NEAR-FIELD SCANNING OPTICAL SPECTROSCOPY - SPATIALLY RESOLVED SPECTRA OF MICROCRYSTALS AND NANOAGGREGATES IN DOPED POLYMERS

Near-field optical techniques make it possible to bypass the optical diffraction limit (''uncertainty principle'') and attain spatial resolution of lambda/50 or better. We present near-field scanning optical spectroscopy (NSOS) data on alpha and beta mixed microcrystals of perylene and on various aggregates of perylene and tetracene doped into PMMA. We use nanofabricated optical fiber tips (aluminum coated) with apertures as small as 100 nm. These can be piezoelectrically scanned close to the sample. Fluorescence spectra easily differentiate between adjoining microcrystallites of alpha- and beta-perylene, giving spectra identical with those of large (>1 cm) single crystals. The apparently homogeneous molecularly doped polymer samples of tetracene/PMMA have regions that fluoresce anywhere between green and red. Thus, the spatially resolved spectra are much sharper and more detailed than the broad and featureless bulk spectra. The different emission spectra are attributed to different aggregates of the tetracene guest embedded in the PMMA host. The spatial resolution is limited by the size of the scanning photon tip and its distance from the sample. NSOS data on samples of perylene doped into PMMA suggest that sample heterogeneities can be recognized down to sizes approximately an order of magnitude smaller than the size of the light probe.