Effects of lower symmetry and dimensionality on Raman spectra in two-dimensional WSe2

作     者：Xin Luo Yanyuan Zhao Jun Zhang Minglin Toh Christian Kloc Qihua Xiong Su Ying Quek 

作者机构：Institute of High Performance Computing 1 Fusionopolis Way #16-16 Connexis Singapore 138632 Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 21 Nanyang Link Singapore 637371 School of Materials Science and Engineering Nanyang Technological University Nanyang Avenue Singapore 639798 Division of Microelectronics School of Electrical and Electronic Engineering Nanyang Technological University Singapore 639798 

出 版 物：《Physical Review B》 (Phys. Rev. B Condens. Matter Mater. Phys.)

年 卷 期：2013年第88卷第19期

页      面：195313-195313页

核心收录：

学科分类：0808[工学-电气工程] 0809[工学-电子科学与技术（可授工学、理学学位）] 07[理学] 0805[工学-材料科学与工程（可授工学、理学学位）] 0702[理学-物理学] 

基　　金：Institute of High Performance Computing Independent Investigatorship Singapore National Research Foundation [NRF-NRFF2013-07, NRF-RF2009-06] Ministry of Education [MOE2012-T2-2-086] Nanyang Technological University [M58113004] Singapore A*STAR Computational Resource Center 

摘      要：We report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe2), induced by the reduction of symmetry going from three-dimensional (3D) to two-dimensional (2D). In general, Raman frequencies in 2D materials follow quite closely the frequencies of corresponding eigenmodes in the bulk. However, while the modes that are Raman active in the bulk are also Raman active in the thin films, the reverse is not always true due to the reduced symmetry in thin films. Here, we predict from group theory and density functional calculations that two intralayer vibrational modes, which are Raman inactive in bulk WSe2 in our experimental configuration become Raman active in thin film WSe2, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at ∼310 and 176 cm−1 in thin film WSe2. Interestingly, the bulk B2g1 mode at ∼310 cm−1 that is Raman inactive can, in fact, be detected in Raman measurements under specific wavelengths of irradiation, suggesting that in this case, crystal symmetry selection rules may be broken due to resonant scattering. Both theory and experiment indicate that the E2g1 and B2g1 modes blueshift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/infrared activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D.