A novel synthesis method and up-conversion properties of hexagonal-phase NaYF4:Er nano-crystals

A novel synthesis method for hexagonal (β)-phase NaYF₄:Er nano-crystals (NCs) which showed up-conversion (UC) from infrared to visible spectral region was developed. The NaYF₄:Er NCs were synthesized in oleic acid (OA) and 1-octadecene (ODE) with Y₂(CO₃)₃· xH₂O, Er₂(CO₃)₃· xH₂O, Na₂CO₃ and NH₄F as precursors. This proposed method was simple and less toxic compared with generally used method so far. The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control of NaYF₄:Er NCs. The UC emission spectra were obtained with the emission wavelength at about 980 nm (⁴I_11/2→⁴I_15/2), 800 nm (⁴I_9/2→⁴I_15/2), 660 nm (⁴F_9/2→⁴I_15/2) and 540 nm (⁴S_3/2→⁴I_15/2) from Er³⁺ ions, by excitation wavelength of 1550 nm. The slope values, n, in the pump-power dependence, showed that the emission at 980 and 800 nm were generated by 2-step UC and at 660 nm and 540 nm were 3-step UC. The optical process for the UC excitation was discussed.