Enhanced NIR-NIR luminescence from CaWO4: Nd3+/Yb3+ phosphors by Li+ codoping for thermometry and optical heating

Rare earth ions doped luminescent materials have demonstrated their potentiality as sensitive thermometers with good accuracy and high spatial resolution. Nevertheless, low luminescence efficiency, especially at high temperature, still limits the development of this thermometry technique. Here, Nd3+/Yb3+ codoped CaWO4 phosphors were synthesized using high temperature solid-state method, and Li+ ions were introduced aiming to improve the near-infrared (NIR) emissions from Nd3+ in the spectral range of 725-950 nm. The influence of Li+ codoping on the structure and NIR luminescence in CaWO4: Nd3+/Yb3+ when excited by a 980 nm laser diode was investigated. NIR-NIR emissions from Li+ codoped samples are successfully strengthened compared with that without Li+ ions, which is mainly attributed to the distortion of the crystal structure and the increase of crystalline size by Li+. The optical temperature sensing behavior of NIR luminescence from Nd3+ was studied based on fluorescence intensity ratio (FIR). Assisted by Li+ ions, the NIR-NIR emissions are observed to be effectively enhanced with the increment of temperature. And a new pair of thermally coupled levels in Nd3+ ions with excellent thermometry characteristics is explored, through which superior measurement sensitivity as well as accuracy could be achieved. In addition, light-induced temperature rise was realized in the phosphors, which makes CaWO4: Nd3+/Yb3+/Li+ excellent candidate for luminescence thermometry and optical heaters.