Scanning Tunneling Research Articles

Plasmon-Exciton Strong Coupling in Single-Molecule Junction Electroluminescence.

Single molecules bridging two metallic electrodes can emit light through electroluminescence when subjected to a bias voltage. Typically, light emission in such devices results from transitions between molecular states, although in the presence of light-matter coupling, the emission can result from a transition between hybrid light-matter states. Here, we create single metal-molecule-metal junctions and simultaneously collect conductance and electroluminescence data using a scanning tunneling microscope (STM) equipped with a custom spectrometer. Through experimental analysis and electronic structure calculations, we provide evidence for a molecule-electrode interfacial exciton coupled to a junction cavity plasmon. Importantly, we find that close to resonant transport conditions, the molecular junction functions as a single emitter that is strongly coupled to the junction cavity mode, leading to characteristic Rabi splitting of the emission spectrum and providing the first example of an electroluminescence-driven single-molecule system in the regime of strong light-matter coupling.

Hydride-Induced Reconstruction of Pd Electrode Surfaces: A Combined Computational and Experimental Study.

Designing electrocatalysts with optimal activity and selectivity relies on a thorough understanding of the surface structure under reaction conditions. In this study, experimental and computational approaches are combined to elucidate reconstruction processes on low-index Pd surfaces during H-insertion following proton electroreduction. While electrochemical scanning tunneling microscopy clearly reveals pronounced surface roughening and morphological changes on Pd(111), Pd(110), and Pd(100) surfaces during cyclic voltammetry, a complementary analysis using inductively coupled plasma mass spectrometry excludes Pd dissolution as the primary cause of the observed restructuring. Large-scale molecular dynamics simulations further show that these surface alterations are related to the creation and propagation of structural defects as well as phase transformations that take place during hydride formation.

Guidance on surface cyclization reactions through coordination structures.

Metal coordination structures, formed through interactions between metal atoms and active functional groups, are crucial in determining the reaction pathway and its products. This study examines 2,3-dibromo-6,7-dicyanonaphthalene (DDN), which contains cyano and halogen groups, deposited on Au(111) and Ag(111) surfaces, using scanning tunneling microscopy at room temperature. The reason for the formation of various cyclization products on the Au(111) surface is that the bromine of DDN and the cyano group have overlapping reaction temperature ranges. This overlap leads to the coexistence of C-C coupling products from the debromination sites and cyclization products from the cyano groups. In contrast, on the Ag(111) surface, the final cyclization reaction produces a single type of polymer directly induced by the metal-coordinated structures. The synergistic effects between coordination structures and the activation temperatures of molecular reaction groups are crucial factors in regulating polymerization reactions.

Multiband Josephson effect in an atomic scale Pb tunnel junction

Multiband superconductivity plays an important role in many emergent novel superconductors and has attracted great interest over the years. Various related experimental aspects have been intensely researched, but a quantitative understanding on the Cooper-pair transport remains still elusive, despite its fundamental and technological importance. We study a Josephson junction with a scanning tunneling microscope (STM), where both tip and sample are Pb, a prototypical type I two-band superconductor. We map the properties of the junction across a wide range of normal state conductances revealing in-gap features originating from multiple Andreev reflections (MARs) and the Josephson effect. We present the theoretical framework to extract the transmission through the transport channels and describe the Cooper-pair tunneling with quantitative precision through two superconducting bands. This paves the way for the understanding of increasingly complicated superconductors. Published by the American Physical Society 2024

Self-Organization of Square Nitrogen Islands on Copper (100) Surfaces: Island Pair Distributions Based on an Ab-Initio DFT and Mesoscopic Interaction Model.

Earlier scanning tunneling microscopy (STM) studies have shown that nitrogen forms square c(2×2) islands on Cu(100) surfaces, the assembly of which depends on coverage. Recent calculations have revealed that adatom-adatom interactions are compatible with the formation of square c(2×2) islands. The pair distribution of these islands is a topic of the current investigation. In addition, the domain structure of islands found recently in STM experiments is discussed and explained using two types of island interactions. The potential used for the interaction between N square islands on copper is a combination of previous ab initio calculations and an updated mesoscopic interaction scheme. The 2D-BGY integral equation is extended to handle binary objects like two different types of islands. It is solved for up to a quarter island coverage using a numerical recursion algorithm. Coverage beyond half is accessible with island vacancy symmetry. The calculation predicts the formation of ordered island assemblies with increasing coverage. These results reasonably explain the previous experimental observations. The handling of binary objects also seems to be applicable to assemblies of two adatom types as found in catalysis. The assumptions and limitations of the model are discussed and open questions are formulated.

Strong-Field Theory of Attosecond Tunneling Microscopy

Attosecond observations of coherent electron dynamics in molecules and nanostructures can be achieved by combining conventional scanning tunneling microscopy (STM) with ultrashort femtosecond laser pulses. While experimental studies in the subcycle regime are under way, a robust strong-field theory description has remained elusive. Here we devise a model based on the strong-field approximation. Valid in all regimes, it provides a surprising analogy to the standard model of STM. We show that the intuitive three-step model of attosecond science directly emerges from our model and we also describe the optimal conditions for attosecond STM experiments. Published by the American Physical Society 2024

Adsorption and Thermal Evolution of the Carbonyl‐Functionalized Ionic Liquid [5‐oxo‐C6C1Im][NTf2] on Pt(111): A Combined IRAS, STM, and DFT Study

The coating of heterogeneous catalysts with ionic liquids enables precise tuning of catalytic activity and selectivity. Recently, the fundamentals of solid catalysts with ionic liquid layers have been extensively studied. So far, investigations have focused on simple ILs without specialized functional groups. In our current work, we aim to involve functionalized ILs to take advantage of the interactions between these functional groups, the catalyst, and the reactants. In this study, we investigated the interaction, and thermal stability of the carbonyl‐functionalized IL [5‐oxo‐C6C1Im][NTf2] on Pt(111) by infrared reflection absorption spectroscopy and scanning tunneling microscopy. In addition, we performed density functional theory calculations to support our interpretation. At 200 K and low coverage, the carbonyl group of the [5‐oxo‐C6C1Im]+ cation is oriented parallel to the Pt(111) surface. With increasing coverage, the alkyl chain detaches from the surface and orients towards the vacuum. The [NTf2]‐ anion adsorbs parallel to the surface via the oxygen atoms of the SO2 groups. At higher coverage, at least one of the SO2 groups completely detaches from the surface. Upon heating to 250 K, we observe decomposition and partial desorption of [5‑oxo‐C6C1Im][NTf2], with further decomposition and desorption occurring between 350 and 400 K.

Structural and electrical characterization of self-assembled Sb4 molecules on metal surfaces.

Antimonene is a promising two-dimensional material that is calculated to have a significant fundamental bandgap usable for advanced applications such as field-effect transistors, photoelectric devices, and the quantum-spin Hall state. Herein, we conducted a comprehensive investigation of self-assembled Sb4 islands on Au and Ag surfaces by scanning tunneling microscopy. The Sb4 molecules form α-phase and β-phase structures on Au and Ag surfaces, respectively, each exhibiting distinct island patterns. We simulated these structures using density functional theory calculations. Finally, we demonstrated the metallic band structure of Sb4 islands and observed the spatial and energy-dependent contributions of Sb4 molecules to the electronic states. This research enhances the understanding of the synthesis process of antimonene on metal surfaces and provides valuable insights for the industrial production of antimonene.

Fabricating model heterostructures of large-area monolayer or bilayer MoS2 on an Au(111) surface under ultra-high vacuum

Fabricating heterojunctions with precisely controlled interfacial structures is crucial for exploring novel low-dimensional physics and for realizing high-performance devices. However, such capabilities are often constrained by contamination from the ambient environment or by the limitations of applicable methods and materials under vacuum conditions. In this study, MoS2/Au(111) heterostructures were fabricated by exfoliating MoS2 thin layers onto a crystallized Au(111) surface using a gold-assisted exfoliation method in an ultra-high vacuum environment. This method yields millimeter-sized monolayer or sub-millimeter-sized bilayers with contamination-free interfaces, which are unattainable for samples made in air. Scanning tunneling microscopy revealed that both the monolayer and the bilayer exhibit uniform and well-ordered moiré superlattices controlled by the twisting angle between the Au(111) surface and the MoS2 overlayer. The direct contact with the Au surface renders the monolayer MoS2 weakly metallic, while a less coupled bilayer is semiconducting, indicating a 0.54 eV Schottky barrier for the MoS2/Au(111) contact. This method is applicable to various combinations of van der Waals materials and metal surfaces. The uniform and controllable heterojunctions can serve as ideal model systems for exploring semiconductor–metal interfaces and atomic structures formed within.

Reversible Sliding Motion by Hole-Injection in Ammonium-Linked Ferrocene, Electronically Decoupled from Noble Metal Substrate by Crown-Ether Template Layer.

Artificial molecular machines, especially when based on wheel-and-axle complexes, can generate mechanical motions in response to external stimuli. Ferrocene (Fc) is a key component, but it decomposes at 300 K on metal surfaces. Here, a novel method is presented to construct and control the molecular complex composed of ammonium-linked ferrocene (Fc-amm) and tetrabrominated crown ether (BrCR) on a Cu(111) surface. Fc-amm molecules are periodically arranged on a BrCR monolayer film and imaged using scanning tunneling microscopy and spectroscopy. A lateral motion of the Fc groups by ≈0.1nm is observed for pairs of "edge-on" Fc-amm molecules upon hole injection. This sliding motion is reversible and controlled by the applied voltage. Theoretical analysis indicates that the motion is caused by increased Coulomb repulsion of the hole-doped Fc-amm+ ions and accompanied by a weakening of CH-π interactions. These findings open new avenues for developing nanomolecular devices using on-surface bottom-up processes.

Mechanistical Study on Substrate-Controlled Highly Selective [2+2] and [2+3] Cycloaddition Reactions.

Polycyclic conjugated hydrocarbons have acquired increased interests recently because of their potential applications in electronic devices. On metal surfaces, the selective synthesis of four- and five-membered carbon rings remains challenging due to the presence of diverse reaction pathways. Here, utilizing the same precursor molecule, we successfully achieved substrate-controlled highly selective cycloaddition reactions towards four- and five-membered carbon rings. A 97% yield for four-membered carbon rings on Au(111), while a 96% yield towards five-membered carbon rings is achieved on Ag(111). The detailed topological structures of the reaction products are carefully examined by bond-resolving scanning tunneling microscopy (BR-STM) imaging with a CO functionalized tip. The underlying mechanism of the novel surface-directed reaction selectivity is elucidated by extensive density functional theory (DFT) calculations. Our study paves the way for high selective synthesis of polycyclic conjugated hydrocarbons with non-benzenoid rings.

Two-Dimensional Self-Assembly of BODIPY Derivatives with Different Functional Groups at the Liquid-Solid Interface.

The 4,4-difluoro-boradiazaindacene (BODIPY) unit possesses a rigid aromatic backbone, which facilitates the formation of self-assemblies and aggregates. However, most current studies on the self-assemblies of BODIPY derivatives have relied on spectroscopic methods to indirectly gather information about the self-assembled structures. In this study, we presented three BODIPY derivatives (B-3OC12, B-3OC12-2I, and B-DOB-2OC12) that shared the same core but were decorated with different functional groups. The self-assembled structures were revealed using scanning tunneling microscopy (STM) in combination with density functional theory (DFT). The results showed that all the molecules self-assembled into lamellar structures. When modified with three dodecyloxy chains or with the introduction of additional halogen atoms, the B-3OC12 and B-3OC12-2I molecules tended to distribute in a staggered form to build a tetramer or dimer. In contrast, the B-DOB-2OC12 molecule, which contains a dioxaborole group, self-assembled in a head-to-tail manner. These results demonstrated that BODIPY derivatives self-assembled into different structures, depending on their distinct patterns of intermolecular interactions influenced by functional groups.

Direct visualization of relativistic quantum scars in graphene quantum dots.

Quantum scars refer to eigenstates with enhanced probability density along unstable classical periodic orbits. First predicted 40 years ago1, scars are special eigenstates that counterintuitively defy ergodicity in quantum systems whose classical counterpart is chaotic2,3. Despite the importance and long history of scars, their direct visualization in quantum systems remains an open field4-10. Here we demonstrate that, by using an in situ graphene quantum dot (GQD) creation and a wavefunction mapping technique11,12, quantum scars are imaged for Dirac electrons with nanometre spatial resolution and millielectronvolt energy resolution with a scanning tunnelling microscope. Specifically, we find enhanced probability densities in the form of lemniscate ∞-shaped and streak-like patterns within our stadium-shaped GQDs. Both features show equal energy interval recurrence, consistent with predictions for relativistic quantum scars13,14. By combining classical and quantum simulations, we demonstrate that the observed patterns correspond to two unstable periodic orbits that exist in our stadium-shaped GQD, thus proving that they are both quantum scars. In addition to providing unequivocal visual evidence of quantum scarring, our work offers insight into the quantum-classical correspondence in relativistic chaotic quantum systems and paves the way to experimental investigation of other recently proposed scarring species such as perturbation-induced scars15-17, chiral scars18,19 and antiscarring20.

Imaging Dihydrogen Bond-Driven Assembly of Borazine on Au(111).

Dihydrogen bondingis a peculiar type of attractive interaction occurring between a partially positively charged hydrogen atom and a partially negatively charged hydrogen atom. Borazine represents a prototypical molecule exhibiting dihydrogen bonding in both gas phase, as well as in its crystalline form. For borazine assemblies on solid surfaces, a direct observation and characterization of dihydrogen bonding has remained elusive, possibly due to an intricate interplay of substrate-molecule and intermolecular interactions. Here we present evidence of dihydrogen bonding occurring in borazine assemblies on a Au(111) surface. By means of low-temperature scanning tunneling microscopy, we unveiled distinct configurations, exhibiting single and double dihydrogen bonding. Density functional theory calculations elucidate the interplay between substrate adsorption and intermolecular interactions to stabilize the formation of borazine dimers on Au(111), being the building blocks for the formation of larger assemblies.

Characterization of Coulomb Interactions in Electron Transport Through a Single Hetero-Helicene Molecular Junction Using Scanning Tunneling Microscopy.

Characterization of the structural and electron transport properties of single chiral molecules provides critical insights into the interplay between their electronic structure and electrochemical environments, providing broader implications given the significance of molecular chirality in chiroptical applications and pharmaceutical sciences. Here, we examined the topographic and electronic features of a recently developed chiral molecule, B,N-embedded double hetero[7]helicene, at the edge of Cu(100)-supported NaCl thin film with scanning tunneling microscopy and spectroscopy. An electron transport energy gap of 3.2 eV is measured, which is significantly larger than the energy difference between the highest occupied and the lowest unoccupied molecular orbitals given by theoretical calculations or optical measurements. Through first-principles calculations, we demonstrated that this energy discrepancy results from the Coulomb interaction between the tunneling electron and the molecule's electrons. This occurs in electron transport processes when the molecule is well decoupled from the electrodes by the insulating decoupling layers, leading to a temporary ionization of the molecule during electron tunneling. Beyond revealing properties concerning a specific molecule, our findings underscore the key role of Coulomb interactions in modulating electron transport in molecular junctions, providing insights into the interpretation of scanning tunneling spectroscopy features of molecules decoupled by insulating layers.

Exploring structural and electronic properties of topological insulator/graphene nano-heterostructures

There is great interest in the study of topological insulator-based heterostructures due to expected emerging phenomena. However, a challenge of topological insulator (TI) research is the contribution of bulk conduction to the TI surface states. Both strain engineering and thickness control routes, which have been proposed to compensate for bulk doping, can be accessed through the use of nano-heterostructures consisting of topological insulator nanostructures grown on 2D materials. In this work, we report the synthesis of TI/graphene nano-heterostructures based on Bi2Te3 and Sb2Te3 nanoplatelets (NPs) grown on single-layer graphene. Various techniques were used to characterize this system in terms of morphology, thickness, composition, and crystal quality. We found that most of the obtained NPs are mainly <30 nm thick with thickness-dependent crystal quality, observed by Raman measurements. Thinner NPs (1 or 2 quintuple layers ) tend to replicate the topography of the underlying single-layer graphene, according to roughness analysis. Finally, we show preliminary studies of their band structure obtained by Low Temperature Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy, and by Density Functional Theory. We observe a highly negative ED value which can be attributed to the presence of defects.

Engineering 2D Materials from Single-Layer NbS2.

Starting from a single layer of NbS2 grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb5/3S3-2D and Nb2S3-2D are created using two different pathways. Either annealing under sulfur-deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase-pure Nb5/3S3-2D followed by Nb2S3-2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy, and X-ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structureformation.

Vibrational and Magnetic States of Point Defects in Bilayer MoSe2.

Defects in two-dimensional materials profoundly impact the physicochemical properties of the systems, whose characterization is highly desirable at the atomic scale. Here, using spectroscopic imaging scanning tunneling microscopy, we elucidate the vibrational and magnetic states of MoSe antisite and VMo vacancy with different charge states embedded in ultrathin MoSe2 bilayers supported on graphene substrate. Stringent vibronic states with multimode coupling are resolved on the defects. The spectral intensities are tunable with the electron tunneling rates and well-reproduced by theoretical modeling. Moreover, first-principles calculations suggest that the defects host a local magnetic moment of 2 μB in their neutral state, which is directly confirmed by our spin-flip inelastic electron tunneling spectroscopy. Our study deepens the understanding of defect properties and paves the way of defect-engineering material functionalities and spin-catalytic applications.

On-Surface Synthesis of Ni-Porphyrin-Doped Graphene Nanoribbons

On-Surface Synthesis of Ni-Porphyrin-Doped Graphene Nanoribbons

Voltage-Gated Switching of Moiré Patterns in Epitaxial Molecular Crystals.

Studying molecular materials at the nanoscale allows us to gain a deeper understanding of supramolecular structure formation and serves as the basis for rationally controlling the resulting interfacial properties. Here, we describe the formation of extended Moiré patterns resulting from the assembly of dipolar π-conjugated molecules on highly oriented pyrolytic graphite at the liquid-solid interface as characterized by scanning tunneling microscopy (STM). By switching the bias of the sample and thus the orientation of the external electric field in the vicinity of the STM junction, structural reorganization of the molecular building blocks and the resulting organic 2D crystal is induced and can conveniently be monitored in situ by the appearance and disappearance of the Moiré patterns. Importantly, the formation and loss of the Moiré patterns are fully reversible, providing exquisite control over epitaxial molecular crystals. Our approach provides fundamental insights into the supramolecular organization and resulting superstructure formation of incommensurable 2D lattices upon applying an electric field and enables the rational tuning of Moiré patterns as a key step toward the potential integration of organic 2D crystals in molecular nanodevices.