Review on one-dimensional ZnO nanostructures for electron field emitters

The emission of electrons from the surface of a solid caused by a high electric field is called field emission (FE). Electron sources based on FE are used today in a wide range of applications, such as microwave traveling wave tubes, e-beam evaporators, mass spectrometers, flat panel of field emission displays (FEDs), and highly efficient lamps. Since the discovery of carbon nanotubes (CNTs) in 1991, much attention has been paid to explore the usage of these ideal one-dimensional (1D) nanomaterials as field emitters achieving high FE current density at a low electric field because of their high aspect ratio and "whisker-like" shape for optimum geometrical field enhancement. 1D metal oxide semiconductors, such as ZnO and WO3 possess high melting point and chemical stability, thereby allowing a higher oxygen partial pressure and poorer vacuum in FE applications. In addition, unlike CNTs, in which both semiconductor and metallic CNTs can co-exist in the as-synthesized products, it is possible to prepare 1D semiconductor nanostructures with a unique electronic property. Moreover, 1D semiconductor nanostructures generally have the advantage of a lower surface potential barrier than that of CNTs due to lower electron affinity and the conductivity could be enhanced by doping with certain elements. As a consequence, there has been increasing interest in the investigation of 1D metal oxide nanostructure as an appropriate alternative FE electron source to CNT for FE devices in the past few years. This paper provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on FE properties and applications of the most widely studied 1D ZnO nanostructures, such as nanowires (NWs), nanobelts, nanoneedles and nanotubes (NTs). We begin with the growth mechanism, and then systematically discuss the recent progresses on several kinds of important nanostructures and their FE characteristics and applications in details. Finally, it is concluded with the outlook and future research tendency in the area. © 2013 Higher Education Press and Springer-Verlag Berlin Heidelberg.