Atypical Exciton–Phonon Interactions in WS2 and WSe2 Monolayers Revealed by Resonance Raman Spectroscopy

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• 作者：E. del Corro ; A. Botello-Méndez ; Y. Gillet ; A. L. Elias ; H. Terrones ; S. Feng ; C. Fantini ; Daniel Rhodes ; N. Pradhan ; L. Balicas ; X. Gonze ; J.-C. Charlier ; M. Terrones ; M. A. Pimenta
• 刊名：Nano Letters
• 出版年：2016
• 出版时间：April 13, 2016
• 年：2016
• 卷：16
• 期：4
• 页码：2363-2368
• 全文大小：358K
• 年卷期：0
• ISSN：1530-6992

文摘

Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton–phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS₂ and WSe₂ using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS₂ monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin–orbit related X_A and X_B excitons), whereas Raman bands of WSe₂ are not enhanced at X_A/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin–orbit coupling. However, we reveal that WS₂ and WSe₂ exhibit quite different exciton–phonon interactions. In this sense, we demonstrate that the interaction between X_C and X_A excitons with phonons explains the different Raman responses of WS₂ and WSe₂, and the absence of Raman enhancement for the WSe₂ modes at X_A/B energies. These results reveal unusual exciton–phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).