Stephan Heer, Karsten Kömpe, Hans-Ulrich Güdel and Markus Haase
Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF₄ Nanocrystals
Adv. Mater. 16, 2102-2105 (2004)                       

Abstract: (Article without Abstract)
Graphical Abstract: Highly efficient photon upconversion in solution is demonstrated for the first time. Transparent colloidal solutions of Yb/Er and Yb/Tm doped NaYF₄ nanocrystals show bright upconversion emission in the red, green, and blue spectral region by excitation around 1 mm with a laser of moderate power. These new systems show a substantial improvement on the order of 10⁸ compared to previously reported systems.

Inside Front Cover Picture: A synthesis route to lanthanide-doped NaYF₄ nanocrystals, an important new class of materials showing efficient multicolor upconversion emission in transparent colloidal solution, is reported. Solutions of Yb/Er- and Yb/Tm-doped NaYF₄ nanocrystals show bright upconversion emission in the red, green, and blue spectral region after excitation with a laser of moderate power. These new systems show a substantial improvement on the order of 10⁸ compared to previously reported systems.