High-Speed Lattice Resolution and Friction Loops on 2D Materials Using a New Interferometric Atomic Force Microscope

Measuring lateral force with lattice resolution is critical for atomic-scale measurements of friction, stick-slip phenomena, and wear reactions, which inform both the tribological and electrical characterization of 2D materials. The atomic force microscope (AFM) is an important tool for nanoscale measurement of friction, down to the level of single atomic defects [1]. Despite these successes, conventional optical beam deflection (OBD) based AFMs have several challenges, including calibration and noise limitations. It is difficult to achieve lattice resolution, especially at high scan speeds, and accurate calibration of the lateral signal is in general possible only with invasive or labor-intensive techniques, all with substantial uncertainty.<br/><br/>In this work, we use a recently introduced AFM [2] with a quadrature phase differential interferometer (QPDI) detector with both an extremely low noise floor and highly accurate positional measurement which enables new capabilities [3,4], including lateral force microscopy at high scan rates. We describe methods to calibrate and measure the lateral force signal using QPDI and apply these methods to measure friction forces on a variety of 2D materials. By positioning the interferometric detection spot along one edge of the cantilever, the AFM takes advantage of the detector’s low noise floor to observe stick-slip friction at scan rates that would be difficult or impossible with optical beam AFMs. Alternatively, the detection spot may be placed on the centerline of the cantilever, using interferometric detection in combination with conventional optical beam detection. This variation allows for lateral force imaging without crosstalk from the lateral signal into the normal signal, which reduces the effects of friction and topography on the applied load. The results demonstrate clearly resolvable stick-slip friction over a range of tip speeds up to 2 µm/s and additionally show the variation of friction with applied load.<br/><br/>A major advantage of interferometric AFM is its ability to perform a complete <i>in situ</i> calibration of the lateral force sensitivity without cumbersome sample changes or modifications to the probe. We demonstrate a method based on a combination of our previous method [5] and a new technique for automated, direct measurement of the tip height. We compare the accuracy of this new calibration technique with the wedge calibration method [6]. The methods and results described here pave the way for accurate probing of in-plane frictional and conservative forces including the effect of defects, load, and rate-and-state friction relationships in velocity ranges that were previously difficult or impossible to explore.<br/><br/>References<br/>[1] Yang, Y., Xu, K., Holtzman, L. N., Yang, K., Watanabe, K., Taniguchi, T., Hone, J., Barmak, K., & Rosenberger, M. R. <i>ACS Nano </i>(2024), 18, 6887–6895.<br/>[2] Oxford Instruments Application Note: QPDI technology in the AR Vero AFM.<br/>[3] Proksch, R. & Wagner, R. Manuscript in preparation.<br/>[4] Proksch, R. et al., Symposium CH03.<br/>[5] Labuda, A., Cao, C., Walsh, T., Meinhold, J., Proksch, R., Sun, Y., & Filleter, T. <i>Rev. Sci. Inst. </i>(2018), 89, 093701.<br/>[6] Ogletree, D. F., Carpick, R. W., & Salmeron, M. <i>Rev. Sci. Inst. </i>(1996), 67, 3298–3306.