39900 2017 eng 6924 6929 12 121 article ACS Publications American Chemical Society 1 -- -- -- The mechanism for red upconversion luminescence of Yb−Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. We have studied β-NaYF4:Yb3+,Er3+ nanoparticles in H2O and D2O, and we propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H2O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D2O, MPR is less effective and the three-photon path involving back energy transfer to Yb3+ is the dominant mechanism. For the green upconversion emission, our results suggest the common two-photon path through the 4F9/2 energy state in both H2O and D2O. Journal of Physical Chemistry C 10.1021/acs.jpcc.7b01019 http://pubs.acs.org/doi/pdf/10.1021/acs.jpcc.7b01019 1932-7447 1932-7455 28.04.2017 Iko Hyppänen Niina Höysniemi Riika Arppe Michael Schäferling Tero Soukka eng uncontrolled Photon upconversion eng uncontrolled Three photon excitation eng uncontrolled Multiphonon relaxation eng uncontrolled Excitation intensity Analytische Chemie Verlagsliteratur Datei im Netzwerk der BAM verfügbar ("Closed Access")