Richard Berndt

Artificial spin chains composed of switchable units are a key objective in molecular spintronics, but their creation on surfaces is a challenge. Spin-crossover molecules allow for the reversible switching between spin states and could provide a means of creating such functional nanostructures through on-surface synthesis. We report the first on-surface thermally activated Ullmann coupling of spin-crossover complexes on a Ag(111) surface. Using scanning tunneling microscopy, we demonstrated the formation of molecular chains with electrically tunable spin states. The conductance spectra of individual molecules in chains exhibit spin-flip excitations. A comparison to a Heisenberg spin chain shows that the spins are antiferromagnetically coupled. Reversible manipulation of the molecules in the chains suppresses the spin excitations, indicating a transition from high spin to low spin S = 0. Overall, these observations confirm that Ullmann coupling on surfaces is a viable strategy for engineering electrically programmable spin chains with a spin-crossover functionality.

First-principles predictions of excitation energies and cross sections in inelastic tunneling spectroscopy are challenging, making it difficult to assign experimental features to specific excitations. There is, however, a need for a reliable identification of conductance features. We investigated the spin- 1 / 2 molecule cobaltocene on superconducting Pb(110) and employ Yu-Shiba-Rusinov states combined with a superconducting tip to detect a large number of sharp spectral features. From an analysis of the peak heights at both bias voltage polarities along with model calculations and density-functional theory results we identify spin, vibrational, and orbital excitations that coexist within a single conductance spectrum. The peak height asymmetry turns out to be a useful indicator to determine the type of excitation.

Lattice geometry plays a fundamental role in the behavior of Bloch electrons in a crystal. The diatomic Kagome lattice, an extension of the honeycomb and Kagome lattices, is predicted to give rise to emergent and topological phenomena, but its experimental investigation has been limited thus far. Here, we fabricate a diatomic Kagome lattice through self-assembly of a triptycene derivative with phenazine moieties (Trip-Phz)─a C3-symmetric, nonplanar π-conjugated molecule. Our scanning tunneling microscopy (STM) observations show that Trip-Phz forms a highly ordered diatomic Kagome lattice terminated by zigzag-type edges on the Pb(111) surface. Combined STM measurements and tight-binding calculations provide direct evidence for the existence of the edge states that correspond to those of graphene. These states are topological edge states dictated by the quantization of the Zak phase and the bulk-edge correspondence. This work reveals an ideal platform for exploring quantum materials with unique lattice geometries using supramolecular technology.

Trioxatriangulenium was functionalized with a rotatable pyridyl moiety (pyridyl-TOTA), which exhibits a large electrostatic dipole moment, and investigated on the Ag(111) surface using low-temperature scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM). The molecules arrange themselves in a complex, well-ordered long-range pattern, namely, a chiral hexagonal mesh of rings composed of six molecules. Submolecularly resolved STM images reveal the orientations of the molecular dipoles, which are confirmed by NC-AFM measurements. Density functional theory (DFT) calculations reproduce the STM images and their intriguing dependence on the junction bias and confirm that the experimental results allow us to fully determine the orientation of the functional units. Comparison with phenyl-TOTA reveals that the dipole–dipole interaction of the pyridyl unit is strong enough to change the structure from a hexagonal close-packed pattern to a chiral hexagonal mesh of supramolecular hexagons. These results show that dipole functionalization may serve to control the formation of molecular assemblies on the surfaces.

Bilayers of the nonplanar complex indium phthalocyanine (InPc) were studied on superconducting Pb(001) using scanning tunneling microscopy and density functional theory calculations. The structure of the bilayer is drastically different from that of a single layer in terms of the unit cell, the azimuthal orientations of the molecules, and the position of the In ion above (In↑Pc) or below (In↓Pc) the macrocycle. Intriguingly, the presence of the second layer appears to induce a geometric transition of the first layer. Moreover, a Yu–Shiba–Rusinov state reflecting a localized spin is observed above all molecules of the bilayer.

Self-assembled monolayers and bilayers of the electron acceptor molecule tetracyanoquinodimethane (TCNQ) were investigated on Pb(100) using low-temperature scanning tunneling microscopy (STM) at 4.5 K and density functional theory (DFT) calculations. The monolayer exhibits an unusually low molecular density. In contrast, both the first and second layers in the bilayer adopt a packing density twice that of the monolayer. Our modeling reveals that Pb atoms, apparently released from step edges, stabilize the bilayer structure. The structural transition is accompanied by a pronounced change in the electronic structure: the bilayer hosts a singly occupied molecular orbital (SOMO) and its corresponding unoccupied partner (SUMO), arising from a spin-splitting of the lowest unoccupied molecular orbital (LUMO) observed in the monolayer.

Magnetic molecules on surfaces provide atomically precise test beds for quantum magnetism and device concepts in spintronics and quantum information. Scanning probe techniques now resolve and control single-molecule spin states, their coupling to substrates and emergent many-body phenomena, including the Kondo effect and Yu–Shiba–Rusinov states. This review summarizes recent progress in combining molecular design, adsorption geometry and interfacial hybridization to create measurable spin anisotropy, excitations and correlations. We first recall the fundamentals of spin formation and surface-induced modifications, then survey key methodologies such as scanning tunneling microscopy, inelastic tunneling spectroscopy, pump–probe protocols, functionalized and superconducting tips, and magnetic exchange force microscopy. Case studies span single-ion magnets, metal-free Kondo-active molecules, molecular magnets on superconductors, and coupled spin architectures assembled by on-surface synthesis. Across these examples, we distill design rules for tuning exchange, anisotropy and charge transfer at the molecule–surface interface. We conclude with open challenges—reproducibility, room-temperature stability and multiscale theory—and outline opportunities in van der Waals integration, ultrafast spin dynamics and data-driven spectroscopy, charting a path from single-molecule experiments to programmable molecular spin devices.

Artificial arrays of tin phthalocyanine molecules have been studied on Pb(100) using scanning tunneling microscopy (STM). The molecules adopt two azimuthal orientations with distinct spin states, which results in a checkerboard pattern in STM images. Upon converting the central molecule by current injection from its pristine state with the Sn ion above the molecular plane to a conformation with Sn between the macrocycle and the substrate, the orientations of all molecules are swapped. This cooperative rotation is modeled by combining the potential energies of the intermolecular and the molecule-substrate interactions.

Spin-crossover complexes exhibit two stable configurations with distinct spin states. The investigation of these molecules using low-temperature scanning tunneling microscopy has opened new perspectives for understanding the associated switching mechanisms at the single-molecule level. While the role of tunneling electrons in driving the spin-state switching has been clearly evidenced, the underlying microscopic mechanism is not completely understood. In this study, we investigate the electron-induced switching of [ Fe ( H 2 B ( pz ) ( pypz ) ) 2 ] (pz = pyrazole, pypz = pyridylpyrazole) adsorbed on Ag(111). The current time traces show transitions between two current levels corresponding to the two spin states. We extract switching rates from these traces by analyzing waiting-time distributions. Their sample-voltage dependence can be explained within a simple model in which the switching is triggered by a transient charging of the molecule. The comparison between experimental data and theoretical modeling provides estimates for the energies of the lowest unoccupied molecular orbitals, which were so far experimentally inaccessible. Overall, our approach offers new insights into the electron-induced switching mechanism and predicts enhanced switching rates upon electronic decoupling of the molecule from the metallic substrate, for example by introducing an ultrathin insulating layer.