Nanocrystalline Zinc Oxide Thin Films Research Articles

Nanocrystalline ZnO Thin Films – Influence of Sol-gel Conditions on the Underlying Chemistry and Film Microstructure and Transparency

Zinc oxide (ZnO) thin films were spin-coated onto glass substrates by a low-cost sol-gel process. The influence of the zinc acetate dihydrate concentration (0.35M and 0.75M) and two different solvents (methanol and 2-methoxyethanol) on the underlying chemical mechanism, surface morphology, microstructure and optical properties of the films was investigated. FTIR results support the formation of [CH3COOZn]+, Zn(OH)2 and Zn-O-CH-CH2-NH2 as intermediate products/by-products, particularly when methanol and higher precursor concentration were used. XRD patterns show a preferred orientation of crystallization of wurtzite along the (002) plane, more evident when 2-methoxyethanol was used, due to the longer available time for the reactions completion and crystal growth. SEM analysis shows a wrinkled surface in the films that is denser when methanol and the highest precursor concentration were used in the synthesis. The ZnO films synthesized with 2-methoxyethanol exhibited higher optical transmittance (≥90%) than the films made with methanol (60-70%). The transparency in the visible range is enhanced in the case of 2-methoxyethanol and higher Zn(II) concentration (≥90%). The optical band gap values of the films were in the range 3.10-3.50eV.

Synthesis and Characterization of Nanocrystalline Zinc Oxide Thin Films for Ethanol Vapor Sensor

ZnO nanoparticles as gas sensors have a property of changing the electrical conductivity of the sensing element on exposure to gas atmospheres. The number, density and size distribution of particles plays a key role in gas sensing applications. The gas sensing mechanism mainly depends upon the morphology, ion occupancy and the operating temperature, which helps in adsorbing the OH- groups on the surface of oxide materials. The ethanol vapor sensing measurements have been carried out on the ZnO pellet with silver pasted electrodes on both the ends. The change in electrical resistance was used as the measure of ethanol vapor response at different temperatures in the range of 25°C to 45°C at an interval of 5°C under an ethanol concentration of 25.02, 50.05, 75.08, 100.11 and 150.17 ppm. These results showed that the indium zinc oxide film based ethanol sensor has high response quick recovery time and selectivity to the ethanol vapor.

Erratum to: Structural and optical properties of Na doped ZnO nanocrystalline thin films synthesized using sol–gel spin coating technique

Sodium (Na) doped Zinc oxide (ZnO) thin films have been deposited on a glass substrate by the sol–gel spin coating method. Effect of doping with various percentages of Na at a particular annealing temperature of 500 °C is studied. The samples were characterized by X-ray diffraction (XRD), micro-photoluminescence, Raman and Polarized Raman spectroscopy. The X-ray diffraction and micro-Raman spectroscopy confirmed the presence of Na substitution in zinc oxide and the wurtzite structure of the lattice is retained. An enhancement of resonant Raman scattering processes as well as longitudinal optical phonon overtones up to the fifth order were observed in the micro Raman spectra. The similar values of depolarization ratios obtained from Polarized Raman studies recommend no change in the symmetry. Photoluminescence showed a strong emission peak in the near UV at 3.2 eV and negligible visible emission.

Thermal annealing-induced formation of ZnO nanoparticles: Minimum strain and stress ameliorate preferred c-axis orientation and crystal-growth properties

Nanocrystalline zinc oxide (ZnO) thin films have been deposited on glass substrates using a sonicated sol–gel dip-coating technique at various thermal annealing (Ta) temperatures. The Ta temperature was varied to range from 300 to 600°C in intervals of 50°C in an open atmosphere. To obtain desirable piezoelectric properties, the correlations between Ta and the characteristics of ZnO thin films (crystallisation, optical and electrical behaviour) were investigated. The as-deposited films have large compressive stresses of 0.49GPa, which relaxed to 0.27GPa as the Ta temperature increased to 500°C. Optical parameters, such as optical transmittance, absorption coefficient and energy band gap, have been studied and discussed with respect to Ta. All films exhibit a transmittance above 50% in the visible region. It was found that the compressive stresses in the films cause a decrease in the optical band gap, whereas the tensile stress reveals an incline pattern with the optical band gap. This result corroborated with the crystallinity along the c-axis plane. The highest crystallinity value was achieved at the lowest stress value. An identical trend was observed for the resistivity values from the I–V measurement. Moreover, an increase in the crystallite size from 10 to 39nm as the level of Ta increased was noticed. The thickness of the films also decreased when Ta increased, and denser films were obtained as a result. A qualified ZnO thin film with good piezoelectric properties has been prepared using a sonicated sol–gel dip-coating technique with various Ta. Experimental results show that Ta has the greatest influence on the final properties of the ZnO thin films.

Qualitative and quantitative differentiation of gases using ZnO thin film gas sensors and pattern recognition analysis

In the present work we have grown highly textured, ultra-thin, nano-crystalline zinc oxide thin films using a metal organic chemical vapor deposition technique and addressed their selectivity towards hydrogen, carbon dioxide and methane gas sensing. Structural and microstructural characteristics of the synthesized films were investigated utilizing X-ray diffraction and electron microscopy techniques respectively. Using a dynamic flow gas sensing measurement set up, the sensing characteristics of these films were investigated as a function of gas concentration (10-1660 ppm) and operating temperature (250-380 °C). ZnO thin film sensing elements were found to be sensitive to all of these gases. Thus at a sensor operating temperature of ~300 °C, the response% of the ZnO thin films were ~68, 59, and 52% for hydrogen, carbon monoxide and methane gases respectively. The data matrices extracted from first Fourier transform analyses (FFT) of the conductance transients were used as input parameters in a linear unsupervised principal component analysis (PCA) pattern recognition technique. We have demonstrated that FFT combined with PCA is an excellent tool for the differentiation of these reducing gases.

Growth and Characterisation of Nanocrystalline ZnO Thin Films by Dip Coating Technique

Nanocrystalline zinc oxide (ZnO) thin films have been obtained by the sol gel process. A stable and homogeneous solution was prepared by dissolving zinc acetate dehydrate as a starting material in a solution of 2-methoxyethanol and monoethanolamine (MEA). The molar concentration of zinc acetate was fixed at 0.6 mol/L while the molar ratio of MEA to zinc acetate was kept at 1:1. The films were deposited by various deposition speeds by dip-coating on glass substrates, and subsequently transformed into nanocrystalline pure ZnO films after a thermal treatment. Various deposition speeds were selected as the parameter to optimize the thin films quality. The structural and optical properties of the ZnO films were studied by X-ray diffraction (XRD), UV-Vis-NIR spectroscopy, respectively. The electrical properties of the ZnO thin films were characterised by dc 2 probing system and power supply (Advantest R6243). It was found that the deposition speed affects the resultant properties of ZnO thin films.

A comparative study of physico-chemical properties of CBD and SILAR grown ZnO thin films

In the present work, nanocrystalline zinc oxide (ZnO) thin films have been successfully deposited onto glass substrates by simple and economical chemical bath deposition (CBD) and successive ionic layer adsorption reaction (SILAR) methods. These films were further characterized for their structural, optical, surface morphological and wettability properties. The X-ray diffraction (XRD) patterns for both CBD and SILAR deposited ZnO thin films reveal the highly crystalline hexagonal wurtzite structure. From optical studies, band gaps obtained are 2.9 and 3.0eV for CBD and SILAR deposited thin films, respectively. The scanning electron microscope (SEM) patterns show growth of well defined randomly oriented nanorods and nanograins on the CBD and SILAR deposited samples, respectively. The resistivity of CBD deposited films (102Ωcm) is lower than that of SILAR deposited films (105Ωcm). Surface wettability studies show hydrophobic nature for both films. From the above results it can be concluded that CBD grown ZnO thin films show better properties as compared to SILAR method.

Improved performance of ZnO thin film solar cells by doping magnesium ions

This study investigates the effect of magnesium doping on the performance of nanocrystalline zinc oxide (ZnO) thin film solar cells. MgxZn1−xO (x = 0.00, 0.05, 0.075 and 0.1 %) nanoparticles were prepared by simple precipitation method. The crystallinity and morphology of the photo-anode was characterized by X-ray diffraction analysis and scanning electron microscopy. The effect of various concentrations of magnesium ions on the structural and optical properties of ZnO nanoparticles was examined. Doping of magnesium ions helps the ZnO nanoparticles to grow larger in size with rod like surface morphologies. Solar cell with 0.075 % magnesium doped ZnO electrode exhibits an enhanced short-circuit current density of 3.36 mA/cm2, open-circuit photo voltage of 0.79 V, fill factor of 0.73, and overall power conversion efficiency of 2.1 %.

High intense violet luminescence in fluorine doped zinc oxide (FZO) thin films deposited by aerosol assisted CVD

Abstract Fluorine doped nanocrystalline zinc oxide (F: ZnO–FZO) thin films with varying fluorine dopant level (0–5%) were prepared on corning glass substrates by aerosol assisted chemical vapor deposition. The films exhibited polycrystalline hexagonal wurtzite structure with preferred orientation along c-axis and crystallinity deteriorated with increasing fluorine. As prepared films were found to be stressed in the range from 1.2 to 1.7 GPa and the amount of stress depends strongly on growth conditions. Undoped ZnO films were in a state of less tensile stress, whereas in FZO, the tensile stress increases with the increasing F. Films exhibit hexagonal platelets morphology analyzed using Field emission scanning electron microscope (FESEM). UV–Vis-NIR transmission studies indicated a increase of energy bandgap and increase of optical transmission with the increase in fluorine. Urbach energy (EU) of films have also been determined. Raman spectroscopy and photoluminescence (PL) analyses identified the origin of stress due to defects introduced by fluorine. A special emphasis was devoted to E2 high phonon mode analysis of Raman spectra. High intense violet PL has been achieved at room temperature and intensity improved vigorously with the dopant concentration due to a radiative transition of electrons from zinc vacancies level to the conduction band.

Influence of Substrate Temperature on ${\rm H}_{2}{\rm S}$ Gas Sensing Properties of Nanocrystalline Zinc Oxide Thin Films Prepared by Advanced Spray Pyrolysis

In this paper, nanocrystalline zinc oxide (ZnO) films are deposited onto glass substrates by a novel advanced spray pyrolysis technique, using non-aqueous zinc acetate solution. ZnO films are grown at various substrate temperatures in the range of 423-523 K, keeping the reaction chamber temperature constant. The influence of substrate temperature on film crystallinity and surface morphology electrical properties are investigated. The films are found to be polycrystalline with a wurtzite hexagonal structure. Surface morphological studies reveal columnar growth with enhanced crystallinity and lower activation energy at higher substrate temperature. The H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S gas sensing properties of ZnO films are studied as a function of substrate temperature, operating temperature, and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S gas concentration. The film grown at a 523 K substrate temeprature exhibits maximum sensitivity (~22%) to 20-ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S exposure at a 573-K working temperature and exhibits good selectivity and stability.

Structural, optical and gas sensing properties of spin coated nanocrystalline zinc oxide thin films

Synthesis of nanocrystalline zinc oxide thin films by sol gel spin coating technique and its application as ammonia gas sensor is presented in this paper. The synthesized sample is pure zinc oxide with hexagonal wurtzite structure. The lattice parameters are: a = 3.2568 A and c = 5.210 A. Average crystallite size is of the order of 58 nm. SEM studies show that growth of the film takes place with folded structure, increasing the open surface area of the film. Optical study revealed that band gap of ZnO is 3.25 eV with direct band to band transitions. Gas sensing characteristics showed that ZnO film is sensitive as well as fast responding to ammonia gas at 573 K. A high sensitivity for ammonia gas indicates that the ZnO films are selective for this gas. The rise time and recovery time are 25 and 80 s, respectively. The mechanism of gas sensing is explained adequately.

Thickness dependent H2S sensing properties of nanocrystalline ZnO thin films derived by advanced spray pyrolysis

Undoped zinc oxide (ZnO) thin films with various thicknesses were deposited onto glass substrates by advanced spray pyrolysis technique by varying the volume of spray solution. All the films were deposited at 473 K substrate temperature. These samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques, respectively. The X-ray diffraction analysis reveals that film growth is preferentially along (0 0 2) plane and thickness independent. All films exhibit hexagonal wurtzite structure, however, noticeable change is observed in crystallite size. An enhancement in the grain size with increase in film thickness was observed from the SEM studies. A two probe resistivity measurements exhibit decrease in resistivity with increase in film thickness. The response of ZnO samples toward H2S gas has been investigated at different operating temperatures and gas concentrations. Highest sensitivity was obtained for the film with 135 nm film thickness at 20 ppm H2S concentration when operating temperature is 573 K. The observed variation in sensitivity is related to the change in grain size with variation in film thickness.

Effect of solvent volume on the physical properties of aluminium doped nanocrystalline zinc oxide thin films deposited using a simplified spray pyrolysis technique

Aluminium doped zinc oxide (AZO) thin films were deposited by employing a low cost and simplified spray technique using a perfume atomizer from starting solutions having different volumes (10, 20,…,50mL) of solvent. The effect of solvent volume on the structural, electrical, optical, photoluminescence (PL) and surface morphological properties was studied. The electrical resistivity of the AZO films is remarkably influenced by the variation in the solvent volume. The X-ray diffraction profiles clearly showed that all the films have preferential orientation along the (002) plane irrespective of the solvent volume. The crystallite size was found to be in the nano range of 35–46nm. The optical transmittance in the visible region is desirably high (>85%). The AFM images show columnar morphology with varying grain size. The PL studies revealed that the AZO film deposited from 50mL of solvent volume has good quality with lesser defect density.

Disorder induced semiconductor to metal transition and modifications of grain boundaries in nanocrystalline zinc oxide thin film

This paper report on the disorder induced semiconductor to metal transition (SMT) and modifications of grain boundaries in nanocrystalline zinc oxide thin film. Disorder is induced using energetic ion irradiation. It eliminates the possibility of impurities induced transition. However, it is revealed that some critical concentration of defects is needed for inducing such kind of SMT at certain critical temperature. Above room temperature, the current-voltage characteristics in reverse bias attributes some interesting phenomenon, such as electric field induced charge transfer, charge trapping, and diffusion of defects. The transition is explained by the defects induced disorder and strain in ZnO crystallites created by high density of electronic excitations.

Role of oxygen vacancy in optical and gas sensing characteristics of ZnO thin films

Highly (002) textured, ultra-thin (100–160nm), nano-crystalline zinc oxide thin films are grown by metal organic chemical vapor deposition. These films are characterized in terms of structure, microstructure, optical and carbon monoxide gas sensing characteristics. The room-temperature photoluminescence (PL) spectra of these films are characterized by two peaks: the first one at 387nm is due to the near band edge UV emission and the other one ∼496nm is green emission from defect levels. The PL intensity ratio of the defect level and near band edge emission (IDL/INBE) is argued to be an indicator of oxygen vacancy concentration of these films. The response (%) of carbon monoxide gas sensing is found to increase with the film thickness and maximized in film ∼130nm thick. With further increase of film thickness the response % is found to be reduced. Good correlation is observed between the variation of the PL intensity ratio (IDL/INBE) and response% as a function of film thickness. It has been argued that the concentration of oxygen vacancies in turn controls the gas sensing characteristics of the synthesized films. Finally, we have elucidated the interrelation between the oxygen vacancy concentration, film microstructure and the gas sensing characteristics of the synthesized films.

Influence of core temperature on physical and H2S sensing properties of zinc oxide thin films

Nanocrystalline Zinc oxide (ZnO) thin films were deposited onto glass at low substrate temperature using advanced spray pyrolysis technique, with zinc acetate as precursor. The films were grown at various core temperatures in the range of 548–648K. Influence of the core temperature on the crystalline, surface morphological and electrical properties of the films have been investigated. XRD studies show that films prepared at core temperatures ⩾598K are polycrystalline and have a wurtzite crystal structure. FE-SEM micrograph observations of the films support the XRD results. The temperature dependence of the electrical resistivity was studied using two-probe method. It was observed that ZnO films exhibit semiconducting behavior. Further, the gas sensing potential of ZnO films towards H2S was examined. The sensing performance was studied at various operating temperatures and gas concentrations. The film deposited at 598K show a good sensing response (∼19%) and selectivity to H2S at an optimal working temperature.

Work function measurements on nano-crystalline zinc oxide surfaces

The work function of nano-crystalline zinc oxide (ZnO) thin films was examined using photoemission spectroscopy (PES). Colloidally dispersed ZnO nano-particles were electrospray-deposited in vacuum to form nano-crystalline thin films. The samples showed an immediate work function reduction by 0.35 eV during ultraviolet photoemission spectroscopy (UPS) measurements. This artifact was detected and quantified through low intensity x-ray photoemission spectroscopy (LIXPS) measurements, which use a very low photon flux. This prevented significant photochemical changes on the measured surface, i.e. the true work function unaffected by the UPS artifact can be measured. Annealing of an identical sample removed all ambient contamination from the ZnO surface with the effect to prevent the work function lowering artifact. This allowed the conclusion that ambient contamination is essential for the artifact to occur, similar to what was observed earlier on indium tin oxide and TiO2 surfaces. In an additional experiment, exposure of the annealed sample to the ambient resulted in a sample that again showed the artifact further demonstrating the necessity of water to be present. This experiment also demonstrated that the solubility enhancing surfactant shell of the nano-crystals does not play a significant role in the artifact, since it was removed during the annealing process.

Photoelectrocatalytic degradation of oxalic acid by spray deposited nanocrystalline zinc oxide thin films

The high quality nano-crystalline zinc oxide thin films are deposited onto corning glasses by spray pyrolysis technique. The influence of reaction temperature onto their photoelectrochemical, structural, morphological, optoelectronic, luminescence and thermal properties has been investigated. The structural characteristics studied by X-ray diffractometry has complemented by resistivity measurements and UV–Vis spectroscopy. The photoelectrochemical activity shows enhancement in short circuit current (Isc=0.357mA) and open circuit voltage (Voc=0.48V). Direct band gap calculated by considering R & T values of ZnO thin films increases from 3.14–3.21eV exhibiting a slight blue shift in band edge. Three characteristic luminescence peaks having near band-edge, blue and green emission are observed in the photoluminescence spectra. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in films. Photocatalytic degradation of oxalic acid followed with reaction mechanism by using zinc oxide photoelectrode under solar illumination has been investigated.

Photodissolution Suppression and Photocatalytic Activity Improvement through Crystallinity Improvement of ZnO Nanocrystalline Thin Films

Zinc oxide (ZnO) is an attractive candidate for semiconductor photocatalysis studies because of its strong oxdizability and wide bandgap. However, ZnO has the problem of photodissolution in the aqueous phase and photocatalytic activity decrease due to dissolution. In this paper, we report the photocatalytic property of nanocrystalline ZnO thin films fabricated by laser ablation. The films grown at different annealing temperatures and times have been characterized using X-ray diffraction and a scanning electron microscope. The photocatalytic activity of ZnO thin films was determined by the decoloration of azo dye OrangeII solution and hydrogen generation. It was shown that the photocatalytic activity of ZnO thin films depends on the annealing temperature and time, and photodissolution of the sample can be inhibited by increasing the crystallinity of ZnO. It was also shown that the ZnO thin layer with high crystallinity generates hydrogen from NaOH solution without applying bias voltage.

On the Mechanism of Electrical Conduction in Cobalt-Doped Zinc Oxide Nanocrystalline Thin Films

ZnO and Co-doped ZnO thin films (3 and 11 at. %) were grown on glass substrates by spin coating method and structurally investigated by X-ray diffraction, X-ray photoelectron spectroscopy and UV–vis absorption spectroscopy. It was established that Co enters into ZnO wurtzite lattice by substitution. The electrical conductivity of undoped and Co-doped ZnO nanocrystalline thin films was measured in the temperature range of 300–425 K. The electrical conduction mechanism of the films is explained on the basis of the multiphonon assisted hopping model with a weak electron–phonon coupling. We found that the conductivity first increases with incorporation in ZnO structure of an amount of 3 at. % Co but then it decreases when the amount of Co is increased to 11 at. %. This situation is well explained by the fluctuation in the hopping rate.