Theoretical Studies Of Two-photon Absorption On Ⅱ-Ⅵ Group Semiconductor Nano Clusters

Due to the quantum confinement effect, semiconductor clusters have more large nonlinear refractive index and two-photon absorption cross section compared with bulk materials. And the absorption peak and absorption coefficient of two-photon absorption are closely related to cluster’s size and structure. The specific surface area, surface defect, surface ligand type and bonding mode of nano clusters have important influence on electronic structures and nonlinear optical properties of the system. So developing new calculation methods on theory, exploring different size structure’ characteristics and changing rules, giving a theoretical guidance for experiment are the important starting points of this article.So far, the semiconductor clusters’ research mainly focus on electronic structure and the second order hyper-polarizability. In this article, through using different functions and basis sets to calculate the two-photon absorption properties of Ⅱ-Ⅵ group small semiconductor clusters and comparing with CCSD a high accuracy but time-consuming method,we provide guidance for large cluster structures when selecting an appropriate calculation method and basis set for the calculation of two-photon absorption. This paper mainly contains the following parts:(1) The initial model is established and the structures of the semiconductor clusters are optimized.(2) Using various methods and basis sets to calculate the first four exciting energy of the structure, the appropriate density functional method and basis set are selected by comparison with precise CCSD method.(3) Two-photon absorption of the clusters are calculated and the structure of the clusters are also analyzed.(4) Explore the effect of NH3 ligand on two-photon absorption of Cd13Se13 quantum dots.Through reasonable calculation and comparative analysis, we obtain an effective method and basis set from small clusters which lays the foundation for the calculation of the structure of large clusters. At the same time, the effects of cluster size, structure and ligand on the two-photon absorption we studied, the rules were summarized and explained theoretically. They all provide theoretical support for the development of related experiments.