Shinta Watanabe

Energy location of Ce3+ 4f level and majority carrier type in Gd3Al2Ga3O12:Ce crystals studied by surface photovoltage spectroscopy

Gd3Al2Ga3O12:Ce (GAGG:Ce) was studied by surface photovoltage spectroscopy using the ultraviolet photoelectron spectroscopy technique with synchrotron radiation and a laser source. The lowest Ce3+ 4f level is located below the conduction band minimum by 3.02 eV. This result is supported by the excitation spectrum for photo-stimulated luminescence and is compatible with the value predicted by the vacuum-referred binding energy scheme for GAGG:Ce. It is also found that GAGG:Ce is of the p-type. The information on the energy location of the Ce3+ 4f level and majority carrier type provides us with hints on how to improve the optical properties of GAGG:Ce for photonic device applications.

Intra- and inter-atomic optical transitions of Fe, Co, and Ni ferrocyanides studied using first-principles many-electron calculations

We have investigated the electronic structures and optical properties of Fe, Co, and Ni ferrocyanide nanoparticles using first-principles relativistic many-electron calculations. The overall features of the theoretical absorption spectra for Fe, Ni, and Co ferrocyanides calculated using a first-principles many-electron method well reproduced the experimental one. The origins of the experimental absorption spectra were clarified by performing a configuration analysis based on the many-electron wave functions. For Fe ferrocyanide, the experimental absorption peaks originated from not only the charge-transfer transitions from Fe2+ to Fe3+ but also the 3d-3d intra-transitions of Fe3+ ions. In addition, the spin crossover transition of Fe3+ predicted by the many-electron calculations was about 0.24 eV. For Co ferrocyanide, the experimental absorption peaks were mainly attributed to the 3d-3d intra-transitions of Fe2+ ions. In contrast to the Fe and Co ferrocyanides, Ni ferrocyanide showed that the absorption p...

Visualizing hidden electron trap levels in Gd3Al2Ga3O12:Ce crystals using a mid-infrared free-electron laser

The energy levels of electron traps (namely, defect complexes associated with oxygen vacancies) in Gd3Al2Ga3O12:Ce (GAGG:Ce) were studied at 12 K using mid-infrared (MIR) light pulses from a free-electron laser (FEL) as the probe light. Ce3+ 5d–4f luminescence was stimulated by the MIR light pulse following an ultraviolet light pulse. Stimulation of Ce3+ 5d–4f luminescence by MIR light pulses was pronounced above 0.31 eV. This result is consistent with that of previous work based on a trap-mediated luminescence model. It is concluded that the electron trap levels are located 0.31 eV below the bottom of the conduction band. This study demonstrates that MIR-FEL is applicable for the determination of hidden electron trap levels.

Shallow electron traps formed by Gd2+ ions adjacent to oxygen vacancies in cerium-doped Gd3Al2Ga3O12 crystals

To clarify the origin of shallow electron traps in cerium-doped Gd3Al2Ga3O12 (Ce:GAGG), the optical properties of cerium-doped Lu3−xGdxAl2Ga3O12 crystals were investigated. Absorption spectra for x = 3 exhibited a prominent band at 12 000 cm−1 when excited by 3.31-eV ultraviolet light. This band has been previously attributed to shallow electron traps at defect complexes associated with oxygen vacancies. When Gd3+ ions were replaced with Lu3+ ions, the 12 000 cm−1 band weakened and disappeared completely for Ce:Lu3Al2Ga3O12. In addition, thermally stimulated luminescence glow curves were observed. Optically stimulated luminescence indicated that the energy of the conduction band minimum did not change by the presence of Lu3+ ions. Thus, the Gd3+ ions were important for the formation of shallow electron traps in Ce:GAGG. First-principles calculations implied that Gd3+ ions responsible for shallow electron traps formed antisite defects at GAGG octahedral sites. Hence, defect complexes of antisite Gd2+ ions adjacent to oxygen vacancies were the most plausible candidates for shallow electron traps in Ce:GAGG.

Spectroscopic and first-principles calculation studies of the chemical forms of palladium ion in nitric acid solution for development of disposal of high-level radioactive nuclear wastes

We have investigated the chemical forms of palladium (Pd) ion in nitric acid solution, using XAFS/UV-vis spectroscopic and first-principles methods in order to develop the disposal of high-level radioactive nuclear liquid wastes (HLLW: radioactive metal ions in 2 M nitric acid solution). The results of theoretical calculations and XAFS/UV-vis spectroscopy indicate that Pd is a divalent ion and forms a square-planar complex structure coordinated with four nitrate ions, [Pd(NO3)4]2-, in nitric acid solution. This complex structure is also thermodynamically predicted to be most stable among complexes [Pd(H2O)x(NO3)4-x]x-2 (x = 0-4). Since the overall feature of UV-vis spectra of the Pd complex was independent of nitric acid concentration in the range 1–6 M, the structure of the Pd complex remains unchanged in this range. Furthermore, we examined the influence of γ-ray radiation on the [Pd(NO3)4]2- complex, using UV-vis spectroscopy, and found that UV-vis spectra seemed not to be changed even after 1.0 MGy ir...

Spectroscopic and theoretical studies on the structural, electronic, and optical properties of zinc octaethylporphyrin/C60 co-deposited films

We have examined the structural, electronic, and optical properties of zinc-octaethylporphyrin [Zn(OEP)]/C60 co-deposited films to elucidate the donor (D)-acceptor (A) interactions at the D/A interface of heterojunction organic solar cells (OSCs), using Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) spectroscopy in combination with first-principles and semi-empirical calculations. The FT-IR and XRD results indicated that Zn(OEP) and C60 were mixed with each other at the molecular level in the co-deposited film. The theoretical calculations suggested that in the interfacial region, it is energetically preferable for the C60 molecule to face the center of the planar structure of Zn(OEP) at a distance of 2.8 Å rather than the edge of the structure at a distance of 5.0 Å. After consideration of the C60 solvent effects, this coordination model for C60-Zn(OEP) adequately explained the line shift of the UV-vis peaks with respect to the proportion of C60 in the co-deposited films. A comparison of the energy level diagrams of Zn(OEP) before and after the interaction with C60 revealed that the LUMO, HOMO, and HOMO-1 were significantly affected by the interaction with C60. In particular, the HOMO-1 wave function became spread over a portion of C60, although the charge transfer between Zn(OEP) and C60 was almost negligible. Since no PL peaks (S1 → S0) from the excited Soret band of Zn(OEP) were observed for the Zn(OEP)/C60 co-deposited films, the D/A mixing layers played a crucial role in completely dissolving the photogenerated excitons to electrons-hole pairs that cause the short-circuit current, which is relevant to improving the energy conversion efficiency of OSCs.

Correlational analysis of Eu3+ charge transfer state using La effective charge in La-based mixed-anion host compounds

A prediction of the Eu3+ charge transfer state (E CT) was attempted in La-based mixed-anion host compounds. We paid attention to La3OF3S2:Eu, since it is expected to have a more covalent La site than La2O2S. The La effective charge (La EC) was proposed as the index factor of covalency and/or ionicity. The correlation between the experimental E CT and the calculated La EC was systematically analyzed for La2S3, LaFS, La2O2S, La2O3, LaOF, and LaF3 host materials, and good approximation was obtained using the single exponential function with a variable number of La ECs. According to the fitting curve, the E CT of La3OF3S2:Eu was predicted to be 5.8 and 2.1 eV for Eu3+ centers activated at ionic and covalent sites, respectively. To confirm the prediction accuracy, La3OF3S2:Eu phosphor powder samples were synthesized by solid-state reaction. From the photoluminescence excitation and absorption measurements, the E CT values of about 4.7 eV (ionic La site) and 2.4 eV (covalent La site) were obtained. Even though the energy difference between the predicted and experimental values is large for the higher E CT, La EC is the useful index factor for estimating E CT. In addition, it indicates that the estimation can be applied to phosphor materials having multication sites.