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

Sci Rep. 2013;3:1608. doi: 10.1038/srep01608.


We report systematic optical studies of WS2 and WSe2 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Chalcogens / chemistry*
  • Computer Simulation
  • Light
  • Luminescent Measurements / methods*
  • Materials Testing
  • Models, Chemical*
  • Scattering, Radiation
  • Spin Labels
  • Tungsten Compounds / chemistry*


  • Chalcogens
  • Spin Labels
  • Tungsten Compounds