Enhanced phase and amplitude image contrasts of polymers in bimodal atomic force microscopy

Abstract

Bimodal atomic force microscopy (AFM), where the first two flexural eigenmodes of the cantilever are simultaneously excited, is an extension of dynamic AFM to enhance the contrast of multicomponent materials. The effects of the operating parameters for both modes are explored by imaging a polymer blend of polystyrene (PS) and low-density polyethylene (LDPE) the in attractive and repulsive regimes. In this study, the phase and amplitude contrasts of the individual eigenmodes for bimodal AFM are quantitatively calculated using a statistical expression. Meaningfully, the higher free amplitudes of the second mode, which were chosen beyond the traditional values, show significantly better compositional contrast, particularly for the phase of the second mode. The phase contrast of the second mode increases by 3–10 times compared with that of the traditional results. The relationship between the contrast and amplitude ratio of the first mode is also researched experimentally in the bimodal mode. Virial and energy dissipation power for the corresponding modes are employed to explain the enhanced phase contrast physically. The meaningful results obtained are applicable to optimize the operating parameters of image contrast and study the surface morphology and properties of soft, even biological materials.