Strong visible cooperative up-conversion emission in ZrO2:Yb3+ nanocrystals

Blue, green, and red emission was observed under infrared excitation in ZrO2:Yb3+ nanocrystals prepared by the sol-gel process. The structural characterization was performed by using XRD and HRTEM, suggesting that the crystalline phase of the nanoparticles is controlled by the active ion concentration being mainly tetragonal for 2 mol% of dopant and mainly monoclinic for 0.5 mol%. The blue emission was explained in terms of the cooperative deexcitation of an Yb-Yb pair, while the green and red bands were associated with the up-conversion of traces of Er ion. The number of photons involved in the luminescence process is analyzed in order to confirm that cooperative emission is produced by the interaction of an Yb pair and that the green and red emission are the results of energy transfer between Yb-Er ions. The high efficiency of all bands is explained in terms of the high surface area of the nanoparticles.