Coordination Geometry‐Dependent Multi‐Band Emission and Atypically Deep‐Trap‐Dominated NIR Persistent Luminescence from Chromium‐Doped Aluminates

An abnormal trap distribution and coordination geometry-dependent multi-band emission are discovered in chromium-doped aluminates. The multi-band and broadband persistent luminescence from 650-1100 nm peaking at 688 and 793 nm from Cr3+-doped SrAl12O19 is systematically studied via structural and spectroscopic analysis. Solid state nuclear magnetic resonance allows the visualization of various coordination configurations in SrAl12O19, thus offering the possibility of tailoring the local geometry of the emission center to trigger the control of the spectral parameter. Deep tissue ex vivo and in vivo imaging in mice both demonstrate that multi-band-emissive SrAl12O19:Cr3+ shows superior, high-quality near-infrared (NIR) bio-imaging in the biological transparency window compared to single band-emissive (with emission only at 688 nm) SrAl2O4:Cr3+, although SrAl2O4:Cr3+ has a higher luminescent intensity and longer duration at 688 nm. Moreover, by measuring the thermoluminescence spectra the driving force of carrier release is discovered to be only from the deep trap (the depth is > 1 eV), which is different from the generally accepted shallow-dependent afterglow-emitting process. These findings pave the way for opening a vista of possible avenues for the enhancement of signal-to-noise ratio, the improvement of imaging quality, as well as the understanding of the trapping and de-trapping process in long-persistent phosphors.