A comparison of the Yb3+ absorption and upconversion excitation spectra for both the cubic and hexagonal phases of NaYF4: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 ~40% (cubic) and ~34% (hexagonal) enhancement of the 2H11/2, 4S3/2 and 4F9/2 visible upconversion fluorescence. The hexagonal phase material exhibits Er3+ upconversion fluorescence which is a factor of 104 brighter than that obtained from the cubic phase nanoparticles, irrespective of their sizes, morphologies and Ln3+ ion distribution. The upconversion excitation spectra show preliminary evidence of excited state absorption features when monitoring 4F9/2 fluorescence for the hexagonal phase material.