Abstract:
Because of their unique optical properties, the lanthanides (Ln) are employed as do-pants in a large manifold of optoelectronic applications. The optical properties of the Ln are influenced by the local coordination environment at an atomistic level. This local environment, in turn, is linked to the (crystallographic) order or disorder present in the lattice of the host material. Energy levels, oscillator strengths, and emission spectra of lanthanides doped into either ordered or disordered host materials may, therefore, vary noticeably. For other effects, such as photon upconversion, that depend on these prop-erties, different responses are also to be expected for ordered and disordered systems.
In this work, a combined theoretical and experimental investigation of the optical properties of three Ln (Er, Tm, and Yb) doped into two typical upconverting host materi-als, ordered LiYF4 and intrinsically disordered β NaYF4, is presented. The objectives of this study were to establish how crystallographic disorder influences the structure of the host material, the local symmetry of doping sites, and the optical response of Ln doped into such sites. In the first part of the study, periodic density functional theory was ap-plied to optimise the examined structures and systematically characterise the conse-quences of cation disorder in β NaYF4. The model was benchmarked by comparing its predictions with data from the literature for both LiYF4 and β NaYF4. In the second part, the optimised structures served as the basis for embedded cluster calculations to pre-dict crystal field splitting, energies, and oscillator strengths via a wavefunction-based approach for the three Ln. Results were compared to values available in the literature and showed very good agreement. In both studies, the disorder experienced by β NaYF4 was considered through a custom weighting scheme of the available disordered struc-tures.
Finally, emission spectra from specifically prepared Ln-doped upconverting nano-particles were recorded and compared to those generated from the computational data sets. Great overlap between the spectra compared was observed, highlighting the va-lidity of the computational approach and the disordered nature of β NaYF4. The study demonstrated that β NaYF4 cannot be adequately described by a single representative structure. Remaining challenges are connecting the observed changes in the emission profiles to the enhanced upconversion yield observed for disordered host materials.
Chemistry