Multi-Color Tunable Luminescence in Yb3+, Er3+ Codoped NaYF4 Nanocrystals Based on Size-Dependent

Lanthanide-doped upconversion nanoparticles have been the focus of a growing body of investigation due to their considerable promise in biological labeling, imaging, and therapeutics. This article reports on investigation of luminescence properties of NaYF4 upconversion nanocrystals codoped with Yb3+/Er3+ by a facile hydrothermal synthesis. The samples have been characterized by powder X-ray diffractometer (XRD), transmission electron microscopy (TEM), photoluminescence spectroscopy and energy-dispersive X-ray spectroscopy (EDS). Intense visible emissions centered at around 525, 550 and 650 nm have been detected in all the samples upon excitation with a 980 nm laser diode, and the involved mechanisms have been explained. Interestingly, size-dependent upconversion emissions have been observed in these nanocrystals. With increasing grain size, an obvious enhancement of red to green emission ratio in the NaYF4: Yb3+/Er3+ nanocrystals is observed. We reason that when the size of nanocrystals increases, the amount of Yb3+ dopants into the NaYF4 host lattice would also increase and the enhanced energy transfer from Yb3+ to Er3+ should subsequently elevate the population in excited levels of F-4(9/2), resulting in the increase of red (F-4(9/2) -> I-4(15/2)) emission. As a consequence, multi-colors such as green, yellow, and red upconversion emissions can be easily realized.