Endohedral metallofullerenes (EMFs) — fullerenes enclosing magnetic ions, such as DySc2_2N@C80_{80} and Dy2_2ScN@C80_{80} — are chemically robust single-molecule magnets (SMMs) whose carbon cage shields the magnetic core from metallic substrates. When derivatized with a thioacetate-terminated arm via 1,3-dipolar cycloaddition and self-assembled on Au(111) from solution, the DySc2_2N@C80_{80} cycloadduct retains magnetic hysteresis up to 10 K, the highest temperature reported for a solution-grafted SMM monolayer on a metal. However, only about 30–40% of the molecules are covalently bound to the gold, the opposite effect of the cycloaddition on the two molecules (improved for DySc2_2N@C80_{80}, degraded for Dy2_2ScN@C80_{80}) lacks a quantitative mechanistic explanation, on-surface magnetic dilution beyond 1:4 could not be probed because of signal-to-noise limits, and zero-field quantum tunneling of magnetization is masked by total-electron-yield measurement artifacts.

Background

Single-molecule magnets (SMMs) are molecules with a large magnetic anisotropy barrier that makes their magnetization relax slowly enough below a blocking temperature TBT_B for hysteresis to be observed at the molecular scale; the slow relaxation proceeds through a combination of Orbach processes (thermal excitation over the barrier via real excited states), Raman processes (two-phonon scattering), and quantum tunneling of magnetization (QTM), a field-independent tunneling through the barrier that is most destructive near zero applied field. Endohedral metallofullerenes (EMFs) are fullerenes whose carbon cage encapsulates metal ions; in the nitride clusterfullerenes DySc2_2N@C80_{80} (one magnetic Dy3+^{3+} ion) and Dy2_2ScN@C80_{80} (two Dy3+^{3+} ions), the nitride ion imposes a strong ligand field so that each Dy3+^{3+} easy axis lies along its Dy–N bond. In bulk, DySc2_2N@C80_{80} has TB≈7T_B \approx 7 K with low-temperature in-field relaxation governed by the Raman mechanism and pronounced QTM near zero field, whereas intramolecular dipolar and exchange coupling between the two Dy ions in Dy2_2ScN@C80_{80} quenches QTM, raises TBT_B to about 8 K, and makes relaxation follow the Orbach mechanism via a state with antiferromagnetically coupled Dy moments. Because the carbon cage absorbs substrate charge redistribution (a Faraday-cage effect), sublimed EMF monolayers on metals retain bulk-like bistability.

Device integration requires monolayers on conducting substrates prepared by scalable chemistry. Self-assembled monolayers (SAMs) are formed by soaking the substrate in a solution of molecules carrying anchor groups; here the EMFs are derivatized by 1,3-dipolar (Prato-type) cycloaddition of a thioacetate-terminated arm (giving M3_3N-R-SAc), which after acid deprotection in solution can bind Au(111) through Au–S bonds. X-ray photoelectron spectroscopy (XPS) in the S 2p region separates Au-bound sulfur (about 161.1 eV) from non-bonded and oxidized sulfur, so the chemisorbed fraction can be quantified; molecules that are merely adsorbed rather than covalently bound (physisorbed) may remain, possibly packing head-to-tail and capping the achievable coverage. Magnetism of such sub-monolayer films is probed by X-ray absorption spectroscopy (XAS), X-ray natural linear dichroism (XNLD, which reports the orientational order of the endohedral cluster), and X-ray magnetic circular dichroism (XMCD, an element-specific probe of magnetization and hysteresis); in the total-electron-yield (TEY) detection mode the XMCD signal near zero magnetic field suffers erratic oscillations that are experimental artifacts, not sample physics.

Earlier stages of this program: solution SAMs of the same EMFs with thioether (–SMe) linkers gave low-coverage, essentially physisorbed, randomly oriented films (Chen et al., Nanoscale 10, 11287, 2018); the thioacetate cycloadduct SAMs (Chen et al., Advanced Science 8, 2000777, 2021) showed hysteresis up to 10 K on Au(111) with partial orientational order, but only about 40% (DySc2_2N) and 30% (Dy2_2ScN) chemisorbed fractions. In parallel, other deposition routes overtook gold chemisorption on temperature: pyrene-functionalized Tb2_2@C80_{80}(CH2_2Ph) solution monolayers on graphene and graphite are magnetically bistable up to 28 K (Spree et al., 2021), and electrosprayed Dy2_2@C80_{80}(CH2_2Ph) on graphene has TBT_B(100 s) = 17 K (Paschke et al., 2021) — but these anchor by π\pi-interactions on graphitic substrates, not by covalent bonds to a metal electrode. The open questions below concern the covalently anchored, gold-based route and the mechanistic understanding of the molecular magnetism it exposes.

Problem Statement

The problem is to convert the demonstrated 10 K magnetic hysteresis of the thioacetate-cycloadduct DySc2_2N@C80_{80} self-assembled on Au(111) into a robust, reproducible, and quantitatively understood platform of covalently anchored single-molecule-magnet monolayers on a metallic electrode, by resolving four interlinked questions left open in ref 1 (Chen et al., Adv. Sci. 8, 2000777, 2021):

(Q1) Deposition chemistry: can the solution-based grafting of M3_3N-R-SAc cycloadducts on Au(111) be optimized so that essentially all molecules are bound through Au–S bonds — a dense, well-packed, (near-)completely chemisorbed monolayer instead of the present sub-monolayer mixture in which only about 30–40% of molecules are chemisorbed and the rest are physisorbed (plausibly head-to-tail)?

(Q2) Mechanism of functionalization effects: what quantitatively causes the opposite influence of exohedral cycloaddition on the two EMF-SMMs — beneficial for DySc2_2N@C80_{80} (TBT_B rising from about 7 K to 8 K with retained or broadened hysteresis) but detrimental for Dy2_2ScN@C80_{80} (TBT_B falling from about 8 K to 4 K with narrowed hysteresis)? Specifically, what are the respective roles of the cycloadduct-induced changes in vibrational density of states, in the balance between Raman-type and Orbach-type relaxation pathways, and in the Dy ligand field — and is the hypothesis proposed in ref 1 correct, that Orbach relaxation via the antiferromagnetically coupled Dy–Dy state is particularly susceptible to hindering of the internal cluster motion by the adduct?

(Q3) On-surface magnetic dilution: does strong magnetic dilution on the surface — diluting DySc2_2N-R-SAc in the diamagnetic analogue Sc3_3N-R-SAc well beyond the 1:4 ratio already tested, at which the hysteresis shape was unchanged — suppress QTM in EMF monolayers as it does in bulk powders, and can the required XMCD detection sensitivity be achieved at sub-monolayer coverage?

(Q4) Zero-field dynamics: what are the true zero-field QTM and relaxation dynamics of EMF monolayers, measured free of total-electron-yield artifacts, using refined protocols such as very low field-sweep rates and continuous X-ray helicity switching?

These four questions jointly define the completion boundary: the problem is resolved when (Q1) a deposition protocol reproducibly yields chemically anchored EMF-SMM monolayers on Au(111) with a controlled, near-quantitative chemisorbed fraction, taken here as an Au-bound S 2p XPS fraction above 90%; (Q2) a microscopic model validated against both molecules and their cycloadducts quantitatively explains the opposite cycloadduct-induced changes in their magnetization dynamics — including the blocking temperatures of about 8 K for DySc2_2N-R-SAc and about 4 K for Dy2_2ScN-R-SAc — and settles whether the hypothesis of ref 1 holds that Orbach relaxation via the antiferromagnetically coupled Dy–Dy state is particularly susceptible to hindering of the internal cluster motion by the adduct; (Q3) the response to on-surface dilution well beyond 1:4 is established, with the required XMCD detection sensitivity demonstrated at sub-monolayer coverage; and (Q4) the true zero-field QTM and relaxation dynamics are measured free of total-electron-yield artifacts.

References

  1. Chia-Hsiang Chen, Lukas Spree, Emmanouil Koutsouflakis, Denis S. Krylov, Fupin Liu, Ariane Brandenburg, Georgios Velkos, Sebastian Schimmel, Stanislav M. Avdoshenko, Alexander Fedorov, Eugen Weschke, Fadi Choueikani, Philippe Ohresser, Jan Dreiser, Bernd Büchner, Alexey A. Popov, 'Magnetic Hysteresis at 10 K in Single Molecule Magnet Self-Assembled on Gold', Advanced Science 8, 2000777 (2021). DOI: 10.1002/advs.202000777, https://doi.org/10.1002/advs.202000777
  2. C.-H. Chen, D. S. Krylov, S. M. Avdoshenko, F. Liu, L. Spree, R. Westerström, C. Bulbucan, M. Studniarek, J. Dreiser, A. U. B. Wolter, B. Büchner, A. A. Popov, 'Magnetic hysteresis in self-assembled monolayers of Dy-fullerene single molecule magnets on gold', Nanoscale 10, 11287–11292 (2018). DOI: 10.1039/c8nr00511g, https://doi.org/10.1039/c8nr00511g
  3. Denis S. Krylov, Sebastian Schimmel, Vasilii Dubrovin, Fupin Liu, T. T. Nhung Nguyen, Lukas Spree, Chia-Hsiang Chen, Georgios Velkos, Claudiu Bulbucan, Rasmus Westerström, Michał Studniarek, Jan Dreiser, Christian Hess, Bernd Büchner, Stanislav M. Avdoshenko, Alexey A. Popov, 'Substrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2ScN@C80 on Metals and Insulators', Angewandte Chemie International Edition 59, 5756–5764 (2020). DOI: 10.1002/anie.201913955, https://doi.org/10.1002/anie.201913955
  4. Lukas Spree, Fupin Liu, Volker Neu, Marco Rosenkranz, Georgios Velkos, Yaofeng Wang, Sandra Schiemenz, Jan Dreiser, Pierluigi Gargiani, Manuel Valvidares, Chia-Hsiang Chen, Bernd Büchner, Stanislav M. Avdoshenko, Alexey A. Popov, 'Robust Single Molecule Magnet Monolayers on Graphene and Graphite with Magnetic Hysteresis up to 28 K', Advanced Functional Materials 31, 2105516 (2021). DOI: 10.1002/adfm.202105516, https://doi.org/10.1002/adfm.202105516
  5. Fabian Paschke, Tobias Birk, Vivien Enenkel, Fupin Liu, Vladyslav Romankov, Jan Dreiser, Alexey A. Popov, Mikhail Fonin, 'Exceptionally High Blocking Temperature of 17 K in a Surface-Supported Molecular Magnet', Advanced Materials 33, 2102844 (2021). DOI: 10.1002/adma.202102844, https://doi.org/10.1002/adma.202102844
  6. Emmanouil Koutsouflakis, Denis Krylov, Nicolas Bachellier, Daria Sostina, Vasilii Dubrovin, Fupin Liu, Lukas Spree, Georgios Velkos, Sebastian Schimmel, Yaofeng Wang, Bernd Büchner, Rasmus Westerström, Claudiu Bulbucan, Kyle Kirkpatrick, Matthias Muntwiler, Jan Dreiser, Thomas Greber, Stas M. Avdoshenko, Harry Dorn, Alexey A. Popov, 'Metamagnetic transition and a loss of magnetic hysteresis caused by electron trapping in monolayers of single-molecule magnet Tb2@C79N', Nanoscale 14, 9877–9892 (2022). DOI: 10.1039/d1nr08475e, https://doi.org/10.1039/d1nr08475e
  7. Ryunosuke Sagehashi, Wei Chuang Lee, Fupin Liu, Alexey A. Popov, Matthias Muntwiler, Bernard Delley, Peter Krüger, Thomas Greber, 'Inferring the Dy-N axis orientation in adsorbed DySc2N@C80 endofullerenes by linearly polarized x-ray absorption spectroscopy', Physical Review Materials 7, 086001 (2023). DOI: 10.1103/PhysRevMaterials.7.086001, https://doi.org/10.1103/PhysRevMaterials.7.086001
  8. Giulia Serrano, Lorenzo Poggini, Matteo Briganti, Andrea Luigi Sorrentino, Giuseppe Cucinotta, Luigi Malavolti, Brunetto Cortigiani, Edwige Otero, Philippe Sainctavis, Sebastian Loth, Francesca Parenti, Anne-Laure Barra, Alessandro Vindigni, Andrea Cornia, Federico Totti, Matteo Mannini, Roberta Sessoli, 'Quantum dynamics of a single molecule magnet on superconducting Pb(111)', Nature Materials 19, 546–551 (2020). DOI: 10.1038/s41563-020-0608-9, https://doi.org/10.1038/s41563-020-0608-9
  9. Ziqi Hu, Shangfeng Yang, 'Endohedral metallofullerene molecular nanomagnets', Chemical Society Reviews 53, 2863–2897 (2024). DOI: 10.1039/d3cs00991b, https://doi.org/10.1039/d3cs00991b
  10. Shuting Liu, Zhenhua Zhu, Peng Zhang, Jinkui Tang, 'Chemisorption of lanthanide single-molecule magnets on surfaces', Fundamental Research 6, 1431–1444 (2026; available online 2023). DOI: 10.1016/j.fmre.2023.10.018, https://doi.org/10.1016/j.fmre.2023.10.018
  11. Matheus Felipe de Souza Barbosa, Wei Yang, Noel Israel, Fupin Liu, Bernd Büchner, Stanislav M. Avdoshenko, Alexey A. Popov, 'Carbene Addition and Its Remote Influence on Dy···Dy Coupling, Relaxation of Magnetization, and Magnetic Frustration in Fullerene Single-Molecule Magnets', JACS Au 5, 6134–6150 (2025). DOI: 10.1021/jacsau.5c01106, https://doi.org/10.1021/jacsau.5c01106
  12. Wei Yang, Matheus Felipe de Souza Barbosa, Noel Israel, Marco Rosenkranz, Fupin Liu, Stanislav M. Avdoshenko, Alexey A. Popov, 'Controlling Magnetic Anisotropy of Endohedral Lanthanide Ions by Carbene Addition: Paramagnetic NMR, Lanthanide Luminescence, and Single-Molecule Magnetism in Adamantylidene Adducts of MSc2N@C80 (M = Nd, Dy)', Journal of the American Chemical Society 147, 33812–33827 (2025). DOI: 10.1021/jacs.5c10147, https://doi.org/10.1021/jacs.5c10147
  13. A. D. Pykhova, S. M. Sudarkova, I. N. Ioffe, 'Some Aspects of the Endohedral Cluster Orientation in the Exohedrally Functionalized DySc2N@C80 and Dy2ScN@C80 Molecules', Russian Journal of Physical Chemistry A 99, 1041–1045 (2025). DOI: 10.1134/S0036024425700098, https://doi.org/10.1134/S0036024425700098
  14. Guillem Gabarró-Riera, E. Carolina Sañudo, 'Challenges for exploiting nanomagnet properties on surfaces', Communications Chemistry 7, 99 (2024). DOI: 10.1038/s42004-024-01183-6, https://doi.org/10.1038/s42004-024-01183-6
  15. Vasilii Dubrovin, Stanislav M. Avdoshenko, 'Conformational preferences of endohedral metallofullerenes on Ag, Au, and MgO surfaces: Theoretical studies', Journal of Computational Chemistry 43, 1614–1620 (2022). DOI: 10.1002/jcc.26962, https://doi.org/10.1002/jcc.26962
  16. Yuzhe Zhang, Wei Si, Xukui Hou, Qinghua Gao, Cong Zhao, Ruizhi Liang, Jie Guo, Mingliang Li, Chuancheng Jia, Xuefeng Guo, 'Single-molecule spin devices: Fundamentals, advances, and prospects', Nano Research 19, 94908552 (2026). DOI: 10.26599/nr.2026.94908552, https://doi.org/10.26599/nr.2026.94908552