Effect of doping Ge into Y2O3:Ho,Yb on the green-to-red emission ratio and temperature sensing

A series of Ge-doped monophase Y2O3:Ho,Yb phosphor materials has been synthesized using solid state reactions. The addition of Ge to the Y2O3 host decreases the Ho green emission (F-5(4)/S-5(2) I-5(8)) and increases the red emission (F-5(5) I-5(8)), providing a new means to tune the green-to-red emission intensity ratio. It is proposed that the Ge-induced multiphonon relaxation process enhances the transition from the intermediate state I-5(6) to I-5(7), which tunes the green and red emission intensities. Most importantly, with the addition of Ge, the non-thermally coupled Ho green and red emitting levels are associated together, and the red-to-green emission intensity ratio becomes sensitive to environmental temperature change. The absolute thermal sensitivity is enhanced by a factor of >5 times that in the absence of Ge. The matched green and red emission intensities, as well as the high thermal sensitivity, make Y2O3:Ho,Yb,Ge an ideal probe for optical temperature sensing at the single particle level in live biological samples. This study demonstrates a new mechanism to channel non-thermally coupled energy levels to achieve high temperature sensitivity.