Spatially Resolved Spectra of Micro-Crystals and Nano-Aggregates in Doped Polymers
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SPATIIALLY R•ESOLVED SPECTRA OF MIICRO-CRYSTALS AND NANO-AGGIREGATES IN DOPED POLYMERS
DUANE BIRNBAUM, SEONG-KEUN KOOK and RAOUL KOPELMAN Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 ABSTRACT Near-field optics techniques make it possible to by-pass the optical diffraction limit ("uncertainty principle) and attain spatial resolution of X/50 or better. We present near-field scanning optical spectroscopy (NSOS) data on a and Bmixed micro-crystals of perylene and on various aggregates of tetracene doped into PMMA. The spatial resolution is limited by the size of the scanning photon tip and its distance from the sample. We use nanofabricated optical fiber tips (aluminum coated) that are as small as 100 nm. These can be piezoelectrically scanned close to the sample. Fluorescence spectra easily differentiate between adjoining microcrystallites of a and Bperylene, 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 hosL INTRODUCTION Near-field scanning optical microscopy (NSOM) has generated considerable interest recently as yet another form of scanning probe microscopy [1,2]. But unlike scanning tunnelling (STM) or atomic force microscopies (AFM), imaging in NSOM is via the interaction of light with the surface by either simple contrast, or absorption and fluorescence mechanisms. Photon scanning tunnelling microscopy (PSTM) is similar, but the use of this technique to image absorbing or fluorescing surfaces can yield misleading results.[3] The advantages of NSOM are its non-invasive nature, its ability to look at nonconducting and soft surfaces, and its addition of a spectral dimension, the latter of which does not exist in either STM or AFM. The main emphasis of this paper is to demonstrate the ability of this technique to examine systems spectroscopically with a spatial resolution greater than that of conventional, diffraction limited microscopies (W2 or 400nm). This technique has considerable potential in applications where it is necessary to extract spectroscopic information from a nanometer-sized area. Examples include the detection of fluorescent labels on biological samples and isolating local nanometer-sized heterogeneities in microscopic samples. The major disadvantage is that its spatial resolution has not yet equalled the above mentioned microscopies. The highest spatial resolution reported for NSOM thus far as been -12 nm.[1] In near-field scanning optical spectroscopy (NSOS), an optical fiber with an emissive aperture that is submicrometer in size is positioned within a distance of the sample that is much less than the wavelength employed in the so-called near-field region. With piezoelectric contro
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