Frequency response of higher cantilever eigenmodes in bimodal and trimodal tapping mode atomic force microscopy

Abstract

We have recently implemented a trimodal tapping mode atomic force microscopy (AFM) imaging scheme for ambient air with which it is possible to simultaneously acquire topographical, phase and frequency shift contrast images. In implementing this method we have identified conditions, such as very low fundamental amplitude setpoints and very low oscillation amplitudes for the higher eigenmodes, for which the stability of the imaging process can be compromised. In this work we use numerical simulations to gain insight into the changes in the frequency response of the higher eigenmodes in bimodal and trimodal operation for different levels of sample stiffness, tip–sample dissipative forces, oscillation amplitudes for each of the eigenmodes and cantilever rest positions above the surface. Although we do not attempt to convey a complete dynamics picture of the system, the results provide general guidelines for the selection of conditions that lead to stable imaging as well as insight into the observed phase and frequency shift contrast, highlighting a few potential imaging artifacts. Our simulation results are in general agreement with our experimental observations.