Laboratoire National des Champs Magnétiques Intenses, CNRS-UGA-UPS-INSA-EMFL, 25, avenue des Martyrs, 38042 Grenoble, France.
Nanoscale. 2017 Sep 14;9(35):13128-13141. doi: 10.1039/c7nr04672c.
We present a comprehensive optical study of thin flakes of tungsten disulfide (WS) with thickness ranging from mono- to octalayer and in the bulk limit. It is shown that the optical band-gap absorption of monolayer WS is governed by competing resonances arising from one neutral and two distinct negatively charged excitons whose contributions to the overall absorption of light vary as a function of temperature and carrier concentration. The photoluminescence response of monolayer WS is found to be largely dominated by disorder/impurity- and/or phonon-assisted recombination processes. The indirect band-gap luminescence in multilayer WS turns out to be a phonon-mediated process whose energy evolution with the number of layers surprisingly follows a simple model of a two-dimensional confinement. The energy position of the direct band-gap response (A and B resonances) is only weakly dependent on the layer thickness, which underlines an approximate compensation of the effect of the reduction of the exciton binding energy by the shrinkage of the apparent band gap. The A-exciton absorption-type spectra in multilayer WS display a non-trivial fine structure which results from the specific hybridization of the electronic states in the vicinity of the K-point of the Brillouin zone. The effects of temperature on the absorption-like and photoluminescence spectra of various WS layers are also quantified.
我们对厚度在单层到八层以及体相范围内的二硫化钨（WS）薄片进行了全面的光学研究。结果表明，单层 WS 的光学带隙吸收由来自一个中性和两个不同的带负电荷激子的竞争共振决定，它们对光的整体吸收的贡献随温度和载流子浓度而变化。发现单层 WS 的光致发光响应主要由无序/杂质和/或声子辅助复合过程主导。在多层 WS 中，间接带隙发光是一个声子介导的过程，其能量随层数的演化出人意料地遵循一个二维限制的简单模型。直接带隙响应（A 和 B 共振）的能量位置与层厚度的依赖性较弱，这强调了激子结合能减小与表观能带隙收缩的近似补偿效应。多层 WS 中的 A-激子吸收型光谱显示出复杂的精细结构，这是由布里渊区 K 点附近电子态的特定杂化引起的。我们还定量研究了温度对各种 WS 层的吸收谱和光致发光谱的影响。
