ABSTRACT
Figure 6.36 (a) A typical SEM image of individual ZnS/ZnO biaxial nanobelts. (b, c) The typical CL spectra obtained at RT and LT from these nanobelts. Reproduced from Nano Lett., 8, pp. 2794-2799 (2008). Copyright ©2008, American Chemical Society [155]. Abbreviations: RT, room temperature; LT, low temperature. See also Color Insert. Very recently, wide-bandwidth random lasing action is observed from the randomly assembled ZnS/ZnO biaxial nanobelt heterostructures under optical excitation [161]. The sample was optically excited by a frequency-quadruplet 266 nm pulsed neodymium doped yttrium aluminum garnet (Nd:YAG) laser with 120 ps pulse width and 10 Hz repetition rate. Emission was collected from the edge of the sample. As shown in the inset of Fig. 6.37, when pumping intensities reached certain pump intensities, shape peaks with line width of ~0.4 nm started to emerge from the emission spectra. Further increase in pump intensities increases the number of sharp peaks. Emissions at 332 nm, 383 nm, 483 nm, and 355 nm, which are attributed to the band-to-band transitions of ZnS, the radiative recombination of free excitons of ZnO, surface states of ZnS or ionized oxygen vacancy in ZnO and ZnS/ZnO heterostructure’s interface, respectively, were observed. Figure 6.37 also plots the
emission intensity of the ZnS/ZnO biaxial nanobelts at wavelengths around 332, 355, 383, and 483 nm versus pump intensities. It is noted that the lowest value of pump threshold, Pth, (i.e., sharp peaks are excited for pump intensity larger than Pth), which is equal to ~0.42 MW/cm2, occurred at a wavelength equal to ~483 nm. Value of Pth for the emission intensity at wavelength around ~383 nm (~332 nm) is ~0.5 MW/cm2 (0.57 MW/cm2). The highest lasing threshold, which is equal to ~0.7 MW/cm2, is detected at ~355 nm. These verified that the ZnS/ZnO biaxial nanobelts can provide high optical gain at wavelengths around 332, 355, 383, and 483 nm simultaneously. These results indicate that the novel ZnS/ZnO biaxial nanobelts are valuable not only with respect to fundamental research but also for new broad-range UV high-performance nanoscale lasers and other potential applications [161].
