Tuning upconversion emission by controlling particle shape in NaYF4:Yb3+/Er3+ nanocrystals

Rod-shaped and plate-shaped NaYF4:Yb3+/Er3+ nanocrystals (NCs) have been synthesized via a facile oleic acid-assisted hydrothermal route. The intensity and color tunability by shape controlling of NCs is demonstrated by up and down frequency conversions in NaYF4:Yb3+/Er3+ NCs. The results obviously show that rod-shaped NCs present much stronger upconversion luminescence intensity and smaller ratio of green to red emission intensity than that of plate-shaped NCs. The underlying reason of luminescence enhancement by shape controlling is explored, and a mechanism of the enhancement based on the decrease of nonradiation relaxation probability and the increase of energy transfer efficiency from Yb3+ to Er3+ is proposed for nanorods.