Harnessing torsional resonances for enhanced atomic force microscopy

B. Pittenger, M. Ye, S. Hu, J. Thornton, P. De Wolf
Bruker Corporation,
United States

Keywords: torsional resonance, friction, AFM, 2d materials, graphene


While most AFM modes are based on measuring flexural bending of the cantilever, it is sometimes more useful to focus on the lateral or torsional motion of the cantilever. These signals can provide insights into the lateral anisotropy of the sample and variations of shear or frictional force. TR Friction Mode (or Dynamic Friction) is the resonant analogue of Lateral Force Microscopy (LFM) -- measuring the amplitude and phase of the torsional resonance with the tip in contact with the surface, and feedback on the flexural deflection. This mode has recently proven an excellent way to characterize the near surface structure of Van der Waals stacks, allowing observation of single layer atomic lattices and moire' structures [1]. Additionally, TR Friction Mode with PLL based resonance tracking can be applied to heterogeneous materials such as polymer composites to study the variations in shear stiffness and damping in these materials. For softer or more fragile samples where tip-sample contact must be minimized to avoid sample damage, contact mode is not appropriate, but TR Mode may be more suitable. As in TappingMode, where the flexural vibrations of the cantilever are damped as the probe comes into contact with the sample, in TR mode the same thing occurs with torsional vibrations. In both cases, a feedback loop can be used to control the amount of damping by adjusting the Z position. In contrast to TappingMode, the TR mode vibration tends to keep the tip at nearly the same distance from the sample throughout the vibrational cycle and the damping occurs very suddenly, enabling atomic resolution imaging in ambient conditions [2, 3]. Finally, the nearly in-plane motion of the torsional resonance can be used to investigate gradients parallel to the sample surface in electrical or magnetic fields by using lift-mode based techniques (TR-MFM, TR-EFM, and TR-KPFM). In this talk, we discuss recent advances in torsional resonance based AFM methods with examples across a wide range of sample types. [1] M. Pendharkar _et al._, “Torsional Force Microscopy of Van der Waals Moires and Atomic Lattices.” arXiv, Aug. 17, 2023. doi: [10.48550/arXiv.2308.08814](https://doi.org/10.48550/arXiv.2308.08814). [2] R. C. Savage, N. Mullin, and J. K. Hobbs, “Molecular Conformation at the Crystal–Amorphous Interface in Polyethylene,” _Macromolecules_, vol. 48, no. 17, pp. 6160–6165, Sep. 2015, doi: [10.1021/ma5025736](https://doi.org/10.1021/ma5025736). [3] A. L. Eichhorn and C. Dietz, “Torsional and lateral eigenmode oscillations for atomic resolution imaging of HOPG in air under ambient conditions,” _Sci Rep_, vol. 12, no. 1, Art. no. 1, May 2022, doi: [10.1038/s41598-022-13065-9](https://doi.org/10.1038/s41598-022-13065-9).