Yb and Er doped M2O2S (M = Y, Gd, La) phosphor was synthesized by solid state flux fusion method and their up conversion spectral properties were studied as a function different Yb concentrations. The solid state flux fusion results in well crystallized hexagonal shaped phosphor particles of average size 4–6 μm. Upconversion spectral studies shows that all the compositions are stronger in green emission with the green emission intensity 1.7 times than the red in composition Gd2O2S:Yb(8)Er(1), Y2O2S:Yb(9)Er(1), La2O2S:Yb(3)Er(7) (All mol%). The internal upconversion efficiency for the green emission bands was calculated to be 74, 62, 100% respectively in Gd2O2S:Yb(8)Er(1), Y2O2S:Yb(8)Er(1), La2O2S:Yb(8)Er(1). Mechanisms of up conversion by two photon and energy transfer processes are interpreted and explained. The x, y color coordinates are measured and the color tunability was analyzed as a function of the 980 nm excitation power. Results shows that all phosphor offers power dependent color tuning properties where the emission color can be tuned from 490 nm to 550 nm by simply changing the 980 nm excitation power from 10–50 mW.
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Kumar et al. (2012) studied this question.