Crystalline Zinc Oxide Research Articles

Atomic Force Microscopy Adhesion Mapping: Revealing Assembly Process in Inorganic Systems

There are still many unknowns regarding assembly processes. In this work, we demonstrate the capability of atomic force microscopy (AFM) adhesion mapping in revealing the conditions that promote the light-induced assembly of nanoparticles (NPs) on nanostructured surfaces in inorganic systems, both in macro- and nanodomains. Gold (Au) NPs and zinc oxide (ZnO) nanostructures are employed as the model materials, and different characterization techniques are used for extracting the relationship between the materials’ crystallinity, stoichiometry, and morphology as well as surface adhesion mapping information. The light-induced assembly of Au NPs is associated with the attraction forces between the opposite surface charges of the NPs and preferential ZnO sites, which can be identified by adhesion mapping. We show that the yield of Au nanoclusters assembled onto the ZnO surface depends on the crystallinity and stoichiometry of ZnO and is not due to the roughness of the surface. The presented experiments demonstrate that AFM adhesion mapping can be used as an invaluable tool for predicting the strength and directions of assembly processes.

Air-Stable Efficient Inverted Polymer Solar Cells Using Solution-Processed Nanocrystalline ZnO Interfacial Layer

In this work, efficient bulk heterojunction (BHJ) organic solar cells (OSC) in inverted configuration have been demonstrated. Power conversion efficiency (PCE) of 3.7% is reported for OSC employing silver top electrodes, molybdenum trioxide (MoO3) as the hole-transport interlayer (HTL), active layer comprising of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as well as a nanocrystalline solution-synthesized zinc oxide (ZnO) nanoparticle (NP) film as the electron-transport layer (ETL). By using solution-processable ZnO crystalline NPs as ETL, we can eliminate the typical high temperature processing/annealing step, which is widely adopted in the conventional ZnO ETL fabrication process via the sol-gel method. Such highly crystalline ZnO NP films can enhance charge collection at the electrodes. It is also found that inverted OSCs exhibit greater air stability and lifetime performance compared to the OSC employing the normal structure.

Crystallinity-dependent substitutional nitrogen doping in ZnO and its improved visible light photocatalytic activity

Increasing visible light absorption of wide-bandgap photocatalysts (for example, ZnO and TiO2) plays a pivotal role in improving their photocatalytic activity. In this work, we show that substitutional nitrogen doping can be realized in semi-crystalline zinc oxide (ZnO) nanoparticles but fails for highly crystalline ZnO by heating the ZnO at a temperature of 400°C in gaseous ammonia atmosphere. The results suggest that substitutional nitrogen for lattice oxygen is strongly dependent on the crystallinity of ZnO. The nitrogen doped ZnO obtained shows an improved visible light photocatalytic activity in the degradation of organic dyes due to its increased visible light absorption. The origin of the increased visible light absorption is theoretically attributed to the formed N 2p localized states in the bandgap.

Gelatin-assisted hydrothermal synthesis of single crystalline zinc oxide nanostars and their photocatalytic properties

Biotemplate-assisted approach is simple and friendly to the environment. With the assistance of gelatin as a soft biotemplate and a structure-directing agent, star-like zinc oxide (ZnO) nanostructures have been prepared by assembly of well-defined nanorods under hydrothermal conditions. Their morphology and structures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The growth mechanism of the ZnO nanostars is also investigated. The as-prepared ZnO nanostars display high photocatalytic activity toward the degradation of methyl orange (MO) under ultraviolet (UV) irradiation.

Arrays of ZnO nanocolumns for 3-dimensional very thin amorphous and microcrystalline silicon solar cells

We report on the hydrothermal growth of high quality arrays of single crystalline zinc oxide (ZnO) nanocolumns, oriented perpendicularly to the transparent conductive oxide substrate. In order to obtain precisely defined spacing and arrangement of ZnO nanocolumns over an area up to 0.5cm2, we used electron beam lithography. Vertically aligned ZnO (multicrystalline or single crystals) nanocolumns were grown in an aqueous solution of zinc nitrate hexahydrate and hexamethylenetetramine at 95°C, with a growth rate 0.5÷1μm/h. The morphology of the nanostructures was visualized by scanning electron microscopy. Such nanostructured ZnO films were used as a substrate for the recently developed 3-dimensional thin film silicon (amorphous, microcrystalline) solar cell, with a high efficiency potential. The photoelectrical and optical properties of the ZnO nanocolumns and the silicon absorber layers of these type nanostructured solar cells were investigated in details.

Ultra-fine UV photoluminescence emission at room temperature from an individual ZnO nanorod

Highly-crystalline zinc-oxide (ZnO) nanorods and nanowires with hexagonal cross sections are grown on a sputtered ZnO thin film as buffer layer, using physical vapor deposition (PVD) technique in the absence of any catalyst. For the experimental setup, a double tubes furnace at atmospheric pressure is utilized. The structural characteristics of the samples are studied by using field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The optical properties of grown nanorods are investigated by using micro photoluminescence (PL) spectroscopy. The results show that high-quality ZnO nanostructures can be obtained by control of the super saturation conditions during the growth process while the XRD and EDS results reveal that the grown nanorods are single-phase hexagonal ZnO with a perfect elemental stoichiometry. Micro PL measurement of a single nanorod results in fine spectra, in which, it exhibits a sharp ultraviolet (UV) emission bands, corresponding to the near-band-edge (NBE) emission of crystalline ZnO, without any peak in the green or the orange region, which is normally related to crystal defects.

Monte Carlo simulation of scintillation behavior of zinc oxide under X-ray illumination using GEANT4

Monte Carlo simulations play an essential role in many of the research activities on new scintillation detectors for X-ray-based medical imagers. The scintillation process, the transport of scintillation photons through the crystal(s) and X-photons interactions have a major influence on the detector performance; hence, considering all of these processes in the model is important to obtain accurate results. In this work, the behavior of crystalline zinc oxide as a scintillator detector under X-ray illumination is simulated by using the GEANT4 Monte Carlo simulation code. The optical and scintillation models of the GEANT4 are employed to investigate the optical response of zinc oxide as the X-ray detector in energies up to 100 keV (suitable for medical imaging applications). The results show that ZnO suffers from high self-absorption of the X-ray-generated optical photons and this problem strongly affects the performance of the ZnO as the X-ray scintillator detector especially for higher energies.

Optical cavity modes of a single crystalline zinc oxide microsphere

A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 µm are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors.

The effects of the pressure and the oxygen content of the sputtering gas on the structure and the properties of zinc oxy-nitride thin films deposited by reactive sputtering of zinc

Zinc nitride and oxy-nitride thin films were prepared by reactive magnetron rf sputtering of zinc in either nitrogen–argon or nitrogen–argon–oxygen ambient. The effects of varying the total sputtering pressure and the oxygen fraction in the total sputtering gas mixture on the microstructure, electrical and optical properties were investigated. With increasing the sputtering pressure, the dominant phase comprising the film material changes from the crystalline zinc nitride phase to crystalline zinc oxide. The characteristic pressure, at which this change in the dominant phase is observed, decreases with the increase of the oxygen fraction in the total sputtering gas mixture. The increase of the oxygen content in the films (from 5 at.% to a maximum of 40 at.%) and the downward shift in the optical absorption edge (from 920 to 400 nm), combined with the x-ray diffraction data, support these observations, indicating the controllable fabrication of an oxy-nitride film material. Correlations between the films’ fabrication conditions, including post-deposition annealing, their structure and composition, and their electrical properties are examined as well.

Micro-Galvanic Corrosion Effects on Patterned Copper-Zinc Samples during Exposure in Humidified Air Containing Formic Acid

Corrosion in indoor atmospheric environments is often triggered by carboxylic acids, especially at higher relative humidity. Micro-galvanic effects can lead to severe corrosion, particularly important in miniature devices with small-sized metallic components. To elucidate the mechanism of micro-galvanic corrosion effects, well-defined zinc-copper patterned samples were investigated during exposure to 100 ppb formic acid (HCOOH) and 80% relative humidity at 20°C. The corrosion effects were monitored quantitatively with in situ infrared absorption spectroscopy, and the corrosion products characterized with scanning electron microscopy, confocal Raman microscopy, and atomic force microscopy. The nature of corrosion on zinc on the patterned samples was compared with that on pure zinc and turned out to result, not only in several times higher corrosion kinetics, but also in different corrosion products with respect to distribution, morphology, and composition. Local electrochemical and chemical gradients across the copper-zinc borders resulted in characteristic hemispherically shaped corrosion products at the zinc-copper junction, and in the formation of zinc formate dihydrate (Zn(HCOO)2·2H2O) and crystalline zinc oxide (ZnO), phases not identified on pure zinc. In all, the micro-galvanic effects on the patterned samples resulted in accelerated corrosion kinetics and in structurally more developed corrosion products.

Nonlinear length dependent electrical resistance of a single crystal zinc oxide micro/nanobelt

We have systematically investigated the intrinsic electrical property of a single crystal zinc oxide (ZnO) micro/nanobelt (MB/NB) using a conductive atomic force microscopy (AFM) technique. By mounting one end of the MB/NB on a flat nonconductive silicon substrate, a platform for performing electrical property characterization using conductive current AFM is established. The quantitative characterization of the electrical resistance of the MB/NBs was performed by acquiring I-V curves for the MB/NB in between the electrode and the conductive AFM tip. The resistance of the single crystalline ZnO MB/NB was measured to be exponentially dependent on the length of the MB/NB. A systematic model based on the anisotropic velocity of the carriers in the crystal planes has been proposed and fits the experimental measurement well. This research reveals that the electrical resistance shows a nonlinear length dependence in the semiconducting single crystal MB/NB, which is significantly different from the bulk counterpart. Understanding such a property could definitely improve the design and the performance of next generation electrical nanodevices.

The Use of Additives to Control the Morphology of Thin Films Synthesized Using Aerosol Assisted Chemical Vapour Deposition

Abstract This paper identifies and discusses the use of additives to control the morphology of nanoscale crystalline zinc oxide films, grown using AACVD, principally from a solution of zinc acetate in methanol. We will discuss the use of both positively charged additives such as tetramethylammonium bromide and cetyltrimethylammonium bromide as well as negatively charged additives like sodium dodecylsulphate. Additives such as these, combined with other experimental parameters such as deposition time, have allowed us to control the size of the particles (from 100 to 400 nm in diameter), their orientation, and the regularity of their shape, from rough edged plates to well defined hexagons. We will show that the additives can be used to control morphology consistently over a temperature range of 250-400 °C, and that this technique can be applied generally to a number of additives and solvents.

Zinc hydroxide sulphate and its transformation to crystalline zinc oxide

The thermal transformation of zinc hydroxide sulphate hydrate to zinc oxide has been examined using synchrotron X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and surface area measurements. By collecting X-ray diffraction data in situ, we found that the dehydration of zinc hydroxide sulphate pentahydrate proceeded in discrete steps to form anhydrous zinc hydroxide sulphate. This compound then decomposed to a mixture of zinc oxide and a compound tentatively identified as Zn3(OH)2(SO4)2 at ~235 °C. At ~360 °C, the final dehydroxylation occurred with the formation of zinc oxy-sulphate, Zn3O(SO4)2, which then decomposed to ZnO at about ~800 °C. Interruption of the dehydration process can be used to synthesize the intermediate compounds.

N-ZnO nanorods/p+-Si (111) heterojunction light emitting diodes

In this study, we report the effects of thermal annealing in nitrogen ambient on the optical and electrical properties of zinc oxide (ZnO) nanorod (NR) arrays for the application in light emission diodes (LED). The single-crystalline ZnO NR array was synthesized on p+-Si (111) substrate without seed layer using simple, low-cost, and low-temperature hydrothermal method. The substrate surface was functionalized by hydrofluoric acid and self-assembled monolayer of octadecyltrimethoxysilane ((CH3 (CH2)17Si(OCH3)3). ZnO NRs were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and micro-photoluminescence (micro-PL). The results of FESEM and XRD indicate that single crystalline ZnO NRs with (002) preferred orientation along the substrate surface is successfully grown on functionalized p+-Si (111) substrate. The current–voltage and electroluminescence (EL) characteristics of the LED show that the most suitable annealing temperature ranges from 400°C to 600°C. Both PL and EL spectra show broadband emissions, ultraviolet and visible (green-yellow) light. The white-like light emission is able to be observed by naked eyes.

Chemically synthesized one-dimensional zinc oxide nanorods for ethanol sensing

One-dimensional (1D), single-phase (002) crystalline zinc oxide (ZnO) nanorod (NR) based conductometric sensors have been developed and investigated toward ethanol (C2H5OH) vapor. The ZnO NRs were chemically deposited onto conductometric transducer of alumina substrates employing a simple hydrothermal method. To facilitate the nucleation of ZnO NRs for oriented growth from the substrate, a seed layer of ZnO nanoparticles was pre-deposited employing filtered cathodic vacuum arc (FCVA) deposition technique onto the substrate containing pre-patterned interdigital electrodes (IDTs). Micro-characterization studies revealed that the NRs have a single crystal 1D structure with an average diameter of 30–50nm. The developed sensors were exposed toward ethanol of different concentrations at temperature up to 330°C. An optimum sensor operational temperature was found between 280 and 310°C. At this temperature range, high sensitivity, fast response and fast recovery in conjunction with a stable baseline occur. The long-term stability test of the developed sensors confirmed that they are highly stable with only less than 3% variation of the dynamic performances (baseline resistance, sensitivity, response and recovery time).

Influence of ammonia, lithium hydroxide, and hexamine on ZnO films synthesized by successive ionic layer adsorption and reaction technique

Various nanostructured zinc oxide (ZnO) films have been developed on non-seeded soda-lime glass substrates by successive ionic layer adsorption and reaction (SILAR) method, employing different baths of aqueous zinc sulfate solution using complexing agents of ammonia, lithium hydroxide, and hexamine. Influence of complexing agents on the structural, morphological, chemical compositional, optical and electrical properties of the films were investigated and compared among and with the samples annealed at 400 °C. Role of complexing agents on the growth process was discussed. X-ray diffraction analysis revealed that the ZnO films were polycrystalline dominating with (100), (002), and (101) oriented crystallites of hexagonal wurtzite structure. The lattice constants, c/a ratio, internal relaxation parameter, anion–cation (Zn–O) bond length, and the tetrahedral angles of ZnO films were determined. Cauliflower, flower, and flower-like nanorod morphologies of the films from the respective baths were exposed by scanning electron microscopic studies. Optical analysis of all films except that prepared from hexamine showed low absorbance and a higher transmittance 70–85 % in the entire visible region. Resistivity of annealed films was one order of magnitude lower than that of the as-grown films. We demonstrated utility of SILAR as an efficient single step soft chemical route for obtaining high-quality crystalline ZnO films having high-specific surface area on non-seeded substrates.

Study of the structural evolution of crystalline zinc oxide films prepared by PLD

AbstractThe structural evolution of nanocrystalline ZnO films on copper substrates synthesized via pulsed laser deposition was observed with increasing thickness controlled by deposition time. Scanning electron microscopy images indicate an increase in the average grain size. All the films are polycrystalline with the hexagonal wurtzite structure. X-ray diffraction patterns prove the orientation of the crystals to become better with an increase in film thickness. Raman measurements show three optical normal modes of crystalline ZnO indicating wurtzite structure. The line-width variation of the E2high mode reveals the structural improvement of the nanocrystalline ZnO films. The spatial correlation model was applied to analyze the structure evolution of the films. Different electron field-emission current densities are observed during the structural evolution of nanocrystalline ZnO films.

Preparation of crystalline zinc oxide films by one-step electrodeposition in Reline

One-step electrodeposition of crystalline zinc oxide thin films on Molybdenum/glass substrates in a low-cost ionic liquid, Reline composed of urea and choline chloride, is presented. X-ray diffraction spectroscopy shows the thin films have wurtzite structure. Raman microspectroscopy indicates a slight oxygen deficit in the film. Both X-ray energy dispersion and X-ray photonelectron spectroscopy reveal a 1:1 stoichiometric ratio of zinc to oxygen. UV–visible absorption spectra illustrate film transparency in the wavelength range of 190–900nm. The optimized film fabrication is attractive for applications in solar cells.

Defect induced high photocurrent in solution grown vertically aligned ZnO nanowire array films

Highly oriented and homogeneously distributed single crystalline zinc oxide nanowires (NWs) are fabricated on amorphous glass substrates using soft solution growth approach. The nanowire films and sol-gel grown ZnO films are devised and tested for UV light detection applying four-probe conductivity measurements. As-grown ZnO NWs film device demonstrates three orders enhancement (sensitivity = 440) in conductivity at room temperature under an illumination of 365 nm UV light, while the sol-gel based thick film reveals two orders of enhancement in device conductance. A clear correlation of conductivity and photoluminescence measurements suggest that surface oxygen vacancies (singly charged/Vo+) which render higher green defect luminescence intensity (IG/IUV = 1.8) in ZnO NWs leads to poor dark conductance and higher photo-conductance. Post growth annealing of nanowire arrays either in air (IG/IUV = 0.85) or oxygen ambience (IG/IUV = 0.38) results in reduction of green defects and corresponding suppression of photocurrent. Higher concentration of surface traps also leads to persistent photocurrent due to ionization of oxygen vacancies and creation of perturb host states under UV light excitation.

Room temperature ultra-sensitive resistive humidity sensor based on single zinc oxide nanowire

A highly sensitive relative humidity (RH) sensor is fabricated by a low-cost and scalable dielectrophoresis method that places a single crystalline zinc oxide nanowire (ZnO NWs) between pre-patterned electrodes in a freestanding device structure. The resistive device demonstrates an exponential sensitivity of more than 5 decades to changes from dry air to 60% RH at room temperature, indebted to a subthreshold carrier modulation due to the high surface-to-volume-ratio and completely exposed NW surface that facilitates surface physisorption of water molecules. Transient and temperature-dependence measurements signify a reproducible sensing performance and an Arrhenius behavior with activation energy of 0.6eV, expected for these metal oxide semiconductors. These results demonstrate the potential of such nanoscale devices for accurate monitoring of RH in different applications, including fuel cells and automotive sensors, smart clothing, smart food packaging and sensor networks.