Chiral Honeycomb Lattices of Nonplanar π-Conjugated Supramolecules with Protected Dirac and Flat Bands.

The honeycomb lattice is a fundamental two-dimensional (2D) network that gives rise to surprisingly rich electronic properties. While its expansion to 2D supramolecular assembly is conceptually appealing, its realization is not straightforward because of weak intermolecular coupling and the strong influence of a supporting substrate. Here, we show that the application of a triptycene derivative with phenazine moieties, Trip-Phz, solves this problem due to its strong intermolecular π-π pancake bonding and nonplanar geometry. Our scanning tunneling microscopy (STM) measurements demonstrate that Trip-Phz molecules self-assemble on a Ag(111) surface to form chiral and commensurate honeycomb lattices. Electronically, the network can be viewed as a hybrid of honeycomb and kagome lattices. The Dirac and flat bands predicted by a simple tight-binding model are reproduced by total density functional theory (DFT) calculations, highlighting the protection of the molecular bands from the Ag(111) substrate. The present work offers a rational route for creating chiral 2D supramolecules that can simultaneously accommodate pristine Dirac and flat bands.

Crystallization with Nodular Aggregation near the Glass Transition Temperature for Syndiotactic Polypropylene.

The isothermal crystallization from the melt state of syndiotactic polypropylene (sPP) has been studied by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and optical microscopy. The WAXD and SAXS results show the crystallization mechanism near the glass transition temperature in which the crystalline and mesomorphic nodules cover the entire sample with the formation of aggregation regions. For the SAXS analysis, the scattering function for the three-component system has been suggested. Furthermore, to analyze the growth kinetics of the aggregation region for sPP, the time-dependent structure factor combined with the homogeneous and inhomogeneous nucleation-and-growth kinetics has been suggested. The analysis shows that the growth kinetics of the aggregation region for sPP is the homogeneous nucleation-and-growth. The growth velocity of the aggregation region is a natural extrapolation of that of spherulite to the high supercooling region. These results might indicate that the crystallization with the nodular aggregation is a fundamental crystallization process near the glass transition temperature for polymers.

Anomalous dewetting growth of Si on Ag(111).

We demonstrate the novel growth of silicene grown on Ag(111) using STM and reveal the mechanism with KMC simulation. Our STM study shows that after the complete formation of the first layer of silicene, it is transformed into bulk Si with the reappearance of the bare Ag surface. This dewetting (DW) during the epitaxial growth is an exception in the conventional growth behavior. Our KMC simulation reproduces DW by taking into account the differences in the activation energies of Si atoms on Ag, silicene, and bulk Si. The growth modes change depending on the activation energy of the diffusion, temperature, and deposition rate, highlighting the importance of kinetics in growing metastable 2D materials.

Tip-Mediated Bandgap Tuning for Monolayer Transition Metal Dichalcogenides.

Monolayer transition metal dichalcogenides offer an appropriate platform for developing advanced electronics beyond graphene. Similar to two-dimensional molecular frameworks, the electronic properties of such monolayers can be sensitive to perturbations from the surroundings; the implied tunability of electronic structure is of great interest. Using scanning tunneling microscopy/spectroscopy, we demonstrated a bandgap engineering technique in two monolayer materials, MoS2 and PtTe2, with the tunneling current as a control parameter. The bandgap of monolayer MoS2 decreases logarithmically by the increasing tunneling current, indicating an electric-field-induced gap renormalization effect. Monolayer PtTe2, by contrast, exhibits a much stronger gap reduction, and a reversible semiconductor-to-metal transition occurs at a moderate tunneling current. This unusual switching behavior of monolayer PtTe2, not seen in bulk semimetallic PtTe2, can be attributed to its surface electronic structure that can readily couple to the tunneling tip, as demonstrated by theoretical calculations.

Scanning tunneling spectroscopy studies of topological materials.

Topological materials have become promising materials for next-generation devices by utilizing their exotic electronic states. Their exotic states caused by spin-orbital coupling usually locate on the surfaces or at the edges. Scanning tunneling spectroscopy (STS) is a powerful tool to reveal the local electronic structures of condensed matters. Therefore, STS provides us with an almost perfect method to access the exotic states of topological materials. In this topical review, we report the current investigations by several methods based on the STS technique for layered topological material from transition metal dichalcogenide Weyl semimetals (WTe2 and MoTe2) to two dimensional topological insulators (layered bismuth and silicene). The electronic characteristics of these layered topological materials are experimentally identified.

Quasiparticle scattering in type-II Weyl semimetal MoTe2.

The electronic structure of type-II Weyl semimetal molybdenum ditelluride (MoTe2) is studied by using scanning tunneling microscopy and density functional theory calculations. Through measuring energy-dependent quasiparticle interference (QPI) patterns with a cryogenic scanning tunneling microscope, several characteristic features are found in the QPI patterns. Two of them arise from the Weyl semimetal nature; one is the topological Fermi arc surface state and the other can be assigned to be a Weyl point. The remaining structures are derived from the scatterings relevant to the bulk electronic states. The findings lead to further understanding of the topological electronic structure of type-II Weyl semimetal MoTe2.

Visualizing Type-II Weyl Points in Tungsten Ditelluride by Quasiparticle Interference.

Weyl semimetals (WSMs) are classified into two types, type I and II, according to the topology of the Weyl point, where the electron and hole pockets touch each other. Tungsten ditelluride (WTe2) has garnered a great deal of attention as a strong candidate to be a type-II WSM. However, the Weyl points for WTe2 are located above the Fermi level, which has prevented us from identifying the locations and the connection to the Fermi arc surface states by using angle-resolved photoemission spectroscopy. Here, we present experimental proof that WTe2 is a type-II WSM. We measured energy-dependent quasiparticle interference patterns with a cryogenic scanning tunneling microscope, revealing the position of the Weyl point and its connection with the Fermi arc surface states, in agreement with prior theoretical predictions. Our results provide an answer to this crucial question and stimulate further exploration of the characteristics of WSMs.

Transport characteristics of a silicene nanoribbon on Ag(110).

We present the transport characteristics of individual silicene nanoribbons (SiNRs) grown on Ag(110). By lifting up a single SiNR with a low-temperature scanning tunneling microscope tip, a nanojunction consisting of tip, SiNR and Ag is fabricated. In the differential conductance spectra of the nanojunctions fabricated by this methodology, a peak appears at the Fermi level which is not observed in the spectra measured either for the SiNRs before being lifted up or the clean Ag substrate. We discuss the origin of the peak as it relates to the SiNR.

Impact of reduced symmetry on magnetic anisotropy of a single iron phthalocyanine molecule on a Cu substrate.

We study the magnetic anisotropy of a single iron phthalocyanine (FePc) molecule on a Cu(110) (2 × 1)-O by using inelastic electron tunneling spectroscopy (IETS) with low-temperature scanning tunneling microscopy. Two inelastic excitations derived from the splitting of the molecular triplet spin state appear as two pairs of steps symmetrically with respect to zero sample voltage. We measured IETS spectra with external magnetic fields perpendicular and parallel to the molecular plane, and we analyzed the spectral evolution with the effective spin Hamiltonian approach. We determined all parameters related with magnetic anisotropy at a single-molecule level, both the easy- and hard-magnetization directions, zero-field splitting constant, D = - 4.0 meV and E = 1.1 meV, the Lande g-tensor gxx, gyy, gzz=(1.82, 2.02, 2.34), and the constant of spin-orbit coupling λ = - 19.1 meV. We stress that the symmetry breaking caused by the adsorption of FePc on the oxidized Cu(110) significantly impacts the magnetic anisotropy.

Serotonin release in the central nucleus of the amygdala in response to noxious and innocuous cutaneous stimulation in anesthetized rats.

We investigated the effect of noxious (pinching) and innocuous (stroking) stimulation of skin on serotonin (5-HT) release in the central nucleus of the amygdala (CeA) in anesthetized rats. 5-HT in the CeA was collected by microdialysis methods. Dialysate output from consecutive 10-min periods was injected into a high-performance liquid chromatograph and 5-HT was measured with an electrochemical detector. Bilateral pinching of the back for 10 min increased 5-HT release significantly; 5-HT release was also increased with stimulation of the forelimb or hindlimb. In contrast, stroking of these areas decreased 5-HT release significantly. Furthermore, simultaneous stroking and pinching produced no change in the 5-HT release. In conclusion, the present study demonstrates that 5-HT release in the CeA is regulated by somatic afferent stimulation in a modality-dependent manner, and that innocuous stimulation can dampen the change in 5-HT release that occurs in response to noxious stimulation.

Controlling orbital-selective Kondo effects in a single molecule through coordination chemistry.

Iron(II) phthalocyanine (FePc) molecule causes novel Kondo effects derived from the unique electronic structure of multi-spins and multi-orbitals when attached to Au(111). Two unpaired electrons in the d(z)(2) and the degenerate dπ orbitals are screened stepwise, resulting in spin and spin+orbital Kondo effects, respectively. We investigated the impact on the Kondo effects of the coordination of CO and NO molecules to the Fe(2+) ion as chemical stimuli by using scanning tunneling microscopy (STM) and density functional theory calculations. The impacts of the two diatomic molecules are different from each other as a result of the different electronic configurations. The coordination of CO converts the spin state from triplet to singlet, and then the Kondo effects completely disappear. In contrast, an unpaired electron survives in the molecular orbital composed of Fe d(z)(2) and NO 5σ and 2π* orbitals for the coordination of NO, causing a sharp Kondo resonance. The isotropic magnetic response of the peak indicates the origin is the spin Kondo effect. The diatomic molecules attached to the Fe(2+) ion were easily detached by applying a pulsed voltage at the STM junction. These results demonstrate that the single molecule chemistry enables us to switch and control the spin and the many-body quantum states reversibly.

Substrate-induced symmetry breaking in silicene.

We demonstrate that silicene, a 2D honeycomb lattice consisting of Si atoms, loses its Dirac fermion characteristics due to substrate-induced symmetry breaking when synthesized on the Ag(111) surface. No Landau level sequences appear in the tunneling spectra under a magnetic field, and density functional theory calculations show that the band structure is drastically modified by the hybridization between the Si and Ag atoms. This is the first direct example demonstrating the lack of Dirac fermions in a single layer honeycomb lattice due to significant symmetry breaking.

Symmetry-driven novel Kondo effect in a molecule.

The Kondo effect caused by the adsorption of iron phthalocyanine (FePc) on Au(111) was investigated by the combination of density functional theory and a numerical renormalization group calculation with scanning tunneling microscopy. We found that a novel Kondo effect is realized for a single FePc molecule on Au(111) by tuning the symmetry of the ligand field through the local coordination to the substrate. For FePc in the on top configuration where fourfold symmetry around the Fe(2+) ion is held, the orbital degrees of freedom survive, resulting in the spin+orbital SU(4) Kondo effect. In contrast, the reduced symmetry in the bridge configuration freezes the orbital degrees of freedom, leading to the spin SU(2) Kondo effect. These results provide a novel example to manipulate the many-body phenomena by tuning the local symmetry.

Combined scanning tunneling microscopy and high-resolution electron energy loss spectroscopy study on the adsorption state of CO on Ag(001).

The adsorption site and vibrational energies of CO on a clean Ag(001) surface were determined using scanning tunneling microscopy, inelastic electron tunneling spectroscopy with a scanning tunneling microscope, and high-resolution electron energy loss spectroscopy. The CO molecules were found to adsorb on the atop site of the Ag(001) surface, which was similar to their adsorption on the Cu(001) surface. The vibrational energy of the CO internal stretching mode was found to be 263 meV, which is only 3 meV less than that of CO in the gas phase. This result indicates that the CO molecules chemisorb very weakly on the Ag(001) surface.

Evolution of Kondo resonance from a single impurity molecule to the two-dimensional lattice.

The successive spectral evolution of the Kondo resonance state was investigated from a single iron(II) phthalocyanine molecule to the two-dimensional lattice on Au(111) by interrogating the individual molecules with a scanning tunneling microscope. A sharp Kondo peak appears in the single-impurity regime, which broadens and splits as the lattice builds up. The origin of spectral evolution together with the electronic ground state of the lattice are discussed based on the competition of the Kondo effect and Rudermann-Kittel-Kasuya-Yosida coupling between the molecular spins.

Nature of electron transport by pyridine-based tripodal anchors: potential for robust and conductive single-molecule junctions with gold electrodes.

We have designed and synthesized a pyridine-based tripodal anchor unit to construct a single-molecule junction with a gold electrode. The advantage of tripodal anchoring to a gold surface was unambiguously demonstrated by cyclic voltammetry measurements. X-ray photoelectron spectroscopy measurements indicated that the π orbital of pyridine contributes to the physical adsorption of the tripodal anchor unit to the gold surface. The conductance of a single-molecule junction that consists of the tripodal anchor and diphenyl acetylene was measured by modified scanning tunneling microscope techniques and successfully determined to be 5 ± 1 × 10(-4)G(0). Finally, by analyzing the transport mechanism based on ab initio calculations, the participation of the π orbital of the anchor moieties was predicted. The tripodal structure is expected to form a robust junction, and pyridine is predicted to achieve π-channel electric transport.

Adsorption-induced switching of magnetic anisotropy in a single iron(II) phthalocyanine molecule on an oxidized Cu(110) surface.

We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.

Synthesis of tripodal anchor units bearing selenium functional groups and their adsorption behaviour on gold.

The synthesis of new selenium-functionalized tripodal anchor units composed by a tetraphenylmethane core with three selenocyanate or selenol arms has been successfully accomplished and CV, XPS and UPS measurements of their monolayers on a gold surface were investigated.

Structure and thermal fluctuation of one-dimensional AgO chains on Ag(110) surfaces studied with density functional theory and Monte Carlo simulations.

The structures of continuous and truncated AgO chains on Ag(110) surfaces are studied by using density functional theory (DFT) calculations and the thermal fluctuations of truncated chains are simulated by using the Monte Carlo method. Although it is known that oxygen elimination by CO from one-dimensional AgO chains takes place exclusively at chain ends when the chains keep a linear structure at low temperatures, the structure of chain ends has been unexplored. The DFT calculations reveal that oxygen-terminated chains are more stable than silver-terminated ones and have an enhanced density of states near the Fermi level at the terminal oxygen, which is consistent with scanning tunneling microscope (STM) observations. The Monte Carlo simulations with pairwise interactions between AgO units reproduce characteristic features observed in STM studies, including the existence of an onset temperature for the chain fluctuations and the oxygen-coverage dependence of average chain length. The onset temperature, on one hand, is largely controlled by attractive interactions in the direction parallel to chain growth. On the other hand, the spatial distribution of fragmented AgO chains depends strongly on repulsive interactions in the direction perpendicular to chains. In particular, the repulsive interactions ranging ten units of the lattice constant in the direction perpendicular to the AgO chains are essential to mimic STM observations, where fragmented chains almost keep the mutual distance inherent to the (nx1)-O phase even under thermal fluctuations.