Self-oscillation technique for AFM in liquids

A simple self-oscillation circuit was built into a commercially available atomic force microscope (AFM) apparatus and used for imaging sample surfaces with a high rigidity in liquid environments. Imaging experiments were conducted with an AFM tip recommended for the normal tapping (intermittent contact) mode in liquids. In spite of the existence of many artificial resonance peaks, the self-oscillation could be successfully achieved with our simple electronic circuits. Force curves obtained in liquids showed that, as the tip contacted the surface, the resonance frequency began to increase steeply without a clear reduction of the vibration amplitude. Image of the sample surface was stably obtained under the condition of the positive frequency shift within a few Hertzs, which corresponded to a contact force comparable with Q-control operation.