Optical properties of B2O3–CaF2 glass-ceramics doped with silver nanoparticles and praseodymium ions
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We report on the synthesis and optical experiments with borate glass-ceramics (B2O3-CaF2 glass enclosing CaF2 nanocrystals), either undoped or doped with praseodymium ions (Pr³⁺) and containing silver nanoparticles (Ag-NPs). The techniques of transmission electron microscopy, X-rays diffraction, linear optical absorption, and photoluminescence (PL), were used to characterize the samples prepared. The glass samples, prepared by using the diffusion method, were heat-treated for nucleation of CaF2 nanocrystals (CaF2-NCs) with cubic structure and dimensions of ∼20 nm, and Ag-NPs (with sizes between ∼2 and ∼50 nm). Optical absorption and PL experiments were performed. The glass-ceramic with Ag-NPs, but without Pr³⁺, exhibited tenfold enhancement (∼1000%) of the blue PL from the CaF2-NCs, in comparison with the samples without Ag-NPs. In the samples with Pr³⁺ the presence of Ag-NPs induced small enhancement of the absorption spectra in the blue region [Pr³⁺ transitions: ³H4→³PJ (J=0, 1, 2), ¹I6] and 75% increase of the emitted intensity in the red region (Pr³⁺ transitions: ¹D2→³H4). On the other hand, the CaF2-NCs luminescence was totally quenched due to the presence of Pr³⁺. This effect was attributed to resonant energy-transfer from the CaF2-NCs to Pr³⁺. The phenomena observed in the experiments were understood considering the contributions of the local electric field in the vicinities of the Ag-NPs and processes of energy-transfer among the CaF2-NCs, Ag-NPs and Pr³⁺.