Tailored up-conversion luminescence output of Al-modulated KYbF4: Er3+ nanocrystals for a low-temperature sensor

The up-conversion features of lanthanide-doped nanocrystals endow them with promising application potential in optical encoding, optical temperature sensing, bioimaging, diagnostics, and therapeutics. Here, tailoring the local structure to enhance the up-conversion luminescence intensity was achieved through chemical substitution with Al in KYbF4: Er3+ up-conversion nanoparticles (UCNPs). Moreover, the as-explored KYb0.98-xAlxF4: 0.02 Er3+ exhibits a superior temperature-dependent up-conversion performance. Herein, the relative sensitivity of the H-2(11/2)/S-4(3/2) thermally coupled levels (TCLs) and F-4(9/2)/S-4(3/2) non-thermally coupled levels (NTCLs), particularly below the room temperature, was evaluated. This indicates the potential application of the as-explored UCNPs with improved up-conversion performance as an optical temperature sensor for low temperature detection.