Facile synthesis of NaYF₄:Yb, Ln/NaYF₄:Yb core/shell upconversion nanoparticles via successive ion layer adsorption and one-pot reaction technique

The facile one-pot synthesis of NaYF4:Yb, Ln/NaYF4:Yb core/shell (CS) upconversion nanoparticles (UCNPs) was firstly developed through the successive ion layer adsorption and reaction (SILAR) technique, which represents an attractive alternative to conventional synthesis utilizing the chloride of Ln as the precursors, where the Ln doped NaYF4 core was firstly purified and then an epitaxial shell of the desired thickness was obtained by the injection of shell precursors to a solution of the purified NaYF4 core. The temperature-dependent shell growth mechanism and upconversion luminescent properties were explored thoroughly. A temperature of 280 degrees C proved to be the optimal shell growth temperature to prepare the compact CS structure with the highest luminescent enhanced efficiency for this preparative system. The shell thickness could be easily tuned using this SILAR technique from about 3 monolayers (ML) to 44 ML. The UC luminescent intensity was found to be increased with increasing shell thickness, and at last the effect of surface on the PL could be completely excluded by an appropriate shell thickness. Furthermore, the performance of the SILAR technique was also demonstrated by comparing Tm3+ and Er3+ doped separately in the core and shell with the Ln ions co-doped in the core.