Effects of laser spot positioning with optical beam deflection method on tapping mode and bimodal AFM

This work focuses on the importance of laser location and its effect on contact mode, tapping mode, and bimodal AFM both theoretically and experimentally. It is found that the current guidelines in the field might lead to mischaracterization of matter especially in bimodal AFM. A numerical study is done for a cantilever with its tip located at the end while interacting with two different polymers of Polystyrene (PS) and Low-density polyethylene (LDPE). Different observables are recorded at the end of the cantilever, 80% of its length, and 60% of its length. These results are verified by experiments in contact mode, tapping mode and bimodal AFM. Bimodal AFM observables are converted to energy quantities (i.e., virial and dissipated power) which are used to characterize soft matter in this field. Similar to simulation, for each of these measurements three different laser locations are selected. Finally, it is found there are certain locations on the cantilever that should be avoided for placing the laser while exciting higher eigenmodes. Hence, locating the laser around 80% of its length while performing bimodal AFM with the first and second eigenmodes on polymer surfaces can decrease phase contrast. It is concluded that: (1) contact mode AFM on stiff surfaces might not be effected by change of laser location, (2) misalignment of laser can cause up to 40% of mischaracterization in tapping mode and (3) 10% of phase contrast reduction in addition to 20% percent of reduction in amplitude oscillations in bimodal AFM. It is shown this can lead to mischaracterization of material. By positioning the laser location closer to the clamped end of the cantilever, surface indentation can occur. Although this is considered as a surface damage in soft matter imaging, it can be considered as a new capability of multifrequency AFM for surface modification.