ZnO – nanostructures , defects , and devices

The interest in ZnO structures has increased drastically in recent years. Intense research by many different groups has focused on novel nanostructures with different shapes ranging from nanowires to nanobelts and even nanosprings. The number of reviews on ZnO is increasing (see for example1-9). The topic of p-doping is especially difficult and has been investigated by different groups10-13. ZnO is a direct wide bandgap (~3.3 eV)2 material with a large exciton binding energy of 60 meV. This makes it interesting as a laser material based on exciton recombination at room temperature or even higher. Such a large amount of research on ZnO nanomaterials makes it hard to summarize the work in a systematic way. In this article, we first describe different synthesis routes toward a large variety of nanostructures. The first approach is chemical vapor deposition; certainly one of the most common routes for the synthesis of nanowires, nanobelts, nanosprings, and nanorings. Aqueous solution growth has also attracted great interest. This method allows growth of nanowires and other nanostructures at low temperatures. We describe this method in some detail. Similar to aqueous solution growth, electrodeposition of ZnO nanocolumns can be performed at low temperatures, and this is also briefly described. A very important issue is the doping of ZnO nanowires. We give an introduction into the defect states of ZnO and describe different examples where doping of ZnO nanowires and thin films has been performed. The last section focuses on applications of ZnO nanowires. We describe their use as sensors and cantilevers, as well as their ZnO has received much attention over the past few years because it has a wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, wide-bandgap semiconductivity, room-temperature ferromagnetism, and huge magnetooptic and chemical-sensing effects. Without much effort, it can be grown in many different nanoscale forms, thus allowing various novel devices to be achieved. We review recent studies of ZnO nanostructures, fabrication, novel device applications, and its potential as an electronacceptor material in hybrid solar cells. Control of its rich defect chemistry, which is critical for controlling properties but has not been widely addressed in the context of novel applications, is also discussed.