Femtosecond laser-induced one-step formation of microstructured ZnO on bulk Zn in ethanol

In this work, we demonstrate a one-step fabrication of microstructured ZnO surfaces via femtosecond laser irradiation of Zn metal immersed in ethanol. The process employs 200 kHz repetition rate pulses at a scanning speed of 1.36 mm s -1 and a line spacing of 1.7 µm. The resulting surface morphologies, characterized by SEM and 3D optical profilometry, reveal a strong dependence on laser fluence. At lower pulse energies, micro-rippled surfaces with superwavelength laser-induced periodic surface structures (LIPSS) dominate the central regions, with a transition from low- to high-spatial-frequency LIPSS toward the periphery. At higher pulse energies, columnar structures prevail across most of the irradiated area, except at the edges where LIPSS reappear. Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) analyses confirm that peripheral regions exhibit higher near-band-edge to defect emission ratios, indicating fewer oxygen vacancies, while central zones show stronger defect-related emission due to higher defect densities. XPS-derived O/Zn ratios corroborate this trend, with higher lattice oxygen content in less ablated areas. These findings reveal that femtosecond laser processing enables tunable defect engineering in ZnO, linking morphology and composition to spatial variations in optical properties.