Growth process of poly (3-dodecyl thiophene) self-assembled monolayers: FTIR-RAS and gravimetric studies

Abstract

The growth process of self-assembled monolayers (SAMs) of poly (3-dodecylthiophene) (PDT) on Au (111) surfaces has been revealed by Fourier-transform infrared reflection absorption spectroscopy (FTIR-RAS) and quartz crystal microbalance (QCM) method. Immersion of the gold substrate into a chloroform solution of PDT induces adsorption of the polymer coils on the Au surface. Both the FTIR-RAS and QCM methods revealed that the overall kinetics of the PDT-SAMs growth obeys the time-dependent Langmuir isotherm. FTIR-RAS clarified that the SAM growth of PDT depends upon the molecular conformational transitions. The PDT coils spread over the surface due to the attractive force between the sulphur atoms of thiophene-ring and Au atoms, and intramolecular interactions. The thiophene backbone of the PDT orients parallel to the gold surface. Simultaneously, the alkyl chain moiety of PDT is oriented perpendicular to the surface. At the equilibrium state, the alkyl chain moiety takes a flat-lying configuration to the surface. QCM defined the high-affinity isotherm, i.e., the first phase in the lower concentration range, where alkyl-chains of PDT is oriented perpendicular to the surface until the saturation coverage. As the coverage increases, the alkyl chains of PDT change from “standing-up” to “flat-lying” configurations, which is in good agreement with the FTIR-RAS data. In the second phase with the higher coverage, PDT forms loops and tails along the thiophene backbone.