Sub-10 nm Yb3+/Er3+-doped NaYF4 core–shell upconversion nanoparticles: precise size control and enhanced luminescence

Highly crystalline and luminescent sub-10 nm NaYF 4 :Yb 3+ /Er 3+ @NaYF 4 core–shell UCNPs are synthesized using the thermal decomposition method. Optimized conditions yield efficient UC and enhanced crystallinity through rapid α to β phase transition.