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

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 (Alq(3)) 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. (C) 2000 American Institute of Physics. [S0021-9606(00)71116-0].