ZnO Nanorods Grown on p-GaN Using Hydrothermal Synthesis and Its Optoelectronic Devices Application

The ZnO nanorods with the length of 1-1.5 μm were deposited on p-GaN by hydrothermal synthesis at low temperature 100°C. The structural and optical properties of the as-grown ZnO rods were investigated by X-Ray diffraction (XRD) and photoluminescence (PL) spectra. After annealing treatment the as-grown films in air at 600°C, 30min, and the ZnO rods showed good crystallinity and optical properties with strong UV emission at 378 nm. In addition, a sharp UV emission peak at 369.45 nm with the FWHM 20 meV, which attributed to the bound exciton recombination, was also observed from the ZnO rods at 80K. Next, the e-beam evaporation method was used to deposit metal contact on n-ZnO and p-GaN. Here, we use Au and Ni/Au as metal contacts for n-ZnO and p-GaN, respectively. The current-voltage characteristics of the fabricated n-ZnO/p-GaN heterojunction revealed rectifying behavior with a leakage current of 10⁻⁸ A at -10V, a forward current 4x10⁻⁶ A at 10V bias. The heterojunction also showed a good photoresponse, with the change of the current – voltage characteristics under ultraviolet illumination. Under UV illumination, the forward turn on voltage changed to 7.5V. This result showed the ability to manipulate the electron transport in the ZnO based heterojunction devices.Singapore-MIT Alliance (SMA

Random laser action in ZnO nanorod arrays embedded in ZnO epilayers

Random laser action with coherent feedback has been observed in ZnO nanorod arrays embedded in ZnO epilayers. The sample was fabricated by depositing a MgO buffer layer and followed by a layer of ZnO thin film onto a vertically well-aligned ZnO nanorod arrays grown on sapphire substrate. Under 355 nm optical excitation at room temperature, sharp lasing peaks emit at around 390 nm with a linewidth less than 0.4 nm has been observed in all directions. In addition, the dependence of the lasing threshold intensity on the excitation area is shown in good agreement with the random laser theory. Hence, it is demonstrated that random laser action can also be supported in ZnO nanorod arrays. (C) 2004 American Institute of Physics.open11173186sciescopu

The pH Response and Sensing Mechanism of n-Type ZnO/Electrolyte Interfaces

Ever since the discovery of the pH-sensing properties of ZnO crystals, researchers have been exploring their potential in electrochemical applications. The recent expansion and availability of chemical modification methods has made it possible to generate a new class of electrochemically active ZnO nanorods. This reduction in size of ZnO (to a nanocrystalline form) using new growth techniques is essentially an example of the nanotechnology fabrication principle. The availability of these ZnO nanorods opens up an entire new and exciting research direction in the field of electrochemical sensing. This review covers the latest advances and mechanism of pH-sensing using ZnO nanorods, with an emphasis on the nano-interface mechanism. We discuss methods for calculating the effect of surface states on pH-sensing at a ZnO/electrolyte interface. All of these current research topics aim to explain the mechanism of pH-sensing using a ZnO bulk- or nano-scale single crystal. An important goal of these investigations is the translation of these nanotechnology-modified nanorods into potential novel applications

Photoluminescence of spray pyrolysis deposited ZnO nanorods

Photoluminescence of highly structured ZnO layers comprising well-shaped hexagonal rods is presented. The ZnO rods (length 500-1,000 nm, diameter 100-300 nm) were grown in air onto a preheated soda-lime glass (SGL) or ITO/SGL substrate by low-cost chemical spray pyrolysis method using zinc chloride precursor solutions and growth temperatures in the range of 450-550°C. We report the effect of the variation in deposition parameters (substrate type, growth temperature, spray rate, solvent type) on the photoluminescence properties of the spray-deposited ZnO nanorods. A dominant near band edge (NBE) emission is observed at 300 K and at 10 K. High-resolution photoluminescence measurements at 10 K reveal fine structure of the NBE band with the dominant peaks related to the bound exciton transitions. It is found that all studied technological parameters affect the excitonic photoluminescence in ZnO nanorods

Microscopic origins of the surface exciton photoluminescence in ZnO nanostructures

Photoluminescence (PL) studies of the surface exciton peak in ZnO nanostructures at ∼3.367 eV are reported to elucidate the nature and origin of the emission and its relationship to nanostructure morphology. Localised voltage application in high vacuum and different gas atmospheres show a consistent PL variation (and recovery), allowing an association of the PL to a bound excitonic transition at the ZnO surface modified by an adsorbate. Studies of samples treated by plasma and of samples exposed to UV light under high vacuum conditions show no consistent effects on the surface exciton peak indicating no involvement of oxygen species. X-ray photoelectron spectroscopy data indicate involvement of adsorbed OH species. The relationship of the surface exciton peak to the nanostructure morphology is discussed in light of x-ray diffraction, scanning and transmission electron microscopy data

Electron Microscopy-based Study of Nanostructured ZnO:Morphological, Structural and Electrical Characterization

In the framework of the scientific plan 2009-2013, ZnO nanostructures have been studied at Center for Space Human Robotics (CSHR) Istituto Italiano di Tecnologia as candidates for tactile sensors and energy harvesting applications, such as dye-sensitized solar cells and mechanical harvesting. Part of this thesis work has been devoted to the morphological and structural characterization of all the ZnO nanostructures that have been synthesized at the CSHR. This activity required the use of electron microscopy; in particular, for the research activity reported in this thesis a dual-beam FIB-SEM workstation has been used for the morphological characterization of the samples and preparation of TEM samples, while TEM has been used for the morphological and structural analysis. Among the different ZnO nanostructures, ZnO nanowires are of particular interest for applications, since they have well-defined geometry at the nanoscale and they have controllable crystalline orientation. Part of this research activity has been devoted to the development of procedures for the in-situ electrical characterization of single ZnO nanowires, based on the electron beam and ion-beam induced deposition of contacts and subsequent two-probe electrical characterization performed in the dual-beam FIB-FESEM chamber by micromanipulators

Zinc Oxide Nanorod Based Ultraviolet Detectors with Wheatstone Bridge Design

This research work, for the first time, investigated metal semiconductor-metal (MSM) zine oxide (ZnO) nanorod based ultra-violet (UV) detectors having a Wheatstone bridge design with a high responsivity at room temperature and above, as well as a responsivity that was largely independent of the change in ambient conditions. The ZnO nanorods which acted as the sensing element of the detector were grown by a chemical growth technique. Studies were conducted to determine the effects on ZnO nanorod properties by varying the concentration of the chemicals used for the rod growth. These studies showed how the rod diameter and the deposition of ZnO nanorods from the solution was controlled by varying the concentration of the chemicals used for the rod growth. Conventional MSM UV detectors were fabricated with ZnO nanorods grown under optimized conditions to determine the dependence of UV response on electrode dimension and rod dimension. These studies gave insights into the dependence of UV response on the width of the electrode, spacing between the electrodes, density of the rod growth, and length and diameter of the rods. The UV responsivity was affected by varying the number of times the seed layer was spin coated, by varying the spin speed of seed layer coating and by varying the annealing temperature of the seed and rod. Based on these studies, optimum conditions for the fabrication of Wheatstone bridge UV ZnO nanorod detectors were determined. The Wheatstone bridge ZnO nanorod UV detectors were fabricated in three different configurations, namely, symmetric, asymmetric, and quasi-symmetric. The transient responses of the symmetric, asymmetric and quasi-symmetric configurations at room temperature and above showed how the response stability differed. At high temperature the responsivity of quasi-symmetric Wheatstone bridge detector configuration did not drop after saturation and the responsivity drifted by 17% to 25% from the room temperature response.The responsivity of quasisymmetric Wheatstone bridge configuration with good temperature stability was 1.16 A/W, while those of conventional MSM UV detectors were approximately 60 A/W. However, the quasi-symmetric Wheatstone bridge with responsivity 1.16 A/W was higher than the commercially available detector having responsivity of only about 0.1 A/W. Though the response of quasi-symmetric Wheatstone bridge detector was higher than the detectors available commercially, the response time was very high. The response time of quasi-symmetric Wheatstone bridge was approximately 159 seconds at room temperature, while that of commercially available detectors is of the order of microseconds. If the quasi-symmetric Wheatstone bridge has to compete with current commercially available detectors, then the response time should be brought down from seconds to microseconds. Based on these studies, an improved design of the quasi-symmetric Wheatstone bridge UV detector with the ZnO rods oriented parallel to the substrate instead of oriented vertical to the substrate was proposed