Probing nanoscale photo-oxidation in organic films using spatial hole burning near-field scanning optical microscopy

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Abstract
Spatial hole burning near-field scanning optical microscopy (SHB–NSOM) is used to locally photopattern three species of organic thin films, poly(2-methoxy, 5-(2′-ethyl hexyloxy)–p-phenylene vinylene) (MEH–PPV), tris-8-hydroxyquinoline aluminum (Alq3) and dye-functionalized polyelectrolyte self-assembled layers, on a 100 nm length scale. In SHB–NSOM the film is illuminated with light from a stationary NSOM tip to induce photo-oxidation. The reduction in the fluorescence yield resulting from this exposure is then mapped using fluorescence NSOM (FL–NSOM). We have examined the localized photo-oxidation as a function of time, position, and environment free from the limits of far-field spatial averaging. In all of the thin film materials studied we find that the long-time diameter of the dark spot is much larger than the tip diameter and is a signature of energy migration. Characteristic lengths of the energy migration are extracted from this data by a simple diffusion model and are found to be of the order of a few hundred nanometers for each of the films studied.