我们将四片布拉格体光栅技术集成到单光栅拉曼光谱仪中, 并且成功地探测到2至19层二硫化钼(MoS2)的层间剪切模和层间呼吸模。我们根据对称性分析和偏振拉曼的结果对所有观察到的模式进行了指认。同时, 我们利用简单的“单原子线性链”模型得到了适用于任何二维层状晶体材料层间振动模的频率随层数变化关系的解析形式。根据此模型, 我们发现层间弱范德瓦耳斯力作用决定了剪切模和呼吸模的频率随层数的变化。我们将此结果推广到ABC堆垛的多层石墨烯材料, 发现光学衬度方法无法鉴别的AB和ABC型石墨烯可以利用能否观察到剪切模来加以鉴别。此研究结果也提供了准确地确定二维层状材料的厚度的一种新方法, 为一般二维层状材料基本性质和器件应用方面的研究奠定了基础。

We integrated four notch filters made from volume Bragg grating into single-stage Raman spectrometer and successfully probed the shear (C) modes and layer breathing modes (LBMs) for 2-19 layer MoS2. The results obtained from symmetry analysis and polarization measurements allow one to assign all the measured Raman modes. A “monatomic chain model” (MCM) for the C mode and LBM was built with an analytic expression, which can be applicable to any layered materials. It is found that the weak interlayer van der Waals forces dominate the frequency evolution of shear and layer breathing modes on the layer number of ultra-thin layered materials. We further extend our model to ABC-stacked multilayer graphenes. AB-stacked and ABC-stacked multilayer graphenes cannot be well-distinguished by optical contrast, however, the disappearance of the shear mode in experimental measurements gives an evidence of the ABC-stacking for measured multilayer graphenes. This allows a new and reliable diagnostic of thickness for two-dimensional layered materials, which lays the foundation for researches in their fundamental properties and device applications.