Improved green upconversion emissions from CaWO4: Er3+-Yb3+ by Cr3+ codoping for optical thermometry

Optical temperature sensors based on rare earth ions activated luminescent materials have presented their capabilities in thermal reading with good stability and high spatial resolution. The ongoing challenge is to improve the luminescence for further enhancing temperature measurement sensitivity and accuracy. Here, several Er 3+ -Yb 3+ codoped CaWO 4 phosphors are prepared and Cr 3+ ions are introduced, trying to modify the upconverion (UC) emissions from Er 3+ ions. Under the excitation of 980 nm laser, green UC luminescence from 2 H 11/2 and 4 S 3/2 states of Er 3+ is successfully strengthened with Cr 3+ codoping. Through the analysis of characterization by scanning electron microscopy, X-ray diffraction, Raman spectroscopy , and time-resolved spectroscopy, it is found that the dopant of Cr 3+ ions could affect the crystallinity and distort the crystal structure of the as-prepared samples. The thermometry behavior based on green emissions of Er 3+ is studied via fluorescence intensity ratio method. It is interesting to observe that the temperature sensing performance has also been influenced by the incorporation of Cr 3+ . When introducing 9 mol% Cr 3+ ions into Er 3+ -Yb 3+ codoped CaWO 4 phosphors, a maximum sensitivity of about 1.91% K −1 has been achieved. Combined with the intense green luminescence, CaWO 4 : Er 3+ -Yb 3+ -Cr 3+ would be a promising candidate for optical thermometry with excellent measurement sensitivity and accuracy.