Physical and optical effect of ZnO nanowalls to nanoflakes on random lasing emission

Random lasing emission was investigated from different ZnO nanostructures: a transition from nanowalls to nanoflakes; prepared on seeded glass substrate using simple chemical bath deposition method. The transition geometry of ZnO nanostructures were achieved simply by tuning the equimolar concentration of the chemical bath solution, zinc nitrate hexahydrate, and hexamethylenetetramine from 0.15 M, 0.20 M to 0.25 M. This leads to the changes in particle size of nanostructures from 168.47 nm to 365.91 nm, and finally to 534.07 nm. The population decreased from 131 nanostructures to 100, and finally to 26 nanostructures based on a surface area of 2.8 µm × 2.5 µm, and accordingly enhanced crystallite size was observed as 90.21 nm, 98.04 nm to 110.79 nm for the nanowalls to nanoflakes respectively. These physical variations of the scattering medium affect random lasing emission threshold whereby the threshold excitation power density was reduced from 40 kW/cm² to 17.14 kW/cm², and finally to 14.29 kW/cm² when the structure was in the form of nanoflakes. The emission wavelength from the structures shows a Stokes shift pattern and self-reliant while maintaining a stable emission at 379.7 nm (0.15 M), 382.7 nm (0.20 M), and 387.2 nm (0.25 M). This work demonstrates that by simply tuning the chemical bath solution molarities, changes in nanostructures from nanowalls to nanoflakes were obtained, and upon optical pumping, nanoflakes showed the lowest threshold for random lasing to occur indicating the best structure for ZnO random lasers.