Gradient and scattering forces in photoinduced force microscopy

A theoretical and experimental analysis of the dominant forces measured in photoinduced force microscopy is presented. It is shown that when operated in the noncontact and soft-contact modes, the microscope is sensitive to the optically induced gradient force (F-g) and the scattering force (F-sc). The reconstructed force-distance curve reveals a tip-dependent scattering force in the 30-60 pN range. Whereas the scattering force is virtually insensitive to the nanoscopic tip-sample distance, the gradient force shows a z(-4) dependence and is manifest only for tip-sample distances of a few nm. Measurements on glass, gold nanowires, and molecular clusters of silicon naphtalocyanine confirm that the gradient force is strongly dependent on the polarizability of the sample, enabling spectroscopic imaging through force detection. The nearly constant F-sc and the spatially dependent F-g give rise to a complex force-distance curve, which varies from point to point in the specimen and dictates the image contrast observed for a given set point of the cantilevered tip.