应用光电流频谱研究二维材料的激子行为

Recently, the two dimensional materials (graphene and transition metal dichalcogenides (TMD), etc.) have been attracted unprecedented research enthusiasm worldwide due to their superior optical and electrical properties. WSe2 is a representative example, the band structure transfers from indirect bandgap to direct bandgap when the material is thinned down from bulk to monolayer. The two-dimensional nature of single-layer TMD results in reduced screening and enhanced Coulomb interaction, giving rise to excitonic complexes such as exciton, trion, and biexciton with binding energy orders of magnitude larger than that of conventional semiconductors. Large spin-orbit coupling leads to the splitting of the valence bands in TMDs, and the resulting valence band minimum with different spin configurations breaks the symmetry at the corners of TMD’s Brillouin zone, i.e., K and K′ valleys. The different valleys can be accessed selectively through circularly polarized light, providing a valley degree of freedom that can be exploited for valleytronics which is analogical to the electronic and spintronics. In the view of this perspective, we propose the application of Photocurrent Spectrum to study the physical properties of different excitonic states and reveal the dissociation mechanism of excitonic particles to generate photocurrent, based on the monolayer, Van de Waals heterostructure, angular-control Van de Waals homostructure and heterostructure as well as 2D ferromagnetic materials coupled with TMD heterostructure. Furthermore, it is of great significance and practical value to disclose the interaction between photocurrent properties and band structure of heterostructure, the long-range periodic potential of Moiré pattern, and the coupling effect with 2D ferromagnetic materials, respectively.

近来，以石墨烯和过渡金属硫族化合物(TMD)为代表的二维材料，因其优越的光学和电学性质，激起世界范围内研究人员空前的研究热情。以WSe2为例，随着材料逐渐由块体减薄至单层，能带结构由间接带隙转为直接带隙，并且由于库伦相互作用的增强和屏蔽效应的减弱，一些比常见半导体拥有更高结合能的激子，带电激子和双激子等都相继出现。此外，由于自旋轨道耦合导致价带劈裂，在K空间存在两类不能简并的能谷，通过激发光的圆偏振特性可以调控和探测谷自由度，这使得能谷也可以成为类似于电荷和自旋的信息载体。鉴于此，本项目提出基于TMD单层二维材料，范德华异质结器件、转角控制范德华结构以及铁磁性二维材料与TMD耦合的光电流频谱，研究各种激子态的物理性质，揭示激子分离形成光电流的机制，探究光电流的性质与异质结能带结构、Moiré图案长程有序周期势场和铁磁性二维材料耦合效应之间的相互作用关系，具有重要的科学意义和实用价值。

近来，以石墨烯和过渡金属硫族化合物(TMD)为代表的二维材料，因其优越的光学和电学性质，激起世界范围内研究人员空前的研究热情，尤其是其独特的激子性质。然而传统激子探测手段还存在诸多限制，因此，本项目基于二维材料光电器件的光电流频谱，并结合光致发光光谱，吸收光谱和电子输运等研究方法，研究各种激子态的本征性质，揭示激子-声子的相互作用关系:1.发现在单层MoSe2光电器件中可以探测到激子A1s，A激子激发态A2s和激子B1s对应的特征峰，更值得关注的是，研究人员还探测到了B激子的激发态B2s，并且这些不同的激子态对栅极电压具有明显的响应；2.在双层WSe2中同样可以探测到激子A1s和激子B1s对应的特征峰，但未发现更高激发态，与单层WSe2发现高达11s的Rydberg激子态具有明显的区别；3.发现动量禁阻的谷间激子（电子和空穴分别位于K'和K）与K点的手性声子耦合，电子通过发射一个手性声子从K'跃迁到K的一个虚拟态，与空穴复合，手性声子的赝角动量传递给发射的光子从而发出圆偏光，谷间激子同时具有长寿命和谷自旋性质的激子态，为二维材料在量子信息科学和新型光电器件的开发和应用奠定理论基础。