Charge dynamics and electronic structures of monolayer graphene with molecular doping

THz absorption and spectroscopic ellipsometry were used to investigate the charge dynamics and electronic structures of monolayer graphene with doping by triazine. The absorption spectrum of the triazine-doped film shows an optical excitation near 5.1 eV that is blue shifted compared with that of undoped analog. THz conductivity displays a coherent response of itinerant charge carriers at zero frequency and a disorder-induced finite frequency peak around 155 cm ^-1. Drude plasma frequency (∼4400 cm ^-1) decreases with decreasing temperature while carrier relaxation time (∼13 fs) is almost temperature independent, implying the semiconducting behavior with thermal activation energy of 3 meV.