  Intrinsic electrical and excitonic properties of monolayer transition metal
dichalcogenides are studied on CYTOP fluoropolymer substrates with greatly
suppressed unintentional doping and dielectric screening. Ambipolar transport
behavior is observed in monolayer WS2 by applying solid states backdates. The
excitonic properties of monolayer MoS2 and WS2 are determined by intricate
interplays between the bandage renormalization, Pauli blocking and carrier
screening against carrier doping.
