SNOM, a new photophysical tool

G. Kaupp, A. Herrmann, J. Schmeyers, J. Boy
Organic Chemistry I, University of Oldenburg, Germany
E-mail: kaupp@kaupp.chemie.uni-oldenburg.de

Introduction

Scanning near field optical microscopy (SNOM) on rough surfaces is a new photophysical tool for the recording of chemical contrast at the submicroscopic level [1]. It complements submicroscopic techniques such as atomic force microscopy (AFM) that record topography and its changes down to the molecular and submolecular scale. The high importance for most surface phenomena is apparent not only in photochemistry [2], biology or materials' sciences. A particular problem in such surface sciences is the local identification of different species in minute quantities if only few molecular layers are involved. We report here on two distinct surface changes if phthalimide crystals (1) are exposed to vaporized water and their analysis by SNOM. These processes deserve scrutiny as they may complicate the study of solid state syntheses [3].

Results and Discussion

The (001) surface of phthalimide crystals (P21/n from acetone [4]) changes when left in moist air and similarly but more rapidly when stored in a vacuum near the saturation pressure of water at room temperature. The course of events may be easily followed with contact AFM. The initial surface in Fig. 1 is very flat (Rms = 0.349 nm; some pre-exposure is apparent), the step heights are at the monomolecular range and some peaks with heights of 2.3 - 3.0 nm persist in moist air.

Figure 1 - click on image for higher resolution
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Fig. 1. 6 µm AFM topographies on (001) of phthalimide (1) (from acetone; z = 10 nm), after 2 days in air (rel. humidity was 50 - 60%; z = 10 nm), and after 16 h in water vapor (z = 200 nm) at room temperature.

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VRML Image of Fig 1a (low resolution - approx 385 kB)
VRML Image of Fig 1a (high resolution - approx 6.089 kB)
VRML Image of Fig 1b (low resolution - approx 385 kB)
VRML Image of Fig 1b (high resolution - approx 6.084 kB)
VRML Image of Fig 1c (low resolution - approx 387 kB)
VRML Image of Fig 1c (high resolution - approx 6.119 kB)

However, numerous monomolecularly sized steps and deeper ditches align along the long crystal axis ([010]) upon the moderate exposure to moisture. More extended exposure to moisture (weeks) or exposure to undiluted vapor of water retained the direction of preference (the ditch in Fig. 1c is ca. 10 nm deep) but additionally volcano-like hills of considerable height (180 nm in Fig. 1c) were formed. Apparently, two different processes with long-range molecular movements are seen. The most likely interpretation is surface hydration for the terraces/ditches and hydrolytic ring opening to give phthalamic acid 2 for the volcanoes.

The extremely minute quantities of material preclude common chemical analyses, however, the photo-tool SNOM is able to differentiate locally the two processes via chemical near-field contrast.

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Fig. 2. Shear-force AFM and simultaneous reflection-back-to-the-fiber SNOM on (001) of a phthalimide crystal (1) that was exposed to water vapor. Optical contrast at 488 nm.

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VRML Image of Fig 2 AFM (low resolution - approx 387 kB)
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VRML Image of Fig 2 SNOM (low resolution - approx 384 kB)
VRML Image of Fig 2 SNOM (high resolution - approx 6.067 kB)

The simultaneous SNOM and AFM experiment [1] shows a strong negative chemical contrast in the optical image. It provides evidence that the volcanoes are chemically different and must therefore consist of phthalamic acid (2), while the hydrated plain of 1 is free of 2. Interestingly, the near-field reflectivity of hydrated 1 is higher than that of 2, but we cannot predict the sign of the relative contrast, yet. The reliability of the SNOM image is indicated by the precise local correspondence with the topographic image [5]. The broad and very minor positive features do not seem to be part of the chemical contrast. An even more advanced hydration/hydrolysis of 1 is analyzed in Fig. 4. In order to secure our interpretation we also changed the wavelength to 633 nm. We have again the distinct negative contrast on the small and large hills (up to 135 nm height). The direction of preference (again along [010]) is still seen.

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Fig. 3. Simultaneous AFM/SNOM on (001) at a more advanced state of exposure of crystal 1 to water vapor. Optical contrast at 633 nm.

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VRML Image of Fig 3 AFM (low resolution - approx 386 kB)
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VRML Image of Fig 3 SNOM (low resolution - approx 384 kB)
VRML Image of Fig 3 SNOM (high resolution - approx 6.078 kB)

The rough plain is not free of negative contrast and it appears that 2 is also present in the minutely damed extended part to the lower right and at further locations. There seems to be a tendency for bright contrast in the depths, where the water content may be higher. Again, the perfect correspondence of optical contrast and topography guarantees the validity of the data [5], that can be explored interactively using the VRML full data.

Phthalamic acid 2 is the product of base catalyzed hydrolysis of 1 in solution, followed by acidification [6] and there remain no other reasonable possibilities for the interpretation of the SNOM data. Thus, the correlation of the AFM/SNOM data with the crystal structure of 1 [4] is of importance. The 3D images in Fig. 4 show the stacking of hydrogen bridged dimers of 1 parallel to (001). A further glide plane is (101) that runs parallel to the b-axis and therefore explains the direction of preference in the AFM images of Figs. 1 - 3.

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Fig. 4. Stereoscopic representations of the crystal packing of 1. Upper pair: (010), turned by 10° around y for a better view. Lower pair: (101). The (001)-face is on top of both images.

One half of the carbonyl groups of 1 remain unbridged. It is therefore comprehensible that the surface may be hydrated and reconstructed, initially in monolayers. Furthermore, the molecules 2 once formed have to move above the (001) surface and leave the lattice, as they do not geometrically fit. An obvious way is along the (101) glide plane, but that is not smooth and makes such movement difficult. The best way for the system is to start at nucleation centers and build islands around it. Thus, the occurence of isolated volcanoes reflects nicely the correlation to the crystal structure.

Conclusions
The applications of SNOM are by no means restricted to photochemistry [2]. The present model for the application of the new photo-tool points to numerous applications in all fields of surface sciences where the submicroscopic elucidation of local surface structure and surface change gains more and more importance [1]. SNOM on rough surfaces opens exciting new possibilities for supermicroscopy with light in the near-field at distances of interaction that are well below 10 nm, including local near-field fluorescence and Raman spectroscopy [7].

Acknowledgements
This work was supported by the German Federal Ministry of Education and Research BMBF as part of the project Nr. 13N7640/3: Optische Nahfeldmikroskopie und -spektroskopie: Optische Nahfeldmikroskopie auf rauhen Oberflächen mit Glasfaserspitzen in Kreuzpolarisation.

References
[1] G. Kaupp, Supermicroscopy in Supramolecular Chemistry: AFM, SNOM, and SXM, in Comprehensive Supramolecular Chemistry, vol. 8, p. 381-423 + 21 color plates; ed. J. E. D. Davies; Elsevier, Oxford, 1996
[2] G. Kaupp, Adv. Photochem. 1995, 19, 119-177
[3] G. Kaupp, J. Schmeyers, J. Boy, Tetrahedron 2000 in press
[4] S. W. Ng, Acta Cryst. 1992, C48, 1694-1695
[5] G. Kaupp, A. Herrmann, M. Haak, J. Phys. Org. Chem. 1999, 12, 797-807
[6] M. H. Aslam, A. G. Burden, N. B. Chapman, J. Shorter, M. Charton, J. Chem. Soc. Perkin Trans. II 1981, 500-508
[7] G. Kaupp, A. Herrmann, G. Wagenblast, Proc. SPIE-Int. Soc. Opt. Eng. 1999, vol. 3607, 16-25