Two-photon Absorption In ZnO/ZnS Core/Shell Quantum Dots

Abstract:We have studied the typically type-II core-shell structure based on ZnO-based nanocomposites materials which is different from the type-I core-shell structure semiconductor quantum dots. The type-Ⅱ core-shell structure semiconductor can control the relative motion of energy level by changing the core radius and shell thickness because of the dislocation structure. What’s more, all the relevant various physical processes can regulate by artificial regulator, such as emission wavelength, radiation lifetime, fluorescence quantum yield, Auger effect, Exciton-exciton interaction. This is one of the most important significance about studying the type-Ⅱ core-shell structure semiconductor quantum dots. Hence, studying the preparation of the type-Ⅱ core-shell structure semiconductor quantum dots and the character of nonlinear optics have important significance not only in theoretical side but also in practical applications. In our paper, we have studied the preparation of based on ZnO-based core-shell structure semiconductor quantum dots and two-photon absorption. The research contents and results are listed as follows:(1) We use the hydrothermal synthesis method preparation of the samples:the ZnO-based of ZnO/ZnS, ZnO/ZnS/Ag core/shell structure quantum dots. With the experimental and the simulated diffraction patterns and X ray diffraction analysis, it can be deduced that the ZnO/ZnS/Ag core/shell nanoparticles consist of ZnO core and ZnS shell with hexagonal structure as well as Ag nano-dots with cubic structure. The high resolution transmission electron microscopy images clearly reveal that the ZnO/ZnS core/shell QDs are crystalline with an average diameter of～9nm.(2) Through the one-photon absorption spectra of the ZnO/ZnS and ZnO/ZnS/Ag core/shell QDs in DMF solution, we can obtain that the lowest excitonic transition is located at～350nm. And compared to the PL spectra of the pure ZnO QDs and ZnO/ZnS QDs, we obtain that the strong emission band (450～650nm) in pure ZnO QDs could be attributed to the carrier recombination of the surface defect states, which are mostly suppressed in ZnO/ZnS core/shell QDs due to the surface passivation.(3) We have studied on two-photon absorption (2PA) in ZnO/ZnS and ZnO/ZnS/Ag Core/Shell quantum dots, determined by both Z-scan and pump-probe techniques with femtosecond laser pulses. The nonlinear optical properties of these samples with different intensity are studied in detail. The results indicate that at the laser pulses of wavelengths660nm, with the increase of the intensity, the same sample of nonlinear absorption is stronger. When the light intensity is the same, nonlinear absorption of ZnO/ZnS/Ag Core/Shell quantum dots is stronger than the ZnO/ZnS Core/Shell quantum dots.In conclusion, the observed TPA in ZnO/ZnS nanocomposites is an instantaneous nonlinear process and is greatly enhanced compared to that in ZnS, ZnSe, and CdS QDs, which demonstrates that ZnO-based nanocomposites might be a promising candidate for photonic device applications at room temperature.