Because fossil fuels are becoming scarce and because of the expected climate change, our standard of living can only be maintained by a significant increase in energy efficiency. Large amounts of energy are consumed for lighting and during operation of displays. Thus, the targets are the development of economical light sources like white-light-emitting diodes and display panels with enhanced efficiency. Solar energy is converted into electricity by solar cells, and their efficiency must be improved considerably. A possible contribution might be delivered by phosphors which allow the conversion of thermal radiation into electrical energy. Although the target of energy efficiency is very important, we must not overlook that medical imaging diagnostic methods require efficient and sensitive detectors. For the solution of these central questions, inorganic solid-state materials doped with rare-earth ions are very promising and are therefore in the focus of current research activities.

Recent Developments in the Field of Inorganic Phosphors

Recent progress in quantum cutting phosphors

The Eu3+ charge transfer energy and the relation with the band gap of compounds

Density-functional calculations using the linearized-augmented-plane-wave method were carried out for the scheelite materials ${\mathrm{CaMoO}}_{4},$ ${\mathrm{CaWO}}_{4},$ ${\mathrm{PbMoO}}_{4},$ and ${\mathrm{PbWO}}_{4}$ in order to determine their ground-state electronic properties. The results indicate that ${\mathrm{CaMoO}}_{4}$ and ${\mathrm{CaWO}}_{4}$ have direct band gaps at the center of the Brillouin zone, while ${\mathrm{PbMoO}}_{4}$ and ${\mathrm{PbWO}}_{4}$ have band extrema at wave vectors away from the zone center with possibly indirect band gaps. The magnitudes of the band gaps increase in the order ${\mathrm{PbMoO}}_{4}{l\mathrm{PbWO}}_{4}{l\mathrm{CaMoO}}_{4}{l\mathrm{CaWO}}_{4}.$ The valence and conduction bands near the band gap are dominated by molecular orbitals associated with the ${\mathrm{MoO}}_{4}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\alpha}}}$ and ${\mathrm{WO}}_{4}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\alpha}}}$ ions, where $\ensuremath{\alpha}\ensuremath{\approx}2.$ The valence-band widths are 5 and 5.5 eV for the Ca and Pb materials, respectively. In the Pb materials, the Pb $6s$ states form narrow bands 1 eV below the bottom of the valence bands, and also hybridize with states throughout the valence bands, while the Pb $6p$ states hybridize with states throughout the conduction bands. In the Ca materials, the Ca $3d$ states contribute to a high density of states 3--4 eV above the bottom of the conduction bands.

Electronic band structures of the scheelite materials CaMoO 4 , CaWO 4 , PbMoO 4 , and PbWO 4

Nanosized Bi 2 WO 6 was synthesized by a hydrothermal crystallization process. The as-prepared samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and porosity measurements, transmission electron microscopy, Raman spectra, and diffuse reflectance spectroscopy. The photoactivities of the as-prepared samples for the rhodamine-B photodegradation were investigated systematically. As a result, the sample prepared at 180 °C exhibited the highest photochemical activity under visible-light irradiation. The further experiments revealed that the catalyst was active in a wide spectral range. Density functional theory calculations suggested that the visible-light response was due to the transition from the valence band formed by the hybrid orbitals of Bi 6s and O 2p to the conduction band of W 5d. The photoactivity of the catalyst in relationship with the hydrothermal temperature, the crystal and band structure were also discussed in detail.

Photocatalytic properties of nanosized Bi2WO6 catalysts synthesized via a hydrothermal process

The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4\char21{}40 eV or electron pulses have been studied in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ single crystals at 8 K. The intrinsic A (7.6 eV) and E emissions (3.77 eV) can be effectively excited in the region of long-wavelength (8.85\char21{}9.1 eV) and short-wavelength (9.1\char21{}9.3 eV) components of exciton absorption doublet, respectively. Fast (\ensuremath{\sim}6 and \ensuremath{\sim}20 ns) and slow (\ensuremath{\sim}150 ns) components of the A emission correspond to the creation of singlet and triplet ${p}^{5}s$ excitons. The efficiency of the A emission in the region of band-to-band transitions is low. The intensity of A emission sharply increases (approximately quadratically) with a rise of the excitation density by nanosecond electron pulses. In ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}:\mathrm{S}\mathrm{c},$ the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations (\ensuremath{\sim}8.5 eV) as well as by the electron recombination with holes localized near ${\mathrm{Sc}}^{3+}$ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of $h\ensuremath{\nu}g25\mathrm{eV}.$ One photon of 26\char21{}29 and 30\char21{}37 eV causes the ionization of the ${2p}^{6}$ or ${2s}^{2}$ shell of the oxygen ion and provides the creation of two or three electron-hole pairs, respectively. Long-term investigations of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ crystals did not lead to the detection of immobile self-trapped holes or electrons. The A emission excited at the direct photocreation of excitons or at the recombination of free electrons and free holes is interpreted by us as the radiative decay of self-shrunk excitons. The theoretical model of Sumi allows the existence of such immobile self-shrunk excitons even if an electron and a hole do not separately undergo the self-trapping.

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Self-trapping and multiplication of electronic excitations in Al 2 O 3 and Al 2 O 3 : S c crystals

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

A systematic spectroscopic study of oriented single ZnWO4 crystals was performed using UV and synchrotron excitation. Emission and phosphorescence spectra in the energy region 1-4 eV as well as reflection spectra and excitation spectra of various emissions and phosphorescence in the energy region 3.5-32eV were studied at 4.2-300K using synchrotron radiation. The results are compared to those available for CdWO4. The peculiarities of the band structure and those of excitonic and electron-hole processes in the systems investigated are discussed.

Energy transfer in ZnWO4 and CdWO4 scintillators

The peculiarities of multiplication of electronic excitations have been investigated in wide-gap alkali halide crystals where, in contrast to semiconductors, free and self-trapped excitons as well as free and self-trapped holes coexist. In particular, the luminescent methods for separation and quantitative investigation of the formation processes of secondary excitons have been elaborated. These methods are based on the investigation of the dependence of the intensity ratio for two different emissions from the same crystal as a function of the energy of exciting photons. The excitation spectra of triplet ($\ensuremath{\pi}$) and singlet ($\ensuremath{\sigma}$) emissions of self-trapped excitons as well as of impurity luminescence have been measured, using synchrotron radiation of 12 to 32 eV, for high-purity KI, KBr, and doped KI:Na, KI:Tl, KBr:I, KBr:Tl crystals at 8 K. The analysis of the intensity ratio spectra for two emissions allowed us to separate the regions of secondary triplet exciton formation in KI (13-15 and 26.5-29 eV) and KBr (15-17 and 28-30 eV). The threshold photon energy for the formation of a secondary exciton has been experimentally determined to be equal to 13.1\ifmmode\pm\else\textpm\fi{}0.2 eV in KI and 15.2\ifmmode\pm\else\textpm\fi{}0.2 eV in KBr. The probability of secondary exciton formation by hot photoelectrons reaches the main maximum at 0.5-0.7 eV above the value of threshold energy. The energy transport to impurity centers by free triplet secondary excitons has been revealed in KI:Na and KBr:I crystals. The decay of $3{p}^{5}4s$ cation excitons in KI and KBr, generated by 20-eV photons, leads to the formation of a double amount of electron-hole pairs.

Aleksandr Lushchik

Papers

Self-trapping and multiplication of electronic excitations in Al 2 O 3 and Al 2 O 3 : S c crystals

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Energy transfer in ZnWO4 and CdWO4 scintillators

Secondary excitons in alkali halide crystals

Luminescence of free and self-trapped excitons in wide-gap oxides