NSOM of Advanced Organic Thin Film Materials

Near Field Scanning Optical Microscopy/Spectroscopy (NSOM) of Advanced Organic Thin Film Materials

Joseph Kerimo, David M. Adams, David A. Vanden Bout, Daniel A. Higgins and Paul F. Barbara*
Department of Chemistry, University of Minnesota, Minneapolis, MN 55455

This informal report briefly describes recent efforts at Minnesota in applying NSOM to the investigation of the structure, spectroscopy and photochemistry of thin film materials. NSOM is a type of scanning probe optical microscopy that breaks the diffraction limit to resolution of ordinary (far field) microscopy by illuminating (or collecting) through a subwavelength aperture in a NSOM probe. NSOM provides high resolution optical and simultaneous topographic images of the sample. The high resolution and single molecule sensitivity of NSOM allows it to be used to probe extremely small spatial regions. Consequently, the optical and electronic properties of materials and their spatial variation can be studied over small distance scales. The bibliography at the end of this article includes representative references of early NSOM research and more recent references from our laboratory and others.

In this short note we will summarize some of our recent applications of NSOM to fluorescent thin film materials. In the last three years we have been able to efficiently and routinely study a broad range of materials without artifacts from film damage by heating, scratching or photochemistry. We have been able to accomplish this by carefully optimizing the NSOM aparatus and the experimental protocol that we use. The overall conclusion of this work is that NSOM clearly offers uniquely useful information on mesostructured organic thin films. It complements such optical methods of thin film analysis as AFM, STM, TEM and X-ray scattering.

NSOM is fundamentally an imaging technique. A graphic representation of the data with a high resolution color scale is critical in visualizing the results.

NSOM investigations in our laboratory employ a modified Topometrix Aurora instrument. The basic microscope has been modified by the addition of a glove box around the scanning head to control the oxygen and moisture level of the sample. The detection and collection optics were modified to allow for polarization analysis and single molecule sensitivity. Figure 1a shows an image of the test sample DI fluorescent molecules on a PMMA film, demonstrating the tremendous sensitivity of the NSOM technique. This fluorescence NSOM image is also an approximate measure of the resolution of the apparatus since the apparent size of the single molecule shape in the image is a measure of the size of the optical field of the probe, which is in the range of 50 to 100 nm typically.

Our group has invested considerable effort in understanding the structure and dynamics of self-assembled fluorescent fibers based on the PIC dye and polyvinyl sulfate (PVS). These samples have offered several opportunities to test the capabilities of NSOM in investigation of mesostructure. The PIC/PVS films assemble into long fluorescent yarnlike aggregates which rest on the surface of a spin-cast film. AFM images show that the fibers are hundreds of microns long (Fig. 1b), and detailed AFM images reveal occasional split ends of the fibers (Fig. 1c). Fluorescence NSOM images demonstrate that the dye is restricted to an aggregated state as shown in Fig. 1d. The fluorescence of the aggregates is highly polarized along the long direction of the fibers, as demonstrated in Figs. 1e and 1f). In our articles on these PIC aggregates the spectroscopy, NSOM images, and ultrafast NSOM data has been used to characterize the molecular arrangement of the aggregates and to understand the energy migration dynamics. Figs. 1h and 1i give another approach to estimating the distance scale for energy (exciton) migration. The NSOM images recorded before and after photobleaching in a localized stripe can be used to prove that the energy migration does not occur over a distance of 50 nm in these aggregates. The use of spot sizes to determine energy migration distances in films is a powerful new technique for measuring migration of excitons that we have denoted by the term spatial hole-burning NSOM.

Figure 1. (A) NSOM Fluorescence Image of DiI Single Molecules; (B and C) Non-Contact AFM Images of PIC/PVS Molecular Yarn; (D through G) Fluorescence NSOM Image of PIC/PVS Molecular Yarn; (H and I) Before and After Images of Spatial Hole-Burning of PIC/PVS Molecular Yarn; (J) Topography of a Small PIC Dye Crystal; (K) NSOM Fluorescene Image of Crystal in Fig. 1J; (L) NSOM Image of 620 nm Emission from Crystal in Fig. 1J.

Figs. 1j and 1k are topography and NSOM images of crystals of the PIC dye. Our NSOM investigation of these materials have shown that they are composed of a super lattice of plate-like single crystal domains which extend through the entire crystal. The spectra of these crystals show two distinct bands whose relative intensity varies depending on point of excitation, as described in detail in our full articles on this subject. Fluorescence excitation images at specific wavelengths, for example 620 nm (Fig. 1l) and Fig. 2a (700 nm), show that the emitting traps are formed in distinct regions. This is a dramatic demonstration of NSOM' s ability to spatially resolve optical and photophysical variations.

In the short format of this article, it is not possible to describe the many types of samples that we have investigated in detail. Rather we will briefly mention some of the results and invite the reader to look at the full articles which are listed along with titles in the bibliography.

We have applied NSOM to the study of conjugated polymer films which are used in LED devices. Fluorescence NSOM images with polarization selection (Figs. 2b and 2c) show that conjugated polymer films of the PPyV type are actually comprised of locally oriented regions of polarized chromophores on the less than 200 nm distance scale. NSOM led to the first definitive data on the existance of such mesostructure in this film type.

One of the powerful modifications of the NSOM experiment is demonstrated in Figs. 2d, 2e and 2f. Here polarization modulation NSOM is used to study the absorption and anisotropy and the direction of the anisotropy for nano-crystals of rhodomine 110. Fig. 2d displays the magnitude of the absorption anisotropy of these crystals, while the direction of the anisotropy is represented in Fig. 2e. Fig. 2f shows polarization modulation and some images recorded at a different wavelength where the crystals did not absorb. This image is a measure of the local variation in the refractive index including its dependence on orientation in the crystal. It is extraordinarily sensitive to nanoscopic crack stresses.

Preliminary results on vapor deposited films of a perylene derivative PPEI are shown in Figs. 2g and 2h which correspond to vertical and horiztonal fluorescence detected NSOM. These measurements uniquely reveal the local polycrystalinity of these vapor deposited materials. They are useful as the synthetic chemists refine their techniques for film preparation. Similar images of layered materials are leading in our laboratory to a spatial resolution of energy transfer and electron transfer in organic thin films. A simple example is shown in Fig. 2i where a NSOM image of a twin rhodomine 110 crystal is shown. We are developing methods to measure the efficiency of energy transfer across the interface between the different crystalline domains. A very different type of sample is shown in Figs. 2j and 2k. These are multilayer structures which have been prepared by Mallouk and coworkers. They involve alternating polymer doped layers with different dyes that are separating by interleaving inorganic layers. Spatial hole-burning and NSOM images of scratched samples has allowed us to verify the basic structural model proposed by Mallouk as well as to begin to characterize the interlayer energy transfer between dye dopants in the polymer layers.

Figure 2. (A) NSOM Image of Emission from Crystal Defects from Crystal in Fig. 1J; (B and C) Horizontal and Vertical Polarized Fluorescence from the Thin Film of PPyV; (D through F) PM-NSOM Images of Rhodamine 110 Crystals; (G and H) Polarized Fluorescence Near-Field Images of Thin Film of PPEI Crystals; (I) Non-resonant NSOM Transmission Image of Rhodamine 110 Crystals; (J and K) NSOM Fluorescence Images of Dye-Labeled ZrP Multilayered Structures; (L) Scanning Confocal Image of Single Luminescent Conjugated Polymer Molecules.

A final example is given in Fig. 2l. This image is a scanning confocal microscopy image of single PPYV derivative polymer molecules. We have been using single molecule imaging and single molecule spectroscopy of multichromophoric conjugated polymer molecules to study the intramolecular energy transfer and photochemistry of these types of materials. Such an approach offers a molecule by molecule examination of the inhomogeneous photochemical aspects of fluorescent polymers and promises to revolutionize the understanding of these systems.

We hope we have teased the reader with this very superficial summary of some of the recent results from our laboratory. We invite the reader to read our full publications using NSOM as listed below.

BIBLIOGRAPHY
(Representative articles on NSOM, single molecule spectroscopy, and the materials described above)

1. "Evolution of Photophysical and Photovoltaic Properties of Perylene Bis(Phenethylimide) Films Upon Solvent Vapor annealing", B. A. Gregg, J. Phys. Chem., 100, 852 (1996).
2. "Imaging and Time-Resolved Spectroscopy of Single Molecules at an Interface", J. J. Macklin, J. K. Trautman, T. D. Harris, L. E. Brus, Science, 272, 255-258 (1996).
3. "Near-Field Optics: Microscopy, Spectroscopy, and Surface Modification Beyond the Diffraction Limit", E. Betzig, J. K. Trautman, Science, 257, 189-195 (1992).
4. "Optical Spectroscopy: Image Recording with Resolution l/20", D. W. Pohl, W. Denk, M. Lanz, Appl. Phys. Lett., 44, 651-653 (1984).
5. "Spatially Resolved Spectral Impurities in Small Molecular Crystals Studied by Near-Field Sanning Optical Microscopy", D. A. Vanden Bout, J. Kerimo, D. A. Higgins, P. F. Barbara, J. Phys. Chem., 100, 11843 (1996).
6. "Excitonic Transitions in J-Aggregates Probed by Near-Field Scanning Optical Microscopy", D. A. Higgins, P. F. Barbara, J. Phys. Chem., 99, 3-7 (1995).
7. "A Molecular Yarn: Near Field Optical Studies of Self Assembled, Flexible Fluorescent Fibers", D. A. Higgins, J. Kerimo, D. A. Vanden Bout, P. F. Barbara, J. Am. Chem. Soc., 118, 4049 (1996).
8. "Structure and Exciton Dynamics in J-Aggregates Studied by Polarization-Dependent Near Field Scanning Optical Microscopy", D. A. Higgins, P. J. Reid, P. F. Barbara, J. Phys. Chem., 100, 1174-1180 (1996).
9. "Polarization-Modulation Near-Field Scanning Optical Microscopy of Mesostrucured Materials", D. A. Higgins, D. A. Vanden Bout, J. Kerimo, P. F. Barbara, J. Phys. Chem., 100, 13794 (1996).
10. "Environment-Dependent Photophysics of Polymer-Bound J Aggregates Determined by Time-Resolved Fluorescence Spectroscopy and Time-Resolved Near-Field Scanning Optical Microscopy", P. J. Reid, D. A. Higgins, P. F. Barbara, J. Phys. Chem., 100, 3892-3899 (1996).
11. "Near Field Fluorescence of LH Complexes and Photosynthetic", R. C. Dunn, G. R. Holtom, L. Mets, X. S. Xie, J. Phys. Chem., 98, 3094-3098 (1994).
12. "Detection and Imaging of Single Molecules by Optical Near-Field Microscopy", T. Basche, Angew. Chem. Int. Ed. Engl., 33, 1723-1725 (1994).

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