Spatially Resolved Spectral Inhomogeneities in Small Molecular Crystals Studied by Near-Field Scanning Optical Microscopy

Abstract

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.

NSOM Investigations of the Spectroscopy and Morphology of Self-Assembled Multilayered Thin Films

“…NSOM measurements were made on a modified Topometrix Aurora near-field microscope using homemade NSOM probes. The apparatus and analogous experimental methods have been described in detail elsewhere. − Sub-wavelength-resolution probes (typically 50−100 nm resolution) were routinely obtained. The NSOM results were observed to be highly reproducible and not subject to artifacts due to sample heating by the NSOM probe, sample-tip contact, or photolysis of the samples by the near-field excitation.…”

“…NSOM is a high-resolution scanning probe technique which provides simultaneous topographical (resolution ≈ 10 nm) and optical (resolution ≥ 30 nm) images of thin-film materials. It has been used to study a wide range of nanostructured thin-film materials, including locally aggregated conjugated polymer thin films, self-assembled aggregate thin films of fluorescent molecules such as porphyrin wheels and J-aggregate yarns, organic nanocrystals, and organic molecular semiconductor heterojunctions. − The local concentration, local electronic structure, and local molecular orientation of nanostructured thin films has been effectively probed by near-field optical spectroscopy (both transmission and emission in conjunction with polarization techniques) via the local electronic spectra (emission and absorption) of these materials.…”

Spectral inhomogeneities and spatially resolved dynamics in porphyrin J-aggregate studied in the near-field

“…Fluorescence spectral measurements in the near-field have been applied to characterize spatial inhomogeneities of optical spectra in organic microcrystals. For example, Vanden Bout et al [7] reported Chemical Physics Letters 381 (2003) 368-375 www.elsevier.com/locate/cplett position dependence of the localized fluorescence spectrum in a crystal of pseudoisocyanine; Miura et al [8] studied a mixture of porphyrin monomers and dimers in polymer film and found excitation energy migration among monomers and energy trapping to dimers. However, J-aggregates of porphyrin are less emissive, which makes fluorescence SNOM studies difficult.…”

Controlled Crystallization of Gold Nanocrystals

“…The usage of these sensors is widely reported in a variety of fields including: liquid crystal orientation [45], molecular motors [46,47], the structure of glasses and polymers [48], and the study of liquids in confined spaces [49].…”