Publication date: 15th May 2025
We demonstrate very different photophysical properties arising from Cr3+-Yb3+ coupling in two families of perovskite-adjacent nanomaterials. Perovskite-adjacent nanocrystals with broadband emission between 750 and 1500 nm are attractive for near infrared (NIR) optoelectronic applications because of their decreased toxicity compared to NIR-emissive III-V quantum dots (e.g., PbS, InAs) and their colloidal stability which makes them easily deployable in NIR light source devices via additive manufacturing. Here, we report the first preparation of Cr3+-based elpasolite and cryolite fluoride nanocrystals (Cs2NaCrF6 and Na3CrF6, respectively). These nanocrystals show broadband NIR luminescence at room temperature that spans the entire NIR-I window (750-1000 nm) and is characteristic of weak-field Cr3+ 4T2g → 4A2g emission. Yb3+ ions were doped into these nanocrystals and sensitized Yb3+ f-f luminescence was observed, caused by resonant energy transfer from excited Cr3+ ions to neighboring Yb3+ ions. The 0D connectivity of both lattices provides only weak, multipolar coupling of Cr3+ and Yb3+ ions. We compare this weak-coupling scenario to that arising from oxo-bridged Cr3+ and Yb3+ ions in the orthochromite lattice, Yb3+:YCrO3. Here, we describe the first investigation of Yb3+ luminescence in any orthochromite. YCrO3 is a canted antiferromagnet with a Néel temperature (TN) of 140 K. We show that the Yb3+ Kramers degeneracies are lifted by magnetic exchange in this lattice, with splittings that depend on lattice spin correlation, highlighting the unique spin-photonic functionality of this material. Variable-temperature photoluminescence spectroscopy is used to quantify the exchange splittings of the ground and lowest-lying excited states of Yb3+ and to probe spontaneous spin ordering. Efficient excitation of Yb3+ f-f luminescence at room temperature is observed via direct simultaneous pair excitation of coupled Yb3+-Cr3+ ions within this lattice.
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