Yuan Long, Huang Libai
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
Nanoscale. 2015 Apr 28;7(16):7402-8. doi: 10.1039/c5nr00383k.
We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10(-3) and (1.88 ± 0.47) × 10(-3) cm(2) s(-1), respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons.
我们通过时间分辨光致发光（TRPL）光谱系统地研究了单层、双层和三层WS2二维（2D）晶体中的激子动力学。在没有激子湮灭的情况下，WS2单层、双层和三层的激子寿命分别确定为806±37 ps、401±25 ps和332±19 ps。通过测量荧光量子产率，我们还确定了直接和间接激子的辐射和非辐射寿命。单层WS2中的激子衰减表现出强烈的激发密度依赖性，这可以用激子-激子湮灭（双粒子俄歇复合）模型来描述。单层、双层和三层WS2的激子-激子湮灭率分别确定为0.41±0.02、(6.00±1.09)×10(-3)和(1.88±0.47)×10(-3) cm(2) s(-1)。值得注意的是，单层中的激子-激子湮灭率比双层和三层快两个数量级。我们将双层和三层中激子-激子湮灭率慢得多归因于多体相互作用的减少以及间接激子的声子辅助激子-激子湮灭。
