以二硫化钼(MoS ₂)为代表的过渡族金属硫族化合物，因其合适的能带结构和光学性质，在光电子器件等用途中有着很好的应用前景。单层过渡金属硫化物堆叠而成的二维异质结，在新型光电子和光伏器件领域具有十分广泛的使用潜力。最近的超快激光实验发现，二维异质结中激发的电子-空穴对的分离过程能够在50 fs的超快时间尺度发生，为极大提高光电器件的响应速度提供了可能。然而，对于决定器件工作性能的微观机理，比如超快电子/空穴动力学、层间能量转移、以及层间热激子的形成等，因缺乏深入的研究而大都处于空白状态。
　　最近，中国科学院物理研究所/北京凝聚态物理国家研究中心表面物理国家重点实验室SF10课题组博士生张进在孟胜研究员的指导下，与哈佛大学Efthimios Kaxiras教授、北京大学刘开辉研究员、帝国理工学院Johannes Lischner教授等合作，发现一种新的界面热电子弛豫机制。他们研究了第二类半导体异质结MoS ₂/WSe ₂界面，特别是其在光激发后的电子/空穴动力学，从时间尺度分析载流子的弛豫和界面传输过程（图1）。第一性原理含时密度泛函动力学模拟表明，单层过渡金属硫化物深能级激发的热载流子弛豫时间尺度大约为数百飞秒。一般认为，二维材料是通过微弱的范德华作用堆垛而成，范德华作用导致的层间电子跃迁应远远慢于因共价键结合的层内电子弛豫时间。但是，电子动力学计算结果表明在二维异质结界面中，光激发产生的热载流子层间跃迁 (~100 fs) 明显快于层内弛豫(~700 fs)，因此热载流子更倾向于通过层间来回跃迁弛豫到带边（图2）。这是一个从未被人们留意的热电子/热空穴弛豫的新机制，在很多二维材料异质结界面、甚至传统半导体的异质界面应该广泛存在。其主要机制源于孟胜课题组此前提出的范德华异质结层间强耦合电子传输机制；该传输机制具有强烈的相干性，甚至能打破费米黄金规则的限制（Adv. Sci. 4, 1700086 (2017)）。该研究发现一种热电子弛豫新机制，建立了过渡金属硫化物异质结中层间相互作用与电子动力学之间的微观图像，为今后范德华多层异质结的设计和性能优化提供了重要的指导原则。主要研究结果发表在Nano Letters 18, 6057-6063 (2018)。
　　本项研究工作得到国家自然科学基金项目（项目批准号11774396, 11474328 和 51522201）、科技部（项目批准号2016YFA0300902, 2015CB921001 和2016YFA0300903)的资助。

图1. 二维材料异质结中光激发诱导载流子的不同弛豫过程。

图2. 二维材料异质结中光激发后不同路径热电子过程的对比。

New Pathway for Hot Electron Relaxation in Two-dimensional Heterostructures

Description: Discovery of a new pathway for the relaxation of photoexcited hot electrons in 2D heterostructures through interlayer hopping, which is significantly faster than intralayer relaxation.

Two-dimensional (2D) materials exhibit intriguing electronic, optical and mechanical properties, which are significantly different from conventional bulk materials. Experimental investigations of optical excitation in bilayer MX₂ heterostructures report ultrafast charge dynamics in these systems. For example, it was reported that ultrafast hole transfer from the MoS₂ to the WS₂ layer within ~50 femtoseconds (fs) after photoexcitation. Interestingly, ultrafast interlayer electron transfer in randomly stacked homo-bilayers occurs also on a sub-picosecond time scale. Developing further insight requires a detailed theoretical investigation to examine different dynamic processes including interlayer charge transfer, energy transfer, and formation of interlayer excitons.
Now a group from Institute of Physics, Chinese academy of sciences led by Prof. Sheng Meng, in collaboration with Prof. Efhimios Kaxiras (Harvard University), Prof. Kaihui Liu (Peking University), and Prof. Johannes Lischner (Imperial College), demonstrates the dynamics of photoexcited electron/hole pairs in 2D heterostructures using time-dependent density functional theory. They report the discovery of a new pathway for the relaxation of photoexcited hot electrons in 2D heterostructures through interlayer hopping in (~100 fs), which is significantly faster than intralayer relaxation (~700 fs). This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast timescale (< 1 ps). They compare the results quantitatively to available experimental data. The work established a concrete link between the interlayer interactions and electron dynamics in MX₂ heterostructures, providing important insights for future design and optimization of van der Waals multilayers.

Reference:
Zhang et al. New Pathway for Hot Electron Relaxation in Two-dimensional Heterostructures. Nano Lett. 18, 6057-6063 (2018).