Scanning Tunneling Research Articles

Interfacial Stereoelectronic Effect Induced by Anchoring Orientation.

Heterogeneous interfaces in most devices play a key role in the material performance. Exploring the atomic structure and electronic properties of metal-molecule interfaces is critical for various potential applications, such as surface sensing, molecular recognition, and molecular electronic devices. This study unveils a ubiquitous interfacial stereoelectronic effect in conjugated molecular junctions by combining first-principles simulation and scanning tunneling microscopy break junction technology. Single-molecule junctions with same-side interfacial anchoring (cis configuration) exhibit higher conductance than those with opposite-side interfacial anchoring (trans configuration). The cis and trans configurations can undergo reversible conversions, resulting in a conductance switching. The stability of these configurations can be adjusted by an electric field, achieving precise regulation of conductance states. Our findings provide important insights for designing high-quality materials and enhancing the device performance.

Electrically‐Driven Light Source Embedded in a GaP Nanowaveguide for Visible‐Range Photonics on Chip

AbstractThe key components of photonic integrated circuits are nanoscale optica emitters and nanowaveguides. III‐V semiconductor nanostructures are considered as the most promising material platform for these components due to highly efficient luminescence and high refractive index, but the problem of emission coupling with waveguide is to be solved. In this work, the use of GaP nanowires (NWs) with different types of doping (GaP:Si or GaP:Be) is proposed as optical waveguides with directly integrated electrically‐driven light sources, solving the problem of emission‐to‐waveguide coupling. Single NWs are integrated with electrodes and pump electroluminescence by a tunnel junction allowing to study emission properties with nanoscale spatial resolution. Basing on the experiments on scanning tunnelling microscopy (STM), electron microscopy, time‐resolved photoluminescence micro‐spectroscopy, X‐ray diffraction, and STM‐induced electroluminescence, it is proven that GaP NWs exhibit different integrated light‐source on doping type of NWs. GaP:Be NWs contain inclusion of the crystalline wurtzite phase with a direct bandgap, and, thus, these NW regions can be considered as electrically‐driven nanoscale sources of light monolithically integrated into GaP NW‐based waveguides. Meanwhile, GaP:Si NWs work as optical waveguides capable of efficient light generation over the entire length of NW. The developed designs are promising for construction of integrated photonic circuits.

Role of Cu Oxide and Cu Adatoms in the Reactivity of CO2 on Cu(110).

We investigate the interaction of CO2 with metallic and oxidized Cu(110) surfaces using a combination of near-ambient pressure scanning tunneling microscopy (NAP-STM) and theoretical calculations. While the Cu(110) and full CuO films are inert, the interface between bare Cu(110) and the CuO film is observed to react instantly with CO2 at a 10 mbar pressure. The reaction is observed to proceed from the interfacial sites of CuO/Cu(110). During reaction with CO2, the CuO/Cu(110) interface releases Cu adatoms which combine with CO3 to produce a variety of added Cu-CO3 structures, whose stability depends on the gas pressure of CO2. A main implication for the reactivity of Cu(110) is that Cu adatoms and highly undercoordinated CuO segments are created on the Cu(110) surface through the interaction with CO2, which may act as reaction-induced active sites. In the case of CO2 hydrogenation to methanol, our theoretical assessment of such sites indicates that their presence may significantly promote CH3OH formation. Our study thus implies that the CuO/Cu(110) interfacial system is highly dynamic in the presence of CO2, and it suggests a possible strong importance of reaction-induced Cu and CuO sites for the surface chemistry of Cu(110) in CO2-related catalysis.

Role of Cu Oxide and Cu Adatoms in the Reactivity of CO2 on Cu(110)

AbstractWe investigate the interaction of CO2 with metallic and oxidized Cu(110) surfaces using a combination of near‐ambient pressure scanning tunneling microscopy (NAP‐STM) and theoretical calculations. While the Cu(110) and full CuO films are inert, the interface between bare Cu(110) and the CuO film is observed to react instantly with CO2 at a 10 mbar pressure. The reaction is observed to proceed from the interfacial sites of CuO/Cu(110). During reaction with CO2, the CuO/Cu(110) interface releases Cu adatoms which combine with CO3 to produce a variety of added Cu−CO3 structures, whose stability depends on the gas pressure of CO2. A main implication for the reactivity of Cu(110) is that Cu adatoms and highly undercoordinated CuO segments are created on the Cu(110) surface through the interaction with CO2, which may act as reaction‐induced active sites. In the case of CO2 hydrogenation to methanol, our theoretical assessment of such sites indicates that their presence may significantly promote CH3OH formation. Our study thus implies that the CuO/Cu(110) interfacial system is highly dynamic in the presence of CO2, and it suggests a possible strong importance of reaction‐induced Cu and CuO sites for the surface chemistry of Cu(110) in CO2‐related catalysis.

Van Hove annihilation and nematic instability on a kagome lattice.

A nematic phase breaks the point-group symmetry of the crystal lattice and is known to emerge in correlated materials. Here we report the observation of an intra-unit-cell nematic order and associated Fermi surface deformation in the kagome metal ScV6Sn6. Using scanning tunnelling microscopy and scanning tunnelling spectroscopy, we reveal a stripe-like nematic order breaking the crystal rotational symmetry within the kagome lattice itself. Moreover, we identify a set of Van Hove singularities adhering to the kagome-layer electrons, which appear along one direction of the Brillouin zone and are annihilated along other high-symmetry directions, revealing rotational symmetry breaking. Via detailed spectroscopic maps, we further observe an elliptical deformation of the Fermi surface, which provides direct evidence for an electronically mediated nematic order. Our work not only bridges the gap between electronic nematicity and kagome physics but also sheds light on the potential mechanism for realizing symmetry-broken phases in correlated electron systems.

Unveiling the surface of carbon black via scanning probe microscopy and chemical state analysis

Carbon black (CB) has wide range of industrial applications, including in the manufacturing of automobile tires, rubber products, inks, and plastics. To improve the properties of the target products and establish recycling systems, it must be fully characterized. However, characterization of CB is challenging owing to its structural complexity and the limitation of conventionally used experimental techniques, especially for surface structures at the nanoscale. In this study, we characterized the surface structures of two commercial CB via atomic force and scanning tunneling microscopy. Analysis of well-dispersed aggregates on atomically flat solid surfaces revealed primary particles of diverse sizes. The particle surfaces lacked edges, grooves, and steps that should be observed between stacked graphene sheets, which contradicts the widely accepted crystallite model. Observed images suggest that the graphene sheets exhibit a size distribution, inferring that multiple non-uniformly sized small graphene sheets are stacked turbostratically, with each sheet displaying a localized curvature rather than the ideal planar form. Varying size of sheets and curvature indicate the presence of a decent number of edges terminated with hydrogen and oxygen-containing functional groups. This interpretation was corroborated by conventional spectroscopic techniques: Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and infrared absorption spectroscopy.

Terahertz spectroscopy of collective charge density wave dynamics at the atomic scale.

Charge density waves are wave-like modulations of a material's electron density that display collective amplitude and phase dynamics. The interaction with atomic impurities induces strong spatial heterogeneity of the charge-ordered phase. Direct real-space observation of phase excitation dynamics of such defect-induced charge modulation is absent. Here, by utilizing terahertz pump-probe spectroscopy in a scanning tunnelling microscope, we measure the ultrafast collective dynamics of the charge density wave in the transition metal dichalcogenide 2H-NbSe2 with atomic spatial resolution. The tip-enhanced electric field of the terahertz pulses excites oscillations of the charge density wave that vary in magnitude and frequency on the scale of individual atomic impurities. Overlapping phase excitations originating from the randomly distributed atomic defects in the surface create this spatially structured response of the charge density wave. This ability to observe collective charge order dynamics with local probes makes it possible to study the dynamics of correlated materials at the intrinsic length scale of their underlying interactions.

Nanoscale Visualization of Symmetry-Breaking Electronic Orders and Magnetic Anisotropy in a Kagome Magnet YMn6Sn6.

A kagome lattice hosts a plethora of quantum states arising from the interplay between nontrivial topology and electron correlations. The recently discovered kagome magnet RMn6Sn6 (R represents a rare-earth element) is believed to showcase a kagome band closely resembling textbook characteristics. Here, we report the characterization of local electronic states and their magnetization response in YMn6Sn6 via scanning tunneling microscopy measurements under vector magnetic fields. Our spectroscopic maps reveal a spontaneously trimerized kagome electronic order in YMn6Sn6, where the 6-fold rotational symmetry is disrupted while translational symmetry is maintained. Further application of an external magnetic field demonstrates a strong coupling of the YMn6Sn6 kagome band to the field, which exhibits an energy shift discrepancy under different field directions, implying the existence of magnetization-response anisotropy and anomalous g factors. Our findings establish YMn6Sn6 as an ideal platform for investigating kagome-derived orbital magnetic moment and correlated magnetic topological states.

Visualization of spatial inhomogeneity in the superconducting gap using micro-ARPES

ABSTRACT Electronic inhomogeneity arises ubiquitously as a consequence of adjacent and/or competing multiple phases or orders in strongly correlated electron systems. Gap inhomogeneity in high- Tc cuprate superconductors has been widely observed using scanning tunneling microscopy/spectroscopy. However, it has yet to be evaluated by angle-resolved photoemission spectroscopy (ARPES) due to the difficulty in achieving both high energy and spatial resolutions. Here, we employ high-resolution spatially-resolved ARPES with a micrometric beam (micro-ARPES) to reveal the spatial dependence of the antinodal electronic states in optimally-doped Bi 2 Sr 2 CaCu 2 O 8 + δ . Detailed spectral lineshape analysis was extended to the spatial mapping dataset, enabling the identification of the spatial inhomogeneity of the superconducting gap and single-particle scattering rate at the micro-scale. Moreover, these physical parameters and their correlations were statistically evaluated. Our results suggest that high-resolution spatially-resolved ARPES holds promise for facilitating a data-driven approach to unraveling complexity and uncovering key parameters for the formulation of various physical properties of materials.

Observation of the sliding phason mode of the incommensurate magnetic texture in Fe/Ir(111)

The nanoscopic magnetic texture forming in a monolayer of iron on the (111) surface of iridium, Fe/Ir(111), is spatially modulated and uniaxially incommensurate with respect to the crystallographic periodicities. As a consequence, a low-energy magnetic excitation is expected that corresponds to the sliding of the texture along the incommensurate direction, i.e., a phason mode, which we explicitly confirm with atomistic spin simulations. Using scanning tunneling microscopy (STM), we succeed to observe this phason mode experimentally. It can be excited by the STM tip, which leads to a random telegraph noise in the tunneling current that we attribute to the presence of two minima in the phason potential due to the presence of disorder in our sample. This provides the prospect of a floating phase in cleaner samples and, potentially, a commensurate-incommensurate transition as a function of external control parameters.

Lanthanum nanoparticle decorated carbon nanotubes: Facile method of synthesis and studies of their redox stability, cytotoxicity and corrosion inhibition on the magnesium alloy in 3.5 % NaCl environment

Functionalized carbon materials can act as high-temperature coatings and eco-friendly composite materials. We have studied lanthanum-doped carbon nanotube (La-CNT) material redox behavior, cytotoxicity, and corrosion inhibition efficiency. The La-CNT material has been characterized by different techniques to confirm the presence of functional groups. The Fourier transform infrared (FTIR) and Raman studies were performed to investigate the functional groups and carbon defects. Field emission scanning electron microscopy (FESEM) and tunneling electron microscopy (TEM) studies were used to investigate the carbon nanotube microstructure, La nanoparticles were shown to interact with the carbon nanotube surface. X-ray diffraction (XRD) techniques were used to confirm the crystallinity of composite material. Electrochemical methods were used to investigate corrosion inhibition and redox stabilities. After corrosion study on the Mg alloy its microstructure was analyzed to confirm the composite material inhibition efficiency. The composite material showed significant cytotoxicity, and corrosion inhibition efficiency was shown to be 82 % as compared with pristine epoxy-coated Mg alloy.XPS results are suggested that alloy oxide layer formed. The DFT study supported the experimental findings.

Single-Molecule Identification of the Isomers of a Lipidic Antibody Activator.

Molecular structural elucidation can be accomplished by different techniques, such as nuclear magnetic resonance or X-ray diffraction. However, the former does not give information about the three-dimensional atomic arrangement, and the latter needs crystallizable solid samples. An alternative is direct, real-space visualization of the molecules by cryogenic scanning tunneling microscopy (STM). This technique is usually limited to thermally robust molecules because an annealing step is required for sample deposition. A landmark development has been the coupling of STM with electrospray deposition (ESD), which smooths the process and widens the scope of the visualization technique. In this work, we present the on-surface characterization of air-, light-, and temperature-sensitive rhamnopolyene with relevance in molecular biology. Supported by theoretical calculations, we characterize two isomers of this flexible molecule, confirming the potential of the technique to inspect labile, non-crystallizable compounds.

Towards revealing intrinsic vortex-core states in Fe-based superconductors through statistical discovery

In type-II superconductors, electronic states within magnetic vortices hold crucial information about the paring mechanism and can reveal non-trivial topology. While scanning tunneling microscopy/spectroscopy (STM/S) is a powerful tool for imaging superconducting vortices, it is challenging to isolate the intrinsic electronic properties from extrinsic effects like subsurface defects and disorders. Here we combine STM/STS with basic machine learning to develop a method for screening out the vortices pinned by embedded disorder in iron-based superconductors. Through a principal component analysis of large STS data within vortices, we find that the vortex-core states in Ba(Fe0.96Ni0.04)2As2 start to split into two categories at certain magnetic field strengths, reflecting vortices with and without pinning by subsurface defects or disorders. Our machine-learning analysis provides an unbiased approach to reveal intrinsic vortex-core states in novel superconductors and shed light on ongoing puzzles in the possible emergence of a Majorana zero mode.

Low‐Defect Quantum Dot Lasers Directly Grown on Silicon Exhibiting Low Threshold Current and High Output Power at Elevated Temperatures

The direct growth of III‐V materials on silicon is a key enabler for developing monolithically integrated lasers, offering substantial potential for ultradense photonic integration in vital communications and computing technologies. However, the III‐V/Si lattice and thermal expansion mismatch pose significant hurdles, leading to defects that degrade lasing performance. This study overcomes this challenge, demonstrating InAs/GaAs‐on‐Si lasers that perform on par with top‐tier lasers on native GaAs substrates. This is achieved through a newly developed epitaxial approach comprising a series of rigorously optimized growth strategies. Atomic‐resolution scanning tunneling microscopy and spectroscopy experiments reveal exceptional material quality in the active region and elucidate the impact of each growth strategy on defect dynamics. The optimized III‐V‐on‐silicon ridge‐waveguide lasers demonstrate a continuous‐wave threshold current as low as 6 mA and high‐temperature operation reaching 165 °C. At 80 °C, critical for data center applications, they maintain a 12 mA threshold and 35 mW output power. Furthermore, lasers fabricated on both Si and GaAs substrates using identical processes exhibit virtually identical average threshold current. By eliminating the performance limitations associated with the GaAs/Si mismatch, this study paves the way for robust and high‐density integration of a broad spectrum of critical III‐V photonic technologies into the silicon ecosystem.

Hamiltonian Cycles on Ammann-Beenker Tilings

We provide a simple algorithm for constructing Hamiltonian graph cycles (visiting every vertex exactly once) on a set of arbitrarily large finite subgraphs of aperiodic two-dimensional Ammann-Beenker (AB) tilings. Using this result, and the discrete scale symmetry of AB tilings, we find exact solutions to a range of other problems which lie in the complexity class NP-complete for general graphs. These include the equal-weight traveling salesperson problem, providing, for example, the most efficient route a scanning tunneling microscope tip could take to image the atoms of physical quasicrystals with AB symmetries; the longest path problem, whose solution demonstrates that collections of flexible molecules of any length can adsorb onto AB quasicrystal surfaces at density one, with possible applications to catalysis; and the three-coloring problem, giving ground states for the q-state Potts model (q≥3) of magnetic interactions defined on the planar dual to AB, which may provide useful models for protein folding. Published by the American Physical Society 2024

Multiple Interaction Forces Construct Two‐Dimensional Self‐assembly Kagome Lattices on Au(111)

Comprehensive SummaryKagome lattices have garnered significant attention due to their promising applications in catalysis, electronics, and magnetics. Although many efforts have been paid to the design and synthesis of Kagome lattices, there is a limited focus on constructing this lattice by multiple interaction forces. In this work, we employ 2,7‐dibromo‐carbazole as precursors to successfully fabricate the two‐dimensional self‐assembly Kagome lattices stabled by multiple interaction forces on Au(111) substrate. Using low‐temperature scanning tunneling microscopy, non‐contact atomic force microscopy and density functional theory calculation, we visualize and identify the four interaction forces within Kagome lattices: Au—N coordination bonds, Au—H hydrogen bonds, Br—Br halogen bonds, and Br—H hydrogen bonds, respectively. This study provides a basic understanding for designing and constructing more complex Kagome lattices.

Chiral Solvent-Induced Homochiral Hierarchical Molecular Assemblies at the Liquid/Solid Interface.

We herein investigate the formation of homochiral hierarchical self-assembled molecular networks (SAMNs) via chirality induction by the coadsorption of a chiral solvent at the liquid/graphite interface by means of scanning tunneling microscopy (STM). In a mixture of achiral solvents, 1-hexanoic acid, and 1,2,4-trichlorobenzene, an achiral dehydrobenzo[12]annulene (DBA) derivative with three alkoxy and three hydroxy groups in an alternating manner forms chiral hierarchical triangular cluster structures through dynamic self-sorting. Enantiomorphous domains appear in equal probability. On the other hand, in chiral 2-methyl-1-hexanoic acid as a solvent, this molecule produces (i) homochiral small triangular clusters at a low solute concentration, (ii) a chirality-biased hierarchical structure consisting of triangular cluster structures with different cluster sizes at a medium concentration, and (iii) a dense structure with no chirality bias at a high concentration. We attribute the concentration-dependent degree of the chirality transmission to the number of coadsorbed solvent molecules in the SAMNs and to the difference in nucleus structure and size in the initial stage of the SAMN formation.

On-Surface Synthesis of Five-Membered Copper Metallacycles Using Terminal Alkynes.

We present our studies on the adsorption, deprotonation, and reactions of 4,4″-diethynyl-1,1':4',1″-terphenyl on Cu(111) under ultrahigh-vacuum conditions using scanning tunneling microscopy combined with density functional theory calculations. Sequential annealing treatments induce deprotonation of pristine molecules followed by chemical reactions, resulting in branched nanostructures. Within the nanostructures, a previously unreported, double-spot linkage is observed. Our density functional theory calculations unravel that this linkage corresponds to a five-membered copper metallacycle.

Synthesis and Characterization of Nanocellulose/Polyacrylonitrile‐based Carbon Nanofibers: An Approach toward Low Cost and Sustainable Agro Waste based Carbon Nanomaterials

AbstractThe cost of polymer precursors and high energy need in carbonization for the preparation of carbon materials are two limiting factors in the commercialization of carbon‐based materials. In this study, carbon nanofiber films were prepared by adding a very small concentration of polymer polyacrylonitrile in nanocellulose (extracted from agro waste i.e., rice straws) with an overall aim of producing carbon nanomaterials with cost effective, renewable and sustainable carbon source. Polyacrylonitrile (PAN) added nanocellulose solution was drop‐casted to prepare Nanocellulose/Polyacrylonitrile‐based nanofibers film (NC/PAN). It was further stabilized in air atmosphere at 280 °C followed by carbonized in nitrogen atmosphere at 700 °C. It was found that addition of NC/PAN have 18 % lower activation energies than PAN. Results of tunneling electron microscopy showed that polyacrylonitrile/nanocellulose films carbonized at 700 °C temperature exhibited amorphous carbon nanofibers structure similar to bare polyacrylonitrile based carbon film carbonized at higher temperature of 1000 °C‐1200 °C. Furthermore, polyacrylonitrile/nanocellulose based carbon nanofibers have showed high degree of carbonization ((ID)/(IG)=0.87) as compared to bare polyacrylonitrile based carbon nanofibers film ((ID)/(IG)=1.02). This may be attributed to chiral nature of nanocellulose which served as a template to orientation of graphene planes and showed efficient carbonization.

Hydrogen exposure-enhanced superconductivity transition in FeSe/SrTiO3 monolayer

Interface-enhanced superconductivity in FeSe/SrTiO3 (FeSe/STO) monolayers provides the record for the highest transition temperature (Tc) in iron-based compounds. Long-term post annealing is the commonly adopted recipe to induce the superconductivity transition in the not-superconductive as-grown FeSe/STO monolayer. Here, we developed a kinetic method, i.e., hydrogen exposure followed by gentle annealing, to enhance the superconductivity of the FeSe/STO monolayer. Our approach is more efficient than the long-term post annealing. Scanning tunneling microscopy (STM) characterization demonstrated the so far largest superconducting gap of ∼22 mV, indicating an enhanced superconductivity. We believe that the hydrogen-induced lattice Fe diffusion facilitates to remove the interfacial excess Se atoms fatal to the superconductivity, resulting in the formation of a second layer FeSe. The subsequent annealing helps to annihilate the generated Fe vacancies and, thus, enhance the superconductivity in the FeSe/STO monolayer.