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 6×10⁵ times enhanced photoluminescence (PL). The excitation is 4×10⁴ 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.