Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides

• Published in: Scientific reports Vol. 3; no. 1; p. 1608
• Main Authors: Zeng, Hualing, Liu, Gui-Bin, Dai, Junfeng, Yan, Yajun, Zhu, Bairen, He, Ruicong, Xie, Lu, Xu, Shijie, Chen, Xianhui, Yao, Wang, Cui, Xiaodong
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 11.04.2013; Nature Publishing Group
• Subjects: 639/301/119; 639/301/119/1000; 639/766/119/1001; 639/766/119/995; Chalcogens - chemistry; Computer Simulation; Efficiency; Graphene; Humanities and Social Sciences; Light; Luminescence; Luminescent Measurements - methods; Materials Testing; Models, Chemical; multidisciplinary; Optical properties; Photons; Physics; Scattering, Radiation; Science; Semiconductors; Spin Labels; Splitting; Symmetry; Tungsten; Tungsten Compounds - chemistry
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep01608

We report systematic optical studies of WS 2 and WSe 2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.