Abstract
Lead halide compounds, including lead halide perovskite nanocrystals (NCs), have attracted the interest of researchers in optoelectronics and photonics because of their high photoluminescence quantum yields (PLQYs) coupled with relatively short PL lifetimes (on the order of a few nanoseconds). However, lead-free metal halides of high PLQY, including double perovskites and their doped NCs, typically possess long PL lifetimes (up to microseconds) that limit their application space. Here, we introduce CsMnBr, NCs, which are leadfree and red-emitting, that combine a high PLQY with an exceptionally short radiative lifetime (on the order of picoseconds). We find that the octahedral coordination of Mn2+ in CsMnBr3 induces a red emission centered at -643 nm with a PLQY of similar to 54% and a fast radiative decay rate. Femtosecond transient absorption and transient PL spectroscopies reveal the existence of a low-lying excited state of Mn2+ that relaxes to the ground state within around 605 ps by emitting light at around 643 nm. At greater excitation energies, higher excited states of Mn2+ relax in the sub-nanosecond time scale to this low-lying excited state. A similarly positioned PL peak with a short picosecond scale PL lifetime and a PLQY of similar to 6.7% was also detected in bulk CsMnBr3 single crystals reported in this study-a relatively high quantum yield for a bulk material. Our experimental results and density functional theory modelling show that the crystal structure and the strong coupling among Mn2+ ions govern those luminescence properties of CsMnBr3 NCs and single crystals. These findings pave the way for new lead-free materials that combine high PLQY and ultrafast luminescence.