Over the past decade, high-quality lanthanide doped upconverting nanocrystals (UPCNCs) have been successfully synthesized with the rapid development of nanotechnology. Several materials (i.e. CaF₂, NaYF₄, NaGdF₄) have been successfully applied so far as potential matrix for Yb³⁺-RE³⁺combinations, where the low crystal field symmetry and low phonon frequency are the main factors defining the best matrix selection. These materials have already found several potential applications in biology, medicine as well as in photovoltaics and photocatalysis – as the most common examples. However, to change the commercial potential of UPCNCs to real devices still a several problems must be solved, where week emission quantum efficiency (EQE typically < 2%) is one of the most important one. The EQE is directly related to other challenges in this field: growth of small size <10 nm UPCNCs, UPCNCs shape and architecture control and control of UPCNCs surface properties. To solve all abovementioned problems the correlation between technological, morphological and physical parameters is needed. This is however a difficult task because the system itself is very complicated and consist number of correlated parameters. Thus, still a better understanding of physical processes responsible for the optical properties of UPCNCs in needed to bring this fascinating material to commercial use.

In this work, we will discuss synthesis and optical properties of NaGdF₄-based up-converting nanocrystals. We will discuss how to control size, shape and core-shell architecture of lanthanide doped NaGdF₄ nanocrystals focusing mainly on Yb-Er co-doping. We will discuss how doping influencing the nanocrystals size, shape and their optical properties (Fig.1.). This will be done based on experimental results obtained from photoluminescence (PL) measurements, PL-excitation spectroscopy, PL-decay measurements, microscopic imaging and structural data obtained from HRTEM and XRD. Our conclusions will be supported by results obtained from numerical Monte Carlo modeling used to model the kinetic properties of nanocrystals emission and to model the ions distribution within the nanocrystals volume. Finally, based on our work on nanocrystals surface functionalization and their toxicity tests we will briefly discuss the potential use of UPCNCs in biology and medicine.

Figure 1. ß-NaGdF₄:Yb, Er nanocrystals with different shapes and architectures.