High quantum yield of the Egyptian blue family of infrared phosphors (MCuSi4O10, M = Ca, Sr, Ba)

The alkaline earth copper tetra-silicates, blue pigments, are interesting infrared phosphors. The Ca, Sr, and Ba variants fluoresce in the near-infrared (NIR) at 909, 914, and 948 nm, respectively, with spectral widths on the order of 120 nm. The highest quantum yield phi reported thus far is ca. 10%. We use temperature measurements in sunlight to determine this parameter. The yield depends on the pigment loading (mass per unit area) omega with values approaching 100% as omega -> 0 for the Ca and Sr variants. Although maximum quantum yield occurs near omega = 0, maximum fluorescence occurs near omega = 70 g m(-2), at which phi = 0.7. The better samples show fluorescence decay times in the range of 130 to 160 mu s. The absorbing impurity CuO is often present. Good phosphor performance requires long fluorescence decay times and very low levels of parasitic absorption. The strong fluorescence enhances prospects for energy applications such as cooling of sunlit surfaces (to reduce air conditioning requirements) and luminescent solar concentrators. Published by AIP Publishing.