International Materials Reviews

ZnO and the related ZnMgO and ZnCdO ternary solid solutions are possible alternatives to GaN-based compounds for the fabrication of ultraviolet (UV)/blue light emitting diodes (LEDs) and injection laser diodes. The ZnO materials system has advantages in terms of the higher binding energy of excitons (60 meV), which leads to efficient electron-hole pair-to-photon conversion at the elevated operating temperatures likely for such devices, the availability of commercially available high-quality ZnO substrates, lower growth temperatures (by at least 200°C) and simplicity of selective wet etching processes for mesa formation in devices. Progress in development of ZnO LEDs has been disappointing due to the difficulty of achieving robust p-type doping and the low crystal quality of heterojunctions and quantum wells. We critically review reports of p-type doping using group V impurities as well as progress in growing ternary ZnMgO and ZnCdO films. We also summarise recent progress and prospects for further advancement of ZnO-based light emitters.

Localised corrosion is a cause of unanticipated and sometimes catastrophic failures of equipment, transport vessels and infrastructures. Therefore, the development of modern corrosion-resistant materials and inhibitors by design is both technically and economically attractive. In the coming decades, industrial components will be engineered from molecular structures. This prospect provides the designer with a truly enormous range of choices in design, which is a situation that demands predictive tools that can link molecular structures with the final component performance. In particular, the development of alloys and inhibitors can replace the use of toxic compounds in protecting metal surfaces. To execute a tailored design programme, it is necessary to understand how corrosion and the associated processes occur from the molecular level to the component level and how the overall system behaviour emerges because of the inherent links among different scales. Therefore, in the present work, the literature on theoretical modelling of localised corrosion and related experimentation are reviewed from a multiscale viewpoint. The review addresses (a) the challenges in the theoretical formulation of the important phenomena that influence localised corrosion and (b) the hurdles facing computational methods. It is shown that (i) the existing models lack the resolution to design effective corrosion-resistant systems, (ii) the numerical strategies for linking the scales are in a state of evolution and (iii) there are gaps in the experimental characterisation of the corrosion system, particularly at the lower end of the scales. Suggestions are provided towards the construction of a multiscale model (MSM) for localised corrosion.