Charge transfer induced chemical reaction of tetracyano-p-quinodimethane adsorbed on graphene

Raman spectroscopy was used to study the interaction of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) deposited on graphene films grown by Chemical Vapor Deposition (CVD). Different thickness layers (1 ML to 7 ML) of TCNQ deposited on single layer graphene were investigated. A distinct blue shift of the G band of graphene was observed depending on the TCNQ coverage, which indicated charge transfer from graphene to TCNQ. No charge transfer phenomenon was observed when TCNQ was adsorbed on graphite, reflecting the intrinsic difference in the electronic structure of graphene and graphite. The vibrational modes of TCNQ were identified. Moreover, new Raman bands not associated with TCNQ were discovered on the TCNQ/graphene sample and assigned to vibrational modes of α,α-dicyano-p-toluoylcyanide (DCTC−1). Our observations indicate that charge transfer occurs between graphene and TCNQ followed by chemical reaction with the atmosphere to form DCTC−1. We estimate the transfer of ≤0.03 electrons per C atom to the TCNQ adlayer. This study clearly demonstrates the potential of TCNQ to produce p-type graphene. It also demonstrates that the charged species at the graphene/organic layer may be readily susceptible to chemical reaction and that the resulting surface species may be more complex than a simple negative (or positive) ionization of the parent adsorbate. The final charge state of the graphene may depend upon the free energy change associated with the surface chemical reaction. Any detailed understanding of the graphene–adsorbate charged interface must include a thorough chemical study of the potential redox reactions that can occur following deposition of the primary organic species.