Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542.
Nanoscale. 2013 Oct 21;5(20):9677-83. doi: 10.1039/c3nr03052k.
Thickness is one of the fundamental parameters that define the electronic, optical, and thermal properties of two-dimensional (2D) crystals. Phonons in molybdenum disulfide (MoS2) were recently found to exhibit unique thickness dependence due to the interplay between short and long range interactions. Here we report Raman spectra of atomically thin sheets of WS2 and WSe2, isoelectronic compounds of MoS2, in the mono- to few-layer thickness regime. We show that, similar to the case of MoS2, the characteristic A1g and E2g(1) modes exhibit stiffening and softening with increasing number of layers, respectively, with a small shift of less than 3 cm(-1) due to large mass of the atoms. Thickness dependence is also observed in a series of multiphonon bands arising from overtone, combination, and zone edge phonons, whose intensity exhibit significant enhancement in excitonic resonance conditions. Some of these multiphonon peaks are found to be absent only in monolayers. These features provide a unique fingerprint and rapid identification for monolayer flakes.
厚度是定义二维（2D）晶体电子、光学和热性能的基本参数之一。最近发现，由于短程和长程相互作用的相互作用，二硫化钼（MoS2）中的声子表现出独特的厚度依赖性。在这里，我们报告了原子薄 WS2 和 WSe2 薄片的拉曼光谱，WS2 和 WSe2 是 MoS2 的等电子化合物，处于单到少数层厚度范围。我们表明，类似于 MoS2 的情况，特征 A1g 和 E2g(1) 模式分别随着层数的增加而变硬和变软，由于原子的质量较大，仅存在小于 3 cm(-1) 的小位移。在一系列由倍频、组合和带边声子产生的多声子带中也观察到厚度依赖性，其强度在激子共振条件下显著增强。其中一些多声子峰仅在单层中缺失。这些特征为单层薄片提供了独特的指纹和快速识别。
