Toshiki Hara

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.

Electrical properties of a two-dimensionally hexagonal C60 photopolymer

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.

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

Field emission from an electron-beam irradiated C60 film

The field-emission characteristics of an electron-beam (EB) irradiated C60 film have been investigated. A C60 film on a stainless-steel substrate was irradiated by a 3 keV EB primarily for 10 h in an ultrahigh vacuum and subsequently for 10 h under a hydrogen atmosphere in order to induce a coalescence reaction between the adjacent C60 molecules and to introduce C–H bonds, which provide a good emission site, to the film surface. The coalescence reaction and C–H bond formation were confirmed by in situ Fourier-transform infrared spectroscopy. It was found that the field emission was enhanced for the 20 h irradiated C60 film. This enhancement was discussed in terms of the structural change in the C60 film.

In-situ infrared spectroscopy of a photoirradiated potassium-doped C60 film

Abstract Photodimerization between C 60 molecules in a potassium-doped C 60 film (a mixture of α-C 60 and K 3 C 60 phases) has been investigated using in-situ Fourier-transform (FT) infrared spectroscopy in combination with tight-binding molecular dynamics calculations. A comparison of the experimental spectrum of C 60 dimers, which was found to be a main product in the photoirradiated K-doped film using an FT mass spectrometer, with the theoretical spectra for several isomers of C 120 suggests that C 120 species with structures similar to a bucky peanut were formed in the photoirradiated film.

Ionization energies, electron affinities, and absorption spectrum of fullerene C60 calculated with the semiempirical HAM/3 and CNDO/S methods

Abstract Ionization energies (PES), excitation energies (UV) and electron affinities (EA) of fullerene C 60 have been calculated with the semiempirical HAM/3 method. The first few ionization energies calculated with HAM/3 show errors as great as 1.4 eV. The HAM/3-CI method has reproduced the observed UV spectrum fairly well. The HAM/3-Δe method has given fairly good energies for the forbidden lowest singlet and triplet transitions but rather poor results for the allowed transitions. The correlation between calculated PES, UV and EA values is discussed. A method to estimate accurate electron affinity is proposed. The CNDO/S method, on the other hand, has given good values for some of the ionization energies and electron affinities.

Two-Electron Excited States of Fullerene-C60: Analysis by the CNDO/S-SDCI Calculations

ABSTRACT Possible existence of the low-lying two-electron excited states of fullerene-C60 is investigated by the singly- and doubly-excited CI calculations in the semi-empirical CNDO/S approximation.

The third-order polarizability γ of C60: The role of low-lying two-electron excited singlet states Ag and Hg

The third-order polarizability γ of the C60 molecule has been calculated for the third harmonic generation at several incident frequencies using two different schemes of the sum-over-state (SOS) method, and the contribution of the low-lying singlet excited states as the second intermediate states has been analyzed. The group-theoretical analysis of the SOS expression clarifies that the 1Ag and 1Hg states are the only states that contribute to the γ as the second intermediate states. For the numerical analysis, the electronic states previously obtained in the semiempirical CNDO/S approximation with the singly and doubly excited configuration interaction method are used. It is found that the inclusion of the doubly excited configurations is essential in evaluating the γ. Those excited 1Ag and 1Hg states which make significant positive contributions to the γ are practically the doubly excited states. In order to secure error-free SOS calculations, equivalent but independent calculations have been also carrie...

Role of the low-lying two-electron excited 1Ag states of C60 in contributing to its hyperpolarizability γ

Abstract The role of the low-lying two-electron excited 1 A g states of C 60 in contributing to its hyperpolarizability γ is investigated by singly and doubly excited CI calculations in the semi-empirical CNDO/S approximation. It is found that inclusion of the doubly excited configurations is essential in evaluating γ. The calculated values of γ zzzz (−3 ω ; ω , ω , ω ) at wavelengths of ∞, 1.9, 1.83 μm sufficiently far from the first resonance are in reasonable agreement with experiment.

In situ x-ray photoelectron spectroscopic and density-functional studies of Si atoms adsorbed on a C60 film

The interaction between C(60) and Si atoms has been investigated for Si atoms adsorbed on a C(60) film using in situ x-ray photoelectron spectroscopy (XPS) and density-functional (DFT) calculations. Analysis of the Si 2p core peak identified three kinds of Si atoms adsorbed on the film: silicon suboxides (SiO(x)), bulk Si crystal, and silicon atoms bound to C(60). Based on the atomic percent ratio of silicon to carbon, we estimated that there was approximately one Si atom bound to each C(60) molecule. The Si 2p peak due to the Si-C(60) interaction demonstrated that a charge transfer from the Si atom to the C(60) molecule takes place at room temperature, which is much lower than the temperature of 670 K at which the charge transfer was observed for C(60) adsorbed on Si(001) and (111) clean surfaces [Sakamoto et al., Phys. Rev. B 60, 2579 (1999)]. The number of electrons transferred between the C(60) molecule and Si atom was estimated to be 0.59 based on XPS results, which is in good agreement with the DFT result of 0.63 for a C(60)Si with C(2v) symmetry used as a model cluster. Furthermore, the shift in binding energy of both the Si 2p and C 1s core peaks before and after Si-atom deposition was experimentally obtained to be +2.0 and -0.4 eV, respectively. The C(60)Si model cluster provides the shift of +2.13 eV for the Si 2p core peak and of -0.28 eV for the C 1s core peak, which are well corresponding to those experimental results. The covalency of the Si-C(60) interaction was also discussed in terms of Mulliken overlap population between them.