  Exciton binding energy and excited states in monolayers of tungsten
diselenide (WSe2) are investigated using the combined linear absorption and
two-photon photoluminescence excitation spectroscopy. The exciton binding
energy is determined to be 0.37eV, which is about an order of magnitude larger
than that in III-V semiconductor quantum wells and renders the exciton excited
states observable even at room temperature. The exciton excitation spectrum
with both experimentally determined one- and two-photon active states is
distinct from the simple two-dimensional (2D) hydrogenic model. This result
reveals significantly reduced and nonlocal dielectric screening of Coulomb
interactions in 2D semiconductors. The observed large exciton binding energy
will also have a significant impact on next-generation photonics and
optoelectronics applications based on 2D atomic crystals.
