Influence of precursor source on sol–gel deposited ZnO thin films properties

Zinc oxide thin films were deposited by sol gel technique on glass substrates using different precursors (zinc acetate, zinc nitrate and zinc chloride). In the present work we investigate the precursor nature influence on structural, morphological, optical, electrical properties and photocatalytic activity of ZnO thin films. For this purpose we have used X-rays diffraction (XRD), atomic force microscopy (AFM), UV–visible spectroscopy and Hall effect measurements for films characterization. The obtained results indicated that ZnO films properties are strongly influenced by the nature of the used precursor as reactant. Films photocatalytic activity was evaluated by the photo-degradation of methylene blue (MB) dissolved in aqueous solution under UV-A light. The obtained results indicated that ZnO thin films prepared from zinc acetate are more efficient than those prepared from zinc nitrate and zinc chloride.     

Growth of a-b-axis orientation ZnO films with zinc vacancies by SSCVD

Zinc oxide (ZnO) films were grown on silicon (1 0 0) substrates by single-source chemical vapor deposition (SSCVD). X-ray diffraction (XRD) showed that ZnO thin films have a polycrystalline hexagonal wurtzite structure with (1 0 0) and (1 0 1) orientation, i.e., a-b-axis orientation. Atomic force microscopy (AFM) and scanning electronic microscopy (SEM) showed the films to be of relatively high density with a smooth surface. X-ray photoelectron spectroscopy (XPS) showed that the deposited films were very close to stoichiometry but contained a small number of zinc instead of O vacancies as normally found with ZnO films produced by other methods. These results were also confirmed by photoluminescence (PL) measurements.     

Sol–gel derived ZnO/PVP nanocomposite thin film for superoxide radical sensor

Pure ZnO films and ZnO nanoparticle-dispersed polyvinylpyrrolidone (PVP) composite films are prepared on Pyrex glass substrates by the sol–gel dip-coating technique utilizing zinc acetate precursor. The thin films are extensively characterized for surface morphology, chemistry, and nanocrystallite size using various advanced analytical techniques including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). For the processing conditions considered, ZnO semiconductor thin films with nanocrystallite size 20–30 nm are obtained. The ZnO nanoparticle size in the PVP composite film increases with increase in ZnO content. The resistance of both the synthesized ZnO and ZnO/PVP thin films decrease significantly after exposure to solution containing superoxide anion radicals (SOR). The results thus indicate that ZnO and ZnO/PVP composite thin films can be used as biosensors for SOR and potentially for characterizing the antioxidant properties of fluids.     

Effect of precursor on epitaxially grown of ZnO thin film on p-GaN/sapphire (0 0 0 1) substrate by hydrothermal technique

Single crystalline ZnO thin film on p-GaN/sapphire (0 0 0 1) substrate, using two different precursors by hydrothermal route at a temperature of 90 °C were successfully grown. The effect of starting precursor on crystalline nature, surface morphology and optical emission of the films were studied. ZnO thin films were grown in aqueous solution of zinc acetate and zinc nitrate. X-ray diffraction analysis revealed that all the thin films were single crystalline in nature and exhibited wurtzite symmetry and c-axis orientation. The thin films obtained with zinc nitrate had a more pitted rough surface morphology compared to the film grown in zinc acetate. However the thickness of the films remained unaffected by the nature of the starting precursor. Sharp luminescence peaks were observed from the thin films almost at identical energies but deep level emission was slightly prominent for the thin film grown in zinc nitrate.     

A comparative study on the NO2 gas sensing properties of ZnO thin films, nanowires and nanorods

ZnO thin films were fabricated using the spin coating method, ZnO nanowires by cathodically induced sol-gel deposition by the means of an anodic aluminum oxide (AAO) template, and ZnO nanorods with the hydrothermal technique. For thin film preparation, a clear, homogeneous and stable ZnO solution was prepared by the sol-gel method using zinc acetate (ZnAc) precursor which was then coated on a glass substrate with a spin coater. Vertically aligned ZnO nanowires which were approximately 65 nm in diameter and 10 μm in length were grown in an AAO template by applying a cathodic voltage in aqueous zinc nitrate solution at room temperature. For fabrication of the ZnO nanorods, the sol-gel ZnO solution was coated on glass substrate by spin coating as a seed layer. Then ZnO nanorods were grown in zinc nitrate and hexamthylenetetramine aqueous solution. The ZnO nanorods are approximately 30 nm in diameter and 500 nm in length. The ZnO thin film, ZnO nanowires and nanorods were characterized by X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). The NO₂ gas sensing properties of ZnO thin films, nanowires and nanorods were investigated in a dark chamber at 200 °C in the concentration range of 100 ppb-10 ppm. It was found that the response times of both ZnO thin films and ZnO nanorods were approximately 30 s, and the sensor response was depended on shape and size of ZnO nanostructures and electrode configurations.     

Control of ZnO thin film surface by ZnS passivation to enhance photoluminescence

To enhance the optical property of zinc oxide (ZnO) thin film, zinc sulfide (ZnS) thin films were formed on the interfaces of ZnO thin film as a passivation and a substrate layer. ZnO and ZnS thin films were deposited by atomic layer deposition (ALD) using diethyl zinc, H₂O, and H₂S as precursors. Investigations by X-ray diffraction and transmission electron microscopy showed that ZnS/ZnO/ZnS multi-layer thin films with clear boundaries were achieved by ALD and that each film layer had its own polycrystalline phase. The intensity of the photoluminescence of the ZnO thin film was enhanced as the thickness of the ZnO thin film increased and as ZnS passivation was applied onto the ZnO thin film interfaces.     

Highly enhanced solar conversion efficiency of novel layer-by-layer PbS:Hg and CdS quantum dots-sensitized ZnO thin films prepared by sol–gel spin coating

Owing to superior optical properties, ZnO thin films have immense potential in solar cell preparation. ZnO thin films were prepared by sol–gel technology. However, this is prolonged technique and it necessitates a complex precursor solution. In the present work, ZnO thin films are prepared by sol–gel spin coating with simple precursor, zinc acetate. A very remarkable feature of the method is that polycrystalline, non-abrasive and translucent films were obtained. Additionally, novel PbS:Hg quantum dots (QDs) and CdS QDs are successfully synthesized. Moreover, both types of QDs are deposited layer-by-layer over pure ZnO and Ag:ZnO thin films. The films are characterized by X-ray diffraction, and crystallinity continuation is observed even after the addition of QDs layer. Presence of synthesized QDs over thin films is also confirmed. The films were also characterized by scanning electron microscopy (SEM) and UV–Vis spectroscopy. Uniform, dense and porous surface morphology is clearly revealed. Sensitized thin films show a huge decline in band gap and large enhancement in efficiency. Superior current density (\(10.87~\hbox {mA}~\hbox {cm}^{-2})\) is achieved with PbS:Hg/CdS/Ag:ZnO, which leads to enhancement in overall solar conversion efficiency by 6.34 times.     

Properties of nanocrystalline zinc oxide thin films prepared by thermal decomposition of electrodeposited zinc peroxide

Zinc peroxide thin films were electrodeposited from aqueous solution at room temperature using H₂O₂ as the oxidation agent. Nanocrystalline zinc oxide thin films were then obtained from thermal decomposition of zinc peroxide thin films. The grain sizes of ZnO through thermal decomposition of ZnO₂ at 200 °C, 300 °C and 400 °C were estimated from the peak width of ZnO(110) obtained from X-ray diffraction and were 6.3 nm, 9.1 nm and 12.9 nm, respectively. The optical properties of zinc oxide thin films have been studied. The photoluminescence results indicate that ZnO thin films have low Stokes blue shift (about 110 meV) and low oxygen vacancies.     

Low temperature growth and optical properties of ZnO nanowires using an aqueous solution method

ZnO nanowires were grown on indium tin oxide (ITO) coated glass substrates at a low temperature of 90 degrees C using an aqueous solution method. The ZnO seeds were coated on the ITO thin films by using a spin coater. ZnO nanowires were formed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2 x 6H2O) and hexamethylenetetramine (C6H12N4). The pH value and concentration of the solution play an important role in the growth and morphologies of ZnO nanowires. The size of ZnO naonowires increased as the concentration of the solution increased. It was formed with a top surface of hexagonal and tapered shape at low and high pH values respectively. Additionally, the single crystalline structure and optical property of the ZnO nanowires were investigated using high-resolution transmission electron microscopy and photoluminescence spectroscopy.     

Effect of acetic acid on ZnO:In transparent conductive oxide prepared by ultrasonic spray pyrolysis

Undoped and indium doped zinc oxide (ZnO) transparent conductive oxide were prepared by a low-cost Ultrasonic Spray Pyrolysis. The influence of acetic acid on properties of the ZnO thin films was investigated. The complex formed by [CH₃COO⁻] and [Zn²⁺] in precursor solution was better for the growth of ZnO film. The acetic acid added in precursor solution can supply [CH₃COO⁻] for both [Zn²⁺] and [In³⁺] to form complexes. That made the [Zn²⁺] and [In³⁺] have similar statement, which can promote the indium doping in the ZnO films. The surface morphology, structural and electrical properties of the ZnO thin films were influenced by the acetic acid adding. The total transmittance of the ZnO thin films is above 80% in the wide wavelength region from 400 nm to 2000 nm.     

Thickness dependency of sol-gel derived ZnO thin films on gas sensing behaviors

ZnO thin films were fabricated by a sol-gel method using Zn(CH₃COO)₂·2H₂O as starting material in order to prepare an acetone gas sensor. A homogeneous and stable solution was prepared by dissolving the zinc acetate in a solution of ethanol and monoethanolamine. The sol-gel solution is coated on alumina substrates with various thicknesses by spin coating technique and heat treated to grow crystalline ZnO thin films. The effect of thickness on physical and electrical properties of as deposited ZnO thin films has been studied. The as deposited ZnO thin films were characterized by X-ray diffraction spectroscopy, field emission scanning electron microscopy and atomic force microscopy. The root mean square surface roughness factors increase with thickness of the films and found 3.9, 6.6, 9.0, and 11.28 nm for 80-, 220-, 450- and 620-nm-thin films respectively. The activation energies of the films are calculated from the resistance temperature characteristics. The sensitivities of the ZnO films towards the acetone gas were determined at an operating temperature of 200 °C. The sensitivity towards acetone vapor is strongly depending on surface morphology of the ZnO thin films.     

A Raman spectroscopic study of structural evolution of electrochemically deposited ZnO films with deposition time

Zinc oxide thin films were fabricated on ITO substrates by electrodeposition method. The electrolyte used was a 0.2 M zinc nitrate aqueous solution. The substrates were maintained at room temperature and the deposition performed for different times between 10 and 30 min. X-ray diffraction measurements indicated the formation of polycrystalline ZnO film with hexagonal wurtzite structure. The structure and crystallinity of the films was also confirmed by Raman spectroscopy. Further, the degree of disorder was estimated both from the phonon correlation length calculated from the Raman spectra using the spatial correlation model and from the intensity ratios of the phonons. The variation with deposition time followed the same trend as the crystallite sizes obtained from X-ray diffraction. X-ray photoelectron spectroscopy measurements indicated oxygen deficiency in the films. A combination of annealing and optimum deposition time improves the quality of the electrodeposited ZnO films.     

Optimization of growth conditions of ZnO nano thin films by chemical double dip technique

Abstract

Zinc oxide (ZnO) nano thin films have been deposited by the chemical double-dip technique using aqueous ZnSO₄ and NaOH solutions. The ZnO films were characterized in terms of surface morphology by x-ray diffraction, energy-dispersive x-ray analysis (EDX), the use of a scanning electron microscope (SEM) and atomic force microscope (AFM) for surface morphology. The films exhibited a smooth morphology. The chemical states of oxygen and zinc in the ZnO nano thin films were also investigated by x-ray photoelectron spectroscopy (XPS). In the present investigations, highly textured ZnO thin films with a preferential (002)-orientation were prepared on glass substrates. The deposition conditions were optimized to obtain device-quality films for practical applications.     

Preferential growth of ZnO thin films by the atomic layer deposition technique

Preferred orientation of ZnO thin films deposited by the atomic layer deposition (ALD) technique could be manipulated by deposition temperature. In this work, diethyl zinc (DEZn) and deionized water (H(2)O) were used as a zinc source and oxygen source, respectively. The results demonstrated that (10.0) dominant ZnO thin films were grown in the temperature range of 155-220?°C. The c-axis crystal growth of these films was greatly suppressed. Adhesion of anions (such as fragments of an ethyl group) on the (00.2) polar surface of the ZnO thin film was believed to be responsible for this suppression. In contrast, (00.2) dominant ZnO thin films were obtained between 220 and 300?°C. The preferred orientations of (10.0) and (00.2) of the ZnO thin films were examined by XRD texture analysis. The texture analysis results agreed well with the alignments of ZnO nanowires (NWs) which were grown from these ZnO thin films. In this case, the nanosized crystals of ZnO thin films acted as seeds for the growth of ZnO nanowires (NWs) by chemical vapor deposition (CVD) process. The highly (00.2) textured ZnO thin films deposited at high temperatures, such as 280?°C, contained polycrystals with the c?axis perpendicular to the substrate surface and provided a good template for the growth of vertically aligned ZnO NWs.     

Preparation and characterization of nanostructured ZnO thin films for photoelectrochemical splitting of water

Nanostructured zinc oxide thin films (ZnO) were prepared on conducting glass support (SnO₂: F overlayer) via sol-gel starting from colloidal solution of zinc acetate 2-hydrate in ethanol and 2-methoxy ethanol. Films were obtained by spin coating at 1500 rpm under room conditions (temperature, 28–35°C) and were subsequently sintered in air at three different temperatures (400, 500 and 600°C). The evolution of oxide coatings under thermal treatment was studied by glancing incidence X-ray diffraction and scanning electron microscopy. Average particle size, resistivity and bandgap energy were also determined. Photoelectrochemical properties of thin films and their suitability for splitting of water were investigated. Study suggests that thin films of ZnO, sintered at 600°C are better for photoconversion than the films sintered at 400 or 500°C. Plausible explanations have been provided.     

Cathodic electrodeposition of nanoporous ZnO thin films from new electrochemical bath and their photoinduced hydrophilic properties

The cathodic electrodeposition of nanocrystalline zinc oxide (ZnO) thin films from an aqueous zinc acetate containing electrochemical bath is reported. Scanning electron microscopic examination of the resultant films reveals the formation of nanoporous ZnO. X-ray diffraction (XRD) analysis indicates that the ZnO thin films deposited with crystallite sizes as small as 10 nm are possible with this method. The surface wettability of the electrodeposited ZnO thin films was also investigated. Reversible hydrophobicity to super-hydrophilicity transition was observed and nicely controlled by alternation of UV light irradiation and dark storage.     

ZnO nanostructure thin films by continuous spray pyrolysis using doped precursor for Si solar cell application

This work presents deposition of Zn solution seed layer assisted growth of zinc oxide (ZnO) nanostructure layers by continuous spray pyrolysis reactor using lanthanides (Er and Eu) and metal (Al) influenced zinc acetate precursor solution. Dopants in precursors have influenced structural property, surface morphology and optical reflectance of resulting ZnO thin films which are supported by X-ray diffractometer, scanning electron microscope and reflectance measurements. Enhanced dispersion amongst nanorods is observed under the influence of Er and Al dopant in ZnO thin film. The change of precursor from Zinc acetate to Titanium tetraisopropoxide for Er doped precursor is helping to achieve better crystalline ZnO nanorods arrangement with increased homogenous growth, which results into improved light reflectance reduction of thin film. The experimental evidences of light reflectance from ZnO nanorods on Si surface is studied with the help of FDTD based Lumerical software package which can be a useful study for designing ZnO nanorods thin film in device purposes. The utility of ZnO layer by this reactor on low efficiency Si solar cell is also explored in improving device efficiency via increase of photocurrent.     

Application of ZnO single-crystal wire grown by the thermal evaporation method as a chemical gas sensor for hydrogen sulfide

A zinc oxide single-crystal wire was synthesized for application as a gas-sensing material for hydrogen sulfide, and its gas-sensing properties were investigated in this study. The gas sensor consisted of a ZnO thin film as the buffer layer and a ZnO single-crystal wire. The ZnO thin film was deposited over a patterning silicon substrate with a gold electrode by the CFR method. The ZnO single-crystal wire was synthesized over the ZnO thin film using zinc and activated carbon as the precursor for the thermal evaporation method at 800 degrees C. The electrical properties of the gas sensors that were prepared for the growth of ZnO single-crystal wire varied with the amount of zinc contained in the precursor. The charged current on the gas sensors increased with the increasing amount of zinc in the precursor. It was concluded that the charged current on the gas sensors was related to ZnO single-crystal wire growth on the silicon substrate area between the two electrodes. The charged current on the gas sensor was enhanced when the ZnO single-crystal wire was exposed to a H2S stream. The experimental results obtained in this study confirmed that a ZnO single-crystal wire can be used as a gas sensor for H2S.     

Microstructural evolution of sol-gel derived ZnO thin films

Zinc oxide thin films, with thicknesses between ∼ 20 and 450 nm, were prepared by spin-coating a sol-gel precursor solution (zinc acetate dihydrate and monoethanolamine in an isopropanol solvent) onto glass substrates, followed by heat treatment at temperatures through 773 K. At 298 and 373 K, the films exhibited the structure of a lamellar ZnO precursor, Layered Basic Zinc Acetate (LBZA). At higher temperatures, LBZA released intercalated water and acetate groups and dehydroxylated to form zinc oxide nanograins with wurtzite structure, which were preferentially oriented in the c-axis direction. Both the degree of the films' c-axis orientation and the topography of their surfaces varied with heat treatment and precursor concentration. For films calcined at 773 K, a minimum of micron-scale surface wrinkles coincided with a maximum in c-axis preference at intermediate concentrations, suggesting that release of mechanical stress during densification of thicker films may have disrupted the ordering process that occurs during heat treatment.     

Influence of annealing on ZnO films grown by metal–organic chemical vapor deposition

Zinc oxide (ZnO) films were deposited on silica glass substrates using metal–organic chemical vapor deposition (MOCVD) with diethyl zinc (DEZn) as the Zn precursor and ethanol as the oxygen source. Annealing was performed at 600°C for 1 h in air. The X-ray diffraction (XRD) patterns of the samples show sharp diffraction peaks for ZnO (0002), which indicates that the films are highly c-axis oriented. The films were also characterized by measuring the optical transmission spectrum, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XPS spectra showed that the ZnO films changed from O-rich to Zn-rich after being annealed.