报 告 人：韩博 博士
报告摘要：2D semiconductors based on Transition Metal Dichalcogenides (TMD), such as MoS2, are very promising nanostructures for optical and electronic applications. 2H-phase TMD materials have a layer thickness dependent band structure: indirect bandgap for a number of layers ≥ 2, while the monolayer form has a direct bandgap located at the K points of the 2D hexagonal Brillouin zone. The optical properties of TMD thin layers are governed by Coulomb bound electron-hole pair, i.e. exciton, with giant binding energy of a few hundreds of meV. In monolayers (ML), the interplay between the absence of inversion symmetry and the spin-obit interaction yields very unique optical selection rules: valley selective circular dichroism arises as a consequence of the coupling between spin and valley physics.
In this talk, an all-dry viscoelastic stamping method is used to fabricate high-quality van der Waals heterostuctures based on TMD materials. Thanks to the use of hexagonal boron nitride (hBN, large indirect gap ~6meV semiconductor) as encapsulation layers, the exciton linewidth of TMD monolayer is narrowed, approaching the homogeneous limit (1-4 meV). (1) This improvement of optical properties allows us to explore the long-disputing exciton fine structure in Molybdenum-based TMD MLs. I performed magneto-photoluminescence experiments up to 30 T to brighten the dark exciton states. The bright excitons are measured 1.4 meV below the dark ones in MoSe2 ML, whereas the ordering is opposite for MoS2 ML with bright states 14 meV above the dark states. (2) Thanks to the reduction of dielectric disorder in encapsulated MoSe2 monolayers, I also demonstrate the control of exciton radiative lifetime and linewidth as a result of the Purcell effect by simply tuning the bottom hBN thickness. (3) Finally we use various optical spectroscopy techniques to reveal the exciton excited states in MoTe2 MLs for the first time. Exciton upconversion is observed in all TMD monolayers that we have investigated: MoS2, MoSe2, MoTe2 and WSe2. It is interpreted as a consequence of very efficient resonant interband Auger processes. These detailed studies help us better understand the physics of 2D TMD semiconductors and pave the way for future photonic and optoelectronic applications.
报告人简介：韩博，2014年和2017年在吉林大学物理学院获得学士和硕士学位，2020年在法国图卢兹大学-国立应用科学学院获得博士学位。硕、博士期间主要从事低维半导体材料的光学和自旋等物理性质研究，采用微区光谱（拉曼散射，反射谱，光致发光谱，光致发光激发谱，时间分辨光致发光谱）结合金刚石对顶砧高压实验技术，对过渡金属硫化物（Transition Metal Dichalcogenides）二维单层材料的光学性质和高压调控做了系统研究，揭示了低温（~4K）、强磁场和静电势对二维单层材料的晶体结构、电子能带的影响，进一步解析了材料的激发性质、动力学以及载流子相互作用，为二维单层材料在接近于物理尺寸极限的光电器件和量子调控领域的应用提供解决方案。