Scanning Tunneling Microscopy Experiments Research Articles

F···Br and F···S Heterohalogen-Bond-Directed 2D Self-Assemblies of a Benzothiadiazole Derivative.

Halogen bonding (XB) is of great importance in fabricating a two-dimensional (2D) self-assembly for its adaptive directionality. However, the XBs involving fluorine (F) have barely been studied due to the absence of an σ-hole on F. Here, 2D self-assemblies of a F-substituted 4,7-bis(5-bromo-4-dodecylthiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (BTZ-BrF) molecule on graphite were investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. STM experiments revealed that the 2D patterns of BTZ-BrF had a clear solvent and concentration dependence, showing a frame-like pattern in aliphatic acid and aliphatic hydrocarbon solvents at high concentrations. At lower concentrations, a bamboo-like and a wave-like pattern were observed in aliphatic acid, whereas small frame-like and large ladder-like domains at high solution concentrations in aliphatic hydrocarbon were observed. As the concentration further decreased, two linear patterns were observed. DFT calculations suggested that the hetero-XBs of F···Br, F···S, Br···S, and Br···N, the homo-XBs of type-II Br···Br, and the S···S interactions synergistically directed and stabilized the polymorphic 2D architectures. This understanding of intermolecular XBs during the molecular assembly at the molecular level may shed light on the ongoing efforts of regulating nanostructures of multifunctional organics.

Anisotropic vortex core in the nematic state in electron-doped iron-pnictide superconductors

We investigated the vortex states in the electron-doped BaFe1.8Co0.2As2 using Bogoliubov-de Gennes equations, based on a phenomenological model with competing nematic order and extended s-wave superconductivity. Specifically, we considered the spin-driven nematic order for the optimally doped compound. Our findings suggest that there are no Andreev bound states at the Fermi energy in the nematic phase, and the local density of states exhibits a double-peak structure, consistent with scanning tunneling microscopy experiments. Moreover, we propose an elliptic vortex structure in the local density of states map, which we attribute to the presence of the nematic order.

Superconducting gap symmetry from Bogoliubov quasiparticle interference analysis on Sr2RuO4

The nature of the superconducting order parameter in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ has generated intense interest in recent years. Since the superconducting gap is very small, high-resolution methods such as scanning tunneling spectroscopy might be the best chance to directly resolve the gap symmetry. Recently, a Bogoliubov quasiparticle interference imaging (BQPI) experiment has suggested that the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ gap symmetry is appropriate for ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. In this work, we use a material-specific theoretical approach based on Wannier functions of the surface of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ to calculate the continuum density of states as detected in scanning tunneling microscopy experiments. We examine several different proposed gap order parameters and calculate the expected BQPI pattern for each case. Comparing to the available experimental data, our results suggest that a ${s}^{\ensuremath{'}}+i{d}_{xy}$ gap order parameter is the most probable state, but the measured BQPI patterns still display features unaccounted for by the theory for any of the states currently under discussion.

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions.

The dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first-principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential- and pH-dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions

AbstractThe dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first‐principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential‐ and pH‐dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

The Role of Surface Hydroxyls in the Mobility of Carboxylates on Surfaces: Dynamics of Acetate on Anatase TiO2(101).

The dynamics of reactive intermediates are important in catalysis for understanding transient species, which can drive reactivity and the transport of species to reaction centers. In particular, the interplay between surface-bound carboxylic acids and carboxylates is important for numerous chemical transformations, including CO2 hydrogenation and ketonization. Here, we investigate the dynamics of acetic acid on anatase TiO2(101) using scanning tunneling microscopy experiments and density functional theory calculations. We demonstrate the concomitant diffusion of bidentate acetate and a bridging hydroxyl and provide evidence for the transient formation of molecular monodentate acetic acid. The diffusion rate is strongly dependent on the position of hydroxyl and adjacent acetate(s). A facile three-step diffusion process is proposed consisting of acetate and hydroxyl recombination, acetic acid rotation, and acetic acid dissociation. This study clearly demonstrates that the dynamics of bidentate acetate could be important in forming monodentate species, which are proposed to drive selective ketonization.

Selective binding and periodic arrangement of magic boron clusters on monolayer borophene

The synthesis and characterization of small boron clusters with unique size and regular arrangement are crucial for boron chemistry and two-dimensional borophene materials. In this study, together with theoretical calculations, the joint molecular beam epitaxy and scanning tunneling microscopy experiments achieve the formation of unique B5 clusters on monolayer borophene(MLB) on a Cu(111) surface. The B5 clusters tend to selectively bind to specific sites of MLB with covalent boron-boron bonds in the periodic arrangement, which can be ascribed to the charge distribution and electron delocalization character of MLB and also prohibits nearby co-adsorption of B5 clusters. Furthermore, the close-packed adsorption of B5 clusters would facilitate the synthesis of bilayer borophene, exhibiting domino effect-like growth mode. The successful growth and characterization of uniform boron clusters on a surface enrich the boron-based nanomaterials and reveal the essential role of small clusters during the growth of borophene.

Flat band and Lifshitz transition in long-range-ordered supergraphene obtained by Erbium intercalation

We report the observation of graphene doped up to the Lifshitz transition obtained solely by the intercalation of Erbium atoms. ARPES measurements show that a wide flat band is generated around the $M$ point of the Brillouin zone. We propose that this is the effect of an induced spin-orbit coupling. Scanning Tunneling Microscopy (STM) experiments reveal a longrange ordered hexagonal superstructure: we argue that this reflects the ordering of the intercalated Er atoms acting as impurities. This system provides a playground to study the interaction between a magnetic order and a divergent density of states at the Fermi level.

Emergence of metallic surface states and negative differential conductance in thin β-FeSi2 films on Si(001)

The electronic properties of the surface of β-FeSi2 have been debated for a long. We studied the surface states of β-FeSi2 films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory calculations. STM simulations using the surface model proposed by Romanyuk et al (2014 Phys. Rev. B 90 155305) reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned by a surface state that appeared in the bulk band gap of the β-FeSi2 film, irrespective of the polarity of the substrate. We also observed negative differential conductance at ∼0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states of β-FeSi2.

Buckling in a rotationally invariant spin-elastic model.

Scanning tunneling microscopy experiments have revealed a spontaneous rippled-to-buckled transition in heated graphene sheets, in absence of any mechanical load. Several models relying on a simplified picture of the interaction between elastic and internal, electronic, degrees of freedom have been proposed to understand this phenomenon. Nevertheless, these models are not fully consistent with the classical theory of elasticity, since they do not preserve rotational invariance. Herein, we develop and analyze an alternative classical spin-elastic model that preserves rotational invariance while giving a qualitative account of the rippled-to-buckled transition. By integrating over the internal degrees of freedom, an effective free energy for the elastic modes is derived, which only depends on the curvature. Minimization of this free energy gives rise to the emergence of different mechanical phases, whose thermodynamic stability is thoroughly analyzed, both analytically and numerically. All phases are characterized by a spatially homogeneous curvature, which plays the role of the order parameter for the rippled-to-buckled transition, in both the one- and two-dimensional cases. In the latter, our focus is put on the honeycomb lattice, which is representative of actual graphene.

Earthquake Countermeasures in Scanning Tunneling Microscopy Experiments Using Earthquake Early Warning

Earthquake Countermeasures in Scanning Tunneling Microscopy Experiments Using Earthquake Early Warning

Quantum confinement of electrons at metal surfaces

Scanning tunneling microscopy and spectroscopy experiments on surface-localized electron states confined to nanometer-scaled resonators are reviewed from the first observations to the recently discovered novel reflection mechanism of electron de Broglie waves. The focus of the presented work is on lateral confinement and on processes leading to finite decay rates of the confined states.

Denoising scanning tunneling microscopy images of graphene with supervised machine learning

Machine learning (ML) methods are extraordinarily successful at denoising photographic images. The application of such denoising methods to scientific images is, however, often complicated by the difficulty in experimentally obtaining a suitable expected result as an input to training the ML network. Here, we propose and demonstrate a simulation-based approach to address this challenge for denoising atomic-scale scanning tunneling microscopy (STM) images, which consists of training a convolutional neural network on STM images simulated based on a tight-binding electronic structure model. As model materials, we consider graphite and its mono- and few-layer counterpart, graphene. With the goal of applying it to any experimental STM image obtained on graphitic systems, the network was trained on a set of simulated images with varying characteristics such as tip height, sample bias, atomic-scale defects, and nonlinear background. Denoising of both simulated and experimental images with this approach is compared to that of commonly used filters, revealing a superior outcome of the ML method in the removal of noise as well as scanning artifacts---including on features not simulated in the training set. An extension to larger STM images is further discussed, along with intrinsic limitations arising from training set biases that discourage application to fundamentally unknown surface features. The approach demonstrated here provides an effective way to remove noise and artifacts from typical STM images, yielding the basis for further feature discernment and automated processing.

Switchable single-molecule electronic and thermoelectric device induced by light in a designed diarylethene molecule

Tremendous efforts have been devoted to studying ordinary and novel properties in molecular-scale. Especially the reversible and controlled electrical and thermoelectric switches, which have been extensively studied experimentally and theoretically. They possess two distinct electronic and thermoelectric transport states actuated by external triggers, such as light, temperature, and electric field. Here, a photochromic diarylethene, linked to a nontriazatriangulene platform covalently, is studied by first-principles calculation. By means of density functional theory, we calculate the density of states and scanning tunneling microscope (STM) images of the designed diarylethene adsorbed on a Au(111) surface. Moreover, the thermoelectric transport properties of diarylethene-based molecular junction with a Au(111) electrode are studied systematically by using nonequilibrium Green's function transport formalism. We illustrate that due to the structure chang e of the designed diarylethene induced by light, the electronic and thermoelectric transport properties of the junction are completely different. Those differences can be visualized by the calculated electronic transmission eigenchannel and transport pathway. Furthermore, we demonstrate that the structure change of the designed diarylethene can be detected by STM experiments. The present study shows that the designed diarylethene could be a desirable candidate for electronic and thermoelectric molecular switch driven by light.

Controlling selective nucleation and growth of dysprosium islands on graphene by metal intercalation

Metal intercalation is an effective method to modify the physical and chemical properties of low-dimensional systems such as epitaxial graphene. Here, we show that the nucleation and growth of metal nanostructures on epitaxial graphene on SiC(0001) can be dramatically changed by metal intercalation. Using scanning tunneling microscopy experiments, we demonstrate that dysprosium (Dy) metal islands are selectively nucleated on the area with Dy intercalation under both graphene and carbon buffer layers, while the adjacent area with only buffer layer intercalated remains relatively bare. Using first-principles calculations based on density functional theory, we show that adsorption of Dy adatom on the preferred nucleation area is energetically more favorable than on other areas. Moreover, changes in the electronic structure and the interlayer spacing upon Dy intercalation obtained from our calculations are also consistent with experimental observations. Our results indicate that metal intercalation is a promising way to manipulate the interaction between graphene and deposited adatoms.

Influence of Multi‐Orbitals and Hund's Coupling Induced Pseudogap on Specific Heat Jump in Iron‐Pnictide High T c Superconductors

A three-orbital per site theoretical model study is attempted to analyze nature of specific heat in iron-based superconductors. Based on recent experimental data in these systems, it is found that Fe five 3d orbitals, Hund’s coupling and intra- and inter-electron orbital correlations play significant role in electronic properties of these systems. Thus, in present work, we employed model Hamiltonian, containing hopping between orbitals, Hund’s coupling and intra- and inter- orbital Coulomb interactions for iron pnictide. Employing Green’s function technique within BCS mean field approximation, expressions of superconducting energy gap parameter and quasiparticle energies are obtained. Considering these parameters, we calculated specific heat and analyzed its variation with temperature and other parameters of model Hamiltonian. Also, a pseudogap (PG) parameter, arising out of multi-orbitals’ Hund’s coupling, as supported by ARPES and STM experiments, is introduced. The expression of specific heat is numerically analyzed to pinpoint its variation below and above transition temperature (T c ). A self-consistent numerical analysis of the specific heat jump as a function of multi-orbital hopping, electron interactions and Hund’s coupling in terms of PG parameter is shown in detail. Finally, we compared outcomes with recent theoretical and experimental data available on specific heat variations in iron superconductors like LaOFeAs system. This article is protected by copyright. All rights reserved.

On the Possibility of Exploring Tip-Molecule Interactions with STM Experiments

We present a theory for analyzing residence times of single molecules in a fixed detection area of a scanning tunneling microscope (STM). The approach is developed for one-dimensional molecule diffusion and can be extended to two dimensions by using the same methodology. Explicit results are derived for an harmonic attractive and repulsive tip-molecule interaction. Applications of the theory allows one to estimate the type and strength of interactions between the STM tip and the molecule. This includes the possibility of an estimation of molecule-molecule interaction when the tip is decorated by a molecule. Despite our focus on STM, this theory can analogously be applied to other experimental probes that monitor single molecules.

Macro and micro methodologies for thermomechanical mirror buckling of freestanding graphene membranes

The thermal mirror buckling behavior of freestanding graphene, which is a particular phenomenon observed in scanning tunneling microscope experiments, has not been analyzed systematically in the existing literature. However, because loads imposed by the scanning tunneling microscope tip are both mechanical and thermal, it is appropriate to regard this behavior as thermomechanical mirror buckling. The mirror buckling behavior is related to geometrical characteristics of a model, such as structure curvature. Therefore, appropriate analytical methods should be determined before examining the deep-seated origin of mirror buckling of graphene membranes. In this study, three methods are used separately in both the macroscale and microscale analyses, namely, continuum mechanics, molecular structural mechanics, and molecular dynamics, to analyze the mirror buckling of graphene. A comparison of the theoretical analysis and experimental results shows that, of the three methods, the molecular dynamics method is the most effective and suitable method for examining the thermomechanical mirror buckling behavior of freestanding graphene. The effects of the load magnitudes and graphene membrane radius on the deflection are also analyzed.

Identifying the alloy structures of germanene grown on Al(111)

While the growth of germanene has been extensively claimed on many substrates, their exact crystal structures remain controversial. Herein, we systematically explore the surface structures of Ge deposition onto the Al(111) surface by theoretical calculations combined with available scanning tunneling microscopy (STM) and low-energy electron diffraction data. We show that the formation of germanene on Al(111) is energetically unfavorable by $ab\phantom{\rule{0.16em}{0ex}}initio$ evolutionary simulations and high-level random-phase approximation calculations, and the two experimental phases are identified as honeycomb alloys ${\mathrm{Al}}_{3}{\mathrm{Ge}}_{3}/\mathrm{Al}(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})$ and ${\mathrm{Al}}_{3}{\mathrm{Ge}}_{4}/\mathrm{Al}(3\ifmmode\times\else\texttimes\fi{}3)$. It is interesting to find a vacancy at the interface of ${\mathrm{Al}}_{3}{\mathrm{Ge}}_{4}/\mathrm{Al}(3\ifmmode\times\else\texttimes\fi{}3)$, which is responsible for the dark clover pattern in STM experiments. Our results clarify the structural controversy of the Ge/Al(111) system and indicate that the fabrication of germanene on Al(111) remains challenging.

Magnetic ground state of supported monatomic Fe chains from first principles

A new computational scheme is presented based on a combination of the conjugate gradient and the Newton–Raphson method to self-consistently minimize the energy within local spin-density functional theory, thus to identify the ground state magnetic order of a finite cluster of atoms. The applicability of the new ab initio optimization method is demonstrated for Fe chains deposited on different metallic substrates. The optimized magnetic ground states of the Fe chains on Rh(111) are analyzed in details and a good comparison is found with those obtained from an extended Heisenberg model containing first principles based interaction parameters. Moreover, the effect of the different bilinear spin–spin interactions in the formation of the magnetic ground states is monitored. In case of Fe chains on Nb(110) spin-spiral configurations with opposite rotational sense are found as compared to previous spin-model results which hints on the importance of higher order chiral interactions. The wavelength of the spin-spiral states of Fe chains on Re(0001) was obtained in good agreement with scanning tunneling microscopy experiments.