Cantilever Dynamics using Small Amplitude AFM

Abstract

The dynamic force mode of spectroscopy measurements on the single molecules using soft cantilevers makes the bending signal easy to detect. But, there are many problems associated with soft cantilevers. The aim and the results of this report are focused on one of the major sources of error when working with a soft cantilever in the dynamic mode of force spectroscopy at the off Resonance frequencies. When the stiffness of the cantilever and the derivative of the tip-sample interaction force is of the same order then, the solution of the cantilever dynamics becomes anharmonic. Here, to present the anharmonicity in a quantitative manner, I have shown that the power present at drive frequency gets transferred to the higher harmonics during force mapping on single molecules. In this report, the power present at 2nd harmonics has been investigated by demodulating the output signal and then sent to two lock-in amplifiers. The experiment was also performed in MATLAB and COMSOL software by creating a virtual environment. The output results agree with the statement made by the experimental results, which suggests avoiding the use of a soft cantilever for the dynamic mode of force spectroscopy.