Phase and morphology evolution of NaGdF4:Yb,Er nanocrystals with power density-dependent upconversion fluorescence via one-step microwave-assisted solvothermal method

Highlights: • Hexagonal NaGdF₄:Yb,Er nanocrystals with fried-egg shape are synthesized via microwave-assisted solvothermal method. • The morphology and phase evolution is investigated. • The nanocrystals show an obvious power density-dependence upconversion emission. • The controlled surface modification of the NaGdF₄:Yb,Er nanocrystals is achieved. Controlled synthesis and modification of nanocrystals is the ultimate frontier of inorganic nanochemistry. Here, hexagonal NaGdF₄:Yb,Er nanocrystals with fried-egg shape were successfully synthesized via a one-step microwave-assisted solvothermal method. The phase and morphology evolution of NaGdF₄:Yb,Er nanocrystals was investigated in detail. We experimentally verified that oleic acid and oleylamine played important roles in the formation of NaGdF₄:Yb,Er nanocrystals. The synthesis of NaGdF₄:Yb,Er nanocrystals was also regulated by microwave-assisted rapid heating and uniform heating at high temperatures. The fluorescence spectra showed that the relative intensity of fluorescence emission peaks of NaGdF₄:Yb,Er nanocrystals had an obvious power density-dependent phenomenon. Meanwhile, two common strategies were used to modify the surface of the nanocrystals. For dopamine-modified nanocrystals, the thickness of the polydopamine shell was well controlled. As for silica-modified nanocrystals, not only dense silica shells with varying thickness have been fabricated, but mesoporous silica shell can also be easily coated on the surface of nanocrystals. It is expected that the control of synthesis, fluorescent property, and modification provides the groundwork for developing the applications of NaGdF₄:Yb,Er nanocrystals.