Sr2+ substitution engineering of Mn5+-activated Ba2TiO4 narrowband NIR-II phosphors for optical thermometry and deep-tissue imaging

The emission peak of Mn 5+ lies in the second near-infrared window (NIR-II) biological transparency window, and it exhibits outstanding application prospects in deep-tissue bioimaging, non-contact optical thermometry, and related fields. In this work, a novel Mn 5+ -activated Ba 1.9 Sr 0.1 TiO 4 NIR-II phosphor is designed and synthesized via a high-temperature solid-state reaction, where partial Sr 2+ substitution for Ba 2+ enables precise crystal field modulation around Mn 5+ ions. Mn K-edge X-ray absorption near-edge structure (XANES) spectroscopy confirms that Mn ions are stabilized in the +5 oxidation state. The optimized BSTO: 0.1 % Mn 5+ phosphor shows efficient 680 nm red light excitation and a characteristic narrowband NIR-II emission at 1190 nm from the 1 E → 3 A 2 transition of Mn 5+ , with a long luminescence lifetime of 184.9 μs and excellent thermal stability. Density functional theory with Hubbard U correction (DFT + U) calculations further reveal that Sr 2+ substitution induces controllable lattice contraction and local structural distortion, which modulates the host electronic structure and enhances the stability of Mn 5+ luminescent centers. Furthermore, the phosphor delivers a maximum relative temperature sensitivity of 2.24 % K −1 via the luminescence intensity ratio technique. A proof-of-concept NIR phosphor-converted light-emitting diode fabricated with the as-prepared phosphor realizes clear NIR penetration imaging.