Nanoscale subsurface imaging via resonant difference-frequency atomic force ultrasonic microscopy

A scanning probe microscope methodology, called resonant difference-frequency atomic force ultrasonic microscopy (RDF-AFUM), has been developed. It employs an ultrasonic wave launched from the bottom of a sample while the cantilever of an atomic force microscope, driven at a frequency differing from the ultrasonic frequency by one of the contact resonance frequencies of the cantilever, engages the sample top surface. The nonlinear mixing of the oscillating cantilever and the ultrasonic wave in the region defined by the cantilever tip–sample surface interaction force generates difference-frequency oscillations at the cantilever contact resonance. The resonance-enhanced difference-frequency signals are used to create images of nanoscale near-surface and subsurface features. An analytical model is presented for assessing the RDF-AFUM phase signal resulting from near-surface variations in the sample contact stiffness and from the interaction of the bulk wave with subsurface structures. The application of the model to RDF-AFUM phase measurements of a 12.7μm thick film of LaRC™-CP2 polyimide polymer containing a monolayer of gold nanoparticles embedded 7μm below the specimen surface reveals variations in the Young modulus of the material of approximately 24% over regions roughly 10–35nm wide. The magnitude of the modulus variations suggests the occurrence of contiguous amorphous and crystalline phases within the bulk of the polymer. The RDF-AFUM micrograph indicates a preferential growth of the crystalline phase in the vicinity of the gold nanoparticles.