Multimodal imaging of heterogeneous polymers at the nanoscale by AFM and scanning near-field ellipsometric microscopy

Abstract

Scanning near field ellipsometric microscopy (SNEM) was used to simultaneously obtain optical images and tapping mode topography images of the microphase separated morphology of PS-b-P2VP block copolymer thin films. Optical images revealed a spatial resolution well below the diffraction limit. The SNEM setup used consisted of an AFM and an ellipsometer that were commercially available and that did not require major changes in their design to build the hybrid microscope. The observed increase in optical contrast for gold coated tips compared to silicon AFM probe tips was in qualitative agreement with the calculated increase in scattering amplitude according to the point dipole model for a gold AFM probe tip compared to a bare silicon AFM probe tip of the same size. The dielectric constant difference between the two blocks of the diblock copolymer was increased by selectively staining the P2VP block with iodine vapor. This resulted in an increase in the optical contrast between the PS and P2VP domains. Furthermore, the decrease in optical contrast as a function of increasing tip-sample separation was studied. It was observed that at 50nm tip sample separation the optical contrast was significantly reduced. The non-linear decay of the near-field amplitude signal as a function of the tip-sample separation calculated with the point dipole model supported this experimental result. The use of tapping mode in SNEM opens novel opportunities to study soft matter down to the macromolecular level.