Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide‐Doped NaYF₄ Nanocrystals

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 μm with a laser of moderate power (see Figure). These new systems show a substantial improvement on the order of 108 compared to previously reported systems. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2004/c0772_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.