Defect engineering via post-annealing in Cu-doped ZnO thin films: Impact on optical and antibacterial performance

Copper-doped zinc oxide (ZnO:Cu) thin films were deposited by vacuum thermal evaporation then annealed at 300°C, 400°C, and 500°C in order to tailor defect states and surface-related functional properties. X-ray diffraction analysis revealed a transition from an amorphous structure in the as-deposited film to a crystallized hexagonal wurtzite phase after annealing. This structural evolution was accompanied by grain growth, lattice distortion, and changes in residual stress. Atomic force microscopy showed a progressive increase in surface roughness with increasing annealing temperature. Optical measurements showed that the transmittance increased in the visible range after annealing, reaching its highest value at 400°C, followed by a slight decrease at 500°C. The optical band gap decreased with increasing annealing temperature, from 3.82 eV for the as-deposited film to 3.45 eV for the film annealed at 500°C. This reduction is attributed to improved crystallinity, and modification of defect-related electronic states. Antibacterial tests against Escherichia coli demonstrated a marked enhancement in antibacterial activity for films annealed at higher temperatures, with complete inhibition of visible bacterial growth achieved at 500°C, highlighting the key role of annealing-induced surface defects and microstructural changes in governing the antibacterial functional performance of the films.