title: Evolution of electronic structure in atomically thin sheets of WS2 and WSe2.
abstract: 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).
authors.affiliation: Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542.
authors.forename: Weijie
authors.initials: W
authors.lastname: Zhao
authors.forename: Zohreh
authors.initials: Z
authors.lastname: Ghorannevis
authors.forename: Leiqiang
authors.initials: L
authors.lastname: Chu
authors.forename: Minglin
authors.initials: M
authors.lastname: Toh
authors.forename: Christian
authors.initials: C
authors.lastname: Kloc
authors.forename: Ping-Heng
authors.initials: PH
authors.lastname: Tan
authors.forename: Goki
authors.initials: G
authors.lastname: Eda
mesh_terms: D004579:Electron Transport; D008422:Materials Testing; D008567:Membranes, Artificial; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D010316:Particle Size; D044367:Phase Transition; D012643:Selenium / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D014414:Tungsten / Q000737:chemistry*
chemical_list: D008567:Membranes, Artificial; D013440:Sulfides; D012643:Selenium; D014414:Tungsten