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Timestamp: 2019-04-19 05:23:02+00:00

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The present work gives a detailed investigation of the dependence of the real time luminescence of Eu3+-doped tin dioxide nanopowder on rare earth (RE) site symmetry and host defects. Ultrafast time-resolved analysis of both RE-doped and undoped nanocrystal powder emissions, together with electronic paramagnetic resonance studies, show that host-excited RE emission is associated with RE-induced oxygen vacancies produced by the non-isoelectronic RE-tin site substitution that are decoupled from those producing the bandgap excited emission of the SnO2 matrix. A lower limit for the host-RE energy transfer rate and a model for the excitation mechanism are given.
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Fig. 1 (a) Low temperature TRFLN spectra corresponding to the 5D0→7F0-4 transitions of a tin dioxide nanopowder (average grain size 40 nm) doped with 0.5 mol % of Eu2O3 obtained with a time delay of 10 μs after the pump pulse. (b) Observed energy levels of Eu3+ in SnO2 corresponding to sites A (A*), B, and C.
Fig. 2 Excitation spectrum of the 0.5 mol% Eu3+-doped SnO2 powder obtained by collecting the 5D0→7F2 emission at 612 nm (a). Emissions of the mentioned sample resulting from direct excitation at the 5D2 level (b). Emissions obtained by pumping this powder sample above the band gap at 300nm (c).
Fig. 3 (a) X-band (9.39 GHz) EPR spectra recorded on a tin dioxide nanopowder doped with 0.5 mol % of Eu2O3. (b) Best fit obtained for the EPR signal of the most abundant VO+ center detected on this nanopowder.
Fig. 4 Variety of possible two nearest neighbor substitutional Eu3+ ions clusters i(CS), ii(CS) and iii(C1- CS) compensated by an oxygen vacancy (VO++, in white). Blue, red, yellow, and white spheres represent Sn, O, Eu, and O vacancies, respectively.
Fig. 5 (a) Donor-aceptor pair and UV pumping photon. (b) Excitation process of the donor-acceptor pair. Under bandgap excitation a hole is trapped by the RE center (RC-) acting as acceptor and an electron is trapped by the V0+ vacancy acting as donor. (c) The nonradiative recombination of the bound exciton produces the Eu3+ excited state. (d) The radiative relaxation of the RE center leads to the Eu3+ emission.
Fig. 6 Normalized emission spectrum of the 0.5 mol% Eu3+-doped SnO2 powder pumped at 800 nm. The inset shows the power dependence of the VIS Eu3+ integrated emission intensity on a log-log scale. The straight line represents the linear fit to the logarithmic data. Within experimental accuracy, the Eu3+ VIS emission shows a slope of 2.5 up to a 0.5 mJ excitation energy.
Fig. 7 (a) Emission spectral profiles extracted over the whole temporal range in the undoped (blue line) and 0.5 mol % Eu3+ doped (red line) SnO2 powder samples under excitation at 800 nm with 100 fs laser pulses (Eexc = 0.5 mJ) by setting the time window of the Streak camera at 1 ns. (b) Temporal profiles extracted over the 420-520 nm spectral range in these undoped (blue points) and doped (red points) powder samples under the above mentioned excitation conditions and time window. The continuous red and blue lines are the best fits to two-exponential functions. (c) Normalized simultaneous emissions of the 0.5 mol % Eu3+ doped SnO2 powder under three photon excitation at 800 nm extracted over the 417-484 nm spectral range (red line) and the two photon excited second harmonic generation extracted over the 395-417 nm spectral interval (blue line). The inset shows the streak camera image from where these temporal profiles where extracted.
Fig. 8 Spectral (a, c) and temporal (b, d) profiles of the 0.5% mol Eu3+ doped SnO2 powder in the 570-660 nm spectral range (red lines), which mainly corresponds to the 5D0→7F1 emission of Eu3+. The spectral (a, c) and temporal emission profiles (b, d) of the pure SnO2 powder are shown in blue. The green profiles of Fig. 8 (b) and (d) correspond to the subtraction of the pure and doped sample temporal profiles. Figure 8(a) and (b) were obtained by using the 1 ms time window whereas (c) and (d) were obtained with the 100 µs time window.

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