Fabrication and photoluminescence studies of porous ZnO nanorods

Zinc-oxide (ZnO) nanorods were grown using the hydrothermal method, and nanosized pores were formed on their surfaces by thermal annealing at 700 A degrees C for 20 min under an argon atmosphere. The structural and the optical properties of the porous ZnO nanorods were investigated using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-dependent photoluminescence. The average diameter and density of the nanosized pores were 32 nm and 7 x 10(10) cm(-2), respectively. The crystal quality, stoichiometry, and optical properties of the porous ZnO nanorods were enhanced in comparison with those of the as-prepared ZnO nanorods. The activation energies for the exciton with increasing temperature in the as-prepared and the porous ZnO nanorods were 27.0 and 37.8 meV, respectively. At temperatures of 50 K < T < 125 K, the free exciton emission peak energy of the porous ZnO nanorods exhibited an anomalous behavior. The values of the fitting parameters in Varshni's empirical formula were alpha = 1 x 10(-3) eV/K and beta = 720 K for the as-prepared ZnO nanorods and alpha = 1 x 10(-3) eV/K and beta = 710 K for the porous ZnO nanorods. With increasing temperature, the exciton radiative lifetime of the porous ZnO nanorods exhibited an inverted S-shape while that of the as-prepared ZnO nanorods increased linearly.