Luminescent materials containing bismuth and emitting in the near-infrared (NIR) spectral region have attracted much interest owing to a broad range of applications such as lasers, bioimaging, and telecommunications. However, it is extremely difficult to realize the rational synthesis of bismuth-activated, near-infrared luminescent materials. Concurrently, the near-infrared luminescence mechanism of such systems remains poorly understood. A team from College of Chemistry, Chemical Engineering and Materials Science of Soochow University has found a novel generic approach to convert Bi-activated visible emitters to those emitting in the NIR region, and made progress in understanding the nature of Bi-related NIR active centers. Their research has been published on March 11^th, 2016 in Angewandte Chemie International Edition.

The team has shown that the low-temperature topochemical reduction strategy can be employed as a powerful method for the synthesis of unconventional phosphors with luminescence covering the biological and/or telecommunication optical windows. Additionally, they have found that this low-temperature topochemical reduction strategy can be readily extended to the manipulation of PL from other classes of Bi-doped compounds.  XANES and EXAFS analyses suggest that the topochemical reduction leads to the evolution of [BiO₆] octahedra in Y_1.992Bi_0.008O₃ to [BiO_6-z] (z<6) polyhedra in reduced phases, resulting in a reduction in the average oxidation states of Bi present. The team confirmed that the change of Bi coordination environments is the reason for the shift of the PL band maxima from the visible to the NIR spectral region.

Figure 1. Bi L_III-edge XANES and EXAFS spectra of the precursor and oxygen-deficient sample. The X-ray absorption spectra were obtained from BSRF. The result shows that topochemical reduction results in the formation of under-coordinated Bi-O polyhedra in the reduced phase (S192) with respect to the precursor.

Figure 2. Schematic of the topochemical treatment of Bi^III-doped Y₂O₃. CaH₂ treatment of Bi^III-doped Y₂O₃ (Y_2-xBi_xO₃) creates randomly distributed oxygen vacancies in the matrix, leading to lowered oxidation states for the Bi centers and a change in the emission profile of the material.