Role of host matrix on NIR emission and ratiometric thermometry properties in Nd3+/Yb3+ co-doped CaGd2(MO4)4 (M = W, Mo) phosphors

The development of ultrasensitive thermometric phosphors has become an active area of research due to their appealing applications in the contactless temperature detection field. Herein, the scheelite-related compounds—CaGd 2 (WO 4 ) 4 : Nd 3+ , Yb 3+ and CaGd 2 (MoO 4 ) 4 : Nd 3+ , Yb 3+ , which exhibit high sensing capacity—are successfully prepared and systematically characterized. The X-ray diffraction analysis reveals that CaGd 2 (WO 4 ) 4 : Nd 3+ , Yb 3+ exhibits the monoclinic structure, whereas CaGd 2 (MoO 4 ) 4 : Nd 3+ , Yb 3+ is in the tetragonal phase. The maximum phonon cut-off energy of CaGd 2 (WO 4 ) 4 : Nd 3+ , Yb 3+ is 929 cm −1 , slightly larger than that of CaGd 2 (MoO 4 ) 4 : Nd 3+ , Yb 3+ (904 cm −1 ). Irradiated by 980 nm laser, PL spectra consist of the Nd 3+ : 4 F j → 4 I 9/2 (j = 7/2, 5/2, 3/2) and Yb 3+ : 2 F 5/2  →  2 F 7/2 emissions located in the near-infrared (NIR) range. Negative and positive thermal quenching behaviors are observed for the emissions of Nd 3+ (754, 802, and 868 nm) and Yb 3+ (1008 nm), respectively, attributed to the cooperative effects of the phonon-assisted energy transfer process of Yb 3+ → Nd 3+ and excited state absorption process of Nd 3+ . Moreover, the host material can affect the luminescence properties, thermal enhancement factor of the NIR anti-Stokes emission, and thermal sensing performance of the Nd 3+ -Yb 3+ pair. Consequently, CaGd 2 (MoO 4 ) 4 : Nd 3+ , Yb 3+ exhibits higher sensing sensitivity (S r-max  = ∼6.00%K −1 ) than CaGd 2 (WO 4 ) 4 : Nd 3+ , Yb 3+ (S r-max  = ∼5.56%K −1 ). This study provides useful information for the construction of high-performance ratiometric thermometers based on the inverse thermal dependence of the NIR emissions of Nd 3+ and Yb 3+ ions.