Ultrapure Green Light-Emitting Diodes Using Two-Dimensional Formamidinium Perovskites: Achieving Recommendation 2020 Color Coordinates.

Pure green light-emitting diodes (LEDs) are essential for realizing an ultrawide color gamut in next-generation displays, as is defined by the recommendation (Rec.) 2020 standard. However, because the human eye is more sensitive to the green spectral region, it is not yet possible to achieve an ultrapure green electroluminescence (EL) with a sufficiently narrow bandwidth that covers >95% of the Rec. 2020 standard in the CIE 1931 color space. Here, we demonstrate efficient, ultrapure green EL based on the colloidal two-dimensional (2D) formamidinium lead bromide (FAPbBr3) hybrid perovskites. Through the dielectric quantum well (DQW) engineering, the quantum-confined 2D FAPbBr3 perovskites exhibit a high exciton binding energy of 162 meV, resulting in a high photoluminescence quantum yield (PLQY) of ∼92% in the spin-coated films. Our optimized LED devices show a maximum current efficiency (ηCE) of 13.02 cd A-1 and the CIE 1931 color coordinates of (0.168, 0.773). The color gamut covers 97% and 99% of the Rec. 2020 standard in the CIE 1931 and the CIE 1976 color space, respectively, representing the "greenest" LEDs ever reported. Moreover, the device shows only a ∼10% roll-off in ηCE (11.3 cd A-1) at 1000 cd m-2. We further demonstrate large-area (3 cm2) and ultraflexible (bending radius of 2 mm) LEDs based on 2D perovskites.

Thermal restructuring of silica-grafted TiCl(x) species and consequences for epoxidation catalysis.

TiCl4 grafted to dehydrated silica is an industrially applied catalyst for the epoxidation of propylene. As with many heterogeneous catalysts, the precise nature of the surface species is not yet fully known, prohibiting reliable structure-activity correlations. In this study, the speciation and restructuring of site-isolated Ti(IV) Lewis acid centers was carefully investigated by using a variety of techniques. The initially formed ≡SiOTiCl3 species were found to restructure upon heating through the transfer of Cl ligands to the silica surface, eventually leading to tripodal (≡SiO)3 TiCl species. The superior activity and stability of such tripodal species is demonstrated for catalytic olefin epoxidation under continuous flow conditions.