Highly efficient NIR to NIR and VIS upconversion in Er3+ and Yb3+ doped in M2O2S (M = Gd, La, Y)

Efficient photon upconversion (UC) is a promising way to utilize the infrared photons, where upconversion quantum yield (QY) is a key parameter to find the potential application of these UC materials for many applications. Here we are reporting the absolute upconversion QYs for different concentrations of M2O2S:Yb3+/Er3+ (M = Gd, Y, La) in comparison with β-NaYF4:20%Yb3+/2%Er3+ for the UC emissions centered at 410, 550, 667 and 822 nm at varying excitation power densities under 980 nm excitation. Concentration dependent upconversion QY analysis has revealed that La2O2S:1%Yb3+/1%Er3+ and Y2O2S:1%Yb3+/1%Er3+ possess almost similar upconversion QY compared to that of 9%Yb3+/1%Er3+:La2O2S. Measurements under identical conditions with respect to the reported most efficient upconverting phosphor NaYF4:20%Yb3+/2%Er3+ show that M2O2S:Yb3+/Er3+ yields a higher absolute upconversion QY at lower excitation power densities compared to that of β-NaYF4:20%Yb3+/2%Er3+. Furthermore, UC spectral studies and lifetime measurements were performed for a series of M2O2S:Yb3+/Er3+ phosphors to analyze the UC mechanisms in these phosphors. We show that upconversion emission from 4F9/2 is mainly influenced by the longer lifetime of metastable state 4I13/2.