FINITE-SIZE EFFECTS OBSERVED IN THE FLUORESCENCE OF ULTRATHIN CRYSTALLINE ORGANIC FILMS GROWN BY ORGANIC MOLECULAR-BEAM DEPOSITION

We study the low-temperature fluorescence spectra of stacks of alternating layers of crystalline organic thin films grown by organic molecular beam deposition. We find that there is an increase in molecular vibrational frequency and a redistribution of fluorescence intensity to higher energy as the layer thickness decreases from 500 to 10 angstrom. These observations indicate that the width of the molecular potential energy well of the ground state and the displacement of the excited and ground states are functions of layer thickness. We explain these results using a model based on electronic wavefunction confinement and a systematic change in the exciton-phonon coupling in ultrathin molecular films.