Site-Bi3+ and Eu3+ dual emissions in color-tunable Ca2Y8(SiO4)6O2:Bi3+, Eu3+ phosphors prepared via sol-gel synthesis for potentially ratiometric temperature sensing

Abstract

A series of Ca2Y8(SiO4)6O2 (CYSO):Bi3+, Eu3+ phosphors were prepared via a Pechini-type sol-gel reaction method. The refinement results for CYSO:Bi3+, Eu3+ phosphors implied that they had a pure phase. The blue-green emission ascribed to Bi3+ 3P1→1S0 transition was generated upon UV excitation in Bi3+ singly-doped CYSO samples. Spectral analysis indicated that two main emission bands around 414 and 494 nm correspond to two kinds of Bi3+ occupying the crystal lattices of 4f and 6 h available for Y3+ in CYSO, denoted as Bi3+(2) and Bi3+(1), respectively. A broad spectral overlap between Bi3+ emission and Eu3+ excitation spectra implied the existence of energy transfer from Bi3+ to Eu3+ ions in CYSO:Bi3+,Eu3+, which resulted in the tunable emission color from blue-green to red. The energy transfer mechanism from Bi3+ to Eu3+ ions was determined to be a dipole-quadrupole interaction. Moreover, the quite different luminescence thermal quenching behaviors between Bi3+(2) and Eu3+ showed good temperature sensing properties with a temperature range of 298–523 K by analyzing the temperature sensitivity of the fluorescent intensity ratio [Bi3+(2)/Eu3+(612)]. The maximum absolute and relative sensitivities reached as high as 0.07174 K-1 (523 K) and 0.958% K−1 (423 K), which can be compared to the highest values of 0.015 K-1 and 1.1%K−1 in reported optical thermometric materials before, respectively, based on the thermally coupled level (TCLs) of Er3+. Meanwhile, the luminescence thermal quenching mechanism in this system was investigated in detail. Results inspire that a feasible method based on site-Bi3+ and Eu3+ emissions is potential as one of candidate strategies for developing novel ratiometric optical thermometry materials.