Scanning Tunneling Research Articles

Diaminotriazinyl-Substituted N-Heterotriangulene: Hydrogen Bonding-Driven Self-Assembly in the Solid State, in the Gas Phase and on Surfaces.

We report the synthesis and comprehensive characterization of a dimethylmethylene-bridged N-heterotriangulene (N-HTA) with diaminotriazinyl groups to guide the self-assembly through directional hydrogen bonding. Mass spectrometry collision-induced dissociation experiments indicated the formation of multiply charged clusters in the gas phase. Single crystal X-ray diffraction studies revealed that the solid state packing is governed by an interplay between the hydrogen bonding and the solvent system used for crystallization. The self-assembly on both semiconducting and insulating surfaces upon simple drop-casting was disclosed by atomic force microscopy and scanning tunneling microscopy. The strong hydrogen bonding leads to robust self-assembly on surfaces, which can be conveniently achieved by simple solution processing techniques under ambient conditions.

Redirecting On-surface Cycloaddition Reactions in a Self-assembled Ordered Molecular Array on Graphite.

The synthesis of atomically precise carbon nanostructures in ultra-high vacuum has seen extensive progress on metal surfaces. However, this remains challenging on chemically inert surfaces. It is because the thermally activated C-C coupling encounters a severe "desorption problem" on weakly interacting substrates. In this study, we report an extraordinary [2+2]+[2+2] cycloaddition triggered by mild annealing (~210°C) in a highly ordered π-conjugated molecular array on graphite using scanning tunneling microscopy. In contrast to irregular dendritic fragments typically obtained on metal substrates, large supramolecular islands are observed here with cycloaddition products and other polymers over 30%, which are embedded as defective individuals or chains (grain boundaries). First-principles calculations reveal that the energy barriers of the multiple dehalogenation, dehydrogenation, and cycloaddition reactions are reduced by catalytic Fe atoms but remain energetically unfavorable. A distinct driving mechanism is proposed for redirecting the reactions on inert surfaces, where additional intermolecular coupling, steric hindrance, and/or interfacial interactions play significant roles. This study introduces a new paradigm for understanding on-surface synthesis on non-metal substrates.

Redirecting On‐surface Cycloaddition Reactions in a Self‐assembled Ordered Molecular Array on Graphite

The synthesis of atomically precise carbon nanostructures in ultra‐high vacuum has seen extensive progress on metal surfaces. However, this remains challenging on chemically inert surfaces. It is because the thermally activated C‐C coupling encounters a severe “desorption problem” on weakly interacting substrates. In this study, we report an extraordinary [2+2]+[2+2] cycloaddition triggered by mild annealing (~210°C) in a highly ordered π‐conjugated molecular array on graphite using scanning tunneling microscopy. In contrast to irregular dendritic fragments typically obtained on metal substrates, large supramolecular islands are observed here with cycloaddition products and other polymers over 30%, which are embedded as defective individuals or chains (grain boundaries). First‐principles calculations reveal that the energy barriers of the multiple dehalogenation, dehydrogenation, and cycloaddition reactions are reduced by catalytic Fe atoms but remain energetically unfavorable. A distinct driving mechanism is proposed for redirecting the reactions on inert surfaces, where additional intermolecular coupling, steric hindrance, and/or interfacial interactions play significant roles. This study introduces a new paradigm for understanding on‐surface synthesis on non‐metal substrates.

Non-Hermitian Boundary in a Surface Selective Reconstructed Magnetic Weyl Semimetal.

Non-Hermitian physics, studying systems described by non-Hermitian Hamiltonians, reveals unique phenomena not present in Hermitian systems. Unlike Hermitian systems, non-Hermitian systems have complex eigenvalues, making their effects less directly observable. Recently, significant efforts have been devoted to incorporating the non-Hermitian effects into condensed matter physics. However, progress is hindered by the absence of a viable experimental approach. Here, the discovery of the surface-selectively spontaneous reconstructed Weyl semimetal NdAlSi provides a feasible experimental platform for studying non-Hermitian physics. Utilizing angle-resolved photoemission spectroscopy (ARPES) measurements, surface-projected density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements, it is demonstrated that surface reconstruction in NdAlSi alters surface Fermi arc (SFA) connectivity and generates new isolated non-topological SFAs (NTSFAs) by introducing non-Hermitian terms. The surface-selective spontaneous reconstructed Weyl semimetal NdAlSi can be viewed as a Hermitian bulk - non-Hermitian boundary system. The isolated non-topological SFAs on the reconstructed surface act as a loss mechanism and open boundary condition (OBC) for the topological electrons and bulk states, serving as non-Hermitian boundary states. This discovery provides a good experimental platform for exploring new physical phenomena and potential applications based on boundary non-Hermitian effects, extending beyond purely mathematical concepts. Furthermore, it provides important enlightenment for constructing topological photonic crystals with surface reconstruction and studying their topologicalproperties.

Unveiling the Correlation between Defects and High Mobility in MoS2 Monolayers.

Defects in semiconductors play a crucial role in modifying their electronic structure and transport properties. In transition metal dichalcogenides, atomic chalcogen vacancies are a primary source of intrinsic defects. While the impact of these vacancies on electrical transport has been widely studied, their exact role remains not fully understood. In this work, we correlate optical spectroscopy, low-temperature electrical transport measurements, scanning tunneling microscopy (STM), and first-principles density functional theory (DFT) calculations to explore the effect of chalcogen vacancies in MoS2 monolayers grown by chemical vapor deposition. We specifically highlight the role of disulfur vacancies in modulating electrical properties, showing that these defects increase the density of shallow donor states near the conduction band, which facilitates electron hopping conduction, as evidenced by low-temperature transport and STM measurements. These findings are further supported by DFT calculations, which reveal that the electronic states associated with these defects are relatively delocalized, promoting hopping conduction and inducing n-type doping. This mechanism accounts for the observed high field-effect mobility (>100 cm2 V-1s-1) in the samples. These findings highlight the potential for defect engineering as a universal approach to customizing the properties of 2D materials for various applications.

Theory of Atomic-Scale Direct Thermometry Using Electron Spin Resonance via Scanning Tunneling Microscopy.

Knowledge of the occupation ratio and energy splitting of a two-level system provides a direct method for temperature readout. This principle was demonstrated for an individual two-level magnetic atom using Electron Spin Resonance via Scanning Tunneling Microscopy (ESR-STM). The temperature determination involves two steps: measuring the energy splitting with ESR-STM and determining the equilibrium occupation of a nearby atom using the peak height ratio in the ESR spectrum. Here we present a theory addressing three aspects: the impact of shot noise and back-action on thermometry precision, the role of spin geometry in enhancing signal-to-noise ratio, and the method's capability to detect thermal gradients as small as 5 mK/nm. We predict ESR-STM thermometry achieves 10 mK resolution at around 1 K temperatures, offering new avenues for nanoscale thermal measurements.

Stripe charge order and its interaction with Majorana bound states in 2M-WS2 topological superconductors.

To achieve logic operations via Majorana braiding, positional control of the Majorana bound states (MBSs) must be established. Here we report the observation of a striped surface charge order coexisting with superconductivity and its interaction with the MBS in the topological superconductor 2M-WS2, using low-temperature scanning tunneling microscopy. By applying an out-of-plane magnetic field, we observe that MBSs are absent in vortices in the region with stripe order. This is in contrast to adjacent underlaying layers without charge order, where vortex-bound MBSs are observed. Via theoretical simulations, we show that the surface stripe order does not destroy the bulk topology, but it can effectively modify the spatial distribution of MBSs, i.e. it pushes them downward, away from the 2M-WS2 surface. Our findings demonstrate that the interplay of charge order and topological superconductivity can potentially be used to tune the positions of MBSs, and to explore new states of matter.

Importance of Ion Size on the Dominance of Water-Ion Versus Water-Water Interactions in Au-Supported Solvatomers.

The solvation of ions at interfaces is important to areas as diverse as atmospheric sciences, energy materials, and biology. Despite the significance, fundamental understanding, particularly at the molecular level, remains incomplete. Here, we probe the initial solvation of two singly charged but differently sized ions (Li and Cs) on a Au(111) by combining low-temperature scanning tunneling microscopy with density functional theory. Real-space molecular scale information reveals that water-ion interactions dominate the Li-water system, whereas water-water interactions dominate in the Cs case, and in both cases, the Au(111) surface confines the formed solvatomers to two dimensions. The difference in prevalent interactions leads to disparate symmetry and binding patterns of the solvation shells observed, as well as significantly different ion-surface interactions. The relationship between water number, geometry, and electronic structure of the solvatomers obtained here is an essential step toward understanding heterogeneous interfaces on the nanoscale.

Coalescence of multiple topological orders in quasi-one-dimensional bismuth halide chains

Topology is being widely adopted to understand and to categorize quantum matter in modern physics. The nexus of topology orders, which engenders distinct quantum phases with benefits to both fundamental research and practical applications for future quantum devices, can be driven by topological phase transition through modulating intrinsic or extrinsic ordering parameters. The conjoined topology, however, is still elusive in experiments due to the lack of suitable material platforms. Here we use scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and theoretical calculations to investigate the doping-driven band structure evolution of a quasi-one-dimensional material system, bismuth halide, which contains rare multiple band inversions in two time-reversal-invariant momenta. According to the unique bulk-boundary correspondence in topological matter, we unveil a composite topological phase, the coexistence of a strong topological phase and a high-order topological phase, evoked by the band inversion associated with topological phase transition in this system. Moreover, we reveal multiple-stage topological phase transitions by varying the halide element ratio: from high-order topology to weak topology, the unusual dual topology, and trivial/weak topology subsequently. Our results not only realize an ideal material platform with composite topology, but also provide an insightful pathway to establish abundant topological phases in the framework of band inversion theory.

Enhancing Chemoselectivity of On-Surface Reactions on Reconstructed Au(110): The Case of a Pentalene-Bridged Polyacene Analogue.

Polyacene analogues, consisting of short acene segments separated by nonbenzenoid rings, offer intriguing electronic properties and magnetic interactions. Pentalene-bridged polyacenes (PPs), in particular, hold promise for enhancing the electrical conductivity and potential open-shell ground states. However, PPs have remained elusive in solution chemistry due to poor solubility and limited synthetic protocols. Here, we report the on-surface synthesis of PPs through the annulation between ortho-xylene groups. Scanning tunneling microscopy and atomic force microscopy reveal that the reconstructed Au(110) surface significantly enhances the chemoselectivity of the annulation process. Scanning tunneling spectroscopy combined with density functional theory suggests that PP exhibits a narrow direct band gap, similar to long acenes. This work demonstrates the potential for band structure engineering in polyacene analogues by incorporating nonbenzenoid rings, paving the way to advancements in organic electronics and spintronics.

Identical Fe-N4 Sites with Different Reactivity: Elucidating the Effect of Support Curvature.

Detailed atomic-scale understanding is a crucial prerequisite for rational design of next-generation single-atom catalysts (SACs). However, the sub-ångström precision needed for systematic studies is challenging to achieve on common SACs. Here, we present a two-dimensional (2D) metal-organic system featuring Fe-N4 single-atom sites, where the metal-organic structure is modulated by 0.4 Å corrugation of an inert graphene/Ir(111) support. Using scanning tunneling microscopy and density functional theory, we show that the support corrugation significantly affects the reactivity of the system, as the sites above the support "valleys" bind TCNQ (tetracyanoquinodimethane) significantly stronger than the sites above the "hills". The experimental temperature stability of TCNQ varies by more than 60 °C, while computations indicate more than 0.3 eV variation of TCNQ adsorption energy across the Fe-N4 sites placed atop different regions of the corrugated graphene unit cell. The origin of this effect is steric hindrance, which plays a role whenever large molecules interact with neighboring single-atom catalyst sites or when multiple reactants coadsorb on such sites. Our work demonstrates that such effects can be quantitatively studied using model SAC systems supported on chemically inert and physically corrugated supports.

Fractional Spinon Quasiparticles in Open-Shell Triangulene Spin-1/2 Chains.

The emergence of spinon quasiparticles, which carry spin but lack charge, is a hallmark of collective quantum phenomena in low-dimensional quantum spin systems. While the existence of spinons has been demonstrated through scattering spectroscopy in ensemble samples, real-space imaging of these quasiparticles within individual spin chains has remained elusive. In this study, we construct individual Heisenberg antiferromagnetic spin-1/2 chains using open-shell [2]triangulene molecules as building blocks. Each [2]triangulene unit, owing to its sublattice imbalance, hosts a net spin-1/2 in accordance with Lieb's theorem, and these spins are antiferromagnetically coupled within covalent chains with a coupling strength of J = 45 meV. Through scanning tunneling microscopy and spectroscopy, we probe the spin states, excitation gaps, and their spatial excitation weights within covalent spin chains of varying lengths with atomic precision. Our investigation reveals that the excitation gap decreases as the chain length increases, extrapolating to zero for long chains, consistent with Haldane's gapless prediction. Moreover, inelastic tunneling spectroscopy reveals an m-shaped energy dispersion characteristic of confined spinon quasiparticles in a one-dimensional quantum box. These findings establish a promising strategy for exploring the unique properties of excitation quasiparticles and their broad implications for quantum information.

Non‐monotonic Variation of Potential‐dependent Surface Diffusion at Electrochemical Interfaces in the Presence of Coadsorbates

The influence of coadsorbed ions on adsorbate diffusion, an inherent effect at solid‐liquid interfaces, was studied for adsorbed sulfur on Ag(100) electrodes in the presence of bromide or iodide. Quantitative in situ high‐speed scanning tunnelling microscopy (video‐STM) measurements were performed both in the potential regime of the c(2×2) halide adlayer at its saturation coverage and in the regime of a disordered adlayer where the halide coverage increases with potential. These studies reveal a surprising non‐monotonic potential dependence of Sad diffusion with an initial increase with halide coverage, followed by a decrease upon halide adlayer ordering into the c(2×2) structure. Density functional theory (DFT) and Monte Carlo (MC) simulations only qualitatively reproduce the rise in Sad mobility with halide coverage, suggesting that many‐adsorbate interactions and the presence of the electrolyte need to be considered.

Weak coupling of buckled germanene with high Fermi velocity on semiconducting Cu2Te

Despite its promise, growing a quasi-freestanding monolayer of germanene with Dirac cone signature remains a significant attention. Synthesizing germanene on semiconductor surfaces is highly desirable to preserve its linear energy dispersion near the K points, which has been experimentally challenging. Here, we report the molecular beam epitaxy of monolayer germanene on semiconducting Cu2Te supported by Cu(111). Scanning tunneling microscopy/spectroscopy (STM) revealed a low-buckled honeycomb lattice of germanene, exhibiting an intrinsic Dirac cone at the K point. By combining STM measurements with theoretical simulations, we confirm that germanene atoms occupy threefold hollow sites on Cu2Te via van der Waals interaction. Remarkably, by dI/dV spectra fitting, we find the prepared germanene owns the Fermi velocity of (6.9 ± 0.1) × 105 m/s, which is slightly higher than the density functional theory calculated 4.6 × 105 m/s with considering the dielectric constant of the underlying Cu2Te, implying the weak coupling of germanene with the substrate. This work provides a platform for further exploring the ballistic charge transport properties of germanene with a Dirac cone.

Hydride Formation and Decomposition on Cu(111) in HClO4.

Cu electrodeposition and the electrocatalysis of hydrogenation reactions thereupon involve significant interactions with adsorbed hydrogen. Electrochemical mass spectrometry (EC-MS) is used to explore the formation and decomposition of surface hydride on Cu(111) in 0.1 mol L-1 HClO4. Hydride formation is associated with two reduction waves that reflect the potential-dependent Hads coverage and its reconstruction. Voltammetric cycling reveals an additional oxidative and reductive feature at ≈ -0.05 V versus the reversible hydrogen electrode (RHE) that reflects the state of the 2D surface hydride. Extending the voltammetric window to more negative potentials results in an increase in Hads coverage and surface reconstruction that subsequently leads to accelerated hydride decomposition at positive potentials. Voltammetric and chronoamperometric analysis of hydride formation indicates a Hads coverage of ≈0.75 monolayers (ML) between -0.225 V vs RHE and -0.275 V vs RHE with further increases in Hads observed with the onset and acceleration of the HER at more negative potentials. Returning to more positive potentials, hydride decomposition begins above -0.05 V vs RHE. Recombination of Hads to form H2 accounts for desorption of ≈0.5 ML of Hads while its oxidation to H3O+ consumes between ≈0.15 and ≈0.4 ML of Hads, depending on the specific electrochemical conditions. The potential-dependent Hads coverage and surface reconstruction are congruent with trends identified in recent computational and electrochemical scanning tunneling microscopy studies. In contrast to perchloric acid, the presence of strongly adsorbing anions, such as sulfate or halides, favors hydride decomposition via the recombination pathway.

Non-monotonic Variation of Potential-dependent Surface Diffusion at Electrochemical Interfaces in the Presence of Coadsorbates.

The influence of coadsorbed ions on adsorbate diffusion, an inherent effect at solid-liquid interfaces, was studied for adsorbed sulfur on Ag(100) electrodes in the presence of bromide or iodide. Quantitative in situ high-speed scanning tunnelling microscopy (video-STM) measurements were performed both in the potential regime of the c(2×2) halide adlayer at its saturation coverage and in the regime of a disordered adlayer where the halide coverage increases with potential. These studies reveal a surprising non-monotonic potential dependence of Sad diffusion with an initial increase with halide coverage, followed by a decrease upon halide adlayer ordering into the c(2×2) structure. Density functional theory (DFT) and Monte Carlo (MC) simulations only qualitatively reproduce the rise in Sad mobility with halide coverage, suggesting that many-adsorbate interactions and the presence of the electrolyte need to be considered.

Magnetic exchange interaction between unpaired π- and d-electrons in nanographene-metal coordination complexes

Abstract The combination of open-shell nanographenes (NGs) and magnetic transition metals holds great promise for generating various new quantum phases applicable in spintronics and quantum information technologies. However, a crucial aspect in accomplishing this is to comprehend the magnetic exchange interactions between unpaired π- and d-electrons, a topic that has been seldom addressed. In this study, we focus on magnetic π-d exchange interactions between open-shell NGs and a magnetic coordination center of Fe or Co by employing scanning tunneling microscopy and spectroscopy. We synthesize two sets of NG-metal coordination complexes on a Au(111) substrate, secured by coordination bonds of carboxyl acid-Fe (Co). Through analysis of the excitation spectra, we observe a characteristic exchange coupling of 9 meV (5 meV) between the unpaired π-electron and the Fe (Co) d-shell electrons. Our experimental findings are qualitatively in agreement with multiconfigurational quantum chemistry calculations. This work evidences that a substantial magnetic exchange coupling can be achieved and engineered in metal-organic coordination systems, paving the way for designing and customizing extended radical metal-organic frameworks with precisely tailored magnetic properties.

Synthesis and Characterization of a Non-planar Cyclophenylene on Au(111).

We report the surface-assisted synthesis of a non-planar cyclophenylene derivative containing four meta- and two para- connected phenylene moieties on Au(111), via hierarchical Ullmann coupling of a 1,10-dibrominated angular [3]phenylene and subsequent C-C bond cleavage at the four-membered rings. Scanning tunneling microscopy and spectroscopy (STM/STS) were used for the characterization of its chemical structure and electronic properties. Density functional theory (DFT) calculations support the experimental observations.

Chalcogen Substitution-Modulated Molecule-Electrode Coupling in Single-Molecule Junctions.

Molecule-electrode interfaces play a pivotal role in defining the electron transport properties of molecular electronic devices. While extensive research has concentrated on optimizing molecule-electrode coupling (MEC) involving electrode materials and molecular anchoring groups, the role of the molecular backbone structure in modulating MEC is equally vital. Additionally, it is known that the incorporation of heteroatoms into the molecular backbone notably influences factors such as energy levels and conductive characteristics. In this work, we report a series of molecular wires that are organized in donor-acceptor-donor configurations, with distinct chalcogen substitutions, including oxygen (BOD), sulfur (BTD), and selenium (BSD). We investigated the electron transport properties using the scanning tunneling microscope break junction (STM-BJ) technique. Our results revealed that both the single-molecule conductance and the junction evolution feature are impacted by the heteroatoms in the benzo(chalcogen)diazole cores. Furthermore, current-voltage (I-V) experiments, combined with theoretical analyses, suggest that MEC plays a dominant role in modulating electron transport behaviors. Overall, our findings provide important insights into the interface-mediated charge transport exerted by chalcogen atoms within molecular devices, thereby enhancing the fundamental comprehension of these critical interactions.

Molecular-Scale Insights into the Heterogeneous Interactions between an m-Terphenyl Isocyanide Ligand and Noble Metal Nanoparticles.

The structural and chemical properties of metal nanoparticles are often dictated by their interactions with molecular ligand shells. These interactions are highly material-specific and can vary significantly even among elements within the same group or materials with similar crystal structure. In this study, we surveyed the heterogeneous interactions between an m-terphenyl isocyanide ligand and Au and Ag nanoparticles (NPs) at the single-molecule limit. Specifically, we found that the ligation behavior with this molecule differs significantly between that of Au and AgNPs. Surface-enhanced Raman spectroscopy measurements revealed unique enhancement factors for two molecular vibrational modes between two metal surfaces, indicating different ligand binding geometries. Molecular-level characterization using scanning tunneling microscopy allowed us to directly visualize these variations between Ag and Au surfaces, which we assign as two distinct binding mechanisms. This molecular-scale visualization provides clear insights into the different ligand-metal interactions as well as the chemical behavior and spectroscopic characteristics of isocyanide-functionalized NPs.