Growth and photoluminescence dynamics of zno nanoparticles produced by pulsed laser ablation in liquid

Abstract

Zinc oxide (ZnO) is a wide band-gap semiconductor material (Eg=3.37 eV) characterized by many promising properties which make it appealing for several technological applications. One of the most attractive features is its high exciton binding energy (60 meV) which allows the observation of the UV excitonic emission at room temperature and favors the excitation of defect-related emission bands in the visible spectral region. In recent years, there has been an increasing interest in ZnO nanostructures due to their variety of morphologies and availability of simple and low cost processing methods. Among the techniques currently available, pulsed laser ablation in liquid (PLAL) offers a simple and versatile route to synthesize nanoparticles (NPs) of extremely high purity. In this Thesis we have reported an experimental study on ZnO NPs produced by PLAL providing a complete characterization of their morphological, structural and optical properties, as well as a deep insight on the formation processes involved during the NPs synthesis. Our investigation is founded on the combined use of several experimental techniques including in situ optical absorption (OA) and photoluminescence (PL), and ex situ force microscopy (AFM), high resolution TEM (HRTEM), Raman and time resolved. Microscopic analysis has evidenced that ns PLAL of zinc target in water produces ZnO nanocrystals with an average size of tens of nm having a wurtzite structure. Their UV-Vis absorption curve exhibits the typical edge of wurtzite ZnO and time resolved PL spectra show the corresponding excitonic PL peaked at 3.32 eV with a single exponential lifetime of 800 ps. ZnO NPs display a further PL peaking at 2.2 eV related to defects, which decays following a power law consistent with a recombination mechanisms where trapped electrons tunnel to recombination centers. Thermal annealing in O2 and in a He atmosphere produces a reduction of the A1(LO) Raman mode at 565 cm-1 associated with oxygen vacancies, accompanied by a decrease of defect-related emission at 2.2 eV. Based on our experimental results the emission at 2.2 eV is proposed to originate from a photo-generated hole in the valence band recombining with an electron deeply trapped in a singly ionized oxygen vacancy. In situ analysis has clarified the oxidation process of the ablated Zn, which occurs out of the plume region, due to the reaction of Zn NPs with water molecules. OA spectra exhibit a peak at 5 eV coming from surface plasmonic resonance (SPR) of Zn NPs and the typical absorption edge of the wurtzite ZnO, thus revealing transient Zn/ZnO core-shell NPs, which are fully oxidized only several hundreds of seconds after the end of PLAL. Defect-related PL arises with afurther delay time (100 s) indicating that the earliest oxidation of Zn essentially produces defect-free ZnO. We have also investigated the effects of laser parameters and of the solvent on the oxidation kinetics. The intensity ofthe SPR peak at 5 eV evidences that the Zn/ZnO ratio at the end of PLAL decreases on decreasing the laser repetition rates. Moreover, the oxidation rate of Zn NPs decreases on varying the mixture of water andethanol from 0 to 100%. These findings can be explained considering that initially produced Zn NPs are oxidized in two phases. An earliest and superficial oxidation, and a later and slower oxidation which is sensitive to the repetition rate, and it is completed only after the end of ablation thus leading to the disappearance of the metal species. Overall, our findings enrich the knowledge on the PL origin and dynamics of ZnO NPs useful in view of their use in new applications which exploit the enhancement of surface to volume ratio favoring the formation of luminescent defects as well as increase the material reactivity with the surrounding environment.