Sub-Tip-Radius Near-Field Interactions in Nano-FTIR Vibrational Spectroscopy on Single Proteins.

Tip-enhanced vibrational spectroscopy has advanced to routinely attain nanoscale spatial resolution, with tip-enhanced Raman spectroscopy even achieving atomic-scale and submolecular sensitivity. Tip-enhanced infrared spectroscopy techniques, such as nano-FTIR and AFM-IR spectroscopy, have also enabled the nanoscale chemical analysis of molecular monolayers, inorganic nanoparticles, and protein complexes. However, fundamental limits of infrared nanospectroscopy in terms of spatial resolution and sensitivity have remained elusive, calling for a quantitative understanding of the near-field interactions in infrared nanocavities. Here, we demonstrate the application of nano-FTIR spectroscopy to probe the amide-I vibration of a single protein consisting of ∼500 amino acid residues. Detection with higher tip tapping demodulation harmonics up to the seventh order leads to pronounced enhancement in the peak amplitude of the vibrational resonance, originating from sub-tip-radius geometrical effects beyond dipole approximations. This quantitative characterization of single-nanometer near-field interactions opens the path toward employing infrared vibrational spectroscopy at the subnanoscale and single-molecule levels.

Highly Durable Spin Filter Switching Based on Self-Assembled Chiral Molecular Motor.

Chiral molecules have recently received renewed interest as highly efficient sources of spin-selective charge emission known as chiral-induced spin selectivity (CISS), which potentially offers a fascinating utilization of organic chiral materials in novel solid-state spintronic devices. However, a practical use of CISS remains far from completion, and rather fundamental obstacles such as (i) external controllability of spin, (ii) function durability, and (iii) improvement of spin-polarization efficiency have not been surmounted to date. In this study, these issues are addressed by developing a self-assembled monolayer (SAM) of overcrowded alkene (OCA)-based molecular motor. With this system, it is successfully demonstrated that the direction of spin polarization can be externally and repeatedly manipulated in an extremely stable manner by switching the molecular chirality, which is achieved by a formation of the covalent bonds between the molecules and electrode. In addition, it is found that a higher stereo-ordering architecture of the SAM of OCAs tailored by mixing them with simple alkanethiols considerably enhances the efficiency of spin polarization per a single OCA molecule. All these findings provide the creditable feasibility study for strongly boosting development of CISS-based spintronic devices that can simultaneously fulfill the controllability, durability, and high spin-polarization efficiency.

Wettability and Surface Tension of Imidazolium, Ammonium, and Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids: Comparison between Pentyl, Ethoxyethyl, and Ethylthioethyl Groups.

This study particularly compares the surface tensions and contact angles for molten bis(fluorosulfonyl)amide salts of imidazolium, ammonium, and phosphonium cations with the pentyl, ethoxyethyl, or ethylthioethyl group. The examined substrate plates for contact angle measurements include silicate glass, platinum, copper, graphene, and polytetrafluoroethylene (PTFE). In addition, quantum chemistry calculations were performed to obtain the optimized structures of the cations and anions of the ionic liquids (ILs) that were studied here along with some typical anions and their dipole moments, mean polarizabilities, and charge distributions. All ILs showed the same order of contact angles with respect to the substrates: PTFE > graphene ≈ copper ≈ platinum > silicate glass. By comparing the three functional groups, i.e., pentyl, ethoxyethyl, and ethylthioethyl, the ILs with the ethylthioethyl group featured a higher work of adhesion than the respective ILs with the pentyl or ethoxyethyl group. The values of the surface tensions of the ILs followed the same trend for the three functional groups. Based on the Fowkes theory, it was found that the larger surface tensions of the ILs with the ethylthioethyl group compared with pentyl and ethoxyethyl groups were because of the increase in both dispersive and nondispersive components. The quantum chemistry calculations of the ions showed a larger dipole moment and mean polarizability for the cations with the ethylthioethyl group as compared with the pentyl and ethoxyethyl groups. This is consistent with the analysis results of the surface tensions based on the Fowkes theory. By comparing other anions, the dispersive component of the surface tension of the ILs with bis(fluorosulfonyl)amide was large, which is attributed to the small dipole moment of the anion.

Atomic-Level Viscosity Distribution in the Hydration Layer.

The viscosity of solvation structures is crucial for the development of energy-efficient biofunctional and electrochemical devices. Elucidating their subnanoscale distributions can cause the formation of a sustainable energy society. Here, we visualize the site-specific three-dimensional damping distribution on a CaCO_{3} surface composed of binary ion species using ultra-low-noise frequency modulation atomic force microscopy. With the support from molecular dynamics simulation, we found a strikingly large damping at the calcium sites, which demonstrates the capability of this methodology to visualize atomic-scale viscosity in the hydration layers. Our finding will expedite the evolutions of various functional devices.

Atomic-Scale 3D Local Hydration Structures Influenced by Water-Restricting Dimensions.

Hydration structures at solid-liquid interfaces mediate between the atomic-level surface structures and macroscopic functionalities in various physical, chemical, and biological processes. Atomic-scale local hydration measurements have been enabled by ultralow noise three-dimensional (3D) frequency-modulation atomic force microscopy. However, for their application to complicated surface structures, e.g., biomolecular devices, understanding the relationship between the hydration and surface structures is necessary. Herein, we present a systematic study based on the concept of the structural dimensionality, which is crucial in various scientific fields. We performed 3D measurements and molecular dynamics simulations with silicate surfaces that allow for 0, 1, and 2 degrees of freedom to water molecules. Consequently, we found that the 3D hydration structures reflect the structural dimensions and the hydration contrasts decrease with increasing dimension due to the enlarged water self-diffusion coefficient and increased embedded hydration layers. Our results provide guidelines for the analysis of complicated hydration structures, which will be exploited in extensive fields.

Interface structure between tetraglyme and graphite.

Clarification of the details of the interface structure between liquids and solids is crucial for understanding the fundamental processes of physical functions. Herein, we investigate the structure of the interface between tetraglyme and graphite and propose a model for the interface structure based on the observation of frequency-modulation atomic force microscopy in liquids. The ordering and distorted adsorption of tetraglyme on graphite were observed. It is found that tetraglyme stably adsorbs on graphite. Density functional theory calculations supported the adsorption structure. In the liquid phase, there is a layered structure of the molecular distribution with an average distance of 0.60 nm between layers.

Unsupported Nanoporous Gold Catalyst for Chemoselective Hydrogenation Reactions under Low Pressure: Effect of Residual Silver on the Reaction.

For the first time, H-H dissociation on an unsupported nanoporous gold (AuNPore) surface is reported for chemoselective hydrogenation of C≡C, C═C, C═N, and C═O bonds under mild conditions (8 atm H2 pressure, 90 °C). Silver doping in AuNPore, which was inevitable for its preparation through a process of dealloying of Au-Ag alloy, exhibited a remarkable difference in catalytic activity between two catalysts, Au>99Ag1NPore and Au90Ag10NPore.The former was more active and the latter less active in H2 hydrogenation, while the reverse tendency was observed for O2 oxidation. This marked contrast between H2 reduction and O2 oxidation is discussed. Further, Au>99Ag1NPore showed a high chemoselectivity toward reduction of terminal alkynes in the presence of internal alkynes which was not achieved using supported gold nanoparticle catalysts and other previously known methods. Reductive amination, which has great significance in synthesis of amines due to its atom-economical nature, was also realized using Au>99Ag1NPore, and the Au>99Ag1NPore/H2 system showed a preference for the reduction of aldehydes in the presence of imines. In addition to this high chemoselectivity, easy recovery and high reusability of AuNPore make it a promising heterogeneous catalyst for hydrogenation reactions.

Atomic defects in titanium dioxide.

The functionality of solid materials is defined by the type and ordering of the constituent atoms. By introducing defects that perturb the ordered structure, new functionality is created within the solid material. Atomic defects in titanium dioxide, such as oxygen vacancies, atomic hydrogen, and interstitial Ti, typically create new functionality. However, the fundamental physical properties of atomic defects in TiO2 are not fully understood and still remain controversial. In this account, the progress and issues for debate regarding the physical properties, electronic structure, and manipulation mechanisms of atomic defects in TiO2 as well as their interaction with gold nanoclusters are described.

Microscopic behavior of nano-water droplets on a silica glass surface.

Recent advancements in computational science and interfacial measurements have sparked interest in microscopic water droplets and their diverse behaviors. A previous study using nonlinear spectroscopy revealed the heterogeneous wetting phenomenon of silica glass in response to humidity. Building on this premise, we employed high-resolution atomic force microscopy to investigate the wetting dynamics of silica glass surfaces at various humidity levels. Our observations revealed the spontaneous formation of nano-water droplets at a relative humidity of 50%. In contrast to the conventional model, which predicts the spreading of nanodroplets to form a uniform water film, our findings demonstrate the coexistence of nano-water droplets and the liquid film. Moreover, the mobility of the nano-water droplets suggests their potential in inducing the transport of adsorbates on solid surfaces. These results may contribute to the catalytic function of solid materials.

Quantitating the lattice strain dependence of monolayer Pt shell activity toward oxygen reduction.

Lattice strain of Pt-based catalysts reflecting d-band status is the decisive factor of their catalytic activity toward oxygen reduction reaction (ORR). For the newly arisen monolayer Pt system, however, no general strategy to isolate the lattice strain has been achieved due to the short-range ordering structure of monolayer Pt shells on different facets of core nanoparticles. Herein, based on the extended X-ray absorption fine structure of monolayer Pt atoms on various single crystal facets, we propose an effective methodology for evaluating the lattice strain of monolayer Pt shells on core nanoparticles. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity.

Selective aerobic oxidation of methanol in the coexistence of amines by nanoporous gold catalysts: highly efficient synthesis of formamides.

Holey gold: Highly selective aerobic oxidation of methanol over alkylamines was achieved with a reusable nanoporous gold (AuNPore) catalyst that was fabricated from a Au-Ag alloy. This excellent chemoselectivity enabled direct N-formylation of alkylamines from a mixture of methanol and amines. The remarkable catalytic activity was attributed to the synergistic effect between gold and the residual silver remaining in the AuNPore.

Enantioselectivity of discretized helical supramolecule consisting of achiral cobalt phthalocyanines via chiral-induced spin selectivity effect.

Enantioselectivity of helical aggregation is conventionally directed either by its homochiral ingredients or by introduction of chiral catalysis. The fundamental question, then, is whether helical aggregation that consists only of achiral components can obtain enantioselectivity in the absence of chiral catalysis. Here, by exploiting enantiospecific interaction due to chiral-induced spin selectivity (CISS) that has been known to work to enantio-separate a racemic mixture of chiral molecules, we demonstrate the enantioselectivity in the assembly of mesoscale helical supramolecules consisting of achiral cobalt phthalocyanines. The helical nature in our supramolecules is revealed to be mesoscopically incorporated by dislocation-induced discretized twists, unlike the case of chiral molecules whose chirality are determined microscopically by chemical bond. The relevance of CISS effect in the discretized helical supramolecules is further confirmed by the appearance of spin-polarized current through the system. These observations mean that the application of CISS-based enantioselectivity is no longer limited to systems with microscopic chirality but is expanded to the one with mesoscopic chirality.

Nanoporous gold catalyst for highly selective semihydrogenation of alkynes: remarkable effect of amine additives.

We report for the first time the highly selective semihydrogenation of alkynes using the unsupported nanoporous gold (AuNPore) as a catalyst and organosilanes with water as a hydrogen source. Under the optimized reaction conditions, the present semihydrogenation of various terminal- and internal-alkynes affords the corresponding alkenes in high chemical yields and excellent Z-selectivity without any over-reduced alkanes. The use of DMF as solvent, which generates amines in situ, or pyridine as an additive is crucial to suppress the association of hydrogen atoms on AuNPore to form H(2) gas, which is unable to reduce alkynes on the unsupported gold catalysts. The AuNPore catalyst can be readily recovered and reused without any loss of catalytic activity. In addition, the SEM and TEM characterization of nanoporosity show that the AuNPore catalyst has a bicontinuous 3D structure and a high density of atomic steps and kinks on ligament surfaces, which should be one of the important origins of catalytic activity.

Atomic-Scale Three-Dimensional Local Solvation Structures of Ionic Liquids.

Room-temperature ionic liquids are promising media for next-generation energy devices because of their various superior characteristics. Because device performance is often dictated by the solvation structures at the solid-liquid interfaces, particularly at the local reactive sites, their atomistic pictures are in great demand. However, there has been no experimental technique for their three-dimensional solvation structures. Here, we first demonstrate the measurement of the atomic-scale ionic liquids using a recently established ultralow-noise three-dimensional frequency-modulation atomic force microscopy technique supported by molecular dynamics simulations. We conducted the experiments in protic and aprotic aqueous solutions and reveal that the aprotic solvation structure exhibits the higher site specificity, which resolves atomic-scale surface charge distribution on mica because of the absence of the H-bonding network. Our methodology is also applicable to pure liquids and would be a breakthrough for expanding their future applications.

Molecular-Scale Solvation Structures of Ionic Liquids on a Heterogeneously Charged Surface.

Understanding the sub-nanoscale solvation structures of ionic liquids is crucial for the development of innovative functional "devices" across numerous fields. We previously demonstrated the atomic-scale solvation measurements using an ultralow noise 3D frequency-modulation atomic force microscopy combined with molecular dynamics simulations. However, to facilitate practical applications, the molecular distribution on a heterosurface must be verified. Here, we unveil the local solvation structures on a heterogeneously charged phyllosilicate surface in an ionic liquid solution and pure liquid. By identifying adsorbed ion species from the molecular sizes and orientations, we experimentally demonstrate that anions and cations preferentially adsorbed onto the positive and negative surfaces exhibit different orientations and water miscibility. Moreover, we reveal that neutral intermediate regions are formed at the boundary region in ionic liquid media as well as a KCl solution. In the future, this technique will be essential for the evolution of ionic-liquid functional "devices".

The electronic structure of oxygen atom vacancy and hydroxyl impurity defects on titanium dioxide (110) surface.

Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate (screen) the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl (OH) impurity defects on rutile TiO(2)(110) surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO(2) surfaces.

Structural Understanding of Superior Battery Properties of Partially Ni-Doped Li2MnO3 as Cathode Material.

We examined the crystal structures of Li2(NixMn1-x)O3(-δ) (x = 0, 1/10, 1/6, and 1/4) to elucidate the relationship between the structure and electrochemical performance of the compounds using neutron and synchrotron X-ray powder diffraction analyses in combination. Our examination revealed that these crystals contain a large number of stacking faults and exhibit significant cation mixing in the transition-metal layers; the cation mixing becomes significant with an increase in the Ni concentration. Charge-discharge measurements showed that the replacement of Mn with Ni lowers the potential of the charge plateau and leads to higher charge-discharge capacities. From a topological point of view with regard to the atomic arrangement in the crystals, it is concluded that substituting Mn in Li2MnO3 with Ni promotes the formation of smooth Li percolation paths, thus increasing the number of active Li ions and improving the charge-discharge capacity.

Tunneling Desorption of Single Hydrogen on the Surface of Titanium Dioxide.

We investigated the reaction mechanism of the desorption of single hydrogen from a titanium dioxide surface excited by the tip of a scanning tunneling microscope (STM). Analysis of the desorption yield, in combination with theoretical calculations, indicates the crucial role played by the applied electric field. Instead of facilitating desorption by reducing the barrier height, the applied electric field causes a reduction in the barrier width, which, when coupled with the electron excitation induced by the STM tip, leads to the tunneling desorption of the hydrogen. A significant reduction in the desorption yield was observed when deuterium was used instead of hydrogen, providing further support for the tunneling-desorption mechanism.

Nitric Oxide Reduction by Carbon Monoxide over Supported Hexaruthenium Cluster Catalysts. 1. The Active Site Structure That Depends on Supporting Metal Oxide and Catalytic Reaction Conditions.

Ruthenium site structures supported on metal oxide surfaces were designed by reacting organometallic Ru cluster [Ru6C(CO)16](2-) or [Ru6(CO)18](2-) with various metal oxides, TiO2, Al2O3, MgO, and SiO2. The surface Ru site structure, formed under various catalyst preparation and reaction conditions, was investigated by the Ru K-edge extended X-ray absorption fine structure (EXAFS). Samples of [Ru6C(CO)16](2-)/TiO2(anatase) and [Ru6C(CO)16](2-)/TiO2(rutile) were found to retain the original Ru6C framework when heated in the presence of NO (2.0 kPa) or NO (2.0 kPa) + CO (2.0 kPa) at 423 K, i.e., catalytic reaction conditions for NO decomposition. At 523 K, the Ru-Ru bonds of the Ru6C framework were cleaved by the attack of NO. In contrast, the Ru site became spontaneously dispersed over TiO2 (anatase). When being supported over TiO2 (mesoporous), MgO, or Al2O3, the Ru6C framework split into fragments in gaseous NO or NO + CO even at 423 K. The Ru6 framework of [Ru6(CO)18](2-) was found to break easily into smaller ensembles in the presence of NO and/or CO at 423 K on support. Taking into consideration the realistic environments in which these catalysts will be used, we also examined the effect of water and oxygen. When water was introduced to the sample [Ru6C(CO)16](2-)/TiO2(anatase) at 423 K, it did not have any effects on the stabilized Ru6C framework structure. In the presence of oxygen gas, however, the Ru hexanuclear structure decomposed into isolated Ru cations bound to surface oxygen atoms of TiO2 (anatase).

The synergistic effect of nanoporous AuPd alloy catalysts on highly chemoselective 1,4-hydrosilylation of conjugated cyclic enones.

The nanoporous AuPd (AuPdNPore) alloy catalyst showed superior chemoselectivity and high catalytic activity for the direct 1,4-hydrosilylation of the conjugated cyclic enones with hydrosilane in comparison with the monometallic nanoporous Au and Pd catalysts. The enhanced catalytic properties of AuPdNPore arise mainly from the nanoporous structure and the synergistic effect of the AuPd alloy.