A comparison of the Yb3+ absorption and upconversion excitation spectra for both the cubic and hexagonal phases of NaYE4:Yb3+/Er3+ nanoparticles

We report on studies of the Yb3+ ion absorption in Er3+ co-doped cubic and hexagonal phase NaYF4 upconverting nanoparticles using high-resolution absorption measurements. The Yb3+ absorption spectra have intense maxima at 10266.1 cm(-1) and 10237.0 cm(-1) for the cubic and hexagonal phases respectively, which is far from the most commonly used laser diode wavelength. Resonant excitation of these absorption maxima yields a similar to 40% (cubic) and similar to 34% (hexagonal) enhancement of the H-2(11/2), S-4(3/2) and F-4(9/2) visible upconversion fluorescence. The hexagonal phase material exhibits Er3+ upconversion fluorescence which is a factor of 10(4) brighter than that obtained from the cubic phase nanoparticles, irrespective of their sizes, morphologies and Ln(3+) ion distribution. The upconversion excitation spectra show preliminary evidence of excited state absorption features when monitoring F-4(9/2) fluorescence for the hexagonal phase material.