NaYF4:Yb,Er-upconversion nanoparticles: Systematic enhancement of the luminescence efficiency by unraveling the processes of energy loss

• Applicants: Professor Dr. Markus Haase; Dr. Ute Resch-Genger
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271522046

The crystalline compound NaYF4 represents the best host lattice known today for Yb3+/Er3+ and Yb3+/Tm3+ doped upconversion materials. Upconversion materials emit visible light upon excitation in the near-infrared spectral region (NIR region). Although upconversion luminescence is a non-linear optical process as the emission of one photon in the visible range requires the absorption of at least two NIR photons, comparatively low light intensities are sufficient to efficiently excite the emission. The reason is that the NIR photons can be absorbed sequentially rather than simultaneously because the upconversion mechanism involves only metastable energy levels of the dopant ions. During the last years synthesis procedures have been developed allowing to prepare colloidal nanocrystals of Yb3+/Er3+ and Yb3+/Tm3+ doped NaYF4 upconversion materials with well defined crystal morphologies and very narrow particle size distributions. Similar to semiconductor nanocrystals (quantum dots), also methods have been developed to deposit a thin shell of undoped NaYF4 onto the surface of upconversion particles. This undoped shell significantly reduces the transfer of excitation energy from the doped particle core to the surface of the core-shell particles since the energy transfer takes place via adjacent dopant ions. Such core-shell particles display much stronger upconversion emission than particles without shell, since the shell significantly reduces energy loss processes on the particle surface. In contrast to other doped nanoparticles or quantum dots, however, the luminescence of the core-shell particles is still weaker than that of the corresponding bulk material. Although electron microscopy images of upconversion core-shell particles verify that shells with very uniform thickness can be prepared, the efficiency of the upconversion emission is still almost an order of magnitude lower than the efficiency of bulk upconversion phosphors. We therefore propose a collaborative research project involving one research group being specialized on the synthesis of upconversion particles and a second research group having a strong background in the determination of absolute luminescence quantum yields and the field of time-dependent luminescence measurements. Within this research project, we are aiming to unravel the reason for the low efficiency of the upconversion nanocrystals by investigating the influence of several different parameters in the particle synthesis. Based on these results, we will try to prepare improved upconversion particles which lack the luminescence quenching mechanisms that reduce the upconversion efficiency of todays nanocrystals.

• DFG Programme: Research Grants