Enhanced Ultraviolet Photon Capture in Ligand-Sensitized Nanocrystals

Abstract

The small absorption cross sections (ε –1 cm–1) characteristic of Laporte-forbidden transitions in the f-elements have limited the practical implementation of lanthanide nanoparticles in solar capture devices. While various strategies designed to circumvent the problems of low f–f oscillator strengths have been investigated, comparatively little work has explored the utility of organic ligands with high absorption coefficients (ε ≈ 103–105 M–1 cm–1) in sensitizing excited states in lanthanide nanocrystals. Here, we detail the photophysics of NaGd1–xEuxF4 nanoparticles featuring surface display of the ligand 3,4,3-LI­(1,2-HOPO), an aromatic antenna functioning as the terminal light absorber in this system. The result is a ligand–nanocrystal hybrid that converts UV (250–360 nm) light into red Eu­(III) luminescence with an external quantum yield of 3.3%. We analyze this sensitization process, responsible for a 104-fold increase in luminescence relative to metal-centered excitation, through a quantitative treatment of energy transfer between ligand and metal states