Structure and properties of the highly conductive Langmuir–Blodgett films based on ditetradecyldimethylammonium-Au(dmit)2 salt

•The structure and properties of the Au(dmit)2 LB film have been studied.•The Au(dmit)2 dimers are partially oxidized even in the as-deposited film.•The number of the oxidized Au(dmit)2 dimers increases with the electro-oxidation.•Some IR-inactive modes are observed by electron-molecular vibration coupling.•The electro-oxidized LB film consists of insulating and conducting domains.

The Langmuir–Blodgett (LB) film based on ditetradecyldimethylammonium-Au(dmit)2 (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) [2C14N+Me2-Au(dmit)2] salt exhibits a high room-temperature conductivity of 40 S/cm (4.0 × 103 S/m) after electro-oxidation. Since the optical transmittance of the 20-layered LB film is more than 80 % in the whole visible region, the thin-film system attracts much attention both from the technological and scientific point of views. Although the electro-oxidation plays a key role to realize the high conductivity, the induced changes in structure and properties of the film had not been fully elucidated. Here we present a comprehensive study on the structure and properties of the 2C14N+Me2-Au(dmit)2 LB film utilizing UV–visible spectroscopy, FT-IR spectroscopy, AFM and XRD. The oxidation of the Au(dmit)2 anions solely proceeds in the earlier stage, while the anion oxidation and a morphological change proceed simultaneously in the later stage. Associated with the electro-oxidation, a broad band appears in a wide IR region and the change in the spectral weight [Δ∫Adk(cm−1)] coincides well with the conductance indicating that conduction electrons are generated by the charge transfer. The order of the hydrocarbon chains improves by the electro-oxidation. The as-deposited film consists of insulating domains with d = 3.2 nm, while the insulating domains with d = 3.2 nm and highly conductive domains with d = 2.9 nm coexist in the electro-oxidized film. The heterogeneity is considered to be the origin of the percolation-type conduction.

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