Reflectance and fluorescence spectroscopy of ultrathin PTCDI-C5 films on muscovite mica

- A wetting layer containing flat-lying PTCDI-C5 molecules forms on the surface of mica.
- The evolution of the fluorescence spectrum resolves a 2D gas to condensed phase transition.
- The π-π stacking emergences after the completion of the PTCDI-C5 monolayer layer.
- The uncompleted PTCDI-C5 monolayer dewets upon air exposure.

We have monitored the evolution of the optical properties of ultrathin N,N′-dipentylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C5) molecular thin films during their growth on muscovite mica surfaces using differential reflectance spectroscopy (DRS) and fluorescence spectroscopy (FL). In the monolayer regime, the DRS shows a monomer-like absorption, whereas the FL reveals a transition from monomeric to excimer-like emission accompanied by a quenching of the intensity with the increase of coverage. The observed evolution of the FL signals can be explained by a two dimensional (2D) gas to condensed phase transition. The characteristic fluorescence of the condensed phase implies an enhancement of the intermolecular coupling and the opening of new channels for the relaxation of the excited state. The π-π stacking, which is manifested by both DR and FL spectroscopies, emergences only after the completion of the first monolayer. Finally, by analyzing the variation of the fluorescence spectrum, we have revealed the dewetting of the uncompleted PTCDI-C5 monolayer upon air exposure. This observation has been verified by atomic force microscopy (AFM).

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