Enhancement of the Biexciton Binding Energy in Laterally Confined CdSe Nanoplatelets

Optical amplification in CdSe nanoplatelets (NPLs) has been linked to biexcitons with a large binding energy ΔBX, preventing dissociation at room temperature. While the exciton binding energy ΔX has been studied extensively, ΔBX in colloidal NPLs is typically inferred using the 2D Haynes rule, ΔBX = 0.228·ΔX. Systematic studies of ΔBX in CdSe NPLs with varying thicknesses and lateral dimensions are still lacking. Here, we used transient absorption spectroscopy to investigate ΔBX, through photoinduced biexciton absorption. ΔBX ranges between 40 and 55 meV and increases by 30% when reducing the width from 9 to 3 nm, while overall 3.5 ML NPLs have only 10% higher ΔBX compared to 4.5 ML NPLs. Fitting the linear absorbance spectra, we extracted ΔX and demonstrate that the ΔBX/ΔX ratio is substantially lower than 0.228. Results confirm that biexcitons in CdSe NPLs are stable at room temperature and have an increased ΔBX in laterally confined NPLs.