Electron Localization Induced by Disordered Anions in an Organic Conductor.

We report on a new organic conductor κ″-(ET)2Cu[N(CN)2]Br (κ″-Br), which is the first polymorph of an organic superconductor κ-(ET)2Cu[N(CN)2]Br (κ-Br), where ET denotes bis(ethylenedithio)tetrathiafulvalene. κ″-Br has a similar κ-type arrangement of ET molecules to κ-Br, but, in contrast to the orthorhombic κ-Br, which has ordered polyanion chains, presents a monoclinic crystal structure with disordered polymeric anion chains. To elucidate the electronic state of κ″-Br, we performed band calculations as well as transport, magnetic, and optical measurements. The calculated band dispersion, magnitude of electron correlation, and room-temperature optical conductivity spectra of κ″-Br were comparable to those of κ-Br. Despite these similarities, the κ″-Br salt exhibited a semiconducting behavior. The electron spin resonance and Raman spectroscopies indicated that there is neither magnetic nor charge order in κ″-Br, suggesting the occurrence of Anderson localization due to disordered anion layers.

Stabilization of Interfacial Polarization and Induction of Polarization Hysteresis in Organic MISIM Devices.

Molecule-based ferroelectrics has attracted much attention because of its advantages, such as flexibility, light weight, and low environmental load. In the present work, we examined an organic metal|insulator|semiconductor|insulator|metal (MISIM) device structure to stabilize the interfacial polarization in the S layer and to induce polarization hysteresis even without bulk ferroelectrics. The MISIM devices with I = parylene C and S = TMB (=3,3',5,5'-tetramethylbenzidine)-TCNQ (=tetracyanoquinodimethane) exhibited hysteresis loops in the polarization-voltage (P-V) curves not only at room temperature but also over a wide temperature range down to 80 K. The presence of polarization hysteresis for MISIM devices was theoretically confirmed by an electrostatic model, which also explained the observed thickness dependence of the I layers on the P-V curves. Polarization hysteresis curves were also obtained in MISIM devices using typical organic semiconductors (ZnPc, C60, and TCNQ) as the S layer, demonstrating the versatility of the interfacial polarization mechanism.

Phase-transition-induced jumping, bending, and wriggling of single crystal nanofibers of coronene.

For decades, it has been reported that some organic crystals suddenly crack, break, or jump when they are heated from room temperature. Recently, such crystals have been intensively studied both in fundamental science and for high-speed mechanical device applications. According to these studies, the sudden crystal motions have been attributed to structural phase transitions induced by heating. Stress created by the phase transition is released through the sudden and rapid motion of the crystals. Here we report that single crystal nanofibers of coronene exhibit a new type of ultrafast motion when they are cooled from room temperature and subsequently heated to room temperature. The nanofibers make centimeter-scale jumps accompanied by surprisingly unique behaviors such as sharp bending and wriggling. We found that the motions are caused by a significantly fast structural phase transition between two polymorphs of coronene. A theoretical investigation revealed that the sudden force generated by the phase transition together with the nanoscale dimensions and elastic properties create dynamical instability in the nanofibers that results in the motions. Our finding demonstrates the novel mechanism that leads to ultrafast, large deformation of organic crystals.

Regular-triangle trimer and charge order preserving the Anderson condition in the pyrochlore structure of CsW2O6.

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report on a phase transition that preserves the cubic symmetry of the ß-pyrochlore oxide CsW2O6, where each of W 5d electrons are confined in regular-triangle W3 trimers. This trimer formation represents the self-organization of 5d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. An electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the strong correlations of the 5d electrons characteristic of the pyrochlore oxides are both likely to play important roles in this charge-orbital-spin coupled phenomenon.

Bis(ethylenedithio)tetrathiafulvalene Cation Radical Salts Composed of Nonuniform Silver(I) Complex Polyanions.

Rational control of the molecular arrangement in solids has been the subject of intense research for many years. In particular, the structural control of bis(ethylenedithio)tetrathiafulvalene (ET) radical cations has attracted special interest because of the primary effect on the electronic properties of the salts. In this study, we obtained the first ET cation radical salts formed with nonuniform silver(I) complex polyanions, which involve multiple kinds of openings in the anionic layer, by an electrocrystallization method. θ-(ET)2Ag2(CN)[N(CN)2]2 (1) with a θ-type ET packing motif contains double helical chains composed of AgN(CN)2, whereas α″-(ET)2Ag2(CN)(SCN)2 (2) with an α″-type ET packing motif contains zigzag ladders composed of AgSCN. Both silver(I)-based tube-like assemblies are connected to each other by a cyano group, affording nonuniform polyanionic structures. Although both salts show semiconducting behavior, there is a distinct difference in their spin geometry, with an S = 1/2 Heisenberg antiferromagnetic square lattice in 1, which is associated with charge disproportionation or dynamical charge fluctuation in the ET layers, and an S = 1/2 Heisenberg anisotropic triangular lattice in 2, which results in spin frustration in the ET layers. The ability of the nonuniform polymeric structures in the anionic layers to act as templates for various arrangements of ET radical cations is demonstrated.

Partial Substitution of Ag(I) for Cu(I) in Quantum Spin Liquid κ-(ET)2Cu2(CN)3, Where ET Is Bis(ethylenedithio)tetrathiafulvalene.

Three mixed crystals, κ-(ET)2Ag2 xCu2(1- x)(CN)3 [ET is bis(ethylenedithio)tetrathiafulvalene; 0.24 < x < 0.71] with a κ-type packing motif of face-to-face ET dimers, were obtained by electrocrystallization. Regardless of the composition, each ET dimer fits into a hexagonal anionic opening (i.e., key-on-hole packing) similar to its parent spin liquid candidate, κ-(ET)2Cu2(CN)3. X-ray diffraction and energy dispersive spectroscopy analyses revealed that Cu and Ag atoms are statistically disordered with a fairly homogeneous distribution in a crystal. A structural variation depending on x is responsible for the change in the calculated band parameters related to intermolecular interactions, electron correlations, and frustrations. A salt with nearly equimolar amounts of Ag and Cu ( x = 0.49) is semiconductive at ambient pressure and undergoes a Mott transition upon application of hydrostatic pressure. Along with the positive pressure dependence of the transition temperature, the temperature-independent amplitude of magnetic torque at low temperatures suggests that the insulating phase is a quantum spin liquid. Further application of pressure results in the appearance of a superconducting phase. Contrary to those of the parent salts, κ-(ET)2Cu2(CN)3 and κ-(ET)2Ag2(CN)3, the transition temperature increases as the pressure increases and eventually reaches 4.5 K at 1.65 GPa.

Isolation of Single-Wired Transition-Metal Monochalcogenides by Carbon Nanotubes.

The successful isolation of single layers from two-dimensional (2D) van der Waals (vdW)-layered materials has opened new frontiers in condensed matter physics and materials science. Their discovery and unique properties laid the foundation for exploring 1D counterparts. However, the isolation of 1D vdW-wired materials has thus far remained a challenge, and effective techniques are demanded. Here we report the facile synthesis of isolated transition-metal monochalcogenide MoTe nanowires by using carbon nanotubes (CNTs) as molds. Individual nanowires are perfectly separated by CNTs with a minimal interaction, enabling detailed characterization of the single wires. Transmission electron microscopy revealed unusual torsional motion of MoTe nanowires inside CNTs. Confinement of 1D vdW-wired materials to the nanotest tubes might open up possibilities for exploring unprecedented properties of the nanowires and their potential applications such as electromechanical switching devices.

Facile Synthetic Route to Atomically Thin Conductive Wires from Single-Species Molecules in One-Dimensionally Confined Space: Doped Conjugated Polymers inside Single-Walled Carbon Nanotubes.

A facile synthetic method for doped conjugated molecules by a heating process is demonstrated. Br-terminated terthiophene precursors are encapsulated in single-walled carbon nanotubes by a vapor-phase reaction, and additional heat treatment promotes the thermal condensation of the precursors. Transmission electron microscopy observations and optical measurements show the successful synthesis of sexithiophenes and their doping (oxidation) by Br dopants generated by the condensation reaction. This study provides a new strategy for the synthesis of the doped conjugated polymers from single-species molecules by only a heating process.

Coronene-based charge-transfer complexes.

Recent developments in the arena of charge-transfer complexes composed of the D 6h-symmetric polycyclic aromatic hydrocarbon, coronene, are highlighted with emphasis on the structural and physical properties of these complexes. Because of the dual electron-donating and -accepting abilities of coronene, this group involves structurally-defined four cation salts and three anion salts. The Jahn-Teller distortions and in-plane motion of coronene molecules in the solids, both of which are closely associated with the high symmetry of coronene molecules, and syntheses of clathrate-type complexes are also presented.

Conducting π Columns of Highly Symmetric Coronene, The Smallest Fragment of Graphene.

Coronene, which is the smallest D6h -symmetric polycyclic aromatic hydrocarbon, attracts particular attention as a basic component of electronic materials because it is the smallest fragment of graphene. However, carrier generation by physical methods, such as photo- or electric field-effect, has barely been studied, primarily because of the poor π-conduction pathway in pristine coronene solid. In this work we have developed unprecedented π-stacking columns of cationic coronene molecules by electrochemical hole-doping with polyoxometallate dianions. The face-to-face π-π interactions as well as the partially charged state lead to electrical conductivity at room temperature of up to 3 S cm(-1) , which is more than 10 orders of magnitude higher than that of pristine coronene solid. Additionally, the robust π-π interactions strongly suppress the in-plane rotation of the coronene molecules, which has allowed the first direct observation of the static Jahn-Teller distortion of cationic coronene molecules.

Optical freezing of charge motion in an organic conductor.

Dynamical localization, that is, reduction of the intersite electronic transfer integral t by an alternating electric field, E(ω), is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3 MV cm(-1) instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor α-(bis[ethylenedithio]-tetrathiafulvalene)(2)I(3). A large reflectivity change of >25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the ~10% reduction of t driven by the strong, high-frequency (ω ≧ t/h) electric field.

Measurement of the nonlinear conducting states of α-(BEDT-TTF)2I3 using electronic Raman scattering.

Nonlinear conducting states in a strongly correlated organic electronic system α-(BEDT-TTF)2I3 [BEDT-TTF=bis(ethylenedithio)-tetrathiafulvalene] are studied by Raman spectroscopy. Wide-range Raman spectra of nonlinear conducing states provide direct information about conducting properties through the electronic Raman process. A comparison between the behaviors of the electronic modes of BEDT-TTF layers and the vibrational mode of I3 molecules reveals the formation of nonequilibrium states in which only the electronic parts show the change of states. We obtained a spatial map of the conducting regions of the nonlinear conducting states by utilizing the electronic Raman intensity as a measure of the highly conducting states. A spatially inhomogeneous formation of nonlinear conducting states was observed.

Near-Infrared photoluminescence in the femtosecond time region in monolayer graphene on SiO2.

We investigate the dynamical properties of photoexcited carriers in a single monolayer of graphene at room temperature in air using femtosecond time-resolved luminescence spectroscopy. The luminescence kinetics are observed in the near-infrared region of 0.7-1.4 eV and analyzed based on the two-temperature model describing the cooling of thermalized carriers via the carrier-optical phonon interaction. The observed luminescence in the range 0.7-0.9 eV is well reproduced by the model. In the range 1.0-1.4 eV, however, the luminescence, which decays in ∼300 fs, cannot be reproduced by this model. These results indicate that the carrier system is not completely thermalized in ∼300 fs. We also show the importance of the carrier-doping effect induced by the substrate and surrounding environment in the carrier cooling dynamics and the predominance of optical phonons over acoustic phonons in the carrier-phonon interactions even at a temperature of ∼400 K.

Development of a robust model system of FRET using base surrogates tethering fluorophores for strict control of their position and orientation within DNA duplex.

Although distance dependence of Förster resonance energy transfer (FRET) is well-studied and FRET has been extensively applied as "molecular ruler", only limited examples of orientation-dependent FRET have been reported. To create a robust FRET system that precisely reflects the orientation between donor and acceptor, donor and acceptor fluorophores were introduced into a DNA via a D-threoninol scaffold. Strong stacking interactions among intercalated dyes and natural base-pairs suppress free movement of the dyes, clamping them in the duplex in a fixed orientation. Pyrene and perylene were used as donor and acceptor, respectively, and both the distance and orientation between these dyes were systematically controlled by varying the number of intervening AT pairs from 1 to 21 (corresponding to two turns of helix). FRET efficiency determined from static fluorescence measurement did not decrease linearly with the number (n) of inserted AT pairs but dropped significantly every 5 base pairs (i.e., n = 8, 13, and 18), corresponding to a half-turn of the B-type helix. This clearly demonstrates that FRET efficiency reflects the orientation between pyrene and perylene. We also measured time-resolved fluorescence spectroscopy with a streak camera and successfully observed the time-course of the energy transfer directly. As expected, the FRET efficiencies determined from the lifetime of pyrene emission were in good agreement with static measurements. Theoretical calculation of FRET efficiency assuming that the DNA duplex is a rigid cylinder with B-type geometry coincided with the experimental results. We believe that our method of using d-threoninol will contribute to further development of FRET-based measurement techniques.

pi-Conjugated multidonor/acceptor arrays of fullerene-cobaltadithiolene-tetrathiafulvalene: from synthesis and structure to electronic interactions.

The synthesis, structure, photoelectrochemical behavior, and nonlinear optical (NLO) properties of a symmetric acceptor-acceptor-donor-acceptor-acceptor array, C(60)-Co-TTF-Co-C(60), have been described. The precursors, namely, cobalt dicarbonyl complexes Co(C(60)Ar(5))(CO)(2) were synthesized from the penta(organo)[60]fullerenes, C(60)Ar(5)H, as starting materials. In the next step, two cobalt-fullerene complexes were connected to a tetrathiafulvalene (TTF) tetrathiolate bridge to obtain the C(60)-Co-TTF-Co-C(60) array. In addition, the monomeric compounds, Co(C(60)Ar(5))(S(2)C(2)R(2)) (R = CO(2)Me and CN) and Co(C(60)Ar(5))(S(2)C(2)S(2) C = CS(2)C(2)R(2)) were synthesized as references. The C(60)-Co-TTF-Co-C(60) array exhibits very strong transitions in the near-infrared region (lambda(max) = 1,100 nm, epsilon = 30 000 M(-1) x cm(-1)) due to a ligand-to-metal-charge-transfer (LMCT) transition and six reversible electron transfer processes. In the crystal, a fullerene/TTF-layered packing structure is evident. Femtosecond flash photolysis revealed that photoexcitation of the array results in a charge separated state involving the strongly interacting cobaltadithiolene and TTF constituents which electronically relax via a resonance effect that extends all throughout the acceptor parts of the C(60)-Co-TTF-Co-C(60) array. The third-order NLO measurement of the array gave the magnitude of the third-order nonlinear susceptibility, |chi((3))|, values to be 9.28 x 10(-12) esu, suggesting the pi-conjugation of donors and acceptors in the array.

Effect of an in-plane ligand on the electronic structures of bromo-bridged nano-wire Ni-Pd mixed-metal complexes, [Ni(1-x)Pd(x)(bn)2Br]Br2 (bn = 2S,3S-diaminobutane).

Single crystals of quasi-one-dimensional bromo-bridged Ni-Pd mixed-metal complexes with 2S,3S-diaminobutane (bn) as an in-plane ligand, [Ni(1-x)Pd(x)(bn)(2)Br]Br(2), were obtained by using an electrochemical oxidation method involving mixed methanol/2-propanol (1:1) solutions containing different ratios of [Ni(II)(bn)(2)]Br(2) and [Pd(II)(bn)(2)]Br(2). To investigate the competition between the electron-correlation of the Ni(III) states, or Mott-Hubbard states (MH), and the electron-phonon interaction of the Pd(II)-Pd(IV) mixed valence states, or charge-density-wave states (CDW), in the Ni-Pd mixed-metal compounds, X-ray structure analyses, X-ray oscillation photograph, and Raman, IR, ESR, and single-crystal reflectance spectra were analyzed. In addition, the local electronic structures of Ni-Pd mixed-metal single crystals were directly investigated by using scanning tunneling microscopy (STM) at room temperature and ambient pressure. The oxidation states of [Ni(1-x)Pd(x)(bn)(2)Br]Br(2) changed from a M(II)-M(IV) mixed valence state to a M(III) MH state at a critical mixing ratio (x(c)) of approximately 0.8, which is lower than that of [Ni(1-x)Pd(x)(chxn)(2)Br]Br(2) (chxn = 1R,2R-diaminocyclohexane) (x(c) approximately 0.9) reported previously. The lower value of x(c) for [Ni(1-x)Pd(x)(bn)(2)Br]Br(2) can be explained by the difference in their CDW dimensionalities because the three-dimensional CDW ordering in [Pd(bn)(2)Br]Br(2) observed by using X-ray diffuse scattering stabilizes the Pd(II)-Pd(IV) mixed valence state more than two-dimensional CDW ordering in [Pd(chxn)(2)Br]Br(2) does, which has been reported previously.

A black body absorber from vertically aligned single-walled carbon nanotubes.

Among all known materials, we found that a forest of vertically aligned single-walled carbon nanotubes behaves most similarly to a black body, a theoretical material that absorbs all incident light. A requirement for an object to behave as a black body is to perfectly absorb light of all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition. We found a material that can absorb light almost perfectly across a very wide spectral range (0.2-200 mum). We attribute this black body behavior to stem from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes.

Direct observation of dark excitons in micelle-wrapped single-wall carbon nanotubes.

We have performed electroabsorption spectroscopy on micelle-wrapped single-wall carbon nanotubes. In semiconducting nanotubes, many oscillating structures composed of the increase and decrease of absorption are observed in the spectra in the region of the first and second absorption bands, E11 and E22. The spectral shape is reproduced mainly by the second-derivative curve of the absorption spectrum, which indicates the presence of nearly degenerate bright and dark excitonic states.

Thermochromism in an organic crystal based on the coexistence of sigma- and pi-dimers.

Transition-metal complexes and organic radical molecules can be used to make electric conductors and ferromagnets, the optical properties of which can be controlled by changing temperature and are used as molecular switches and sensors. Whereas a number of organic radicals in solution show temperature-dependent optical properties, such behaviour in crystalline forms is more rare. Here, we show a fully reversible continuous thermochromism with a unique mechanism in purely organic crystals of diazaphenalenyl radical. This behaviour is based on changes in the diazaphenalenyl dimers coexisting in the crystal. From the X-ray crystal structure analyses and temperature-dependent visible spectra, we conclude the presence of a thermal equilibrium between sigma-bonded and pi-bonded dimers, which are separated by 2.62(6) kcal mol(-1). This conclusion is supported by room-temperature electron spin resonance spectra of the solid, which showed signals that are attributable to a thermally accessible triplet state of the pi-dimer structure. This proves the coexistence of two dimers of different bonding natures in the crystal, causing it to demonstrate thermometer-like behaviour.

Dynamical valence fluctuation at the charge-density-wave phase boundary in iodide-bridged Pt compound [Pt(chxn)2I]I2.

We synthesized a novel iodo-bridged linear chain platinum compound, having the quasi-two-dimensional charge-density-wave (CDW) ground state and the smallest band gap. In this compound, we discovered an anomalous valence state in the boundary region at which the CDW phase alternates in the crystal by means of ESR, X-ray diffuse scattering, STM, and electrical resistivity. This anomalous state can be explained by the fast fluctuation between Pt(IV)-I...Pt(II) and Pt(II)...I-Pt(IV) in the double well potential. This is the first observation of the dynamical fluctuation of the CDW phase among the quasi one-dimensional halogen-bridged complexes.