Fabrication of TTF-TCNQ charge-transfer complex self-assembled monolayers: Comparison between the coadsorption method and the layer-by-layer adsorption method

We propose a novel technique to fabricate self-assembled monolayers (SAMs) of 2D charge-transfer complexes by coadsorption of thiol-functionalized tetrathiafulvalene derivatives (TTF-CH₂SH) with 7,7,8,8-tetracyano-quinodimethane (TCNQ) on a gold substrate. For the "coadsorption" method, the gold substrates are immersed into the TTF-CH₂SH and TCNQ mixed acetonitrile solution under optimum conditions for TTF-TCNQ bulk crystalline growth. TTF-CH₂SH/TCNQ SAMs are also prepared by the conventional "layer-by-layer" adsorption method to compare the film properties with those of the coadsorbed SAMs, where the gold substrates are exposed to TCNQ solution after TTF-CH₂SH (single component) SAM formation. For both TTF-CH₂SH/TCNQ SAMs, the adsorption process and the optical thickness are characterized by surface plasmon resonance (SPR) measurements. The coadsorbed TTF-CH₂SH/TCNQ SAMs form slightly thicker films (15.9 Å) than do the SAMs prepared by the layer-by-layer adsorption method (15.1 Å) because they incorporate the TCNQ molecules more efficiently in the film. The FTIR-RAS data reveal that all TCNQ molecules in the coadsorbed SAMs are in the mixed valence state, whereas no intermolecular charge transfer is present in the SAMs prepared by the layer-by-layer adsorption method. From the C≡N absorption bands in the IR spectra, the degree of charge transfer is estimated to ≃0.6 for the coadsorbed SAMs, which is comparable to the value for the bulk TTF-TCNQ crystals.