Morphology-controlled random lasing in ZnO nanorods and nanowalls

Random lasing in zinc oxide (ZnO) is strongly influenced by nanoscale morphology. Here, one-dimensional (1D) ZnO nanorods were synthesized by chemical bath deposition on a ZnO seed layer, while two-dimensional (2D) ZnO nanowalls were grown on a DC-sputtered Al metal seed layer. Both morphologies exhibit comparable crystallite sizes (∼56 nm) and a preferred (002) orientation. Photoluminescence shows dominant near-band-edge emission, indicating good optical quality. The optical band gaps extracted from PL are 3.28 eV for nanorods and 3.20 eV for nanowalls, while Tauc analysis from UV-Vis spectra gives apparent optical band gaps of 3.22 eV and 3.75 eV, respectively. The higher apparent value obtained for the nanowalls reflects changes in the absorption edge, which may be influenced by band-tail disorder, surface states, and Al-related carrier effects. Under optical pumping, the 1D nanorods show predominantly single-mode random lasing with thresholds of 3.82-4.20 mJ/cm². In contrast, the 2D nanowalls exhibit position-dependent single-mode, dual-mode, and three-mode random lasing with generally lower thresholds of 2.55-3.48 mJ/cm². Fourier transform analysis reveals position-dependent optical cavity lengths, indicating that different recurrent scattering paths are formed at different excitation positions. These results suggest that the interconnected 2D ZnO nanowalls interconnected network contributes to more efficient optical feedback than the 1D nanorods, contributing to reduced random lasing thresholds and diverse lasing mode behavior. This work highlights morphology-driven design routes for low-cost ZnO-based photonic light sources.