Development of near-field scanning optical microscopy for studies of heterogeneity in organic thin films

Near-Field Scanning Optical Microscopy (NSOM) is combined with Time-Correlated Single Photon Counting (TCSPC) to study the heterogeneity of organic thin films of poly (9,9′-dialkylfluorene) and a dye doped polyvinyl alcohol via fluorescence lifetime NSOM imaging. Fluorescence lifetime imaging can be a powerful tool to separate species possessing similar or overlapped emission spectra. NSOM is an excellent technique for a nano-scale optical and topographical image acquisition. Here we have combined these two useful techniques to get images containing optical, topographical and lifetime information simultaneously. Polyfluorene has attracted much interest in light emitting device research application since it has high quantum efficiency, highly ordered structure and easy processing characteristics. Polyfluorene films have two distinct lifetimes, 200 ps at 430 nm and 7 ns at 550 nm for intra- and inter-polymer species, respectively. vii Lifetime imaging with high spatial resolution has been performed. As a demonstration, pristine films of poly (9,9′-dioctylfluorene) have been imaged. Images indicate that the two species are evenly distributed throughout the film. Our further effort represents the most advanced coupling of time-resolved fluorescence with NSOM. At each pixel we collect a full histogram of the time decay. The most powerful advantage of this technique is the ability to measure high signal to noise time decays for selected regions of interest. Once areas of the sample have been identified we can bin the decays by combining different pixels in the image to create a composite lifetime of a particular region. Images of poly (vinylalcohol) (PVA) films doped with the fluorescent dye, cascade blue, show that the two lifetimes differ by only 0.3 ns with a spatial resolution of less than 100 nm. Both pristine and thermally annealed poly (9,9′-hexylfluorene) films have been studied and the inter- and intra polymer species are evenly distributed throughout the film. A standard sample for NSOM has been developed to measure the size of the NSOM probe apertures. Fluorescent 59 nm latex spheres are dispersed on a substrate and imaged for fluorescence standards. For transmission standards, spheres on a glass substrate are coated with a thin layer of aluminum then dissolved out subsequently to leave perforated holes. The size of the NSOM apertures can be determined from a deconvolution of the image size and the known sphere size.