows the broad visible emission with no detectable ultraviolet (UV) peak, while the PL result of the Sn-doped sample shows two emission bands, one related to UV emission with a strong peak at 376 nm that is blue-shifted by 4 nm in comparison to pure ZnO nanobelts, and another related to green emission with a weak peak. A weak peak in the UV region at 383 nm appeared after annealing the S-doped ZnO nanobelts at 600 °C. Additionally, the annealed S-doped nanobelts show a stronger peak in the visible emission region in comparison to that observed prior to annealing. The Sn-doped ZnO nanobelts are also affected by annealing, as the UV emission peak is blue-shifted to 372 nm after annealing.     

Effect of the annealing environment on the optical properties of ZnO/GaAs grown by MOCVD

The optical properties of ZnO grown on (1 0 0) GaAs substrate using metalorganic chemical vapor deposition are investigated by photoluminescence (PL) spectroscopy. Postgrowth annealing in nitrogen and oxygen was performed for different times and temperatures in order to incorporate As from the substrate into the ZnO thin films. The PL spectra of the samples annealed in different ambients reveal that the effect of As diffusion into the ZnO thin films is more pronounced when the annealing is performed in oxygen at 550 °C. The 11 K PL spectra show the appearance of a transition at ∼3.35 eV after annealing in oxygen at 550 °C for 1 h. A further increase in the annealing temperature leads to the disappearance of this line, while for annealing times longer than 2 h at 550 °C, it is no longer prominent. The increase in intensity of this new transition is also accompanied by the enhancement of radiative centers related to structural defects, such as the stacking fault-related transition at 3.31 eV and the Y-line. Temperature dependent PL illustrates the excitonic nature of the new transition at ∼3.35 eV, which is therefore assigned to (A⁰, X) transition, where the acceptor is possibly the 2V_Zn-As_Zn complex, with an activation energy E_A in the range of 160-240 meV. Furthermore, the enhancement of the radiative centers related to structural defects is regarded as evidence that As atoms tend to segregate in the vicinity of structural defects to relieve local strain.     

Galvanic deposition of ZnO nanorods and thermal annealing effects on their optical properties

The effects of NaCl electrolyte concentrations in the range 6-48 mM on the galvanic deposition of ZnO in Zn(Ac)₂ electrolyte is presented. Effects of thermal annealing on their structural and optical properties have been investigated by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) microanalysis and photoluminescence (PL). The results show that the increase of NaCl electrolyte concentration not only results in the increase of the diameter of ZnO nanorods, but also promotes the blue-shift of UV emission of ZnO. After air annealing at 200 °C, 300 °C and 400 °C, the UV emission is first enhanced then quenched sharply, while the visible emission tends to be enhanced tremendously. It can be ascribed to the new defect states introduced in ZnO after annealing at high temperature.     

ZnO nanoparticles obtained by mechanochemical technique and the optical properties

Crystalline ZnO nanoparticles were synthesized by mechanochemical method. Mechanochemical processing involves the mechanical activation of solid-state displacement reactions at low temperatures in a ball mill. Statistical design was used to investigate the effect of main parameters (i.e. time, milling rate and calcination temperature) on ZnO crystallite size and morphology. After annealing at 400 °C in air, zinc oxide (ZnO) nanoparticles were obtained. The milled powders are analyzed by X-ray diffraction (XRD), TG/DTA and transmission electron microscope (TEM).The crystallite size of ZnO samples calculated from XRD is consistent with the TEM images and estimated to be less than 20 nm. The optical properties of the samples were studied by UV-vis spectrophotometer.     

Effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles

We explore the effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles synthesized by the sol-gel method. X-ray diffraction and x-ray photoelectron spectroscopy data show evidence that Cr has been incorporated into the wurtzite ZnO lattice as Cr²⁺ ions substituting for Zn²⁺ ions without any detectable secondary phase in as-synthesized Zn_0.97Cr_0.03O nanopowders. The room temperature magnetization measurements reveal a large enhancement of saturation magnetization M_s as well as an increase of coercivity of H₂-annealed Zn_0.97Cr_0.03O:H samples. It is found that the field-cooled magnetization curves as a function of temperature from 40 to 400 K can be well fitted by a combination of a standard Bloch spin-wave model and Curie–Weiss law. The values of the fitted parameters of the ferromagnetic exchange interaction constant a and the Curie constant C of H₂-annealed Zn_0.97Cr_0.03O:H nanoparticles are almost doubled upon H₂-annealing. Photoluminescence measurements show evidence that the shallow donor defect or/and defect complexes such as hydrogen occupying an oxygen vacancy H_o may play an important role in the origin of H₂-annealing induced enhancement of ferromagnetism in Cr-H codoped ZnO nanoparticles.     

Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films

XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 °C, while reduction of Ni²⁺ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 °C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 °C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni²⁺ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M_s) in the films was observed after annealing in air (M_s = 0.036 μ_B/Ni) or Ar (M_s = 0.033 μ_B/Ni) at 600 °C compared to that in as-deposited film (M_s = 0.017 μ_B/Ni). An even higher M_s value was observed in the film annealed in Ar at 800 °C (M_s = 0.055 μ_B/Ni) compared to that at 600 °C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 °C, which was also attributed to the diffusion of Ni.     

Influence of Fe-doping on the structural,optical and magnetic properties of ZnO nanoparticles

Zn_1–xFe_xO (x=0–0.05) nanoparticles were synthesized without a catalyst by a two-step method. Fe was doped into ZnO by a source of metallic Fe sheets in a solid–liquid system at 80 °C, and the Zn_1−xFe_xO nanoparticles were obtained by annealing at 300 °C. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy were used to characterize the structural properties of the as-grown Zn_1−xFe_xO. The optical properties were determined by Infrared and Ultraviolet–visible spectroscopy. The results confirm that the crystallinity of the ZnO is deteriorated due to Fe-doping. XPS results show that there is a mixture of Fe⁰⁺ and the Fe³⁺ in the representative Zn_0.95Fe_0.05O sample. The optical band gap of Zn_1−xFe_xO is enhanced with increasing of Fe-doping. Room temperature ferromagnetism was observed in all the Fe-doped ZnO samples.     

The influence of annealing temperature on the slip plane activity and optical properties of nanostructured ZnO films

The zinc oxide films were prepared by the sol-gel method on the ordinary glass substrates. The activity of slip systems were evaluated by X-ray diffraction line broadening analysis using convolution multiple whole profile (CMWP) fitting procedures. It was found that in all temperatures the 〈a〉 type dislocations is dominating and its fraction increases with the rise of annealing temperature in the range of 350-600 °C. The investigation on the optical properties of films showed that the optical band gap energy increases linearly with the annealing temperature and crystallite size but decreases with the lattice strain.     

Al掺杂原子分数及退火温度对ZnO薄膜光学特性的影响

 采用溶胶凝胶法在（0001）Al₂O₃衬底上制备了不同掺杂原子分数的ZnO:Al薄膜,在Ar气氛中进行了600～950 ℃不同温度的退火处理,研究了掺杂原子分数和退火温度对薄膜光致发光、光吸收和透射的影响。结果显示,薄膜的紫外峰强度随掺杂原子分数和退火温度的提高而增强,与缺陷相关的绿光强度却随着掺杂原子分数和退火温度的提高而降低;薄膜光学带隙随掺杂原子分数的提高从3.21 eV增大到3.25 eV;光吸收在可见光区随着退火温度的升高而增大,在紫外区却随着退火温度的升高而减小,透射与吸收的变化规律相反;薄膜吸收边随退火温度的升高出现轻微的红移。     

Surface and bulk thermal annealing effects on ZnO crystals

Annealing at temperatures up to 1000 °C is shown to decrease band edge photoluminescence in bulk ZnO crystals and increase deep level-related emission. The surface roughens for anneals in the range of 600-800 °C as O is lost preferentially from the surface, but at 900 °C the morphology improves as excess Zn is also lost from the surface. Splitting of the peak in the rocking curve of the ZnO (0 0 2) plane after annealing at 900-1000 °C indicates that the substrate is a mosaic of two or more crystals oriented slightly differently from one another and we are detecting differences in orientation of some of the grains in different areas or small changes due to annealing. There was no significant change in bulk conductivity of the ZnO for anneals up to 1000 °C, suggesting that ion implantation followed by annealing may be an effective approach for doping in this.     

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 atomic oxygen treatment on structures, morphologies and electrical properties of ZnO:Al films

ZnO:Al (ZAO) film has a potential application in providing spacecrafts the protection against atomic oxygen (AO) erosion. To advance the understanding of the AO resisting mechanisms and the relationships between the structures, morphologies and conductive properties of ZAO film, direct current magnetron sputtered ZAO films with different thicknesses were treated with AO in a ground-based simulation facility. The microstructure, surface chemical state, morphologies and electrical properties of pristine films and irradiated ones were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy and Hall measurement. It is found that AO exposure produces novel, oriented recrystallization of the surface particles. It also increases the content of oxygen ions in fully oxidized stoichiometric surroundings on the surface, resulting in the decrease of the conductivity. As the thickness of ZAO film increases, the crystallinity, conductivity and resistance to AO erosion are all improved.     

退火温度对Cr掺杂ZnO薄膜的微结构与机械性能的影响研究

本研究利用射频磁控溅镀法在玻璃基板上制备3 at.%的Cr掺杂ZnO薄膜,再以300℃～500℃温度退火处理25 mins,并探讨了退火温度对Al掺杂ZnO薄膜的微观结构与机械性能的影响.微观结构分析结果表明Cr掺杂ZnO薄膜的结晶方向为(002),且沿(002)方向的成长随退火温度升高而越加明显,但薄膜的表面却随退火温度升高而变得越来越粗糙.机械性能分析结果揭示晶粒尺寸随退火温度升高而增大,导致差排原子的动能随之降低,致使Cr掺杂ZnO薄膜的硬度随退火温度升高而增大,但对其对杨氏模量却没有太大的影响;此外,Cr掺杂ZnO薄膜的耐磨性与韧性均随退火温度升高而增强,表明退火处理对该薄膜的抗塑性形变能力有很大帮助.     

Effect of La doping on the electronic structure and optical properties of ZnO

The electronic structure and optical properties of ZnO doped with La have been investigated using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that the La doping increases the bandgap of ZnO, in agreement with the experimental results; while the Fermi level shifts into the conduction band, revealing the so-called Burstein-Moss effect. In comparison to pure ZnO, a new peak appears in the imaginary part of dielectric function in the system doped with La and the optical absorption edge has been obviously changed. Moreover, the covalent property of Zn_1−xLa_xO is found to weaken with the increase of La concentration.     

Effects of deposition temperatures and annealing conditions on the microstructural, electrical and optical properties of polycrystalline Al-doped ZnO thin films

Al-doped ZnO (AZO, ZnO:Al₂O₃ = 98:2 wt%) films are deposited on different substrates by an RF magnetron sputtering and subsequently annealed at three different conditions to investigate the microstructural, electrical, and optical properties. X-ray diffraction and scanning electron microscope results show that all the samples are polycrystalline and the samples rapid-thermal-annealed at 900 °C in an N₂ ambient contain larger grains compared to the furnace-annealed samples. It is shown that the sample deposited at room temperature on the sapphire gives a resistivity of 5.57 × 10⁻⁴ Ω cm when furnace-annealed at 500 °C in a mixture of N₂:H₂ (9:1). It is also shown that the Hall mobility vs. carrier concentration (μ-n) relation is divided into two groups, depending on the annealing conditions, namely, either rapid-thermal annealing or furnace annealing. The relations are described in terms of either grain boundary scattering or ionized impurity scattering mechanism. In addition, the samples produce fairly high transmittance of 91-96.99% across the wavelength region of 400-1100 nm. The optical bandgaps of the samples increase with increasing carrier concentration.     

An influence of deposition temperature on structural,optical and electrical properties of sprayed ZnO thin films of identical thickness

Nanocrystalline ZnO thin films were deposited at different temperatures (T_s = 325 °C–500 °C) by intermittent spray pyrolysis technique. The thickness (300 ± 10 nm) independent effect of T_s on physical properties was explored. X-Ray diffraction analysis revealed the growth of wurtzite type polycrystalline ZnO films with dominant c-axis orientation along [002] direction. The crystallite size increased (31 nm–60 nm) and optical band-gap energy decreased (3.272 eV–3.242 eV) due to rise in T_s. Scanning electron microscopic analysis of films deposited at 450 °C confirmed uniform growth of vertically aligned ZnO nanorods. The films deposited at higher T_s demonstrated increased hydrophobic behavior. These films exhibited high transmittance (>91%), low dark resistivity (～10^?2 Ω-cm), superior figure of merit (～10^?3 Ω^?1) and low sheet resistance (～10² Ω/□). The charge carrier concentration (η -/cm³) and mobility (μ – cm²V^?1s^?1) are primarily governed by crystallinity, grain boundary passivation and oxygen desorption effects.     

Electronic structure and optical properties of substitutional and interstitial phosphor-doped ZnO

Using first-principles method, electronic structure and optical properties of phosphorus-doped ZnO for the possible substitutional (P_Zn, P_O) and interstitial (P_tet, P_oct) doping are investigated. P_O gives p-type conductivity, but others show n-type. P_O and P_tet has a significant difference in optical properties due to the contribution of P 3p states at Fermi level.