Resonant heterodyne force microscopy provides elasticity contrast on the molecular level using a standard cantilever

Dana group has developed an imaging technique that uses nonlinear tip-sample interactions to convert the frequency components of periodic tip-sample interaction forces to frequencies where they can be resonantly detected. One flexural mode of a cantilever is used for tapping-mode imaging and another flexural mode is used for detection of forces converted in presence of an externally injected mechanical oscillation at the difference frequency of the detecting mode and a harmonic of the tapping mode. Material contrast in attractive and repulsive regimes are demonstrated on samples with polymethyl methacrylate patterns and with deoxyribonucleic acid strands on silicon.

Material contrast can be observed through upconversion in the attractive regime on PMMA patterns on silicon as seen above. Material contrast can observed through down-conversion in the repulsive regime even on the molecular scale, for example with plasmid DNA strands on silicon:  Softer material gives darker signature.

Reference: Abak et al. , Applied Physics Letters 92, 223113 2008