Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF4Nanocrystals Heer, S. ; Kömpe, K. ; Güdel, H. -U. ; Haase, M. Abstract Publication: Advanced Materials Pub Date: December 2004 DOI: 10.1002/adma.200400772 Bibcode: 2004AdM....16.2102H