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:
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:
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):
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:
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).