J. Phys. Chem. 1993.97, 3091-3094
3091
Near-Field Scanning Optical Spectroscopy: Spatially Resolved Spectra of Microcrystals and Nanoaggregates in Doped Polymers Duane Bimbaum, Seong-Keun Kook, and Raoul Kopelman' Department of Chemistry, University of Michigan, Ann Arbor, Michigan 481 09 Received: January 8, 1993
Near-field optical techniques make it possible to bypass 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 0 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 a- and 0-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.
Introduction Near-field scanningoptical microscopy (NSOM) is yet another form of scanning probe microscopy that has recently been developed and is generating considerable interest.',* It bypasses the optical diffraction limit (X/2 or 200 nm) through the use of a small light source which effectively focuses photons through a tiny aperture that may be as small as X/50. For a short distance from this aperture (much less than the wavelength of light employed), the emissive photons are highly collimated. The region of collimated light is known as the "near-field" region, and its diameter will define the resolution of an image generated from such a small probe. Unlike scanningtunneling (STM) or atomic force microscopies (AFM), imaging in NSOM is via the interaction of light with the surface by a simple contrast mechanism such as absorption or fluorescence. The advantages of NSOM are its noninvasive nature, its ability to look at nonconducting and soft surfaces, and the addition of a spectral dimension. The latter generally does not exist in either STM or AFM. It is this potential for extracting spectroscopic information from a nanometer-sizedarea that makes it particularly attractive. Examples include the detection of fluorescent labels on biological samples and isolating local nanometer-sized heterogeneities in microscopic samples. The major disadvantage of NSOM is that its spatial resolution has not yet equaled the above-mentioned microscopies. The highest spatial resolution reported for NSOM thus far has been -12 nm.' Photon scanning tunneling microscopy (PSTM) is similar to NSOM in that the spectral dimension exists, but the mechanism is quite differ~nt.3.~ It is essentially the optical analog of STM. In PSTM, a small optical probe (usually a chemically etched fiber) is brought close to a sample which is immemed in an evanescent field. Photons then 'tunnel" from the field to the tip, and if the signal is kept constant by varying the distance of the tip from the surface, information regarding the sample surface topolagy is recorded. A disadvantageof this technique is that the sample must be a thin film that does not fully attenuate the exponentially decaying evanescent field. PSTM has been shown to be particylarly useful for looking at dielectric^.^ However, for sample surfaces that are not well-defined there is the obvious problem of distinguishing between absorption or fluorescence 0022-3654/93/2097-309 1S04.00/0
and local sample thickness. PSTM's greatest and perhaps only advantageover NSOM is that a feedbackmechanismfor knowing how close the probe is to the sample is inherent in the system. In near-field scanningoptical spectroscopy (NSOS), an optical fiber with an emissive aperture that is submicrometer in size is positioned such that the sample is within the near-field region. With piezoelectriccontrolofthe fiber tip, the tipcan beaccurately positioned over a fluorescing region of the sample and a spectrum recorded (Figure 1). Excitation of the sample can be either external with detection through the fiber tip or excitation with the fiber tip itself and subsequentdetection of the emittingphotons. This means that it is not necessary for the sample to be of any particular thickness or opacity; however,it should havea relatively smooth surface. The resolution of an NSOM image is limited by the size of the light probe. Optical fiber tips are easily fabricated to sizes of 50 nm or smaller in our lab, and the smallest nanofabricated optical fiber tipreported todateisabout 20nm.I At thissize, thenumber of photons emitted from the fiber tip is too low to excite a large enough number of fluorophores such that their subsequent emission can be detected. Of course, then it is impossible to detect a single molecule. Alternative light sources have been proposed for obtaining molecular resolution, most notably, the molecular exciton sourp.s Such a light source incorporates a nanometer-sizedorganic crystal at the end of a pulled micropipet with an inner diameter of a few nanometers. Excitation of the crystal then results in the creation of excitonswhose travel through the crystal is only limited by the crystal size, which potentially could beof a molecular dimension. Excitation of a sample would then proceed through a Forster type energy-transfer machanism, which may be orders of magnitude more efficient than emission and subsequentreabsorptionby the samplc6 However,fabrication of such a source at molecular dimensions has not yet been realized due to the confined region in which a crystal must be grown. To date, relatively little work has concentrated on the spectroscopic dimension inherent to NSOM and its ability to study nanometer-sized regions in various samples. In this paper we present the potential for addressing spectroscopy on a submicrometer scale. We have studied systems of tetracene and perylene doped in PMMA as well as microscopiccrystals in order 63 1993 American Chemical Society
3092
Letters
The Journal of Physical Chemistry, Vol. 97, No. 13, I993 TOLPW aDClec(0c
4
HeCd or Ar+ laser
Microscope Optics To Dclcetor
niuror
Figure 1. Illustration of near-field scanning optical spectroscopy. Excitation can come via the fiber tip (internal) or from an external source.
OMA
lens
to demonstrate that nanoscopic inhomogeneities can be detected in what might at first appear to be a homogeneous sample. The eventualgoal is to obtain spectroscopicinformation with a spatial resolution approaching molecular sizes.
Near-field Microscope
piezo
Experimental Section Our procedure for fabricating nanometer-sized optical fiber light sources is based on the technique developed by Betzig et al.' Radiation (10.6 pm) from a COz (Synrad) laser is focused onto a single-mode optical fiber (3M or Newport, 2-3-pm core, 80100-pm cladding) which is secured in a micropipet puller (Sutter Instrument Co.). Upon pulling, the fiber tapers (taper length
