A novel ultrahigh-sensitive optical thermometer based on inverse thermal responses of Nd3+and Yb3+emissions in BaGd2(MoO4)4 phosphor

Nowadays, lots of efforts have been vested on the exploitation of novel luminescent thermometric materials to achieve remote temperature readout with ultra-high sensitivity and sub-degree resolution. Herein, the BaGd2(-MoO4)4: Yb3+, Nd3+ phosphors were successfully prepared via a solid-state reaction method, and their concentration/temperature-dependent luminescence behaviors were investigated systematically. Upon 980 nm excitation, the photoluminescence (PL) spectra consisted of three emission bands centered at 754 nm, 805 nm and 872 nm from Nd3+, together with an emission band peaked at 1008 nm from Yb3+. As the temperature elevated, the Yb3+: 2F5/2 -> 2F7/2 emission weakened sharply, whereas the promoted phonon-assisted energy transfer process from Yb3+ to Nd3+ and excited-state absorption process of Nd3+ led to an intense thermal enhancement of the Nd3+: 4Fj -> 4I9/2 (j = 7/2, 5/2, 3/2) transitions. Moreover, the thermally enhancing factor was found to depend on the Nd3+ concentration. By utilizing the luminescence intensity ratio (LIR) technique, the temperature sensing behaviors based on thermally coupled levels (Nd3+: 4Fj, j = 7/2, 5/2, 3/2) and non-thermally coupled levels (Nd3+: 4Fj and Yb3+: 2F5/2) were studied at the different doping concentrations. When the temperature sensing was based on LIR between Nd3+: 4F3/2 -> 2I9/2 and Yb3+: 2F5/2 -> 2F7/2 transitions, the BaGd2(MoO4)4: 3 mol%Nd3+, 15 mol%Yb3+ sample displayed excellent thermal sensitivity (2.28-8.42 %K- 1) and temperature resolution (0.1-0.22 K) at temperatures ranging from 298 K to 573 K, which were superior to those of many other reported luminescence materials. The present work might give a new input for developing near-infrared (NIR) luminescence materials with desirable temperature sensing performances.