Single Nanocrystals Studies of Upconverting Nanoparticles

Abstract

Upconverting nanocrystals (UCNC) such as NaYF4:Er3+, Yb3+ have been recently a subject of intensive investigation due to their unique optical properties and high potential in applications in biology and medicine as multifunctional diagnostic probes. Despite the level of attention paid to this material, their very low emission quantum yield is still an important problem excluding them from clinical applications. Thus, understanding of UCNC emission intensity limitation together with a development in growth protocols are crucial challenges to bring these materials to clinical practice. One of the main obstacles, which slowdown the progress in this field, is the complicity of the emission quantum yield measurements. Moreover, even the relative emission intensity measurements (from sample to sample), became a very challenging experiment in case of UCNC. These two difficulties make the optimization of growth process and understanding of experimental data very challenging tasks.In this paper, we will discuss the influence of relative ions concentration, size and the shape of UCNC on their optical properties. We will also show our results obtained for a single UCNC, measured however in the wide field mode giving also the possibility of statistical analysis of UCNC properties. These results will include both the optical imaging as well spectroscopic data. With these results we will show, that this type of experiment enables to measure the relative UCNC emission intensity from sample to sample, what allows for optimization of nanocrystals growth as well as enables to understand the physical limitation of UCNC emission intensity. In particular, we will investigate the effects of Yb3+ ions clustering and its impact on NaYF4:Yb3+, Er3+ UCNC emission intensity. We will also support our conclusions with numerical data.Based on our results we will explain the influence of Yb3+ concentration on optical properties of UCNC in details. Moreover, based on obtained experimental and numerical results we will explain the existence of optimal Yb3+ concentration in UCNC often reported in the literature.