  Organic charge-transfer complexes (CTCs) formed by strong electron acceptor
and strong electron donor molecules are known to exhibit exotic effects such as
superconductivity and charge density waves. We present a low-temperature
scanning tunneling microscopy and spectroscopy (LT-STM/STS) study of a
two-dimensional (2D) monolayer CTC of tetrathiafulvalene (TTF) and fluorinated
tetracyanoquinodimethane (F4TCNQ), self-assembled on the surface of
oxygen-intercalated epitaxial graphene on Ir(111) (G/O/Ir(111)). We confirm the
formation of the charge-transfer complex by dI/dV spectroscopy and direct
imaging of the singly-occupied molecular orbitals. High-resolution spectroscopy
reveals a gap at zero bias, suggesting the formation of a correlated ground
state at low temperatures. These results point to the possibility to realize
and study correlated ground states in charge-transfer complex monolayers on
weakly interacting surfaces.
