Enhancing phase contrast for bimodal AFM imaging in low quality factor environments

Since the invention of atomic force microscopy (AFM), there have been extensive studies with the goal of expanding its capabilities beyond topographical imaging. These studies have made AFM capable of providing material compositional mapping, magnetic or electrical properties, subsurface imaging not only in air environment but also in the liquid environment. Being able to perform AFM imaging in the liquid environment is advantageous since it can provide the true environment for various samples. However, it can also cause experimental challenges while tuning the dynamics of the cantilever for proper imaging. This issue can also be more problematic when performing multifrequency AFM in liquid environments. This paper focuses on providing both numerical and experimental procedures for selection of oscillation amplitudes for bimodal AFM in liquid with the goal of enhancing the higher eigenmode's phase channel. The numerical work has considered different models (vdW and DLVO) for long-range attraction models and DMT for short-range interactions. The oscillation amplitudes for both eigenmodes that provide the highest 2nd eigenmode's phase contrast have been selected. Simulations are done on three different polymers: low-density polyethylene (LDPE), polystyrene (PS) and polytetrafluoroethylene (PTFE). Correspondingly, the experimental results have verified that the selected configuration enhances the phase signal. The experiments are done on a polyethylene (LDPE) and polystyrene (PS) polymer blends in both air and liquid environment.