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

Abstract

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 (TCNQ 0 ) 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.

High-Endurance Magneto-Electronic Switchable Molecular Electronic Crystal

“…23 The energy gap in K-TCNQ is estimated to be ∼1 eV whereas the applied electric field does not break the gap under dielectric breakdown. 20 This further suggests the role of electronic correlation and collective charge dynamics in understanding the transition in K-TCNQ. 1,22 In situ Raman spectra are measured for the charge sensitive C−CN wing stretching model.…”

“…The collective nature of charge carriers and the role of correlation among them are inferred from the temperature dependence of E th . , The strength of E th is comparatively higher than that observed in a typical charge density wave system (0.01–0.1 V cm –1 ) . The energy gap in K-TCNQ is estimated to be ∼1 eV whereas the applied electric field does not break the gap under dielectric breakdown . This further suggests the role of electronic correlation and collective charge dynamics in understanding the transition in K-TCNQ. , …”

“…17,18 Quasi-one-dimensional alkali-7,7,8,8-tetracyanoquinodimethan (TCNQ) crystals undergo a paramagnetic Mott insulator to diamagnetic spin-Peierls phase transition, showing strong spin-charge-lattice coupling. 1,19,20 Here, we select strongly correlated K-TCNQ electronic crystal as a model system due to its high critical temperature (T c ) of 395 K. 1 We demonstrate an electronically switchable magneto-electric transition in molecular K-TCNQ electronic crystal up to 10 5 cycles, where the noise and optical spectroscopies reveal the charges are carried through an inhomogeneous mixed state. Electron spin resonance (ESR) shows the transition is cooperative between spin and charge.…”