Crystal size dependence of fluorescence spectra from perylene nanocrystals evaluated by scanning near-field optical microspectroscopy

The fluorescence spectra of perylene nanocrystals, the crystal size of which is in the range of about 300 nm to 1 mum, were measured with the scanning near-field optical microspectroscopy. It has become apparent for the first time that the emission peaks from the self-trapped exciton state were evidently blue-shifted with decreasing crystal size. On the other hand, the emission spectra from the free exciton state were scarcely dependent on crystal size. These spectral changes were discussed semi-quantitatively on the basis of the strong coupling model between excitons and lattice for a molecular crystal. The influence of the substrate was also suggested.