Surface Of ZnO Films Research Articles

Polarity of homoepitaxial ZnO films grown by Nd:YAG pulsed laser deposition

We investigate the stability of the polar surface of ZnO films grown homoepitaxially on atomically flat ZnO (0001¯) O-face substrates by neodymium yttrium aluminum garnet (Nd:YAG) pulsed laser deposition (PLD). For films grown in the temperature range from 500 to 700 °C, ion scattering spectroscopy showed that the film surface termination was the same as the ZnO substrate. Even for a Mg0.2Zn0.8O/ZnO superlattice, no polarity reversal occurred, indicating that the ZnO (0001¯) O-face is highly stable, despite the film surface sputtering caused by the high kinetic energy of the PLD plume generated by the Nd:YAG laser.

The influence of annealing environment of ZnO thin film on its optical, structural and photovoltaics performance

For inverted organic solar cells (IOSC), the interface contacts between the ZnO electron transport layer and the organic active layer play an important role in device performance and stability. The light absorption effect is one of the major disadvantages of IOSC when metal oxides are used as the electron transport layer (ETL). Oxide ETL primarily causes the above effect due to the energy barrier, deep level defects and excess carrier tunneling. In this work, we studied the effects of annealing ZnO films under various environmental conditions. ZnO films were synthesized by the sol-gel method, and the films were annealed at a temperature of 450 °C in air, an inert nitrogen gas (N2), oxygen (О2) and vacuum. SEM images and EDX spectra of the surface of thin ZnO films showed a change in thickness and the quantitative content of Zn/O elements. It was found that the annealing environment has a great influence on the morphology and thickness of ZnO films. Numerical calculations of the extinction, absorption, and scattering spectra of ZnO nanoparticles of different sizes are in good agreement with the results of measurements. Volt-ampere and electrical impedance measurements showed that ZnO films not only change the overall electrical resistance of devices to the contribution of the ZnO film resistance, but also change the electrical properties of the P3HT-PC61BM heterojunction.

Wrinkle type nanostructured of Al-Ce co-doped ZnO thin films for photocatalytic applications

ZnO:Al:Ce thin film photocatalysts were prepared by sol-gel spin coating method for the photocatalytic degradation of Methylene Blue (MB). The effect of Al-Ce co-doping on the structural, surface, optical and photocatalytic properties of ZnO films was analyzed. The crystallite size of the films calculated using the Williamson-Hall method decreased with Al-Ce co-doping. The enhanced photocatalytic activities of ZnO:Al:Ce wrinkle type nanostructured films can be associate with the increased surface area due to the crystallite size reduction by the Al-Ce co-doping. In the morphology analyzes of the films, the wrinkle network structure was observed in the pure ZnO and (6 %) Al-Ce co-doped ZnO films. All ZnO:Al:Ce films exhibited high transparency in the visible light (> 90%). Photoluminescence (PL) spectra depict near band edge (NBE), violet, blue, blue-green and green emission in deposited films. The photocatalytic test shows that the presence of Al-Ce enhances MB degradation efficiency up to 94 %. As a result, it has been observed that the co-existence of Al and Ce has significant effects on both the physical and photocatalytic properties of ZnO films.

Stable Superhydrophobic and Antimicrobial ZnO/Polytetrafluoroethylene Films via Radio Frequency (RF) Magnetron Sputtering.

In this study, superhydrophobic ZnO/Polytetrafluoroethylene (ZnO/PTFE) films with water droplet contact angles (CA) observed as high as 165° and water droplet sliding angles of (SA) <1° have been prepared on glass substrates by RF magnetron sputtering. The PTFE was wrapped on a nano-rod made of a ZnO film with superhydrophobic properties while providing excellent UV resistance compared to hexadecyltrimethoxysilane (HDTMS) hydrophobic agents. The upper surface of the rough ZnO film was coated with PTFE, and most of the underlying coating was bare ZnO, which could well make contact with bacteria. For the Gram-negative strain, E. coli, the cell viability count of the ZnO/PTFE sample (3.5 log reduction, 99.96%) was conspicuously lower than that of the ZnO/HDTMS sample (1.2 log reduction, 93.87%) under 1 h illumination of UV light, which showed that the ZnO/PTFE sample has a better photocatalytic property than the ZnO/ HDTMS films. The ZnO/PTFE films also showed good mechanical robustness, which is an important consideration in their widespread real-world adoption.

Preparation and evaluation of Zn(O,S) thin film layer by sulfurization method

In this paper, we propose a method of incorporating sulfur on the ZnO film surface, which can effectively alter the properties of the ZnO. Through sulfurization, a ZnS cubic crystal structure is formed in the ZnO film, with the ZnS peak increasing as the sulfur weight increases. This approach may pave the way for the successful production of Zn(O,S) film which may suit as an alternative buffer layer for CIGS solar cell structure.

Enhanced performance of inverted polymer solar cells by adding benzyl viologen dichloride into ZnO electron transport layer

An organic-inorganic hybrid electron transport layer (ETL) is developed using benzyl viologen dichloride (BV–2Cl) as a dopant for the preparation of sol-gel ZnO precursor solution. The inverted polymer solar cells (I–PSCs) based on PTB7-Th:PC71BM were prepared by introducing hybrid ZnO:BV-2Cl as ETL, and the effect of BV-2Cl on the photovoltaic performance of PSCs was closely investigated. Consequently, the device with hybrid ZnO:BV-2Cl (1% wt%) as ETL achieves the power conversion efficiency (PCE) of 9.31%, which is 16.5% more efficient than the device using pure ZnO as ETL. As the results indicate, hybrid ZnO:BV-2Cl films can modulate the interfacial compatibility of the hydrophilic surface of conventional ZnO films, hence enhancing charge transport and exciton dissociation. Our results indicate that using BV-2Cl as a dopant for the preparation of ZnO ETL is a facile and efficient method of fabricating high-performance I–PSCs.

AZO work function enhanced by oxygen plasma immersion ion implantation

The surface of aluminum-doped ZnO (AZO) films was modified using the oxygen plasma immersion ion implantation method, which is also called O-PIII. The modified AZO films were analyzed by an X-ray photoelectron spectroscopy system (XPS), X-ray diffraction (XRD), a UV–visible spectrophotometer, atomic force microscopy (AFM), and a Kelvin probe instrument (KP). The results showed that O-PIII modification can further enhance the work function (WF) of AZO based on oxygen plasma (O-ICP) treatment. The optical characteristics of AZO thin films are nearly entirely preserved, and the surface morphology is enhanced. The variation in WF was strongly linked to the surface stoichiometry. The reduction of oxygen vacancies should be attributed to the high WF. The WF of the modified AZO films decreased rapidly with exposure time in the air promptly following O-ICP and O-PIII modification. Nonetheless, the O-PIII treatment had a longer-lasting effect.

High-Performance Colloidal Quantum Dot Photodiodes via Suppressing Interface Defects.

PbS colloidal quantum dot (CQD) infrared photodiodes have attracted wide attention due to the prospect of developing cost-effective infrared imaging technology. Presently, ZnO films are widely used as the electron transport layer (ETL) of PbS CQDs infrared photodiodes. However, ZnO-based devices still suffer from the problems of large dark current and low repeatability, which are caused by the low crystallinity and sensitive surface of ZnO films. Here, we effectively optimized the device performance of PbS CQDs infrared photodiode via diminishing the influence of adsorbed H2O at the ZnO/PbS CQDs interface. The polar (002) ZnO crystal plane showed much higher adsorption energy of H2O molecules compared with other nonpolar planes, which could reduce the interface defects induced by detrimentally adsorbed H2O. Based on the sputtering method, we obtained the [002]-oriented and high-crystallinity ZnO ETL and effectively suppressed the adsorption of detrimental H2O molecules. The prepared PbS CQDs infrared photodiode with the sputtered ZnO ETL demonstrated lower dark current density, higher external quantum efficiency, and faster photoresponse compared with the sol-gel ZnO device. Simulation results further unveiled the relationship between interface defects and device dark current. Finally, a high-performance sputtered ZnO/PbS CQDs device was obtained with a specific detectivity of 2.15 × 1012 Jones at -3 dB bandwidth (94.6 kHz).

Improved epitaxy of ZnO films by regulating the layers of graphene

Growth of ZnO film on graphene buffered sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE) was investigated. Specifically, the surface morphology, crystal orientation and crystal quality of ZnO epitaxial layer and nucleation layer deposited on graphene with various thickness were analyzed by atomic force microscope (AFM), scanning electron microscope (SEM), high resolution X-ray diffraction (HRXRD), and high-resolution transmission electron microscope (HRTEM). The experiments showed that after buffered with double-layer graphene (DLG), six-fold symmetrical pits with the same orientation appeared on the successive ZnO film surface and the full width of half maximum (FWHM) of ZnO (002) plane rocking curve decreased to 769.3 arcsec. By comparing ZnO nucleation layer on graphene with different layers and checking the graphene by Raman spectroscopy, the mechanism for improving crystal quality of ZnO film grown on DLG was revealed. This study demonstrates the potential that high-crystal-quality ZnO film can be grown on graphene by plasma-assisted remote heteroepitaxy without limitation of the crystal mismatch and thermal mismatch between epitaxial film and substrate. It could be meaningful for the development of simplified growth technology of ZnO based films and devices.

Stabilization of the Surface of ZnO Films and Elimination of the Aging Effect.

Zinc oxide is a promising multifunctional material. The practical use of nano- and polycrystalline ZnO devices faces a serious problem of instability of electrical and luminescent characteristics, due to the adsorption of oxygen by the surface during aging. In this paper, the aging effect in ZnO films and nanorod arrays was studied. It was found that ZnO samples demonstrate different behavior of the degradation process, which corresponds to at least two different types of adsorbing surface sites for O2, where O2 adsorption is of a different nature. The first type of surface sites is rapidly depassivated after hydrogen passivation and the aging effect takes place due to these centers. The second type of surface sites has a stable structure after hydrogen passivation and corresponds to HO–ZnO sites. The XPS components of these sites include the Zn2p3/2 peak at 1022.2 ± 0.2 eV and Zn2p1/2 peak at 1045.2 ± 0.2 eV, with a part of the XPS O1s peak at 531.5 ± 0.3 eV. The annealing transforms the first type of site into the second one, and the subsequent short-term plasma treatment in hydrogen results in steady passivation, where the degradation of characteristics is practically reduced to zero.

Formation of Zn-rich ZnO films with improved bulk and surface characteristics by approach of magnetron sputtering technique

In this work Zn-rich ZnO films were formed by unconventional magnetron sputtering approach where oxygen flow cyclically varied between two different values during deposition process. Specifically, ZnO films were deposited using radio frequency power source with Zn target, while argon flow was set to keep total deposition pressure at constant and oxygen flow cyclically varied between stoichiometric ZnO and intermediate Zn/ZnO formation regimes. Such modification of physical vapor deposition method allowed to form complex structure ZnO films with altered bulk and surface characteristics. X-ray diffraction analysis of deposited films revealed highly expressed wurtzite structure with existence of small fraction of metallic Zn phase. In comparison to regular ZnO film surface morphology analysis of complex structure Zn-rich ZnO films showed significant increment of grain size and more than three times larger surface roughness. The reduction of optical transmittance as well as band gap value were observed by analysing optical properties of deposited films. Urbach energy analysis revealed that deposited Zn-rich ZnO films had higher level of disorder, dopants or defects comparing to the regular ZnO. Such combination of deposited films properties leads to the significant improvement of photocatalysis efficiency measured by the bleaching of methylene blue solution.

Influence of isovalent Cd doping concentration and temperature on electric and dielectric properties of ZnO films

In this study, pure zinc oxide and lightly cadmium doped zinc oxide (Zn1-xCdxO; x = 0.01, 0.03, 0.05 and 0.07) films were synthesized by chemical bath deposition to mainly investigate the possible impact of isovalent (in particular Cd) doping ratio and temperature on their electric and dielectric features. X-ray diffraction patterns revealed that all produced films have a dominance of ZnO hexagonal wurtzite structure with the emergence of a minor CdO cubic phase at x ≥ 0.03, and predicted the decrease in average crystallite sizes with Cd doping. Cd content in the films was verified by energy dispersive X-ray analysis. Images of scanning electron microscopy revealed the formation of nanorods and spheres on the surface of pure ZnO film which changed to porous/agglomerative spheres with Cd doping. Then a comprehensive electric and dielectric analysis was carried out as a function of frequency in a wide temperature range (300–700 K) using two separate experimental data sets, (Z, θ) and (C, G). The results demonstrated the critical effect of temperature and Cd doping ratio on the electrical and dielectric properties of ZnO films. Among the investigated films, Zn0·97Cd0·03O film recorded highest conductivity and enhanced dielectric properties which was attributed to the equal activation of grains and grain boundaries in the film structure verified by the estimation of activation energies from impedance spectrum. However, the effect of Cd doping on electric and dielectric properties was prominent only below 500 K, beyond which the doping effect became negligible which might be correlated with the effective dominance of grain boundaries at high temperatures as was witnessed by modulus spectrum.

An Efficiency of 16.46% and a T80 Lifetime of Over 4000 h for the PM6:Y6 Inverted Organic Solar Cells Enabled by Surface Acid Treatment of the Zinc Oxide Electron Transporting Layer.

For the inverted organic solar cells (OSCs), the interface contacts between the ZnO electron transporting layer and the organic active layer play an important role in the device performance and stability. Since the solution-processed ZnO surface always contains some base or zinc salt contaminants, we explored how the surface pH conditions influence the performance and stability of the nonfullerene acceptor (NFA) cells. A tight relationship between the surface pH condition and the device performance and stability was established. Specifically, device performance and stability were improved by treating the ZnO films with acid solutions but worsened after base treatment. The large number of hydroxyl groups on the surface of the solution-processed ZnO films was proved to be the main reason for the surface pH condition-related performance, which caused oxygen-deficient defects and unfavorable vertical phase separation in the blend films, hindered the photogenerated charge transfer and collection, and consequently resulted in low short-circuit current density and power conversion efficiency (PCE). The surface -OH groups also boosted the photocatalytic activity and led to fast degradation of the nonfullerene acceptor. Removal of the surface -OH groups can alleviate such problems. Different acid solutions, ZrAcac, 2-phenylethylmercaptan (PET), and glutamic acid (GC), were used to treat the ZnO films, and PET treatment was the most effective treatment for performance improvement. An efficiency of 16.46% was achieved for the PM6:Y6 cells and the long-term stability under continuous illumination conditions was significantly improved with a T80 lifetime of over 4000 h (4410 h), showing the excellent long-term stability of this heterojunction solar cell. Our understanding of the surface pH condition-related device performance and stability would guide the development of a feasible method for solving the interface problems in OSCs. We also provide a practical strategy to modify ZnO with acid solutions for high-performance and stable NFA OSCs.

Redistribution of Tb and Eu ions in ZnO films grown on different substrates under thermal annealing and its impact on Tb-Eu energy transfer

In (Tb, Eu) co-doped ZnO films grown on p-Si and Al2O3 substrates, the annealing at 900 °C was found to result in the diffusion and segregation of rare earth (RE) dopants to ZnO grain boundaries and film surface. The segregation of RE dopants to film/substrate interface was observed in the film on p-Si and explained by the drift of positively charged RE ions in the electric field of n-ZnO/p-Si junction. The annealing promoted also formation of additional crystal phases, namely Tb2O3, TbO2, TbxEuySiO7, Zn2SiO4 and ZnAl2O4. In the photoluminescence spectra, it caused the appearance of narrow and broader Eu3+ emission lines. The broader lines emerged only for the film on p-Si substrate and were ascribed to Eu3+ in TbxEuySiO7 inclusions formed at the ZnO/p-Si interface. The narrow lines were attributed to Eu3+ ions in Tb2O3 inclusions located at ZnO grain boundaries and film surface. The energy transfer from Tb3+ to Eu3+ was demonstrated in the annealed films only. A theoretical approach that allowed to describe the RE ion surrounding and to calculate the energy spectra of Tb3+ and Eu3+ in various site positions was developed. The crystal field parameters were also estimated.

Carrier Transport Mechanism and Barrier Height of B-, Al- and B-Al-Ion-Doped ZnO Film/Graphene Schottky Contacts Prepared Using the Sol–Gel Method

In this article, graphene/ZnO Schottky contacts with different ion doping were fabricated by the sol–gel method. The results showed that the nanoparticles growth on the surface of ZnO film was limited by ion doping, and the number and size of nanoparticles decreased for B ion doping. Further, the ZnO film band gap presented a decreasing trend with B, Al and B-Al ion doping. The electrical properties of the graphene/ZnO Schottky contact incorporating an ion-doped ZnO film were investigated, where the results suggested that ion doping could effectively improve the barrier height and reduce the leakage current. This phenomenon can be explained by the reduction of the oxygen vacancies on the surface of the ZnO film by ion doping, which leads to a reduction of the defect level at the interface and weakened Fermi level pinning.

Effect of annealing temperature on the performance of photoconductive ultraviolet detectors based on ZnO thin films

ZnO films prepared by sol–gel method and the ultraviolet detectors based on this material have been investigated in this paper. X-ray diffraction (XRD) patterns showed ZnO films present hexagonal wurtzite structure with a preferential orientation of (002) plane, and the crystallite size of films gradually increased from 44.3 to 54.8 nm as the annealing temperature increased from 400 to 600 °C. Ultraviolet–visible (UV–Vis) spectra indicated that the optical band gap decreases gradually with the increase of annealing temperature, and the minimum band gap is 3.06 eV at 600 °C. Photoluminescence (PL) spectra revealed that increasing annealing temperature can significantly reduce the defects and improve the crystallinity. Finally, gold (Au) coplanar interdigital electrodes were deposited on ZnO film surface and used to fabricate the ultraviolet photodetectors. The response performance of the devices improved as the annealing temperature of ZnO films increased, and the fastest response speed with a rise time of 4.172 s and a fall time of 11.012 s was obtained.

Highly efficient dye-sensitized solar cells by TiCl4 surface modification of ZnO nano-flower thin film

In dye-sensitized solar cells (DSSCs), the semiconductor photo-anode film plays a significant role in enhancing the overall power conversion efficiency. ZnO is considered as the futuristic hope for photoanodes in DSSCs due to manifold properties over TiO2. However, the power conversion efficiency of ZnO-based DSSCs is still low due to its poor chemical stability and surface defects. In this work, we reported the synthesis of ZnO nano-flowers as well as its surface modification of with TiCl4 at different concentration. In DSSCs, the enhancement in power conversion efficiency results suggested that surface modification of ZnO film by TiCl4 leads to the deposition of TiO2 which subsequently increases the roughness factor of film as well as scattering layer. This preferential surface modification of ZnO film facilitates the accumulation of large number of photo-injected electrons in the HOMO of the photoanode with rapid transfer of charge carriers to FTO via ZnO layer by lowering the recombination of photo-injected electrons with the redox electrolyte as well as oxidized dye. The intensity-modulated photocurrent spectroscopy (IMVS) and intensity-modulated photovoltage spectroscopy (IMPS) study also indicated that the recombination rate decreased considerably during the electron transportation. The ZnO film surface modified by TiCl4 achieved a power conversion efficiency of 4.48%, which is two times higher than that of the non-modified ZnO photoanode.

Ion-beam-induced surface modification of solution-derived indium-doped zinc oxide film for a liquid crystal device with stable and fast switching properties

The characteristics of a solution-derived indium-doped zinc oxide (In:ZnO) film exposed to ion-beam (IB) irradiation as a liquid crystal (LC) alignment layer were investigated. Solution processing was conducted to deposit a thin layer on a substrate and irradiation at various IB irradiation energies was used as an LC alignment method, all of which achieved uniform and homogeneous LC alignment. Atomic force microscopy using three-dimensional imaging and numerical analyses showed that the IB irradiation reduced the surface roughness. Through the X-ray photoelectron spectroscopy, it was revealed that the modification of the chemical composition due to the IB irradiation affected the state of the LC alignment. Increased polarizability of the surface and oxygen vacancies induced strong van der Waals forces between the In:ZnO film surface and the LC molecules, thereby subsequently achieving uniform and homogeneous LC alignment. The electro-optical (EO) characteristics of a twisted-nematic cell made with IB-irradiated In:ZnO film at an IB irradiation energy of 2200 eV, with which outstanding EO performance was observed. Therefore, using the solution-derived In:ZnO film with the IB irradiation as an alignment shows remarkable potential for use in LC device.

Fabrication of perovskite solar cells with ZnO nanostructures prepared on seedless ITO substrate

The nanostructured ZnO thin films have been fabricated on seedless SnO2: In (ITO) substrate by using an inexpensive and uncovered hydrothermal method. These prepared ZnO films are used as electron transporting materials to fabricate the perovskite solar cells (PeSCs). The performance of PeSC has been varied with ZnO-deposition temperatures ranges from 55 to 105 °C. The ZnO films show the high crystalline wurtzite structure. The whole surface of ZnO films fabricated at the higher deposition temperatures has covered with the hexagonal nanorods/nanotubes-like structures. The maximum efficiency of 4.55% is obtained for PeSC with ZnO films prepared at 85 °C. The photocurrent increases from 8.5 to 13.9 mA/cm2 with the ZnO-deposition temperature increases from 55 to 105 °C. The PeSC with ZnO films prepared at 95 and 105 °C shows the lower efficiency than the PeSC with ZnO-85 °C, which may be caused due to the large series and shunt resistance as well as the carrier leakage in PeSCs. The surface morphological, structural, and optical properties of ZnO electrodes fabricated at various deposition temperatures have been correlated with the photovoltaic performances of PeSC prepared by these ZnO electrodes.

Flexible one diode-one resistor composed of ZnO/poly (fluorene-alt-benzothiadiazole) (PFBT) heterojunction diode and TiO2 resistive memory

One diode–one resistor (1D1R) memory is an effective structure to suppress sneak current and read interference of crossbar network. Organic–inorganic complex plays an important factor in the field of flexible microelectronic memory due to integrated flexibility and excellent electronic properties. Herein, we demonstrated a 1D1R based on the n–n heterojunction diode of ZnO/polymer and Al/TiO2/Al resistor. The resistive switching performance of flexible 1D1R decreased with increase of bending times. Atomic force microscopy results show that the cracks on the ZnO film surface were related to the bending cycles. The scanning electron microscopy indicates crack growth path was along the grain boundaries. The finite element studies confirmed that channel crack of ZnO film and delamination between ZnO and polymethyl methacrylate interface are the main failure mode for the 1D1R under bending, which leads to a lower forward current in the low resistance state and weaken the ON/OFF ratio of 1D1R.