Check out the new publication from Emilia if you want to know more about the upconversion nanoparticle synthesis.

Palo, E., Tuomisto, M., Hyppänen, I., Swart, H.C., Hölsä, J., Soukka, T., and Lastusaari, M., “Highly uniform up-converting nanoparticles: Why you should control your synthesis even more”, J. Lumin. 185 (2017) 125–131.
Read online: http://dx.doi.org/10.1016/j.jlumin.2016.12.051