Jun Onoe

Structural and electrical properties of an electron-beam-irradiated C60 film

The structural and electrical properties of an electron-beam (EB)-irradiated C60 film have been examined at room temperature, using in situ infrared (IR) spectroscopy and ex situ four-probe measurements. IR results show that the irradiated film is neither graphite nor carbon nanotube-like but a peanut-shaped C60 polymer. Current–voltage curve shows that the polymer exhibits a metallic property with a drastically reduced resistivity of 7 Ω cm in comparison with 108–1014 Ω cm for solid C60. This indicates the possibility of applying C60 molecules in EB nanofabrication processes and large potential for developing carbon-based nanodevices.

Observation of Riemannian geometric effects on electronic states

Einstein first applied Riemannian geometry to develop the general theory of relativity almost one hundred years ago and succeeded in understanding astronomical-scale phenomena such as the straining of time-space by a gravitational field. Whether or not Riemannian space affects the electronic properties of condensed matters on a much smaller scale is of great interest. Although Riemannian geometry has been applied to quantum mechanics since the 1950s, nobody has yet answered this question, because the electronic properties of materials with Riemannian geometry have not been examined experimentally. We report here the first observation of Riemannian geometrical effects on the electronic properties of materials such as Tomononaga-Luttinger liquids, which were previously theoretically predicted by our group. We present in situ high-resolution ultraviolet photoemission spectra of a one-dimensional metallic C60 polymer with an uneven periodic peanut-shaped structure.

Relativistic effects on the electronic structure and chemical bonding of UF6

We have studied the relativistic effects in the electronic structure and chemical bonding for the ground state of UF6, using the relativistic and nonrelativistic discrete‐variational Xα molecular orbital calculations. It is found that two relativistic effects appear in the valence levels; the energy level splitting and upward shift of energies of the molecular orbitals. From the Mulliken population analysis of the valence levels, it is shown that the level splitting is due to mixing of the uranium atomic orbitals with a strong spin–orbit interaction, such as U6p, and the upward shift due to the increase in the screening of the nuclear charge and charge redistribution. The strength of U–F bonding remarkably increases for the relativistic case, because the changes in the radial distributions due to the relativistic effects induce both the decrease in the antibonding interactions and the increase in the bonding ones.

PHOTOINDUCED PRODUCTS IN A C60 MONOLAYER ON

Irradiation of ultraviolet-visible light onto a C60 monolayer on resulted in a chemical reaction between adjacent C60 molecules, as confirmed by scanning tunneling microscopy (STM) images. The major photoinduced products were double-bonded (two covalent C–C bonds between molecules) C60 dimers and linear trimers. We also found single-bonded (single covalent bond between molecules) C60 dimers and linear trimers, and two-dimensional C60 oligomers as minor products. Observed atomic geometry of the nonbonding contact between a C60 dimer (or a trimer) and a neighboring dimer (or a trimer), which are parallel to each other regarding their axes, is different from that reported between C60 molecules in three-dimensional C60 crystals, where there is no interaction with a substrate.

{\rm Si}(111) (\sqrt{3}\times \sqrt{3})\hbox{-}{\rm Ag}

Time-resolved temperature-dependent transmission changes were measured for both pristine C60 and electron-beam-irradiated C60 polymer films using an optical pump-probe technique. Only the signals obtained for the C60 polymer show a temperature-dependent slow decay, which appears in the low temperature region below ∼60K. This slow decay component exhibits a monotonic increase in both relaxation time and amplitude with decreasing temperature, providing evidence of gap formation associated with ordering fluctuations.

: AN STM STUDY

We investigated the electrical properties of a two-dimensionally (2D) hexagonal C60 photopolymer, using four-probe measurements in air at room temperature. The current–voltage curve shows that the photopolymer exhibits nondoped semiconducting behavior. This behavior is consistent with the theoretical prediction that a 2D hexagonal C60 polymer with a crosslinkage of a [2+2] cycloadditional four-member ring between adjacent C60 molecules is semiconducting [S. Okada and S. Saito, Phys. Rev. B 59, 1930 (1999)]. The origin of these semiconducting properties is discussed on the basis of density-functional calculations for the valence molecular orbitals of the dumbbell C120, which is regarded as a basic unit of the [2+2] crosslinkage of the 2D photopolymer.

Femtosecond carrier dynamics in electron-beam-irradiated C60 film

The reaction between C60 molecules in a 3-keV electron-irradiated C60 thin film has been studied using in situ Fourier-transform infrared (FTIR) spectroscopy and ex situ laser-desorption FT-mass spectrometry (LD-FTMS). The results of FTIR and LD-FTMS measurements indicate that coalesced C120 species were formed as a main product in the electron-beam (EB)-irradiated film. We studied the kinetics of the reaction by examining the decrease in the amount of C60 molecules as a function of irradiation time and found that the reaction rate exhibits a linear dependence on the reactant concentration and a third-power dependence on the electron dose rate.

Electrical properties of a two-dimensionally hexagonal C60 photopolymer

Relativistic effects on covalent bonding, in particular the role of individual valence atomic orbitals, have been investigated for diatomic (AuH and Pb 2 ) and hexafluoride (XF 6 : X=S, Se, Mo, Ru, Rh, Te, W, Re, Os, Ir, Pt, Po, Np, U and Pu) molecules, by analysis of bond overlap population using both nonrelativistic and relativistic DV-Xα molecular orbital methods. The contributions of valence atomic orbitals to the relativistic effects on covalent bonding for the molecules are clarified. The present approach is applied to interpret the physical picture of the relativistic effects on bond length of the AuH molecule. The origin of the relativistic bond-length contraction is discussed in comparison with previously reported results obtained by bonding energy calculations.

In Situ Fourier-Transform Infrared Study of Electron-Irradiation-Induced Reaction in a C60 Film

Abstract Relativistic molecular orbital calculations of the valence electronic structure of UF 6 were done using the discrete-variational Dirac-Slater method (DV-DS). Theoretical ionization energies of the UF 6 valence levels were evaluated by means of Slaters transition-state method. The ionization energies and relative X-ray photoelectron spectrum intensities of UF 6 valence levels calculated in the present work are in good agreement with the experimental X-ray photoelectron spectrum.

Relativistic Effects on Covalent Bonding: Role of Individual Valence Atomic Orbitals

The correlation between open-circuit voltage (VOC) and built-in potential (Vbi) upon photo-irradiation in the vicinity of the donor/acceptor (D/A) interface in zinc-octaethylporphyrin [Zn(OEP)]/C60 heterojunction photovoltaic (OPV) cells is investigated by examining the capacitance–voltage (C–V) characteristics. Charge accumulation of photogenerated carriers in the vicinity of the D/A interface is found to enlarge Vbi of both D [Zn(OEP)] and A (C60) films in the vicinity of the interface. In addition, it is noted that VOC is in good agreement with the sum of Vbi of each film in the vicinity of the D/A interface. This suggests that charge accumulation of photogenerated carriers in the vicinity of the D/A interface plays a key role in determining VOC for solar cells.