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 spatially resolve mesoscopic inhomogeneous spectral features in small crystals of the dye 1,1'-diethyl-2,2'-cyanine iodide (PIC). The small crystals show strong absorption perpendicular to their long direction of growth and no absorption in either of the two other orthogonal directions. This polarization is seen uniformly throughout the crystals. Topographic images reveal the crystals are composed of platelike single-crystalline domains separated by defect planes. The individual plates share a common orientation that gives the crystal its single polarization. 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 crystals. 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 become more abundant over long periods of time (months). In addition to the dual wavelength emission from the crystals, the fluorescence spectra taken in the near field are shown to be significantly narrower than those taken in the far field. The near-field spectra reveal that approximately 100 cm(-1) of the broadening observed in the far-field fluorescence spectra can be attributed to inhomogeneous broadening due to crystal features which exist over distances as large as 100 nm. These results may be general to small molecular crystals but would have been nearly impossible to detect with any method other than NSOM.