Photothermal Expansion of Nanostructures in Photo-induced Force Microscopy

Abstract: Powerful mid-infrared illumination combined with mechanical detection via force microscopy provides access to nanoscale spectroscopic imaging in Materials and Life Sciences. Photo-induced force microscopy (PiFM) employs pulsed illumination and noncontact force microscopy resulting in unprecedented spatial and high spectral resolution. The near-field-enhanced light absorption in the materials leads to thermal expansion affecting the distance-dependent weak van der Waals (VdW) force acting between the tip and the sample. We model the non-linear impact of material characteristics and surface shape on the tip-sample interaction, the heat generation from the presence of a photo-induced electric field, the associated thermal expansion under different illumination conditions including light polarization and the feedback to the dynamic tip motion due to the expansion. Comparison of the results with our experimental investigation of a polymer nanosphere shows good agreement, contributing new insights into the understanding required for a quantitative analysis of nanostructured materials imaged using PiFM.