Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, United Kingdom.
Department of Physics, Beihang University , Beijing 100191, People's Republic of China.
ACS Nano. 2016 Feb 23;10(2):2176-83. doi: 10.1021/acsnano.5b06678. Epub 2016 Jan 13.
Monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors, and their 2D structure results in large binding energies for excitons, trions, and biexcitons. The ability to explore many-body effects in these monolayered structures has made them appealing for future optoelectronic and photonic applications. The band structure changes for bilayer TMDs with increased contributions from indirect transitions, and this has limited similar in-depth studies of biexcitons. Here, we study biexciton emission in bilayer WS2 grown by chemical vapor deposition as a function of temperature. A biexciton binding energy of 36 ±4 meV is measured in the as-grown bilayer WS2 containing 0.4% biaxial strain as determined by Raman spectroscopy. The biexciton emission was difficult to detect when the WS2 was transferred to another substrate to release the stain. Density functional theory calculations show that 0.4% of tensile strain lowers the direct band gap by about 55 meV without significant change to the indirect band gap, which can cause an increase in the quantum yield of direct exciton transitions and the emission from biexcitons formed by two direct gap excitons. We find that the biexciton emission decreases dramatically with increased temperature due to the thermal dissociation, with an activation energy of 26 ± 5 meV. These results show how strain can be used to tune the many-body effects in bilayered TMD materials and extend the photonic applications beyond pure monolayer systems.
单层过渡金属二卤族化合物（TMD）是直接带隙半导体，其二维结构导致激子、三激子和双激子的结合能很大。在这些单层结构中探索多体效应的能力使它们成为未来光电和光子应用的理想选择。随着双层 TMD 中来自间接跃迁的贡献增加，其能带结构发生变化，这限制了对双激子的类似深入研究。在这里，我们研究了化学气相沉积生长的双层 WS2 中双激子发射随温度的变化。通过拉曼光谱确定，在含有 0.4%双轴应变的原始双层 WS2 中，测量到 36±4 meV 的双激子结合能。当 WS2 转移到另一个衬底以释放应变时，很难检测到双激子发射。密度泛函理论计算表明，0.4%的拉伸应变使直接带隙降低约 55 meV，而间接带隙没有明显变化，这可能导致直接激子跃迁的量子产率增加，以及由两个直接带隙激子形成的双激子发射增加。我们发现，由于热离解，双激子发射随温度升高而急剧下降，其激活能为 26±5 meV。这些结果表明应变如何用于调节双层 TMD 材料中的多体效应，并将光子应用扩展到纯单层系统之外。
