High-resolution imaging of local oxidation in polyfluorene thin films by nonlinear near-field microscopy

The authors demonstrate nonlinear near-field two-photon photoluminescence imaging of organic semiconductors by coupling femtosecond light pulses to a scanning near-field optical microscope (SNOM) based on hollow-pyramid cantilevered probes. Two-photon excitation of the blue-emitting poly(9,9-dioctylfluorene) (PFO) is found to selectively address keto-defect units, which are responsible for a shifted green-orange emission. This effect is exploited to map oxidized sites in PFO thin films with high contrast and spatial resolution, introducing nonlinear SNOM as an effective technique to characterize oxygen-induced degradation in electroluminescent materials.