Institut de Physique et Chimie des Matériaux de Strasbourg and NIE, UMR 7504, Université de Strasbourg and CNRS , 23 rue du Lœss, BP43, Strasbourg 67034 Cedex 2, France.
ACS Nano. 2016 Feb 23;10(2):2752-60. doi: 10.1021/acsnano.5b07844. Epub 2016 Feb 3.
We investigate the interlayer phonon modes in N-layer rhenium diselenide (ReSe2) and rhenium disulfide (ReS2) by means of ultralow-frequency micro-Raman spectroscopy. These transition metal dichalcogenides exhibit a stable distorted octahedral (1T') phase with significant in-plane anisotropy, leading to sizable splitting of the (in-plane) layer shear modes. The fan-diagrams associated with the measured frequencies of the interlayer shear modes and the (out-of-plane) interlayer breathing modes are perfectly described by a finite linear chain model and allow the determination of the interlayer force constants. Nearly identical values are found for ReSe2 and ReS2. The latter are appreciably smaller than but on the same order of magnitude as the interlayer force constants reported in graphite and in trigonal prismatic (2Hc) transition metal dichalcogenides (such as MoS2, MoSe2, MoTe2, WS2, WSe2), demonstrating the importance of van der Waals interactions in N-layer ReSe2 and ReS2. In-plane anisotropy results in a complex angular dependence of the intensity of all Raman modes, which can be empirically utilized to determine the crystal orientation. However, we also demonstrate that the angular dependence of the Raman response drastically depends on the incoming photon energy, shedding light on the importance of resonant exciton-phonon coupling in ReSe2 and ReS2.
我们通过超低频微拉曼光谱研究了 N 层二硒化铼（ReSe2）和二硫化铼（ReS2）中的层间声子模式。这些过渡金属二卤化物表现出稳定的扭曲八面体（1T'）相，具有显著的面内各向异性，导致（面内）层剪切模式的显著分裂。与测量的层间剪切模式和（面外）层间呼吸模式的频率相关的扇形图可以通过有限线性链模型得到很好的描述，并允许确定层间力常数。ReSe2 和 ReS2 的值几乎相同。后者明显小于但在同一数量级上，与石墨和三角棱柱（2Hc）过渡金属二卤化物（如 MoS2、MoSe2、MoTe2、WS2、WSe2）中的层间力常数报告值相当，证明了范德华相互作用在 N 层 ReSe2 和 ReS2 中的重要性。面内各向异性导致所有拉曼模式的强度具有复杂的角依赖性，这可以通过经验利用来确定晶体取向。然而，我们还证明，拉曼响应的角度依赖性强烈依赖于入射光子能量，这揭示了共振激子-声子耦合在 ReSe2 和 ReS2 中的重要性。
