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 u sed to locally photopattern three species of organic thin films, poly(2-met hoxy, 5-(2'-ethyl hexyloxy)-p-phenylene vinylene) (MEH-PPV), tris-8-hydroxy quinoline aluminum (Alq(3)) and dye-functionalized polyelectrolyte self-ass embled layers, on a 100 nm length scale. In SHB-NSOM the film is illuminate d with light from a stationary NSOM tip to induce photo-oxidation. The redu ction in the fluorescence yield resulting from this exposure is then mapped using fluorescence NSOM (FL-NSOM). We have examined the localized photo-ox idation as a function of time, position, and environment free from the limi ts of far-field spatial averaging. In all of the thin film materials studie d 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 len gths of the energy migration are extracted from this data by a simple diffu sion 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].