EPR and Luminescence of ${\hbox {F}}^{+}$ Centers in Bulk and Nanophosphor Oxyorthosilicates

Abstract

The main thermally stimulated luminescence glow peak in irradiated oxyorthosilicates occurs near 360-400 K and has been postulated to comprise an electron trapped at an oxygen vacancy (F + center). We have used electron paramagnetic resonance spectroscopy to identify this defect in Ln 2 SiO 5 : Ce (Ln = Lu and Y) and show that it consists of a single electron trapped at a non-silicon-bonded oxygen vacancy in both bulk and nanophosphor oxyorthosilicates. Both Lu-and Y-based nanophosphors form seven-and nine-oxygen coordinated structures (P2 1 c) whereas the bulk phosphors form six-and seven-oxygen coordinated structures (C2/c). In each case the F + center predominately forms at the larger oxygen site. A typical resonance comprises a single Gaussian line broadened by hyperfine interaction with g-values near the free electron value and hyperfine coupling 0.4 mT. The F + center can be annealed and radiation-induced, consistent with the thermally stimulated luminescence glow peak behavior.

Luminescence properties of Ce-doped oxyorthosilicate nanophosphors and single crystals

“…A similar shift to lower temperatures was reported in SCS-prepared Y 2 O 3 when compared to samples prepared by a solid state reaction [45]. More importantly, the TL intensity of nanophosphors was lower than the TL from single crystals for the same dose, as reported earlier [24]. One of the possible explanations is a low concentration of trapping centers, which together with higher Ce 3 + concentration would explain the relatively high RL output because of reduced competition for charge carrier localization.…”

“…Vedda et al [32] suggested that the trapping centers responsible for the four TL peaks in the 340-560 K in LSO and LYSO (Lu x Y 2 À x SiO 5 ) are related to oxygen vacancies, the TL originating from electron tunneling from the excited state of the vacancy to the Ce 4 + defect, creating a Ce 3 + defect in the excited state which relaxes by photon emission. As pointed out by Cooke et al [24], the SCS process is oxygen rich, whereas Czochralski method is relatively oxygen deficient, therefore leading to a lower concentration of oxygen defects in the nanophosphors. According to the same authors, this conclusion is also supported by preliminary measurements of spin concentration of F + centers [24].…”

“…Several studies on Y 2 SiO 5 :Pr 3 + nanocrystals have shown that the reduced dimensionality leads to suppression of phonon emission within the 1 D 2 energy levels of Pr 3 + , resulting in intense fluorescence from the 1 D 2 to the 3 H 4 energy levels of Pr 3 + and surprisingly sharp lines even at room temperature [21][22][23]. EPR studies of YSO and LSO prepared by solution combustion synthesis (SCS) showed peak broadening due to structural disorder [24,25]. Also, it was shown that the concentration quenching for nanoscale YSO and LSO is peaked around 1 at% [26], above the maximum Ce concentration of 0.05 at% achievable in single crystals produced by the Czochralski method [27].…”

Complex oxide scintillators: Material defects and scintillation performance

“…The similarity of trap depths suggests a possible thermally assisted tunneling radiative recombination process between electron trap and Ce 4+ hole center [122] similarly as found in Lu x Y 1-x AP:Ce earlier [138]. Very recently, an electron captured in oxygen vacancy in the form of F + center was evidenced in LSO:Ce by ESR [139] and bleaching of this center was interconnected with the mentioned afterglow phenomena. A very nice confirmation of the intrinsic origin of traps was the finding that all oxyorthosilicates possessing C2/c structure (YSO, LSO, YbSO, ErSO) are characterized by similar TSL glow curves above RT, irrespective of their doping [140,141] so that crystal structure seems to play an important role in trap properties.…”

Nanophosphor aluminum oxide: Luminescence response of a potential dosimetric material

“…The intense OSL response of bulk Al 2 O 3 :C to ionizing radiation is believed to be governed by the presence of oxygen vacancies, F, and F + centers in the material due to its growth conditions [21], and the reduced OSL (and TL) intensity of the nanophosphor samples may be due to fewer oxygen vacancies in the crystal lattice as the SCS process has been shown to produce fewer oxygen vacancies in other oxides [25]. However, the reduced luminescence intensities of the nanophosphors could be affected by other situations such as the creation of competing and possibly non-luminescent traps or the incorporation of other luminescent phases such as aluminum nitrides [20,26].…”