Unravelling design principles for efficient NIR-emitting inorganic phosphors

• Applicant: Professor Dr. Markus Suta
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554302036

The huge success in the design of narrow-band emitting and efficient Eu2+-activated LED phosphors of the UCr4C4 structure type such as e.g. Sr[LiAl3N4]:Eu2+ (SLA:Eu2+), Sr[Li2Al2O2N2]:Eu2+ (SALON:Eu2+), or various alkali lithosilicates demonstrate the potential of inorganic structural chemistry for selected material properties. While there exist almost perfected phosphors for this visible spectral range by now, promising candidates for phosphor-converted LEDs emitting in the invisible near infrared (NIR) range are still lacking, however. These are currently of great interest for night vision, analytics and sensor technology. Broadband-emitting NIR phosphors have a fundamental tendency towards non-radiative relaxation even at lower temperatures. In the context of this project, the structural success principles of the visibly emitting inorganic phosphors will be transferred to potential NIR-emitting inorganic phosphors and adapted to the modified challenges. For this purpose, the focus lies on the 3d3 ion Cr3+ (near-infrared broadband emitter) in complex oxides and oxometallates. The aim is to learn from the experience of coordination chemistry and to electronically influence the emission energy of this NIR emitter with the help of next-neighbouring highly charged ions with d0 or d10 configuration in the solid by means of covalent binding contributions. Ligand field modelling within the framework of the old, but in this case very useful "angular overlap model" is used in a complementary way, which explicitly includes covalent contributions to the metal-ligand bond. By clever use of high-pressure chemistry and condensed structures with tetrahedral oxoanions of the p-block such as borates, silicates, phosphates, or sulfates, a high quantum yield due to structural rigidity is also targeted. This project particularly focusses on a mechanistic understanding of the control parameters of the radiationless channel, which is crucial for thermal quenching of the luminescence of the NIR-emitting Cr3+ ions. In addition, the influence of the activator concentration and the incident power density of blue light on thus found brightness NIR-emitting phosphors will be investigated in more detail. The so-called "shell model" of energy transfer is used for this purpose, which explicitly takes into account the discrete distribution of adjacent cation layers around a given Cr3+ ion in a crystal structure under consideration. Based on these fundamental studies, reliable design principles are supposed to be developed that could be important for the future development of phosphor-converted NIR LEDs.

• DFG Programme: Research Grants
• Major Instrumentation: Closed-Cycle He-Kryostat ; NIR-Photomultiplier
• Instrumentation Group: 5820 Elektronenvervielfacher ; 8550 Spezielle Kryostaten (für tiefste Temperaturen)