Few-layer graphene/ZnO nanowires based high performance UV photodetector

A graphene and zinc oxide nanowires (G/ZnO NWs) based ultraviolet (UV) photodetector presents excellent responsivity and photocurrent gain with detectivity. Graphene due to higher charge carrier transport mobility induces faster response to UV illumination at the interface between ZnO and graphene with improved response and decay times as compared to a ZnO NWs device alone. A linear increase is revealed for both the responsivity and photocurrent gain of the G/ZnO NWs device with the applied bias. These results suggest that the G/ZnO NWs device exhibits great promise for highly efficient UV photodetectors.

Synthesis of ZnO nanorods on a flexible Phynox alloy substrate: influence of growth temperature on their properties

A novel flexible alloy substrate (Phynox, 50 mm thick) was used for the synthesis of zinc oxide (ZnO) nanorods via a low-temperature solution growth method. The growth of ZnO nanorods was observed over a low temperature range of 60-90 degrees C for a growth duration of 4 hours. The as-synthesized nanorods were characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) for their morphology, crystallinity, microstructure and composition. The as-grown ZnO nanorods were observed to be relatively vertical to the substrate. However, the morphology of the ZnO nanorods in terms of their length, diameter and aspect ratio was found to vary with the growth temperature. The morphological variation was mainly due to the effects of the various relative growth rates observed at the different growth temperatures. The growth temperature influenced ZnO nanorods were also analyzed for their wetting (either hydrophobic or hydrophilic) properties. After carrying out multiple wetting behaviour analyses, it has been found that the as-synthesized ZnO nanorods are hydrophobic in nature. The ZnO nanorods have potential application possibilities in self-cleaning devices, sensors and actuators as well as energy harvesters such as nanogenerators.

ZnO quantum dots and graphene based heterostructure for excellent photoelastic and highly sensitive ultraviolet photodetector

Heterostructures comprised of zinc oxide quantum dots (ZnO QDs) and graphene are presented for ultraviolet photodetectors (UV PD). Graphene-ZnO QDs-graphene (G-ZnO QDs-G) based PD demonstrated an excellent UV photoresponse with outstanding photoelastic characteristics when illuminated for several cycles with a periodicity 5 s. PD demonstrated faster detection ability with the response and recovery times of 0.29 s in response to much lower UV illumination. A direct variation in photoresponse is revealed with the bias voltage as well as UV illumination intensity. A drastic reduction in the dark current is noticed due to potential barrier formation between adjacent ZnO QDs and the recombination rate reduces by directly transferring photogenerated charge carriers from ZnO QDs to graphene for enhanced the charge mobility.

Origin of giant dielectric constant and conductivity behavior in Zn1-xMgxO (0 <= x <= 0.1) ceramics

Zn1-xMgxO ( <= x <= 0.1) ceramics were fabricated by conventional solid-state reaction of co-precipitated zinc oxide and magnesium hydroxide nanoparticles. Structural and morphological properties of the fabricated ceramics were studied using X-ray diffraction and scanning electron microscopic analysis. The dielectric measurements of the ceramics were carried out as a function of frequency and temperature respectively. Interestingly, Mg doped ZnO (MZO) samples exhibited colossal dielectric response (similar to 1 x 10(4) at 1 kHz) with Debye like relaxation. The detailed dielectric studies and thermal analyses showed that the unusual dielectric response of the samples were originated from the defected grain and grain boundary (GB) conductivity relaxations due to the absorbed atmospheric water vapor (moisture). Impedance spectroscopy was employed to determine the defected grain and GB resistances, capacitances and which supported Maxwell-Wagner type relaxation phenomena. (C) 2015 Elsevier Ltd. All rights reserved.

Instability in an amorphous In-Ga-Zn-O field effect transistor upon water exposure

The instability of an amorphous indium-gallium-zinc oxide (IGZO) field effect transistor is investigated upon water treatment. Electrical characteristics are measured before, immediately after and a few days after water treatment in ambient as well as in vacuum conditions. It is observed that after a few days of water exposure an IGZO field effect transistor (FET) shows relatively more stable behaviour as compared to before exposure. Transfer characteristics are found to shift negatively after immediate water exposure and in vacuum. More interestingly, after water exposure the off current is found to decrease by 1-2 orders of magnitude and remains stable even after 15 d of water exposure in ambient as well as in vacuum, whereas the on current more or less remains the same. An x-ray photoelectron spectroscopic study is carried out to investigate the qualitative and quantitative analysis of IGZO upon water exposure. The changes in the FET parameters are evaluated and attributed to the formation of excess oxygen vacancies and changes in the electronic structure of the IGZO bulk channel and at the IGZO/SiO2 interface, which can further lead to the formation of subgap states. An attempt is made to distinguish which parameters of the FET are affected by the changes in the electronic structure of the IGZO bulk channel and at the IGZO/SiO2 interface separately.

Sandwiched assembly of ZnO nanowires between graphene layers for a self-powered and fast responsive ultraviolet photodetector

A heterostructure of graphene and zinc oxide (ZnO) nanowires (NWs) is fabricated by sandwiching an array of ZnO NWs between two graphene layers for an ultraviolet (UV) photodetector. This unique structure allows NWs to be in direct contact with the graphene layers, minimizing the effect of the substrate or metal electrodes. In this device, graphene layers act as highly conducting electrodes with a high mobility of the generated charge carriers. An excellent sensitivity is demonstrated towards UV illumination, with a reversible photoresponse even for a short period of UV illumination. Response and recovery times of a few milliseconds demonstrated a much faster photoresponse than most of the conventional ZnO nanostructure-based photodetectors. It is shown that the generation of a built-in electric field between the interface of graphene and ZnO NWs effectively contributes to the separation of photogenerated electron-hole pairs for photocurrent generation without applying any external bias. Upon application of external bias voltage, the electric field further increases the drift velocity of photogenerated electrons by reducing the charge recombination rates, and results in an enhancement of the photocurrent. Therefore, the graphene-based heterostructure (G/ZnO NW/G) opens avenues to constructing a novel heterostructure with a combination of two functionally dissimilar materials.

Sandwiched assembly of ZnO nanowires between graphene layers for a self-powered and fast responsive ultraviolet photodetector

A heterostructure of graphene and zinc oxide (ZnO) nanowires (NWs) is fabricated by sandwiching an array of ZnO NWs between two graphene layers for an ultraviolet (UV) photodetector. This unique structure allows NWs to be in direct contact with the graphene layers, minimizing the effect of the substrate or metal electrodes. In this device, graphene layers act as highly conducting electrodes with a high mobility of the generated charge carriers. An excellent sensitivity is demonstrated towards UV illumination, with a reversible photoresponse even for a short period of UV illumination. Response and recovery times of a few milliseconds demonstrated a much faster photoresponse than most of the conventional ZnO nanostructure-based photodetectors. It is shown that the generation of a built-in electric field between the interface of graphene and ZnO NWs effectively contributes to the separation of photogenerated electron-hole pairs for photocurrent generation without applying any external bias. Upon application of external bias voltage, the electric field further increases the drift velocity of photogenerated electrons by reducing the charge recombination rates, and results in an enhancement of the photocurrent. Therefore, the graphene-based heterostructure (G/ZnO NW/G) opens avenues to constructing a novel heterostructure with a combination of two functionally dissimilar materials.

Localized Charge Carrier Transport Properties of Zn1-x Ni (x) O/NiO Two-Phase Composites

We report the localized charge carrier transport of two-phase composite Zn1-x Ni (x) O/NiO (0 a parts per thousand currency sign x a parts per thousand currency sign 1) using the temperature dependence of ac-resistivity rho (ac)(T) across the N,el temperature T (N) (= 523 K) of nickel oxide. Our results provide strong evidence to the variable range hopping of charge carriers between the localized states through a mechanism involving spin-dependent activation energies. The temperature variation of carrier hopping energy epsilon (h)(T) and nearest-neighbor exchange-coupling parameter J (ij)(T) evaluated from the small poleron model exhibits a well-defined anomaly across T (N). For all the composite systems, the average exchange-coupling parameter (J (ij))(AVG) nearly equals to 70 meV which is slightly greater than the 60-meV exciton binding energy of pure zinc oxide. The magnitudes of epsilon (h) (similar to 0.17 eV) and J (ij) (similar to 11 meV) of pure NiO synthesized under oxygen-rich conditions are consistent with the previously reported theoretical estimation based on Green's function analysis. A systematic correlation between the oxygen stoichiometry and, epsilon (h)(T) and J (ij)(T) is discussed.

A Method for the Deposition of Transparent Conducting Thin-Films of Tin Oxide

A modified method has been developed for the deposition of transparent semiconducting thin films of tin oxide, involving the chemical vapour phase oxidation of tin iodide. These films show sheet resistances greater than 100 Ω/□ and an average optical transmission in the visible range exceeding 80%. The method avoids uncontrolled contamination, resulting in better reproducibility of the films. The films showed direct and indirect transitions and the possibility of an indirect forbidden transition. X-ray diffraction studies reveal that the films are polycrystalline. The low mobility values of the films have been attributed to the grain boundary scattering effect.

Microstructural evolution of tungsten oxide thin films

Tungsten oxide thin films are of great interest due to their promising applications in various optoelectronic thin film devices. We have investigated the microstructural evolution of tungsten oxide thin films grown by DC magnetron sputtering on silicon substrate. The structural characterization and surface morphology were carried out using X-ray diffraction and Scanning Electron Microscopy (SEM). The as deposited films were amorphous, where as, thin films annealed above 400 degrees C were crystalline. In order to explain the microstructural changes due to annealing, we have proposed a ``instability wheel'' model for the evolution of the microstructure. This model explains the transformation of mater into various geometries within them selves, followed by external perturbation.

High contrast laser writing in insitu textured germanium films via a novel chemical oxide formation

A high contrast laser writing technique based on laser induced efficient chemical oxidation in insitu textured Ge films is demonstrated. Free running Nd-YAG laser pulses are used for irradiating the films. The irradiation effects have been characterised using optical microscopy, electron spectroscopy and microdensitometry. The mechanism for the observed contrast has been identified as due to formation of GeO2 phase upon laser irradiation using X-ray initiated Auger spectroscopy (XAES) and X-ray photoelectron spectroscopy (XPS). The contrast in the present films is found to be nearly five times more than that known due to GeO phase formation in similar films.

Structural, optical and electrical characteristics of transparent bismuth vanadate films deposited on indium tin oxide coated glass substrates

Bismuth vanadate (BVO) thin films were fabricated on indium tin oxide (ITO) coated glass substrates using pulsed laser ablation technique and investigated their structural, optical and electrical properties. The use of the indium tin oxide coated glass substrate resulted in reducing the leakage current characteristics of crystalline BVO thin films. The X-ray diffraction (XRD) studies confirmed the monophasic nature of the post annealed (500 A degrees C/1 h) films. The atomic force microscopy indicated the homogeneous distribution of crystallites in the as-deposited films. The as-deposited and the post annealed films were almost 90% transparent (380-900 nm) as confirmed by optical transmission studies. Dielectric constant of around 52 was attained accompanied by the low dielectric loss of 0.002 at 10 kHz for post annealed films. The leakage current of the post annealed BVO films on ITO coated glass substrates measured at room temperature was 8.1 x 10(-8) A at an applied electric field of 33 kV/cm, which was lower than that of the films with platinum and SrRuO3 as the bottom electrodes.

Novel mechanism for high speed growth of transparent and conducting tin oxide thin films by spray pyrolysis

A novel mechanism is proposed for efficient manipulation of transport forces acting on the droplets during spray pyrolytic deposition of thin films. A ‘‘burst mode’’ technique of spraying is used to adjust the deposition conditions so as to transport the droplets under the new mechanism. Transparent, conducting thin films of undoped tin oxide prepared by this method showed significant improvement in growth rate. The films are found to be of fairly good quality with optical transmission of 82% and sheet resistance of 35 Ω/☒. The films are chemically homogeneous and grow preferentially along 〈200〉 direction.