Superbroad near-to-mid-infrared luminescence from Bi5(3+) in Bi5(AlCl4)3

Superbroad near-to-mid infrared (NIR-MIR) photoluminescence was observed from Bi5(AlCl4)3 at room temperature, spanning the spectral range of about 1000 to 4000 nm. On the basis of structural considerations and dynamic analyses, Bi5(3+) clusters were identified as the optically active species, inherently differing from the species which is typically believed to be active in NIR-emitting Bi-doped glasses. In comparison to most other NIR-luminescent Bi-doped materials, the MIR-part of the luminescence spectrum is still present at room temperature. Emission intensity and excited state lifetime were found to exhibit abnormal temperature dependence, where the former increases with temperature up to a critical value of about 150 K. This behavior is related to a temperature-dependent overlap between ground state and excited states. The observed stabilization of MIR photoemission at room temperature may be a starting point for the development of Bi-based NIR-MIR light sources with superbroad emission spectrum, where Bi5(3+) or similar polycationic species act as optical gain medium.