Luminescence of ultrathin organic films: Transition from monomer to excimer emission

We investigate high-quality evaporated dye layers of N,N'-dimethylperylene-3,4:9,10-bis-discarboximide of varying thicknesses, using temperature-dependent luminescence. Layers with a nominal thickness much less than a monolayer show monomer emission similar to that of dilute solutions. With increasing layer thickness, the luminescence is dominated by excimer transitions. The excimer linewidth decreases markedly with decreasing temperature. We show that the aggregation of molecules can be controlled by variation of the surface roughness. (C) 1996 Optical Society of America