Improved photoluminescence emission and gas sensor properties of ZnO thin films

In this article, we report a comparative study of the influence of pressure-assisted (1.72 MPa) versus ambient pressure thermal annealing on both ZnO thin films treated at 330 °C for 32 h. The effects of pressure on the structural, morphological, optical, and gas sensor properties of these thin films were investigated. The results show that partial preferential orientation of the wurtzite-structure ZnO thin films in the [002] or [101] planes is induced based on the thermal annealing conditions used (i.e., pressure assisted or ambient pressure). UV–vis absorption measurements revealed a negligible variation in the optical -band gap values for the both ZnO thin films. Consequently, it is deduced that the ZnO thin films exhibit different distortions of the tetrahedral [ZnO₄] clusters, corresponding to different concentrations of deep and shallow level defects in both samples. This difference induced a variation of the interface/bulk-surface, which might be responsible for the enhanced optical and gas sensor properties of the pressure-assisted thermally annealed film. Additionally, pressure-assisted thermal annealing of the ZnO films improved the H₂ sensitivity by a factor of two.     

Effect of structural defects,surface roughness on sensing properties of Al doped ZnO thin films deposited by chemical spray pyrolysis technique

In this article, the gas sensing properties of Al-doped ZnO thin films have been reported where the nanocrystalline ZnO based thin films were well deposited by a simple and inexpensive ‘chemical spray pyrolysis (CSP)’ technique. Films have been found to be uniform, pinhole free and well adherent to the substrate. The morphology, structures, and surface roughness of the deposited Al-doped ZnO thin films were studied by various types of characterization techniques. In addition, the authors have observed that the sensor response and selectivity towards CO gas is improved by the Al doping at a low operating temperature. XRD results showed that the obtained films are nanocrystalline in nature with hexagonal wurtzite phase. Further, the annealed films were used for detection of CO in the air and maximum response was observed at 175 °C. The improvement in sensor response of Al-doped ZnO thin films to CO gas attributed to the defect chemistry, crystallite size and surface roughness.     

Boron and nitrogen co-doped ZnO thin films for opto-electronic applications

The transparent conductive oxides such as ZnO have been widely studied due to their potential applications. As a promising wide band gap semiconductor, ZnO thin films with various dopants are important in fabricating the photonic devices to meet the various needs. In this study, boron and nitrogen co-doped ZnO thin films were fabricated at different temperatures (100–600 °C) on sapphire (0 0 1) substrates using pulsed laser deposition technique. X-ray diffractometer, atomic force microscope, spectrophotometer and spectrometer were used to characterize the structural, morphological and optical properties of the thin films. Hall measurements were also carried out to identify the electrical properties of the thin films.     

Structural and optical properties of a radio frequency magnetron-sputtered ZnO thin film with different growth angles

This study introduces optical properties of a columnar structured zinc oxide [ZnO] antireflection coating for solar cells. We obtained ZnO films of columnar structure on glass substrates using a specially designed radio frequency magnetron sputtering system with different growth angles. Field-emission scanning electron microscopy was utilized to check the growth angles of the ZnO films which were controlled at 0°, 15°, and 30°. The film thickness was fixed at 100 nm to get a constant experiment condition. Grain sizes of the ZnO films were measured by X-ray diffraction. A UV-visible spectrometer was used to measure the transmittance and reflectance of the ZnO film columnar structures as a function of the growth angles.     

Interfacially polymerized thin film nanofiltration membranes on TiO2 coated polysulfone substrate

In this study, a new approach was developed to prepare the novel thin film composite nanofiltration membranes. In this new approach, nanoparticles were coated completely under the polymeric thin film layer. Thin film composite (TFC) membranes were fabricated by interfacial polymerization on polysulfone (PSf) sublayer using m-phenylenediamine (MPD) and trimesoyl chloride (TMC) respectively as amine monomer and acid chloride monomer. Scanning electron microscopy and atomic force microscopy were used to study surface morphology and roughness properties of NF membranes. Energy dispersive X-ray microanalysis (EDX) was used to analyze the elemental change before and after filtration experiment. Chemical structure and thickness of polyamide formed on TFC membranes were observed by Fourier transmission infrared attenuated total reflectance (FTIR-ATR) spectroscopy. Permeability, salt rejection and pepsin macromolecule rejection of prepared membranes were tested using dead end filtration cell. Antifouling behavior of the membranes was studied by filtering pure water before and after pepsin solution filtration. A smoother and thicker surface without any defect appeared as the concentration of nanoparticle was increased. NaCl rejection was increased from 70% for neat nanofiltration membrane to 84% for 0.5 wt% TiO₂ modified nanofiltration membrane. Antifouling and permeability behavior of the prepared membranes were improved in the new approach. Antibacterial property of prepared membranes was improved as a result of photocatalytic characteristic of TiO₂ nanoparticles.     

碳包覆氧化锌材料在锌镍二次电池中的应用研究

为了提高锌镍二次电池的电化学性能,将高能球磨法制备的碳包覆氧化锌(ZnO/C)材料作为负极活性物质,与纯氧化锌比,碳包覆氧化锌作为负极活性物质的电池,具有更低的充电平台电压以及更高的放电平台电压,并能有效减小电极的欧姆电阻与反应电阻,显著提高了锌镍电池的循环性能。因此,碳包覆后的氧化锌负极材料能够有效的提高锌镍二次电池的电化学性能。     

Synthesis,characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles

Herein, we report the simple synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles prepared by facile hydrothermal method. The synthesized nanoneedles were characterized through different characterization techniques to examine its crystallinity, phase structure, morphological, compositional, optical, vibrational and gas sensing properties. The detailed morphological studies revealed that the Ag-doped ZnO nanoneedles are assembled into non-homogeneously distributed flower-shaped structures which are grown in high density. Further characterizations confirmed that the synthesized nanoneedles are pure, possessing well-crystallinity and exhibiting good optical and vibrational properties. The synthesized Ag-doped ZnO nanoneedles were used as functional material to fabricate high sensitive acetone gas sensors. The effect of operating temperature and concentration of the acetone were analyzed for detailed sensing performance of synthesized nanoneedles. By detailed sensing experiments, the response and recovery times of 10 s and 21 s, respectively were calculated at acetone concentration of 100 ppm at an optimized operating temperature of 370 °C. A maximum sensitivity of 30.233 was recorded at 370 °C operating temperature for 200 ppm of acetone for the fabricated acetone sensor based on Ag-doped ZnO nanoneedles.     

Synthesis of ZnO/Cu micro and nanostructures via a vapor phase transport method using different tube systems

ZnO micro and nanostructures were grown on copper coated silicon substrates using two different systems: an opened system (both ends opened tube) and a closed system (one closed end tube). The thermodynamic conditions of the systems made a significant difference in boundary layer and super-saturation between the systems. The results indicate that diffusion of the gaseous species through the boundary layers at low and high pressures controls the final formation of the morphologies. The ZnO nanostructures which have been grown in a restricted place have larger diameters and lengths. The structure of the products was analyzed by X-ray diffractometer (XRD) and it was found that the good crystalline quality of the samples was obtained in a closed system. To study the optical properties, photoluminescence (PL) and ultra violet–visible (UV–vis) spectroscopy were employed. It was observed that a decrease in the growth temperature of the opened system caused a broad and dominant visible emission covering the blue and green emission in the PL spectra.     

Laser irradiation of ZnO:Al/Ag/ZnO:Al multilayers for electrical isolation in thin film photovoltaics

Laser irradiation of ZnO:Al/Ag/ZnO:Al transparent contacts is investigated for segmentation purposes. The quality of the irradiated areas has been experimentally evaluated by separation resistance measurements, and the results are complemented with a thermal model used for numerical simulations of the laser process. The presence of the Ag interlayer plays two key effects on the laser scribing process by increasing the maximum temperature reached in the structure and accelerating the cool down process. These evidences can promote the use of ultra-thin ZnO:Al/Ag/ZnO:Al electrode in large-area products, such as for solar modules.     

Effect of nickel doping on structural,optical, electrical and ethanol sensing properties of spray deposited nanostructured ZnO thin films

Undoped and nickel (Ni)-doped ZnO thin films were spray deposited on glass substrates at 523 K using 0.1 M of zinc acetate dihydrate and 0.002–0.01 M of nickel acetate tetrahydrate precursor solutions and subsequently annealed at 723 K. The effect of Ni doping in the structural, morphological, optical and electrical properties of nanostructured ZnO thin film was investigated using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), UV–vis Spectrophotometer and an Electrometer respectively. XRD patterns confirmed the polycrystalline nature of ZnO thin film with hexagonal wurtzite crystal structure and highly oriented along (002) plane. The crystallite size was found to be increased in the range of 15–31 nm as dopant concentration increased. The SEM image revealed the uniformly distributed compact spherical grains and denser in the case of doped ZnO thin films. All the films were highly transparent with average transmittance of 76%. The measured optical band gap was found to be varied from 3.21 to 3.09 eV. The influence of Ni doping in the room temperature ethanol sensing characteristics has also been reported.     

Sandwich-structured ZnO-MnO2-ZnO thin film varistors prepared via magnetron sputtering

ZnO varistors are widely used to protect electronic circuits form transient voltages. However, it is difficult to prepare varistors with voltage less than 10 V using ZnO ceramics. Here we prepared a ZnO-MnO₂-ZnO (ZMZ) sandwich thin film via magnetron sputtering and subsequent annealing at 200-500 °C. With the increase of annealing temperature, the manganese oxide sandwich layer reacts with the upper and lower ZnO layer and becomes thinner. After annealed at 500 °C, because of ZnO grain growth, the upper and lower ZnO layers joined together. The electrical properties of ZMZ films annealed at 400 °C show strong nonlinear I-V characteristics. A ZMZ low voltage thin film varistor with planar boundary potential barrier was obtained whose nonlinear coefficient α and varistor voltage V1 mA are about 30 and 6.0 V, respectively. The stable and excellent nonlinear characteristics make it a promising candidate for overvoltage protection in low operating voltage circuits.     

Fabrication of nanostructured ZnO film as a hole-conducting layer of organic photovoltaic cell

We have investigated the effect of fibrous nanostructured ZnO film as a hole-conducting layer on the performance of polymer photovoltaic cells. By increasing the concentration of zinc acetate dihydrate, the changes of performance characteristics were evaluated. Fibrous nanostructured ZnO film was prepared by sol-gel process and annealed on a hot plate. As the concentration of zinc acetate dihydrate increased, ZnO fibrous nanostructure grew from 300 to 600 nm. The obtained ZnO nanostructured fibrous films have taken the shape of a maze-like structure and were characterized by UV-visible absorption, scanning electron microscopy, and X-ray diffraction techniques. The intensity of absorption bands in the ultraviolet region was increased with increasing precursor concentration. The X-ray diffraction studies show that the ZnO fibrous nanostructures became strongly (002)-oriented with increasing concentration of precursor. The bulk heterojunction photovoltaic cells were fabricated using poly(3-hexylthiophene-2,5-diyl) and indene-C60 bisadduct as active layer, and their electrical properties were investigated. The external quantum efficiency of the fabricated device increased with increasing precursor concentration.     

Improvement of optical transmittance and electrical properties for the Si quantum dot-embedded ZnO thin film

A Si quantum dot (QD)-embedded ZnO thin film is successfully fabricated on a p-type Si substrate using a ZnO/Si multilayer structure. Its optical transmittance is largely improved when increasing the annealing temperature, owing to the phase transformation from amorphous to nanocrystalline Si QDs embedded in the ZnO matrix. The sample annealed at 700°C exhibits not only high optical transmittance in the long-wavelength range but also better electrical properties including low resistivity, small turn-on voltage, and high rectification ratio. By using ZnO as the QDs’ matrix, the carrier transport is dominated by the multistep tunneling mechanism, the same as in a n-ZnO/p-Si heterojunction diode, which clearly differs from that using the traditional matrix materials. Hence, the carriers transport mainly in the ZnO matrix, not through the Si QDs. The unusual transport mechanism using ZnO as matrix promises the great potential for optoelectronic devices integrating Si QDs.     

Influence of yttrium doping on the structural,morphological and optical properties of nanostructured ZnO thin films grown by spray pyrolysis

This study reports on the deposition of highly transparent, n-type ZnO thin films on glass substrate at 450?°C using spray pyrolysis processing, with the simultaneous insertion of yttrium (Y) at different percentages (0, 2, 5, 7?at%) as a dopant. The effect of Y doping on the structure, morphology and optical properties of Y doped ZnO (ZnO:Y) was investigated for optoelectronic applications. The obtained thin films were characterized by means of X-ray diffraction, field-emission scanning electron microscopy (FESEM), UV–visible absorbance measurements, photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. The as-prepared films exhibit well-defined hexagonal wurtzite structure grown along [002]. Field emission scanning electron microscope micrographs of the pure ZnO and ZnO:Y showed that the films acquired a dominance of hexagonal-like grains, the morphology was influenced by Y incorporation. All the films showed high transparency in the visible domain with an average transmittance of 83%. The band gap energy, E_g, increased from 3.12?eV to 3.18?eV by increasing the Y doping concentration up to 5?at% and then decreased to 3.15?eV for 7?at% Y content. The PL and CL measurements reveal a strong ultraviolet (UV) emission, suggesting that the as-prepared ZnO:Y thin films can potentially be used in optoelectronic devices.     

Synthesis and characterization of ZnO NRs with spray coated GO for enhanced photocatalytic activity

Zinc oxide nanorods, ZnO NRs, were synthesized on a clean glass and coated with graphene oxide (GO) using spray coating method to enhance the photocatalytic activity in wastewater treatment. The ZnO NRs were synthesized using the solution process synthesis that was optimized using Taguchi method. Several synthesis parameters have been optimized and studied to determine the best synthesis parameter to grow ZnO NRs for the photodegradation of organic contaminants. Field emission scanning electron microscopy (FESEM) with EDX, X-ray diffraction (XRD), Raman, ultraviolet visible near-infrared (UV-VIS-NIR), and photoluminescence (PL) spectroscopies were used to investigate the structural and optical properties of the produced nanorods. FESEM images revealed the vertical growth of ZnO NRs as well as layers of GO covering the ZnO NRs' top surface. The Raman study demonstrates the combination peak of GO and ZnO, hence proving the GO layer's successful coating. After the GO coating, decrease in the bandgap of the synthesized photocatalyst was detected by PL and UV–Vis absorption measurements. Under UVC exposure with treatment time of 6 h, the degradation of MB with ZnO NRs/GO photocatalyst reached a degradation percentage of 97.86%, which is greater than the degradation percentage achieved using pristine ZnO NRs, which is 93.28%. The results validated that the coating of GO enhances the photocatalytic activity of the host material, ZnO NRs.     

Characterization and gas sensing properties of bead-like ZnO using multi-walled carbon nanotube templates

We report bead-like ZnO nanostructures for gas sensing applications, synthesized using multi-walled carbon nanotube (MWCNT) templates. The ZnO nanostructures are grown following a two-step process: in the first, ZnO nanoparticles are synthesized on MWCNTs by thermal evaporation of a Zn powder; and in the second, the hybrid nanostructures are heat-treated at 800 °C. Scanning and transmission electron microscopy images indicate that the bead-like ZnO nanostructures have surface protuberances with nanoparticle sizes ranging from 20 to 60 nm, and a well-crystallized hexagonal structure. Gas sensors based on multiple-networked bead-like ZnO showed considerably enhanced electrical responses and better stability to both oxidizing (NO₂) and reducing (CO) gases compared with previously reported nanostructured gas sensors, even if the response to CO gas was slow to increase. Both the NO₂ and CO gas sensing properties increased dramatically when the working temperature was increased up to 300 °C. The response sensitivities measured were 2953%, 5079%, 9641%, 3568%, and 3777% to 20 ppm NO₂ at 200, 250, 300, 350 and 400 °C, respectively. For CO gas on the other hand, the response sensitivities were 107%, 110%, 114%, 118%, and 122% at 5, 10, 20, 50, and 100 ppm concentrations, respectively. For concentrations between 5 and 20 ppm, the recovery time of the oxidizing gas was much shorter than the response time. The origin of the NO₂/CO gas sensing mechanism of the bead-like ZnO nanostructures is discussed.     

UV photodetection properties of pulsed laser deposited Cu-doped ZnO thin film

Nanocrystalline undoped and 2 at% copper (Cu) doped zinc oxide (ZnO) thin films were successfully grown onto SiO₂/n-Si substrates at 600 °C by using pulsed laser deposition (PLD) technique. The influence of Cu incorporation on structural, surface morphological, elemental composition and UV detection properties of ZnO film was investigated. X-ray diffraction studies of thin films show that they are polycrystalline and have a hexagonal wurtzite structure; however, Cu doping improves the preferential orientation along c-axis. The chemical state of constituent elements was analysed by X-ray photoelectron spectroscopy (XPS). It indicates the presence of Cu ions in the doped film that exist in a mixed univalent and bivalent state. FE-SEM observations support the crystallographic results. The effective incorporation of Cu ions into the lattice of the ZnO nanostructure without changing its wurtzite structure was confirmed by an energy dispersive X-ray spectroscopic analysis (EDX). The UV photodetection characteristics of both films were further studied in metal-semiconductor-metal (MSM) planar configurations at room temperature and are found to be greatly influenced by Cu doping. The incorporation of Cu into ZnO lattice increases the resistivity of thin film; which leads to lower dark current. As a result, the Cu-doped ZnO film based UV PD demonstrates improved UV sensitivity of about 66.92 upon 2 mW/cm² UV illumination at 365 nm peak wavelengths and 5 V applied bias. The reproducible UV detection performance of MSM devices was also ensured by periodically switching UV light on and off at fixed time intervals.     

High carrier mobility low-voltage ZnO thin film transistors fabricated at a low temperature via solution processing

Wide-bandgap ZnO TFTs have many potential applications in large-area, flexible electronics and transparent devices because of their low cost, high performance and excellent optical transmittance. High-performance ZnO TFTs fabricated via simple solution processing have been widely studied. However, the key issues of solution-processable ZnO TFTs are the relatively high processing temperature (> 300?°C) and the high operating voltage for achieving the desired electrical properties. Here, we successfully fabricated low-voltage ZnO TFTs at an annealing temperature of ≤?250?°C. The resulting ZnO transistors with high-k terpolymer P(VDF-TrFE-CFE) showed a mobility of up to 5.3?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. Furthermore, the influence of the dielectric constant on the carrier mobility was investigated. A lower k-value dielectric resulted in a high carrier mobility under the same carrier density. Therefore, with a low-k CYTOP dielectric applied to modify the interface between the ZnO semiconductor and the P(VDF-TrFE-CFE) layer, ZnO transistors annealed at 250?°C showed an electron mobility of 13.6?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. To the best of our knowledge, this mobility is the highest value reported to date among the low-voltage solution-processable undoped ZnO TFTs annealed at temperatures of ≤?300?°C.     

Polystyrene-poly(2-ethylhexylmethacrylate) block copolymers: Synthesis,bulk phase behavior,and thin film structure

Anionic polymerization was employed to synthesize well-defined diblock copolymers of polystyrene and poly(2-ethylhexylmethacrylate), PS-PEHMA. Diblock morphologies in bulk and in substrate-supported thin films were characterized by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), respectively. PS-PEHMA diblocks exhibited thermotropic order-disorder transitions; one diblock showed a thermoreversible transition between lamellae and a higher-temperature morphology assigned as perforated lamellae. Unlike PS-poly(alkylmethacrylate) diblocks where the alkyl group is n-butyl or n-pentyl, PS-PEHMA diblocks showed a typical decreasing Flory interaction parameter with increasing temperature. Thin films of PS-cylinder-forming PS-PEHMA diblocks showed a strong preference for the cylinders to lie in the plane of the film; films of incommensurate thickness readily formed terraces. Films of commensurate thickness were easily aligned over macroscopic areas through the application of mechanical shear.     

Ultrasensitive NO2 gas sensing performance of two dimensional ZnO nanomaterials: Nanosheets and nanoplates

Highly sensitive NO₂ gas sensors with low detection limit are vital for practical application in air pollution monitoring. Here, the NO₂ gas sensing performance of porous ZnO nanosheets and nanoplates were investigated, with different shape and thickness. It was found that ultra-thin ZnO nanoplates had a higher sensitivity than coral-like ZnO nanosheets. The results were attributed to the high specific surface and very small thickness of the ultrathin nanoplates. The nanoplates have indeed a thickness of 15 nm compared to that of the nanosheets which is 100 nm, and a BET surface area of 75 m²/g, while that of the nanosheets is 6 m²/g. The chemosensor based on ultra-thin ZnO nanoplates shows a response (calculated as the ratio between the resistance of the sensor in the presence of the gas and in its absence) of 76 to 0.5 ppm of NO₂ at 200 °C, with a theoretical detection limit of 3 parts per trillion and a selectivity higher than 760 towards acetone, ethanol, isopropyl alcohol, triethylamine, SO₂ and CO. The specific surface and the small thickness of the ultra-thin nanoplates contribute to its highly improved sensing performance, making it ideal for NO₂ gas sensing.