Scanning Tunneling Microscopy Observations Research Articles

Interaction of yttrium atoms with a C60 fullerite film

Using scanning tunneling microscopy (STM) observation, interaction of yttrium (Y) atoms with the C60 fullerite films has been studied. It has been found that adsorbed Y atoms avoid clustering and are incorporated in the C60 film as individual atoms occupying the octahedral voids of the fullerite crystal. As a result of a charge transfer from the Y atom, the three neighboring surface fullerenes look more bright in the filled-state STM images. Yttrium atoms are tightly bound in their sites and neither move laterally nor float to the film surface upon subsequent overgrowth of C60 layers.

Landscape of Charge Puddles in Graphene Nanoribbons on Hexagonal Boron Nitride

Recently, graphene nanoribbons (GNRs) on hexagonal boron nitride (h‐BN) substrates have been studied to develop high‐mobility devices or devices based on a 1D Moiré superlattice. For this purpose, a device‐level understanding of the charge‐puddle landscape of a GNR/h‐BN structure is needed when the charge puddles function as scattering sources for mobile charge carriers. Here, a puddle landscape is constructed on the basis of an analysis of the temperature dependencies of the conductance of GNR/h‐BN devices at various gate‐voltage values. For low‐, intermediate‐, and high‐temperature regions near the charge‐neutral point (CNP), the puddle size (50–200 nm), distance between neighboring puddles (40–170 nm), and potential depth of the puddles (in a range of 10 meV) in ∼100 nm wide GNR/h‐BN devices are obtained on the basis of Coulomb blockade, 1D variable‐range hopping, and thermally activated hopping, respectively. Based on the constructed puddle landscape, it is also concluded that the confinement‐gap energy for an ∼100 nm wide GNR is similar to that of the thermal activation energy near the CNP in the GNRs. The constructed puddle landscape for GNR/h‐BN devices is consistent with that obtained from scanning tunneling microscopy observations of graphene on an h‐BN structure.

Kagome-like silicene: A novel exotic form of two-dimensional epitaxial silicon

Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome-like lattice of silicene on aluminum (111) substrates. We provide evidence of such an exotic 2D Si allotrope through scanning tunneling microscopy (STM) observations, high-resolution core-level (CL) and angle-resolved photoelectron spectroscopy (ARPES) measurements, along with Density Functional Theory calculations.

Instability of skyrmions in magnetic fields

In this combined experimental and theoretical work, we report on the evolution of the skyrmion radius and its destruction in the system Co/Ru(0001) when an out-of-plane magnetic field is applied. At low fields, skyrmions are metastable and display an elliptical instability in which along the short axis, the spin texture approaches that of the spin-spiral phase and the long axis expands in order to go back to the spin-spiral ground state. At high fields, we observe round skyrmions of finite size up to the collapse field Bc, where they are destroyed and the topological charge is annihilated. We estimate Bc via numerical methods based on magnetization dynamics simulations parametrized by density functional theory calculations and compare it to experimental scanning tunneling microscopy observations obtained at ≈ 30 mK.

Effect of hydrogen-gas treatment on the local structure of graphene-like graphite

Graphene-like graphite (GLG) has been expected as a new candidate for a large-capacity negative electrode material of a lithium-ion battery. Recently, our group has reported that hydrogen-gas treatment of GLG lowered the mean-discharge potential and increased its capacity. In order to clarify the origin of this improvement, scanning tunneling microscopy (STM) was conducted together with simulation using density functional theory calculation. A GLG mono-sheet suitable for STM observation was synthesized from highly oriented pyrolytic graphite (HOPG) as a raw material. A distorted honeycomb region surrounded by triangular one was observed for GLG mono-sheets and it was well simulated using a model consisting of a topmost graphene with pairwisely introduced ether groups underneath. The above honeycomb region disappeared for GLG treated with hydrogen gas (GLG-H), and it was also well simulated using a model in which each oxygen atom introduced in the graphene was substituted by two hydrogen atoms. Since the resulting local structure of GLG-H was similar to that of graphite which shows low discharge potential, this would lead to the increased accessible capacity under 2.0 V of cut-off voltage.

Au-induced reconstructions of the Si(111) surface with ordered and disordered domain walls

Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) observations combined with the density functional theory (DFT) calculations were used to elucidate atomic arrangement of Au/Si(111) reconstructions forming at Au coverage around one monolayer. They include the Si(111)$6\ifmmode\times\else\texttimes\fi{}6$-Au with ordered domain walls, the Si(111)$\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au and the Si(111)$\ensuremath{\beta}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au surfaces with disordered domain walls of various density, and the Si(111)$2\sqrt{21}\ifmmode\times\else\texttimes\fi{}2\sqrt{21}$-Au surface which structure is relevant to those of other reconstructions, albeit cannot be ascribed solely to the domain walls. Using detailed comparison of the STM and ARPES data with the results of DFT calculations, we explicitly proved the atomic model of Si(111)$6\ifmmode\times\else\texttimes\fi{}6$-Au surface proposed by Grozea et al. [Surf. Sci. 418, 32 (1998)]. This model provided us a hint for constructing an atomic model of domain walls which demonstrates a proper coincidence between experimental and simulated STM images. The model contains five Au atoms per $2\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ unit cell, i.e., 1.25 monolayers of Au. For the $\ensuremath{\alpha}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au and $\ensuremath{\beta}\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au surfaces, this finding allows an overall description of their atomic arrangement and, hence, accurate determination of their Au coverages. Knowledge on the atomic arrangement of the Si(111)$6\ifmmode\times\else\texttimes\fi{}6$-Au surface and domain walls also allowed us to construct an atomic model of the Si(111)$2\sqrt{21}\ifmmode\times\else\texttimes\fi{}2\sqrt{21}$-Au surface. The obtained data were unified in the refined formation phase diagram for the Au/Si(111) system.

CO adsorption on Fe3O4(1 1 1) with regular and biphase terminations

We present studies on CO adsorption on an Fe3O4(1 1 1) surface with a regular tetrahedral termination (Fetet1) and with different hexagonal superstructures called ‘biphase’ structures. All these terminations were precisely controlled on a 70 Å magnetite layer epitaxially grown on a Pt(1 1 1) single crystal. The experiments, including low energy electron diffraction, scanning tunnelling microscopy (STM) and temperature-programmed desorption (TPD), were carried out in situ under ultrahigh vacuum conditions. During the TPD measurements, the Fe3O4(1 1 1) surfaces were exposed to 0.1–20 L of CO at 105 K, and then the samples were heated with a linear temperature rise to 600 K. We found that as the Fetet1-terminated area decreases, as confirmed by STM observations, the surface becomes less active in CO adsorption. This means there are fewer adsorption states on the biphase structure than on Fe3O4(1 1 1) with regular termination.

Supramolecular Self-Assembly: Molecular Polymorphs and Their Transitions Triggered Electrically via Water Assistance at the Liquid/Graphite Interface

Supramolecular phase transitions triggered by external stimuli constitute a fascinating topic in materials science and supramolecular chemistry. Trimesic acid (TMA) represents a simple molecular building block, and its polymorphs on a solid surface have been extensively explored by scanning probe microscopy and computational simulations. The phase transformations of TMA electrically induced by scanning tunneling microscopy (STM) were previously observed yet noticed to take place rather sporadically. Here, we show by STM that the presence of water at the liquid/solid interface promotes the occurrence of the electrically triggered phase transition of TMA. DFT simulations were performed to investigate the mechanism of the deprotonation reactions occurring at the interface and to reveal the role of solvents. The Gibbs activation energy for the deprotonation of TMA was calculated to be the smallest for the OH– species present in water, which supports the STM observations.

Enantiospecific equilibrium adsorption and chemistry of d-/l-proline mixtures on chiral and achiral Cu surfaces.

A fundamental understanding of the enantiospecific interactions between chiral adsorbates and understanding of their interactions with chiral surfaces is key to unlocking the origins of enantiospecific surface chemistry. Herein, the adsorption and decomposition of the amino acid proline (Pro) have been studied on the achiral Cu(110) and Cu(111) surfaces and on the chiral Cu(643)R&S surfaces. Isotopically labelled 1-13 C-l-Pro has been used to probe the Pro decomposition mechanism and to allow mass spectrometric discrimination of d-Pro and 1-13 C-l-Pro when adsorbed as mixtures. On the Cu(111) surface, X-ray photoelectron spectroscopy reveals that Pro adsorbs as an anionic species in the monolayer. On the chiral Cu(643)R&S surface, adsorbed Pro enantiomers decompose with non-enantiospecific kinetics. However, the decomposition kinetics were found to be different on the terraces versus the kinked steps. Exposure of the chiral Cu(643)R&S surfaces to a racemic gas phase mixture of d-Pro and 1-13 C-l-Pro resulted in the adsorption of a racemic mixture; i.e., adsorption is not enantiospecific. However, exposure to non-racemic mixtures of d-Pro and 1-13 C-l-Pro resulted in amplification of enantiomeric excess on the surface, indicative of homochiral aggregation of adsorbed Pro. During co-adsorption, this amplification is observed even at very low coverages, quite distinct from the behavior of other amino acids, which begin to exhibit homochiral aggregation only after reaching monolayer coverages. The equilibrium adsorption of d-Pro and 1-13 C-l-Pro mixtures on achiral Cu(110) did not display any aggregation, consistent with prior scanning tunneling microscopy (STM) observations of dl-Pro/Cu(110). This demonstrates convergence between findings from equilibrium adsorption methods and STM experiments and corroborates formation of a 2D random solid solution.

Electrostatically Driven Guest Binding in Self-Assembled Molecular Network of Hexagonal Pyridine Macrocycle at the Liquid/Solid Interface: Symmetry Breaking Induced by Coadsorbed Solvent Molecules.

We present here the construction of a self-assembled two-dimensional network at the liquid/solid interface using a hexagonal pyridine macrocycle which binds an organic cation in its intrinsic porous space by electrostatic interactions. For this purpose, a hexagonal pyridinylene-butadiynylene macrocycle (PyBM) having six octyloxymethyl groups, PyBM-C8, was synthesized. As guests, tropylium (Tr) tetrafluoroborate and trioxatriangulenium (TOTA) hexafluorophosphate were used. In this study, we focused on (i) the network patterns of PyBM-C8 which change in response to its concentration and (ii) the position of the guest immobilized in the porous space of the macrocycle. Scanning tunneling microscopy (STM) observations at the interface of 1,2,4-trichlorobenzene (TCB) and highly oriented pyrolytic graphite (HOPG) revealed that PyBM-C8 formed four different polymorphs, oblique, loose hexagonal, linear, and rectangular, depending on the solute concentration and annealing treatment. Solvent TCB molecules are likely coadsorbed to not only the intrinsically porous space of PyBM-C8 (internal TCB) but also the space outside of the macrocycle between its alkyl chains (external TCB) in most of the cases. Upon adding the guest cation, whereas small Tr was not visualized in the pore due to size mismatching, larger TOTA was clearly observed in each pore. In addition, based on high-resolution STM images of the rhombus packing pattern of PyBM-C8, we revealed experimentally that TOTA was placed at an off-center position of the deformed hexagonal macrocyclic core in the rhombus pattern. On the basis of the molecular mechanics calculations, we hypothesize that the off-center location of TOTA is due to deformation of the hexagonal macrocycle through interaction with two external TCB molecules located at opposite edges of the macrocyclic core. Symmetry breaking of the macrocyclic host framework induced by coadsorbed surrounding solvent molecules thus plays a significant role in host-guest complexation at the liquid/solid interface.

Superconductor-insulator transition in an anisotropic two-dimensional electron gas assisted by one-dimensional Friedel oscillations: (Tl,Au)/Si(100)−c(2×2)

The recently found $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}(100)c(2\ifmmode\times\else\texttimes\fi{}2)$ two-dimensional compound which possesses spin-split surface states with peculiar spin texture and strong band anisotropy was examined using low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy observations and in situ four-probe transport measurements. Two types of the one-dimensional incommensurate Friedel oscillations were found to develop near the surface defects with the wavelengths corresponding to the nesting vectors of the $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}(100)c(2\ifmmode\times\else\texttimes\fi{}2)$ Fermi surface. In transport measurements, samples show strong localization behavior when density of defects is sufficiently high. With low density of defects, the system demonstrates a metallic behavior with fingerprints of superconducting transition at estimated ${T}_{c}=1.33\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Thus, the $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}(100)c(2\ifmmode\times\else\texttimes\fi{}2)$ system hosts a combination of fascinating properties and effects, including one-dimensional Friedel oscillations, superconductivity, and a spin-split anisotropic Fermi surface, that opens an avenue for prospective investigations of the interplay of collective electronic phenomena in atomic layer material.

Structure and phase transition of a uniaxially incommensurate In monolayer on Si(111)

We have studied the atomic structure and phase transition of a dense metallic monolayer of In on Si(111). Although the monolayer phase was previously considered to have a $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ periodicity, low-energy electron diffraction and scanning tunneling microscopy observations have revealed that the phase has an incommensurate structure with the In overlayer uniaxially contracted by 2% from $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$. The uniaxially contracted structure was found to be more stable than the commensurate $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ structure by first-principles calculations. We also observed a phase transition to a $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})$ phase at 250--210 K upon cooling. Angle-resolved photoelectron spectroscopy and macroscopic four-point-probe conductivity measurements demonstrated that the transition induces the disappearance of metallic surface states and a sharp drop in sheet conductivity, respectively. These results indicate an electronic metal-insulator transition.

Effect of amino, hydroxyl, and carboxyl terminal groups of alkyl chains of self‐assembled monolayers on the adsorption pattern of gold nanoparticles

The adsorption pattern of gold nanoparticles (AuNPs) on functionalized self‐assembled monolayers (SAMs) produced on a Au(111) surface was characterized. The Au(111) was modified with 11‐amino‐1‐undecanethiol hydrochloride (AUT), 11‐mercapto‐1‐undecanol (MUT), or 11‐mercaptoundecanoic acid (MUA) at an elevated temperature and pressure. The AuNPs aggregated on the AUT‐SAM surface, whereas they were well dispersed on the MUT‐SAM surface and localized on the MUA‐SAM surface. The results suggest that interactions between AuNPs differ according to the degree of peeling of citrate‐layer‐capped AuNPs. The degree of peeling, which is related to both the surface randomness of the SAMs and the functional characteristics of the terminal group of each SAM, was discussed on the basis of scanning tunneling microscopy observations, X‐ray photoelectron spectroscopic analyses, and contact angle measurements. Our study shows that AuNP patterns can be controlled by changing the terminal group of the alkyl thiol SAM on a Au(111) surface.

On-Surface Crystallization Behaviors of H-Bond Donor-Acceptor Complexes at Liquid/Solid Interfaces.

Two-dimensional (2D) crystallization behaviors of A-TPC n ( n = 4, 6, 10), T3C4, and hydrogen-bonded complexes T3C4@TPC n ( n = 4, 6, 10) are investigated by means of scanning tunneling microscope (STM) observations and density functional theory (DFT) calculations. The STM observations reveal that A-TPC4, A-TPC10, and T3C4 self-organize into dumbbell-shaped structures, well-ordered bright arrays, and zigzag structures, respectively. Interestingly, T3C4@TPC10 fails to form the cage-ball structure, whereas T3C4@TPC4 and T3C4@TPC6 co-assemble into cage-ball structures with the same lattice parameters. The filling rates of the balls of these two kinds of cage-ball structures depend heavily on the deposition sequence. As a result, the filling rates of the cages in T3C4/A-TPC n ( n = 4, 6) with deposition of T3C4 anterior to A-TPC n are higher than those in A-TPC n/T3C4 ( n = 4, 6) with the opposite deposition sequence. Furthermore, lattice defects formed by T3C4 coexist with the cage-ball structures. Moreover, the similar energy per unit area of lattice defects (-0.101 kcal mol-1 Å-2) and the two cage-ball networks (-0.194 and -0.208 kcal mol-1 Å-2, respectively), illustrating the similar stabilities of lattice defects and cage-ball networks, demonstrates the rationality of lattice defects. Combining STM investigations and DFT calculations, this work could provide a useful approach to investigate the 2D crystallization mechanisms of supramolecular liquid crystals on surfaces.

Method combining scanning tunneling microscopy and atom probe tomography for observing inside of materials

In this paper, we demonstrate the feasibility of a method combining scanning tunneling microscopy (STM) and atom probe tomography (APT). In this method, an atomically smooth or flat surface for STM observation is prepared by field evaporation using an APT system. The chemical composition of the surface obtained by APT can be used to analyze the results of STM. By developing this method, it is expected that local and very fine structures inside materials can be observed with true atomic resolution. We also demonstrate that nanoscale corrugation, which has a strong effect on the reconstruction procedure in the analysis of APT data, can be observed by STM.

Reversing the Handedness of Self-Assembled Porous Molecular Networks through the Number of Identical Chiral Centres.

Scanning tunnelling microscope observations at the 1-phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self-assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.

Coinage Metal–Sulfur Complexes: Stability on Metal(111) Surfaces and in the Gas Phase

We provide a comprehensive theoretical assessment at the level of density functional theory (DFT) of the stability of various coinage metal–sulfur complexes, both in the gas phase and also for the complexes adsorbed on the (111) surface of the same coinage metal. Our primary interest lies in the latter where earlier scanning tunneling microscopy (STM) experiments were interpreted to suggest the existence of adsorbed S-decorated metal trimers, sometimes as a component of more complex adlayer structures. Recent STM studies at 5 K directly observed other isolated adsorbed metal–sulfur complexes. For these adsorbed species, we calculate various aspects of their energetics including a natural measure of stability corresponding to their formation energy from sulfur adsorbed on terraces and from metal atoms that are in thermal equilibrium with the substrate. From this perspective, our DFT analysis shows that all of Ag2S3, Ag3S3, and many larger complexes on Ag(111) are strongly stable, Cu2S3 is stable, and some larger complexes are marginally stable on Cu(111), but only Au4S4 on Au(111) is stable. Results are consistent with STM observations for Cu(111) and Ag(111) surfaces but appear to deviate slightly for Au(111). A systematic analysis relating stability in the gas phase with that of adsorbed species is achieved within the framework of Hess’s law. This analysis also unambiguously elucidates various energetic contributions to stability.

Tomonaga-Luttinger Liquid in a Box: Electrons Confined within MoS2 Mirror-Twin Boundaries

Scanning tunneling microscope observations reveal for the first time the discrete energy spectrum of a truly 1D conductor, providing a crucial tool for testing the limits of the Tomonaga-Luttinger liquid theory that describes interacting electrons.

Formation and Structure of Highly Ordered Self-Assembled Monolayers by Adsorption of Acetyl-Protected Conjugated Thiols on Au(111) in Tetrabutylammonium Cyanide Solution

The surface structure and binding conditions of self-assembled monolayers (SAMs) on Au(111) derived from 1-acetylthio-4-[(phenyl)ethynyl]benzene (OPE2-SAc) without and with tetrabutylammonium cyanide (TBACN) as a deprotection reagent were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM observation revealed that OPE2-S SAMs on Au(111) formed from direct adsorption of OPE2-SAc in 1 mM methanol solution at room temperature (RT) for 24 h were composed of short-range, ordered phase separated by a disordered phase. In contrast, adsorption of OPE2-SAc SAMs on Au(111) at a higher solution temperature of 50 °C for 24 h led to formation of a fully ordered phase with slightly increased domain size. The structural quality of OPE2-S SAMs on Au(111) was remarkably enhanced when TBACN was used. OPE2-S SAMs at RT had a well-ordered (√3 × √7)R30° structure with a domain size larger than 80 nm. The SAMs deposited at 50 °C contained very uniform and highly ordered domains w...

Direct observation of copper-induced role on Ullmann reaction by scanning tunneling microscopy

Ullmann coupling of 4,4′'-dibromo-p-terphenyl (DBTP) on Ag(110) surface has been investigated by introducing guest copper atoms in two ways. Scanning tunneling microscopy observations unambiguously demonstrate that the Ullmann reaction on Ag(110) surface can be improved by introducing copper atoms in both ways, and the effect is more remarkable when copper atoms are introduced on a annealing surface covered by molecules.