Microstructured ZnO Research Articles

Successive ionic layer adsorption and reaction (SILAR) synthesis of micro-structured Cu-doped ZnO thin films with enhanced photocatalytic activity

Successive ionic layer adsorption and reaction (SILAR) synthesis of micro-structured Cu-doped ZnO thin films with enhanced photocatalytic activity

Study of The Electrical, Optical and Morphological Properties in Submicron and Microstructured ZnO Thin Films Obtained by Spin Coating and Chemical Bath Deposition

In the present work the synthesis of ZnO semiconductor thin films was performed successively using spin coating and chemical bath deposition techniques. The deposition was made by varying the concentration of zinc acetate and hexamethylenetetramine (HMTA: ZnAc) in the precursor solution. This process led to two preferred growth directions (002) and (101), both with very similar texture coefficients, too; a noticeable change in morphology of structured surface, variation in unit cell parameters and crystalline grain size. All the films turned out homogeneously submicro and microstructured and with a wurtzite-type hexagonal crystalline structure. Using pre-loaded Mathematica 11.3 software functions and an algorithm developed in it, the micrographies were analyzed to calculate the percentage of substrate-covered area which was always greater than 80%. Likewise, it also found that resistivity decreases at a higher percentage of covered area and that the variation in the shape of the photo luminescent emission spectrum can be considered as a qualitative indication of the concentration of charge carriers.

Threshold Voltage Adjustment in Hybrid-Microstructural ITO-Stabilized ZnO TFTs via Gate Electrode Engineering

When metallic Al was replaced by conductive ITO as the gate electrode (GE) material, the positive threshold voltage ( ${V} _{\textsf {th}}$ ) shift of top-gated metal oxide (MO) thin-film transistors (TFTs) was found to be much larger than the work function (WF) difference of the GEs. Using secondary ion mass spectrometry and X-ray photoelectron spectroscopy techniques, it was noted that the permeability of GE materials for hydrogen diffusion out of and oxygen diffusion into the active layers during post-annealing could also affect ${V} _{\textsf {th}}$ significantly. The hybrid-phase microstructural ITO-stabilized ZnO TFTs with ITO top GEs exhibited relatively good electrical characteristics with a typical ${V} _{\textsf {th}}$ of 0.5 V and an on-off ratio of over 1010 in enhancement operation mode, and robust reliability against gate-bias stress. This letter initiated a new perspective to adjust ${V} _{\textsf {th}}$ of top-gated MO TFTs via GE engineering.

A facile electrochemical–hydrothermal synthesis and characterization of zinc oxide hierarchical structure for dye sensitized solar cell applications

A facile hierarchical microstructured ZnO thin films consisting of microspheres and nanorods were deposited on zinc oxide–titanium oxide (ZnO–TiO2) seeded FTO substrate by applying a two-step chemical techniques. At first, ZnO microsphere was electrochemically deposited on ZnO–TiO2 seeded FTO substrate by varying the deposition potentials in the range of − 1.1 to − 1.3 V. Followed by, the nanorods were hydrothermally grown on the optimized ZnO microsphere (ZnO electrodeposited at − 1.3 V). SEM displays the ZnO microsphere and nanorod for films deposited by electrodeposition of − 1.3 and hydrothermal technique, respectively. The suppression of ZnO growth along the (0001) orientation is noticed due to electrostatic absorption of citrate ions. TEM images confirm the hexagonal structure of microstructures and nanorods oriented along the lateral and c-axis growth direction, respectively. The appearance of high-intensity XRD peaks, Raman E2 (high) mode and UV emission confirms the hierarchical structure possessing a higher crystalline nature with lesser atomic defects. The hierarchical structure has high dye absorption, light absorption and scattering ability. The efficiency (η) of dye sensitized solar cells (DSSCs) consisting microsphere and hierarchical structures is found to be 3.13 and 4.64%, respectively. The DSSC consisting of hierarchical structure has the higher charge transfer recombination resistance (Rrec) and electron lifetime (τr) than other DSSC.

Hybrid-Phase Microstructural ITO-Stabilized ZnO TFTs With Self-Aligned Coplanar Architecture

In this letter, hybrid-phase microstructural ITO-stabilized ZnO thin films, which were capped by silane-sourced or tetraethyl-orthosilicate-sourced plasma-enhanced chemical vapor deposition (PECVD) SiO2 layers, were found to present remarkably different resistivity variation after O2 annealing treatment. Besides hydrogen-related factors, the silicon-doping phenomenon along with the silane-sourced PECVD process was also responsible for the durable low-resistivity state of the hybrid-phase microstructural ITO-stabilized ZnO thin films. Via a combination of differentiatedPECVD andO2 annealingprocesses, the self-aligned coplanar TFTs were fabricated, and they exhibited good electrical characteristics with an average field-effect mobility of 19.56 cm2/Vs, and a subthreshold swing of 105 mV/decade, as well as robust stability against gate-bias stress.

Exploring New Mechanisms for Effective Antimicrobial Materials: Electric Contact-Killing Based on Multiple Schottky Barriers.

The increasing threat of multidrug-resistance organisms is a cause for worldwide concern. Progressively microorganisms become resistant to commonly used antibiotics, which are a healthcare challenge. Thus, the discovery of new antimicrobial agents or new mechanisms different from those used is necessary. Here, we report an effective and selective antimicrobial activity of microstructured ZnO (Ms-ZnO) agent through the design of a novel star-shaped morphology, resulting in modulation of surface charge orientation. Specifically, we find that Ms-ZnO particles are composed of platelet stacked structure, which generates multiple Schottky barriers due to the misalignment of crystallographic orientations. We also demonstrated that this effect allows negative charge accumulation in localized regions of the structure to act as "charged domain walls", thereby improving the antimicrobial effectiveness by electric discharging effect. We use a combination of field emission scanning electron microscopy (FE-SEM), SEM-cathodoluminescence imaging, and Kelvin probe force microscopy (KPFM) to determine that the antimicrobial activity is a result of microbial membrane physical damage caused by direct contact with the Ms-ZnO agent. It is important to point out that Ms-ZnO does not use the photocatalysis or the Zn2+ released as the main antimicrobial mechanism, so consequently this material would show low toxicity and robust stability. This approach opens new possibilities to understand both the physical interactions role as main antimicrobial mechanisms and insight into the coupled role of hierarchical morphologies and surface functionality on the antimicrobial activity.

High-performance staggered top-gate thin-film transistors with hybrid-phase microstructural ITO-stabilized ZnO channels

In this paper, the ITO-stabilized ZnO thin films with a hybrid-phase microstructure were introduced, where a number of nanocrystals were embedded in an amorphous matrix. The microstructural and optical properties of thin films were investigated. It was found that the grain boundary and native defect issues in the pristine polycrystalline ZnO could be well suppressed. Meanwhile, such thin films also possessed relatively smooth surface and high transmittance in the visible range. Afterwards, the corresponding staggered top-gate thin-film transistors (TFTs) were fabricated at a temperature of 300 °C and exhibited fairly high electrical characteristics, especially with a field-effect mobility of nearly 20 cm2 V−1 s−1 and a subthreshold swing as low as 0.115 V/decade. In addition, the electrical uniformity and the stability of devices were also examined to be excellent. It is expected that the staggered top-gate TFTs with hybrid-phase microstructural ITO-stabilized ZnO channels are promising in the next-generation active-matrix flat panel displays.

ZnO for photocatalytic air purification applications

Nano and micro-structured ZnO coatings onto various substrates were grown by chemical methods and optimized with respect to their photocatalytic activity against in-doors common air pollutants. Excellent quality coatings with high stability and photocatalytic efficiency were obtained with the scope to be integrated in a novel air-purification system.

Synthesis of Microstructured ZnO in Hydrazine Hydrate Using a Hydrothermal Method and its Optical Properties

Microstructured ZnO have been synthesized by a simple hydrothermal approach, using ZnCl2as the zinc source and N2H4·H2O as the amine precursor in water-ethylene glycol solution at 120 °C for 8 h. The X-ray diffractrometer pattern indicates the pure phase formation of ZnO. Scanning electron microscope images show that the aspect ratio of the ZnO microrod increases by the amount of N2H4·H2O ranging from 0.5 to 1mL, but continuing to 3ml, the aspect ratio of the ZnO microrods decreases. Moreover, the optical band gaps calculated through UV spectroscopy are found to increase from 2.94 (±0.02) to 3.0 (±0.02) eV for samples synthesized at different amounts of hydrazine hydrate ranging from 0.5 to 1mL, respectively.

Design and Trial Production of Microstructured ZnO Gas Sensor

Design and Trial Production of Microstructured ZnO Gas Sensor

Adsorption of Cu(II) from aqueous solution by micro-structured ZnO thin films

Micro-structured thin films of ZnO and Zn(OH)2 on glass slides were prepared using microwave irradiation as a rapid and simple method. The effect of pH on the structure and composition of films were studied by scanning electron microscope and X-ray diffraction methods. Formation of flower-like ZnO structure starts with formation of nano-rods. The application of thin films as adsorbent was studied by Cu(II) adsorption of from aqueous solution. The adsorption equilibrium and kinetic data were modeled with classical and recently developed models. The obtained ZnO film is an effective adsorbent for removing of Cu(II) with high adsorption capacity of 692mg/g.

Fabrication of Micro-Structural ZnO Crystal and the Concentration Fields of the CVD Reactor

Arrayed ZnO crystals were fabricated with Zn as raw material by thermal chemical vapor deposition in a tubular reactor. The characterizations indicate the product grown at upstream is the well-arrayed ZnO submicro-column with good crystal properties, that at downstream is well-arrayed nanoneedle with more defects. In combination of mass field analysis with Fluent, it is revealed that the concentration fields at the upstream and downstream positions of Zn source are not identical even in an identical temperature field. It is the diffusion that drives Zn vapor move to upstream position where the O2 is rich, and the amount diffuse to upstream is rare due to the transportation of working gas. Most of O2 is consumed when passes the Zn source, leading to more lean O2 but rich Zn at the downstream position of Zn source. Our results support that lower concentration of Zn and O2 is in favor of the formation of perfect crystal as happened at upstream, but in such condition that more Zn and rare O2, defects of O vacancy is liable to form during ZnO growth.

Biomimetic fabrication and tunable wetting properties of three-dimensional hierarchical ZnO structures by combining soft lithography templated with lotus leaf and hydrothermal treatments

Three-dimensional hierarchical ZnO films with lotus-leaf-like micro/nano structures were successfully fabricated via a biomimetic route combining sol–gel technique, soft lithography and hydrothermal treatments. A PDMS mold replicated from a fresh lotus leaf was used to imprint microscale pillar structures directly into a ZnO sol film. Hierarchical ZnO micro/nano structures were subsequently fabricated by a low-temperature hydrothermal growth of secondary ZnO nanorod arrays on the micro-structured ZnO film. The morphology and size of ZnO hierarchical micro/nano structures can be easily controlled by adjusting the hydrothermal reaction time. Wettability of hierarchical ZnO thin films was found to convert from superhydrophilicity to hydrophobicity after a low-surface-energy fluoroalkylsilane modification. Improved wetting properties from hydrophobic to superhydrophobic can be tuned by increasing the growth of ZnO nanorod structures.

Hydrogen peroxide generation and photocatalytic degradation of estrone by microstructural controlled ZnO nanorod arrays

The strong oxidant, hydrogen peroxide (H2O2), generated by ZnO nanorod arrays under UV light irradiation was monitored by fluorescence analysis. The ZnO nanorod arrays were synthesized via a low temperature hydrothermal method and their dimensions, i.e., diameter and height, can be controlled by adjusting the concentration of zinc nitrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT). The morphology, nanostructure, surface roughness and optical property were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and transmittance spectra, respectively. The ZnO nanorod arrays were applied in the degradation of estrone, which is an emerging steroid estrogen contaminant. The results revealed that the ZnO nanorod array produced from 25mM Zn2+ and HMT had the highest aspect ratio, the largest surface roughness and the lowest band gap energy, which was beneficial to the efficiency of UV light utilization, photocatalytic degradation of estrone and H2O2 generation.

Nanostructured ZnO synthesis and its application for effective disinfection of Escherichia coli micro organism in water

Nanostructured ZnO photo catalyst was synthesized by precipitation method and was applied in conjunction with 355 nm pulsed laser irradiation for effective disinfection of the water contaminated with Escherichia coli micro organism. The morphological studies using X-Ray Diffractometer (XRD) and Transmission Electron Microscope (TEM) were carried out on the synthesized nano-ZnO, and these studies indicated that the catalyst has the crystallographic structure of hexagonal wurtzite and has the grain size of around 20–40 nm. The bacteria decay rate constants were estimated for nine different concentrations of nano-ZnO in infected water. The parametric optimization was carried out, and we could reach the decay rate constant as high as 0.24 min , −1 which is higher than micro-structured ZnO and the familiar TiO2 photo catalysts under similar experimental condition.

Fabrication of p-type CuSCN/n-type micro-structured ZnO heterojunction structures

Micro-sized ZnO rods on a SnO2 coated glass substrate were obtained by the spray pyrolysis method. Then a p-type CuSCN layer was deposited on this micro-sized n-ZnO to produce a p–n heterojunction. Temperature dependent current–voltage characteristics were measured in the temperature range 150–300K with a step of 25K. The current–voltage characteristics exhibit electrical rectification behavior. The zero bias barrier height Φb0 increases and the ideality factor n decreases with an increase in temperature. The apparent Richardson constant and mean barrier height were found to be 0.0028A cm−2 K−2 and 0.228eV respectively in the range 150–300K. After a barrier height inhomogeneity correction, the Richardson constant and the mean barrier height were obtained as 65.20A cm−2 K−2 and 0.840eV, respectively.

A controllable hydrothermal synthesis of uniform three-dimensional hierarchical microstructured ZnO films

ZnO films composed of uniform three-dimensional (3D) hierarchical microstructures on the conductive glass were synthesized by a one-step polyethylene glycol-assisted hydrothermal route with excellent reproducibility. The morphology of the ZnO hierarchical structured films was controlled by adjusting hydrothermal reaction conditions, i.e., time, temperature, reactant concentration, Zn source and surfactant. A specific growth mechanism for the ZnO hierarchical microstructured films is proposed, and the hydrothermal time is found to be a crucial role in the formation of the hierarchical structures either from the initial nucleation and growth of the primary rods or secondary/tertiary nucleation and growth on the the column facets of the primary rods. As-fabricated ZnO films show a strong photoluminescence emission peak at 435 nm, and a superhydrophobic adhesive surface with a water contact angle of 154.1° and a high contact angle hysteresis.

Co-existence of strongly and weakly localized random laser modes

Recent theoretical work on random lasing predicts that the occurrence of narrow lasing spikes can be caused by both localized and extended modes of light1,2,3. Typical random lasing spikes have been observed in powders of zinc oxide nanoparticles4, but identifying the possible degrees of localization of such modes has until now been an open issue5. In this Letter we present an experimental procedure that directly extracts the area of localization of the modes. This method relies on the investigation of micro-structured fields of zinc oxide powder, which also allows direct correlation to the local structural properties of the ensemble of subwavelength particles by scanning electron microscopy. We find that lasing from both localized and extended modes can be observed simultaneously. Our observation also corroborates the prediction that localized modes have a lower loss rate than that of extended modes5. Whether the electromagnetic fields in random lasers are localized or extended is a topic of ongoing debate. Now, the localization of modes in micro-structured ZnO powder is experimentally determined and lasing from both kinds of modes (localized and extended) shown to exist simultaneously.

Electrothermal Model Evaluation of Grain Size and Disorder Effects on Pulsed Voltage Response of Microstructured ZnO Varistors

Time‐dependent, two‐dimensional, electrothermal simulations based on random Voronoi networks have been developed to study the internal heating, current distributions and breakdown effects in ZnO varistors in response to high‐voltage pulsing. The simulations allow for dynamic predictions of internal failures and to track the progression of hot‐spots and thermal stresses. The focus is on internal grain‐size variations and relative disorder including micropores. Our results predict that parameters such as the hold‐off voltage, internal temperature, and average dissipated energy density would be higher with more uniform grains. This uniformity is also predicted to produce lower thermal stresses and to allow for the application of longer duration pulses. It is shown that the principal failure mechanism arises from internal localized melting, while thermal stresses are well below the thresholds for cracking. Finally, detrimental effects of micropores have been quantified and shown to be in agreement with experimental trends.

Electro-thermal Simulation Studies for Pulsed Voltage Induced Energy Absorption and Potential Failure in Microstructured ZnO Varistors

A time-dependent, two-dimensional model is used to study internal heating effects and possible device failure in ZnO varistors in response to a high-voltage pulse. The physics and qualitative trends discussed here should hold for materials with internal microstructured grain boundaries. Our analysis is based on an electro-thermal, random Voronoi network. It allows for the dynamic predictions of internal failure and to track the progression of hot-spots and thermal stresses. Results here show that application of high voltage pulses can lead to the attainment of Bi2O3 melting temperatures in the grain boundaries and an accelerated progression towards failure. Comparisons between uniform and normally distributed barrier breakdown voltage showed relatively small difference. Physically, this is shown to be associated with the applied bias regime and grain size. It is argued that reduction in grain size would help lower the maximum internal stress. This is thus a desirable feature, and would also work to enhance the hold-off voltage for a given sample size.