Direct Measurement of Photoinduced Force for Nanoscale Infrared Spectroscopy and Chemical-Sensitive Imaging

Measurement of photoinduced mechanical responses by atomic force microscopy (AFM) because of sample infrared resonances enables chemical-sensitive microscopy with spatial resolution orders of magnitude better than the optical diffraction limit. However, the origin of noncontact photoinduced force between the AFM tip and the samples of infrared resonances is still under debate. To investigate its origin, we implemented synchronized infrared pulse excitation and mechanical detection with a stationary cantilever of AFM in the peak force tapping mode. An attractive photoinduced force at nanonewton level between the AFM tip and the sample is observed. A spectral profile of the force corresponds to the infrared absorption of the sample. To interpret these findings, we modified the expected intermolecular force profile by adding the photothermal expansion as a contributing factor. The direct measurement of photoinduced force enables a new analytical method of nanoscale infrared spectroscopy and imaging with 10 nm spatial resolution on soft materials.