ABSTRACT
Due to the fact that there are tens of thousands of publications related to the II-VI semiconductor nanostructures, we are unable to list all the exciting achievements. We choose just a small part of particularly representative and instructive works in this field to reflect recent progress in the synthesis and application of II-VI semiconductor nanostructures, including ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, nanoscale, hetero-, core/shell, and hierarchical nanostructures, and their ternary alloys. 6.2 Zinc Oxide NanostructuresZnO crystallizes in the hexagonal WZ-type structure. It has a polar hexagonal plane, the c-plane [18, 20]. The lack of a centre of symmetry in WZ, combined with large electromechanical coupling, results in strong piezoelectric and pyroelectric properties and the consequent use of ZnO in mechanical actuators and piezoelectric sensors [18]. In addition, ZnO has a wide band gap (3.37 eV) with high exciton-binding energy (60 meV), which makes ZnO suitable for short-wavelength optoelectronic applications [6, 18]. Such unique properties make ZnO one of the few dominant nanomaterials for nanotechnology. ZnO has a wide range of applications in optics, optoelectronics, sensors, energy, biomedical sciences, and spintronics [6, 18]. Hitherto, ZnO has a most diverse group of growth morphologies, such as nanowires [21, 22], nanobelts [23], nanorings [24], nanocombs [25], nanohelixes, and nanosprings [26]. In the following section, we will discuss several pioneering works on ZnO nanostructures and their applications. 6.2.1 ZnO Nanowire Arrays in Anodic Alumina
MembranesZhang et al. have initially explored the synthesis of ZnO nanowire arrays in anodic alumina membranes (AAMs) and the measurements of their photoluminescence (PL) properties [21]. The synthesis of the ZnO nanowire arrays through an AAM-based method can be divided into two main steps. First, the pure zinc nanowires were electrodeposited into the AAM; then, the zinc nanowires embedded in the AAM were oxidized in air. The heat treatment was performed under different conditions. X-ray diffraction (XRD) characterization
of these samples shows that the zinc nanowires had been oxidized completely after heat treatment at 300°C for 35 hours. Figure 6.1 shows a transmission electron microscopy (TEM) (JEM-200 CX) image of an AAM with ZnO nanowires in its channels. Compared with the blank AAM, the dark and bright areas are corresponding to ZnO nanowires and the Al2O3 supporting frame, respectively. The ZnO nanowires with diameters equal to those of the nanochannels are distributed in the AAM periodically with a constant interval and form a parallely aligned array. The ZnO nanowires are polycrystalline, and the diameters range from 15 to 90 nm [21].
