TY - JOUR A1 - Hyppänen, Iko A1 - Höysniemi, Niina A1 - Arppe, Riika A1 - Schäferling, Michael A1 - Soukka, Tero T1 - Environmental impact on the excitation path of the red upconversion emission of nanocrystalline NaYF4:Yb3+,Er3+ JF - Journal of Physical Chemistry C N2 - 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. KW - Photon upconversion KW - Three photon excitation KW - Multiphonon relaxation KW - Excitation intensity PY - 2017 UR - http://pubs.acs.org/doi/pdf/10.1021/acs.jpcc.7b01019 DO - https://doi.org/10.1021/acs.jpcc.7b01019 SN - 1932-7447 SN - 1932-7455 VL - 121 IS - 12 SP - 6924 EP - 6929 PB - ACS Publications AN - OPUS4-39900 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -