Near-Infrared (NIR) to Red and Green Up-Conversion Emission from Silica Sol−Gel Thin Films Made with La<sub>0.45</sub>Yb<sub>0.50</sub>Er<sub>0.05</sub>F<sub>3</sub> Nanoparticles, Hetero-Looping-Enhanced Energy Transfer (Hetero-LEET): A New Up-Conversion ProcessSri Sivakumar, P. Stanley May, Frank C. J. M. van Veggel|Journal of the American Chemical Society|2006Cited by 196