Preparation and characterization of bowl-like porous ZnO film by electrodeposition using two-dimensional photonic crystal template

A rapid and low-cost method combining electrodeposition with two-dimensional (2D) photonic crystal template technique to prepare large scale bowl-like porous ZnO films is described. The 2D photonic crystal templates were fabricated by self-assembly of monodispersed polystyrene (PS) microspheres on indium-tin-oxide coated glass substrates using spinning coating method. The interstitial spaces among the spheres of the templates were filled with ZnO via electrodeposition from an aqueous solution containing 0.02 M zinc nitrate as electrolyte under a constant potential of ?1.0 V at 65 °C for 10 min. After removal of the PS photonic crystal template, bowl-like porous ZnO film was obtained. The entire process can be accomplished within 30 min. Scanning electron microscopic images showed good homogeneity in morphology, X-ray diffraction spectra demonstrated the wurtzite structure of the obtained ZnO film, and transmission electron microscopy indicated the single-crystallinity of the ZnO. Ultraviolet–visible (UV–vis) spectrophotometer was used to detect the absorption in UV–vis region of the PS template, opal ZnO-PS composite and inverse opal ZnO respectively. Two strong emission bands at 400 and 550 nm were displayed in photoluminescence spectrum.     

Effects of buffer layer thickness on properties of ZnO thin films grown on porous silicon by plasma-assisted molecular beam epitaxy

ZnO thin films with different buffer layer thicknesses were grown on Si and porous silicon (PS) by plasma-assisted molecular beam epitaxy (PA-MBE). The effects of PS and buffer layer thickness on the structural and optical properties of ZnO thin films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The ZnO buffer layers, the intensity of the (002) diffraction peak for the ZnO thin films and its full width at half maximum (FWHM) decreased with an increase in the thickness of the ZnO buffer layers, indicating an improvement in the crystal quality of the films. On introducing PS as a substrate, the grain sizes of the ZnO thin films became larger and their residual stress could be relaxed compared with the ZnO thin films grown on Si. The intensity ratio of the ultraviolet (UV) to visible emission peak in the PL spectra of the ZnO thin films increased with an increase in buffer layer thickness. Stronger and narrower UV emission peaks were observed for ZnO thin films grown on PS. Their structural and optical properties were enhanced by increasing the buffer layer thickness. In addition, introduction of PS as a substrate enhanced the structural and optical properties of the ZnO thin films and also suppressed Fabry-Perot interference.     

Structural and photoluminescence investigation of ZnSe nanocrystals dispersed in organic and inorganic matrices

In this work, we report on the investigation of the effect of dispersion of zinc selenide (ZnSe) nanocrystallites into polystyrene (PS) and silica (SiO₂) thin films on their structural, morphological and photoluminescence properties. The ZnSe/PS nanocomposites thin films were synthesized by a direct dispersion of ZnSe crystallites into polymers solution, whereas the ZnSe–SiO₂ films were prepared on glass substrates by the sol–gel dip-coating technique. X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-rays (EDX), UV–visible spectrophotometry and photoluminescence spectroscopy (PL) techniques have been used to study the structural, morphological and optical properties of the prepared nanocomposite thin films. XRD patterns have demonstrated the incorporation of cubic ZnSe in both organic and inorganic matrices. SEM micrographs have indicated that ZnSe dispersion in the films is homogeneous. UV–visible absorption spectra of the nanocomposite thin films have put into evidence that the dispersion of ZnSe nanocrystals in the thin film matrices improved their optical absorption. Room temperature PL spectra have shown that the addition of ZnSe enhanced the UV emission of PS and all the emission of SiO₂ thin films.     

Study of optoelectronics and microstructures on the AZO/nano-layer metals/AZO sandwich structures

In this study, growth nano-layer metals (Al, Cu, Ag) and Al-doped ZnO (AZO) thin films are deposited on glass substrates as the transparent conducting oxides (TCOs) to form AZO/nano-layer metals/AZO sandwich structures. The conductivity properties of thin films are enhanced when the average transmittance over the wavelengths 400–800 nm is maintained at higher than 80 %. A radio frequency magnetron sputtering system is used to deposit the metal layers and AZO thin films of different thickness, to form AZO/Al/AZO (ALA), AZO/Cu/AZO (ACA) and AZO/Ag/AZO (AGA) structures. X-ray diffraction and field emission scanning electron microscopy are used to analyze the crystal orientation and structural characteristic. The optical transmission and resistivity are measured by UV–VIS–NIR spectroscopy and Hall effect measurement system, respectively. The results show that when the Ag thickness is maintained at approximately 9 nm, the TCOs thin film has the lowest resistivity of 8.9 × 10^?5 Ω-cm and the highest average transmittance of 81 % over the wavelengths 400–800 nm. The crystalline Ag nano-crystal structures are observed by high-resolution transmission electron microscopy. In addition, the best figure of merit for the AZO/Ag/AZO tri-layer film is 2.7 × 10^?2 (Ω^?1), which is much larger than that for other structures.     

One-step preparation of polystyrene colloidal crystal films with structural colors and high hydrophobicity

The formation of polystyrene (PS) sphere nanostructures through vertical deposition can provide large surface roughness, which effectively enhances the hydrophobicity of the films. Moreover, well-ordered PS sphere arrays bring about structural colors, which can be controlled through the sphere diameter. All the water contact angles of colloidal crystal films prepared from monodisperse PS sphere solutions with diameters from 225 to 605 nm were larger than 120°, showing highly hydrophobic character.     

A selective ethanol gas sensor based on spray-derived Ag–ZnO thin films

A simple and cost-effective spray pyrolysis technique was employed to synthesize silver-doped zinc oxide (Ag–ZnO) thin films on the glass substrates from aqueous solutions of zinc acetate and silver nitrate precursors at 450 °C. The effects of Ag doping on structural, morphological, and gas-sensing properties of films were examined. The X-ray diffraction spectra of the Ag–ZnO films showed the polycrystalline nature having hexagonal crystal structure. Scanning electron microscopy (SEM) images of the pure ZnO films revealed the uniform distribution of the spherical grains (~80 nm size). Tiny Ag nanoparticles are clearly visualized in the SEM of Ag–ZnO films. The investigation of the effect of Ag doping on the gas-sensing properties of the Ag–ZnO revealed that the 15 % Ag-doped ZnO sample has the highest gas sensitivity (85 %) and excessive Ag doping in ZnO degraded the gas sensitivity. A possible mechanism of Ag–ZnO-based sensor sensitivity to the target gas is also proposed.     

衬底温度对PLD方法生长的ZnO薄膜结构和发光特性的影响

在不同的衬底温度下, 通过脉冲激光淀积的方法在Si衬底上生长出c轴高度取向的ZnO薄膜. ZnO薄膜的结构和表面形貌通过X射线衍射和原子力显微镜表征. 同时以He-Cd激光和同步辐射作为激发源来测试样品的发光特性. 实验结果表明, 在衬底温度为500℃时生长的ZnO薄膜具有非常好的晶体质量, 并且表现出很强的紫外发射. 在用同步辐射为激发源的低温(18K)光致发光谱中, 还观察到了一个位于430nm处的紫光发射, 我们认为这个紫光发射与存在于晶粒间界的界面势阱所引起的缺陷态有关, 这个势阱可能起源于Zn填隙(Zn _i)     

Optical properties of electrochemically deposited ZnO thin films on colloidal crystal film of SiO2 microspheres

The optical properties of electrochemically deposited ZnO thin films on colloidal crystal film of SiO2 microspheres structures were studied. Colloidal crystal film of SiO2 microspheres were self-assembled by evaporation using SiO2 in solution at a constant 0.1 wt%. ZnO in thin films was then electrochemically deposited on to colloidal crystal film of SiO2 microspheres. During electrochemical deposition, the content of Zn(NO3)2 x 6H2O in solution was 5 wt%, and the process's conditions were varied between of 2-4 V and 30-120 s at room temperature, with subsequent heat-treatment between 200 and 400 degrees C. A smooth surface and uniform thickness of 1.8 microm were obtained at 3 V for 90 s. The highest PL peak intensity was obtained in the ZnO thin film heat-treated at 400 degrees C. The double layered ZnO/SiO2 colloidal crystals showed clearly better emission properties than the SiO2/ZnO and ZnO structures.     

Preparation and optimization of epitaxial growth of transparent ZnO nanotip thin films by hydrothermal method

Zinc oxide (ZnO) nanotip thin films were prepared on ZnO coated nanocrystalline ITO/glass substrates by hydrothermal method. In order to obtain the ZnO nanotip arrays with high aspect ratio, the experimental conditions were optimized. The scanning electron microscope images showed that the surface morphology of ZnO thin films could be easily manipulated by changing the seed layer thickness and growth time. The ZnO nanotip thin films were grown epitaxially on ZnO seed layer coated ITO/glass substrates. The surface morphology of ZnO thin films on ITO/glass substrate changed from nanorods with a flat-top end to nanotips as the growth time was increased from 3 to 15 h. The ZnO thin films prepared under these deposition conditions were highly oriented along (002) direction. The as-prepared sample (15 h) was annealed at different temperatures (30, 100, 150, and 270 degrees C). The surface morphologies of annealed ZnO thin films did not show any remarkable change and the best crystallinity was observed at 100 degrees C. The photoluminescence spectra showed that the near band edge emission shifted to shorter wavelength as the annealing temperature was increased from 30 to 270 degrees C, it was due to the intrinsic stress in the films. This was confirmed by X-ray diffraction analyses. NPB thin films were prepared on ITO/clay and ITO/glass substrates by thermal evaporation method. The electrical properties of Ag/NPB/ITO/Clay showed the Ohmic characteristics (J proportional V(1.0)). The J-V characteristic of Ag/NPB/PMMA/ZnO/ITO/Glass showed good rectification behaviour with a diode-ideality factor of 1.36.     

Electrodeposition and properties of nanocrystalline ZnO films prepared in the presence of anionic surfactant SDS and ionic liquid 1-butyl-3-methylimidazolium methylsulfate

ZnO thin films were potentiostatically electrodeposited on transparent tin oxide conducting glass substrates at ?1.0 V (vs. Ag/AgCl) in the presence of anionic surfactant, sodium dodecyl sulphate (SDS) and hydrophobic ionic liquid, 1-butyl-3-methylimidazolium methylsulfate [bmim][CH₃SO₄]. X-ray diffraction (XRD) investigation confirm the formation of nanocrystalline and stoichiometric ZnO thin films. Scanning electron microscopy (SEM) results reveal that the grain size of the deposited ZnO film is reduced from 85 to 28 nm when small amount of ionic liquid is added to the deposition bath. Photoluminescence study at room temperature shows a clear absorption edge in the ultra violet (UV) region confirming the high quality, nanocrystalline and stoichiometric nature of the deposited ZnO films.     

Effect of solution concentration on surface morphology and photocatalytic activity of ZnO thin films synthesized by hydrothermal methods

ZnO thin films were synthesized via hydrothermal method on silicon substrate at various solution concentrations. The thin films were characterized by X-ray diffraction, field-emission scanning electron microscope and UV–Vis spectrophotometer. The results show that the thin films are polycrystalline with wurtzite hexagonal structure. The T _c values of (101) surface of the thin film increase from 0.929 to 1.840 at first, and then decrease to 0.779 with increasing solution concentration. The preferential orientation along the (101) crystal surface can be controlled by changing the solution concentration. Solution concentration has a significant effect on surface morphology of the thin films. The optical band gap of the thin films decreases, when the solution concentration of zinc nitrate hexahydrate increases from 0.01 to 0.06 mol/L and then increases when the solution concentration of zinc nitrate hexahydrate further increases to 0.08 mol/L. Photocatalytic activity of the thin films on degradation of methyl orange under UV light irradiation was studied in detail. The ZnO thin film with many cracks prepared from 0.01 mol/L shows the higher photocatalytic activity but the tower-like ZnO thin film prepared from 0.08 mol/L reveals the lower photocatalytic activity. Therefore, the photocatalytic activity of the thin films are mainly relate to surface morphology and crystallographic orientation.     

Post annealing effect on structural and optical properties of ZnO thin films derived by sol–gel route

Zinc oxide thin films have been spun coated on p-Si (100) substrates by sol–gel route. These films were annealed at different annealing temperatures from 300 to 1,000 °C in the oxygen ambient. In this way a suitable annealing temperature window for the sol–gel derived ZnO films exhibiting minimum defects (points and dislocations) and better quality (crystal and optical) was investigated. The structural and optical features of ZnO thin films have been examined by X-ray diffraction, atomic force microscopy, UV–Vis spectroscopy, and photoluminescence spectra. The results revealed that the crystallization in the films initiated at 300 °C, improved further with annealing. All the deposited films exhibited wurtzite phase with c-axis orientations. The variations in the position of characteristic (002) peak, stress, strain and lattice parameters are investigated as a function of annealing temperature. The optical band gap is not significantly affected with annealing as observed by UV–Vis transmission spectroscopy. The Photoluminescence spectra exhibited three luminescence centers. The near band edge esmission was observed in UV region which enhanced with the heat treatment, is an indication of improvement in the optical quality of films. The other two visible emissions are related to native defects in ZnO lattice were appeared only for higher annealing (≥700 °C).     

Influence of annealing temperature on the structural and optical properties of highly-oriented Al and Er co-doped ZnO films

High-quality c-axis oriented 7 mol% Al and 1.5 mol% Er co-doped ZnO films (ZEAO) were prepared on the quartz glass substrates by using sol–gel method. The influence of the annealing temperature on the crystal orientation, microstructure and optical properties of the ZEAO thin films were studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and UV–vis transmittance spectrophotometer, respectively. XRD results revealed that all the samples were polycrystalline with hexagonal structure and exhibited (002) preferential orientation. With increasing the annealing temperature, the grain size and orientation extent increased. The optical studies showed each ZEAO film had a relatively high transmittance above 85 %. The transmittance as high as 95 % was obtained at the annealing temperature of 800 °C, and the corresponding average grain size was about 50 nm. The cathodoluminescence (CL) spectra of these films were also used to characterize the luminescence properties. Strong UV emission centered at 380 nm was observed in the CL spectra taken for the pure ZnO. For the ZEAO sample, the blue-green emission is related to the 4f shell transition in the Er³⁺ ions of ZnO matrices, corresponding to a transition from the excited states (⁴F_5/2).     

Photocatalytic properties of porous TiO2/Ag thin films

In this study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol-gel spin-coating technique. By introducing polystyrene (PS) spheres into the precursor solution, porous TiO₂/Ag thin films were prepared after calcination at a temperature of 500 °C for 4 h. Three different sizes (50, 200, and 400 nm) of PS spheres were used to prepare porous TiO₂ films. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry (XRD) and by scanning electron microscopy to reveal structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.When PS spheres of different sizes were introduced after calcination, the as-prepared TiO₂ films exhibited different porous structures. XRD results showed that all TiO₂/Ag films exhibited a major anatase phase. The photodegradation of porous TiO₂ thin films prepared with 200 nm PS spheres and doped with 1 mol% Ag exhibited the best photocatalytic efficiency where ∼ 100% methylene blue was decomposed within 8 h under UV exposure.     

Growth and temperature dependent characterization of pulsed laser deposited Ag/n-ZnO/p-Si/Al heterojunction

The Ag/n-ZnO/p-Si(100)/Al heterojunction diodes were fabricated by pulsed laser deposition of zinc oxide (ZnO) thin films on p-type silicon. The X-ray diffraction analysis shows the formation of ZnO thin film with hexagonal structure having strong (002) plane as preferred orientation. The energy band gap of ZnO films simultaneously deposited on quartz substrate was calculated from the measured UV–Visible transmittance spectra. High purity vacuum evaporated silver and aluminum thin films were used to make contacts to the n-ZnO and p-silicon, respectively. The current–voltage and capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterostructures were measured over the temperature range of 80–300 K. The Schottky barrier height and ideality factor were determined by fitting of the measured current–voltage data into thermionic emission diffusion equation. It is observed that the barrier height decreases and the ideality factor increases with decrease of temperature and the activation energy plot exhibit non-linear behavior. This decrease in barrier height and increase in ideality factor at low temperature are attributed to the occurrence Gaussian distribution of barrier heights. The capacitance–voltage characteristics of Ag/n-ZnO/p-Si(100)/Al heterojunction diode were also studied over the wide temperature range. Capacitance–voltage data are used to estimate the barrier height and impurity concentration in n-type ZnO.     

Photonic crystal-assisted light extraction from a colloidal quantum dot/GaN hybrid structure

We present a study of the light extraction from CdSe/ZnS core/shell colloidal quantum dot thin films deposited on quantum well InGaN/GaN photonic crystal structures. The two-dimensional photonic crystal defined by nanoimprint lithography is used to efficiently extract the guided light modes originating from both the quantum dot thin films and the InGaN quantum wells. Far-field photoluminescence spectra are used to measure the extraction enhancement factor of the quantum dot emission (x1.4). Microphotoluminescence measurements show that the guided mode effective extraction lengths range between 70 and 180 microm, depending on the wavelength of light.     

Effect of ethylene glycol monomethyl ether ratio in mixed solvent on surface morphology,wettability and photocatalytic properties of ZnO thin films

Zinc oxide (ZnO) thin films with different ethylene glycol monomethyl ether (EGME) ratio were prepared on Si substrates using a two-step process. The results show that they possess a polycrystalline hexagonal wurtzite crystal structure. The topography of the ZnO thin films evolves from nanoparticles to hexagonal nanorods with the decrease of EGME content. The photoluminescence spectra consist of a near-band-edge emission and two visible emissions. The optical band gap energy decreases first and then increases with the increase of EGME ratio in mixed solvent, the broadening of the optical band gap can be explained by Moss–Burstein effect. The wetting behavior of all the samples can switch from hydrophobicity to hydrophilicity through UV illumination. The degradation efficiency of the thin films increases with decreasing EGME content, photocatalytic reaction mechanism of the ZnO thin films is discussed in detail.     

Investigation of the some physical properties of Ge-doped ZnO thin films deposited by thermionic vacuum arc technique

We exhibit the first nano-crystalline Ge–ZnO thin films deposited on glass and PET substrates by a thermionic vacuum arc technique. The effect of Ge doping on the structural, morphological and optical properties of ZnO:Ge films were investigated. An X-ray diffraction (XRD), atomic force microscopy, field emission scanning electron microscopy (FESEM) and UV–Vis spectrophotometer were used for the analysis. XRD patterns show the polycrystalline structure of the films in the range of 20°–80°. The roughness value for the ZnO:Ge on PET substrate was increased due to agglomeration of the grains. The results are in a good agreement with the FESEM images. Using Filmetrics F20 tool, the thickness values of the deposited thin films were obtained as 60 and 80 nm on glass and PET substrates, respectively. The optical properties of the films such as transmittance, absorbance, refractive index, and reflectance were determined. The band gap values were obtained as to be 3.43 and 3.38 eV glass and PET substrates, respectively. It was found that band gap variation of ZnO is very small with Ge doping.     

Mn掺杂对ZnO薄膜结构和光学性质的影响

利用脉冲激光淀积的方法在Si衬底上生长出了c轴高度取向的ZnO和Zn0.9Mn（0.1）O薄膜.光致发光结果显示了Mn的掺杂引起了薄膜的带边发射蓝移,强度减弱,紫光发射几乎消失,但绿光发射增强.利用X射线衍射,X射线吸收精细结构和X射线光电子能谱等实验技术对Mn掺杂的ZnO薄膜的结构及其对光学性质影响进行了研究.结果表明：Mn掺入到ZnO薄膜中形成了Zn0.9Mn0.1O合金薄膜,Mn以＋2价的价态存在,这就导致了掺Mn以后的薄膜带隙变大,在发光谱中表现为带边发射的蓝移.同时由于掺入的Mn与薄膜中的填隙Zn反应,导致薄膜的结晶性变差,薄膜中的填隙Zn减少,O空位增多,引起带边发射和紫光发射减弱,绿光发射增强.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.