Synthesis, characterization, and design of molecular materials, especially molecular conductors (including superconductors ), have been undertaken. Molecular conductors exhibit a variety of physical properties which can be systematically understood on the basis of "simple" and "clear" electronic structures. From a chemical point of view, the most fascinating character of the molecular conductor is its "designability", that is, we can finely control solid state properties with chemical modifications of the molecule. The newly synthesized materials are characterized by the X-ray diffraction method and physical measurements (electrical conductivity...etc.). The electronic structure is investigated by the simple band structure calculation. All these results are devoted to the design of new molecular materials.
- Magnetic susceptibility measurements of the Pd(dmit)2 salts under pressure
- Mott metal-insulator transition and the field-induced transition in a quantum spin solid with a triangular lattice, EtMe3P[Pd(dmit)2]2
- Supramolecular Ni(dmit)2 system with multifunctional molecular π-electrons
- Development of molecular conductors based on novel metal dithiolene complexes
(1) Physical properties of molecular conductors based on unsymmetrical square planar metal dithiolene complex [(ppy)AuIII(C8H4S8)] (ppy- = C-deprotonated-2- phenylpyridine)
(2) Development of Molecular Conductors Based on Multimetallic Dithiolene complexes - Transport property of massless Dirac fermions state in organic conductors
- Temperature dependence of the electrical resistivity for the organic superconductor under the anisotropic extension
- Charge disproportionate state around the phase boundaries among the insulator, superconductor, and metallic phases of the weakly dimerized (or non-dimerized) 3/4-filled and 2/3-filled organic molecular conductors
- Frustration of site charges in dmit complexes
- Research on molecular conductors using photoemission spectroscopy
- Development of supramolecular nanowire
- Nano-/micro-crystal formation of molecular conductors on silicon substrate
- Development of novel organic semiconductors for organic field effect transistors (OFETs)
(1) Liquid crystalline oligothiophenes
(2) Thienyl-substituted pyrenes - Anisotropy of nonlinear optical responses in organic molecules and semiconductor nanocrystals
- Application of laser ablation to the fullerene science
- Magnetic susceptibility measurements of the Pd(dmit)2 salts under pressure
- Mott metal-insulator transition and the field-induced transition in a quantum spin solid with a triangular lattice, EtMe3P[Pd(dmit)2]2
- Supramolecular Ni(dmit)2 system with multifunctional molecular π-electrons
- Development of molecular conductors based on novel metal dithiolene complexes
- Transport property of massless Dirac fermion state in organic conductors
- Temperature dependence of the electrical resistivity for the organic superconductor under the anisotropic extension
- Charge disproportionate state around the phase boundaries among the insulator, superconductor, and metallic phases of the weakly dimerized (or non-dimerized) 3/4-filled and 2/3-filled organic molecular conductors
- Frustration of site charges in dmit complexes
- Research on molecular conductors using photoemission spectroscopy
- Development of supramolecular nanowire
- Nano-/micro-crystal formation of molecular conductors on silicon substrate
- Development of novel organic semiconductors for organic field effect transistors (OFETs)
- Anisotropy of nonlinear optical responses in organic molecules and semiconductor nanocrystals
- Application of laser ablation to the fullerene science
Magnetic susceptibility measurement under pressure is one of the frontiers in the research field of molecular conductors. We have applied this technique to the pressure-induced phenomena observed in Pd(dmit)2 salts. Most of them are Mott insulators under ambient pressure, and typically undergo changes into metallic state. We could observe little change in susceptibility through the insulator-to-metal transition, which indicates the strong correlation operating even in the pressure-induced metallic state. For EtMe3P[Pd(dmit)2]2 (P21/m phase), which shows pressure-induced superconductivity in the vicinity of the spin-gapped insulating phase, we have carried out magnetic characterization of the superconducting phase using a single crystal. A P-T phase diagram has been obtained. Pressure dependence of the complete charge separation in the Et2Me2Sb salt has been also examined.
In the spin-1/2 triangular-lattice Mott insulator EtMe3P[Pd(dmit)2]2 (P21/m phase), the translational symmetry breaking with a structural distortion makes a spin-singlet ground state with a finite spin-excitation gap. The spin state is called "quantum spin solid" (or valence bond solid) in contrast to quantum spin liquid (or resonating valence bond) without any symmetry breaking. The quantum spin solid in two dimensional system is known in an orthogonal dimer system SrCu2(BO3)2 and a purple pigment BaCuSi2O6 invented in ancient China, while the Mott transition and the superconductivity emerging from the quantum state have been longed for a long time in solid state physics. Our electric resistivity and magnetoresistivity measurements under finely-tuned pressures give evidence that the quantum spin solid neighbors the metallic or superconducting state across the first-order Mott transition line in EtMe3P[Pd(dmit)2]2.
We have constructed a pressure-temperature phase diagram of the Mott transition in EtMe3P[Pd(dmit)2]2, based on the resistivity measurements under hydrostatic pressures. The spin-dimerization transition, which had been detected in static susceptibility and X-ray diffraction measurements, was observed as an increase in the activation energy obtained from the temperature dependence of resistivity. The transition is gradually suppressed with increasing pressure and meets the Mott transition around 4 kbar. At the pressure, once the resistivity settles into a metallic state, it jumps into insulating state again at low temperature. It suggests an occurrence of the first-order reentrant Mott transition from the (Fermi-liquid) metal to the spin-gapped insulator with the smaller spin entropy. In fact, the Mott transition was suppressed by application of the external field and disappeared at 10 T. We also observed a field-induced Mott transition from the insulator to the metal. The results indicate that the Zeeman splitting energy overcomes the spin-singlet formation and stabilizes the metallic state. These results demonstrate that the quantum spin solid persists until the Mott transition and the superconducting phase appear under pressure.
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Ni(dmit)2 molecules can associated with halogenated pyridinium cataions to form supramolecules. We have found two kinds of the supramolecular Ni(dmit)2 anion radical system combined with dihalopyridinium cations (Me-3,5-DIP = N-methyl-3,5-diiodopyridinium and Me-3,5-BIP = N-methyl-3,5-bromoiodopyridinium). This system has two crystallographically independent layers of Ni(dmit)2 anions (Layers I and II), which has localized spins and conducting electrons, respectively. The measurements of electrical resistivity and magnetic susceptibility revealed that both conducting electrons and localized spins with antiferromagnetic interactions coexist in the two salts down to 4.2 K. These properties are solely derived from molecular π-electrons of Ni(dmit)2 anions contained separately in each layer. However, the magnetic properties below 10K are different. Me-3,5-DIP salt shows the antiferromagnetic transition, whereas free-spin-like behavior is observed in Me-3,5-BIP salt. This suggests the possibility that the chemical modification could control their physical properties.
(1) Physical properties of molecular conductors based on unsymmetrical square planar metal dithiolene complex [(ppy)AuIII(C8H4S8)] (ppy- = C-deprotonated-2- phenylpyridine)
We have studied the electronic properties of the cation radical salts derived from organometallic mixed-ligand complexes, [(ppy)Au(C8H4S8)] (ppy- = C-dehydro-2- phenylpyridine(-); C8H4S82- = 2-{(4,5-ethylenedithio)-1,3-dithiole-2-ylidene}-1,3-dihtiole- 4,5-dithiolate(2-)) with Au(III)-C ρ-bond. A 2:1 salt [(ppy)Au(C8H4S8)]2[PF6] is a semiconductor under ambient pressure (ρr.t. = 2.6 Ωcm, Ea = 0.03 eV). Magnetic measurements show it is a Mott insulator close to the metal-insulator boundary. In fact, it becomes metallic by pressure (0.8 GPa < P < 1.6 GPa).
(2)Development of molecular conductors based on multimetallic dithiolene complexes
The development of multimetallic complexes with tto bridging ligands has led to the preparation of novel trimetallic nickel complexes. Our synthesis has yielded crystals of conducting trimetallic complexes (ρr.t. = 1.0 ×104 Ωcm，Ea = 0.28 eV). These new trimetallic complexes are good candidates for single-component molecular metals: the calculated HOMO-LUMO gap of 2 is very small (0.15 eV), and the HOMO and the LUMO of 2 has highly symmetry.
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Zero-gap state with the Dirac cone type energy dispersion has been found in an organic conductor α-(BEDT-TTF)2I3 under high hydrostatic pressures. This is the first two-dimensional zero-gap state discovered in bulk crystals with layered structure. In contrast to graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermions with anisotropic Fermi velocity. From the galvanomagnetic measurements, the density and mobilities of electrons and holes were determined in the temperature region between 77 K and 2 K. In this region, the carrier density (n) depends on temperature (T) as n ∝ T2 and decreases by about four orders of magnitude. On the other hand, the sheet resistance per BEDT-TTF layer (RS) stays almost constant in the region. The value is written as RS=gh/e2 in terms of the quantum resistance h/e2=25.8 kΩ, where g is a parameter that depends weakly on temperature.
There have been few efforts to give the anisotropic extension for the molecular conductors since most of the molecular conductors are fragile. Based on the instruments for the anisotropic extension we had previously developed, we have developed the instrument for the temperature dependence of the electrical resistivity under the anisotropic extension, magnetic field, and low temperature. Newly developed instrument is applied to β-(BEDT-TTF)4[Ga(ox)3H2O]PhNO2, which is the organic superconductor under the ambient pressure. The temperature of the insulator-superconducting transition is increased with increasing anisotropic extension parallel to the highly conducting direction. This result indicates that the increase in the transition temperature is attributed to the increase in the inter-molecular Coulomb repulsion as well as the decrease in transfer integral. In order to elucidate our conjecture, we have conducted further investigation, which is described in the following section.
We are convinced that our experimental result is a pioneer of a new field in Condensed Matter Physics, named "Physical properties of molecular crystal under the negative pressure". [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene, ox=oxalate anion, PhNO2=nitrobenzene]
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There has been some theoretical interest in the insulating state for weakly dimerized (or non-dimerized) 3/4-filled molecular conductors from the viewpoint of the nearest neighbor Coulomb repulsion. On the other hand, there is great lack of theoretical and experimental study for the weakly dimerized molecular conductors around the phase boundary among the insulator, superconductor, and metallic phases. We have studied the role of the nearest neighbor Coulomb repulsion of the β-type BEDT-TTF salts including a superconductor.
We can expect that the molecular charges more or less deviate from a uniform value owing to the nearest neighbor Coulomb repulsion. The vibrational spectra is a powerful method to investigate the fluctuation (frustration) of the site charges in the weakly localized state. The insulator phase is the charge ordered state whose molecular charge largely deviates from the uniform value. The time averaged molecular charges approaches to the uniform value for the metallic compounds. Interestingly, the molecular charges just above the insulator-superconductor transition temperature exhibit an intermediate behavior.
The degree of frustration is determined from degeneracy of the distribution of the site charges rather than the magnitude of the nearest neighbor Coulomb repulsion. The conducting behavior of the β-type BEDT-TTF salts is mapped with the Temperature-Δρ phase diagram where Δρ is the degree of frustration in the molecular charges. In order to confirm our phase diagram, we have measured pressure dependence of the vibrational spectra for the 2/3-filled organic molecular conductors, which exhibits the insulator-superconductor-metal transition under the hydrostatic pressure. As expected from our phase diagram, the Δρ is reduced with increasing pressure.
Most of the dmit complexes, which undergo superconductor phase under pressure, are the antiferromagnetic insulator at the ambient pressure. This observation is interpreted from the viewpoint of the 1/2-filled Mott insulator. On the other hand, some newly synthesized dmit complexes are non-magnetic insulator at the ambient pressure although the electrical resistivity exhibits the superconducting behavior under pressure. In analogy to the β''-type BEDT-TTF salts, we have thought that the frustration of the site charges contributes to the conducting behavior of the non-magnetic dmit complexes. We have measured the vibrational spectra in the non-magnetic phase of some dmit complexes. Actually, the frustration of the site charges is observed for the non-magnetic phase which neighbors with the superconducting phase. [dmit=1,3-Dithiol-2-thione-4,5-dithiolate]
It is well known that the variety of properties that conducting materials display stems from the electronic structure especially near Fermi level, therefore the electronic structure is very important for designing/applying materials. However, there is less research of electronic structure on molecular conductors that expected for applying owing to the difficulty of measurements and so on. Direct observation of electronic structures and comparison with the band calculation and getting a guide to calculate more in detail/accurately would make great progresses on designing new molecular conductors. To study electronic structure of two-dimensional molecular conductors, we have performed photoemission spectroscopy and angel resolved photoemission spectroscopy using He discharge lamp (HeIα: 21.218eV) and newly developed laser (6.994 eV) which gives less radiation damage to samples as photon sources on (BEDT-TTF)3Br(pBIB) which has small correlation and shows quite good metalicity.
Consequently, in laser photoemission measurements, we have been succeeded in getting ARPE spectra and clear Fermi edge for the first time in the world. And also we have succeeded in observe band dispersions in He lamp measurements. Accordingly, comparison between observed electronic structures and band calculation become to be possible.
Compared with tight-binding calculation, the band dispersion shows a good agreement, that implies that the tight-binding calculation is valid for estimating 1stπ band structure which composes Fermi surface.
Also it becomes possible to verify whole valence band structure to compare with first-principal calculation by Dr. Tsuyosi Miyazaki. It shows good agreement in the formation of the band dispersions, but the energy scale seems slightly under estimated. These results can attribute to the progress of first-principal calculation method, which is quite important for designing materials.
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The crystal and electronic structures for six supramolecular nanowire crystals, (EDT-TTF)4BrI2(TIE)5, (EDST)4I3(TIE)5, (MDT-TTF)4BrI2(TIE)5, (HMTSF)2Cl2(TIE)3, (PT)2Cl(DFBIB)2, and (TSF)Cl(HFTIEB), were analyzed and the necessary conditions for the nanowire structure formation are deduced. The firmness of the supramolecular assembly that allows the channel structure originates from the directionality of the Lewis acid/base interaction between neutral iodine and halide anions. The flexibility in both the coordination numbers of halide anions and the angles that acid/base interactions around these anions can adopt is, on the other hand, also important for the generation of the supramolecular insulating cover. At the same time, structural compatibility between the donor molecules and the supramolecular assembly is crucial for the crystal formation. The band calculation and resistance-array calculation were also carried out. (EDT-TTF = ethylenedithiotetrathiafulvalene, TIE = tetraiodoethylene, EDST = ethylenedithiodiselenadithiafulvalene, MDT-TTF = methylenedithiotetrathiafulvalene, HMTSF = hexamethylenetetraselenafulvalene, PT = bis(propylenedithio)tetrathiafulvalene, DFBIB = 1,4-difluoro-2,5-bis(iodoethynyl)benzene, TSF = tetraselenafulvalene, HFTIEB = 1,1',3,3',5,5'-hexafluoro-2,2',4,4'-tris(iodoethynyl)-biphenyl)
Nano-/Micro-crystals of molecular conductors were directly formed on SiO2/Si substrates. (DMe-DCNQI)2Ag single crystal attached to a gold electrode exhibited rectifying switch behavior, which is a new type of bistability. Four-probe resistivity measurement was also performed on (DMe-DCNQI)2Ag single crystal. The M-I transition formally observed in two-probe measurement was confirmed.
α-(BEDT-TTF)2I3 single crystal was also grown on SiO2/Si substrates. The critical temperature for M-I transition which is known to occur at 135 K in bulk phase shifted to 150 K for the micro-crystal on the substrate. The crystal also worked as field effect transistor. The efficiency of the transistor reached maximum at approximately 80 K.
The control parameters for the position, size, and direction of the crystal growth were surveyed. The direction, polarity, and strength of the electrochemical current were important parameters. (DMe-DCNQI = 2,5-Dimethyl-N,N'-Dicyanobenzoquinonediimine, BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene)
We have prepared new organic semiconductors and investigated their FET performance, in which we focus on the molecular order in their thin films or single crystals distinguished from inorganic materials like amorphous silicon.
(1) Liquid crystalline oligothiophenes
An organic OFET, showing a mobility 10-2 cm2/Vs and on/off ratio 104, which are comparable to those of a dry-processed OFET of oligothiophenes, has been built by simple drop cast using a solution of a newly synthesized quaterthiophenes-based liquid crystalline molecule. X-ray diffraction patterns of thin film indicate well-defined molecular layers, and these layers would come from their liquid crystalline orders.
(2) Thienyl-substituted pyrenes
We have directly evaluated the FET performance associated with molecular packing by using single-crystal based device. The estimation of the orbital overlap integral provides an implication of minimum overlap integral of 10-2 eV required for the detection of FET characteristics by carrier hopping mechanism.
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Microscopic molecular symmetry and macroscopic molecular orientation affect nonlinear optical responses and their anisotropy. Molecular orientation is also affected by the molecular symmetry. In this fiscal/academic year, first-order electroabsorption spectroscopy was applied to investigate molecular orientation of vacuum-deposited tris(8-hydroxyquinolinato) aluminum(III) (Alq3) thin films which show spontaneous buildup of giant surface potential (GSP) as high as 28 V at 560 nm thickness. Dependence on film deposition method was investigated, and it was found that vaccum-evaporated films present noncentrosymmetric molecular orientation and that films deposited by wet methods (spin coating or casting method) do not present noncentrosymmetry. In the vacuum-evaporated films, evaporation conditions (whether light was irradiated or not during deposition, and deposition rate) and postprocessing (thermal treatment) did not affect noncentrosymmetry of the films. These results indicate that the vacuum evaporation process plays a dominant roll for formation of noncentrosymmetric molecular orientation.
By femtosecond laser ablation (fsLA), molecules are easily dissociated into atomic ions. Ionization and fragmentation of solid C60 dispersed on a silicon plate are investigated by fsLA. Bimodal mass distribution with large fragment ions C60-2n+ (0 ≤ n ≤ 11) and small fragment ions Cn+ (13 ≤ n ≤ 28), formation of dimer ion (C60)2+, and delayed ionization of C60 have been observed. Metastable dissociation of small fragment ions Cn+ has been observed for the first time, which suggests different structures of fragment ions compared with those of well-studied carbon cluster ions. From these observations, strong coupling of laser energy to electronic degrees of freedom of solid C60 has been revealed for fsLA as compared with excitation in the gas phase.
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