Department of Physics, National University of Singapore , 2 Science Drive 3, Singapore 117542.
Acc Chem Res. 2015 Jan 20;48(1):91-9. doi: 10.1021/ar500303m. Epub 2014 Dec 17.
Two-dimensional (2D) crystals derived from transition metal dichalcogenides (TMDs) are intriguing materials that offer a unique platform to study fundamental physical phenomena as well as to explore development of novel devices. Semiconducting group 6 TMDs such as MoS2 and WSe2 are known for their large optical absorption coefficient and their potential for high efficiency photovoltaics and photodetectors. Monolayer sheets of these compounds are flexible, stretchable, and soft semiconductors with a direct band gap in contrast to their well-known bulk crystals that are rigid and hard indirect gap semiconductors. Recent intense research has been motivated by the distinct electrical, optical, and mechanical properties of these TMD crystals in the ultimate thickness regime. As a semiconductor with a band gap in the visible to near-IR frequencies, these 2D MX2 materials (M = Mo, W; X = S, Se) exhibit distinct excitonic absorption and emission features. In this Account, we discuss how optical spectroscopy of these materials allows investigation of their electronic properties and the relaxation dynamics of excitons. We first discuss the basic electronic structure of 2D TMDs highlighting the key features of the dispersion relation. With the help of theoretical calculations, we further discuss how photoluminescence energy of direct and indirect excitons provide a guide to understanding the evolution of the electronic structure as a function of the number of layers. We also highlight the behavior of the two competing conduction valleys and their role in the optical processes. Intercalation of group 6 TMDs by alkali metals results in the structural phase transformation with corresponding semiconductor-to-metal transition. Monolayer TMDs obtained by intercalation-assisted exfoliation retains the metastable metallic phase. Mild annealing, however, destabilizes the metastable phase and gradually restores the original semiconducting phase. Interestingly, the semiconducting 2H phase, metallic 1T phase, and a charge-density-wave-like 1T' phase can coexist within a single crystalline monolayer sheet. We further discuss the electronic properties of the restacked films of chemically exfoliated MoS2. Finally, we focus on the strong optical absorption and related exciton relaxation in monolayer and bilayer MX2. Monolayer MX2 absorbs as much as 30% of incident photons in the blue region of the visible light despite being atomically thin. This giant absorption is attributed to nesting of the conduction and valence bands, which leads to diversion of optical conductivity. We describe how the relaxation pathway of excitons depends strongly on the excitation energy. Excitation at the band nesting region is of unique significance because it leads to relaxation of electrons and holes with opposite momentum and spontaneous formation of indirect excitons.
二维（2D）过渡金属二卤化物（TMD）晶体是一种有趣的材料，为研究基本物理现象以及探索新型器件提供了独特的平台。半导体族 6 TMD 如 MoS2 和 WSe2 以其大的光吸收系数和在高效光伏和光电探测器中的应用潜力而闻名。这些化合物的单层薄片是柔性、可拉伸和柔软的半导体，具有直接带隙，而其众所周知的体晶体则是刚性和间接带隙半导体。最近，由于这些 TMD 晶体在最终厚度范围内具有独特的电学、光学和机械性能，研究变得非常活跃。作为一种在可见光到近红外频率范围内具有带隙的半导体，这些 2D MX2 材料（M=Mo，W；X=S，Se）表现出明显的激子吸收和发射特征。在本综述中，我们讨论了这些材料的光学光谱如何允许研究它们的电子性质和激子的弛豫动力学。我们首先讨论了二维 TMD 的基本电子结构，强调了色散关系的关键特征。借助理论计算，我们进一步讨论了直接和间接激子的光致发光能量如何提供一个理解电子结构随层数变化的指南。我们还强调了两个竞争导带谷的行为及其在光学过程中的作用。碱金属对族 6 TMD 的插层导致结构相变，相应地发生半导体到金属的转变。通过插层辅助剥落获得的单层 TMD 保留了亚稳金属相。然而，温和的退火会使亚稳相失稳，并逐渐恢复原始半导体相。有趣的是，在单个单晶薄片中可以共存半导体 2H 相、金属 1T 相和电荷密度波状 1T'相。我们进一步讨论了化学剥落的 MoS2 再堆积薄膜的电子性质。最后，我们关注单层和双层 MX2 的强光学吸收和相关激子弛豫。尽管单层 MX2 很薄，但它在可见光的蓝色区域吸收了多达 30%的入射光子。这种巨大的吸收归因于导带和价带的嵌套，这导致了光学电导率的偏离。我们描述了激子的弛豫途径如何强烈依赖于激发能量。在带嵌套区域的激发具有独特的意义，因为它导致具有相反动量的电子和空穴的弛豫以及自发形成间接激子。
