https://www.repository.cam.ac.uk/bitstream/1810/264501/1/Fu_et_al-2017-Progress_in_Material_Science-VoR.pdf

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

Homogenous crystallization in solution, in the absence of external influences, is expected to lead to growth that is symmetric at least in two opposite facets. Such was not the case when we attempted to synthesize ZnO nanostructures by employing a solvothermal technique. The reaction product, instead, consisted of bullet-shaped tiny single crystals with an abrupt hexagonal base and a sharp tip. A careful analysis of the product and the intermediate states of the synthesis reveals that one of the reaction intermediates with sheet-like morphology acts as a self-sacrificing template and induces such unexpected and novel growth. The synthesis was further extended to dope the nanobullets with nitrogen as previous studies showed this can induce p-type behavior in ZnO, which is technologically complementary to the naturally occurring n-type ZnO. Herein, a soft-chemical approach is used for the first time for this purpose, which is otherwise accomplished with high-temperature techniques. Cathodoluminesce (CL) investigations reveal stable optical behavior within a pure nanobullet. On the other hand, the CL spectra derived from the surfaces and the cores of the doped samples are different, pointing at a N-rich core. Finally, even though N-doped ZnO is known to have high electrical conductivity, the study now demonstrates that the field-emission properties of ZnO can also be greatly enhanced by means of N doping.

Solvothermal synthesis, cathodoluminescence, and field-emission properties of pure and N-doped ZnO nanobullets

We build sensors, capable of detecting and measuring ultraviolet (UV) light, by depositing zinc oxide (ZnO) powder from a solvent suspension over common white paper. Although these sensors are easy to fabricate and require inexpensive materials, they feature characteristics similar to those of UV sensors made with complex and expensive procedures. The good performance in terms of conductivity change of our simple devices can be attributed to the conductivity and porosity properties of paper, which effectively binds the ZnO crystals. We perform analyses using quantum chemistry methods to describe possible mechanisms that explain the conductivity changes observed on the ZnO surface due to doping interactions with interstitial hydrogen and doping depletion caused by oxygen adsorption.

ZnO-Paper Based Photoconductive UV Sensor

An ultraviolet photosensor has been successfully constructed by the in situ lift-out method in a focused ion beam system. The prototype consists of individual naturally self-assembled crossed ZnO nanorods grown by an aqueous solution process. The current–voltage (I–V) characteristics show linear behavior. The photosensor exhibits a response of ∼15 mA/W for UV light (361 nm) under 1 V bias. Response measurements showed that such a photosensor is suitable for low levels of ultraviolet detection. The method is simple, rapid and applicable to research prototypes for further studies of crossed ZnO nanorods for nano-device applications.

https://physics.ucf.edu/~lc/X_ZnO_Sensor_09.pdf

Crossed zinc oxide nanorods for ultraviolet radiation detection

We report a simple and inexpensive way for the preparation of highly transparent ZnO thin films and their application as active layer in UV ray sensor devices. ZnO thin films were deposited on glass substrates by thermal evaporation of pure ZnO powder. The as-deposited films were then annealed at different temperatures (100, 200, 300 and 400 °C) for various time durations (5, 15, 25 and 35 min) to make optically transparent in the visible region. The films annealed at 300 °C for 15 min show very good visible transparency and other material properties. These films were used as the active material for Ag/ZnO/Ag UV sensor devices. The sensor devices are photo conductive type and only sensitive in the UV region of the electromagnetic spectrum. Maximum photo-current gain of the UV sensor device is ∼2. Possible sensing mechanism has been discussed.

Preparation of transparent ZnO thin films and their application in UV sensor devices

Homoepitaxial growth of non-polar ZnO (112¯0) films on off-angle ZnO substrates by MOCVD

Photoconductivity of ZnO single crystals grown by a hydrothermal method for ultraviolet light was measured. The ZnO single crystals have photo response in a range of wavelengths shorter than 380 nm. O-faces of ZnO single crystals of the c -plane showed higher photocurrents and wider spectral response than did Zn-faces. It is thought that band bending at the surface regions of ZnO crystals is large at the O-face but small at the Zn-face, because the surface state density for electron capture at the O-face was higher than that of the Zn-face and, therefore, carriers generated at the surface region by UV light were effectively separated at the O-face region before their recombination. The difference between photoresponces of the O-face and the Zn-face is also useful for determining the polarity of a ZnO crystal. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

F. Masuoka

Papers

Homoepitaxial growth of non-polar ZnO (112¯0) films on off-angle ZnO substrates by MOCVD

Applicability of ZnO single crystals for ultraviolet sensors

Comparison of non-polar ZnO (112¯0) films deposited on single crystal ZnO (112¯0) and sapphire (011¯2) substrates

Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method