Radiative transitions in MgAl2O4 spinel single crystal were investigated after irradiation with He+ ions of fluence similar to 10(17) particles/cm(2). Photoluminescence (PL), PL excitation spectra and PL decay curves were measured at cryogenic temperatures of 8 K. It is shown that PL decay kinetics of 5 eV (250 nm) and 3 eV (420 nm) bands are similar because of the common excited state. Furthermore, after irradiation PL band at similar to 5 eV preserved characteristic behaviour of the donor-acceptor pair transitions. The PL channels involve electronic transitions of antisite defects, Mg-Al (shallow acceptor) and Al-Mg (shallow donor), and oxygen vacancy V-O (deep donor), while Al-Mg served an intermediate of the electronic excitation channel. The intense emissions were assigned to V-O(center dot)& lowast; -> V-O(center dot) (3 eV) and V-O(center dot)& lowast; -> Mg-Al(x) (5 eV), which involve respectively energy and energy-electron transfer. Participation of the doubly-ionized oxygen vacancy V-O(center dot center dot) in the energy/electron transfer is suggested. The obtained data enabled a generalised scheme of electronic transitions in MgAl2O4 spinel in presence of intrinsic defects.

The luminescent properties of CeF3 single crystals and nanoparticles have been investigated. The single crystals exhibit intense luminescence associated with the emission of 5d-4f Frenkel excitons at 283 and 305 nm (4.38 and 4.07 eV), as well as "perturbed" cerium ions with a maximum at approximately 340 nm (3.64 eV). The transition from a single crystal to nanoparticles results in a significant reduction in exciton luminescence intensity, giving way to defect-related luminescence. The excitation spectrum maxima of exciton luminescence in nanoparticles with size of a = 26 nm correlate with the dips in the excitation spectrum of the single crystals and the maxima of cerium ion luminescence excitation in LaF3-Ce, due to the absence of light absorption saturation effects in the thin layer of the nanoparticle. The reduction of the exciton luminescence decay time constant from 16.1 for single crystals to 1.7 ns for nanoparticles with size of 12 nm indicates the predominance of non-radiative decay mechanisms associated with surface defects. The quenching rate of exciton luminescence is higher than that of "perturbed" centers luminescence, which is due to an additional channel of exciton luminescence quenching through the diffusion of excitons to surface defects. The quenching rate of exciton luminescence exceeds that of "perturbed" center luminescence due to exciton diffusion to surface defects. In nanoparticles with a = 8 nm, exciton luminescence is absent, however, the defect luminescence excitation spectrum still suggests exciton formation. The absence of delay in the rise time of the defect luminescence pulse for nanoparticles with a = 8 nm confirms the radiative nature of the interaction between excitons and "perturbed" cerium centers.

In this paper, the effect of Yb dopant on the optical and photoelectric properties of CsPbCl3:Yb single crystals is studied. The position of the main energy level of Yb3+ ions relative to the energy band was determined. Two channels of electron photoionization into the conduction band were identified, which are photo-stimulated transition of valence electrons and transition of electrons from the ground level of the Yb3+ dopant ion. The crucial role of Yb2+ ions in the manifestation of the quantum cutting effect was shown. A non-uniform distribution of ytterbium dopant in the CsPbCl3:Yb single crystal volume was revealed. It was established that the absolute quantum yield of ytterbium luminescence in CsPbCl3:Yb single crystals in the near-infrared region at Yb concentration of 2 mol.% reaches 86%.

The search for and fabrication of materials for the near-infrared range of the spectrum, including telecommunication windows, is an extremely important task in modern technology. This work presents the results of studies on the photoluminescence of CsPbCl3 single crystals with varying contents of ytterbium and erbium impurities, grown using the Bridgman method. The existence of electrically neutral Yb3+-V-Pb-Er3+ complexes within the crystal structure was confirmed. Resonant absorption of excitation radiation lambda = 980 nm by Yb3+ ions reveals a three-step energy transfer channel from Yb3+ to Er3+ ions within a single complex. The estimated quantum yield of up-conversion luminescence at lambda = 524 nm (0.02%) associated with erbium ions indicates the presence of electronic transitions to higher energy levels, with the possibility of subsequent emission within the third telecommunication window.

This work is dedicated to investigation of the luminescent properties of the prospective photoconversion material based on the crystal of Ce3+ doped Ca3Sc2Si3O12 (CSSG) garnet. The GSSG:Ce crystal was grown using the micropulling-down (mu PD) method. The CSSG:Ce crystal exhibited an intensive photoluminescence (PL) emission band with two sub-bands peaked at 504 and 545 nm, corresponding to 5d-4f (2F5/2;7/2) transitions. Furthermore, we have investigated also the formation of cerium multicenters in the GSSG:Ce crystal using analyses of the structure of Ce3+ photoluminescence emission and excitation spectra under excitation of the luminescence of this crystal by synchrotron radiation. The formation of Ce3+-multicenters in CSSG:Ce garnet is caused by the local inhomogeneity of the dodecahedral sites of garnet lattice due to localization of the hetero-valent Sc3+ and Si4+ cations in the octahedral and tetrahedral positions of the garnet host. The existence of Ce3+ multicenters resulted in a significant enhancement of the Ce3+ emission band in the red range and improving the performance of conventional YAG:Ce phosphor. The next task of our work was to evaluate the possibility of application of the GSSG:Ce crystal as a light phosphor-converter (pc) for white light-emitting diodes (WLEDs). In the frame of this task, we have successfully developed a prototype of WLED by employing the CSSG:Ce crystal as a phosphorconverter (pc) with blue 450 emitting LED as well as investigated the color characteristics of this pc-WLED.

Optical and luminescence properties of Y3Al5O12 (YAG) single crystals preliminary irradiated by swift heavy ions were studied. Swift heavy Xe ions with fluences ranging from 6·1010 to 2·1012 ions/cm2 were utilized for the irradiation of nominally undoped YAG single crystals. A stable strong induced absorption observed in the 200–600 nm spectral range correlates with the irradiation fluence. It is suggested that several centers are responsible for this induced absorption in YAG single crystals and their possible origin (F-type centers) is proposed and discussed. The swift heavy ions irradiation strongly modifies the luminescence properties of YAG, namely, the excitonic emission at liquid helium temperature is drastically suppressed in heavily irradiated crystals.

A comparative analysis of the luminescent parameters of the CeF 3 nanoparticles and CeF 3 sing crystals was performed. CeF 3 single crystals exhibit characteristic luminescence attributed to the radiative emission of 5d-4f Frenkel excitons at 283 and 305 nm, and “perturbed” cerium ions with a maximum at $\sim 360 \mathrm{~nm}$. In the case of nanoparticles, the intensity of excitonic luminescence decreases and “perturbed” cerium ion luminescence becomes dominant. The maxima of the excitation spectrum of the excitonic luminescence for nanoparticles coincides with the dips in the excitonic luminescence excitation spectrum of single crystal. This feature of the excitation spectra is caused by the absence of saturation effects of light absorption in the thin layer of nanoparticle. A significant decrease of the decay time constant of the excitonic luminescence upon transition from single crystals to nanoparticles indicates a non-radiative mechanism of luminescence quenching mainly by the nanoparticle surface defects. For nanoparticles with $\mathrm{a}=8 \mathrm{~nm}$ the coincidence of dips in the excitation spectra of “perturbed” cerium luminescence with the maxima of the luminescence excitation spectra suggests the formation of free excitons even in the absence of excitonic luminescence. This indicates that excitons completely non-radiatively recombine with defects in the nanoparticle during their lifetime. The absence of efficient excitation of “perturbed” cerium centers in the $4 \mathrm{f}-5 \mathrm{~d}$ absorption region and no delay at the stage of increasing intensity of the luminescence kinetics curve for nanoparticles with $\mathrm{a}=8 \mathrm{~nm}$ indicate the radiative nature of the interaction between excitons and “perturbed” centers.

The luminescence of the well-known spinel-structured functional compound MgAl2O4 was studied at cryogenic temperatures below 10 K in a wide spectral range from near-IR to VUV under synchrotron radiation and electron beam excitation. The most intense luminescence band at similar to 5 eV, which origin remained largely unknown up to now, showed characteristic behaviour of the donor-acceptor pair (DAP) centres and was assigned to the so-called antisite defects. The antisite DAPs are formed by position-swapped cations Al-Mg(center dot) and Mg-Al', which define the inversion degree of the spinel structure and, to the best of our knowledge, always exceed 10% (>1.5 x 10(21) cm(-3)) in the synthetic solids. Full data set allowed confirming that extremely high concentration of these intrinsic defects is the main reason for the strong 5-eV luminescence, which mechanism includes the electron transfer from Mg-Al' to Al-Mg(center dot) (excitation) following by that from V-O(center dot) to Mg-Al(x) (emission). Irradiation with He (+) ions up to fluences of 10(17) cm(-2) randomly displaced cations to empty sites of the spinel lattice resulting in more intragap defect levels, which induce a non-radiative energy relaxation channel quenching closely spaced while unaffecting distant DAP luminescence.

Cd2+ cation doped CsPbCl3 single crystals were synthesized, and their luminescent properties were investigated under excitation with synchrotron radiation quanta with energy greater than Eg and at a sample temperature of 12 K. Pure single crystals exhibit a narrow band of exciton emission at 416.7 nm. Doping the single crystals leads to a high-energy shift of the excitonic luminescence, the appearance of new luminescent bands, the broadening of the luminescent bands, an increase in the decay time constants, and a significant enhancement of the luminescence intensity. The observed changes in luminescent parameters upon doping are discussed in terms of the crystal structure ordering, a decrease in the number of defects responsible for capturing charge carriers at the stage of electronic excitation thermalization, and a decrease in defects that quench excitonic luminescence. Doped single crystals with decay times of the order of 0.4 ns and intensities that significantly exceed the intensity of pure single crystals can be promising scintillation materials for positron emission tomography in the time-of-flight mode.

A set of Pr3+-doped KLuS2 samples with different Pr concentrations are synthesized in the form of transparent crystalline hexagonal platelets by chemical reaction under a flow of hydrogen sulfide. Their structural, optical, and scintillation properties are investigated thoroughly by means of x-ray powder diffraction, time-resolved luminescence spectroscopy, scintillation light yield, and decay measurements to assess the spectroscopic properties of Pr3+ ions in the KLuS2 host. Charge-trapping processes are further investigated by electron paramagnetic resonance. The fundamental absorption band edge of the KLuS2 host is located at 303 nm, the absorption due to the Pr3+ 4f -> 5d transition is found at 347 nm, the emission maximum is given by the 4f 15d1 -> 3H4 transition of the Pr3+ ion at 380 nm, and the leading photoluminescence and scintillation decay time is around 1 ns at room temperature. The moderate scintillation light yield reaches 7200 ph/MeV. A phenomenological model is fitted to the measured temperature dependences of the photoluminescence emission spectra and photoluminescence decay times to better understand the dynamics of the 5d excited state of the Pr3+ center in the KLuS2 host. The number of photons emitted in the first nanosecond of the scintillation response is evaluated and found to be considerably higher (1.6-2.5 times) than that in the commercial (Lu;Y)2(SiO4)O:Ce, Ca scintillator, which shows the potential of Pr-doped KLuS2 for fast-timing scintillator applications.

Short-wavelength synchrotron radiation excitation has been an indispensable tool in the studies of the properties of wide gap materials using time-resolved low-temperature luminescence spectroscopy. In recent years, several setups for such investigations have been launched at MAX IV Laboratory and Photon Science at DESY. Two permanently stationed time-resolved luminescence setups at FinEstBeAMS and P66 beamlines are in operation at MAX IV 1.5 GeV and Petra III storage rings, respectively. Mobile luminescence setups have been developed for studies at FemtoMAX and P23 beamlines. FinEstBeAMS, P66 and P23 provide time resolution from ∼160 to 100 ps. The FemtoMAX photon source based on an in-vacuum undulator getting an electron beam from the 3 GeV linear accelerator provides an exceptional time resolution of ∼30 ps, limited by time response of the photodetector. The performance of the setups, achieved milestones and research challenges are discussed for four new luminescence stations available for the research community with the main focus on time-resolved techniques.

Radiation effects in cerium doped Gd3(Al,Ga)5:O12 (or GGAG) single crystals irradiated by swift heavy ions with fluences ranging from 6·1010 to 2·1012 ions/cm2 have been studied. A stable strong induced absorption observed in the spectral range 200–350 nm correlates with the irradiation fluence. It is suggested that several centers are responsible for this induced absorption in GGAG single crystals and their possible origin (F-type centers and V-centers or holes trapped near cation vacancies) is proposed and discussed. The swift heavy ions irradiation strongly modifies the luminescence properties of GGAG, namely, the excitation spectra of the Ce3+ emission, which have been measured over a wide spectral range including vacuum ultraviolet diapason. In particular, it was shown that the formation of the stable radiation defects under swift heavy ions irradiation leads to the effective Ce4+ → Ce3+ transformation in the Mg2+ co-doped GGAG single crystals. The reasons leading to the alteration in the luminescence properties of irradiated GGAG single crystals are elucidated and discussed.

Photoluminescence and excitation spectra of microcrystalline and nanocrystalline nickel tungstate (NiWO4) were measured using UV-VUV synchrotron radiation source. The origin of the bands is interpreted using comparative analysis with isostructural ZnWO4 tungstate and based on the results of recent first-principles band structure calculations. The influence of the local atomic structure relaxation and of Ni2+ intra-ion d–d transitions on the photoluminescence band intensity are discussed.

Titanium dioxide (TiO2) has rich physical properties with potential implications for both fundamental physics and new applications. To date, the main focus of applied research is to tune its optical properties, which is usually done via doping and/or nanoengineering. However, understanding the role of d electrons in materials and possible functionalization of d-electron properties are still major challenges. Herewith, within a combination of an innovative experimental technique, high-energy optical conductivity, and state-of-the-art ab initio electronic structure calculations, we report an emerging, novel resonant exciton in the deep ultraviolet region of the optical response. The resonant exciton evolves upon low-concentration Ta substitution in anatase TiO2 films. It is surprisingly robust and related to strong electron-electron and electron-hole interactions. The d- and f-orbital localization, due to Ta substitution, plays an unexpected role, activating strong electronic correlations and dominating the optical response under photoexcitation. Our results shed light on a new optical phenomenon in anatase TiO2 films and on the possibility of tuning electronic properties by Ta substitution.

Complex investigations of thermostimulated luminescence (TSL) and radioluminescence properties of Li2B4O7 (LTB), LTB:Cu, LTB:Ag and LTB:Cu, Ag crystals suitable for tissue equivalent dosimeters were carried out in the temperature range 4.2–700K. TSL, cathodoluminescence and X-ray excited luminescence spectra are compared to those measured under photoexcitation. The emission band at 4.6eV in LTB:Ag is reliably related to Ag+ ions based on the comparison of the results of optical spectroscopy studies and first principle calculations. Energy transfer from the relaxed exited state of the Ag+ ion to the Cu+ ion in double-doped LTB:Cu, Ag crystals is demonstrated. Thermostimulated recombination of charge carriers in irradiated crystals is seen to take place mainly at oxygen sites at low temperatures and at impurity sites at high temperatures. For the first time, the appearance of the low-temperature TSL peak at 90K is assigned to ionic processes in LTB crystals. The appearance of pyroelectric flashes due to the lattice relaxation in the temperature region 90–240K is demonstrated and their surface-related nature clarified. In accordance with EPR studies the dosimetric TSL peaks in copper and silver doped LTB crystals are attributed to thermally released electrons recombining with Cu2+ and Ag2+ centres.

The excitation spectra for the emissions of chromium-containing centres have been measured at 10K using synchrotron radiation of 4–32eV in MgO single crystals with different content of Cr3+ (5–850ppm) and Ca2+ impurity ions. Both virgin crystals and the samples preliminarily irradiated with x-rays at 295K have been studied. The role of complex chromium centres containing two Cr3+ and a cation vacancy (sometimes nearby a Ca2+ ion) on the luminescence processes and the transformation/creation of structural defects has been analysed. Such anharmonic complex centres could serve as the seeds for the creation of 3D defects that facilitate the cracking and brittle destruction of MgO crystals under their irradiation with ∼GeV heavy ions providing extremely high excitation density within cylindrical ion tracks.

The optical conductivity (σ1) of SrTiO3 for various vacancies has been systematically studied using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry. For cation (Ti) vacancies, σ1 shows large spectral weight transfer over a wide range of energy from as high as 35 eV to as low as 0.5 eV and the presence of mid-gap states, suggesting that strong correlations play an important role. Meanwhile, for anion (O) vacancies, σ1 shows changes from 7.4 eV up to 35 eV.

In condensed matter physics the quasi two-dimensional electron gas at the interface of two different insulators, polar LaAlO 3 on nonpolar SrTiO 3 (LaAlO 3 /SrTiO 3 ) is a spectacular and surprising observation. This phenomenon is LaAlO 3 film thickness dependent and may be explained by the polarization catastrophe model, in which a charge transfer of 0.5 e − from the LaAlO 3 film into the LaAlO 3 /SrTiO 3 interface is expected. Here we show that in conducting samples (≥4 unit cells of LaAlO 3 ) there is indeed a ~0.5 e − transfer from LaAlO 3 into the LaAlO 3 /SrTiO 3 interface by studying the optical conductivity in a broad energy range (0.5–35 eV). Surprisingly, in insulating samples (≤3 unit cells of LaAlO 3 ) a redistribution of charges within the polar LaAlO 3 sublayers (from AlO 2 to LaO) as large as ~0.5 e − is observed, with no charge transfer into the interface. Hence, our results reveal the different mechanisms for the polarization catastrophe compensation in insulating and conducting LaAlO 3 /SrTiO 3 interfaces.

MgO nano-powder with an average crystallite size of nanoparticles ranging 10-15 nm was synthesized using the extractive-pyrolytic method and was studied by room temperature VUV spectroscopy under synchrotron radiation excitation. Comparative analysis of their luminescent properties with that of macrocrystalline powder analogues and an MgO single crystal, grown by the arc-fusion method, has been performed under excitation by pulsed VUV synchrotron radiation. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. A considerable blue shift of about, similar to 0.3 eV in the excitation spectra of 2.95 eV emission band, was revealed in nanocrystalline MgO samples. (C) 2013 Elsevier B.V. All rights reserved.