Engineering dual-mode near-infrared ratiometric thermometers based on rare earth-doped molybdates

Near-infrared optical thermometers have sparked great interest for their ability to provide non-destructive testing and high-resolution. However, the restricted relative sensitivity and single temperature measurement mode represent the current limitations of luminescent thermometers. Herein, near-infrared dual-mode ratiometric thermometers with high sensitivity in La₂(MoO₄)₃: Yb³⁺, Ln³⁺ (LMO: YbLn, Ln = Er, Ho, Nd) phosphors were designed. Interestingly, excellent thermal enhancement of near-infrared emission (Nd³⁺: ⁴F_3/2 → ⁴I_9/2) over 492 times was observed through effective phonon-assisted energy transfer process, resulting in an ultra-high relative sensitivity of 2.84 % K^-1 at 313 K. Furthermore, a near-infrared ratiometric thermometer was fabricated utilizing the ⁵I₆ energy level of Ho³⁺ (1200 nm) and the ⁴I_13/2 energy level of Er³⁺ (1550 nm). Relying on the luminescence thermal enhancement of Er³⁺ and the luminescence thermal burst of Ho³⁺, a relative sensitivity of 0.31 % K^-1 at 573 K was obtained. The excellent near-infrared emission of the sample was reasonably confirmed through spectral monitoring of penetrating chicken breast of different thicknesses. These findings provide a viable approach for the design of near-infrared phosphors, which has the potential to impact the field of near-infrared temperature sensing.