Unusual upconversion emission from single NaYF4:Yb3+/Ho3+ microrods under NIR excitation

Rare earth ion-doped upconversion materials show great potential applications in optical and optoelectronic devices due to their novel optical properties. In this work, the NaYF4:Yb3+/Ho3+ microrods have been successfully prepared by a hydrothermal method assisted with EDTA (ethylenediaminetetraacetic acid). Time controlled experiments were carried out and the growth mechanism of dissolution–recrystallization of the microrods is proposed. It is important that the unusual upconversion emission with a candy-like pattern from single NaYF4:Yb3+/Ho3+ microrods can be detected by confocal microscopy under NIR 980 nm excitation. The influence of the particle number on the UC emission spectra and the overall luminescence color was investigated systematically. We noted that the multicolor emission from the single NaYF4:Yb3+/Ho3+ microrods can be successfully obtained only by adjusting the excitation strength or the size of the microrods. The present study is of great importance for multicolor tuning of RE-doped NaYF4 nano/micromaterials as well as for understanding the up-mechanism. In addition, the luminescence from single microparticles can provide more precise spectral information on three-dimensional displays and biological labels.