Fabrication, Characterization, and Electrical Properties of Langmuir−Blodgett Films of an Acid Terminated Phenylene−Ethynylene Oligomer

This paper describes the preparation of Langmuir−Blodgett (LB) films comprised of an oligomeric phenylene−ethynylene (OPE) derivative, 4-[4-(phenylethynyl)-phenylethynyl] benzoic acid (BPEBA). Analysis of the surface pressure and surface potential vs area per molecule isotherms reveal that good quality monolayer films can be formed at surface pressures of 15 mN/m. The monolayers were transferred onto solid substrates with a Z-type deposition and a transfer ratio of 1. Raman and surface-enhanced Raman spectroscopy (SERS) studies reveal that the films are physisorbed onto silver metal substrates. The morphology of the deposited films was analyzed by means of atomic force microscopy (AFM), revealing the formation of homogeneous layers free of three-dimensional defects. The optical and emissive properties of the LB films were determined, with significant blue-shifted absorption spectra indicating the formation of two-dimensional H aggregates in the films. In addition, a significant Stokes shift in the excitation and emission spectra of the films is indicative of a distribution of molecular conformations around the long molecular axis in the solidlike monolayer environment. Scanning tunneling microscopy (STM) studies of single layer BPEBA LB films were performed. The tip−sample distance has been calibrated carefully to obtain I−V curves above the LB film. I−V curves are unexpectedly symmetrical in spite of the asymmetric contacts of the molecule with the tip and the substrate. Single molecule conductance for BPEBA has also been determined and the similarity of these results to I−V data for BPEBA incorporated in LB films indicates that lateral (intermolecular) conductance is negligible for electrical measurements using the STM configuration.