Spatially Resolved Investigation of Mixed Valence and Insulator-to-Metal Transition in an Organic Salt.

Using scanning tunneling microscopy/spectroscopy (STM/STS), we investigate the evolution of electronic structures across the boundaries of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and K-TCNQ assemblies on a weakly interacting substrate. Despite the semiconducting/insulating nature of TCNQ (TCNQ0) and K-TCNQ (TCNQ-1), a continuum metallic-like density of states extending deep (∼1.5 nm) into the TCNQ assembly is observed near the domain boundary. We attribute the formation of these states to the abrupt change of molecular valence, which perturbs the electrostatics of the junction and creates local electric fields as evidenced by the band bending near the domain boundary. To the best of our knowledge, this study provides the first microscopic understanding of the crucial physics occurring near domain boundaries of mixed valence in K-TCNQ, or broadly speaking charge-transfer complexes, which highlights these boundaries as potential "weak" points to initiate the electric field-induced insulator-to-metal transition.