Unifying Theory of Quantitative Atomic Force Microscopy Using Piezo Excitation in Liquids

Base-excitation of microcantilevers using a dither piezoelectric element, also known as acoustic excitation, is one of the most popular methods for dynamic atomic force microscopy (AFM) because it is inexpensive, easy to use and does not require special cantilevers. However, in liquid environments there are problems using this method for quantitative force spectroscopy. The problems arise due spurious peaks in the driving spectrum (also known as “forest of peaks”) caused by piezo and fluid cell resonances, as well as a large base motion, which make it very hard to quantify the exciting forces. Although some groups have tried to overcome these limitations, it is has generally been accepted that acoustic excitation is unsuitable for quantitative force spectroscopy in liquids. In this work the authors show that a thorough understanding of the excitation forces and base motions reveals a method by which quantitative analysis is in fact possible with acoustic excitation in liquid environments, thus opening this popular method for quantitative dynamic AFM in liquids. This method is validated by experiments using a scanning laser Doppler vibrometer, which can measure the actual base motion. Finally, the method is demonstrated by performing force spectroscopy on solvation shells of octamethylcyclotetrasiloxane (OMCTS) molecules on mica.