Comparative studies of upconversion luminescence and optical temperature sensing in Tm(3+)/Yb(3+) codoped LaVO(4) and GdVO(4) phosphors

Tm(3+)/Yb(3+) codoped LaVO(4) and GdVO(4) phosphors are successfully synthesized using solid state reaction methods and then upconversion emission studies are performed. X-ray diffraction has confirmed a pure monoclinic phase of LaVO(4) and a tetragonal phase of GdVO(4). Upconversion emission through 980 nm laser diode excitation has shown a strong blue band at 475 nm and two weak red bands at 647 and 700 nm originating from (1)G(4) → (3)H(6), (1)G(4) → (3)F(4) and (3)F(3) → (3)H(6) transitions of Tm(3+) ions, respectively. Non-thermally coupled levels viz.(3)F(3) (700 nm) and (1)G(4) (475 nm) in both the phosphors are used for fluorescence intensity ratio based optical thermometric studies and a comparison is made. The FIR data against temperature were fitted with polynomial and exponential fittings. The results show that polynomial fitting has a higher absolute sensitivity of 21.2 × 10(−3) K(−1) at 653 K for the LaVO(4): Tm(3+)/Yb(3+) phosphor than the exponential fitting sensitivity of 19.0 × 10(−3) K(−1) at 653 K, while in the case of the GdVO(4): Tm(3+)/Yb(3+) phosphor both fitting functions provided the same value of absolute sensitivity, that is 13.0 × 10(−3) K(−1) at 653 K. A comparison of the sensitivity values shows that the LaVO(4): Tm(3+)/Yb(3+) phosphor provides higher sensitivity than the GdVO(4): Tm(3+)/Yb(3+)phosphor but the latter one is too high in upconversion emission.