Wu Lingling, Chen Yuzhong, Zhou Hongzhi, Zhu Haiming
ACS Nano. 2019 Feb 26;13(2):2341-2348. doi: 10.1021/acsnano.8b09059. Epub 2019 Feb 6.
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have shown great potential in ultrathin and flexible optoelectronic and photonics devices. Besides emissive bright excitons, they also possess rich non-emissive dark excitons including momentum-forbidden indirect excitons and spin-forbidden triplet-like excitons, which could be dominant species under optical or electrical excitation in 2D optoelectronic and photonic devices. Efficient harvesting of both bright and dark excitons from TMDs and understanding the exciton-transfer mechanism consequently are not only of fundamental interest but also a technological challenge. Here, by combining steady-state photoluminescence spectroscopy and ultrafast transient reflectance spectroscopy, we show efficient exciton harvesting by ultrafast energy transfer in WSe/MoTe van der Waals heterostructures, leading to the photoluminescence enhancement of MoTe. The energy transfer occurs with near-unity yield and in an ultrafast (∼200 fs) manner for both bright and dark excitons, suggesting a dominant Dexter-type energy-transfer process consisting of simultaneous transfer of both electron and hole in van der Waals coupled 2D layers at ultimate proximity. This result is beyond the conventional dipole-dipole coupling mechanism typically assumed at 2D interfaces and offers a path to high speed and enhanced light harvesting and emission applications based on 2D heterostructures.
二维（2D）过渡金属二硫属化物（TMDs）在超薄和柔性光电器件及光子器件中展现出了巨大潜力。除了发光的亮激子外，它们还拥有丰富的非发光暗激子，包括动量禁阻的间接激子和自旋禁阻的类三重态激子，在二维光电器件和光子器件的光激发或电激发下，这些暗激子可能占主导地位。因此，从TMDs中高效收集亮激子和暗激子并理解激子转移机制不仅具有重要的基础研究意义，也是一项技术挑战。在此，通过结合稳态光致发光光谱和超快瞬态反射光谱，我们展示了在WSe/MoTe范德华异质结构中通过超快能量转移实现的高效激子收集，这导致了MoTe的光致发光增强。亮激子和暗激子的能量转移产率接近100%，且以超快（约200飞秒）的方式发生，这表明在范德华耦合的二维层中，由电子和空穴同时转移组成的主导德克斯特型能量转移过程在极近距离内发生。这一结果超出了通常在二维界面假设的传统偶极 - 偶极耦合机制，并为基于二维异质结构的高速、增强光收集和发射应用提供了一条途径。
