Novel techniques for nanoscale characterization of thin-film materials: Single molecule fluorescence detection and near-field scanning optical microscopy

We have made significant progress in developing two new techniques for characterization of materials on the nanometer length scale, near field scanning optical microscopy and single molecule detection. This includes designing, testing, and characterizing new NSOM configurations as well as contributing to the understanding of NSOM image contrast. In particular, a dual mode NSOM in which the NSOM probe is used to both illuminate the sample and to collect reflected light from the sample has been introduced. The simplified contrast and a well defined relationship between the detected signal and the tip-sample gap of the dual mode configuration can be used to separate the topography induced contrast from true optical contrast. Reflection NSOM techniques have been applied extensively to metal-phosphonate self-assembled monolayer and multilayer films and has revealed defect structures in those films.; Single molecule fluorescence detection and spectroscopy experiments have lead to a new understanding of the dynamics and photophysics of dye molecules, a step toward the use of single molecules as probes of local environment. We have learned that conformational transitions of single dye molecules immobilized on a surface can be monitored directly via the accompanying changes in the intersystem crossing rate, triplet lifetime, and fluorescence spectrum. A wide range of phenomena such as spectral shapes and intensity fluctuation characteristics from one molecule to the next are observed and methods of quantifying these parameters have been developed.