Synthesis of multi-color luminescent ZnO nanoparticles by ultra-short pulsed laser ablation

Crystalline ZnO nanoparticles (NPs) are synthesized by ultra-short femtosecond (fs) pulsed laser ablation (PLA) of a zinc plate in deionized water, and are investigated by optical absorption and time resolved luminescence spectra in combination with the morphology and structure analysis. The comparison with previous experiments based on short nanosecond (ns) PLA highlights that pulse duration is a crucial parameter to determine the size and the optical properties of ZnO NPs. While short PLA generates NPs with average size (S) over bar of (similar to)30 nm, ultrashort PLA allows to achieve much smaller NPs, (S) over bar <= 10 nm, that evidence weak quantum confinement effects on both the absorption edge and the UV excitonic emission. Moreover, the defective nature of synthesized ZnO NPs gives rise to two visible photoluminescence bands: the first, centered around 2.3 eV and commonly associated with oxygen vacancies, unexpectedly blue-shifts on decreasing the NPs size; the second, peaked at 2.8 eV, shows a Lorentzian shape characteristic of a mobile exciton. Overall, these results demonstrate that PLA is a successful method to synthesize ZnO NPs with controlled luminescence properties, thus increasing their application in lighting technologies.