Tetracyanoquinodimethane Thin Films on Cu, Au, Pt, and SnS₂: Characterization by X-ray Photoelectron Spectroscopy

Tetracyanoquinodimethane (TCNQ) thin films, prepared by vacuum deposition on Cu, Au, and Pt and the layered semiconductor SnS₂, have been characterized using X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy studies showed that TCNQ reduction occurred, to distances within the TCNQ film which corresponded to coverages beyond monolayer, on Cu and Au thin films prepared by evaporation onto freshly cleaved mica, and on the layered compound SnS₂. Chemical shifts of the C(1s) and N(1s) lines were consistent with the formation of at least two forms of metal/TCNQ charge transfer salt, and the coverage of this product on these smooth surfaces suggests that there is facile charge exchange over several monolayers of TCNQ. At a critical coverage of TCNQ (ca. 20 Å on the Cu/mica surface), the bulk form begins to predominate, setting an upper coverage limit on this charge exchange process. Similar reactivities were seen for TCNQ on SnS₂ surfaces, with no indication of intercalation. On clean Pt and Au surfaces prepared by sputter cleaning of the metal foils, reduction of TCNQ occurred by a similar mechanism, but conversion to the bulk form of TCNQ occurred at much lower apparent coverages, which we propose is due to the roughened nature of those surfaces. A reproportionation mechanism is proposed which rationalizes the reaction of incoming TCNQ with the multiple anion forms present in the first monolayer.