Masao Ichida

OPTICAL TRANSITION AND IONICITY OF C60/AMINE CHARGE-TRANSFER COMPLEXES STUDIED BY OPTICAL SPECTROSCOPY

We have investigated absorption spectra of charge-transfer (CT) complexes of C60 with various aromatic amines with different ionization potentials. An absorption band due to CT transition is observed in the visible region; with increasing ionization potential of amines, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. We extract degrees of charge transfer by analyzing the transition energy of the CT band as a function of ionization potential and Raman spectra. The experimental results show that the CT complexes studied here have a neutral character in their ground state.

Observation of triplet state of charge-transfer excitons in C60 thin film

Abstract We have studied formation and energy structure of triplet states of charge-transfer excitons in C 60 thin film by means of pump and probe spectroscopy in a vast time domain from 100 fs to 100 ms. In solid C 60 , absorption bleaching which lasts up to several hundred milliseconds is observed slightly below the absorption band due to charge-transfer excitons (CT excitons). This provides a spectroscopic evidence for existence of an inter-molecular triplet state due to CT excitons in solid C 60 . The energy splitting between singlet and triplet CT exciton states is 0.09 eV, and the lifetime of triplet CT excitons is ∼50 ms.

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.

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.

Processing of β-BaB2O4 thin films through metal organics

Abstractβ-BaB2O4 (β-BBO) thin films were successfully synthesized by the sol-gel method using metallo-organic compounds. A stable BBO precursor solution was prepared from barium metal and boron triethoxide or 2,4,6-triethoxycyclotriboroxane in a mixture solvent of ethanol and 2-ethoxyethanol. As-precipitated powder formed by hydrolysis of the precursor solutions crystallized to ψ phase, which was transformed to β phase at higher temperatures. The transformation temperatures of powders from ψ to β phase of the ethoxide system and the boroxane system were 600 and 680°C, respectively. The calcination of precursor films in a mixture gas of water and oxygen was found to decrease the crystallization temperature of β-BBO films on Pt sheet substrates. The precursor films prepared from the ethoxide system and the boroxane system crystallized to β-BBO on Pt (111)/glass substrates at 500 and 550°C, respectively. The BBO films on Pt(111)/glass substrates showed the strong (006) preferred orientation. The β-BBO films on Pt(111)/glass substrates showed the second harmonic generation (SHG) of the 532 nm light on irradiation with 1064 nm light. The SH power from the BBO films was correlated with the fundamental power through the square-law proportionality based on the theory. The SHG efficiency of the BBO films was dependent upon the film thickness.

Cathodoluminescence and photoluminescence decay behaviors of CdSe dots embedded in ZnSe

Abstract We have investigated cathodoluminescence (CL) and decay dynamics of photoexcited states in CdSe/ZnSe single-fractional monolayer structures grown by molecular beam epitaxy on a GaAs(0xa00xa01) substrate. CL spectra measured at 10xa0K by scanning the focused electron beam with a diameter of 200xa0nm show sharp peak structures superimposed on a broad spectrum. The observed spectra are ascribed to exciton luminescence from individual CdSe-based dots in two different lateral size ranges: large dots (15–40xa0nm) and small ones (6–10xa0nm). Decay behaviors of photoluminescence measured in the picosecond time region have been interpreted in terms of exciton migration in the nanostructures.

Ultrafast relaxation of excitons and photopolymerization in C60 and C70

Abstract We have studied relaxation dynamics of photoexcitation in C 60 and C 70 molecules and solids by means of a femtosecond pump and probe spectroscopy. With the excitation of the dipole allowed transition at 3 eV, the photoexcitations in C 60 solution relax into the triplet state in 900 ps. For solid C 60 , however, the fast relaxation occurs in 1 ps, which is interpreted in terms of the self-trapping of charge-transfer (CT) excitons in the simple cubic lattice. Such a self-trapped CT exciton seems to be a precursor to the photopolymerization. A similar dynamical behavior has been observed for solid C 70 .

Room-Temperature Photoswitching in La0.6Sr0.4MnO3 Film

We have observed a transient transmission change (ΔT) induced by photoirradiation in La0.6Sr0.4MnO3 film at room temperature. The magnitude of ΔT is enhanced with a decrease of the probe photon energy hbarω, suggesting a resonance feature to the Drude-like component. We interpreted the photoswitching effect in terms of the photoinduced suppression of the spin correlation in the half-metallic double-exchange system.

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.

Luminescence spectra and structure of novel MnF2 heterostructures

Abstract Cathodo- and photoluminescence of MnF 2 thin epitaxial layers and MnF 2 –CaF 2 superlattices on Si(1xa01xa01) has been studied at low temperatures. A blue shift of the main Mn 2+ broad band in MnF 2 films thinner than three monolayers is revealed. The shift is likely to be due to the change of the crystal structure from the tetragonal rutile-type characteristic of bulk crystals and thick layers to the cubic fluorite-type structure, as measured by RHEED and EXAFS.