Tunable flower-like upconversion emission and directional red radiation in a single NaYF4:Yb3+/Tm3+ microcrystal particle

Rear earth doped upconversion (UC) luminescence materials have attracted considerable attentions in color display and biological labeling due to their tunable emission colors property. In this report, we designed and fabricated NaYF4: Yb3+/Tm3+ microcrystals with novel morphology, which exhibit some unique UC luminescence phenomena. The experimental findings show that a flower-like blue UC emission pattern and the directional red radiation are readily manipulated by controlling the focal point position of the excitation laser on the single microcrystal particle (MP). The unique phenomena are attributed to optical waveguide effect with total internal reflection of the UC luminescence at the microcrystal-air interface. The findings not only provide a new strategy for designing UC nano-/microcrystals with particular and tunable luminescence properties, but also open a promising route for the fabrication of new optoelectronic devices and the construction of three-dimensional color display systems.