二维层状半导体系统中激子态及其动力学过程的理论研究

Owing to the large exciton-binding energy, the effects of excitons and their dynamical processes dominate the optical properties of two-dimensional layered semiconductor materials. Although the binding energies of neutral excitons in two-dimensional layered semiconductors have been accurately calculated by using ab-initio approach, it is still very difficult to study the dynamical processes of such excitons at the level of ab initio. Moreover, the properties of charged excitons cannot be studied by using the state-of-the-art ab-initio method. In this project, post-processes are combined with ab-initio calculations to study excitons and their dynamical processes in layered semiconductors. With the single particle electronic and vibraitonal eigenmodes obtained from ab-initio calculations, the binding energies and wavefunctions of the neutral and the charged exciton states along with the exciton-vibration coupling matrix elements are calculated using configuration interaction and frozen-phonon approaches, respectively. Based on these calculations, the ultrafast radiation and non-radiation processes of excitons along with the quantum dephasing process of excitons induced by lattice vibrations in various two-dimensional layered semiconductor systems will be studied. The achievement of this project will provide a useful theoretical approach to study excitons and their dynamical processes in two-dimensional layered semiconductors and reveal the mechanisms of the time-dependent evolution of exciton states in such systems. This project will also give a theoretical guidance for the development of the two-dimensional layered material based semiconductor optoelectronic devices.

由于二维层状半导体材料中的强激子束缚能，激子态及其动力学过程成为影响这类材料光学性质的关键因素。现有的第一原理方法虽然能够精确地计算出二维层状半导体系统中电中性激子态的束缚能，但却很难描述其动力学过程，同时也无法处理带电激子态问题。本项目将第一原理与后续计算方法相结合，在第一原理计算得到的二维层状半导体系统单电子态及晶格振动态基础上，应用组态相互作用及冻结声子方法分别计算得到电中性及带电激子态的束缚能及波函数，以及激子态与晶格振动之间相互作用矩阵元。在此基础上，分别计算多种二维层状半导体系统中激子态的发光辐射及非辐射跃迁过程以及由晶格振动导致的失相等超快动力学过程。本项目的顺利实施将为研究二维层状半导体系统中激子态及其动力学过程提供新的理论方法，揭示激子态在这类材料中随时间演化过程的物理机制，并为设计新型二维层状半导体光电器件提供理论依据。

由于二维层状材料中强量子受限效应及介电受限效应，在二维层状半导体材料中存在着室温稳定的激子态，这使得激子及其动力学过程成为影响这类材料光学及电学性质的关键因素。现有的第一原理方法虽然能够精确计算出二维层状半导体系统中电中性激子态的束缚能，但却很难描述其动力学过程，同时也无法处理带电激子态问题。本项目将第一原理计算与后续计算相结合，发展出一套计算电中性及带电激子态束缚能及其波函数的新方法。应用这一理论计算了多种二维层状半导体材料及其异质结系统中的激子态及其在晶格振动及外电场作用下的演化过程，并将此方法推广至由二维层状半导体材料与II-VI族半导体量子点所组成的混合异质结体系中。此外，在本项目中基于第一原理方法分别计算了二维石墨烯量子点中晶格振动态及激子-声子相互作用，分析了石墨烯量子点尺寸、几何结构及边界形态对晶格振动及激子-声子相互作用的影响。为了将我们的理论方法推广至近年来广受关注的二维横向异质结及层间扭角异质结体系，我们还在第一原理计算基础上发展出一套用于计算大尺寸二维半导体异质结体系电子能态结构的原子有效赝势方法及一套计算晶格振动态的经验力场模型。在本项目的资助下我们还开展了半导体纳米团簇中激子-声子相互作用及表面声学波调制二维层状半导体异质结表面等离子体辐射等研究工作。本项目研究成果为研究二维层状半导体系统中激子态及其动力学过程提供新的理论方法，揭示了激子态在这类材料中随时间演化过程的物理机制，并为设计新型二维层状半导体光电器件提供相应理论依据。