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 Δ, preventing dissociation at room temperature. While the exciton binding energy Δ has been studied extensively, Δ in colloidal NPLs is typically inferred using the 2D Haynes rule, Δ = 0.228·Δ. Systematic studies of Δ in CdSe NPLs with varying thicknesses and lateral dimensions are still lacking. Here, we used transient absorption spectroscopy to investigate Δ, through photoinduced biexciton absorption. Δ 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 Δ compared to 4.5 ML NPLs. Fitting the linear absorbance spectra, we extracted Δ and demonstrate that the Δ/Δ ratio is substantially lower than 0.228. Results confirm that biexcitons in CdSe NPLs are stable at room temperature and have an increased Δ in laterally confined NPLs.