Concentrating and Recycling Energy in Lanthanide Codopants for Efficient and Spectrally Pure Emission: The Case of NaYF4:Er3+/Tm3+ Upconverting Nanocrystals

In lanthanide-doped materials, energy transfer (ET) between codopant ions can populate or depopulate excited states, giving rise to spectrally pure luminescence that is valuable for the multicolor imaging and simultaneous tracking of multiple biological species. Here, we use the case study of NaYF4 nanocrystals codoped with Er3+ and Tm3+ to theoretically investigate the ET mechanisms that selectively enhance and suppress visible upconversion luminescence under near-infrared excitation. Using an experimentally validated population balance model and using a path-tracing algorithm to objectively identify transitions with the most significant contributions, we isolated a network of six pathways that combine to divert energy away from the green-emitting manifolds and concentrate it in the Tm3+:F-3(4) manifold, which then participates in energy transfer upconversion (ETU) to populate the red-emitting Er3+:F-4(9/2) manifold. We conclude that the strength of this ETU process is a function of the strong coupling of the Tm3+:F-3(4) manifold and its ground state, the near-optimum band alignment of Er3+ and Tm3+ manifolds, and the concentration of population in Tm3+:F-3(4). These factors, along with the ability to recycle energy not utilized for red emission, also contribute to the enhanced quantum yield of NaYF4:Er3+/Tm3+. We generalize a scheme for applying these energy concentration and recycling pathways to other combinations of lanthanide dopants. Ultimately, these ET pathways and others elucidated by our theoretical modeling will enable the programming of physical properties in lanthanide-doped materials for a variety of applications that demand strong and precisely defined optical transitions.