Yoshiki Wada

Charge Transfer Exciton in Halogen-Bridged Mixed-Valent Pt and Pd Complexes: Analysis Based on the Peierls-Hubbard Model

Polarized reflection and luminescence have been measured for the single crystals of [MA 2 ][MX 2 A 2 ](ClO 4 ) 4 (M=Pt, Pd, X=Cl, Br, I and A=ethylenediamine, cyclohexanediamine). The strong absorption bands due to the charge-transfer (CT) exciton transitions between the mixed-valent metal ions have been investigated in detail in the visible or infrared energy regions. The dependence of the CT excitation energies on the species M and X is shown to be consistent with the prediction by the Peierls-Hubbard model which incorporates the effect of the electron-electron correlation on inter-metal sites. The oscillator strength of the CT excitons are observed to be enhanced by substituting heavier halogen ions. This enhancement is interpreted by a halogen-linked super-transfer mechanism. The unusually large values of the oscillator strength can be qualitatively explained in terms of the trimer CT model.

Yellowish-white luminescence in codoped zinc oxide

The broad-band visible emission of codoped ZnO was studied. The codoped ZnO specimens were intentionally and simultaneously doped with IIIa elements (donors) and Li (acceptor). Broad-band emission covering nearly the whole visible range was achieved. The emission was found to be yellowish white to the naked eye. The visible band was composed of two components, i.e., a green emission having a peak at 2.2 eV and a yellow emission having a peak at 2.0 eV. The peak at 2.2 eV was distinct from the nonstructured green emission at 2.45 eV. The 2.2 and 2.0 eV peaks were attributed to donor-acceptor pair transitions involving the zinc vacancy and lithium, respectively.

Optical Properties of Halogen-Bridged Mixed-Valance Complexes, [M(en)2][PtX2(en)2](ClO4)4, (M=Pt, Pd and Ni; X=Cl, Br and I): Effects of Metal-Alternation

The effect of metal alternation (Pt and M) in linear-chain halogen-bridged mixed-valence complexes, [M(en) 2 ][PtX 2 (en) 2 ](ClO 4 ) 4 , (X=Cl, Br and I; M=Pt, Pd and Ni; en=ethylenediamine), has been investigated using polarized reflection measurements. The electronic absorption spectra of these materials, obtained by applying the Kramers-Kronig relation to the reflectivity data, show two distinctive charge-transfer exciton bands associated with the inter-metal transitions, “M–Pt”, and the transitions from halogen ions to metal ions, “X–Pt”. On substitution of M=Pt by Pd or Ni, the absorption intensity of the “X–Pt” exciton was enhanced in contrast to the reduction of the “M–Pt” exciton intensity. The intensity borrowing between these two excitons is consistent with a numerical calculation based on a trimer model of the CT exciton transitions.

Mott-Hubbard State in One-Dimensional Iodo-Bridged Binuclear Metal Dithioacetato Complexes, M2(dta)41 (M=Pt and Ni)

Abstract Polarized reflection, electrical conductivity, magnetic susceptibility measurements and X-ray crystal structure predetermination have been carried out on one- dimensional iodo-bridged binuclear metal dithioacetato complexes, M2(dta)4I (M=Pt and Ni; dta=CH3CS2 −) in order to elucidate their electronic property. The chain structure consists of -M2-5+-M2,5+-I-M2-5+-M2-5+- with no Peierls distortion, and two metal atoms in binuclear moieties are bridged by four dithioacetato ligands to give a cage structure. The magnetic behavior of these compounds revealed very strong anti ferromagnetic coupling between electronic spins (S=1/2) localized on the M-M dimers. They show semi conducting behavior with high conductivities and small activation energies compared with those of the MX compounds. From these results, it may be concluded that the compounds are one-dimensional Mott-Hubbard insulators, in contrast to Peierls insulators of R4[Pt2(pop)4X].nH20 complexes (…M2+-M2+…X-M3+ -M3+-X‥).

Syntheses, Structures, and Solid State Properties of One-Dimensional Halogen-Bridged Ni111−X-Ni111 Compounds (X˭Cl and Br)

Abstract Halogen-bridged linear-chain compounds, {[Ni(R,R-chxn)2X]X2}∞ (X˭C1 and Br), were synthesized and their structures have been determined by low-temperature X-ray diffraction analyses. The novel one-dimensional Ni111 -X-Ni111 structure with no Peierls distortion, which is an extreme limit of the MII -X-MIV mixed-val

An aqueous solution process and subsequent UV treatment for highly transparent conductive ZnO films

High electric conductivity was achieved in ZnO films prepared by a low-temperature (<100 °C) wet-chemical process and subsequent UV treatment by a commercial blacklight-blue lamp with a central wavelength of 360 nm and output power of 2.0 mW cm−2. The UV treatment to the as-deposited film successfully decreased the electric resistivity of the film by three orders of magnitude from 11 to 4.4 × 10−3 Ω cm. The resistivity was not restored to the initial value even after dark storage for 50 days, indicating that the decrease of the resistivity has not mainly originated from conventional persistent photoconductivity. The results of Hall measurement and Fourier transformed infrared measurement showed that an increase of carrier concentration and decomposition of organic impurities of the films occurred simultaneously, which indicates that the phenomenon was caused by introduction of donor impurities such as hydrogen atoms, carbon species and oxygen vacancies into the ZnO lattice by the photocatalytic decomposition of residual organic impurities remaining in the film.

Deep Level Transient Spectroscopy Analysis of an Anatase Epitaxial Film Grown by Metal Organic Chemical Vapor Deposition

The deep level transient spectroscopy (DLTS) study of anatase-type TiO2 material was performed for the first time. The anatase film was epitaxialy grown on a conductive Nb-doped single-crystalline SrTiO3 (100) substrate by metal organic chemical vapor deposition. The photoluminescence characteristics of this anatase film were identical to those in previous reports, where they were attributed to the radiative recombination of self-trapped excitons. According to the DLTS analysis, it was revealed that this anatase film had a characteristic deep level located at 0.96 eV below the bottom of the conduction band with large concentration (6.5×1016/cm3) and capture cross section (8.3×10-13 cm2). Since the capture cross section of this level appeared to be too large to be caused by point defects, the origin of this deep level was attributed to line defects such as dislocations.

Charge instability in 1D halogen-bridged binuclear metal complex

Abstract Single crystals of 1D halogen-bridged binuclear platinum complex, K 4 [Pt 2 (pop) 4 Cl]·3H 2 O, where pop=P 2 O 5 H 2 2− , have been investigated by defused X-ray scattering, reflectivity and luminescence measurements. Compared with the halogen-bridged mononuclear metal complexes, several unusual behaviors are observed, which are responsible for an additional charge modulation in the charge density wave state. From the viewpoint of the extended Peierls-Hubbard model, they are reduced to the result of a competition between the electron-electron repulsive energy in binuclear Pt-Pt sites and the energy gain due to chlorine ion displacement.

Optical properties of quasi-one-dimensional MMX-chain compounds: R4[Pt2(pop)4X].nH2O, (R=K and NH4, X=Cl and Br)

Abstract Polarized reflection and luminescence spectra of single crystals of quasi-one-dimensional halogen bridged binuclear metal complexes (MMX-compounds), R 4 [Pt 2 (pop) 4 X].nH 2 O (R=K and NH 4 ; X=Cl and Br; n=0 (R=NH 4 ) and 3 (R=K); pop=P 2 O 5 H 2 2− ) have been measured. Intense asymmetric Lorenzian-like absorption bands caused by charge transfer excitons from Pt 2+ Pt 2+ ions to the neighboring Pt 3+ Pt 3+ ions polarized parallel to the chain axis have been observed. A board luminescence band with large Stokes sift energy caused by the radiative recombination of the self-trapped excitons has been observed. These features are the characteristics features of the charge density wave systems. In K 4 [Pt 2 (pop) 4 Cl].3H 2 O, coexistence of two CT-absorption bands have been observed.

Evaluation of thermal diffusivity for thin gold films using femtosecond laser excitation technique

We evaluated the dependence of the thermal diffusivity upon the thickness of the thin films and the substrates by means of the transient reflecting gratings (TRGs) method. The wave length of the laser beam was 800 nm, and the width of the pulse 200 fs. The materials of the samples are pure gold thin films on silicon wafer or quartz glass substrates. We have performed TRG measurements for various thickness thin films, and made analysis on changes in the thermal diffusivity and the acoustic wave. The result of our analysis shows that the change in the thermal diffusivity, in the thickness range of 25 and 500 nm, depends upon both the thin film thickness and the substrates. It is also revealed that the acoustic-wave frequency changes depend upon both the thin film thickness and the substrates. These dependencies suggest that information on both the thin films and substrates are mixed up in the TRG data.