Dresden 2020 – scientific programme

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O: Fachverband Oberflächenphysik

O 105: Scanning Probe Techniques II: Method Development (joint session O/CPP)

O 105.2: Talk

Thursday, March 19, 2020, 10:45–11:00, WIL C107

Atomic Force Microscopy study of the complex surface unit cell of CaF₂(111) with a CO-terminated tip — •Alexander Liebig¹, Prokop Hapala², Alfred J. Weymouth¹, and Franz J. Giessibl¹ — ¹Institute of Experimental and Applied Physics, University of Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany — ²Department of Applied Physics, Aalto University, 00076 Aalto, Espoo, Finland

A chemically inert tip apex of an atomic force microscope (AFM), such as a carbon-monoxide (CO) molecule, allows to enter the repulsive interaction regime without destroying tip or sample. Imaging in a gentle repulsive regime has led to unprecedented spatial resolution of organic molecules [1] and opened the way for numerous results on different sample systems [2,3]. Here, we probe the ionic CaF₂ (111) surface with a CO-terminated tip over a range of distances, starting from a regime, where short-range electrostatics dominate the AFM contrast, down to distances, where Pauli repulsion and CO bending lead to a complete reversal of the atomic-scale contrast. By comparing the data to simulations, we demonstrate that a mechanical model using the overlap of electron densities [4] as well as the Lennard-Jones potential [5] is reproducing experimental AFM images even at tip-sample distances, where strong lateral forces cause strong lateral deflection of the CO molecule at the tip apex. [1] L. Gross et al., Science 325, 1110 (2009). [2] P. Jelínek, J. Phys.: Condens. Matter 29, 343002 (2017). [3] F. Huber et al., Science 366, 235 (2019). [4] M. Ellner et al., ACS Nano 13, 786 (2019). [5] P. Hapala et al., PRB 90, 085421 (2014).