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

Abstract

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—CaGd2(WO4)4: Nd3+, Yb3+ and CaGd2(MoO4)4: Nd3+, Yb3+, which exhibit high sensing capacity—are successfully prepared and systematically characterized. The X-ray diffraction analysis reveals that CaGd2(WO4)4: Nd3+, Yb3+ exhibits the monoclinic structure, whereas CaGd2(MoO4)4: Nd3+, Yb3+ is in the tetragonal phase. The maximum phonon cut-off energy of CaGd2(WO4)4: Nd3+, Yb3+ is 929 cm−1, slightly larger than that of CaGd2(MoO4)4: Nd3+, Yb3+ (904 cm−1). Irradiated by 980 nm laser, PL spectra consist of the Nd3+: 4Fj → 4I9/2 (j = 7/2, 5/2, 3/2) and Yb3+: 2F5/2 → 2F7/2 emissions located in the near-infrared (NIR) range. Negative and positive thermal quenching behaviors are observed for the emissions of Nd3+ (754, 802, and 868 nm) and Yb3+ (1008 nm), respectively, attributed to the cooperative effects of the phonon-assisted energy transfer process of Yb3+ → Nd3+ and excited state absorption process of Nd3+. Moreover, the host material can affect the luminescence properties, thermal enhancement factor of the NIR anti-Stokes emission, and thermal sensing performance of the Nd3+-Yb3+ pair. Consequently, CaGd2(MoO4)4: Nd3+, Yb3+ exhibits higher sensing sensitivity (Sr-max = ∼6.00%K−1) than CaGd2(WO4)4: Nd3+, Yb3+ (Sr-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 Nd3+ and Yb3+ ions.