Surface Of ZnO Thin Films Research Articles

Deposition of Alumina Films on Interdigital Transducer/ZnO/Glass Substrates by Electron Beam Evaporation to Improve the Characteristics of Surface Acoustic Wave Devices

ZnO films with c-axis (0002) orientation have been successfully grown by radio frequency magnetron sputtering on glass substrates. The alumina films were deposited on interdigital transducers/ZnO/glass substrates by electron beam evaporation to enhance the performance of the ZnO thin film surface acoustic wave (SAW) devices. The crystalline structure and surface roughness of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. The phase velocity and coupling coefficient of SAW device were apparently increased when we increased the thickness of alumina film. The experimental result is beneficial to improve the characteristics of the ZnO thin film SAW devices on cheap glass substrates.

Influence of water content in mixed solvent on surface morphology, wettability, and photoconductivity of ZnO thin films.

ZnO thin films have been synthesized by means of a simple hydrothermal method with different solvents. The effect of deionized water content in the mixed solvents on the surface morphology, crystal structure, and optical property has been investigated by scanning electron microscopy, X-ray diffraction, and UV-Vis spectrophotometer. A large number of compact and well-aligned hexagonal ZnO nanorods and the maximal texture coefficient have been observed in the thin film, which is grown in the mixed solvent with x = 40%. A lot of sparse, diagonal, and pointed nanorods can be seen in the ZnO thin film, which is grown in the 40-mL DI water solution. The optical band gap decreases firstly and then increases with the increase of x. Reversible wettability of ZnO thin films were studied by home-made water contact angle apparatus. Reversible transition between hydrophobicity and hydrophilicity may be attributed to the change of surface chemical composition, surface roughness and the proportion of nonpolar planes on the surface of ZnO thin films. Photocurrent response of ZnO thin films grown at different solvents were measured in air. The response duration of the thin film, which is grown in the solvent with x = 40%, exhibits a fast growth in the beginning but cannot approach the saturate current value within 100 s. The theoretical mechanism for the slower growth or decay duration of the photocurrent has been discussed in detail.

Complex dielectric constant of various biomolecules as a function of wavelength using surface plasmon resonance

Present study focuses on determination of complex dielectric constant of biomolecules as function of frequency by means of surface plasmon resonance (SPR) technique without losing their biofunctionality. Surface plasmon modes have been excited in Kretschmann configuration at interface of ZnO-Au thin films. Various biomolecules (glucose oxidase, cholesterol oxidase, urease, and uricase) have been immobilized successfully on surface of ZnO thin film by electrostatic interaction. SPR reflectance curves for all biomolecules were recorded separately at different wavelengths (407–635 nm). Complex dielectric constant was determined by fitting the experimental SPR data with Fresnel's equations. Dielectric constant of all biomolecules shows frequency dispersion and attributed to ionic polarization.

Role of Au Incorporated on the Surface of ZnO Thin Film in Enhancing the UV Photoresponse

Role of Au Incorporated on the Surface of ZnO Thin Film in Enhancing the UV Photoresponse

High-efficiency micro-energy generation based on free-carrier-modulated ZnO:N piezoelectric thin films

The free-carrier-modulated ZnO:N thin film-based flexible nanogenerators (NZTF-FNGs) are proposed and experimentally demonstrated. The suggested flexible nanogenerators (FNGs) are fabricated using N-doped ZnO thin films (NZTFs) as their piezoelectric active elements, which are deposited by a radio frequency magnetron sputtering technique with an N2O reactive gas as an in situ dopant source. Considerable numbers of N atoms are uniformly incorporated into NZTFs overall during their growth, which would enable them to significantly compensate the unintentional background free electron carriers both in the bulk and at the surface of ZnO thin films (ZTFs). This N-doping approach is found to remarkably enhance the performance of NZTF-FNGs, which shows output voltages that are almost two orders of magnitude higher than those of the conventionally grown ZnO thin film-based FNGs. This is believed to be a result of both substantial screening effect suppression in the ZTF bulk and more reliable Schottky barrier formation at the ZTF interfaces, which is all mainly caused by the N-compensatory doping process. Furthermore, the NZTF-FNGs fabricated are verified via charging tests to be suitable for micro-energy harvesting devices.

Bendable ZnO thin film surface acoustic wave devices on polyethylene terephthalate substrate

Bendable surface acoustic wave (SAW) devices were fabricated using high quality c-axis orientation ZnO films deposited on flexible polyethylene terephthalate substrates at 120 °C. Dual resonance modes, namely, the zero order pseudo asymmetric (A0) and symmetric (S0) Lamb wave modes, have been obtained from the SAW devices. The SAW devices perform well even after repeated flexion up to 2500 με for 100 times, demonstrating its suitability for flexible electronics application. The SAW devices are also highly sensitive to compressive and tensile strains, exhibiting excellent anti-strain deterioration property, thus, they are particularly suitable for sensing large strains.

Plasmonic enhancement of optical absorption of UV radiation by Au nanoparticles dispersed on ZnO thin film

Ultraviolet photoconductivity in sol gel-derived ZnO thin film loaded with gold (Au) nanoparticle (NPs) is investigated. Au-NPs loaded ZnO thin film (Au-NPs/ZnO) is found to exhibit high photoconducting gain (K) of about 4.12 × 103 as compared to the bare ZnO thin film-based photodetector (4.9 × 101). The enhanced photoconductive gain of Au-NPs/ZnO detector has been achieved due to simultaneous lowering of dark current which occurs due to Schottky barrier effect originated after dispersing the Au NPs on the surface of ZnO thin film. The enhancement in photocurrent upon UV illumination is due to trapping of UV radiation via plasmonic effect caused by Au-NPs and subsequently coupling of absorbed light with the optical modes of underlying semiconducting ZnO surface.

Effect of surface morphology on the field emission property of ZnO films

AbstractStable and high crystalline quality of ZnO nanocones have been successfully fabricated on glass substrates at room temperature. In 3 min irradiation by argon ion on ZnO thin film surface, ZnO nanocones with a number density of 8‐15 µm‐2 can be observed from SEM images. For a high performance of field emission (FE) device, an emission current stability test was carried out at a fixed electric field of 12 V/µm for 60 min. A stable current of 2.5 µA was observed after 30 min without any obvious irregular current flow. The current density of 1 µA cm‐2 was achieved at 9.8 V/µm and 7.8 V/µm before and after stability test, respectively. Even though some of the ZnO nanocones tip were melted due to Joule heating induced by high intensity of electric field, the field enhancement factor are still comparable with other types of nanostructure emitters. The surface morphology of ZnO nanocones after the stability test showed the nanocones of shorter length are not affected by the high electric field at the emission area in which this revealed the robustness of ZnO compared to CNTs/CNFs. The ZnO nanocones at out of the emission area are also not affected by the high electric field or by any heat generated in the vacuum chamber during field emission. Thus, due to the high thermal and mechanical stability, ZnO nanocones is reliable to be as a promising field emitter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence Spectra of ZnO Thin Film Composed Nanoparticles on Silicon and Porous Silicon

ZnO thin film was successfully deposited on different substrate by sol-gel spin coating. Zinc acetate dihydrates, diethanolamine and isopropyl were used as starting material, stabilizer and solvent respectively. Two different substrate used in this work are p-type silicon wafer and porous silicon. Porous silicon was prepared by electrochemical etching. In order to study the surface morphology, field emission scanning electron microscopy (FESEM) was employed. It is found that, ZnO thin film was composed by ZnO nanoparticles. The averages size ZnO nanoparticle is 23.5 nm on silicon and 17.76 nm on porous silicon. Based on Atomic Force Microscopy (AFM) topology analysis, surface of ZnO thin films on porous silicon was rougher compared to ZnO thin films on silicon due to substrate surface effect. Photoluminescence spectra shows two peaks are appear for ZnO thin film on silicon and three peaks are appear for ZnO thin film on porous silicon. PL spectra peaks of ZnO thin film on silicon are correspond to ZnO and ZnO native defects while peaks of PL spectra on porous silicon are corresponds to ZnO, ZnO native defects and porous silicon.

Microstructure evolution and optical properties of c-axis-oriented ZnO thin films incorporated with silver nanoisland layers

Abstract Highly c -axis-oriented ZnO thin films incorporated with various numbers of Ag nanoisland layers were fabricated in this study by alternating sputtering-deposited ZnO and Ag layers on Si substrates. Two such ZnO–Ag layered structures were fabricated: the first by using a 150-nm ZnO film–Ag island layer/Si, and the second by using three applications of a 50-nm ZnO film–Ag island layer/Si. The crystallographic features of the as-deposited ZnO thin film on the Si substrate exhibited a pure c -axis orientation. However, the degree of the c -axis texture of the ZnO thin films slightly decreased after inserting Ag island layers. The ZnO thin-film surface became quite rough because the surface morphology was transformed from being dense and flat to exhibit a loosely columnar grain structure. Transmittance electron microscopy (TEM) images showed that the Ag islands were clearly distributed on the ZnO/substrate and ZnO/ZnO interfaces. The photoluminescence measurement results indicated that the near-band-edge (NBE) emission peak intensity was enhanced by nearly two orders of magnitude in the 150-nm ZnO film–Ag island layer/Si compared with that of the as-deposited 150-nm ZnO thin film without an inserted Ag island layer. Inserting additional Ag island layers did not increase the NBE emission intensity; instead, the visible emission band intensity was markedly increased. This was attributed to the substantial change in the ZnO thin film microstructure caused by inserting the Ag island layers.

Structural and morphological studies of zinc oxide incorporating single-walled carbon nanotubes as a nanocomposite thin film

Pure zinc-oxide and a composition of zinc oxide-single walled carbon nanotubes (ZnO-SWCNTs) thin films were prepared by using a sol–gel doctor blade technique. A precursor of zinc acetate dehydrate (Zn(CH3COO)2·2H2O), absolute ethanol (C2H5OH) and triethanolamine were mixed in one solution. Non-acid treatment SWCNTs were doped in the prepared solution. Structural and morphological properties of ZnO and ZnO-SWCNTs thin films were studied by means of X-ray diffractometer (XRD), field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). XRD measurements indicated that the crystallite size of ZnO was bigger than the crystallite size of ZnO-SWCNTs; 0.4331 and 0.3386 nm, respectively. The FESEM images showed the hexagonal and nanorod structures of ZnO thin film and a broccoli-like ZnO nanostructures coated with CNTs for ZnO-SWCNTs thin film. The AFM analysis revealed smoother surface morphology of ZnO-SWCNTs thin film compared to the surface of pure ZnO thin film. TEM results captured the inner structures of ZnO and ZnO-SWCNTs. Inner and outer diameter of non-acid treatment SWCNTs were recorded about 5.09 and 14.95 nm, respectively. Photovoltaic performance of ZnO-SWCNTs based dye-sensitized solar cell (DSSC) showed high power conversion efficiency of 0.102 % compared to ZnO based DSSC (0.019 %). This study suggests that SWCNTs should be acid-treated to produce highly porous structure and greater surface area for better photovoltaic performance of the DSSCs.

Fabrication and properties of ZnO/GaN heterostructure nanocolumnar thin film on Si (111) substrate

Zinc oxide thin films have been obtained on bare and GaN buffer layer decorated Si (111) substrates by pulsed laser deposition (PLD), respectively. GaN buffer layer was achieved by a two-step method. The structure, surface morphology, composition, and optical properties of these thin films were investigated by X-ray diffraction, field emission scanning electron microscopy, infrared absorption spectra, and photoluminiscence (PL) spectra, respectively. Scanning electron microscopy images indicate that the flower-like grains were presented on the surface of ZnO thin films grown on GaN/Si (111) substrate, while the ZnO thin films grown on Si (111) substrate show the morphology of inclination column. PL spectrum reveals that the ultraviolet emission efficiency of ZnO thin film on GaN buffer layer is high, and the defect emission of ZnO thin film derived from Zni and Vo is low. The results demonstrate that the existence of GaN buffer layer can greatly improve the ZnO thin film on the Si (111) substrate by PLD techniques.

Plasmonic Enhancement of Optical Absorption of UV Radiation in ZnO Thin Film Based Ultraviolet Photodetectors

ABSTRACTUltraviolet (UV) photoconductivity in pure ZnO thin films and metal (Ag, Au, Pt) nanoparticles (NPs) dispersed on ZnO thin films based UV photodetectors biased at 5 V for ultra violet radiation of λ = 365 nm and intensity = 24 µwatt/cm2 has been studied. All the three metal (Ag, Au, Pt) NPs synthesized by Polyol process when dispersed on the surface of 100 nm thin ZnO film results in enhanced photoconductive gain (K) in comparison to pure ZnO (3.1×103). An increase of about an order in K has been obtained in the case of Ag NPs/ZnO and Au NPs/ZnO UV photodetectors ( K = 6.9×104 and 5.3×104 respectively). On the other hand, Pt NPs enhance K by about two orders (5.0×105). Such an enhanced photoconductive gain has been achieved due to the lowering of dark current after dispersing the metal NPs on the surface of ZnO and increased photocurrent upon UV illumination. This may be attributed to the plasmon propagating property in metal NPs which enhances the light trapping through optical absorption in ZnO thin film surface (high photo current).

Comparative study of ZnO nanorods and thin films for chemical and biosensing applications and the development of ZnO nanorods based potentiometric strontium ion sensor

In this study, the comparative study of ZnO nanorods and ZnO thin films were performed regarding the chemical and biosensing properties and also ZnO nanorods based strontium ion sensor is proposed. ZnO nanorods were grown on gold coated glass substrates by the hydrothermal growth method and the ZnO thin films were deposited by electro deposition technique. ZnO nanorods and thin films were characterised by field emission electron microscopy [FESEM] and X-ray diffraction [XRD] techniques and this study has shown that the grown nanostructures are highly dense, uniform and exhibited good crystal quality. Moreover, transmission electron microscopy [TEM] was used to investigate the quality of ZnO thin film and we observed that ZnO thin film was comprised of nano clusters. ZnO nanorods and thin films were functionalised with selective strontium ionophore salicylaldehyde thiosemicarbazone [ST] membrane, galactose oxidase, and lactate oxidase for the detection of strontium ion, galactose and l-lactic acid, respectively. The electrochemical response of both ZnO nanorods and thin films sensor devices was measured by using the potentiometric method. The strontium ion sensor has exhibited good characteristics with a sensitivity of 28.65±0.52mV/decade, for a wide range of concentrations from 1.00×10−6 to 5.00×10−2M, selectivity, reproducibility, stability and fast response time of 10.00s. The proposed strontium ion sensor was used as indicator electrode in the potentiometric titration of strontium ion versus ethylenediamine tetra acetic acid [EDTA]. This comparative study has shown that ZnO nanorods possessed better performance with high sensitivity and low limit of detection due to high surface area to volume ratio as compared to the flat surface of ZnO thin films.

Surface and photocatalytic properties of ZnO thin film prepared by sol–gel method

ZnO films were prepared on glass substrates by a sol–gel dip-coating technique. The films showed a polycrystalline phase without any preferable orientation. By decreasing the withdrawal speed, the surface of the ZnO films became denser because of a decrease in particle sizes. This reduces the distance between the supported solids under the water droplet that could increase the degree of the pinning effects, and leads to increase the water contact angle. Furthermore, these prepared ZnO films showed photocatalytic properties indicating by photocatalytic degradation of methylene blue under a blacklight illumination. By increasing the calcination temperature, the water contact angle value decreases due to the grain coalescence which increases the gap between these supported solids. On the other hand, this enhances the photocatalytic activity caused by the improving of the crystallinity and the surface roughness of ZnO thin films with an increase in calcination temperature.

Effects of Working Pressure on the Microstructure and Properties of ZnO Thin Films Prepared by DC Magnetron Sputtering

ZnO thin films were prepared by DC magnetron sputtering. The effects working pressures on the microstructure, optical properties and the photoluminescent properties were studied. The results show that ZnO thin films were successfully prepared with preferred orientation growth, showing structure of single crystal. The transmission of all the ZnO thin films kept above 85%. With increasing the working pressure, the surface of ZnO thin film became coarse, the intensity of X-ray diffraction peak decreased and the transmission of the film decreased and then increased. The intensity of the two photoluminescence peak of ZnO thin films one ultraviolet peak at 400 nm and one blue peak at 466 nm increased with increasing the working pressure. The ultraviolet peak was originated from the transition emission of the electrons from the conduction band to the valence band while the blue peak was originated from the defects in ZnO thin films.

Elevated temperature dependence of energy band gap of ZnO thin films grown by e-beam deposition

We report the surface, structural, electronic, and optical properties of the epitaxial ZnO thin films grown on (0001) sapphire substrate at 600 °C by an electron-beam deposition technique. ZnO thin films have been deposited in an oxygen environment and post-deposition annealed to improve the stoichiometry and the crystal quality. In order to investigate the free exciton binding energy and the temperature dependence of the energy bandgap, we carried out variable temperature (78–450 K) transmittance measurements on ZnO thin films. The absorption data below the energy bandgap have been modeled with the Urbach tail and a free exciton, while the data above the gap have been modeled with the charge transfer excitations. The exciton binding energy is measured to be E0 = 64 ± 7 meV, and the energy band gaps of the ZnO film are measured to be Eg ~ 3.51 and 3.48 eV at 78 and 300 K, respectively. The temperature dependence of the energy gap has been fitted with the Varshni model to extract the fitting parameters α = 0.00020 ± 0.00002 eV/K, β = 325 ± 20 K, and Eg (T = 0 K) = 3.516 ± 0.0002 eV.

Enhancement of characteristics of ZnO thin film surface acoustic wave device on glass substrate by introducing an alumina film interlayer

ZnO films with c-axis (0002) orientation have been successfully grown by RF magnetron sputtering on Al2O3/glass substrates. The alumina films were deposited on glass substrates by electron beam evaporation. The crystalline structure and surface roughness of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. The phase velocity and coupling coefficient of SAW device were apparently increased when we increased the thickness of alumina film. Besides, an excellent temperature coefficient of frequency of SAW device was obtained with increasing the thickness of alumina film. The experimental result is beneficial to improve the performance of the ZnO thin film SAW devices on cheap glass substrate with easier process and lower cost.

Manufacturing of Multifunctional Nanocrystalline ZnO Thin Films

Optical, surface and structural properties of ZnO thin films fabricated by reactive radio- frequency (rf) magnetron sputtering and sol-gel coating methods are comparatively investigated. The optical properties of films produced by both techniques have very similar characteristics, however; the surface morphology and degree of crystallinity have different behaviors. The nanostructure columnar zinc oxide thin films can be synthesized by sol-gel coating methods which can have numerous applications requiring larger surface area. Also, the process scalability and large-scale manufacturing of these materials are discussed. It indicated that the nanostructure ZnO thin films can be synthesized with sol-gel methods at wafer levels with nano-grains and improved surface properties compared with reactive rf magnetron sputtering deposition.

Fabrication of superhydrophobic surface of hierarchical ZnO thin films by using stearic acid

Flower-like hierarchical ZnO microspheres were successfully synthesized by a simple, template-free, and low-temperature aqueous solution route. The morphology and microstructure of the ZnO microspheres were examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The bionic films with hydrophobicity were fabricated by the hierarchical ZnO microspheres modified by stearic acid. It was found that the hydrophobicity of the thin films was very sensitive to the added amount of stearic acid. The thin films modified with 8% stearic acid took on strong superhydrophobicity with a water contact angle (CA) almost to be 178° and weak adhersion. The remarkable superhydrophobicity could be attributed to the synergistic effect of micro/nano hierarchical structure of ZnO and low surface energy of stearic acid.