Unusually Strong Bi-Exciton Repulsion Detected in Quantum Confined CsPbBr3 Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy

Abstract

Transient absorption measurements were conducted on pristine and on monoexciton saturated<br>CsPbBr3 nanocrystals varying in size within the regime of strong quantum confinement. Once<br>the difference spectra were translated to absolute transient changes in absorption cross section,<br>a single exciton is shown to completely bleach the band edge absorption peak, and induce a<br>new absorption roughly two times weaker ~100 meV to the blue. Difference spectra obtained<br>during Auger recombination of biexcitons demonstrate that addition of a second exciton, rather<br>than double the effect of a first, bleaches the blue induced absorption band without producing<br>net stimulated emission at the band edge. Accompanied by high time resolution transient<br>absorption spectra pumping at the lowest exciton band, these results identify the blue induced<br>absorption as the second transition to 1Se1Sh which is shifted in energy due to unusually strong<br>and promptly rising biexciton repulsion. <br>