Felix Raoul Fischer

Isolated, undercoordinated metal atoms exhibit enhanced reactivity central to catalysis, yet their stabilization on surfaces remains challenging. In heterogeneous single-atom catalysis, activity derives from interactions between individual atoms and a host matrix, but the absence of stabilizing ligands often leads to diffusion, aggregation, and sintering, obscuring structure–activity relationships. On metal surfaces, reactivity is typically limited to sparse, dynamic, undercoordinated sites lining step edges or lattice defects. Here we report a defect-engineering strategy to stabilize isolated adatoms on dilute metal alloy substrates. Scanning tunneling microscopy reveals a high density of uniformly distributed adatoms on Au–Cu(111) terraces that persist under thermal cycling. Combined experiments and modeling attribute this stability to alloy-induced subsurface dislocation networks that trap adatoms and suppress diffusion, forming a two-dimensional frozen gas. These adatoms act as a reactive reservoir, enabling on-surface coordination chemistry inaccessible on pure metals, and providing insights into atomic-scale processes on the surfaces of defect-engineered alloys with implications for heterogeneous catalysis.

Carbon-based synthetic lattices offer a versatile platform for the realization and control of correlated quantum effects at the nanoscale. Advances in on-surface synthesis have enabled the fabrication of increasingly complex molecular frameworks, providing opportunities for the exploration and engineering of custom tailored electronic and magnetic phases. Here, we report the design, on-surface synthesis, and characterization of a two-dimensional (2D) covalent organic framework (COF) assembled from D3h symmetric S = 3/2 [4]triangulene building blocks arranged in a honeycomb structure. Combined scanning probe microscopy and spectroscopy, corroborated by first-principles density functional theory (DFT) and GW calculations, establish an antiferromagnetic (AFM) insulating ground state. Low energy spectroscopy resolves spin excitations in both one-dimensional (1D) chains and extended 2D networks, providing direct insight into the emergent spin dynamics and establishing a modular platform to explore quantum magnetism in a π-conjugated organic system.

Van der Waals (vdW) heterostructures provide a versatile platform for exploring emergent quantum phenomena, yet their fabrication is often limited by interfacial contamination and incompatibility with surface-sensitive characterization. Here, we demonstrate muscovite mica as a polymer-free platform for assembling and patterning vdW heterostructure devices. Mica-mediated transfer enables clean exposed surfaces suitable for atomic-resolution scanning tunneling microscopy (STM) following mild thermal annealing, without aggressive postprocessing. Mica also serves as a mechanically robust support for sequential pickup assembly, eliminating intermediate release steps. Furthermore, exfoliated mica flakes function as removable shadow masks for contact deposition, enabling straightforward patterning without conventional lithography. We quantify the temperature dependence of mica-mediated graphene pickup and demonstrate gate-dependent transport in a three-terminal graphene/hBN device fabricated using the approach. By integrating transfer, sequential assembly, and contact patterning within a single materials platform, mica provides a simple and accessible route to unencapsulated vdW heterostructures for surface-sensitive spectroscopy and quantum-device applications.

Surface metal‐organic frameworks (SMOFs) are promising platforms for catalysis, yet their structural response to electrochemical bias is often overlooked. We report the potential‐driven activation of an N‐heterocyclic carbene (NHC)‐based SMOF on Au(111) for the hydrogen evolution reaction (HER). Operando electrochemical scanning tunneling microscopy (EC‐STM) reveals that the framework, initially “frozen” in a strongly interconnected 1D chain morphology at open circuit potential, transitions into a dynamic phase under cathodic bias. This reconfiguration involves a dynamic cycle of metal abstraction and release, generating a high density of undercoordinated sites. DFT calculations confirm that while internal bridging Au nodes are inert (ΔG _H* = 0.99 eV), the potential‐induced decoupling of the chains creates terminal Au nodes and lattice vacancies with near‐thermoneutral adsorption energies of 0.05 and 0.07 eV, respectively. This study demonstrates that superior HER performance originates from the SMOF's ability to dynamically transform into a fluxional active state, providing a strategy for designing adaptive interfacial catalysts.

Engineering sublattice imbalance within the unit cell of bottom-up synthesized graphene nanoribbons (GNRs) represents a versatile tool for realizing custom-tailored quantum nanomaterials. The interaction between low-energy zero-modes (ZMs) not only contributes to frontier bands but can form the basis for magnetically ordered phases. Here, we present the bottom-up synthesis of a noncentrosymmetric GNR that places all ZMs on the majority sublattice sites. Scanning tunneling microscopy and spectroscopy reveal that strong electron-electron correlations, leading to the Stoner magnetic instability, drive the system into a ferromagnetically ordered insulating ground state featuring a sizable band gap of E_g ∼1.2 eV. At higher temperatures, a chemical transformation induces an insulator-to-metal transition that quenches the ferromagnetic order. Tight-binding (TB), density functional theory, and GW calculations corroborate our experimental observations. This work showcases how control over molecular symmetry, sublattice polarization, and ZM hybridization in bottom-up synthesized nanographenes can open a path to the exploration of many-body physics in rationally designed quantum materials.

This research program established the scientific foundation for the rational, bottom-up design, synthesis, isolation, and investigation of symmetry-protected topological (SPT) electron spin qubits embedded in graphene nanoribbons (GNRs). The work focused on integrating atomically precise low-dimensional carbon nanostructures with emerging quantum logic architectures, providing a pathway toward scalable quantum materials for next-generation computing and sensing technologies. A central component of the program was the elucidation of fundamental relationships between real-space molecular architecture, local spin density distributions, electronic band dispersion, and energy level alignment in atomically precise GNR systems. These correlations define key operational parameters of SPT qubits and were systematically investigated to establish quantitative benchmarks against established molecular and solid-state spin qubit platforms. Attention was given to properties critical for quantum device performance, e.g. decoherence times, spectral sharpness of energy transitions, and tunable exchange interactions between spin states. The research demonstrated that these parameters can be engineered with atomic precision through scalable bottom-up synthetic strategies. Theory-guided design played a central role in identifying candidate structures hosting topologically protected spin states. Experimental validation was performed using both ensemble measurements and single-molecule characterization. In addition to advances in quantum materials synthesis, the program developed and applied spin-sensitive scanning probe microscopy techniques capable of directly probing quantum states and dynamic processes with atomic-scale spatial resolution. These capabilities enabled direct observation and characterization of quantum structures at the single-atom level. While the research activities were primarily hypothesis-driven fundamental investigations, the program adopted a comprehensive materials-by-design framework aimed at translating scientific discoveries into technological concepts compatible with scalable and intelligent manufacturing approaches.

π-Conjugated covalent organic frameworks provide a versatile molecular scaffold for the realization of designer quantum nanomaterials. Strong electron–electron correlation within these artificial lattices can give rise to exotic phases of matter. Their experimental realization, however, requires precise control over orbital symmetry, charge localization and band dispersion, all arising from the effective hybridization between molecular linkers and nodes. Here we present a modular strategy for constructing diatomic kagome lattices from aza-[3]triangulene nodes, in which a D_3h-symmetric ground state is stabilized through resonance contributions from a cumulenic linker. First-principles density functional theory and scanning tunnelling spectroscopy reveal that the hybridization of a sixfold-degenerate set of edge-localized Wannier functions in the unit cell gives rise to orbital-phase-frustration-induced non-trivial flat bands. These results establish a general design principle for engineering orbital interactions in organic lattices and open a pathway towards programmable covalent-organic-framework-based quantum materials with correlated electronic ground states.

Structural distortions in low-dimensional π-conjugated systems profoundly influence their electronic properties, but the control of such behavior in laterally extended systems remains challenging. Here we demonstrate that a one-dimensional conjugated polymer─poly-(difluorenoheptalene-ethynylene) (PDFHE)─undergoes a pronounced out-of-plane backbone distortion, equivalent to a spontaneous symmetry breaking (SSB) of its mirror symmetry. We synthesized PDFHE on noble metal surfaces and characterized its structure and electronic states using low-temperature scanning tunneling microscopy (STM). Rather than adopting a planar, high-symmetry conformation, PDFHE relaxes into nonplanar isomers stabilized by a pseudo-Jahn-Teller (PJT) distortion having mirror-odd out-of-plane character. The distortion lowers the total energy and increases the band gap, providing a concise rationale for the observed symmetry breaking. Density functional theory calculations corroborate these findings, providing a microscopic explanation for the SSB. Our results show that even in mechanically robust extended π-systems, subtle electron-lattice coupling can spontaneously drive significant structural rearrangements, even in mechanically robust extended π-systems.

Pi-conjugated covalent organic frameworks (COFs) provide a versatile platform for the realization of designer quantum nanomaterials. Strong electron-electron correlation within these artificial lattices can give rise to exotic phases of matter. Their experimental realization however requires precise control over orbital symmetry, charge localization, and band dispersion all arising from the effective hybridization between molecular linkers and nodes. Here, we present a modular strategy for constructing diatomic Kagome lattices from aza-[3]triangulene (A[3]T) nodes, in which a D3h symmetric ground state is stabilized through resonance contributions from a cumulenenic linker. First-principles density-functional theory and scanning tunnelling spectroscopy reveal that the hybridization of a sixfold degenerate set of edge-localized Wannier functions in the unit cell gives rise to orbital-phase frustration-induced non-trivial flat bands. These results establish a general design principle for engineering orbital interactions in organic lattices and open a pathway toward programmable COF-based quantum materials with correlated electronic ground states.

Structurally distorting low-dimensional π-conjugated systems can profoundly influence their electronic properties, but controlling such behavior in extended-width systems remains challenging. Here we demonstrate that a one-dimensional conjugated polymer, poly-(difluorenoheptalene-ethynylene) (PDFHE), undergoes a pronounced out-of-plane backbone distortion, equivalent to a spontaneous symmetry breaking (SSB) of its mirror symmetry. We synthesized PDFHE on noble metal surfaces and characterized its structure and electronic states using low-temperature scanning tunneling microscopy. Rather than adopting a planar, high-symmetry conformation, PDFHE relaxes into non-planar isomers stabilized by a Jahn-Teller-like mechanism that relieves an electronic instability relative to the gapped planar structure. Density functional theory calculations corroborate these findings, revealing that distortion lowers the total polymer energy and enlarges the bandgap, providing a microscopic explanation for the SSB. Our results show that even in mechanically robust extended π-systems, subtle electron-lattice coupling can spontaneously drive significant structural rearrangements.