Shinjae Nam

@article{Nam2025,

title = {Sliding friction over individual covalent bonds correlates with bond order},

author = {Shinjae Nam and Lukas Hörmann and Oliver Gretz and Oliver Hofmann and Franz Giessibl and Alfred Weymouth},

url = {https://www.researchsquare.com/article/rs-6127312/v1},

doi = {10.21203/rs.3.rs-6127312/v1},

year  = {2025},

date = {2025-05-08},

urldate = {2025-05-08},

journal = {Research Square},

abstract = {Unveiling the dynamics and energy dissipation involved in atomic scale motion is key to understanding surface catalysis1–3, molecular motors4,5, and single molecule manipulation6,7. Despite significant progress in nanoscale friction8–10 studies, due to challenges in atomistically defining the sliding surfaces, there are outstanding problems regarding reproducibility, isolating nonconservative interactions, and in general reconciling atomistic theory with experimental results11. This has prompted investigations with precise control at the single atom scale12–15. We use a single atom asperity16 as one sliding surface. High spatial resolution allows us to investigate individual chemical bonds and address the question of how the nature of a chemical bond affects sliding friction. Surprisingly, we find a large variety in sliding friction over covalent bonds. Density functional theory-based simulations yield excellent agreement with the data and reveal that sliding friction is correlated to bond order. Finally, we show that over hydrogen bonds, the maximum magnitude of sliding friction can be similar to friction over covalent bonds, however that interaction is not with a bond (increased electron density between the atoms). These findings offer new insights into atomic-scale motion and show that the frictional properties of advanced materials17 and nanodevices can be tuned by selecting the nature and order of chemical bonds at surfaces.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Nam2024,

title = {Exploring in-plane interactions beside an adsorbed molecule with lateral force microscopy},

author = {Shinjae Nam and Elisabeth Riegel and Lukas Hörmann and Oliver T. Hofmann and Oliver Gretz and Alfred J. Weymouth and Franz J. Giessibl},

url = {https://www.pnas.org/doi/abs/10.1073/pnas.2311059120},

doi = {10.1073/pnas.2311059120},

year  = {2024},

date = {2024-01-03},

urldate = {2024-01-03},

journal = {Proceedings of the National Academy of Sciences},

volume = {121},

number = {2},

pages = {e2311059120},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Jung2021,

title = {Atomic-scale intermolecular interaction of hydrogen with a single VOPc molecule on the Au(111) surface},

author = {Jinoh Jung and Shinjae Nam and Christoph Wolf and Andreas J Heinrich and Jungseok Chae},

doi = {10.1039/d0ra08951f},

issn = {20462069},

year  = {2021},

date = {2021-02-01},

urldate = {2021-02-01},

journal = {RSC Advances},

volume = {11},

number = {11},

pages = {6240–6245},

publisher = {Royal Society of Chemistry (RSC)},

abstract = {Molecular dynamics of hydrogen molecules (H2) on surfaces and their interactions with other molecules have been studied with the goal of improvement of hydrogen storage devices for energy applications. Recently, the dynamic behavior of a H2at low temperature has been utilized in scanning tunnelling microscopy (STM) for sub-atomic resolution imaging within a single molecule. In this work, we have investigated the intermolecular interaction between H2and individual vanadyl phthalocyanine (VOPc) molecules on Au(111) substrates by using STM and non-contact atomic force microscopy (NC-AFM). We measured tunnelling spectra and random telegraphic noise (RTN) on VOPc molecules to reveal the origin of the dynamic behavior of the H2. The tunnelling spectra show switching between two states with different tunnelling conductance as a function of sample bias voltage and RTN is measured near transition voltage between the two states. The spatial variation of the RTN indicates that the two-state fluctuation is dependent on the atomic-scale interaction of H2with the VOPc molecule. Density functional theory calculations show that a H2molecule can be trapped by a combination of a tip-induced electrostatic potential well and the potential formed by a VOPc underneath. We suggest the origin of the two-state noise as transition of H2between minima in these potentials with barrier height of 20-30 meV. In addition, the bias dependent AFM images verify that H2can be trapped and released at the tip-sample junction.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}