Chemical Imaging of the Surface of Polymeric Nanostructures Using Apertureless Near-Field IR Microscopy

The chemical composition of the surfaces of thin films of polystyrene-poly (methyl methacrylate) (PS-PMMA) diblock copolymers are investigated using apertureless near-field IR microscopy. In this technique a tunable IR beam is scattered from an oscillating atomic force microscopy tip. The scattered light is enhanced using a reference signal with the same optical frequency (homodyne) or slightly shifted IR frequency (heterodyne) and detected after demodulation in order to eliminate the background scattering. Using this technique a lateral chemical imaging resolution of $<$20 nm is achievable. It is demonstrated that this technique can be successfully used to image the surface of PS-PMMA diblock copolymers. It is shown that an increase in the IR absorption is observed in the PMMA rich domains with a wavenumber dependence that is consistent with the bulk absorption spectrum. The results indicate that even though a topography induced artifact can be observed, when homodyne detection technique is used, the chemical signature of the sample can be detected clearly. This technique can be further used in a variety of different systems to detect the surface structure of polymers or proteins.