Sol Gel Derived ZnO Thin Films Research Articles

Sol-Gel Derived ZnO Thin Films with Nonvolatile Resistive Switching Behavior for Future Memory Applications

Herein we report on a facile sol-gel spin-coating technique to fabricate ZnO thin films that grow preferentially along the (002) plane on FTO substrates. By employing the magnetron sputtering technique to deposit a tungsten (W) top metal electrode onto these ZnO thin films, we successfully realize a W/ZnO/FTO memory device that exhibits self-rectifying and forming-free resistive switching characteristics. Notably, the as-prepared device demonstrates impressive nonvolatile and bipolar resistive switching behavior, with a high resistance ratio (RHRS/RLRS) exceeding two orders of magnitude at a reading voltage of 0.1 V. Moreover, it exhibits ultralow set and reset voltages of approximately +0.5 V and −1 V, respectively, along with exceptional durability. In terms of carrier transport properties, the low resistance state of the device is dominated by ohmic conduction, whereas the high resistance state is characterized by trap-controlled space-charge-limited current conduction. This work highlights the potential of the ZnO-based W/ZnO/FTO memory device as a promising candidate for future high-density nonvolatile memory applications.

Faster photoswitching and amplified NBE emission from mg doped sol-gel derived ZnO thin films

ABSTRACT Photoconductivity and photoluminescence of highly transparent predominantly c-axis oriented hexagonal wurtzite nanocrystalline Mg doped zinc oxide thin films are reported here. These films prepared by sol-gel spin coating are useful for switching and UV detector applications. The increase in dopant concentration tunes the optical band-gap from 3.15 to 3.41 eV. Amplified NBE emission with low defect emission is obtained in appropriately doped sample along with high suitability for switching applications since the photocurrent sharply falls to a very low value as the illumination is put off and the rise and decay pattern is reproducible for several consecutive cycles. The films have crystallite size less than 16 nm and rod-like structures of diameter 50 to 100 nm are seen to grow out of spherical granular background. Doping reduces the size of grains and rods as well as the surface roughness.

Sol–gel derived ZnO thin film as a transparent counter electrode for WO3 based electrochromic devices

Electrochromic devices (ECDs) have the ability to show color even when electrical power is disconnected. However, challenges such as simple and cost-effective electrochromic material preparation, insufficient coloration, slow switching times, and poor cycling stability have yet to be overcome. The paper describes an electrochromic device with WO3 thin film as the electrochromic material for the working electrode and ZnO thin film as a counter electrode fabricated on FTO glass substrates by a simple sol–gel spin coating method. The optical contrast at 600nm of ZnO counter electrode based ECD (12.4%) is almost double that of graphite counter electrode based ECD (6.5%), showing higher efficiency and better electrochromic response. The properties of ECD devices were also examined using cyclic voltammetry and chronoamperometry measurements with results showing that the devices were stable and the charge required to tint the device (5.6mAs) is reduced compared to typical graphite-coated counter electrode. These types of devices are promissing candidates to be used as smart windows.

Effect of monovalent dopant ionic radius and concentration on the growth orientation and optical properties of the sol–gel-derived ZnO thin films

Effect of monovalent dopant ionic radius and concentration on the growth orientation and optical properties of the sol–gel-derived ZnO thin films

The Synthesis of Nanophosphors YPxV1–xO4 by Spray Pyrolysis and Microwave Methods

За счет редкоземельного допирования фосфаты и ванадаты являются ведущими материалами для синтеза люминофоров благодаря их термостойкости, низкой температуре спекания, химической стабильности. Особый интерес привлекают люминофоры в наноразмерном состоянии. Простой, быстрый и масштабируемый синтез нанолюминофоров высокой химической гомогенности является актуальной задачей. Целью работы являлся синтез порошков смешанных кристаллов ванадат-фосфата иттрия различного состава методами соосаждения под действием микроволнового излучения и спрей-пиролиза, а также сравнительный анализ характеристик полученных образцов.Методами соосаждения под действием микроволнового излучения и спрей-пиролиза в различных режимах синтезированы образцы YVхP1–хO4 разного состава. Данные рентгенофазового анализа в случае синтеза ванадат-фосфата иттрия YVхP1–хO4 методом спрей-пиролиза с последующим отжигом подтверждают образование однофазных нанопорошков. Методами просвечивающей электронной микроскопии и растровой электронной микроскопии выявлены морфологические характеристики образцов. В зависимости от режима отжига образцы представляют собой ограненные либо сферические частицы размером менее 100 нм. Состав образцов YVхP1–хO4, синтезированныхметодом соосаждения под действием микроволнового излучения, сильно зависит от рН раствора прекурсоров.Минимальное содержание примесных фаз достигается при рН, равном 9.Метод спрей-пиролиза позволяет синтезировать нанопорошки ванадат-фосфата иттрия YVхP1–хO4 высокой химической гомогенности с размером частиц менее 100 нм. При синтезе YVхP1–хO4 методом соосаждения под действием микроволнового излучения максимальной химической однородности порошков ванадат-фосфата иттрия удается достигнуть при рН = 9. Однако дисперсия частиц по размеру велика, лежит в интервале 2–60 мкм. ЛИТЕРАТУРА 1. Wu C., Wang Y., Jie W. 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Effect of the bivalent dopant ionic radius, electronegativity and concentration on the physical properties of the sol–gel-derived ZnO thin films

This study deals with the influence of the concentration and ionic radius of the bivalent dopant on the structural, morphological and optical properties of the sol–gel-derived ZnO thin films. For that, different concentrations of dopants with different radii (Ni2+, Cu2, Fe2+) have been chosen to synthesize many ZnO thin films. The X-ray diffraction confirms that all the obtained films exhibit a pure hexagonal wurtzite structure. Meanwhile, it appears that both the preferential film orientation and the crystal quality are strongly affected when varying the ionic radius or the concentration of dopant. We note also that the lattice parameters decrease when the electronegativity of the dopant increases. This behavior has been explained at the light of a simple schematic model, based on the electrostatic forces involved in chemical bonding. The Scherer formula reveals that the average size of the crystallites is ranged between 20 and 35 nm. On the other hand, as confirmed by the SEM analysis, all the deposited films present a compact, uniform and nanocrystalline morphology. Nonetheless, the optical analysis indicates that the Ni- and Fe-doped films are highly transparent (greater than 90%) in the visible region, while the Cu-doped ones exhibit the lowest transmission (about 76%). Additionally, a blue shift of the bandgap is noticed when doping with Ni or Fe, whereas the doping with Cu induces a red shift.

Effect of the microwave-assisted thermal annealing on structural and optical properties of the sol–gel-derived ZnO thin films

ZnO thin films were prepared by sol–gel spin coating technique, and the effect of microwave-assisted thermal annealing on structural and optical properties was investigated. Structural features, microstructural morphology and optoelectronic properties were studied using XRD, FE-SEM, UV–visible spectrophotometry and four-point probes measurement system. During microwave-assisted annealing, rapid volumetric heating preserved nanostructure along with pore-free dense morphology. Also, this annealing technique induced larger tensile strain and increased preferential orientation along (002). Power consumption for sintering was reduced as obvious from the decreased annealing time. An increase in optical band gap with a decrease in electric resistivity by one order of magnitude can be attributed to its nanostructured pore-free structure, and the induced strain resulted from microwave-assisted annealing.

Sn–Ga co-doping in sol-gel derived ZnO thin films: Studies of their microstructural, optical, luminescence and electrical properties

Sn–Ga co-doped ZnO films with different proportions synthesized by the sol-gel spin-coating technique have been investigated for their microstructural, optical, luminescence, and electrical properties using a variety of characterization techniques. All the films exhibit c-axis orientation along the (0 0 2) plane having a hexagonal wurtzite structure of zinc oxide. The crystallinity of the films improves upon doping and co-doping. Morphological images reveal that the films possess the granular morphologies with varying features. The co-doped films have a higher root mean square (RMS) roughness in comparison to the undoped and doped ZnO films. In general, relative to the undoped film, the transmittance of the film improves upon doping and co-doping. The direct energy band-gap has a maximum value of 3.26 eV for the undoped ZnO film, which decreases upon doping and co-doping. In the visible region, the refractive index exhibits a minimum value of 2.12 for the 1T3GZO film at the wavelength of 600 nm. The photoluminescence spectra of the films consist of one near-band edge (NBE) emission peak related to the recombination of free excitons, and a few broad emission peaks linked to the native defects in the films. Among the co-doped ZnO films, the 1 at% Sn–Ga co-doped film (1T1GZO) has the optimum transmittance and electrical conductivity.

Morphological and Structural Properties of Sol-Gel Derived ZnO Thin Films Spin-Coated on Different Substrates

In this work, ZnO thin films were prepared by the low-cost sol-gel deposition method onto six different substrates (glass, ITO coated glass, sapphire (Al2O3), p-Si, p-GaN and polyethylene terephthalate (PET)) to study the effects of these substrates on the morphological and structural properties of the produced films. Precursor solution is Zinc acetate dihydrate based dissolved in ethanol with monoethanolamine (C2H7NO) added to act as a stabilizing agent to the sol. The corresponding ZnO thin films were characterized using field emission scanning electron microscopy (FESEM), high resolution X-ray diffraction (XRD) and atomic force microscopy (AFM). Results revealed distinct morphological and structural properties of ZnO thin films deposited on each substrate. The most uniform morphology was identified on glass, owing to the acquisition of the averagely stable grain sizes (58 nm – 61 nm) and thin film thicknesses (280 nm – 325 nm). High resolution XRD analysis showed that the films deposited on glass, ITO, p-Si, and p-GaN were attributed to hexagonal crystallite structures while the films deposited on sapphire and PET substrates exhibited amorphous phases. Amongst the samples, the ZnO thin film spin coated on p-Si demonstrated preferred orientation in (002) direction.

Controlling the extremely preferred orientation texturing of sol–gel derived ZnO thin films with sol and heat treatment parameters

In this article, material and process variables of sol–gel method are considered to investigate the texturing growth model of ZnO thin film in order to achieve ordinary, highly, and extremely preferred ZnO thin films. All the samples were fabricated by deposition of newly developed sol on conventional glass substrate via spin coating method and were further heat treated at various patterns. In sol variable, increasing the molar ratio of triethylamine to Zn increases preferred orientation along c-axis without using special heat treatment pattern. Various heat treatment parameters including drying and calcination are studied. The results demonstrate that drying at 300 °C is essential for achieving highly preferred ZnO thin film. In addition, calcination time, temperature, and repetition leads to more preferred sample to some extent. Furthermore, increasing the drying time at 300 °C causes significant increase of c-axis orientation and extremely preferential growth. In general, selection of an appropriate heat treatment pattern together with an accurate sol composition is necessary to achieve extremely preferred ZnO thin film with less energy conservation without using any special substrate. This is noteworthy that all the ZnO thin films are highly transparent in visible region and their band gap was around 3.3 eV.

Effect of Indium Doping on Optical Parameter Properties of Sol–Gel-Derived ZnO Thin Films

Abstract Zinc oxide (ZnO) has gained great interest for two decades, and its structural, optical, and electrical properties have been investigated by scientists for technological applications. The optical properties of ZnO provide an opportunity for its application in solar cells, lasers, and light-emitting diodes. We prepared ZnO thin films with various In doping levels by using a spin coating technique, and characterised their morphological and detailed optical properties for optical applications. According to the morphological properties obtained by atomic force microscopy, the film surfaces are homogeneous and the In doping level affects the surface morphology of the films. The optical properties of the films were investigated using an ultraviolet–visible spectrometer, and some optical parameters such as band gap, refractive index, extinction coefficient, single oscillator parameters, real and imaginary functions of the dielectric coefficient, and optical conductivity were calculated and discussed in detail. The various In doping levels affected the optical properties, and the In-doped ZnO thin films can find applications in industry.

Effect of Annealing Temperature on Structural and Optical Properties of Sol–Gel-Derived ZnO Thin Films

Nanocrystalline ZnO thin films were deposited on glass substrate via sol–gel dip-coating technique then annealed at 300°C, 400°C, and 500°C for 1 h. Their optical, structural, and morphological properties were studied using ultraviolet–visible (UV–Vis) spectrophotometry, x-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM). XRD diffraction revealed that the crystalline nature of the thin films increased with increasing annealing temperature. The c-axis orientation improved, and the grain size increased, as indicated by increased intensity of the (002) plane peak at 2θ = 34.42° corresponding to hexagonal ZnO crystal. The average crystallite size of the thin films ranged from 13 nm to 23 nm. Increasing the annealing temperature resulted in larger crystallite size and higher crystallinity with increased surface roughness. The grain size according to SEM analysis was in good agreement with the x-ray diffraction data. The optical bandgap of the thin films narrowed with increasing annealing temperature, lying in the range of 3.14 eV to 3.02 eV. The transmission of the thin films was as high as 94% within the visible region. The thickness of the thin films was 400 nm, as measured by ellipsometry, after annealing at the different temperatures of 300°C, 400°C, and 500°C.

Inverted polymer solar cells using CdS fabricated by thermal decomposition of cadmium xanthate precursor as electron transporting layer

In this manuscript, CdS thin films fabricated by thermal decomposition of the solution processable cadmium xanthate precursor were used as electron transporting layer (ETL) in inverted polymer solar cells (IPSCs) with device structure ITO/CdS/poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PC61BM)/MoO3/Ag. The effects of CdS thickness and the annealing temperature of cadmium xanthate precursor on the photovoltaic performance of IPSCs were investigated. The highest power conversion efficiency (PCE) value of 3.22% was obtained when CdS thin film thickness was 10nm and the annealing temperature of cadmium xanthate precursor was 160°C. This value is slightly higher than that of the IPSC with device structure ITO/ZnO/P3HT:PC61BM/MoO3/Ag using sol-gel derived ZnO thin films as ETL. The results indicate that CdS films deposited by thermal decomposition of metal xanthate precursor are excellent ETL for IPSCs.

Reduction in point defects of sol–gel derived ZnO thin films with oxygen ambient

The influence of oxygen as an annealing ambient on the native point defects of ZnO thin films has been investigated. ZnO films were derived by sol-gel spin coating method. Films were annealed at 700°C in vacuum, air and with different oxygen flow rates from 1.5 to 5.0 Standard Cubic Centimetres per Minute (SCCM). Intensity of characteristic XRD peak (002) increased intensively with the increment in oxygen, which indicates the improvement in crystal stoichiometry. In photoluminescence (PL) measurements yellow-green emission reduced while near band edge ultraviolet emission enhanced gradually, which is a sign of reduction in native defects (VO/Zni) and improvement in optical quality of films. Resistivity of the films, measured in Van der Pauw geometry, increased significantly as the oxygen flow increased during the annealing. Ferromagnetism in the films was also observed, although at very low scale, which may be due to one or more types of native defects. It is found that saturated magnetic moment (Ms) reduced with the availability of oxygen during annealing.

ZnO nanowalls grown at low-temperature for electron collection in high-efficiency perovskite solar cells

This paper reports on the fabrication of ZnO nanowalls for use as an electron collecting layer (ECL) in CH3NH3PbI3 perovskite solar cells (PSCs). Two-dimension ZnO nanowalls were grown using a low-temperature chemical bath method with a thin Al film as a seed layer. We compared electron collecting layers based on nanowalls and sol–gel derived ZnO thin film in PSCs and sought to identify the mechanism underlying the collection of electrons. The proposed ZnO nanowalls achieved a fill factor significantly higher than that of ZnO thin films, which translated into a remarkable improvement in power conversion efficiency, reaching 13.6% under AM 1.5G illumination.

Acid Surface Treatment of Sol-Gel Derived ZnO Thin Films for Anti-Reflection Coating Application

Transparent ZnO films were deposited by sol-gel spin coating process. HCl acid was used in the wet etching process to modify its surface morphology that can be used as an anti-reflective layer of optoelectric devices. The effect of etching time on its structure, surface morphologies and optical properties were scrutinized. The overall results indicated that HCl acid has obviously effect on significant change in surface morphologies and its roughness varied in the range of 0.5 nm to 4.9 nm.

Counterpoise-assisted annealing effects on enhanced photoluminescence and electrical properties of sol–gel-derived ZnO thin films grown on polyimide substrates

In order to improve the characteristics of sol–gel-derived ZnO thin films synthesized on polyimide (PI) substrates by controlling their degree of bending, we have devised counterpoise-assisted annealing (CAA) in which an external force is applied to the thin films by placing a counterpoise (0, 5, 20, or 50 g) on them. It was found that CAA could suppress the outward bending of the ZnO thin films during the post-annealing process. In addition, the crystallinity, photoluminescence, and electrical properties of the ZnO thin films exhibited significant improvements with an increase in the counterpoise mass to 20 g. Thus, we expect that the CAA method will emerge as an alternative route for synthesizing and optimizing flexible ZnO-based devices.

Fabrication and experimental characterization of a sol–gel derived nanostructured n-ZnO/p-Si heterojunction diode

In this paper, fabrication and characterization of a sol–gel derived n-ZnO nanoparticle/p-Si heterojunction diode has been presented. A strong diffraction peak (002) obtained from XRD spectra of sol–gel derived ZnO thin film indicates that the growth of hexagonal wurtzite ZnO nanoparticles are preferably along the c-axis. The surface morphological features extracted from AFM and SEM images predict the uniform growth of ZnO nanoparticles over the p-Si substrate. The optical properties of ZnO nanoparticles have been measured in the spectral range of 300–1000 nm using ellipsometer and photoluminescence spectroscopy. The optical transmittance of ZnO nanoparticles has been observed between 81 and 92.66 % in visible region and the optical band gap has been found out to be 3.24 eV. Finally, metallic contacts were deposited on top of ZnO layer and bottom of Si layer to perform electrical characterization of nanostructured n-ZnO/p-Si heterojunction diodes. Electrical parameters including reverse saturation current, ideality factor, barrier height, rectification ratio have been calculated to explore the potential of the device for futuristic nanoelectronic and optoelectronic device applications.

Structural, Morphological and Optical Properties of Sol-Gel Derived ZnO Thin Film for UV Sensing Application

In the current research we have studied the structural, morphological, optical and ultraviolet (UV) light sensing properties of ZnO thin film. The film was prepared by sol-gel technique and deposited on glass substrate by spin coating method. The structural analysis was performed by X-ray diffractometer (XRD) while the surface morphology and optical properties of the deposited thin film were monitored using field emission scanning electron microscope and UV-VIS spectrophotometer. The FESEM image displayed the homogenous coverage of ZnO nanoparticles over the surface of glass substrate. The deposited thin film shows an average transmittance value of more than 90%, in the visible light spectrum. The calculated value for optical band-gap was 3.26 eV. Refractive indices and dielectric constants were calculated by utilizing the bandgap value by considering different relations. Current to voltage characterization was conducted by using Kiethley sourcemeter connected with ultraviolet (UV) light illuminated LED’s. The excellent photo to dark current (Iph/Id) ratio (5.49) and rapid rise and decay time (15 s and 6 s) and the measured responsivity (1 A/W) suggest that the deposited ZnO thin film can be used for ultraviolet (UV) light sensing application.

High efficiency inverted polymer solar cells with solution-processed ZnO buffer layer

In this work, we report the application of a sol–gel derived ZnO thin film as a buffer layer for high efficiency inverted polymer solar cells. ZnO films are widely used in such devices because they have a relatively high electron mobility, high transparency and environmental stability. The ZnO precursor was prepared by dissolving zinc acetate and ethanolamine in the 2-methoxyethanol. ZnO thin films were then deposited on indium tin oxide (ITO)/glass substrates by spin coating the above solution. Inverted polymer solar cells with the configuration ITO/ZnO/photoactive layer/MoOx/Ag were realized in order to investigate the performance of ZnO thin film. The photoactive layer is a blend of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl C71 butyric acid methyl ester ([70]PCBM) (1:1.5 w/w). We made a comparative study of the photovoltaic behavior of devices with ZnO films deposited using different sol–gel recipes. In particular, we varied the zinc acetate/ethanolamine molar ratio to have ZnO films with different trace amounts of starting materials. In addition we also prepared ZnO films annealed at 200 °C for different times (from 5′ to 60′) in order to evaluate this effect on the trace amount removal.