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Photograph of the upconversion emission in oxysulphide phosphors (top) and their colloids in de ionized water (bottom) at a concentration of 1mg/3ml. (a) Gd2O2S:Yb (8) Er (1), (b) La2O2S:Yb (3) Er (7) observed through red filter, (c) Gd2O2S:Yb (2) Tm (1), (d) La2O2S:Yb (9) Er (1)
