二维层状纳米材料的谷极化与谷电子/自旋弛豫动力学研究

In recent years, the graphene-like novel two dimensional (2D) layered transition metal dichalcogenides, such as MoS2 and WSe2, have attracted great attention in the research fields of physics, chemistry, material, electronics so on due to their novel electronic and optical properties. The 2D layered crystalline materials build the bridge between the valleytronics and optoelectronics, as well as the spintronics. This proposal will focus on the doped or undoped monolayer, bilayer and few-layer 2D crystalline materials fabricated by the mechanical exfoliation with adhesive tape from bulk crystals onto 300nm-thick SiO2 on Si substrates. The exciton and related photoluminescence properties as well as the valley polarization controlled by excitation light helicity will be investigated first, then the ultrafast dynamics of the valley polarization, valley electron/spin at different temperature, excitation wavelength and power density, external fields, as well as different material design will be further investigated experimentally in detail. The studied key physics is the influence of intravalley and intervalley electron-electron scattering, electron-phonon scattering; and the electron-hole exchange interaction, spin-orbit coupling, as well as the defects and disorder of the material on the exciton dynamics and the lifetimes of valley polarization and valley electron/spins. The ultimate goal of this proposal is to understand the fundamental physics of the excitonic radiative dynamics; the information about the lifetimes of valley polarization and valley electron/spins and the related physical mechanisms. The research output of this proposal will be fundamentally important aiming for the development of valley-dependent novel optoelectronics and spintronics based on 2D layered crystalline materials.

类石墨烯新型二维层状原子晶体材料由于其独特的光、电性质，构筑了谷电子学、光电子学与自旋电子学的应用桥梁，近年来引起物理、化学、材料、电子等众多领域研究人员的广泛关注。本申请项目拟以机械剥离的掺杂或者非掺杂单层、双层和多层新型二维层状原子晶体材料WSe2、MoS2等为研究对象，通过对其激子及其荧光性质和能谷极化响应的研究，以及谷极化、谷电子/自旋在不同温度、光激发波长（能量）与功率密度，以及外场和材料设计调控下的弛豫动力学过程研究，以期获得谷内与谷间电子－电子散射、电－声子散射，以及电子－空穴交换耦合、自旋－轨道耦合、材料缺陷和无序等影响激子复合寿命、谷极化寿命和谷电子/自旋寿命的信息，从而对激子辐射复合发光动力学过程以及激子、谷电子/自旋弛豫过程和相应的物理机制有深入的理解，为实现基于二维原子晶体谷电子功能的新型光电器件和自旋电子器件奠定基础。

二维层状材料中丰富的激子效应及其独特的光电性质，是构筑未来新型谷电子、微纳光电子与自旋电子器件应用的重要物理基础研究。本项目对以WSe2和MoSe2为代表的机械剥离单层、双层和体相的二维材料中不同激子及其能谷极化响应、谷极化寿命、以及谷电子/自旋在不同温度、光激发能量与功率、外场调控下的弛豫动力学过程开展了研究，阐明谷内与谷间电子－声子散射、谷间电子－空穴交换耦合、自旋－轨道耦合、激子－激子相互作用、以及材料缺陷等对激子辐射发光动力学过程、谷极化寿命、电子/自旋弛豫过程的影响。建立了共聚焦显微双色时间分辨泵浦-探测磁光克尔与反射测试平台，确定了单层WSe2中自由激子与荷电激子的谷极化寿命（自由激子谷弛豫时间仅为2～6皮秒, 而荷电激子则可长达3.7纳秒）；阐明谷间电子－空穴库伦交换耦合是导致单层二维材料中谷极化弛豫的重要机制。研究了单层MoSe2中荷电激子与中性激子间相干与非相干耦合多体效应：发现荷电激子的形成与弛豫过程与自由激子的带填充和能带重整化动力学过程密切相关；比较研究了单层、双层和体相WSe2 中A、B激子间的相干耦合以及电子－声子散射等对不同层WSe2中自旋产生、堆积及弛豫动力学过程的影响。在实验上确定了双层与体WSe2中均可实现载流子的自旋极化，并对其自旋弛豫动力学过程及其相关物理机制进行了研究：得益于WSe2中强的自旋−轨道耦合，即使在体WSe2中也可实现光生载流子的自旋极化以及类似常规半导体材料中电场与磁场对其自旋弛豫过程的调控。其自旋弛豫机制与传统Ш-V族半导体材料如GaAs中的D’yakonov-Perel’ 机制类似。外加横向磁场使得其自旋寿命变短，与传统半导体的Hanle效应类似。本项目的研究结果为深入理解二维层状材料中丰富的激子效应、独特的谷极化性质及其寿命和相关物理机理奠定了物理基础，为基于二维层状材料谷电子的新型光电器件和自旋电子器件应用提供了重要参考。