Efficient broadband-emitting inorganic NIR phosphors

While Eu²⁺-activated condensed oxidosilicates or nitridosilicates and aluminates are now promising phosphors for energy-saving white light sources and displays in the visible spectral range, there are hardly any satisfactory solutions for such phosphors for the near-infrared range (750 nm - 1200 nm). Such phosphors are relevant for non-destructive food analysis, health-promoting interior lighting and also for night vision in headlights in the automotive industry. However, NIR-emitting broadband emitters suffer from the fundamental problem that with decreasing transition energy, non-radiative transitions become thermally very likely at ever lower temperatures. However, this can be limited by the appropriate choice of host compounds with mechanically very rigid structures with a high degree of condensation.

An intrinsically very efficient activator with luminescence in the NIR range is the 3d³ ion Cr³⁺. With a sufficiently weak ligand field, this ion shows broadband luminescence. At the same time, insertion into a mechanically rigid structure with a high degree of condensation often results in an amplification of the ligand field. For this reason, we try to utilize electronic effects to specifically tailor the covalent parts of the Cr-ligand chemical bond and thus bring the emission energy and efficiency in the desired direction. In particular, orbital effects due to atoms in the second coordination sphere are utilized for this purpose. In addition to the synthesis and spectroscopy of these potential NIR phosphors, we use the semiempirical angular overlap model (AOM) of ligand field theory to gain a more fundamental understanding of how to target Cr³⁺-based luminescence in the solid state. Stronger theoretical methods such as multi-reference-based cluster calculations (CASSCF, NEVPT2) are also in focus here.