Thickness-dependent Dirac dispersions of few-layer topological insulators supported by metal substrate.

The surface states protected by time-reversal symmetry in 3-dimensional topological insulators have recently been confirmed by angle-resolved photoemission spectroscopy, scanning tunneling microscopy, quantum transport and so on. However, the electronic properties of ultra-thin topological insulator films have not been extensively studied, especially when the films are grown on metal substrates. In this paper, we have elucidated the local behaviors of the electronic states of ultra-thin topological insulator Bi2Se3 grown with molecular beam epitaxy on Au(111) using scanning tunneling microscopy/spectroscopy. We have observed linear dispersion of electron interference patterns at higher energies than the Fermi energy that were not accessible by conventional angle-resolved photoemission spectroscopy. Moreover, the dispersion of the interference patterns varies with the film thickness, which is explained by band bending near the interface between the topological insulator and the metal substrate. Our experiments demonstrate that interfacial effects in thin topological insulator films on metal substrate can be sensed using scanning tunneling spectroscopy.

"Multipoint Force Feedback" Leveling of Massively Parallel Tip Arrays in Scanning Probe Lithography.

Nanoscale patterning with massively parallel 2D array tips is of significant interest in scanning probe lithography. A challenging task for tip-based large area nanolithography is maintaining parallel tip arrays at the same contact point with a sample substrate in order to pattern a uniform array. Here, polymer pen lithography is demonstrated with a novel leveling method to account for the magnitude and direction of the total applied force of tip arrays by a multipoint force sensing structure integrated into the tip holder. This high-precision approach results in a 0.001° slope of feature edge length variation over 1 cm wide tip arrays. The position sensitive leveling operates in a fully automated manner and is applicable to recently developed scanning probe lithography techniques of various kinds which can enable "desktop nanofabrication."

Supramolecular Cl⋠⋠⋠H and O⋠⋠⋠H interactions in self-assembled 1,5-dichloroanthraquinone layers on Au(111).

The role of halogen bonds in self-assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5-dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl⋅⋅⋅H and O⋅⋅⋅H hydrogen bonds with marginal contributions from Cl-related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br- and I-related halogen bonds, Cl-related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.

Recovery and local-variation of Dirac cones in oxygen-intercalated graphene on Ru(0001) studied using scanning tunneling microscopy and spectroscopy.

Methods to decouple epitaxial graphene from metal substrates have been extensively studied, with anticipation of observing unperturbed Dirac cone properties, but its local electronic structures were rarely studied. Here, we investigated the local variations of Dirac cones recovered using oxygen intercalation applied to epitaxial graphene on Ru(0001) using scanning tunneling microscopy and spectroscopy (STM and STS). New V-shaped features, which appear in the STS data at the oxygen-intercalated graphene regions, are attributed to the signatures of recovered Dirac cones. The Dirac point energy was observed at 0.48 eV below the Fermi level, different from previous photoemission results because of different oxygen coverages. The observed spatial variations of Dirac point energy were explained by the weakly protruding network structures caused by a small net strain in graphene. Our study shows that oxygen-intercalated graphene provides an excellent platform for further graphene research at the nano-meter scale with unperturbed Dirac cones.

Electronic structures of one-dimensional metal-molecule hybrid chains studied using scanning tunneling microscopy and density functional theory.

The electronic structures of self-assembled hybrid chains comprising Ag atoms and organic molecules were studied using scanning tunneling microscopy (STM) and spectroscopy (STS) in parallel with density functional theory (DFT). Hybrid chains were prepared by catalytic breaking of Br-C bonds in 4,4″-dibromo-p-terphenyl molecules, followed by spontaneous formation of Ag-C bonds on Ag(111). An atomic model was proposed for the observed hybrid chain structures. Four electronic states were resolved using STS measurements, and strong energy dependence was observed in STM images. These results were explained using first-principles calculations based on DFT.

Metal-supported high crystalline Bi2Se3 quintuple layers.

Atomically flat thin films of Bi(2)Se(3) were grown on Au(111) metal substrate using molecular beam epitaxy. Hexagonal atomic structures and quintuple layer steps were observed at the surfaces of grown films using scanning tunneling microscopy. Multiple sharp peaks from (003) family layers were characterized by x-ray diffraction measurements. The atomic stoichiometry of Bi and Se was considered using x-ray photoemission spectroscopy. Moiré patterns were obtained at the surfaces of one quintuple layer films due to lattice mismatch between Bi(2)Se(3) and Au. Our experiments suggest that Au is a reasonable material for electrodes in Bi(2)Se(3) devices.

Achieving chiral resolution in self-assembled supramolecular structures through kinetic pathways.

Chiral phase transitions were studied in a self-assembled 2,6-dibromoanthraquinones supramolecular system prepared on Au(111) using scanning tunneling microscopy. As the molecules were deposited at about 150 K, they formed heterochiral chevron structures (a racemate) consisting of two alternating prochiral molecular rows. When the as-deposited sample was warmed to 300 K followed by cooling to 80 K, phase-separated homochiral structures (a conglomerate), as well as the chevron structures, were observed. We propose molecular models for the structures that are in good agreement with ab initio studies and can be explained by hydrogen bonds and halogen bonds. We found that heterochiral chevron structures were more stable than homochiral structures due to two additional [Formula: see text] halogen bonds per molecule. We considered kinetic pathways for the phase transitions that were made possible via a disordered liquid phase entropically stabilized at 300 K. We show how chiral resolution can be achieved by exploiting kinetic paths allowed in supramolecular systems.

O2, NO2 and NH3 coordination to Co-porphyrin studied with scanning tunneling microscopy on Au(111).

The coordination structure between small molecules and metalloporphyrins plays a crucial role in functional reactions such as bio-oxidation and catalytic activation. Their vertical, tilting, and dynamic structures have been actively studied with diffraction and resonance spectroscopy for the past four decades. Contrastingly, real-space visualization beyond simple protrusion and depression is relatively rare. In this paper, high-resolution scanning tunnelling microscopy (STM) images are presented of di-, tri-, and tetra-atomic small molecules (O2, NO2, and NH3, respectively) coordinated to Co-porphyrin on Au(111). A square ring structure was observed for O2, a rectangular ring structure for NO2, and a bright-center structure for NH3 at 80 K. The symmetries of experimental STM images were reproduced in density functional theory (DFT) calculations, considering the precession motion of the small molecules. Thus, this study shows that the structure of small molecules coordinated to metalloporphyrins can be visualized using high-resolution STM and DFT calculations.

Dirac electrons in a dodecagonal graphene quasicrystal.

Quantum states of quasiparticles in solids are dictated by symmetry. We have experimentally demonstrated quantum states of Dirac electrons in a two-dimensional quasicrystal without translational symmetry. A dodecagonal quasicrystalline order was realized by epitaxial growth of twisted bilayer graphene rotated exactly 30°. We grew the graphene quasicrystal up to a millimeter scale on a silicon carbide surface while maintaining the single rotation angle over an entire sample and successfully isolated the quasicrystal from a substrate, demonstrating its structural and chemical stability under ambient conditions. Multiple Dirac cones replicated with the 12-fold rotational symmetry were observed in angle-resolved photoemission spectra, which revealed anomalous strong interlayer coupling with quasi-periodicity. Our study provides a way to explore physical properties of relativistic fermions with controllable quasicrystalline orders.

Axial coordination and electronic structure of diatomic NO, CO, and O2 molecules adsorbed onto Co-tetraphenylporphyrin on Au(111), Ag(111), and Cu(111): a density-functional theory study.

Based on density functional theory calculations, we investigated the axial bindings of diatomic molecules (NO, CO, and O2) to metalloporphyrins and their spin switching behaviors. These binding reactions provide the ways to control molecular states and spins in metalloporphyrin systems that can be used in bio-sensing and spintronic applications. To microscopically understand the non-trivial axial binding structures and spin-switching behaviors of diatomic molecules (NO, CO, and O2) adsorbed onto Co-tetraphenylporphyrin (CoTPP), we performed spin-polarized density functional theory (DFT) calculations for various CoTPP systems on Au(111), Ag(111), and Cu(111) substrates. We also systematically evaluated the effects of van der Waals and Hubbard U corrections by directly comparing the electronic structure with the results of scanning tunnelling microscopy. From the calculation results, we have found that a NO molecule, almost 60° tilted away from the axial direction, is coordinated with CoTPP with a binding energy of 1.2-1.7 eV depending on substrates, and the spin state of CoTPP is completely switched off due to the charge transfer from NO to CoTPP. On the other hand, CO and O2 molecules rather weakly interact with a binding energy of 0.4-0.8 eV, and a spin polarization of ∼1µB is still present at CoTPP. A CO molecule is expected to be almost straightly coordinated along the axial direction of CoTPP, but O2 is tilted similarly to the NO case. Regarding the substrate effects, we have found that there is noticeable charge transfer from Ag(111) and Cu(111) to CoTPP, but no significant charge transfer from Au(111) to CoTPP. These findings of the axial coordination and spin states for NO, CO, and O2 adsorbed CoTPP systems will be useful for understanding bio-sensing mechanisms and designing molecular spintronic systems.

Probing Single-Molecule Dissociations from a Bimolecular Complex NO-Co-Porphyrin.

Axial coordinations of diatomic NO molecules to metalloporphyrins play key roles in dynamic processes of biological functions such as blood pressure control and immune response. Probing such reactions at the single molecule level is essential to understand their physical mechanisms but has been rarely performed. Here we report on our single molecule dissociation experiments of diatomic NO from NO-Co-porphyrin complexes describing its dissociation mechanisms. Under tunneling junctions of scanning tunneling microscope, both positive and negative energy pulses gave rise to dissociations of NO with threshold voltages, +0.68 and -0.74 V at 0.1 nA tunneling current on Au(111). From the observed power law relations between dissociation rate and tunneling current, we argue that the dissociations were inelastically induced with molecular orbital resonances by stochastically tunneling electrons, which is supported with our density functional theory calculations. Our study shows that single molecule dissociation experiments can be used to probe reaction mechanisms in a variety of axial coordinations between small molecules and metalloporphyrins.

Closed-loop ARS mode for scanning ion conductance microscopy with improved speed and stability for live cell imaging applications.

Scanning ion conductance microscopy (SICM) is an increasingly useful nanotechnology tool for non-contact, high resolution imaging of live biological specimens such as cellular membranes. In particular, approach-retract-scanning (ARS) mode enables fast probing of delicate biological structures by rapid and repeated approach/retraction of a nano-pipette tip. For optimal performance, accurate control of the tip position is a critical issue. Herein, we present a novel closed-loop control strategy for the ARS mode that achieves higher operating speeds with increased stability. The algorithm differs from that of most conventional (i.e., constant velocity) approach schemes as it includes a deceleration phase near the sample surface, which is intended to minimize the possibility of contact with the surface. Analysis of the ion current and tip position demonstrates that the new mode is able to operate at approach speeds of up to 250 µm s(-1). As a result of the improved stability, SICM imaging with the new approach scheme enables significantly improved, high resolution imaging of subtle features of fixed and live cells (e.g., filamentous structures & membrane edges). Taken together, the results suggest that optimization of the tip approach speed can substantially improve SICM imaging performance, further enabling SICM to become widely adopted as a general and versatile research tool for biological studies at the nanoscale level.

Anisotropic Terahertz Emission from Bi2Se3 Thin Films with Inclined Crystal Planes.

We investigate the surface states of topological insulator (TI) Bi2Se3 thin films grown on Si nanocrystals and Al2O3 substrates by using terahertz (THz) emission spectroscopy. Compared to bulk crystalline Bi2Te2Se, film TIs exhibit distinct behaviors in the phase and amplitude of emitted THz radiation. In particular, Bi2Se3 grown on Al2O3 shows an anisotropic response with a strong modulation of the THz signal in its phase. From x-ray diffraction, we find that the crystal plane of the Bi2Se3 films is inclined with respect to the plane of the Al2O3 substrate by about 0.27°. This structural anisotropy affects the dynamics of photocarriers and hence leads to the observed anisotropic response in the THz emission. Such relevance demonstrates that THz emission spectroscopy can be a sensitive tool to investigate the fine details of the surface crystallography and electrostatics of thin film TIs.

Catalytic transparency of hexagonal boron nitride on copper for chemical vapor deposition growth of large-area and high-quality graphene.

Graphene transferred onto h-BN has recently become a focus of research because of its excellent compatibility with large-area device applications. The requirements of scalability and clean fabrication, however, have not yet been satisfactorily addressed. The successful synthesis of graphene/h-BN on a Cu foil and DFT calculations for this system are reported, which demonstrate that a thin h-BN film on Cu foil is an excellent template for the growth of large-area and high-quality graphene. Such material can be grown on thin h-BN films that are less than 3 nm thick, as confirmed by optical microscopy and Raman spectroscopy. We have evaluated the catalytic growth mechanism and the limits on the CVD growth of high-quality and large-area graphene on h-BN film/Cu by performing Kelvin probe force microscopy and DFT calculations for various thicknesses of h-BN.

Switching and sensing spin states of co-porphyrin in bimolecular reactions on Au111 using scanning tunneling microscopy.

Controlling and sensing spin states of magnetic molecules at the single-molecule level is essential for spintronic molecular device applications. Here, we demonstrate that spin states of Co-porphyrin on Au(111) can be reversibly switched over by binding and unbinding of the NO molecule and can be sensed using scanning tunneling microscopy and spectroscopy (STM and STS). Before NO exposure, Co-porphryin showed a clear zero-bias peak, a signature of Kondo effect in STS, whereas after NO exposures, it formed a molecular complex, NO-Co-porphyrin, that did not show any zero-bias feature, implying that the Kondo effect was switched off by binding of NO. The Kondo effect could be switched back on by unbinding of NO through single-molecule manipulation or thermal desorption. Our density functional theory calculation results explain the observations with pairing of unpaired spins in dz(2) and ppπ* orbitals of Co-porphyrin and NO, respectively. Our study opens up ways to control molecular spin state and Kondo effect by means of enormous variety of bimolecular binding and unbinding reactions on metallic surfaces.

Atomic and electronic structure of an alloyed topological insulator, Bi1.5Sb0.5Te1.7Se1.3.

Bi2-xSbxTe3-ySey has been argued to exhibit both topological surface states and insulating bulk states, but has not yet been studied with local probes on the atomic scale. Here we report on the atomic and electronic structures of Bi1.5Sb0.5Te1.7Se1.3 studied using scanning tunnelling microscopy (STM) and spectroscopy (STS). Although there is significant surface disorder due to alloying of constituent atoms, cleaved surfaces of the crystals present a well-ordered hexagonal lattice with 10 Å high quintuple layer steps. STS results reflect the band structure and indicate that the surface state and Fermi energy are both located inside the energy gap. In particular, quasi-particle interference patterns from electron scattering demonstrate that the surface states possess linear dispersion and chirality from spin texture, thus verifying its topological nature. This finding demonstrates that alloying is a promising route to achieve full suppression of bulk conduction in topological insulators whilst keeping the topological surface state intact.

A platform for large-scale graphene electronics--CVD growth of single-layer graphene on CVD-grown hexagonal boron nitride.

Direct chemical vapor deposition (CVD) growth of single-layer graphene on CVD-grown hexagonal boron nitride (h-BN) film can suggest a large-scale and high-quality graphene/h-BN film hybrid structure with a defect-free interface. This sequentially grown graphene/h-BN film shows better electronic properties than that of graphene/SiO2 or graphene transferred on h-BN film, and suggests a new promising template for graphene device fabrication.

Polymorphic porous supramolecular networks mediated by halogen bonds on Ag(111).

Intermolecular structures of porous two-dimensional supramolecular networks are studied using scanning tunnelling microscopy combined with density functional theory calculations. The local configurations of halogen bonds in polymorphic porous supramolecular networks are directly visualized in support of previous bulk crystal studies.

Epitaxial Mn(5)Ge(3) nano-islands on a Ge(001) surface.

The growth behavior and atomic structure of Mn germanide, grown on Ge(001), is studied with x-ray diffraction and scanning probe microscopy. The amorphous clusters of as-deposited Mn are crystallized into Mn(5)Ge(3) nano-islands with a size of ∼100 nm by solid phase epitaxy. At low coverage, the shape of the nano-islands is plateau-like, while at increased coverage it becomes mound-like. At the flat top of the plateau-like nano-islands, the hexagonal atomic structure is resolved. It is interpreted, with the help of first-principles study, as a Mn-terminated Mn(5)Ge(3)(0001) structure.

Adsorption, manipulation and self-assembling of TBrPP-Co molecules on a Ag/Si(111) surface by scanning tunnelling microscopy.

Individual adsorption and two-dimensional assembling of 5,10,15,20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecules on a Si(111)-[Formula: see text] Ag reconstructed surface have been studied using low-temperature scanning tunnelling microscopy (STM). All the isolated molecules are observed in a planar shape with slight distortion. The isolated molecules can be controllably rotated with an STM tip to the orientation along the trigonal lattice ([Formula: see text] direction) of the substrate. With an increased coverage (0.07 ML) and appropriate annealing, the molecules assemble to form three types of ordered phase. The long-range ordered structures, however, disappear at higher coverage (0.75 ML).