Real-time changes in the optical spectrum of organic semiconducting films and their thickness regimes during growth

We present real-time in situ studies of optical spectra during thin film growth of several prototype organic semiconductors (pentacene, perfluoropentacene, and diindenoperylene) on SiO2. These data provide insight into surface and interface effects that are of fundamental importance and of relevance for applications in organic electronics. With respect to the bulk, the different molecular environment and structural changes within the first few monolayers can give rise to significant optical changes. Similar to interface-driven phenomena in, e.g., magnetism, spectral changes as a function of thickness d are a very general effect, decaying as 1/d in the simplest approximation. We observe energy shifts of 50-100 meV, rather small changes of the exciton-phonon coupling, and new transitions in specific systems, which should be considered as general features of the growth of organics.

Nanoscale spectroscopic imaging of organic semiconductor films by plasmon-polariton coupling

Tip-enhanced near-field optical images and correlated topographic images of an organic semiconductor film (diindenoperylene, DIP) on Si have been recorded with high optical contrast and high spatial resolution (17 nm) using a parabolic mirror with a high numerical aperture for tip illumination and signal collection. The DIP molecular domain boundaries being one to four molecular layers (1.5-6 nm) high are resolved topographically by a shear-force scanning tip and optically by simultaneously recording the 6x10{5} times enhanced photoluminescence (PL). The excitation is 4x10{4} times enhanced and the intrinsically weak PL-yield of the DIP-film is 15-fold enhanced by the tip. The Raman spectra indicate an upright orientation of the DIP molecules. The enhanced PL contrast results from the local film morphology via stronger coupling between the tip plasmon and the exciton-polariton in the DIP film.