Characterization and modeling of a ZnO nanowire ultraviolet photodetector with graphene transparent contact

We report the demonstration of a ZnO nanowire ultraviolet photodetector with a top transparent electrode made of a few-layered graphene sheet. The nanowires have been synthesized using a low-cost electrodeposition method. The detector is shown to be visible-blind and to present a responsivity larger than 10(4) A/W in the near ultraviolet range thanks to a high photoconductive gain in ZnO nanowires. The device exhibits a peak responsivity at 370 nm wavelength and shows a sub bandgap response down to 415 nm explained by an Urbach tail with a characteristic energy of 83 meV. The temporal response of the detector and the power dependence are discussed. A model of the photoconductive mechanism is proposed showing that the main process responsible for the photoconductive gain is the modulation of the conducting surface due to the variation of the surface depletion layer and not the reduction of recombination efficiency stemming from the electron-hole spatial separation. The gain is predicted to decrease at high incident power due to the flattening of the lateral band bending in agreement with experimental data. (C) 2013 AIP Publishing LLC

Titanium dioxide surface energy levels tuning by self-assembled monolayers

Tailoring the work function of functional layers in an optoelectronic device is an important means for performance improvement, as it results in changes in charge extraction or recombination. One way to proceed is to adsorb molecules with varying dipole moment strengths and signs. In this communication, the surface of anatase TiO2 is modified using different self-assembled monolayers (4-chlorobenzoic acid, 4-nitrobenzoic acid, 4-methoxybenzoic acid, and beta-alanine) and the energetics at the interface is determined using a series of photoelectron spectroscopy techniques, namely, ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, and inverse photoemission spectroscopy. The observed changes in work function are correlated with the dipole moments of the respective acids, calculated by density functional theory. Finally, the relevance of this interfacial engineering for controlling the charge extraction from an optoelectronic device is illustrated. Published under an exclusive license by AIP Publishing

Highly Sensitive and Selective Hydrogen Single-Nanowire Nanosensor

Metal oxides such as ZnO have been used as hydrogen sensors for a number of years. Through doping, the gas response of zinc oxide to hydrogen has been improved. Cadmium-doped ZnO nanowires (NWs) with high aspect ratio have been grown by electrodeposition. Single doped ZnO NWs have been isolated and contacted to form a nanodevice. Such nanosystem demonstrates an enhanced gas response and selectivity for the detection of hydrogen at room temperature compared to previously reported H2 nanosensors based on pure single-ZnO NWs or multiple NWs. A dependence of the gas response of a single Cd–ZnO nanowire on the NW diameter and Cd content was observed. It is shown that cadmium-doping in single-crystal zinc oxide NWs can be used to optimize their response to gases without the requirement of external heaters. The sensing mechanisms responsible for such improved response to hydrogen are discussed

Effects of annealing on properties of ZnO thin films prepared by electrochemical deposition in chloride medium

The development of cost-effective and low-temperature synthesis techniques for the growth of high-quality zinc oxide thin films is paramount for fabrication of ZnO-based optoelectronic devices, especially ultraviolet (UV)-light-emitting diodes, lasers and detectors. We demonstrate that the properties, especially UV emission, observed at room temperature, of electrodeposited ZnO thin films from chloride medium (at 70 °C) on fluor-doped tin oxide (FTO) substrates is strongly influenced by the post-growth thermal annealing treatments. X-ray diffraction (XRD) measurements show that the films have preferably grown along (0 0 2) direction. Thermal annealing in the temperature range of 150–400 °C in air has been carried out for these ZnO thin films. The as-grown films contain chlorine which is partially removed after annealing at 400 °C. Morphological changes upon annealing are discussed in the light of compositional changes observed in the ZnO crystals that constitute the film. The optical quality of ZnO thin films was improved after post-deposition thermal treatment at 150 °C and 400 °C in our experiments due to the reducing of defects levels and of chlorine content. The transmission and absorption spectra become steeper and the optical bandgap red shifted to the single-crystal value. These findings demonstrate that electrodeposition have potential for the growth of high-quality ZnO thin films with reduced defects for device applications

Comparative study of hydrothermal treatment and thermal annealing effects on the properties of electrodeposited micro-columnar ZnO thin films

We report a comparison of the role played by different sample treatments, namely, a low-temperature hydrothermal treatment by hot H2O vapor in an autoclave versus thermal annealing in air on the properties of ZnO films grown by electrochemical deposition (ECD). Scanning electron microscopy studies reveal a homogeneous micro-columnar morphology and changes in the film surface for the two different treatments. It is found that post-growth hydrothermal treatments of ECD ZnO films at 150 °C under an aqueous environment enhance their structural and optical properties (photoluminescence, transmission, Raman spectra, etc.) similar to thermal annealing in air at higher temperatures (> 200 °C). The modifications of the structural and optical properties of ZnO samples after thermal annealing in air in the temperature range of 150–600 °C are discussed. The removal of chlorine from the films by the hydrothermal treatment was evidenced which could be the main reason for the improvement of the film quality. The observation of the enhanced photoluminescence peak at 380 nm demonstrates the superior properties of the hydrothermally treated ZnO films as compared to the films annealed in air ambient at the same or higher temperature. This post-growth hydrothermal treatment would be useful for the realization of high performance optoelectronic devices on flexible supports which might not withstand at high temperature annealing treatments