ABSTRACT

ZnSe with a wide direct band gap (bulk crystal, ~2.7eV) is a wellknown II-VI semiconductor material, which is considered a good material for short-wavelength optoelectronic devices, such as LEDs and diode lasers working in the blue-light region, and photodetectors [82-85]. Controlling the size and dimensions of ZnSe may lead to further novel properties. For instance, ZnSe quantum dots show size-dependent optical properties [86-90]. Furthermore, ZnSe exhibits significantly larger exciton-binding energy (21 meV), which makes it an ideal candidate for efficient room temperature exciton devices and devices with improved temperature characteristics [91]. Triggered by this, investigation on ZnSe nanostructures, including one-dimensional nanostructures, and complex structures, especially the nanoparticles and quantum dots as they have many potential applications, has attracted much attention [83-85]. Similar to ZnO, ZnS, and CdS, ZnSe nanostructures with various morphologies, such as nanoparticles, quantum dots, nanowires, nanobelts (or nanoribbons), nanotubes, nanosaws, etc., have been successfully synthesized using gas-phase and/or solution-based methods, including CVD, MOCVD, sol-gel, precipitation, synthesis in reverse micelles, hot-injection method, solve-thermo, and so on [82, 84-86, 88-95]. To avoid repeating, in this section, we review some interesting growth behaviors of ZnSe nanostructures, namely, the size-dependent growth of ZnSe nanostructures and the controlled synthesis of ZnSe nanoparticles as they have very promising applications. Their novel size-dependent optical property and applications as LEDs will be also discussed. 6.5.1 Size-Dependent Growth of ZnSe Nanostructures

In, 2004, Li and coworkers have reported the interesting growth behavior of ZnSe nanowires, namely, the size-dependent growth of twinned ZnSe nanowires [89]. The ZnSe nanowires were grown via the VLS mechanism (the details can be found in Ref. [89]). Figure 6.22 is the TEM characterization of the ZnSe nanowires showing that the periodic light/dark contrast (low-magnification) results from the twins along the cubic ZnSe <111> crystalline

direction. By checking more than 100 nanowires, it is interesting to find that the diameter (D) varies as the twining period (P) changes. Figure 6.22a-l gives the typical low-magnification TEM images of the nanowires with diameters ranging from 150 nm to 36 nm. The corresponding twinning period of these nanowires changes from 12.0 nm to 1 nm, as clearly demonstrated by the HRTEM images (Fig. 6.22HR-a-l) [89].