Investigation of mesoscopic domains in thin organic films using near-field optical absorption mapping

We have used a near-field scanning optical microscope (NSOM) to conduct optical absorption mapping of a thin organic film of poly(phenylene vinylene) (PPV) and tris(8-hydroxy)quinoline aluminum (Alq). Using 488 nm light, NSOM transmission images revealed 100–300 nm `darkʹ features scattered randomly across the film. However, these `darkʹ features were not observed using 633 nm light. Simultaneously acquired topographic images revealed no significant changes in surface morphology over the `darkʹ features and indicated that the optical variation was not related simply to variations in film thickness or optical scattering. The origin of these `darkʹ features is believed to be related to ordered domains in the thin organic film which has a different absorption spectrum as compared to a disordered film. The change in contrast observed using 488 and 633 nm light was related to the optical absorption difference between the ordered and disordered regions at the two wavelengths.