SPATIALLY-RESOLVED SPECTRAL INHOMOGENEITIES IN SMALL MOLECULAR-CRYSTALS STUDIED BY NEAR-FIELD SCANNING OPTICAL MICROSCOPY

Near-field scanning optical microscopy (NSOM) has been employed to spa tially resolve mesoscopic inhomogeneous spectral features in small cry stals of the dye 1,1'-diethyl-2,2'-cyanine iodide (PIC). The small cry stals show strong absorption perpendicular to their long direction of growth and no absorption in either of the two other orthogonal directi ons. This polarization is seen uniformly throughout the crystals. Topo graphic images reveal the crystals are composed of platelike single-cr ystalline domains separated by defect planes. The individual plates sh are a common orientation that gives the crystal its single polarizatio n. Despite the uniformity of the polarization throughout the crystal, dramatic spatial variation is seen in the fluorescence spectrum. NSOM images and spectra reveal two distinct emission sources in the crystal s. Strong self-absorption is seen in the PIC emission, and red-shifted fluorescence is also observed from lower energy emissive traps. These traps are seen to form in distinct regions within the crystal and bec ome more abundant over long periods of time (months). In addition to t he dual wavelength emission from the crystals, the fluorescence spectr a taken in the near field are shown to be significantly narrower than those taken in the far field. The near-field spectra reveal that appro ximately 100 cm(-1) of the broadening observed in the far-field fluore scence spectra can be attributed to inhomogeneous broadening due to cr ystal features which exist over distances as large as 100 nm. These re sults may be general to small molecular crystals but would have been n early impossible to detect with any method other than NSOM.