Spatially Resolved, Site-Dependent Charge Transfer and Induced Magnetic Moment in TCNQ Adsorbed on Graphene

A site-dependent charge transfer to 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) adsorbed on a single layer of periodically rippled graphene grown epitaxially on Ru(0001), identified by X-ray photoemission techniques, can be spatially resolved using Scanning Tunneling Microscopy, which can also detect the formation of magnetic moments. The molecules adsorbed on the lower part of the ripples are charged with electrons donated from the doped graphene overlayer and develop a magnetic moment, while those at the upper part of the ripples are neutral. On the other hand, TCNQ adsorbed on graphene on Ir(111) shows negligible charge transfer and no magnetic moment. These observations explain the spatially dependent long-range magnetic order observed recently for TCNQ on gr/Ru(0001).