Influence of SiO2 coating on morphology, phase and upconversion luminescence properties of NaYF4:Yb3+,Er3+ submicrocubes during annealing

• NaYF4:Yb3+,Er3+@SiO2 submicrocubes were synthesized using a modified Stöber procedure.
• Silica shells protect NaYF4:Yb3+,Er3+ cores from aggregating and growing up during annealing.
• Intense green light was observed in annealed submicrocubes excited with a 980 nm laser source.

NaYF4:Yb3+,Er3+ submicrocubes were synthesized by a facile method at room temperature, and they were coated with SiO2 using a modified Stöber procedure. Then bare NaYF4:Yb3+,Er3+and NaYF4:Yb3+,Er3+@SiO2 submicrocubes were both annealed at 300 °C, 400 °C and 500 °C for 5 h under N2 protection. The influence of SiO2 shells on morphology, phase and upconversion luminescence properties of NaYF4:Yb3+,Er3+ submicrocubes during annealing was studied in detail. The results showed that silica shells can protect NaYF4:Yb3+,Er3+ cores from aggregating and growing up. Interestingly, SiO2 shells can also prevent phase change of NaYF4:Yb3+,Er3+ submicrocubes from cubic (α-) to hexagonal (β-) phase during annealing, which provides a new way to control the phase of NaYF4 nanocrystals. NaYF4:Yb3+,Er3+cores became nanoporous after annealing, making them possess a feature of ‘cavity’ structure for the storage/delivery of therapeutic drugs. After annealing, NaYF4:Yb3+,Er3+ and NaYF4:Yb3+,Er3+@SiO2 submicrocubes both emit bright green light excited with a 980 nm laser source, and the upconversion luminescence properties were investigated in detail.