Prabakaran Shankar

Racetrack Effect on the Dissimilar Sensing Response of ZnO Thin Film-An Anisotropy of Isotropy.

The isotropic nature of the sensing elements decides the overall sensing performance of metal oxide gas/chemical sensors. Even a minimum deviation in the morphological and electrical characteristics of the sensing surface will lead to a nonuniform sensing performance, which in turn results in undesired figure of merits. With this background, the inhomogeneity of plasma discharge due to the racetrack effect of the magnetic field orbit in the planar magnetron and its significant influence on the formation of nanostructured ZnO thin films with desired uniformity has been investigated. The effect of the intensity of plasma discharges on the structural studies was a change in crystallite size from 11 to 35 nm. Anisotropic characteristics of the film influenced the mobility of carriers (10 and 220 cm(2) V(-1) s(-1)) by populating the carrier concentration (2.13 × 10(11) and 3.87 × 10(7) cm(-2)) in the nanostructures. Furthermore, the influence of this anisotropic surface of the obtained film on the room-temperature ethanol-sensing behavior is reported. The first observation of the racetrack effect on the sensing gradient of the sputter-deposited ZnO thin film has brought out the challenge in preparing an isotropic sensing element without anisotropy.

Electrospun tailored ZnO nanostructures – role of chloride ions

A novel way of transforming ZnO nanospheres to nanorods using an electrospinning technique has been identified as a simple method to control the growth of nanostructures. In this work, ZnO nanospheres and pencil like nanorods were successfully grown using an electrospinning technique with a precursor solution containing polyvinyl alcohol (PVA) and zinc acetate in the desired weight ratio. Also, to control the growth of ZnO nanostructures, chloride ions were used as a capping agent. The X-ray diffraction (XRD) pattern confirmed the influence of chloride ion concentration on the growth of ZnO nanostructures. Field emission scanning electron micrographs (FE-SEM) of as-deposited samples showed the formation of uniform and defect free ZnO–PVA composite nanofibers and the calcined samples revealed the growth of nanospherical and pencil like nanorod shaped ZnO. The growth mechanism, electrical characteristics as well as the room temperature ethanol sensing characteristics of the calcined ZnO nanostructures were investigated. The influence of morphology, grain and grain boundary resistances, activation energy of ZnO nanostructures and dissociation bond energy of ethanol molecules on the receptor and transduction functions of ZnO sensing elements has been discussed.

Room temperature ethanol sensing properties of ZnO nanorods prepared using an electrospinning technique

In recent years, the design of room temperature gas sensors has received major attention from researchers considering their deployment for real-time monitoring and power consumption. In this context, a novel and simple electrospinning route has been proposed to control the growth of ZnO nanorods with definite planes for achieving an enhanced room temperature response to ethanol vapor. The composite nanofibers [polyvinyl alcohol (PVA)–zinc oxide (ZnO)] were transformed to hexagonal wurtzite structured ZnO nanorods with flat, sharp pencil and blunt end shaped morphologies. X-ray diffraction patterns revealed the formation of high c-axis orientation with (002) (polar), (100) (nonpolar), and (101) (semi-polar) plane orientation towards the flat and pencil tip ended ZnO nanorods, respectively. The electrical parameters like carrier concentration, mobility, grain and grain boundary resistances and activation energy of the nanorods were measured and correlated with the room temperature (299 K) ethanol sensing performance of the ZnO nanorods. The flat-end loosely populated nanorods showed a higher sensing response of 26.4 for 500 ppm of ethanol, which is comparatively higher than previously reported responses.

Monomer: Design of ZnO Nanostructures (Nanobush and Nanowire) and Their Room-Temperature Ethanol Vapor Sensing Signatures

Ethanol serves as a biomarker as well as a chemical reagent for several applications and has been predominantly used as an alternative fuel (E10 and E85). Development of sensors for the detection and monitoring of ethanol vapor at lower operating temperatures has gathered momentum in the recent past. In this work, we reported the synthesis of self-assembled ZnO nanowires using electrospun technique without using any external surfactants or capping agents and their room temperature ethanol sensing properties. An inherent template namely monomer of the polymer poly(vinyl alcohol) (PVA) with two different molecular weights (14 000 and 140 000 g mol-1) was used along with the precursor zinc acetate dihydrate. The ZnO-PVA nanofibers have been tranformed to ZnO nanospheres and nanowires after calcination. The ratio of zinc precursor concentration to PVA polymer led to the enhanced carrier concentration of the resultant ZnO nanowire that enhanced, in turn, the sensing response toward ethanol vapor. The developed sensing elements have been systematically characterized to correlate their structural, morphological, and electrical properties with the respective room-temperature ethanol-sensing characteristics. The role of grain features and low activation energy of ZnO nanowires in coordination with the low dipole moment of ethanol resulted in the excellent response of 78 toward 100 ppm at room temperature with ultra-sensitive response and recovery times (9 and 12 s, respectively).