Infrared-vibrational probing of organic nanocomposites

The expansion of scattering-type scanning near-field optical microscopy (s-SNOM) into the IR-vib spectral region has emerged as a promising technique due to its ability to achieve all-optical spatial resolution down to the several nanometer range in combination with the chemical sensitivity provided by infrared spectroscopy.

In our work we have brought the technique a big step forward both in terms of spatial resolution and sensitivity. We achieved imaging contrast even between nanodomains of spectrally similar chemical constituents.

Here, we demonstrate the capabilities for the investigation of nanodomains of diblock-copolymer thin films:

Chemical structure and schematics off the thin film morphology of the block-copolymer poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) consisting of a regular micellar arrangement of P2VP cores with a PS corona.

The contrast in s-SNOM relies on the optical coupling between the probe tip and the sample, that is, how the light scattered by the tip is affected by a change of the dielectric function in its immediate environment:

Generation of infrared vibrational contrast between the IR-vib spectrally dissimilar polymer consituents: calculated spatial variation (vertically and laterally) of the effective optical polarizability of the scanning probe tip in proximity to the two polymers for a probe wavelength of 3.39 (2950 cm-1). The contrast is due to the both - the spectral variation of the vibrational resonances (expresses in terms of the dielectric constants epsilon_PS and epsilon_P2VP) and the dependence of the near-field optical coupling on the tip-sample distance.

The oscillating tip projects the local optical tip-sample interaction into the radiating far-field - thus providing the imaging contrast and ultimately carrying information on the optical properties of the sample with a spatial resolution only limited by the tip radius. In addition, the degree of near-field localization and enhacement of the optical field at the tip apex determines resolution and sensitivity.

The microphase separation of the block-copolymer and the resulting lateral variation of the chemical composition can then be resolved. The figure shows simulataneously recorded topographic (left) and IR s-SNOM images at 3.39 m (2950 cm) (middle):

Topographic (left) , IR s-SNOM (3.39 m, middle) and vis s-SNOM image (right, 632.8 nm) of PS-b-P2VP. The protruding regions represent the P2VP domains.

The data demonstrate a spatial resolution of <10 nm. With a signal change of 1% being readily detectable a difference of several 103 C-H groups would suffice to provide the contrast observed. The sensitivity can be further improved probing chemically specific modes which distinguish the respective consitutents. In contrast, probing at 632.8 nm, i.e. off the vibrational resonance (right), no definite optical contrast is observed.

The thin film morphology of the block-copolymer as prepared represents a thermodynamically metastable configuration. The following demonstrates the capability of IR s-SNOM for the spatially resolved study of a thermally induced structural phase transition:

Topographic and corresponding IR s-SNOM image of PS-b-P2VP after several minutes of ex situ annealing near the glass transition temperature. The surface largely flattens due to a spatilly rearrangement of the polymer chains, yet as seen from the s-SNOM scan the laterally phase separated domains still persist.

This work has been in collaboration with Dong Ha Kim (Max-Planck-Institute for Polymer Research, Mainz) and Karsten Hinrichs (Institute for Analytical Sciences, ISAS, Berlin).

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