Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil.
Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, State College, Pennsylvania 16802, USA.
Nat Commun. 2017 Mar 9;8:14670. doi: 10.1038/ncomms14670.
Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS.
双共振拉曼散射是一种灵敏的探针，可用于研究晶体中的电子-声子散射途径。对于二维过渡金属二卤化物半导体，双共振拉曼过程涉及布里渊区中的不同谷和声子，目前尚未完全理解。在这里，我们结合密度泛函理论计算，展示了在单层和体相 MoS 中的多次能量激发拉曼研究，揭示了双层共振拉曼散射过程。结果表明，当改变激发能量时，一些拉曼特征的频率会发生变化，第一性原理模拟证实了这些能带源于不同的声子，连接不同的谷态。双共振拉曼过程受到间接到直接带隙跃迁的影响，在单层和体相的结果比较允许对布里渊区 M 或 K 点附近的每个拉曼特征进行分配。我们的工作强调了声子电子谷间散射的基本物理，这对于 MoS 中的谷去极化至关重要。
