Language：English   Publishing type：Research paper (scientific journal)  

<jats:p>Graphene, a transparent two‐dimensional conductive material, has brought extensive new perspectives and prospects to various aqueous technological systems, such as desalination membranes, chemical sensors, energy storage, and energy conversion devices. Yet, the molecular‐level details of graphene in contact with aqueous electrolytes, such as water orientation and hydrogen bond structure, remain elusive or controversial. Here, we employ surface‐specific heterodyne‐detection sum‐frequency generation (HD‐SFG) vibrational spectroscopy to re‐examine the water molecular structure at a freely suspended graphene/water interface. We compare the response from the air/graphene/water system to that from the air/water interface. Our results indicate that the χ(2)yyz spectrum recorded from the air/graphene/water system arises from the topmost 1‐2 water layers in contact with the graphene, with the graphene itself not generating a significant SFG response. Compared to the air/water interface response, the presence of monolayer graphene weakly affects the interfacial water. Graphene weakly affects the dangling O‐H group, lowering its frequency through its interaction with the graphene sheet, and has a small effect on the hydrogen‐bonded O‐H group. Molecular dynamics simulations confirm our experimental observation. Our work provides molecular insight into the interfacial structure at a suspended graphene/water interface, relevant to various technological applications of graphene.</jats:p>

DOI： 10.1002/anie.202319503

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DOI： 10.1021/jacs.3c14012

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DOI： 10.1002/smll.202305997

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DOI： 10.1039/D3NA00343D

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.cej.2023.147862

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Proton exchange membranes with high proton conductivity and low crossover of fuel molecules are required to realize advanced fuel‐cell technology. The selective transportation of protons, which occurs by blocking the transportation of fuel molecules across a proton exchange membrane, is crucial to suppress crossover while maintaining a high proton conductivity. In this study, a simple yet powerful method is proposed for optimizing the crossover‐conductivity relationship by pasting sulfanilic‐functionalized holey graphenes onto a Nafion membrane. The results show that the sulfanilic‐functionalized holey graphenes supported by the membrane suppresses the crossover by 89% in methanol and 80% in formate compared with that in the self‐assembled Nafion membrane; an ≈60% reduction is observed in the proton conductivity. This method exhibits the potential for application in advanced fuel cells that use methanol and formic acid as chemical fuels to achieve high energy efficiency.

DOI： 10.1002/advs.202304082

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DOI： 10.1039/D3TA02443A

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We report in situ generation of a 6,6 '-biindeno[1,2-b]anthracene (BIA) derivative as an open-shell biaryl with high diradical character, which could be identified by mass spectrometry, NMR spectroscopy, single-crystal X-ray analysis, UV-vis-NIR absorption spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. Theoretical calculations by various methods and variable-temperature EPR analyses were performed to tackle the elusive ground state of BIA diradical, suggesting a singlet ground state with a nearly degenerate triplet state. These results provide insight into the design of unique open-shell biaryls.

DOI： 10.1021/jacs.2c13890

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We report in situ generation of a 6,6 '-biindeno[1,2-b]anthracene (BIA) derivative as an open-shell biaryl with high diradical character, which could be identified by mass spectrometry, NMR spectroscopy, single-crystal X-ray analysis, UV-vis-NIR absorption spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. Theoretical calculations by various methods and variable-temperature EPR analyses were performed to tackle the elusive ground state of BIA diradical, suggesting a singlet ground state with a nearly degenerate triplet state. These results provide insight into the design of unique open-shell biaryls.

DOI： 10.1021/jacs.2c138903891

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DOI： 10.1002/adma.202207466

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DOI： 10.1002/adma.202205986

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

We present here the synthesis and in-depth physicochemical characterization of a double hetero[7]helicene fused with four triazole rings at both helical ends. The comparison of this triazole-fused double helicene with the previously reported all-carbon and thiadiazole-fused analogs revealed the huge impact of the embedded aromatic rings on the photophysical features. The small structural variation of the terminal rings from thiadiazole to triazole caused a dramatic change of the photoluminescence quantum yields (PLQYs) from <1 % to 96 %, while the replacement of the terminal benzene rings with triazole rings induced a tenfold enhancement of the circularly polarized luminescence dissymmetry factor. These observations were well corroborated with transient absorption analysis and/or theoretic calculations. In addition, the triazole-fused double helicene exhibited ambipolar redox behavior, enabling the generation of radical cation and anion species by electrochemical and chemical methods and showing its potential for spin-related applications.

DOI： 10.1002/chem.202202243

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The Seebeck effect of a molecular junction in a hopping regime or tunneling-to-hopping transition remains uncertain. This paper describes the Seebeck effect in molecular epitaxy films (OPIn where n = 1-9) based on imine condensation between an aryl amine and aldehyde and investigates how the Seebeck coefficient (S, mu V/K) varies at the crossover region. The S value of OPIn linearly increased with increasing the molecular length (d, nm), ranging from 7.2 to 38.0 mu V/K. The increasing rate changed from 0.99 to 0.38 mu V.K-1 & Aring;(-1) at d = 3.4 nm (OPI4). Combined experimental and theoretical studies indicated that such a change stems from a tunneling-to-hopping transition, and the small but detectable length-dependence of thermopower in the long molecules originates from the gradual reduction of the tunneling contribution to the broadening of molecular orbital energy level, rather than its relative position to the Fermi level. Our work helps to bridge the gap between bulk and nanoscale thermoelectric systems.

DOI： 10.1021/acs.nanolett.2c03083

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DOI： 10.26434/chemrxiv-2022-xfrpb-v2

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Graphene-covering is a promising approach for achieving an acid-stable, non-noble-metal-catalysed hydrogen evolution reaction (HER). Optimization of the number of graphene-covering layers and the density of defects generated by chemical doping is crucial for achieving a balance between corrosion resistance and catalytic activity. Here, we investigate the influence of charge transfer and proton penetration through the graphene layers on the HER mechanisms of the non-noble metals Ni and Cu in an acidic electrolyte. We find that increasing the number of graphene-covering layers significantly alters the HER performances of Ni and Cu. The proton penetration explored through electrochemical experiments and simulations reveals that the HER activity of the graphene-covered catalysts is governed by the degree of proton penetration, as determined by the number of graphene-covering layers.

DOI： 10.1038/s41467-020-20503-7

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Geometric structures of carbon networks are key in designing their material properties. In particular, optimization of curved structures through the introduction of topological defects and doping of heteroatoms in the lattice is crucial to the design of carbon-based, non-noble-metal-free catalysts. A simple and practical mathematical model based on discrete geometric analysis is proposed to describe the geometric features of carbon networks and their relationships to their material properties. This model can pre-screen candidates for novel material design, and these candidates can be further examined by the density functional theory (DFT). Inspired by observations regarding the preferential doping of heteroatoms at local curved sites, the important characteristics of the candidate material were experimentally realized, and its enhanced catalytic activity facilitated by chemical dopants was confirmed in the designed carbon network.

DOI： 10.1016/j.carbon.2021.06.004

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DOI： 10.1021/acscatal.1c02646

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DOI： 10.1021/acsaem.1c01207

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Electrochemical CO2 reduction is a key technology to recycle CO2 as a renewable resource, but adsorbing CO2 on the catalyst surface is challenging. We explored the effects of reduced graphene oxide (rGO) in Sn/rGO composites and found that the CO2 adsorption ability of Sn/rGO was almost 4-times higher than that of bare Sn catalysts. Density functional theory calculations revealed that the oxidized functional groups of rGO offered adsorption sites for CO2 toward the adjacent Sn surface and that CO2-rich conditions near the surface facilitated the production of formate via COOH∗ formation while suppressing CO∗ formation. Scanning electrochemical cell microscopy directly indicated that CO2 reduction was accelerated at the interface, together with the kinetic suppression of undesirable and competitive hydrogen evolution at the interface. Thus, the synergism of Sn/rGO ensures a substantial/rapid supply of CO2 from the functional groups to the Sn surface, thereby enhancing the Faradaic efficiency 1.8-times compared with that obtained with bare Sn catalysts.

DOI： 10.1021/acscatal.0c04887

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DOI： 10.1021/jacs.0c10560

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/smll.202006709

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Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C6H4-NCHN-C6H4-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF3) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10-5 to 10-4G0 region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO-LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO-LUMO gaps. This journal is

DOI： 10.1039/D1SC02407H

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DOI： 10.1021/acssuschemeng.0c07114

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE RESEARCH  

Coupling between vibrational modes is essential for energy transfer and dissipation in condensed matter. For water, different O-H stretch modes are known to be very strongly coupled both within and between water molecules, leading to ultrafast dissipation and delocalization of vibrational energy. In contrast, the information on the vibrational coupling of the H-O-H bending mode of water is lacking, even though the bending mode is an essential intermediate for the energy relaxation pathway from the stretch mode to the heat bath. By combining static and femtosecond infrared, Raman, and hyper-Raman spectroscopies for isotopically diluted water with ab initio molecular dynamics simulations, we find the vibrational coupling of the bending mode differs significantly from the stretch mode: the intramode intermolecular coupling of the bending mode is very weak, in stark contrast to the stretch mode. Our results elucidate the vibrational energy transfer pathways of water. Specifically, the librational motion is essential for the vibrational energy relaxation and orientational dynamics of H-O-H bending mode. Vibrational energy transfer in water involves intermolecular coupling of O-H stretching modes, but much less is known about the role of the bending modes. Here the authors, combining static and femtosecond infrared, Raman, and hyper-Raman spectroscopy and ab initio molecular dynamics simulations, provide insight into the energy dynamics of the bend vibrations.

DOI： 10.1038/s41467-020-19759-w

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DOI： 10.1039/c9cp06335h

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DOI： 10.1021/acs.chemrev.9b00512

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DOI： 10.1021/acs.jpclett.0c00329

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<p>The water bending mode vibrational spectroscopy provides a new avenue for unveiling the hydrogen bonding structure of interfacial water at complex aqueous interfaces such as solid–water and bio–water interfaces.</p>

DOI： 10.1039/d0cp01269f

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The electrical properties of a single-molecule junction of spiropyran are investigated through the break junction (BJ) method, and the current-voltage (I-V) characteristics are switched from rectified to symmetric through mechanical stimulus. This phenomenon indicates isomerization from spiropyran to merocyanine. In addition, an increase in the conductance associated with isomerization is observed.

DOI： 10.1039/d0nr00277a

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DOI： 10.1021/acscatal.9b04134

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Density functional theory-based molecular dynamics simulations are increasingly being used for simulating aqueous interfaces. Nonetheless, the choice of the appropriate density functional, critically affecting the outcome of the simulation, has remained arbitrary. Here, we assess the performance of various exchange-correlation (XC) functionals, based on the metrics relevant to sum-frequency generation spectroscopy. The structure and dynamics of water at the water-air interface are governed by heterogeneous intermolecular interactions, thereby providing a critical benchmark for XC functionals. We find that the XC functionals constrained by exact functional conditions (revPBE and revPBEO) with the dispersion correction show excellent performance. The poor performance of the empirically optimized density functional (M06-L) indicates the importance of satisfying the exact functional condition. Understanding the performance of different XC functionals can aid in resolving the controversial interpretation of the interfacial water structure and direct the design of novel, improved XC functionals better suited to describing the heterogeneous interactions in condensed phases.

DOI： 10.1021/acs.jpclett.9b01983

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DOI： 10.1021/acs.jpclett.9b02059

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DOI： 10.1021/acs.jctc.9b00253

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/00268976.2019.1620358

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Other Link： https://www.tandfonline.com/doi/pdf/10.1080/00268976.2019.1620358

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Carbon-based metal-free catalysts for the hydrogen evolution reaction (HER) are essential for the development of a sustainable hydrogen society. Identification of the active sites in heterogeneous catalysis is key for the rational design of low-cost and efficient catalysts. Here, by fabricating holey graphene with chemically dopants, the atomic-level mechanism for accelerating HER by chemical dopants is unveiled, through elemental mapping with atomistic characterizations, scanning electrochemical cell microscopy (SECCM), and density functional theory (DFT) calculations. It is found that the synergetic effects of two important factors—edge structure of graphene and nitrogen/phosphorous codoping—enhance HER activity. SECCM evidences that graphene edges with chemical dopants are electrochemically very active. Indeed, DFT calculation suggests that the pyridinic nitrogen atom could be the catalytically active sites. The HER activity is enhanced due to phosphorus dopants, because phosphorus dopants promote the charge accumulations on the catalytically active nitrogen atoms. These findings pave a path for engineering the edge structure of graphene in graphene-based catalysts.

DOI： 10.1002/advs.201900119

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DOI： 10.1021/acs.jpclett.9b00747

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jctc.8b00567

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

DOI： 10.1039/C8TA11250A

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

<p>Single-molecule rectifiers with perpendicularly connected metal porphyrin–imide dyads showed high rectification ratio, which could be tuned by the central metal inside the porphyrin. The features can be explained with a three sight model.</p>

DOI： 10.1039/C9NR07105A

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.chempr.2018.08.020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society  

Free O-H groups of water are often found at the water-hydrophobic medium interface, e.g. for water contact with hydrophobic protein residues, or at the water-air interface. In surface-specific vibrational spectroscopic studies using sum-frequency generation (SFG) spectroscopy, free O-H groups are experimentally well characterized in the O-H stretch region by a sharp 3700 cm-1 peak. Although these free O-H groups are often defined as the O-H groups which are not hydrogen-bonded to other water molecules, a direct correlation between such non-hydrogen-bonded O-H groups and the 3700 cm-1 SFG response has been lacking. Our data show that commonly used hydrogen bond definitions do not adequately capture the free O-H groups contributing to the 3700 cm-1 peak. We thus formulate a new definition for capturing the subensemble of the surface free O-H groups using the intermolecular distance and the angle formed by the water dimer, through the comparison of the ∼3700 cm-1 SFG response and the responses from the selected free O-H groups at the HOD-air interface. Using these optimized free O-H group definitions, we infer the fraction of interfacial water molecules with free O-H groups of 28%, a vibrational lifetime of the free O-H groups of 1.3 ps, and the angle formed by the free O-H groups and the surface normal of 67° at the water-air interface. We expect that this improved free O-H group definition can be helpful in exploring the structure and dynamics of the interfacial water.

DOI： 10.1021/acs.jctc.7b00566

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DOI： 10.1021/acsenergylett.8b00739

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society  

Six novel donor-acceptor-donor organic dyes containing a Si-Si moiety based on triarylamine functionalities as donor units were prepared by Pd-catalyzed arylation of hydrosilanes. Their photophysical, electrochemical, and structural properties were studied in detail. Most of the compounds showed attractive photoluminescence (PL) and electrochemical properties both in solution and in the solid state because of intramolecular charge transfer (ICT), suggesting these compounds could be useful for electroluminescence (EL) applications. The aggregation-induced emission enhancement (AIEE) characteristics of 1 and 3 were examined in mixed water/THF solutions. The fluorescence intensity in THF/water was stronger in the solution with the highest ratio of water because of the suppression of molecular vibration and rotation in the aggregated state. Single-crystal X-ray diffraction of 4 showed that the reduction of intermolecular π-π interaction led to intense emission in the solid state and restricted intramolecular rotation of the donor and acceptor moieties, thereby indicating that the intense emission in the solid state is due to AIEE. An electroluminescence device employing 1 as an emitter exhibited an external quantum efficiency of up to 0.65% with green light emission. The emission comes solely from 1 because the EL spectrum is identical to that of the PL of 1. The observed luminescence was sufficiently bright for application in practical devices. Theoretical calculations and electrochemical measurements were carried out to aid in understanding the optical and electrochemical properties of these molecules.

DOI： 10.1021/acsami.7b14802

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The development of noble-metal-free hydrogen evolution reaction (HER) materials for electrochemical water splitting is the key to achieving low-cost and efficient electrocatalysis that drives electrochemical hydrogen evolution. However, the electrocatalytic activities of most non-noble metals decrease in acidic electrolytes. Here, we have fabricated non-noble-metal electrodes using a bicontinuous and open porous NiMo alloy covered by nitrogen-doped (N-doped) graphene with nanometer-sized holes. This noble-metal-free HER catalyst exhibits performance almost identical with that of a Pt/C electrode, while its original catalytic activity is preserved even in acidic electrolytes. Density functional theory calculations indicate that the interfacial fringes between the nanoholes and NiMo surface induce charge transfer and promote hydrogen adsorption and desorption. The nanometer-sized holes simultaneously provide minimal surface area for chemical reactions, while delaying NiMo dissolution in excessive amounts of acidic electrolyte. Our method for the fabrication of the NiMo alloy provides a route to a promising class of electrochemical hydrogen-producing electrodes.

DOI： 10.1021/acscatal.7b04091

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

DOI： 10.1103/physrevlett.121.246101

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry  

We simulate sum-frequency generation (SFG) spectra of isotopically diluted water at the water-graphene and water-hexagonal boron-nitride (hBN) sheet interfaces, using ab initio molecular dynamics simulations. A sharp 'dangling' O-D peak around ∼2640 cm-1 appearing in both simulated SFG spectra evidences that both graphene and hBN are hydrophobic. The dangling O-D peak is 10 cm-1 red-shifted at the water-hBN interface relative to the peak at the water-graphene interface. This frequency difference gives a stronger O-D⋯N intermolecular interaction between water and hBN than the O-D⋯C interaction between water and graphene. Accordingly, the anisotropy decay of such a dangling O-D group slows down near hBN compared with near graphene, illustrating that the dynamics of the dangling O-D group are also affected by the stronger O-D⋯N interaction than the O-D⋯C interaction. We discuss molecular-level insights into the structure and dynamics of interfacial water in the context of the friction of hBN and graphene.

DOI： 10.1039/c8cp01351a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

Current-voltage characteristics of single molecule junctions are governed both by the energy level alignment of molecular orbitals with respect to the Fermi level of the electrodes and by the hybridization of electronic structures at the interface between the molecule and the electrodes. While there have been many studies on tuning the former, only a few works intended to control the latter. In the present study, we demonstrate that molecular junctions based on carbazole oligomers showed a current rectification behavior due to asymmetric-symmetric control of electronic hybridization between the molecule and electrodes at the both terminals. The carbazole oligomers originally showed an asymmetric molecular orbital and, hence, electronic hybridization with the electrodes because of the electric dipole moment. Symmetric electronic hybridization was achieved when the applied electric field between electrodes deformed molecular orbital to be symmetric. This is a novel way to control charge transport in single-molecule junctions.

DOI： 10.1039/c8nr06049e

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We studied the effect of impurities on the magnetoresistance of sexithiophene-based diodes using impedance spectroscopy. The impurities were introduced by doping pentacene molecules into a sexithiophene film through a co-evaporation process. The pentacene molecules act as charge-scattering centers, which trigger the negative magnetoresistance of the device. This makes it possible to tune the value of magnetoresistance from positive to negative by increasing the applied voltage. The beneficial properties induced by impurities suggest a potential route to integrate additional functions into organic devices. Published by AIP Publishing.

DOI： 10.1063/1.5006547

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

A new 2:1 donor (D):acceptor (A) mixed-stacked charge-transfer (CT) cocrystal comprising isometrically structured dicyanodistyrylbenzene-based D and A molecules is designed and synthesized. Uniform 2D-type morphology is manifested by the exquisite interplay of intermolecular interactions. In addition to its appealing structural features, unique optoelectronic properties are unveiled. Exceptionally high photoluminescence quantum yield (Phi(F) approximate to 60%) is realized by non-negligible oscillator strength of the S-1 transition, and rigidified 2D-type structure. Moreover, this luminescent 2D-type CT crystal exhibits balanced ambipolar transport (mu(h) and mu(e) of approximate to 10(-4) cm(2) V-1 s(-1)). As a consequence of such unique optoelectronic characteristics, the first CT electroluminescence is demonstrated in a single active-layered organic light-emitting transistor (OLET) device. The external quantum efficiency of this OLET is as high as 1.5% to suggest a promising potential of luminescent mixed-stacked CT cocrystals in OLET applications.

DOI： 10.1002/adma.201701346

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Single molecule conductance (SMC) of a series of porphyrin derivatives, whose porphyrin units are connected through a phenylene bridge that have one or two thiolate end groups, was measured by a scanning tunneling microscopic break junction (STM-BJ) technique. The observed conductance values were analyzed by partial transmission probabilities (T-i) for the molecular parts using the equation G=G(0)Pi T-N(i)i. Unique combination of Ti was obtained, which indicates the validity of this analysis. By the analysis, the free-base porphyrin ring interacting with the gold electrode was found to show a greater transmission probability Tpor-Au than that of the bridge between thiolate and the gold electrode (TS-Au).

DOI： 10.1002/slct.201701015

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Electrochemical surface activity arises from the interaction and geometric arrangement of molecules at electrified interfaces. We present a novel electrochemical tip-enhanced Raman spectroscope that can access the vibrational fingerprint of less than 100 small, non-resonant molecules adsorbed at atomically flat Au electrodes to study their adsorption geometry and chemical reactivity as a function of the applied potential. Combining experimental and simulation data for adenine/Au(111), we conclude that protonated physisorbed adenine adopts a tilted orientation at low potentials, whereas it is vertically adsorbed around the potential of zero charge. Further potential increase induces adenine deprotonation and reorientation to a planar configuration. The extension of EC-TERS to the study of adsorbate reorientation significantly broadens the applicability of this advanced spectroelectrochemical tool for the nanoscale characterization of a full range of electrochemical interfaces.

DOI： 10.1002/anie.201704460

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Both symmetrical and unsymmetrical cyclophanes containing disilane units, tetrasila[2.2]cyclophanes 1-9, were synthesized. The syn and anti conformations and the kinetics of inversion between two anti-isomers were investigated by X-ray diffraction and variable-temperature NMR analysis, respectively. The flipping motion of two aromatic rings was affected by the bulkiness of the aromatic moiety (1 vs 6), the phase (solid vs solution), and the inclusion by host molecules (1 vs 1 subset of[Ag2L](2+)). The photophysical, electrochemical, and structural properties of the compounds were thoroughly investigated. Unsymmetrical tetrasila[2.2]cyclophanes 5-8 displayed blue-green emission arising from intramolecular charge transfer. Compound 6 emitted a brilliant green light in the solid state under 365 nm irradiation and showed a higher fluorescence quantum yield in the solid state (Phi = 0.49) than in solution (Phi = 0.05). We also obtained planar chiral tetrasila[2.2]cyclophane 9, which showed interesting chiroptical properties, such as a circularly polarized luminescence (CPL) with a dissymmetry factor of vertical bar g(lum)vertical bar = ca. 2 X 10(-3) at 500 nm. Moreover, an organic green light-emitting diode that showed a maximum external quantum efficiency (eta(ext)) of ca. 0.4% was fabricated by doping 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl with 6.

DOI： 10.1021/jacs.7b05671

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DOI： 10.1002/ange.201704460

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DOI： 10.1002/anie.201104700

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The interfacial structure of water in contact with TiO2 is the key to understand the mechanism of photocatalytic water dissociation as well as photoinduced superhydrophilicity. We investigate the interfacial molecular structure of water at the surface of anatase TiO2 using phase-sensitive sum frequency generation spectroscopy together with spectra simulation using ab initio molecular dynamic trajectories. We identify two oppositely oriented, weakly and strongly hydrogen-bonded subensembles of O-H groups at the superhydrophilic UV irradiated TiO2 surface. The water molecules with weakly hydrogen-bonded O H groups are chemisorbed, i.e. form hydroxyl groups, at the TiO2 surface with their hydrogen atoms pointing toward bulk water. The strongly hydrogen-bonded O H groups interact with the oxygen atom of the chemisorbed water. Their hydrogen atoms point toward the TiO2. This strong interaction between physisorbed and chemisorbed water molecules causes superhydrophilicity.

DOI： 10.1021/acs.jpclett.7b00564

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The osmolyte molecule trimethylamine-N-oxide (TMAO) stabilizes the structure of proteins. As functional proteins are generally found in aqueous solutions, an important aspect of this stabilization is the interaction of TMAO with water. Here, we review, using vibrational spectroscopy and molecular dynamics simulations, recent studies on the structure and dynamics of TMAO with its surrounding water molecules. This article ends with an outlook on the open questions on TMAO-protein and TMAO-urea interactions in aqueous environments.

DOI： 10.1039/c6cp07284d

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The interfacial structure of room temperature ionic liquids (RTILs) controls many of the unique properties of RTILs, such as the high capacitance of RTILs and the efficiency of charge transport between RTILs and electrodes. RTILs have been experimentally shown to exhibit interfacial molecular layering structures over a 10 angstrom length scale. However, the driving force behind the formation of these layered structures has not been resolved. Here, we report ab initio molecular dynamics simulations of imidazolium RTIL/air and RTIL/graphene interfaces along with force field molecular dynamics simulations. We find that the pi(+)-pi(+) interaction of imidazolium cations enhances the layering structure of RTILs, despite the electrostatic repulsion. The length scales of the molecular layering at the RTIL/air and RTIL/graphene interfaces are very similar, manifesting the limited effect of the substrate on the interfacial organization of RTILs.

DOI： 10.1039/c6cp07034e

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We used first-principles calculations to investigate the hole mobility of metal-containing poly(phenylene ethynylene) insulated molecular wires. The metal-organic bond effects were considered using ruthenium(II) porphyrin-pyridyl and platinum(II) acetylide as the organometallic moieties. We found that high hole mobility can be achieved even when the metal-organic d pi-p pi interaction is weak. The weak metal-organic interaction reduces the structural deformation that accompanies hole hopping and compensates the reduced conjugation inside the molecular wire. Our results suggest a new principle for the design of functionalized metallopolymers with high carrier mobilities.

DOI： 10.1021/acs.jpcc.6b08557

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

By combining heterodyne-detected sum-frequency generation (SFG) spectroscopy, ab initio molecular dynamics (AIMD) simulation, and a post-vibrational self-consistent field (VSCF) approach, we reveal the orientation and surface activity of the amphiphile trimethylamine-N-oxide (TMAO) at the water/air interface. Both measured and simulated C-H stretch SFG spectra show a strong negative and a weak positive peak. We attribute these peaks to the symmetric stretch mode/Fermi resonance and antisymmetric in-plane mode of the methyl group, respectively, based on the post-VSCF calculation. These positive and negative features evidence that the methyl groups of TMAO are oriented preferentially toward the air phase. Furthermore, we explore the effects of TMAO on the interfacial water structure. The O-H stretch SFG spectra manifest that the hydrogen bond network of the aqueous TMAO-solution/air interface is similar to that of the amine-N-oxide (AO) surfactant/water interface. This demonstrates that, irrespective of the alkyl chain length, the AO groups have a similar impact on the hydrogen bond network of the interfacial water. In contrast, we find that adding TMAO to water makes the orientation of the free O-H groups of the interfacial water molecules more parallel to the surface normal. Invariance of the free O-H peak amplitude despite the enhanced orientation of the topmost water layer illustrates that TMAO is embedded in the topmost water layer, manifesting the clear contrast of the hydrophobic methyl group and the hydrophilic AO group of TMAO.

DOI： 10.1021/acs.jpcc.6b04852

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We report calculations on the surface tension of the water-air interface using ab initio molecular dynamics (AIMD) simulations. We investigate the influence of the cell size on surface tension of water from force field molecular dynamics simulations. We find that the calculated surface tension increases with increasing simulation cell size, thereby illustrating that a correction for finite size effects is essential for small systems that are customary in AIMD simulations. Moreover, AIMD simulations reveal that the use of a double-zeta basis set overestimates the experimentally measured surface tension due to the Pulay stress while more accurate triple and quadruple-zeta basis sets give converged results. We further demonstrate that van der Waals corrections critically affect the surface tension. AIMD simulations without the van der Waals correction substantially underestimate the surface tension while the van derWaals correction with the Grimme's D2 technique results in a value for the surface tension that is too high. The Grimme's D3 van derWaals correction provides a surface tension close to the experimental value. Whereas the specific choices for the van derWaals correction and basis sets critically affect the calculated surface tension, the surface tension is remarkably insensitive to the details of the exchange and correlation functionals, which highlights the impact of long-range interactions on the surface tension. Our simulated values provide important benchmarks, both for improving van derWaals corrections and AIMD simulations of aqueous interfaces. Published by AIP Publishing.

DOI： 10.1063/1.4951710

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Understanding aqueous interfaces at the molecular level is not only fundamentally important, but also highly relevant for a variety of disciplines. For instance, electrode water interfaces are relevant for electrochemistry, as are mineral water interfaces for geochemistry and air water interfaces for environmental chemistry; water lipid interfaces constitute the boundaries of the cell membrane, and are thus relevant for biochemistry. One of the major challenges in these fields is to link macroscopic properties such as interfacial reactivity, solubility, and permeability as well as macroscopic thermodynamic and spectroscopic observables to the structure, structural changes, and dynamics of molecules at these interfaces. Simulations, by themselves, or in conjunction with appropriate experiments, can provide such molecular-level insights into aqueous interfaces. In this contribution, we review the current state-of-the-art of three levels of molecular dynamics (MD) simulation: ab initio, force field, and coarse-grained. We discuss the advantages, the potential, and the limitations of each approach for studying aqueous interfaces, by assessing computations of the sum-frequency generation spectra and surface tension. The comparison of experimental and simulation data provides information on the challenges of future MD simulations, such as improving the force field models and the van der Waals corrections in ab initio MD simulations. Once good agreement between experimental observables and simulation can be established, the simulation can be used to provide insights into the processes at a level of detail that is generally inaccessible to experiments. As an example we discuss the mechanism of the evaporation of water. We finish by presenting an outlook outlining four future challenges for molecular dynamics simulations of aqueous interfacial systems.

DOI： 10.1021/acs.jpcb.6b01012

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We present a combined experimental sum-frequency generation (SFG) spectroscopy and ab initio molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine N-oxide (AO) surfactant monolayers. Simulated O-H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the similar zwitterionic AO interface. The positive peaks at the water/ PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen bond network to reach, with its (up-oriented) O-H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group in the interfacial water structure at the membrane interface, namely, stabilizing the water hydrogen bond network.

DOI： 10.1021/acs.jpclett.5b02141

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Interfacial water structures have been studied intensively by probing the O-H stretch mode of water molecules using sum-frequency generation (SFG) spectroscopy. This surface-specific technique is finding increasingly widespread use, and accordingly, computational approaches to calculate SFG spectra using molecular dynamics (MD) trajectories of interfacial water molecules have been developed and employed to correlate specific spectral signatures with distinct interfacial water structures. Such simulations typically require relatively long (several nanoseconds) MD trajectories to allow reliable calculation of the SFG response functions through the dipole moment-polarizability time correlation function. These long trajectories limit the use of computationally expensive MD techniques such as ab initio MD and centroid MD simulations. Here, we present an efficient algorithm determining the SFG response from the surface-specific velocity-velocity correlation function (ssVVCF). This ssVVCF formalism allows us to calculate SFG spectra using a MD trajectory of only similar to 100 ps, resulting in the substantial reduction of the computational costs, by almost an order of magnitude. We demonstrate that the O-H stretch SFG spectra at the water-air interface calculated by using the ssVVCF formalism well reproduce those calculated by using the dipole moment-polarizability time correlation function. Furthermore, we applied this ssVVCF technique for computing the SFG spectra from the ab initio MD trajectories with various density functionals. We report that the SFG responses computed from both ab initio MD simulations and MD simulations with an ab initio based force field model do not show a positive feature in its imaginary component at 3100 cm(-1). (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4931106

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Ab initio molecular dynamics (AIMD) simulations in trimethylamine N-oxide (TMAO)-D2O solution are employed to elucidate the effects of TMAO on the reorientational dynamics of D2O molecules. By decomposing the O-D groups of the D2O molecules into specific subensembles, we reveal that water reorientational dynamics are retarded considerably in the vicinity of the hydrophilic TMAO oxygen (O-TMAO) atom, due to the O-D center dot center dot center dot O-TMAO hydrogen-bond. We find that this reorientational motion is governed by two distinct mechanisms: The O-D group rotates (1) after breaking the O-D center dot center dot center dot O-TMAO hydrogen-bond, or (2) together with the TMAO molecule while keeping this hydrogen-bond intact. While the orientational slow-down is prominent in the AIMD simulation, simulations based on force field models exhibit much faster dynamics. The simulated angle-resolved radial distribution functions illustrate that the O-D center dot center dot center dot O-TMAO hydrogen-bond has a strong directionality through the sp(3) orbital configuration in the AIMD simulation, and this directionality is not properly accounted for in the force field simulation. These results imply that care must be taken when modeling negatively charged oxygen atoms as single point charges; force field models may not adequately describe the hydration configuration and dynamics.

DOI： 10.1021/acs.jpcb.5b02579

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Publishing type：Research paper (scientific journal)  

Organic magnetoresistance (OMAR) can be caused by either single carrier (bipolaron) or double carriers (electron-hole)-based mechanisms. In order to consider applications for OMAR, it is important to control the mechanism present in the device. In this paper, we report the effect of traps on OMAR resulting of disorder at the interface between the organic active layer with the hole injection layer [poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate): PEDOT:PSS]. It has been found that while the single carriers OMAR is enhanced by the presence of traps, the double carriers OMAR is totally removed in a sample with a high interface trap density. The reasons for these results are discussed based on the impedance spectroscopy measurements. First, the mechanism (single or double carriers) responsible of the OMAR was determined with the support of the capacitance measurement. Then, the influence of traps was discussed with the Nyquist diagrams and phase angle-frequency plots of the samples. The results suggested that with a rough interface and thus high disorder, the presence of traps enhanced the bipolaron formation. Traps also acted as recombination centers for electron-hole pairs, which prevented the double carriers OMAR in devices with a rough interface. On the other hand, with a low trap density, i.e., with a smooth surface, the single carrier OMAR decreased, and double carriers OMAR appeared. The sign of the OMAR could then be controlled by simply sweeping the bias voltage. This work demonstrated that the roughness at the interface is important for controlling OMAR and its reproducibility, and that the combination of OMAR measurement and impedance spectroscopy is helpful for clarifying the processes at the interface.

DOI： 10.1063/1.4913272

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INDERSCIENCE ENTERPRISES LTD  

We investigated the organic magnetoresistance (OMAR) mechanism in poly(3-hexylthiophene-2,5-diyl) (P3HT)-based diodes, by using impedance spectroscopy. We have prepared layered structures consisting of indium tin oxide, poly(3,4-ethylenedioxythiophene): poly(styrenesulphonate) (PEDOT:PSS), P3HT and aluminium and measured the bias voltage dependence of the OMAR and of the impedance spectra. The capacitance deduced from the impedance data indicated that the OMAR was explained by the single-carrier (bipolaron) model at lower bias voltages as well as double-carrier models at higher bias voltages. The impedance response also suggested that the OMAR from the single-carrier model was governed by the PEDOT: PSS/P3HT interface whereas the double-carrier OMAR was mainly related to the P3HT layer.

DOI： 10.1504/IJNT.2015.067209

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

We examine the temperature dependence of the interfacial molecular structure at the water-air interface by combining experimental and simulated sum-frequency generation (SFG) spectroscopy. The experimental SFG spectra of the OH-stretching mode show a decrease in the amplitude at similar to 3300 cm(-1) with increasing temperature, while the 3700 cm(-1) 'free OH' SFG feature is insensitive to temperature changes. The simulated spectra are in excellent agreement with experiment. A comparison between interfacial SFG spectra and bulk infrared/Raman spectra reveals that the variation of the SFG signal due to the temperature change is not caused by a temperature-dependent OH bond orientation of the interfacial water molecules, but can be fully accounted for by the temperature dependence of the optical response of water. These results indicate that while the thickness of the interfacial region varies with temperature, the molecular organization of interfacial water at the water-air interface is surprisingly insensitive to temperature changes.

DOI： 10.1039/c5cp04022a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We have performed thermoelectric measurements of benzenedithiol (BDT) and C-60 molecules with Ni and Au electrodes using a home-built scanning tunneling microscope. The thermopower of C60 was negative for both Ni and Au electrodes, indicating the transport of carriers through the lowest unoccupied molecular orbital in both cases, as was expected from the work functions. On the other hand, the NiBDTNi junctions exhibited a negative thermopower, whereas the AuBDTAu junctions exhibited a positive thermopower. First-principle calculations revealed that the negative thermopower of NiBDTNi junctions is due to the spin-split hybridized states generated by the highest occupied molecular orbital of BDT coupled with s- and d-states of the Ni electrode.

DOI： 10.1021/nl502305e

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report molecular dynamics (MD) simulations of the water/clean rutile TiO2 (110) interface using polarizable and non-surface polarity force field models. The effect of surface polarity on the water dynamics near the TiO2(110) surface is addressed, specifically by calculating the water hydrogen bond and reorientational dynamics. The hydrogen bond lifetime of interfacial water molecules is several times longer than that of bulk water due to the strong water-TiO2 interactions. A comparison of the dynamics simulated with the polarizable and non-surface polarity models shows that, while the hydrogen bond lifetime between the interfacial water and TiO2 surface is insensitive to the surface polarity, the reorientational dynamics around this hydrogen bond axis is significantly influenced by the surface polarity; the surface polarity of the TiO2 increases the water-TiO2 interactions, stabilizing the local structure of the interfacial water molecules and restricting their rotational motion. This reorientation occurs predominantly by rotation around the O-H group hydrogen bonded to the TiO2 surface. Furthermore, we correlate the dynamics of the induced charge on the TiO2 surface with the interfacial water dynamics. Our results show that the timescale of correlations of the atom charges induced by the local electric field in bulk water is influenced by the rotational motion, hydrogen bond rearrangement and translational motion, while the induced charge dynamics of the TiO2 surface is governed primarily by the rotational dynamics of the interfacial water molecules. This study demonstrates that the solid surface polarity has a significant impact on the dynamics of water molecules near TiO2 surfaces.

DOI： 10.1088/0953-8984/26/24/244102

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged pi-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

DOI： 10.1021/ja413104g

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Superior long-range electric transport has been observed in several organometallic wires. Here, we discuss the role of the metal center in the electric transport and examine the possibility of high thermoelectric figure of merit (ZT) by controlling the quantum resonance effects. We examined a few metal center (and metal-free) terpyridine-based complexes by first-principles calculations and clarified the role of the metals in determining the transport properties. Quasi-resonant tunneling is mediated by organic compounds, and narrow overlapping resonance states are formed when d-electron metal centers are incorporated. Distinct length (L) and temperature (T) dependencies of thermopower from semiconductor materials or organic molecular junctions are presented in terms of atomistic calculations of ZT with and without considering the phonon thermal conductance. We present an alternative approach to obtain high ZT for molecular junctions by quantum effect.

DOI： 10.1021/ja407662m

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

A series of phenyl-capped cyclohexa[c]thiophene derivatives (nCHT-TEG, n = 2, 4, or 6) have been synthesized. nCHT-TEG are well-soluble in common organic solvents. The absorption spectra of neutral nCHT-TEG oligomers indicated a shorter effective conjugation length than conventional oligothiophenes based on the non-coplanarity of the thiophene rings. nCHT-TEG can be oxidized/reduced reversibly. The results of cyclic voltammetry and UV-Vis-NIR spectroscopy of the oxidized nCHT-TEG revealed that the effective conjugation length increases at the higher oxidation state. Density functional theory (DFT) calculations indicate that the quinoidal structure of the oxidized nCHT contributes to the improved effective conjugation length. nCHT-TEG radical cations and dications were characterized by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopies, respectively. NMR results of nCHT-TEG dication revealed that 2CHT-TEG(2+) have closed-shell bipolaron structure, while 4CHT-TEG(2+) and 6CHT-TEG(2+) are the mixture of closed-shell bipolaron and open-shell two-polaron structures. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.synthmet.2013.08.005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We study the transport properties of single-walled carbon nanotubes (SWCNTs) using the nonequilibrium Green's function method based on first-principles calculations. We compared three SWCNTs with different chiralities (3, 3), (5, 0), and (4, 2), and found that the thermal conductance varies significantly with the chirality, especially at low temperatures. Such differences are attributed to the dependence on the chirality of the frequency of the lowest optical mode and phonon-phonon interaction with the semi-infinite leads. To obtain accurate low-vibrational frequencies, a force constant correction based on the Lagrange undetermined multiplier method was employed. The phonon-phonon interaction was analyzed in terms of the projection of the phonon coupling with the semi-infinite leads onto the normal modes of the center region. Our result indicates that high optical mode frequency and weak phonon coupling on the armchair (3, 3) SWCNT are the origin of the long quantized plateau found in the experimental thermal conductance. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4816476

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

Melamine on Cu(001) is mechanically unstable under the current of a scanning tunneling microscope tip and can switch among configurations. However, these are not equally accessible, and the switching critical current depends on the bias polarity. In order to explain such rich phenomenology, we have developed a scheme to evaluate the evolution of the reaction paths and activation barriers as a function of bias, which is rooted in the nonequilibrium Green's function method implemented within density functional theory. This, combined with the calculation of the inelastic electron tunneling spectroscopy signal, allows us to identify the vibrational modes promoting the observed molecular conformational changes. Finally, once our ab initio results are used within a resonance model, we are able to explain the details of the switching behavior, such as its dependence on the bias polarity, and the noninteger power relation between the reaction rate constants and both the bias voltage and the electric current. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.87.205439

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We present a novel scheme to construct a polarizable force field for liquid/solid interfaces, which takes into account the effect of the surface polarity induced by liquid-solid interactions explicitly. We extend the charge response kernel (CRK) method for molecules to solid surfaces by introducing the surface CRK The CRK parameters are systematically determined by the first-principles calculations in the slab model with the dipole-correction method. Our methodology is applied to the water/clean rutile TiO2(110) interface. Structures and induced charges of a single water molecule attached to the TiO2 surface optimized by our polarizable force field show good agreement with those predicted by the first-principles calculations. Further, we carried out MD simulations for the liquid water/TiO2 interface and found three stable structures of water attached to the TiO2 surface. Two of them are predicted by both the polarizable and the nonpolarizable force fields, while the polarizable force field model predicts a structure of water with the hydrogen and oxygen atoms interacting with the oxygen atom of the surface TiO2 and the hydrogen atom of the other water molecule, respectively, which was reported by the previous first-principles MD simulation. This indicates that the dipole moments of water and TiO2 induced by the water-TiO2 interactions have significant impact on molecular conformations of the water/TiO2 interface.

DOI： 10.1021/ct300998z

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We performed first-principles transport calculations of the contact consisting of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) molecular layers and metal electrodes using the nonequilibrium Green's function method combined with the density functional theory. To analyze roles of organic/metal interfacial states for transport, we examined two kinds of electrodes: Ag(111) and Al(111). By quantitative evaluation of the coupling strength between PTCDA molecular orbitals and electrodes, we found the creation of the Shockley-type state at the interface of PTCDA and Ag(111). In contrast, the Al(111) surface formed a strong chemical bond with PTCDA. A clear Shockley-type state was not created, and an ohmic bias voltage (V) and electric current (I) behavior was found for contacts consisting of thin PTCDA layers and Al(111) electrodes. We also predicted that further stacking of PTCDA layers will make I-V characteristics more Schottky-like for both Ag and Al electrodes, regardless the different microscopic mechanism.

DOI： 10.1103/PhysRevB.84.045417

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We investigated the effect of anchoring group position on the formation and electric conductance of single molecule junctions for benzenedithiol and benzenediamine by the scanning tunneling microscope break junction technique. The conductances of the single 1,4-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenediamine, and 1,3-benzenediamine molecules were 0.005 (+/- 0.001) G(0) (G(0)=2e(2)/h), 0.004 (+/- 0.001) G(0), 0.01 (+/- 0.003) G(0), and 0.005 (+/- 0.002) G(0), respectively. No 1,2-disubstituted benzene molecules formed junctions. While the 1,4-position provided larger conductance than the 1,3-position for both anchoring groups, the effect of the anchoring position on conductance was clearer for benzenediamine than benzenedithiol. The resulting anchoring position and its stability are discussed in consideration of the formation of the single molecular junction. The relationship between conductance and anchoring group (position) was analyzed based on ab initio transport calculations. The deformation and change of the energy alignment of the "conductive" molecular orbital give clearer insight to the anchoring position effect than to quantum interference.

DOI： 10.1021/jp1095079

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We performed a first-principles density-functional calculation to investigate the difference in the mechanisms of phase transitions for In/Cu(001) and W(001). We also explain the origins of surface states of In, Pb, and Bi/Cu(001)-0.5 ML having c(2x2) structure that cause a phase transition via the generation of a charge-density wave (CDW). The perfect theoretical/experimental agreement for the wave numbers of nesting vectors corresponding to the well-nested surface states of In and Pb/Cu(001)-0.5 ML is reported. Band-structure analysis suggests a common mechanism of CDW transition for In and Pb/Cu(001)-0.5 ML. The absorption of In and Pb on Cu(001) leads to a reduction in the energy of the band composing the edge of bulk band gap. As a result, the band enters the bulk band gap and has the character of the surface state. Since these surface states compose a well-nested Fermi surface, CDW transition would be induced. In addition, we studied the relationship between the lattice distortions and the band dispersions of the surface states of In/Cu(001)-0.5 ML and W(001) in terms of the Jahn-Teller effect (JTE). While we find promoting modes that resolve the degeneracy of surface states of W(001), any surface-localized mode of In/Cu(001) does not split the band of surface state. Therefore, the static electron-phonon interaction of In/Cu(001) is weak. We propose that the dynamic JTE contributes to the CDW transition of In/Cu(001) and the dynamic nature is related to the spatial coherence length of the CDW.

DOI： 10.1103/PhysRevB.82.155415

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