Investigation of Ca2al2sio7: Pr3+/Gd3+ Uvb Upconverting Phosphor and its Optical Thermometric Properties

Abstract

On basis of fluorescence intensity ratio (FIR) over certain narrow coupled levels of rare-earth ions, up-conversion of RE-doped materials perform excellent temperature-sensing abilities for thermometry applications. A series of Pr3+/Gd3+ co-doped Ca2Al2SiO7 phosphors (Ca2Al2SiO7:Pr3+/Gd3+) were prepared by a conventional high-temperature solid-state reaction. Under excitation of blue ~ 445 nm, the Ca2Al2SiO7:Pr3+/Gd3+ phosphor yields intense photoluminescence (PL) bands around 489, 528, 537, 580, 604, 613, and 646 nm due to electronic transition of 3P0 → 3H4, 3P0 → 3H5, 3P1 → 3H5, 3P1 → 3H6, 1D2 → 3H4, 3P0 → 3H6, and 3P0 → 3F2 of Pr3+, respectively. Their relative intensities are sensitively determined by Pr3+ contents, which, from 0.1% to 5%, tune the lighting color from pink to warm white under excitation of 445 nm. It is of great interest that intense irradiation using a blue ~ 445 laser does make efficient ultraviolet B (UVB) up-conversion at about 297, 306, and 313 nm, just stemming from the 6P3/2 → 8S7/2, 6P5/2 → 8S7/2, and 6P7/2 → 8S7/2 of Gd3+, respectively. A typical two-photon absorption process was proved to account for the UVB up-conversion, i.e. successive absorption of two blue ~ 445 nm photons populating Pr3+ 4f5d excited state, which further efficiently transfer its energy to the 6IJ/6PJ states of Gd3+ neighbor for UVB emission. Moreover, the thermally coupled levels (TCLs) of Gd3+: 6P3/2-6P7/2, and Gd3+: 6P5/2-6P7/2 were thoroughly investigated for temperature sensing in 298-598 K. Maximum relative sensitivity of the 6P3/2-6P7/2 TCLs, i.e. Sr(I306/I313), is proved to be 0.96% K-1 at 298 K, and that of the 6P5/2-6P7/2 TCLs, i.e. Sr(I297/I313), is 1% K-1 at 373 K. The relevant thermal stabilities of the Ca2Al2SiO7:Pr3+/Gd3+ thermometric candidate was well validated with 10 cycles of heating and cooling processes in 298-598 K. Such continuous explorations of Pr3+/Gd3+ co-doping system will significantly promote the development of robust UVB up-converting phosphors as well as their practical applications in the field of high-resolution optical thermometry.

Keywords: up-conversion, Pr3+/Gd3+, ultraviolet B, optical thermometry, silicate-matrix