Imaging of surface nanobubbles by atomic force microscopy in liquids: Influence of drive frequency on the characterization of ultrasoft matter.

Imaging of soft matter with atomic force microscopy (AFM) is challenging due to tip-induced deformation, which convolutes with the measurement. The challenges are generally more serious in liquid environments due to a severe loss of sensitivity of the vibrating microcantilever to external forces, as well as due to the presence of undesirable mechanical resonances when piezoelectric excitation systems are used. Furthermore, the choice of imaging parameters can have a significant impact on the quality of the results, such that the customary practices used for tuning the cantilever are not always appropriate. Here we explore the influence of the chosen drive frequency on the imaging of ultrasoft matter, using surface gas nanobubbles on gold-coated glass and highly oriented pyrolytic graphite (HOPG) as a test platform. We carry out single- and multifrequency AFM experiments using both the traditional amplitude-peak method and the recently proposed phase-slope-peak method for tuning the cantilever, as well as piezoelectric and photothermal excitation, providing an extensive discussion on the factors governing the level of tip-induced sample deformation and the quality of the phase contrast obtained for each of the methods. The general conclusion is that there is no "one-size-fits-all" approach for tuning the cantilever for low-impact tapping-mode AFM, although rational optimization of the imaging process is generally possible, whereby the choice of the drive frequency plays a prominent role. The physical insight and guidelines provided here can be extremely useful for the gentle imaging of a wide range of biological and other soft materials in liquid environments. Microsc. Res. Tech. 80:41-49, 2017.