Abstract：Bimodal amplitude modulation atomic force microscopy (AM-AFM) is widely used in micro/nanoscale mechanical imaging. However, the contrast inversion which probably appears during imaging makes it difficult to understand the imaging results. Combining the finite difference method and the in-phase and quadrature method, the influences of the selection of probes with different force constants, the mechanical properties of the sample components, and the settings of the imaging parameters on contrast inversion of the amplitude and phase of the second mode are investigated numerically in bimodal AM-AFM. Results show that for a stiff probe, the contrasts of the amplitude and phase of the second mode for components with different viscosity coefficients would not reverse with increasing elastic moduli. The contrast of the amplitude of the second mode for components with different moduli would be inverted with increasing viscosity coefficients, while contrast inversion of the phase of the second mode will not occur. For a soft probe, the increase of the elastic moduli or the viscosity coefficient would result in the transition of the interaction regimes, giving rise to discontinuous jumps of the response. For a soft cantilever, the contrast of the phase of the second mode for components with a higher modulus and different viscosity coefficients or the contrast of the phase of the second mode for components with a small viscosity coefficient and different moduli is lower compared to that of a stiff cantilever. The contrast of the amplitude of the second mode for components with different elastic moduli will reverse as the free amplitude of the second mode increases. However, the contrast at different viscosity coefficients does not reverse. Furthermore, the smaller the free amplitude of the second mode, the higher the contrast of the phase of the second mode to the elastic moduli or viscosity coefficients of the components. The amplitude or phase contrast of the second mode may be inverted in different interaction regimes and the interaction regime should be kept in the repulsive regime to achieve a higher imaging contrast.
2020, Vol. 41 
