The upper panel shows the topographical measurement of a diindenoperylene film. Lighter shades stand for higher areas, darker ones for lower areas. In the lower panel the topographical and the optical measurements are superimposed – the latter one in the red and yellow colour range; the brighter the colour, the higher the luminescence intensity. (Image: Research Group of Prof. Meixner, University of Tübingen)
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 6×10^5 times enhanced photoluminescence (PL). The excitation is 4×10^4 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.