Multilayer ZnO Research Articles

Influence of number of deposited layers on the microstructure, hydrophobicity and electro-optical properties of electrospun Al-doped ZnO thin films

The present study focused on the electro-optical properties of mono and multilayer Al-doped ZnO (AZO) thin films (TFs) using the electrospinning method. The microstructures of 1, 3, and 5 layered TFs were studied using field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques. The results revealed that the monolayered TF was formed by attaching homogeneous nanoparticles (NPs) with a size of 21 nm. With increasing the number of layers, the agglomerates of AZO NPs covered the surface of TFs, the homogeneity decreased, the thickness of TFs increased from 340 nm to 1.8 µm, and the transparency decreased from 97.3% to 85.3%. With the deconvolution of photoluminescence spectra, it was found that the point defects between valence and conduction bands of AZO TFs did not change drastically. However, their optical band gap increased from 3.24 eV monolayered TFs to 3.32 in multilayered TFs. The sheet resistance of mono and multilayered TFs was 17.3 kΩ•sq−1 and 102.2 kΩ•sq−1, respectively. This study suggests electrospinning as a controllable method for the fabrication of transparent conductive oxides (TCOs).

Effects of Sn Doping on Properties of Multilayered ZnO Films Deposited by Spin Coating/Sol–Gel Method

The effects of the Sn doping density and multilayer deposition on the structural, optical, and electrical properties of ZnO films prepared by a spin coating/sol–gel method have been investigated. The film characteristics were studied using x-ray diffraction (XRD) analysis, electronic absorption (ultraviolet–visible) spectroscopy, atomic force microscopy, and measurements using the four-probe method. The XRD patterns of both the five- and ten-layered ZnO films confirmed Sn substitutional incorporation, where Sn4+ ions replace Zn2+ ions while preserving the hexagonal wurtzite structure. The results show that both the doping density and number of layers affect the crystallite size, film thickness, conductivity, and surface morphology, with films having doping densities of 1.5 at.% to 2 at.% showing preferred characteristics. Among the different films, the ten-layered film doped with 1.5 at.% Sn exhibited preferred properties in terms of root-mean-square surface roughness (5.1055 nm), average transmittance (93%), and notably conductivity (7.94 × 10−3 Ω−1 cm−1).

Tuning Photocatalytic Performance of Multilayer ZnO for Water Splitting by Biaxial Strain Composites

Novel two-dimensional (2D) materials have received extensive attention in the field of photocatalysis due to their unique properties. Traditional ZnO material with wurtzite structure transforms into a stable graphite-like structure that has the characteristics of 2D material when its thickness is less than a few atomic layers. In this work, using first-principles calculations, we investigated the potential of multilayer graphite-like ZnO as a photocatalyst for water splitting. The results showed that multilayer ZnO is a series of direct bandgap semiconductors, and their band edge positions all straddle the redox potential of water. Increasing with the number of layers, the bandgap of multilayer ZnO decreased from 3.20 eV for one layer to 2.21 eV for six layers, and visible light absorption capacity was significantly enhanced. Hence, multilayer ZnO was indeed promising for photocatalytic water splitting. Furthermore, suitable biaxial tensile strain could decrease the bandgap and maintain the stable graphite-like structure at a broader thickness range. In contrast, excessive biaxial tensile strain could change the redox capacity of multilayer ZnO and prevent it from catalyzing water splitting. Our theoretical results show that six-layer ZnO under 1% biaxial strain had direct bandgap of 2.07 eV and represents the most excellent photocatalytic performance among these multilayer ZnO materials.

Photocatalytic and Gas Sensitive Multiwalled Carbon Nanotube/TiO2-ZnO and ZnO-TiO2 Composites Prepared by Atomic Layer Deposition.

TiO2 and ZnO single and multilayers were deposited on hydroxyl functionalized multi-walled carbon nanotubes using atomic layer deposition. The bare carbon nanotubes and the resulting heterostructures were characterized by TG/DTA, Raman, XRD, SEM-EDX, XPS, TEM-EELS-SAED and low temperature nitrogen adsorption techniques, and their photocatalytic and gas sensing activities were also studied. The carbon nanotubes (CNTs) were uniformly covered with anatase TiO2 and wurtzite ZnO layers and with their combinations. In the photocatalytic degradation of methyl orange, the most beneficial structures are those where ZnO is the external layer, both in the case of single and double oxide layer covered CNTs (CNT-ZnO and CNT-TiO2-ZnO). The samples with multilayer oxides (CNT-ZnO-TiO2 and CNT-TiO2-ZnO) have lower catalytic activity due to their larger average densities, and consequently lower surface areas, compared to single oxide layer coated CNTs (CNT-ZnO and CNT-TiO2). In contrast, in gas sensing it is advantageous to have TiO2 as the outer layer. Since ZnO has higher conductivity, its gas sensing signals are lower when reacting with NH3 gas. The double oxide layer samples have higher resistivity, and hence a larger gas sensing response than their single oxide layer counterparts.

Frequency Spectra of Coupling Vibration in High-Frequency Thickness-Shear ZnO Thin Film Resonator Applied in Sensing Field Based on the Hamilton Principle.

Mode coupling vibration of a multilayered ZnO thin film bulk acoustic wave resonator (FBAR) with its c-axis along the wave propagation direction operating at the fundamental thickness-shear (TSh) mode is studied in this paper. The accurate dispersion curve from 3-D piezoelectric theory indicates that the operating TSh mode could couple to the flexural mode, which influences the performance of a sensing resonator and needs to be eliminated through the so-called frequency spectrum analysis. Frequency spectra of coupling vibration in this FBAR can be obtained by substituting the displacement solutions consisting of the mode branches in the dispersion curve into several weak boundary conditions, which gives the integral boundary conditions across the plate thickness at the side edge of FBARs. The results show that the content of mode couplings to spurious modes depends on the plate length/thickness ratios. The character of desirable values of plate aspect ratios under which the operating TSh mode dominates among all modes is stated specifically, which provides fundamental guidance to the design of structural parameters of FBARs.

Sensitivity analysis of multilayered C-axis inclined zigzag zinc oxide thin-film bulk acoustic resonators for CO2 concentration monitoring

This paper proposes a theoretical analysis of an innovative C-axis inclined zigzag multilayered ZnO thin-film bulk acoustic resonator (FBAR) for CO2 concentration monitoring. Analytical study is carried out with 3-D linear piezoelectric equations with appropriate boundary conditions. Numerical results show that the thickness of FBAR has a constant influence on the sensitivity while C-axis inclined angle has a significant influence on the sensitivity for the longitudinal mode of multilayered (double-layered in this paper) FBAR CO2 sensor. The selectivity and sensitivity of the proposed sensor can be further improved by the dopant in ZnO FBAR for real environment monitoring.

Influence of Al/Cu thickness ratio and deposition sequence on photoelectric property of ZnO/Al/Cu/ZnO multilayer film on PET substrate prepared by RF magnetron sputtering

ZnO/Al/Cu/ZnO multilayer films were deposited on flexible polyethylene terephthalate (PET) substrates by radio frequency (RF) magnetron sputtering. The overall thickness of the intermediate Al and Cu metal layers maintained at 8 nm. The four different Al/Cu thickness ratios were 7:1, 6:2, 5:3 and 4:4. Other four reference samples were prepared by using the same process, i.e. ZnO single-layer film, ZnO/Al8 (8-nm-thick Al layer, the same below)/ZnO, ZnO/Cu8/ZnO and ZnO/Cu2/Al6/ZnO multilayer films. The effects of Al/Cu thickness ratio and deposition sequence of Al and Cu layers were investigated. The results revealed that with the increase of Cu layer thickness, sheet resistance of the multilayer film decreased, while transmittance increased first and then decreased. The ZnO/Al6/Cu2/ZnO multilayer film with a sheet resistance of 108 Ω/sq and an average visible transmittance of 84.73% had the optimum overall photoelectric property, being confirmed by the highest figure of merit of 1.77 × 10–3 Ω–1. Furthermore, ZnO/Cu2/Al6/ZnO and ZnO/Al6/Cu2/ZnO multilayer films exhibited almost the same average visible transmittance and sheet resistance, implying that the deposition sequence of Al and Cu layers has little influence on photoelectric properties. It is demonstrated that ZnO/Al/Cu/ZnO multilayer flexible films have great potential in various fields.

Optical Properties of ZnO/Al/Ag/ZnO Multilayers as Near Infrared Cut-off Filters

Various ZnO/metal/ZnO multilayers were fabricated and their optical properties were investigated. Top and bottom ZnO layers were formed by sol-gel method and mid-metal layers were deposited by thermal evaporation for precise control of thickness. After the fabrication of ZnO/Ag/ZnO structure, the transmittance increased to 72%. Adding Al to middle Ag layer, the ZnO/Al/Ag/ZnO structure reduced the near IR transmission in half with 40% optical transmittance in entire visible region. This oxide/double metal/oxide multilayers show the huge potential for building integrated photovoltaics as near IR cut-off filters.

Dependence of phonon transport properties with stacking thickness in layered ZnO

The thermoelectric property of ZnO is severely limited due to its high thermal conductivity, in spite of its enormously extraordinary characteristics. Two-dimensional (2D) materials usually act as a good strategy to suppress heat transfer by introduction of interfacial scattering in low dimensional systems. The dependence of thermal conductivity on thickness in 2D ZnO is investigated using a first-principle study, combined with the Boltzman phonon transport equation. The thermal conductivity of ZnO for the first three layers sharply decreases with the thickness increasing, which is attributed to the broken phonon selection rule and stronger anharmonicity. The suppressed processes can be reactivated in contrast to the monolayer, such as ZA + ZA/ZO ZA/ZO and ZA + LA/TA/LO/TO LA/TA/LO/TO. The scattering channels for flexural phonons increase, which suppresses the phonon transport and decreases the thermal conductivity. In addition, the decrease of in-plane interactions outweighs the enhancement of out-of-plane interactions, which leads to the reduced acoustic-optical gap. More A + A O phonon scattering may take place and hinder thermal transfer due to a smaller phonon gap. Furthermore, the variation of bond strength in-plane and out-of-plane in multilayer ZnO leads to the reduction of phonon group velocity and the enhancement of anharmonicity.

Highly Transparent and Conductive W-Doped ZnO/Cu/W-Doped ZnO Multilayer Source/Drain Electrodes for Metal-Oxide Thin-Film Transistors

Here, we demonstrate transparent and conductive W-doped ZnO (WZO)/Cu/WZO multilayer films as source/drain (S/D) electrodes for metal-oxide thin-film transistors (TFTs). The effect of the Cu interlayer thickness on the electrical and optical properties of the films was first investigated. Optimized WZO/Cu/WZO films with an excellent sheet resistance of $4.1~\Omega $ /sq. and an average transmittance of 81.7% in the visible range were obtained and employed as the S/D electrodes. Compared with indium–gallium–zinc oxide (IGZO) TFTs with Cu or WZO S/D electrodes, TFTs with multilayer WZO/Cu/WZO S/D electrodes exhibit much better performance with the current ON/OFF ratio, threshold voltage, subthreshold swing, and electron mobility of 105, 0.2 V, 0.1 V/dec, and 31.2 cm2/Vs, respectively. Owing to the suppression of copper diffusion and low contact resistance between IGZO and WZO/Cu/WZO multilayer films, they can be used as novel S/D electrodes for high-performance fully transparent TFTs.

Infrared metamaterial by RF magnetron sputtered ZnO/Al:ZnO multilayers

Hyperbolic metamaterials create artificial anisotropy using metallic wires suspended in dielectric media or alternating layers of a metal and dielectric (Type I or Type II). In this study we fabricated ZnO/Al:ZnO (AZO) multilayers by the RF magnetron sputtering deposition technique. Our fabricated multilayers satisfy the requirements for a type II hyperbolic metamaterial. The optical response of individual AZO and ZnO films, as well as the multilayered film were investigated via UV-vis-IR transmittance and spectroscopic ellipsometry. The optical response of the multilayered system is calculated using the nonlocal-corrected Effective Medium Approximation (EMA). The spectroscopic ellipsometry data of the multilayered system was modeled using a uniaxial material model and EMA model. Both theoretical and experimental studies validate the fabricated multilayers undergo a hyperbolic transition at a wavelength of 2.2 μm. To our knowledge this is the first AZO/ZnO type II hyperbolic metamaterial system fabricated by magnetron sputtering deposition method.

Atomic layer deposited dielectric and/or semiconducting oxide bilayers for crystalline silicon surface passivation

This study deals with the silicon surface passivation by thermal ALD deposited oxides bilayer of HfO2/Al2O3 as well as single layer of Al rich ZnO (AZO) in the form of ZnO and Al2O3 multilayers. A significant improvement in the effective minority carrier lifetime (τeff ~ 1.3ms) of n-type Si is observed after deposition of HfO2(5nm)/Al2O3(5nm)/Si compared to that of the single layer Al2O3(10nm)/Si (~0.6ms). Better surface passivation is due to a significant increase in the effective charge density in the bilayer system. Also, the sequence of the dielectric layers is found to play an important role in the quality and effectiveness of the passivation. Further, high τeff (~1.5ms) is also realized for p-type Si passivated by ALD deposited AZO films as compared to single layer of pure ZnO (~35μs) when annealed in hydrogen ambient at 450℃ for 30mins.

Binary Compound Bilayer and Multilayer with Vertical Polarizations: Two-Dimensional Ferroelectrics, Multiferroics, and Nanogenerators.

Vertical ferroelectricity in two-dimensional (2D) materials is desirable for high-density data storage without quantum tunneling or high power consumption/dissipation, which still remains elusive due to the surface-depolarizing field. Herein, we report the first-principles evidence of 2D vertical ferroelectricity induced by interlayer translation, which exists extensively in the graphitic bilayer of BN, AlN, ZnO, MoS2, GaSe, etc.; the bilayer of some 2D ferromagnets like MXene, VS2, and MoN2 can be even multiferroics with switchable magnetizations upon ferroelectric switching, rendering efficient reading and writing for high-density data storage. In particular, the electromechanical coupling between interlayer translation and potential can be used to drive the flow of electrons as nanogenerators for harvesting energy from human activities, ocean waves, mechanical vibration, etc. A ferroelectric superlattice with spatial varying potential can be formed in a bilayer Moire pattern upon a small twist or strain, making it possible to generate periodic n/p doped-domains and shape the periodicity of the potential energy landscape. Finally, some of their multilayer counterparts with wurtzite structures like a ZnO multilayer are revealed to exhibit another type of vertical ferroelectricity with greatly enhanced polarizations.

Structure and thickness-dependent gas sensing responses to NO2 under UV irradiation for the multilayered ZnO micro/nanostructured porous thin films

The structure and thickness of the chemiresistive thin films can significantly affect their gas sensing performances for the heating-typed sensors. Under light irradiation, however, their influences are still to be addressed. In present paper, the multilayered ZnO porous thin films with different (three types) micro/nanostructures and controllable thickness are fabricated via layer by layer construction of the self-assembled colloidal-layers. The structural and thickness effects of such films on the gas sensing performances to NO2 under ultraviolet (UV) illumination are experimentally studied. It has been found that under UV irradiation, the responses of the ZnO porous thin films to NO2 increase upto the maxima with the rising film thickness. Further increasing the thickness would lead to the insignificantly or gradually decreasing responses. The film thicknesses corresponding to the maximal responses are associated with the porous structures and the porosity of the thin films. The films with the higher porosity would lead to the higher maximal responses and the larger corresponding film-thicknesses, or vice versa. Such thickness and porous-structure dependences of the responses are attributed to the ever-decaying light intensity (and hence ever-decreasing photo-generated carrier concentration) in the films along the depth from the films’ surface. This study is of importance in design and development of the light illuminating-typed gas sensing devices with high performances.

Optical Reflectivity of Spin-Coated Multilayered ZnO and Al:ZnO Thin Films

Controlling growth, doping, crystallization, thickness of thin films of thin film transistor (TFT) channel materials is required in order to improve and control physical properties, primarily electronic conductivity and optical transparency. With the advent of flexible electronics and curved TFT-based display panels, low cost, solution-processed methods are important and provide scalable coating methods on a range of substrates. This work demonstrates the changes to the morphology, crystalline structure, optical reflectivity and electrical conductance of solution-processed ZnO thin films by the inclusion of an aluminium dopant during spin-coating. The measurements also determine the compositional chemical state of the Al:ZnO structures compared to ZnO using X-ray photoelectron spectroscopy in conjunction with detailed X-ray diffraction and transmission electron microscopy examination of the film morphology.

Highly-Ordered Growth of Solution-Processable ZnO for Thin Film Transistors

Zinc oxide is an important optoelectronic material, of particular interest due to its wide band gap (Eg ~ 3.3 eV at 300 K), large exciton binding energy (~66 meV) and especially for the variety of methods by which it can be processed. Moreover, ZnO is readily able to alloy with other metals in the oxide form and has a lattice that can facilitate interstitial doping. This gives ZnO a key role in the area of optoelectronics, metal oxide thin films and thin film transistor (TFT) technologies. We demonstrate that crystalline, epitaxial-like and highly ordered ZnO thin films can be achieved from a precursor liquid at relatively low temperature via spin-coating. The synthesised films are smooth, stoichiometric ZnO with controllable thickness. An iterative layer-by-layer coating schematic is employed to demonstrate the effects of film thickness on structure, morphology as well as the surface and internal defects. Characterisation of the crystallinity, morphology, O-vacancy formation, stoichiometry, surface roughness and thickness variation was determined through X-ray diffraction, scanning and transmission electron and atomic force microscopy, X-ray photoelectron and photoluminescence spectroscopy, and the data of multi-layered ZnO correlated to defect formation and electrical conductivity. It is imperative that high crystal quality epitaxial-like thin films can be formed from solution processing to compete with physical deposition methods. We demonstrate that iterative spin-coating of deposited ZnO films results in a transition in crystal texture with increasing thickness (number of layers) from the [] m-plane to the [] c-plane. The films attain a c-axis preferential orientation, with no other crystalline peaks present. Results show that the film’s surface morphology was very smooth, with average rms roughness <0.15 nm. Examination of these films also shows the consistency of the surface composition and defect level while highlighting the

Aqueous norfloxacin sonocatalytic degradation with multilayer flower-like ZnO in the presence of peroxydisulfate

Multilayer ZnO nanoflowers were synthesized through a simple precipitation method and characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and nitrogen absorption-desorption techniques. The FE-SEM images show the integrated morphology of an individual flower-like ZnO nanostructure, which is made of nano-platelets with uniform thickness (20–30nm). The average pore size and Brunauer-Emmet-Teller (BET) surface area of the as-synthesized ZnO were 27.25nm and 13.53m2/g. The sonocatalytic ability of the prepared samples was evaluated through norfloxacin (NF) degradation in an aqueous system using ultrasound (US) irradiation. To improve degradation efficiency, peroxydisulfate (Na2S2O8) was introduced to develop a US/ZnO/peroxydisulfate system, which exhibited an excellent synergistic effect. The effects of ZnO dosage, Na2S2O8 concentration, pH, and initial NF concentration were studied to determine the performances of the US/ZnO/peroxydisulfate process. Corresponding results showed that NF degradation rate increased with the increase of ZnO dosage but decreased with the increase of initial NF concentration. Under the optimal Na2S2O8 concentration of 0.1gL−1 at pH 9, the best degradation efficiency can be achieved. Moreover, based on the scavenging experiment results and literatures, NF degradation through US/ZnO/peroxydisulfate system is majorly induced by OH and SO4− radicals.

Influence of AZO amorphous structure on n-AZO/p-Cu2O heterojunction diode photoluminescence properties

In this work two samples of Ag/Cu/Cu2O and Al:ZnO (AZO) multilayers, on glass substrates were prepared. The Ag/Cu layers were obtained by the physical vapor deposition (PVD) method, while the Cu2O and AZO layers were deposited by DC reactive magnetron sputtering. The structural and morphology properties of the considered samples were studied. The X-ray diffraction (XRD) showed that the first junction signifies a crystal structure of compounds of Ag, Cu and Cu2O, but the latter sample showed an amorphous structure. It was shown that the roughness of glass/Ag/Cu/Cu2O layers was substantially lower than the roughness of AZO layer deposited on glass substrate. In addition, the microstructure consideration revealed that the glass/Ag/Cu/Cu2O surface morphology was highly homogenous with average grain size of 35 nm. Moreover, we investigated the properties of the Cu2O/AZO heterojunction. The low turn-on voltage of about 0.64 V was obtained, which indicate that the heterojunction acts as a rectifier diode. The ideality factor was determined to be 10.28.

Fabrication and Characterization of Cu Doped ZnO/Bi Doped ZnO Nanolaminates as Mirror for Application in Optical Devices

Cu doped ZnO/Bi doped ZnO nanolaminated multilayer mirror has been designed for visible window wavelength (∼ 550–750 nm). For the sake of designing multilayer mirror ZnO has been doped with Cu and Bi and alternating layer of as doped ZnO thin film has been deposited on n-silicon substrate using sol-gel spin coating method. One set of alternating layer has been termed as bilayer stack, henceforth the design mirror consists of three bilayer stacks of Cu doped ZnO and Bi doped ZnO. The thickness and reflectance spectra of the deposited layer have been determined using variable angle ellipsometery. Further, the effect of varying angle of incidence of the light incident upon the mirror has also been determined in terms of refractive index. In order to confirm the growth of nanolaminated structure XRD has been performed for all three bilayer stacks that show polycrystalline growth of grain. The experimental outcome of the Cu doped ZnO/Bi doped ZnO multilayer mirror indicated that a high reflectivity of >95% was attainable at wavelength range from 550 to 750 nm and at an incident angle of 20°, 40° and 80° from normal incidence. This approach allows as to simultaneously obtaining a high quality active layer together with a high photonic confinement as demonstrated through microphotoluminescence and reflectivity.

Multi-layer ZnO assembled microspheres, microstars and microflowers with high photocatalytic performance

Hierarchical ZnO microspheres, microstars and microflowers were synthesized via hydrothermal process. After calcination, the zinc nitrate and urea reacted and formed into microspheres. Addition of sucrose made the consisting layers with porosity and assembled to microstars. Surfactant trisodium citrate further altered the hierarchical structure to microflowers. The phase structure, surface morphology and optical absorption of the as prepared samples were analyzed by X-ray diffraction, field emission scanning electron microscopy and ultraviolet–visible spectrophotometer. It turned out that the samples were hexagonal ZnO with high purity and crystalinity, and showed intense absorption in the UV region. Photocatalytic studies indicated that the samples exhibited excellent photocatalytic activity towards methyl orange (MO) under UV light irradiation. The sample with microflowers performed the best in the photodegradation, and its recycling tests also gave satisfactory results for five cycles, which could be attributed to the large specific surface area and strong UV absorption. We expect this work could widen the application of the promising ZnO-based photocatalyst in environmental protection and control.