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

Abstract

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 Er3+-Yb3+ codoped CaWO4 phosphors are prepared and Cr3+ ions are introduced, trying to modify the upconverion (UC) emissions from Er3+ ions. Under the excitation of 980 nm laser, green UC luminescence from 2H11/2 and 4S3/2 states of Er3+ is successfully strengthened with Cr3+ 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 Cr3+ ions could affect the crystallinity and distort the crystal structure of the as-prepared samples. The thermometry behavior based on green emissions of Er3+ 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 Cr3+. When introducing 9 mol% Cr3+ ions into Er3+-Yb3+ codoped CaWO4 phosphors, a maximum sensitivity of about 1.91% K−1 has been achieved. Combined with the intense green luminescence, CaWO4: Er3+-Yb3+-Cr3+ would be a promising candidate for optical thermometry with excellent measurement sensitivity and accuracy.