Ionic disproportionation of charge transfer salt driven by surface epitaxy

Epitaxial growth of organic charge-transfer salts composed of molecular cations and anions will potentially allow the synthesis of thin films of molecular conductors with strong electron correlations and electron-phonon interactions, highly distinct properties compared to thin organic semiconductors and traditional self-assembled monolayers. Here, we report on ionic decomposition of the charge transfer salt β″-(BEDT-TTF) ₂SF₅CH₂CF₂SO₃ into two surface-supported phases on Ag(111), each of which has a cation/anion ratio different from the 2:1 stoichiometry of the bulk. The films were grown by thermal evaporation of the bulk crystal. Subsequent reassembly of the constituent bis(ethylenedithio)tetrathiafulvalene molecules (BEDT-TTF) and the SF₅CH₂CF₂SO₃ anions into long-range ordered structures on the Ag(111) surface produced well-ordered molecular islands with either 1:1 or 3:1 stoichiometric ratios of BEDT-TTF:SF ₅CH₂CF₂SO₃. Tunneling spectroscopy revealed that both surface structures could be considered insulating. However, the charge-transfer interaction between cations and anions still persists, producing electronic states that are distinct from those of pure BEDT-TTF molecules on a silver surface. Density functional theory calculations of adsorbed molecules show that they remain ionic, with adsorption and intermolecular binding energies comparable to that of bulk cohesive energies. The diversity of surface-supported multicomponent molecular structures derived from charge-transfer salts and the perseverance of cation and anion molecules despite thermal decomposition of the bulk all provide rich opportunity toward achieving correlated electron properties in ultrathin molecular films.