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Investigation of Ca2Al2SiO7: Pr3+/Gd3+ UVB upconverting phosphor and its optical thermometric properties

Shanshan Zhao,Saima Kanwal,Weiwen Su,Hui Lin,Zhaoxia Han,Songlin Zhuang,Dechao Yu,Dawei Zhang

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 Pr 3+ /Gd 3+ co-doped Ca 2 Al 2 SiO 7 phosphors (Ca 2 Al 2 SiO 7 :Pr 3+ /Gd 3+ ) were prepared by a conventional high-temperature solid-state reaction. Under excitation of blue ∼445 nm, the Ca 2 Al 2 SiO 7 :Pr 3+ /Gd 3+ phosphor yields intense photoluminescence (PL) bands around 489, 528, 537, 580, 604, 613, and 646 nm due to electronic transition of 3 P 0 → 3 H 4 , 3 P 0 → 3 H 5 , 3 P 1 → 3 H 5 , 3 P 1 → 3 H 6 , 1 D 2 → 3 H 4 , 3 P 0 → 3 H 6 , and 3 P 0 → 3 F 2 of Pr 3+ , respectively. Their relative intensities are sensitively determined by Pr 3+ contents, which, from 0.1 % to 5 %, tune the emission 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 6 P 3/2  →  8 S 7/2 , 6 P 5/2  →  8 S 7/2 , and 6 P 7/2  →  8 S 7/2 of Gd 3+ , 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 Pr 3+ 4f5d excited state, which further efficiently transfer its energy to the 6 I J / 6 P J states of Gd 3+ neighbor for UVB emission. Moreover, the thermally coupled levels (TCLs) of Gd 3+ : 6 P 3/2 - 6 P 7/2 , and Gd 3+ : 6 P 5/2 - 6 P 7/2 were thoroughly investigated for temperature sensing in 298–598 K. Maximum relative sensitivity of the 6 P 3/2 - 6 P 7/2 TCLs, i.e. S r (I 306 /I 313 ), is proved to be 0.96 % K −1 at 298 K, and that of the 6 P 5/2 - 6 P 7/2 TCLs, i.e. S r (I 297 /I 313 ), is 1 % K −1 at 373 K. The relevant thermal stabilities of the Ca 2 Al 2 SiO 7 :Pr 3+ /Gd 3+ thermometric candidate was well validated with 10 cycles of heating and cooling processes in 298–598 K. Such continuous explorations of Pr 3+ /Gd 3+ 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.