SF microscope from Munster

Chemical and Conformational Imaging of Interfaces

Mathias Flörsheimer, University of Münster, Germany

The age of the film imaged here is two weeks. The texture is typical for an aged film. The dark areas were not observed in fresh monolayers. The density and size of the dark spots increases gradually. The change of the texture may be due to two reasons. Either dewetting occurs or the molecular order of the dark regions has changed. Using the spectroscopic information in the SF images, the Münster group could find the actual reason for the pattern evolution. In the SF imaging experiment, the monolayer is illuminated with two intense laser beams in order to generate an SF signal. One of the lasers provides visible light (532 nm). The other one is tunable to the IR absorption bands of the specific functional groups of the molecules. In the image of Figure 1, the IR light is in resonance with the asymmetric stretch vibration of the CH3 groups at 2962 cm-1 (3376 nm). The nonlinear signal, radiated from the surface, occurs in the visible spectrum (460 nm). The monolayer is then imaged in the light of this weak signal using a conventional linear optical polarization microscope and an intensified CCD camera. The local brightness in the image is a measure of the local density and orientation of the CH3 groups with respect to the polarisers. The transformation of the IR information into the visible spectrum is the reason for the improved spatial resolution limit.
	Figure 1: Sum frequency (SF) micrograph of an LB monolayer of arachidic acid. The temporal evolution of the dark areas during aging is not due to local molecular reordering but to dewetting as can be shown using the spectroscopic information in the SF images.

In order to investigate if the small intensity in a part of the CH3 image is due to molecular reordering, Christof Brillert in Flörsheimer's team tuned the IR laser also to the symmetric stretch band of the CH2 groups (2850 cm-1, 3509 nm). In fresh films of arachidic acid, only a small SF intensity was then observed. This can be explained with the straight chains of these fatty acid films. The CH2 groups of an all-trans chain are in an almost centrosymmetric order. Under these conditions the generation of a second-order optical signal is symmetry-forbidden. If, however, gauge defects would be introduced during aging of the monolayer, an increase of the CH2-signal would be expected. From films of other lipids such as oleic acid with high densities of kinks, the Münster team actually observed high CH2-signals. But in the case of arachidic acid, the small CH2-intensities from the fresh films did not increase significantly during aging. This means that the bright areas in a CH3-image (Figure 1) represent the original monolayer with almost straight chains and the dark areas in such SF micrographs are due to dewetting.

Additional small spots can be observed in the interior of the dark areas of Figure 1. It can be expected that these spots are due to three-dimensional crystallites and aggregates which act as traps for the fatty acid molecules. The molecules can diffuse over the surface as long as they are not attached to an aggregate. The Münster group verified this interpretation applying a metal decoration technique to the organic films [3]. This technique allows to transform the two-dimensional distribution of the molecular density into a linear optical polarization microscopy contrast. In contrast to the decoration technique, the novel SF microscope requires no staining or decoration of the specimens. It provides direct information on the chemical composition and molecular order of the interface.

References

1. M. Flörsheimer, Ch. Brillert, H. Fuchs, Langmuir 15 (1999) 5437
2. M. Flörsheimer, Ch. Brillert, H. Fuchs, Materials Science and Engineering C 8-9 (1999) 335
3. Ch. Bubeck, Adv. Mater. 2 (1990) 537

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