Imaging Intermolecular Exciton Coupling in Metal-Free Phthalocyanine Nanofilms Using Tip-Enhanced Near-Field Optical Microscopy

Spatially correlated topography, fluorescence, and Raman scattering signals of metal-free phthalocyanine (H2Pc) nanofilms were imaged using confocal and tip-enhanced (fluorescence and Raman) near-field optical microscopy at lateral spatial resolutions of 500 nm and similar to 25 nm, respectively. The far-field fluorescence intensity was found to vary inversely with film thickness up to 10 nm, with the strongest emission coming from isolated H2Pc molecules on samples with submonolayer coverage. Changes in the fluorescence energy distribution were imaged and were consistent with exciton coupling predictions for H2Pc molecules lying edge-to-edge on the surface. Thicker H2Pc films (>10 nm) were found to consist entirely of nonfluorescing aggregates and crystal domains. Tip enhanced near-field measurements were carried out with W and Au (plasmonic) tips to investigate the local optical properties of H2Pc films and nanostructures. In general, fluorescence increased by a factor of 2-3X compared to the far-field when the tip was engaged; these signal levels suggest a local enhancement factor of 500x. However, the fluorescence was seen to increase by similar to 50% when the tip was lifted 10 nm above the surface, indicating strong coupling of molecular emissions to the metal probe over short distances (i.e., fluorescence quenching). Raman signals were strongly enhanced (>2-5X) over tip-surface distances <20 nm. X-ray diffraction and UV-vis absorption data were also acquired to help further elucidate changes in H2Pc packing and electronic structure during the transition from submonolayer coverage to continuous films. Overall, this work provides emission based measurements of bimolecular exciton coupling in solid-state H2Pc films, before the onset of complete fluorescence quenching, which has only been previously observed for select phthalocyanine species in solution.