Shangke Pan

Temperature dependence of luminescence characteristics of Lu2(1−x)Y2xSiO5:Ce3+ scintillator grown by the Czochralski method

In this paper, we investigate the temperature dependence of luminescence characteristics of highly efficient cerium-doped scintillator, lutetium-yttrium orthosilicate Lu2(1−x)Y2xSiO5:Ce3+. The photoluminescence excitation and emission spectra have been measured in a broad temperature range. The temperature dependence of the Ce1 and Ce2 decay times shows the onset of decay time decrease at about 300 K. This observation demonstrates that unlike so far reported, the Ce2 center is not substantially quenched until room temperature. The 5d1 excited-state ionization of both Ce1 and Ce2 centers is studied by purely optical methods.

Structure and thermal stability of δ-Bi2O3 thin films deposited by reactive sputtering

Nanocrystalline ?-Bi2O3 thin films have been deposited onto Si (100) and quartz substrates by reactive sputtering. The structural characteristics and thermal stability of the thin films have been investigated by x-ray diffraction and Raman spectra. It was found that the ?-Bi2O3 thin films could exist stably below 200??C and undergo a phase transition sequence ? ? ? ? ? with increasing annealing temperature beyond 200??C. These phase transitions were further confirmed by optical constant and optical band gap studies.

Annealing effects on Czochralski grown Lu2Si2O7:Ce3+ crystals under different atmospheres

Cerium-doped lutetium pyrosilicate Lu2Si2O7:Ce3+ (LPS:Ce) single crystal samples grown via Czochralski method were annealed under air and H2 atmospheres from 350to1600°C. The x-ray excited luminescence spectra and UV as well as thermal stimulated luminescence spectra were measured. It was found that the luminescence efficiency after annealing in air at 1400°C was significantly enhanced compared to the unannealed samples. This phenomenon was suggested to be caused by the existence of some oxygen vacancies in the LPS:Ce crystals, and these oxygen vacancies can be effectively eliminated by means of annealing in air. Although the annealing will lead to the oxidization of cerium ions from Ce3+ ions to Ce4+, which is harmful to the luminescent efficiency, the annealing treatment does enhance the luminescent efficiency of LPS:Ce, without changing its luminescence mechanics.

Air Atmosphere Annealing Effects on LSO:Ce Crystal

Cerium-doped lutetium oxyorthosilicate Lu2SiO5:Ce crystal is a well-known scintillator with an excellent figure-of-merit. In this paper, air annealing was used to further improve its scintillation properties. The samples cut from Czochralski grown Lu2SiO5:Ce crystals were annealed in air atmosphere at 1400°C for 10 h. Based on the analysis of the difference in the crystals spectra (such as absorption, emission, and thermoluminescence) obtained before and after annealing, we investigate the nature of the observed effects. To investigate the annealing effects on the valence state of cerium ion, synchrotron radiation was used in this work as an energy source to measure the X-ray absorption near edge spectroscopy of cerium ions. It is supposed that annealing in air atmosphere helps to reduce the content of deep traps, which possibly include oxygen vacancy, lower the intensity of afterglow, shorten the decay time of afterglow, and increase the luminescence intensity of Ce2 (6-oxygen-coordinated). However, this may also cause cerium ion to be oxidized to +4 charge state, which may introduce an additional nonradiative trap center, thereby weakening luminescence.

Study on the cerium oxidation state in a Lu0.8Sc0.2BO3host

Lu0.8Sc0.2BO3:Ce powders doped with different forms of cerium [CeF3, CeO2 or Ce(NO3)3] were synthesized by solid-state methods under different conditions (carbon powders, nitrogen or air) and studied by a combination of X-ray excited luminescence spectra, optical diffuse reflectance spectra, fluorescence decay curves and X-ray absorption near edge spectroscopy. The optical diffuse reflectance spectra and fluorescence decay curves confirmed the different Ce3+/Ce4+ ratios in the samples of interest. Based on the Ce LIII-edge XANES spectra, we finally concluded that the differences in the scintillation intensities of the samples of interest originate from the varied Ce3+/Ce4+ ratios, which explains why the sample doped with CeF3 and synthesized under carbon powder conditions shows an optimal scintillation emission. The specific Ce3+/Ce4+ ratio was also qualitatively determined by fitting the Ce LIII-edge XANES spectra. More importantly, this study further demonstrates the effectiveness of using XANES spectroscopy to investigate the cerium oxidation state in cerium-doped scintillators.

Effects of scandium on the bandgap and location of Ce3+ levels in Lu1−xScxBO3:Ce scintillators

Transparent Lu1−xScxBO3:Ce (x = 0.3, 0.5, 0.7, 0.8) scintillation crystals were grown by Czochralski method. Vacuum ultraviolet excitation spectra and thermoluminescence glow curves revealed the narrowing of bandgap and the redshift of the first electric dipole-allowed fd transition of Ce3+ with the increase of Sc content. The ionization energy evaluated from the photoluminescence decays as a function of temperature indicated that, with increasing Sc content, the thermal ionization probability occurring from 5d1 relaxed state of Ce3+ was enhanced, which degraded the scintillation efficiency of Lu1−xScxBO3:Ce crystals.

CsI:Tl+,Yb2+: ultra-high light yield scintillator with reduced afterglow

The afterglow problem has been preventing CsI:Tl single crystal scintillators from being used for applications in the field of computer tomography and high-speed imaging. We show that Yb2+ codoping in CsI:Tl can reduce it at least by one order of magnitude after 50 ms from X-ray cut-off compared to ordinary CsI:Tl. After optimization of the Yb2+ and Tl+ concentrations, the doubly doped CsI:Tl,Yb crystal exhibits an ultra-high light yield of 90000 ± 6000 photons MeV−1, energy resolution 7.9%@511 keV and low afterglow level of about 0.035% at 80 ms. Simultaneous improvement in afterglow, light yield and energy resolution in CsI:Tl-based scintillators paves the way to its application in computer tomography and high-speed imaging. The physical mechanism and role of ytterbium ions in afterglow suppression are proposed.

Study on the Luminescence and Energy Level of Lanthanide Ions in Lu0.8Sc0.2BO3 Host

Emission and excitation spectra of undoped and Ln(3+) (Ln = Ce, Pr, Nd, Yb)-doped Lu(0.8)Sc(0.2)BO(3) were studied by vacuum ultraviolet spectroscopy, and the X-ray excited luminescence spectra were measured as well at room temperature. The undoped specimen presented two different emissions upon excitation at energies in the vicinity of the band gap or with X-ray. The emission at the highest energy side, located at 4.77 eV, was ascribed to self-trapped exciton, which is anchored by electron capture around the Sc site. We also observed three broad emission bands at 4.43, 3.02, and 2.10 eV, which might be attributed to the trapped exciton, defect -related emissions, or both. Energy transfer processes to the doped lanthanide ions in Lu(0.8)Sc(0.2)BO(3) via exciton state or sequential electron-hole capture are discussed. Finally, the energy level diagram for divalent and trivalent lanthanides in Lu(0.8)Sc(0.2)BO(3) was constructed using the obtained spectroscopic parameters and the three parameters method (Dorenbos, P. J. Phys.: Condens. Matter 2003, 15, 8417).

Research on phase transition behavior of lutetium orthoborate LuBO3

Cerium-doped lutetium orthoborate LuBO3 : Ce crystal is a promising scintillator with excellent properties, but the single crystal is difficult to grow because of the phase transition. In this article, the reported high-temperature (HT) vaterite phase was proved to exist at a low temperature (below 1200°C). Moreover, the real HT phase of LuBO3 was identified for the first time. The observed HT phase structure of LuBO3 was isostructural with the hexagonal structure of YbBO3 with space group P6322(182). The temperature dependence of the lattice constant and unit cell of the above phase structures were also investigated. The relative stability of calcite phase of LuBO3 could associate with fitness of the radius of Lu3+ ion for the six-coordination structure.

Angle-dependent photoluminescence of [1 1 0]-oriented nitrogen-doped SnO2 films

The orientation growth and angle-dependent photoluminescence (PL) of nitrogen-doped SnO2 films grown on Si substrate by reactive magnetron sputtering have been studied. It was found that the orientation of the nitrogen-doped SnO2 films depends strongly on their thickness, and a highly [1 1 0]-oriented film was achieved with a seed layer. An angle-dependent PL of the [1 1 0]-oriented films was observed, and the PL peak position shifts towards the longer wavelength side and the PL peak width decreases as the observation angle decreases. This angle-dependent PL was attributed to the Fabry–Perot optical interference effect.