Zinc Oxide Single Crystals Research Articles

Blue-emitting photoluminescence of rod-like and needle-like ZnO nanostructures formed by hot-water treatment of sol–gel derived coatings

The morphological evolution of the zinc oxide (ZnO) nanostructures generated by hot-water treatment (HWT) of sol–gel derived coatings as a function of temperature from 30 to 90°C was investigated. With increasing HWT temperature, the ZnO crystals evolved from nanoparticles to rod-like and needle-like nanostructures. High-resolution transmission electron microscope observations of rod-like and needle-like nanostructures generated at 60 and 90°C indicated single crystal ZnO wurtzite structure was obtained. All the hot-water treated samples exhibited blue emission at approximately 440nm in room temperature. The intensity of blue emission increased with higher HWT temperatures. The unique photoluminescence emission characteristic remained even after heat-treatment at 400°C for 1h. As the emission peak obtained in our work is approximately 440nm (2.82eV), the emission peak is corresponding to the electron transition from the interstitial Zn to the top of valence band. This facile formation of blue-emitting ZnO nanostructures at low-temperature can be utilized on substrate with low thermal stability for optoelectronic applications such as light emitting devices and biological fluorescence labeling.

The interaction of 193 nm excimer laser radiation with single-crystal zinc oxide: Generation of long lived highly excited particles with evidence of Zn Rydberg formation

In past studies, we have observed copious emissions of ionic and atomic Zn from single-crystal ZnO accompanying irradiation of single-crystal ZnO with 193-nm excimer laser irradiation at fluences below the onset of optical breakdown. The Zn+ and ground state Zn° are studied using time-of-flight techniques and are mass selected using a quadrupole mass spectrometer. Simultaneously, we have observed emitted particles that are detectable with a Channeltron electron multiplier but cannot be mass selected. It is a reasonable hypothesis that these particles correspond to a neutral atom or molecule in highly excited long lived states. We provide strong evidence that they correspond to high lying Rydberg states of atomic Zn. We propose a production mechanism involving laser excitation via a two photon resonance excitation of Zn°.

Optical and structural characterization of antimony doped ZnO single crystal.

We present the optical and structural study of the antimony doped zinc oxide (ZnO) performed by atomic diffusion. Diffusion is carried out at a temperature of 10000C for periods of 1, 2 and 4 hrs from a solid source prepared by partial oxidation of antimony, characterizing each step of the process was performed by photoluminescence (PL) and x-ray diffraction (XRD). The characterization by photoluminescence shows the effect of doping with antimony in radiative transitions caused by the creation of impurity levels. Doped samples were analyzed by high resolution diffraction and reciprocal space mapping which revealed the structural modification of the quality of the single crystal produced by the introduction of antimony. The results indicate that the antimony is introduced fully the volume of ZnO and does not modify the quality of the crystal.

Investigation of structural and photoluminescence properties of gas and metal ions doped zinc oxide single crystals

We report the structural and photoluminescence properties of annealing dependent and gas and metal ion implanted zinc oxide (ZnO) single crystals. Gd, Na and N ions were implanted into ZnO at an average implantation depth of 12, 24, and 38nm, respectively from the surface layer. The samples were annealed under high vacuum or oxygen at 650°C and their effects were studied. Raman spectra of Gd implanted and oxygen annealed ZnO revealed the A1 longitudinal optical, A1(LO) mode, usually assigned to intrinsic defects, which suggested a partial recovery of implantation induced disorders, unlike ion implanted and vacuum annealed ZnO. Low temperature photoluminescence spectra from unimplanted and Gd implanted ZnO revealed transitions from neutral bound exciton, two electron satellites (TES) and LO phonon replicas. Deep level transitions in unimplanted and Gd implanted ZnO were affected by the annealing atmosphere, and oxygen annealing reduced the deep level emission, suggesting oxygen vacancy related emissions. A small red-shift in the near-band edge emission and a blue-shift in deep level emissions were observed in N and Na implanted and annealed ZnO, indicating slight changes in their band gap energies.

Surface nanostructuring by ion-induced localized plasma expansion in zinc oxide

Creation of hillock-like nanostructures on the surface of zinc oxide single crystals by irradiation with slow highly charged ions is reported. At constant kinetic energy, the nanostructures were only observed after irradiation with ions of potential energies above a threshold between 19.1 keV and 23.3 keV. The size of the nanostructures increases as a function of potential energy. A plasma expansion approach is used to explain the nanostructures creation. The calculations showed that the surface nanostructures became taller with the increase of ionic temperature. The influence of charged cluster formation and the relevance of their polarity are discussed.

Physical model construction for electrical anisotropy of single crystal zinc oxide micro/nanobelt using finite element method

Based on conductivity characterization of single crystal zinc oxide (ZnO) micro/nanobelt (MB/NB), we further investigate the physical mechanism of nonlinear intrinsic resistance-length characteristic using finite element method. By taking the same parameters used in experiment, a model of nonlinear anisotropic resistance change with single crystal MB/NB has been deduced, which matched the experiment characterization well. The nonlinear resistance-length comes from the different electron moving speed in various crystal planes. As the direct outcome, crystallography of the anisotropic semiconducting MB/NB has been identified, which could serve as a simple but effective method to identify crystal growth direction of single crystal semiconducting or conductive nanomaterial.

ZnO Nanodisk Based UV Detectors with Printed Electrodes

The fabrication of highly functional materials for practical devices requires a deep understanding of the association between morphological and structural properties and applications. A controlled hydrothermal method to produce single crystal ZnO hexagonal nanodisks, nanorings, and nanoroses using a mixed solution of zinc sulfate (ZnSO4) and hexamethylenetetramine (HMTA) without the need of catalysts, substrates, or templates at low temperature (75 °C) is introduced. Metal-semiconductor-metal (MSM) ultraviolet (UV) detectors were fabricated based on individual and multiple single-crystal zinc oxide (ZnO) hexagonal nanodisks. High quality single crystal individual nanodisk devices were fabricated with inkjet-printed silver electrodes. The detectors fabricated show record photoresponsivity (3300 A/W) and external quantum efficiency (1.2 × 10(4)), which we attribute to the absence of grain boundaries in the single crystal ZnO nanodisk and the polarity of its exposed surface.

Effects of gas flow rate on structural features of ZnO nanowire arrays fabricated on silicon substrate

Arrays of single-crystal zinc oxide (ZnO) nanowires have been synthesized on silicon substrates by vapor–liquid–solid growth techniques. The effect of growth conditions including the substrate temperature and Ar gas flow rate on the growth properties of ZnO nanowire arrays were studied. Structural and optical characteristics were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. The XRD measurements showed that the single-crystal nanowires exhibited growth in the (002) direction. SEM images of the ZnO nanowire arrays grown at various Ar gas flow rates indicated that the alignment and structural features of the ZnO nanowires were affected by the gas flow rate. The PL of the ZnO nanowire arrays exhibited strong UV emission at 380 nm and weak green emission at approximately 510 nm. A blue shift and broadening of the UV emission were observed with increasing Ar gas flow rate.

Constrained, aqueous growth of three-dimensional single crystalline zinc oxide structures

We study low temperature (90 °C) aqueous growth of single crystal zinc oxide structures through patterned PMMA molds of different sizes, shapes, and orientations. We demonstrate the ability to create 3D shapes with smooth vertical sidewalls. Although the unconstrained growth is influenced by the hexagonal geometry of the underlying crystal structure, the ZnO is shown to conform exactly to any shape patterned. Using electron backscatter diffraction and scanning electron microscopy we show that the mold orientation, in conjunction with control of the growth rates of the c and m planes of the ZnO, is crucial in determining the final structure shape.

Slip systems in wurtzite ZnO activated by Vickers indentation on [formula omitted] and [formula omitted] surfaces at elevated temperatures

Dislocations were introduced in wurtzite zinc oxide single crystals by Vickers indentations on {21¯1¯0} and {101¯0} surfaces at elevated temperatures, and their slip systems were determined by transmission electron microscopy combined with etch pit observations. The observed system for {21¯1¯0} indentations was {101¯1¯}〈1¯21¯0〉 or (0001)〈1¯21¯0〉, while that for {101¯0} indentations was {101¯0}〈1¯21¯0〉. Activation of the slip systems was briefly discussed in terms of the Peierls stress and resolved shear stress acting on the slip systems.

Photosensitization of Single-Crystal ZnO by a Conjugated Polyelectrolyte Designed to Avoid Aggregation

A conjugated polyelectrolyte (CPE) based on a poly(phenylene ethynylene) backbone designed to avoid interchain aggregation was adsorbed onto n-type zinc oxide (0001) single crystals. Photophysical, atomic force microscopy, and photoelectrochemical measurements confirm the absence of aggregation in solution and on ZnO single-crystal surfaces. At high surface coverage on the ZnO surface, individual polymer chains are resolved, and photocurrent efficiency measurements suggest that charge injection occurs with modest efficiency.

The interaction of 193-nm excimer laser radiation with single-crystal zinc oxide: The generation of atomic Zn line emission at laser fluences below breakdown

The production of gas phase atomic and ionic line spectra accompanying the high laser fluence irradiation of solid surfaces is well known and is most often due to the production and interaction of high densities of atoms, ions, and electrons generated from laser-induced breakdown. The resulting plasma expands and moves rapidly away from the irradiated spot and is accompanied by intense emission of light. This type of “plume” is well studied and is frequently exploited in the technique of chemical analysis known as laser induced breakdown spectroscopy. Here, we describe a similar but weaker emission of light generated in vacuum by the laser irradiation of single crystal ZnO at fluences well below breakdown; this emission consists entirely of optical line emission from excited atomic Zn. We compare the properties of the resulting laser-generated gas-phase light emission (above and below breakdown) and describe a mechanism for the production of the low-fluence optical emission resulting from a fortuitous choice of material and laser wavelength.

The interaction of 193 nm excimer laser radiation with single-crystal zinc oxide: Neutral atomic zinc and oxygen emission

We report mass-resolved time-of-flight measurements of neutral particles from the (101¯0) surface of single-crystal ZnO during pulsed 193-nm irradiation at laser fluences below the threshold for avalanche breakdown. The major species emitted are atomic Zn and O. We examine the emissions of atomic Zn as a function of laser fluence and laser exposure. Defects at the ZnO surface appear necessary for the detection of these emissions. Our results suggest that the production of defects is necessary to explain intense sustained emissions at higher fluence. Rapid, clean surface etching and high atomic zinc kinetic energies seen at higher laser fluences are also discussed.

Influence of the Aggregation of a Carbazole Thiophene Cyanoacrylate Sensitizer on Sensitized Photocurrents on ZnO Single Crystals

Dye sensitization of zinc oxide single crystals by a carbazole thiophene cyanoacrylate (MK-2) sensitizer deposited from THF and mixtures of THF and water was investigated. AFM images show the formation of larger aggregates, with the maximum size of 20-30 nm from mixtures of THF and water, compared with 8-12 nm from pure THF. Sensitized photocurrent spectra were correlated with the morphological results from AFM imaging and indicate that aggregation in water results in less efficient sensitization of the ZnO substrate. The presence of the aggregation in solution due to water content was confirmed by absorbance and fluorescence spectroscopies.

ZnO nanocrystals with a high percentage of exposed [formula omitted] reactive facets for enhanced gas sensing performance

Zinc oxide single crystals with a high percentage of exposed {4 2¯ 2¯ 3¯} reactive facet were prepared by a facile hydrothermal route. X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission microscopy confirmed the faceted single crystal structure. Through the density functional theory (DFT) calculations, it is validated that the {4 2¯ 2¯ 3¯} surface of ZnO crystals has high surface energy. When used as a sensing material in gas sensors, ZnO crystals with dominating exposed {4 2¯ 2¯ 3¯} planes exhibited a superior sensitivity toward toxic and flammable gases.

Two-electron-satellite transition of donor bound exciton in ZnO: Radiative Auger effect

Two-electron-satellite (TES) transition of donor bound exciton (D0X) is an interesting many-body quantum process. In this letter, precise luminescence spectra and temperature behaviors of the TES transition of aluminum bound exciton in two kinds of zinc oxide single crystals were investigated in detail. It is found that the TES transition can be treated as a radiative Auger process in which the temperature dependence of the emission intensity of the transition can be fitted very well with a model taking into account the temperature dependent Auger term and thermal dissociation of the D0X excitons.

2D XAFS-XEOL Spectroscopy – Some recent developments

The use of optical photons to measure the modulation of the absorption coefficient upon X-ray excitation, or optical XAFS, is of particular interest for application to the study of light emitting semiconducting nanomaterials due to the additional information that may be gained. The potential for site-selectivity, elemental and excitation energy specific luminescence decay channels, and surface vs. bulk effects all make the use of X-ray excited optical luminescence (XEOL) desirable as a detection method. Previous experiments have made use of a monochromator to select the optical emission wavelength used to monitor optical XAFS. This method of detection suffers from the primary limitation of only being able to monitor the optical response at one emission wavelength. By combining the high resolution soft X-ray Spherical Grating Monochromator beam-line at the Canadian Light Source with an Ocean Optics QE 65000 fast CCD spectrophotometer and custom integration software we have developed a technique for collecting 2D XAFS-XEOL spectra, in which the excitation energy is scanned and a XEOL spectra is collected for every energy value. Herein we report the development of this technique and its capabilities using the study of the luminescence emitted from single crystal zinc oxide as an example.

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.

Growth of Single Crystal Zinc Oxide Beaded Nanowires

Reported here is the growth of single crystal zinc oxide beaded nanowires. The beaded nanowires are composed of nanobeads and ultrafine joint nanorods, which alternatively epitaxially grow along [001] direction of zinc oxide wurtzite structure. The growth mechanism was discussed and a zinc-rich process was proposed. Due to the size confinement, the bandgap along the beaded nanowires is modulated by their diameter and like multiple quantum well structure. Different from traditional multiple quantum well structures through alternate heterogeneous epitaxial growth of semiconductors, this kind of quasi-one-dimensional homogeneous multiple quantum well structures is composed of same substance. Their bandgap is modulated through alternately adjusting their diameter along single crystal nanowires. Further studies exhibit that they are of special properties on photoluminescence and Raman spectra. The demonstration would open the route to synthesize homogeneous multiple quantum well structures, study their fundamental physical phenomena, and exploit their applications.

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.