Cooperative effects of zinc interstitials and oxygen vacancies on violet-blue photoluminescence of ZnO nanoparticles: UV radiation induced enhanced latent fingerprint detection

Understanding the mechanisms of the violet-blue (VB) photoluminescence (PL) emissions is very important for possible applications of ZnO nanoparticles (NPs) in optoelectronics and forensic. Origin of the VB PL emissions of ZnO NPs of sizes of 27–54 nm synthesized in water using chemical co-precipitation method at a temperature of ∼100 oC for heating duration of 1–3 h, respectively, has been established here. For both below band gap excitation (BBE) and above band gap excitation (ABE), narrow ultraviolet and broad visible PL emissions including VB emissions have been observed. For BBE, VB emission is due to the direct transition of electrons from Zni/ex-Zni to valence band edge. The weak VB emission in case of ABE, usually ignored in earlier studies, has been studied here very carefully and systematically. The two predominant defect states and their cooperative effects are clearly found to be responsible for VB emissions. The Zni and Vo defect centers have been confirmed from the Raman and electron paramagnetic resonance spectroscopic measurements. As an important forensic applications, detection of latent finger prints (taken on different substances) using as-synthesized ZnO NPs through PL emissions has been demonstrated by illuminating using ultraviolet light even after 40 days.