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 (4I11/2→4I15/2), 800 nm (4I9/2→4I15/2), 660 nm (4F9/2→4I15/2) and 540 nm (4S3/2→4I15/2) 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.