Masatoshi Sakai

Fabrication and Electrical Characterization of Tetrathiafulvalene-tetracyanoquinodimethane Molecular Wires

We have fabricated tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) molecular wires between a parallel cathode and anode (100 µm electrode gap) using the co-evaporation technique with an applied electric field. Wire-shaped microcrystals of TTF-TCNQ grew along the electric field. We have succeeded in fabricating connected wires with an optimum applied voltage. A narrow-gap (20 µm) electrode was used to grow wires with a low applied voltage, which make it possible to fabricate the connected wires easily. The measured I-V curve indicated Ohmic contact between the gold electrode and TTF-TCNQ. Semiconductor-like conduction of the fabricated TTF-TCNQ sample was shown by the temperature dependence of conductivity. Some of the co-evaporated TTF-TCNQ wires may tend to be semiconducting due to nonstoichiometry of TTF1-δ-TCNQ1.

LUMINESCENCE DUE TO INTRA- AND INTER-MOLECULAR TRANSITION IN C70 SINGLE CRYSTALS

Abstract We have investigated absorption and luminescence spectra and temporal behaviors of luminescence in solutions and solids of C 70 . The luminescence bands observed for C 70 in solution originate from the S 1 –S 0 transition. The luminescence decay kinetics of C 70 single crystal exhibits two-component decay with decay times of ∼400 ps and ∼1.4 ns. The fast component is assigned to the intra-molecular S 1 –S 0 transition, and the slow one is attributable to the self-trapped state of charge-transfer exciton. It is found that there are two relaxation channels of photoexcitations in C 70 crystals: intra-molecular and inter-molecular relaxations.

Organic nanochannel field-effect transistor with organic conductive wires

The authors fabricated an organic nanochannel field-effect transistor (FET) that is self-wired with highly conductive organic conductors. The advantages of the transistor are a short channel (approximately 400nm in length) and spontaneous formation of an active layer of the FET. Further, in principle, the carrier-injection barrier is absent at the interface of the organic metal and organic semiconductor. Thus, the transistor is highly conductive despite the narrow cross section of the channel. The FET characteristics of the nanochannel transistor exhibit the n-channel enhancement mode behavior.

Raman scattering study of photopolymerization kinetics in C60 crystals

Abstract Photopolymerization kinetics of C 60 crystals has been investigated by means of Raman scattering measurements changing sample temperature, laser irradiation intensity and time. It is found that the observed Raman spectrum is composed of monomer, dimer and higher photoproduct components. The dimer is formed as an intermediate phase during the course of the photopolymerization process. The variation of Raman intensity with laser irradiation time has been well explained by a kinetic model taking into account photocreation and thermal decomposition of monomers, dimers and trimers. Photochemical reaction coefficients of dimers and trimers have been obtained from the kinetic model analysis.

Visible effects of static electric field on physical vapor growth of lead phthalocyanine crystals

To elucidate the effects of a static electric field on the crystal growth of a molecule with both a molecular dipole and a quadrupole moment, we performed physical vapor growth of monoclinic lead phthalocyanine (PbPc) crystals under quasithermal equilibrium conditions and observed distinct effects of the applied electric field on the drift motion of the molecules that diffused over the substrate surface. The density of crystals grown on the Au electrode with a high electric potential exceeds the crystal density on both an electrically grounded electrode and a glass substrate surface. In addition, this difference in crystal density increases with applied voltage. This biased distribution of crystal growth is explained by the drift motion of diffusing molecules, which is induced by the interaction of the electric field with molecular dipole and quadrupole moments. However, the long-range oriented growth that is clearly seen in the physical vapor growth with an electric field for copper phthalocyanine is not...

Luminescence properties and relaxation dynamics of photoexcited states in C60 solids

Abstract We report huminescence spectra and their temporal behaviors for C 60 single crystals. Two types of luminescence bands are observed. The origin of weak luminescence band can be assigned to the intramolecular HOMO—LUMO transition. We discuss the origin of the main luminescence band connecting with a self-trapped state of a charge-transfer exciton.

Ferroelectriclike dielectric response and metal-insulator transition in organic Mott insulator-gate insulator interface

A ferroelectriclike dielectric response was observed in a field-effect transistor using oriented bis(ethylenedithio)tetrathiafulvalene-tetracyanoquinodimethane crystals. Phase transitions at 285 and 320 K were clearly observed in the temperature dependence of field-effect electron mobility. The phase transition at 320 K corresponds to the metal-insulator transition previously reported in a bulk crystal. On the other hand, the field-effect electron and hole mobilities exhibited an abrupt increase at 285 K, which had not been discovered by other physical measurements in the bulk crystal and is nonetheless sufficiently stable and reproducible. In addition, the abrupt increase in carrier mobilities was clearly correlated with the decrease in the dielectric response. The temperature variation in difference hysteresis curves demonstrated the feature of ferroelectric transition.

Spontaneous Activation Process for Self-aligned Organic Nanochannel Transistors

A spontaneous activation method for self-aligned organic nanochannel transistors with organic conductive wires was developed. With the continuous application of an alternating electrical current through connected organic wires that did not exhibit field-effect transistor characteristics, n-channel field-effect transistor characteristics appeared with a decrease in the electrical conductance of the wires. In principle, this process spontaneously ceased without breaking the organic wires. The yield rate of self-aligned nanotransistors should be effectively improved by using this technique.

PHOTOPOLYMERIZATION AND THERMAL DECOMPOSITION OF POLYMERIZED PHASE IN C60 CRYSTALS UNDER STRONG LASER ILLUMINATION

We have investigated the photopolymerization and thermal decomposition of photochemical products with high density excitation (11–480 mW/mm2) by means of time-gated Raman scattering experiments in C60 single crystal. The temperature of laser-illuminated area was measured by Stokes and anti-Stokes Raman scattering of C60 Hg(1) mode, which shows the local temperature rise with increase of laser power density. The Raman intensity of Ag(2) mode rapidly decreases with irradiation time for I<170 mW/mm2 indicating the rapid decrease in C60 monomer density due to photopolymerization. For higher power densities, however, the Raman intensity increases after showing a minimum, which suggests a dissociation of photopolymers. The results are well explained by the rate equation model taking into account photochemical generation and thermal decomposition of photo-produced dimers. These results indicate the strong laser illumination simultaneously induces the photopolymerization and thermal decomposition.

Deformation potentials of C60 and C70 crystals in low temperature frozen phase studied by photoluminescence spectroscopy under high pressure

We have investigated variation of luminescence spectra with hydrostatic pressure up to 2.3 GPa for C60 and C70 crystals in the frozen phase at 77 K. Red shifts of luminescence peaks are observed for both crystals, and the pressure coefficients are obtained to be 20.059 and 20.070 eV/GPa for C60 and C70 crystals, respectively. The estimated deformation potentials of C60 and C70 crystals in the low temperature frozen phase are 0.78 and 0.90 eV, respectively. q 2001 Elsevier Science Ltd. All rights reserved.