Evolution of electronic structure in atomically thin sheets of WS2 and WSe2

ACS Nano. 2013 Jan 22;7(1):791-7. doi: 10.1021/nn305275h. Epub 2012 Dec 28.


Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS(2)) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS(2) opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS(2) and WSe(2) that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS(2) and WSe(2), respectively. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepared by a chemical vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe(2).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Electron Transport
  • Materials Testing
  • Membranes, Artificial*
  • Nanostructures / chemistry*
  • Nanostructures / ultrastructure*
  • Particle Size
  • Phase Transition
  • Selenium / chemistry*
  • Sulfides / chemistry*
  • Tungsten / chemistry*


  • Membranes, Artificial
  • Sulfides
  • Selenium
  • Tungsten