Subsurface imaging of cavities in liquid by higher harmonic atomic force microscopy

Subsurface imaging capability of liquid-environment higher-harmonic atomic force microscopy (AFM) was investigated using a reference artifact. A series of cylindrical cavities with pre-known dimensions were fabricated on a silicon substrate via electron beam lithography and then covered by a set of highly oriented pyrolytic graphite (HOPG) pieces with different thicknesses. Experiments on these structures demonstrated that the higher-harmonic amplitude sensitivity to the local stiffness in liquids was at least an order of magnitude larger than that in ambient air under the same parameter settings. The harmonic AFM in liquids could detect the cavities beneath over a 200 nm thick HOPG cover. Theoretical analyses based on the cantilever dynamics and the membrane mechanical properties well interpreted the experimental results. Furthermore, it was verified that the momentary excitation of the non-driven higher eigenmode in a low-Q environment could play a critical role in the enhanced subsurface imaging capability of harmonic AFM in liquids.