Emergent Relativistic Effects in Condensed Matter
From Fundamental Aspects to Electronic Functionality

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20.06.2024

Exploring in-plane interactions beside an adsorbed molecule with lateral force microscopy


S. Nam, E. Riegel, L. Hörmann, O.T. Hofmann, O. Gretz, A.J. Weymouth, F.J. Giessibl

PNAS

Hydrogen, the smallest and most abundant element in nature, plays a vital role in many molecular
interactions. Their positions can determine the interactions with neighboring molecules in the form of
hydrogen bonds. While atomic force microscopy can image the internal structure of flat-lying molecules, H-
atoms are difficult to directly image due to their size. We directly image these H-atoms using lateral force
microscopy (LFM). Furthermore, we determine a metric of when the assumption of purely radial atomic
interactions breaks down and an additional angular component is required to account for the additional
electrostatic interaction from the metal tip apex. The application of LFM to the sides of molecules
demonstrates how in-plane molecular interactions can be directly investigated.
Atomic force microscopy with a CO-functionalized tip can be used to directly image the internal structure of
a planar molecule and to characterize chemical bonds. However, hydrogen atoms usually cannot be directly
observed due to their small size. At the same time, these atoms are highly important, since they can direct
on-surface chemical reactions. Measuring in-plane interactions at the sides of PTCDA (3,4,9,10-
perylenetetracarboxylic dianhydride) molecules with lateral force microscopy allowed us to directly identify
hydrogen atoms via their repulsive signature, which we confirmed with a model incorporating radially
symmetric atomic interactions. Additional features were observed in the force data and could not be
explained by H-bonding of the CO tip with the PTCDA sides. Instead, they are caused by electrostatic
interaction of the large dipole of the metal apex, which we verified with density functional theory. This
calculation allowed us to estimate the strength of the dipole at the metal tip apex. To further confirm that
this dipole generally affects measurements on weakly polarized systems, we investigated the archetypical
surface adsorbate of a single CO molecule. We determined the radially symmetric atomic interaction to be
valid over a large solid angle of 5.4 sr, corresponding to 82°. We therefore find that in both the PTCDA and
CO systems, the underlying interaction preventing direct observations of H-bonding and causing a collapse
of the radially symmetric model is the dipole at the metal apex, which plays a significant role when
approaching closer than standard imaging heights.

https://www.pnas.org/doi/abs/10.1073/pnas.2311059120

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