Near-Infrared-to-Near-Infrared Downshifting and Near-Infrared-to-Visible Upconverting Luminescence of Er3+-Doped In2O3 Nanocrystals

Luminescent lanthanide-doped nanoparticles (NPs) excitable in the near-infrared (NIR) spectral region are highly desired as new optical bioprobes in the fields of biological assays and medical imaging. Er 3+ -doped In 2 O 3 NPs, synthesized via a facile sol–gel solvothermal method, exhibit intense and well-resolved NIR-to-NIR downshifting (DS) and NIR-to-visible upconversion (UC) dual-mode luminescence upon NIR excitation at 808 and 980 nm, respectively. Forty-one crystal-field levels below 23 000 cm –1 were identified for Er 3+ at a single lattice site of C 2 symmetry in In 2 O 3 NPs by means of high-resolution site-selective NIR spectroscopy at 10 K and room temperature. Furthermore, the luminescence dynamics for the UC emissions were systematically investigated, and various UC processes were clearly distinguished based on excited state dynamics and rate equation analysis. Mediated by the long-lived intermediate states of 4 I 11/2 and 4 I 13/2, the decay times of 4 F 9/2 (or 4 S 3/2 ) due to energy transfer upconversion (ETU) were found to be approximately 1 order of magnitude larger than that of DS luminescence, namely, increasing from ∼20 μs to ∼0.51 ms (or ∼0.24 ms). It was revealed that the contribution from the ETU process to the UC luminescence of 4 S 3/2 and 4 F 9/2 increased significantly along with the increase of the ETU probability of Er 3+ at high doping concentration.