Indirect Optical Absorption of Single Crystalline beta-FeSi2

We investigated optical absorption spectra near the fundamental absorption edge of beta-FeSi2 single crystals by transmission measurements. The phonon structure corresponding to the emission and absorption component was clearly observed in the low-temperature absorption spectra. Assuming exciton state in the indirect allowed transition, we determined a phonon energy of 0.031 +- 0.004 eV. A value of 0.814 eV was obtained for the exciton transition energy at 4K.Comment: 10 pages with 3 figure

Efficient host excitation in thiosilicate phosphors of lanthanide(III)-doped Y4(SiS4)3

Lanthanide (Ln)-doped yttrium thiosilicate (Y1−x Ln x )4(SiS4)3 is synthesized, and its optical properties are studied. In (Y1−x Tb x )4(SiS4)3, the green photoluminescence band corresponding to the intra 4f transition of 5D4  →  7F5 appears at 545 nm and becomes the maximum for x  =  0.2 in the range x  =  0.01 to 1. The internal quantum efficiency is higher (11% for x  =  0.01) for the thiosilicate host excitation (360 nm) than for the direct excitation (1.6%) of the intra 4f transition of 5D4  ←  7F6 (489 nm). A time-resolved photoluminescence study shows that the luminescence of defect states of thiosilicate hosts decays faster (typically 10–30 ns) for higher Tb3+ concentration x. In addition, the rise time of Tb3+ photoluminescence is shorter (10–40 ns) for greater x. Energy transfer from the thiosilicate host to Tb3+ is discussed using these results. For all of (Y1−x Ln x )4(SiS4)3 (x  =  0.01, Ln  =  Pr, Nd, Dy, Er or Tm), the internal quantum efficiency is higher for the host excitation (11–21%) than for the direct excitation of intra 4f transitions (1.1–12%). A photoluminescence excitation study reveals broad host absorption in 300–400 nm for Ln luminescence. These results show the promising characteristics of the host absorption of (Y1−x Ln x )4(SiS4)3 phosphors and their optical properties

Broad luminescence of Ce3+ in multiple sites in (La,Ce,Y)6Si4S17

We have developed novel broad luminescent phosphors (La,Ce,Y)6Si4S17 including multiple substituted sites of Ce3+ . They have formed a triclinic structure (P  −  1) with three kinds of coordination structures around Ce3+ ions. The broad photoluminescence (PL) spectra at the range from 430 nm to 700 nm were observed, and they can be respectively decomposed into three PL bands. Moreover, the correlation between the three PL bands and the three substitution sites for Ce3+ was confirmed through the PL spectra at 78 K and Van Uitert\u27s universal equation

Crystal structure and photoluminescence of (Gd,Ce)4(SiS4)3 and (Y,Ce)4(SiS4)3

Structural and photoluminescence (PL) properties of undoped and Ce3+-doped rare-earth thiosilicate (Gd1-xCex)4(SiS4)3 (0⩽x⩽0.1) and (Y1-xCex)4(SiS4)3 (0⩽x⩽0.3) are reported. They maintain a monoclinic structure (P2_1/n) for the whole range of x. Increases in lattice constants appear with the increase in x because of the replacement of Gd3+ and Y3+ by larger Ce3+. Yellow-orange PL originating from the 5d1-4f1 (2FJ, J=5/2,7/2) transition of Ce3+ is obtained. From the measurement of PL spectra at 20 K, red shifts of the peak wavelength with the increase in x would be understood by the change of relative intensity for two luminescent centers of Ce3+ in PL bands. The maximum internal quantum efficiency is 62% for (Y_{0.85} Ce_{0.15})4(SiS4)3. These phosphors have higher water-resistance than alkaline-earth metal thiosilicate phosphors such as Ba2SiS4

Exciton localization and decomposition dynamics in cuprous halide nanocrystals

We report temporal changes of luminescence and absorption (differential transmission) of excitons in nanometer-size semiconductor crystals (nanocrystals) of CuCl embedded in NaCl or in glass, and CuBr nano- crystals embedded in glass. In CuCl nanocrystals in NaCl, an exciton relaxes nonradiatively to some localized state. In CuCl and CuBr nanocrystals in glass, the temporal changes of the differential transmission have a longer decay component in addition to the fast decay component which agrees with the luminescence decay. This result suggests exciton decomposition and the existence of an electron or a hole remainder in the nano- crystals. The decay time of the longer decay component increases by the accumulation of photoexcitation. This phenomenon indicates persistent trapping of carriers in the glass matrix, which is concerned with persistent spectral hole burning in nanocrystals

Complementary detection of confined acoustic phonons in quantum dots by coherent phonon measurement and Raman scattering

Coherent acoustic phonon oscillation is observed in PbSe quantum dots embedded in phosphate glass by femtosecond pump-and-probe. The size dependence of the oscillation is investigated. Distinct low-frequency peaks are observed in Raman spectrum for the same samples. The size-dependence of the frequencies is well explained by elastic sphere model, but the observed modes are different to each other for coherent phonon and Raman scattering. Coherent phonon measurement and Raman scattering are found to give complementary information on confined acoustic phonons in this system

Electronic structures of two types of TiO2 electrodes: inverse opal and nanoparticulate cases

We present a comparison between the electronic structures of inverse opal (IO) and nanoparticulate (NP)-TiO2 electrodes. The electronic structure details were obtained from optical absorption, fluorescence, and valence band studies in order to clarify the nature of the higher photovoltaic performance observed in sensitized solar cells using IO-TiO2 electrodes. We used photoacoustic (PA) and photoluminescence (PL) spectroscopy to characterize the optical absorption and fluorescence properties, respectively. Photoelectron yield (PY) spectroscopy was applied to characterize the position of the valence band maximum (VBM) of the IO- and NP-TiO2 electrodes. The PA spectrum for IO-TiO2 is different to that for NP-TiO2, indicating differences in the exciton–phonon interactions and the density of states in the conduction band. PL measurements showed that the curvature of the valence band structure of IO-TiO2 is different to that of NP-TiO2. Also, PL measurements showed that the oxygen vacancy in IO-TiO2 is different to that in NP-TiO2. Moreover, PY measurements showed VBM in IO-TiO2 to be at a higher position than that in NP-TiO2, suggesting a correlation with the increased open circuit voltage (Voc) in sensitized solar cells

Red persistent luminescence excited by visible light in CaS:Eu2+,Tm3+

Excitation wavelength dependence of red persistent luminescence in CaS:Eu2+,Tm3+ phosphor is reported. Persistent luminescence appears under visible light excitation in the wavelength region of 400–600nm. Photon energy from white light-emitting diode lamps is possibly stored in this material. This sulfide phosphor is synthesized using iodine vapor. Under iodine vapor, Eu2+ and Tm3+ are found to be efficiently included in CaS. The concentration dependence of Eu2+ is studied, and the optimum concentration is 0.05%. Trap depth of 0.27–0.33eV contributing to persistent luminescence is evaluated by using thermoluminescence

Enhanced O-GlcNAcylation Mediates Cytoprotection under Proteasome Impairment by Promoting Proteasome Turnover in Cancer Cells

The proteasome is a therapeutic target in cancer, but resistance to proteasome inhibitors often develops owing to the induction of compensatory pathways. Through a genome-wide siRNA screen combined with RNA sequencing analysis, we identified hexokinase and downstream O-GlcNAcylation as cell survival factors under proteasome impairment. The inhibition of O-GlcNAcylation synergistically induced massive cell death in combination with proteasome inhibition. We further demonstrated that O-GlcNAcylation was indispensable for maintaining proteasome activity by enhancing biogenesis as well as proteasome degradation in a manner independent of Nrf1, a well-known compensatory transcription factor that upregulates proteasome gene expression. Our results identify a pathway that maintains proteasome function under proteasome impairment, providing potential targets for cancer therapy