Abstract
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.
Highlights
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
Organic charge-transfer complexes (CTCs) formed by electron-donor and -acceptor molecules are an intriguing and broad class of materials that can exhibit phenomena related to strong electron correlations and electron−phonon coupling such as charge and spin density waves, Mott metal−insulator transitions, charge ordering, spinliquid phases, and superconductivity.[1−6] In bulk CTC crystals, donor and acceptor molecules typically stack in rows that maximize π−π electronic overlap along the rows only.[7]
We grow a near-monolayer coverage of graphene on Ir(111) by a combination of temperature-programmed growth (TPG) and chemical vapor deposition (CVD), as described previously,[21−23] followed by oxygen intercalation to electronically decouple graphene from the underlying substrate.[24]
Summary
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)). 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. Despite the broad spectrum of intriguing physical phenomena that have been reported in bulk CTCs, their two-dimensional (2D) films have been much less studied.[8−16] In particular, the studies have been confined to metal substrates, which strongly interact with the molecular layer and mask the intrinsic electronic properties of the CTCs. The CTC formed out of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) molecules is an archetypal example of a CTC. High-resolution tunneling spectra exhibit a dip at Fermi energy below a temperature of 20 K that may be attributed to the formation of a correlated ground state in the CTC monolayer
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