Scanning Tunneling Research Articles

Absence of bulk charge density wave order in the normal state of UTe2

A spatially modulated superconducting state, known as pair density wave (PDW), is a tantalizing state of matter with unique properties. Recent scanning tunneling microscopy (STM) studies revealed that spin-triplet superconductor UTe2 hosts an unprecedented spin-triplet, multi-component PDW whose three wavevectors are indistinguishable from a preceding charge-density wave (CDW) order that survives to temperatures well above the superconducting critical temperature, Tc. Whether the PDW is the mother or a subordinate order remains unsettled. Here, based on a systematic search for bulk charge order above Tc using resonant elastic X-ray scattering (REXS), we show that the structure factor of charge order previously identified by STM is absent in the bulk within the sensitivity of REXS. Our results invite two scenarios: either the density-wave orders condense simultaneously at Tc in the bulk, in which case PDW order is likely the mother phase, or the charge modulations are restricted to the surface.

Single‐Molecule Mechanoresistivity by Intermetallic Bonding

AbstractThe metal‐electrode interface is key to unlocking emergent behaviour in all organic electrified systems, from battery technology to molecular electronics. In the latter, interfacial engineering has enabled efficient transport, higher device stability, and novel functionality. Mechanoresistivity – the change in electrical behaviour in response to a mechanical stimulus and a pathway to extremely sensitive force sensors – is amongst the most studied phenomena in molecular electronics, and the molecule‐electrode interface plays a pivotal role in its emergence, reproducibility, and magnitude. In this contribution, we show that organometallic molecular wires incorporating a Pt(II) cation show mechanoresistive behaviour of exceptional magnitude, with conductance modulations of more than three orders of magnitude upon compression by as little as 1 nm. We synthesised series of cyclometalated Pt(II) molecular wires, and used scanning tunnelling microscopy – break junction techniques to characterise their electromechanical behaviour. Mechanoresistivity arises from an interaction between the Pt(II) cation and the Au electrode triggered by mechanical compression of the single‐molecule device, and theoretical modelling confirms this hypothesis. Our study provides a new tool for the design of functional molecular wires by exploiting previously unreported ion‐metal interactions in single‐molecule devices, and develops a new framework for the development of mechanoresistive molecular junctions.

Mechanisms for the Spin-State Switching of Strapped Ni-Porphyrin Complexes Deposited on Metal Surfaces: Insights from Quantum Chemical Calculations.

The incorporation of molecular switches on nanodevices requires both the intactness of the molecule once deposited on a substrate and the persistence of the reversible switching feature. Recently, the reversible spin-switching of strapped Ni(II)-porphyrin complexes deposited on Ag(111) surface is demonstrated with low-temperature scanning tunneling microscopy (STM). The spin transition is accompanied by the coordination change of the metal center, a phenomenon denominated in coordination-induced spin-state switching (CISSS). In this contribution, the spin switching of the deposited strapped Ni-porphyrin molecules using different quantum chemistry approaches is explored. This calculations inform about the geometry and electronic structure of the adsorbed molecules and the origin of the voltage-dependent switching promoted by the STM tip. Two different mechanisms are inspected to elucidate the key role of the tip, mainly the electron injection between the tip and the molecule and the differential stabilization of the two spin states by the applied electric field between the tip and the silver surface. This study puts in evidence the relevance of the pyridine ligand contained in the strap in the transport properties as in the CISSS process itself.

Stacking of charge-density waves in 2H−NbSe2 bilayers

We employ electronic-structure calculations to investigate the charge-density waves and periodic lattice distortions in bilayer 2H−NbSe2. We demonstrate that the vertical stacking can give rise to a variety of patterns that may lower the symmetry of the charge-density waves exhibited separately by the two composing 1H−NbSe2 monolayers. The general tendency to a spontaneous symmetry breaking observed in the ground state and the first excited states is shown to originate from a non-negligible interlayer coupling. Simulated images for scanning tunneling microscopy as well as geometric structure factors show signatures of the different stacking orders. This may not only be useful to reinterpret past experiments on surfaces and thin films, but it may also be exploited to devise ad hoc experiments for the investigation of the stacking order in 2H−NbSe2. We anticipate that our analysis not only applies to the 2H−NbSe2, but is also relevant for thin films and bulk, whose smallest centrosymmetric component is indeed the bilayer. Finally, our results illustrate clearly that the vertical stacking is not only important for 1T structures, as exemplified by the metal-to-insulator transition observed in 1T−TaS2, but seems to be a general feature of metallic layered transition metal dichalcogenides as well. Published by the American Physical Society 2024

Room-temperature selective cyclodehydrogenation on Au(111) via radical addition of open-shell resonance structures

Cyclodehydrogenation is an important ring-formation reaction that can directly produce planar-conjugated carbon-based nanomaterials from nonplanar molecules. However, inherently high C–H bond energy necessitates a high temperature during dehydrogenation, and the ubiquity of C − H bonds in molecules and small differences in their bond energies hinder the selectivity of dehydrogenation. Here, we report a room-temperature cyclodehydrogenation reaction on Au(111) via radical addition of open-shell resonance structures and demonstrate that radical addition significantly decreases cyclodehydrogenation temperature and further improves the chemoselectivity of dehydrogenation. Using scanning tunneling microscopy and non-contact atomic force microscopy, we visualize the cascade reaction process involved in cyclodehydrogenation and determine atomic structures and molecular orbitals of the planar acetylene-linked oxa-nanographene products. The nonplanar intermediates observed during progression annealing, combined with density functional theory calculations, suggest that room-temperature cyclodehydrogenation involves the formation of transient radicals, intramolecular radical addition, and hydrogen elimination; and that the high chemoselectivity of cyclodehydrogenation arises from the reversibility and different thermodynamics of radical addition step.

Atomically Sharp 1D Interfaces in 2D Lateral Heterostructures of VSe2─NbSe2 Monolayers.

van der Waals heterostructures have emerged as an ideal platform for creating engineered artificial electronic states. While vertical heterostructures have been extensively studied, realizing high-quality lateral heterostructures with atomically sharp interfaces remains a major experimental challenge. Here, we advance a one-pot two-step molecular beam lateral epitaxy approach and successfully synthesize atomically well-defined 1T-VSe2─1H-NbSe2 lateral heterostructures. We demonstrate the formation of defect-free lateral heterostructures and characterize their electronic structure by using scanning tunneling microscopy and spectroscopy together with density functional theory calculations. We find additional electronic states at the 1D interface as well as signatures of Kondo resonances in a side-coupled geometry. Our experiments explored the full potential of lateral heterostructures for realizing exotic electronic states in low-dimensional systems for further studies of artificial designer quantum materials.

Fermi Arcs Dominating the Electronic Surface Properties of Trigonal PtBi2

AbstractMaterials combining topologically non‐trivial behavior and superconductivity offer a potential route for quantum computation. However, the set of available materials intrinsically realizing these properties are scarce. Recently, surface superconductivity has been reported in PtBi2 in its trigonal phase and an inherent Weyl semimetal phase has been predicted. Here, based on scanning tunneling microscopy experiments, the signature of topological Fermi arcs are revealed in the normal state patterns of the quasiparticle interference. It is shown that the scattering between Fermi arcs dominates the interference spectra, providing conclusive evidence for the relevance of Weyl fermiology for the surface electronic properties of trigonal PtBi2.

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(2x2) 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(2x2) 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.

Realizing one-dimensional moiré chains with strong electron localization in two-dimensional twisted bilayer WSe2

Two-dimensional (2D) moiré systems based on twisted bilayer graphene and transition metal dichalcogenides provide a promising platform to investigate emergent phenomena driven by strong electron-electron interactions in partially filled flat bands. A natural question arises: Is it possible to expand the 2D correlated moiré physics to one-dimensional (1D) that electron-electron correlation is expected to be further enhanced? This requires selectively doping of 1D moiré chain, which seems to be not within the grasp of today's technology. Therefore, an experimental demonstration of the 1D moiré chain with partially filled electronic states remains absent. Here, we show that we can introduce 1D boundaries, separating two regions with different twist angles, in twisted bilayer WSe2 (tWSe2) by using scanning tunneling microscopy (STM) and demonstrate that the electronic states of 1D moiré sites along the boundaries can be selectively filled. The strong localized charge states of correlated moiré electrons in the 1D moiré chain can be directly imaged and manipulated by combining a back-gate voltage with the STM bias voltage. Our results open the door for realizing new correlated electronic states of the 1D moiré chain in 2D systems.

Protein Quakes in Redox Metalloenzymes: Clues to Molecular Enzyme Conductivity Triggered by Binding of Small Substrate Molecules.

Multicentre redox metalloproteins undergo conformational changes on electrochemical surfaces, or on enzyme substrate binding. The two-centre copper enzymes, laccase (Type I and TypeII/III Cu) and nitrite reductase (CuNIR) (Type I and Type II Cu) are examples. With some exceptions, these enzymes show no non-turnover voltammetry on Au(111)-surfaces modified by thiol based self-assembled molecular monolayers, but dioxygen or nitrite substrate triggers strong electrocatalytic signals. Scanning tunnelling microscopy also shows high conductivity only when dioxygen or nitrite is present. Atomic force microscopy shows constant CuNIR height but pronounced structural expansion in the electrocatalytic range on nitrite binding. We have recently offered a rationale, based on ab initio quantum chemical studies of water/nitrite substitution in a 740-atom CuNIR fragment. Presently we provide much more detailed structural assignment mapped to single-residue resolution. NO2 --binding induces both a 2 Å Cu-Cu distance increase, and pronounced frontier orbital delocalization strongly facilitating ET between the Cu regions. The conformational changes transmit from the catalytic Type II centre to the electron inlet Type I centre, via the His129-Cys130 ligands, and via Type I-Cys130 or Type I-His129 ending at Type II Asp92. The ET patterns are reflected in different atomic Mulliken charges in the water and nitrite CuNIR fragment.

Structural Manipulation of Spin Excitations in a Molecular Junction.

Single metallocene molecules act as sensitive spin detectors when decorating the probe of a scanning tunneling microscope (STM). However, the impact of the atomic-scale electrode details on the molecular spin state has remained elusive to date. Here, a nickelocene (Nc) STM junction is manipulated in an atomwise manner showing clearly the dependence of the spin excitation spectrum on the anchoring of Nc to Cu(111), a Cu monomer, and trimer. Moreover, while the spin state of the same Nc tip is a triplet with tunable spin excitation energies upon contacting the surface, it transitions to a Kondo-screened doublet on a Cu atom. Notably, the nontrivial magnetic exchange interaction of the molecular spin with the electron continuum of the substrate determines the spectral line shape of the spin excitations.

Molecular Evidence for the Axial Coordination Effect of Atomic Iodine on Fe‐N4 Sites in Oxygen Reduction Reaction

AbstractWe present a molecular‐scale investigation of the axial coordination effect of atomic iodine on Fe‐N4 sites in the oxygen reduction reaction (ORR) by electrochemical scanning tunneling microscopy (ECSTM). A well‐defined model catalytic system with explicit and uniform iodine‐coordinated Fe‐N4 sites was constructed facilely by the self‐assembly of iron(II) phthalocyanine (FePc) on an I‐modified Au(111) surface. The electrocatalytic activity of FePc for the ORR shows notable enhancement with axial iodine ligands. The modulation of the electronic structure of Fe sites to evoke a higher spin configuration by axial iodine was evidenced. The interaction strength between oxygen‐containing species and active centers becomes weaker due to the presence of iodine ligands, and the reaction is thermodynamically preferable. Furthermore, the reaction dynamics of FePc on I/Au(111) were explicitly determined via in situ ECSTM potential pulse experiments. In contrast, axial atomic iodine was found inefficacious for improving the activity of Co‐N4 sites, and electron rearrangement was found to be marginal, demonstrating that adequate interactions between axial ligands and metal sites for optimizing electronic structures and catalytic behaviors are prerequisites for the impactful role of axial ligands.

Recent Progress of Imaging Chemical Bonds by Scanning Probe Microscopy: A Review.

In the past decades, the invention of scanning probe microscopy (SPM) as the versatile surface-based characterization of organic molecules has triggered significant interest throughout multidisciplinary fields. In particular, the bond-resolved imaging acquired by SPM techniques has extended its fundamental function of not only unraveling the chemical structure but also allowing us to resolve the structure-property relationship. Here, we present a systematical review on the history of chemical bonds imaged by means of noncontact atomic force microscopy (nc-AFM) and bond-resolved scanning tunneling microscopy (BR-STM) techniques. We first summarize the advancement of real-space imaging of covalent bonds and the investigation of intermolecular noncovalent bonds. Beyond the bond imaging, we also highlight the applications of the bond-resolved SPM techniques such as on-surface synthesis, the determination of the reaction pathway, the identification of molecular configurations and unknown products, and the generation of artificial molecules created via tip manipulation. Lastly, we discuss the current status of SPM techniques and highlight several key technical challenges that must be solved in the coming years. In comparison to the existing reviews, this work invokes researchers from surface science, chemistry, condensed matter physics, and theoretical physics to uncover the bond-resolved SPM technique as an emerging tool in exploiting the molecule/surface system and their future applications.

Boosting the Photoresponse of Azobenzene Single-Molecule Junctions via Mechanical Interlock and Dynamic Anchor.

As the most classic photoisomerization system, azobenzene has been widely utilized as a building unit in various photoswitching applications. However, attempts to build azobenzene-based single-molecule photoswitches have met with limited success, giving low on/off ratios. Herein, we demonstrate two designs of azobenzene-based photoresponsive single-molecule junctions, based on mechanically interlocked diazocine and azobenzene-based dynamic anchors, respectively. Molecular conductance measurements using the scanning tunneling microscope breaking junction (STMBJ) technique revealed dramatic conductance changes upon photoillumination, achieving a high on/off ratio of ∼3.7. Using density functional theory (DFT), we revealed peculiar quantum interference (QI) effects in the diazocine molecular switch, indicating that diazocine is an excellent candidate for molecular photoswitches. The asymmetric azobenzene devices with a dynamic anchor exhibit switching behavior between a fully off state and a highly conductive state associated with the trans/cis conformation transition. The findings of this work not only present the design and development of functional molecular devices based on azobenzene units but also provide insight into the fundamental properties of light-induced quantum interference in azobenzene-based molecular devices.

Electronically Seamless Domain Wall of Chiral Charge Density Wave in 1T-TiSe2.

Domain walls (DWs) have been recognized to play crucial roles in various interesting properties and functionalities in quantum materials. A notable example is DWs in charge density wave (CDW), which are believed to provide metallic electron channels for emerging superconductivity. However, electronic states of DWs and the microscopic mechanism toward superconductivity have been elusive. Here, we clarify the atomic/electronic structure of DWs of the chiral CDW emerging in 1T-TiSe2, using scanning tunneling microscopy and density functional calculations. We reveal unambiguously the microscopic origin of chiral CDW as the C2 distortion in Se layers and its interlayer coupling. We further identify unique DWs connecting CDW domains of opposite chirality. The DWs are endowed with no in-gap state due to the characteristic multibands around the band gap, which defies the widely believed notion for CDW DWs. These results provide an important insight into the role of DWs in emerging superconductivity.

Solution Versus On‐Surface Synthesis of Peripherally Oxygen‐Annulated Porphyrins through C−O Bond Formation

AbstractThis study investigates the synthesis of tetra‐ and octa‐O‐fused porphyrinoids employing an oxidative O‐annulation approach through C−H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra‐high vacuum (UHV) conditions. X‐ray photoelectron spectroscopy, scanning tunneling microscopy, and non‐contact atomic force microscopy elucidated the preferential formation of pyran moieties via C−O bond formation and subsequent self‐assembly driven by C−H⋅⋅⋅O interactions. Furthermore, the O‐annulation process was found to reduce the HOMO–LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O‐atoms.

Solution Versus On-Surface Synthesis of Peripherally Oxygen-Annulated Porphyrins through C-O Bond Formation.

This study investigates the synthesis of tetra- and octa-O-fused porphyrinoids employing an oxidative O-annulation approach through C-H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra-high vacuum (UHV) conditions. X-ray photoelectron spectroscopy, scanning tunneling microscopy, and non-contact atomic force microscopy elucidated the preferential formation of pyran moieties via C-O bond formation and subsequent self-assembly driven by C-H⋅⋅⋅O interactions. Furthermore, the O-annulation process was found to reduce the HOMO-LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O-atoms.

Hierarchical zero- and one-dimensional topological states in symmetry-controllable grain boundary

Structural imperfections can be a promising testbed to engineer the symmetries and topological states of solid-state platforms. Here, we present direct evidence of hierarchical transitions of zero- (0D) and one-dimensional (1D) topological states in symmetry-enforced grain boundaries (GB) in 1T′–MoTe2. Using a scanning tunneling microscope tip press-and-pulse procedure, we construct two distinct types of GBs, which are differentiated by the underlying symmorphic and nonsymmorphic symmetries. The GBs with the nonsymmorphic rotation symmetry harbor first-order topological edge states protected by a nonsymmorphic band degeneracy. On the other hand, the edge state of the symmorphic GBs attains a band gap. More interestingly, the gapped edge state realizes a hierarchical topological phase, evidenced by the additional 0D boundary states at the GB ends. We anticipate our experiments will pioneer the material platform for the hierarchical realization of first-order and higher-order topology.

Large-scale simulations of vortex Majorana zero modes in topological crystalline insulators

Topological crystalline insulators are known to support multiple Majorana zero modes (MZMs) at a single vortex, their hybridization is forbidden by a magnetic mirror symmetry MT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$M_{T}$\\end{document}. Due to the limited energy resolution of scanning tunneling microscopes and the very small energy spacing of trivial bound states, it remains challenging to directly probe and demonstrate the existence of multiple MZMs. In this work, we propose to demonstrate the existence of MZMs by studying the hybridization of multiple MZMs in a symmetry breaking field. The different responses of trivial bound states and MZMs can be inferred from their spatial distribution in a vortex. However, the theoretical simulations are very demanding since it requires an extremely large system in real space. By utilizing the kernel polynomial method, we can efficiently simulate large lattices with over 108 orbitals to compute the local density of states which bridges the gap between theoretical studies based on minimal models and experimental measurements. We show that the spatial distribution of MZMs and trivial vortex bound states differs drastically in tilted magnetic fields. The zero-bias peak elongates when the magnetic field preserves MT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$M_{T}$\\end{document}, while it splits when MT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$M_{T}$\\end{document} is broken, giving rise to an anisotropic magnetic response. Since the bulk of SnTe are metallic, we also study the robustness of MZMs against the bulk states, and clarify when can the MZMs produce a pronounced anisotropic magnetic response.

Scanning tunneling microscopy of ultrathin indium intercalated between graphene and SiC using confinement heteroepitaxy

Large-scale and air-stable two-dimensional metal layers intercalated at the interface between epitaxial graphene and SiC offer an appealing material for quantum technology. The atomic and electronic details, as well as the control of the intercalated metals within the interface, however, remain very limited. In this Letter, we explored ultrathin indium confined between graphene and SiC using cryogenic scanning tunneling microscopy, complemented by first-principle density functional theory. Bias-dependent imaging and tunneling spectroscopy visualize a triangular superstructure with a periodicity of 14.7 ± 3 Å and an occupied state at about −1.6 eV, indicating proof of highly crystalline indium. The scanning tunneling microscopy tip was used to manipulate the number of indium layers below graphene, allowing to identify three monatomic In layers and to tune their corresponding electronic properties with atomic precision. This further allows us to attribute the observed triangular superstructure to be solely emerging from the In trilayer, tentatively explained by the lattice mismatch induced by lattice relaxation in the topmost In layer. Our findings provide a microscopic insight into the structure and electronic properties of intercalated metals within the graphene/SiC interface and a unique possibility to manipulate them with atomic precision using the scanning probe technique.