Area-Independence of the Biexciton Oscillator Strength in CdSe Colloidal Nanoplatelets.

Abstract

Colloidal CdSe nanoplatelets (NPLs) are unique systems to study two-dimensional excitons and excitonic complexes. However, while absorption and emission of photons through exciton formation and recombination have been extensively quantified, few studies have addressed the exciton-biexciton transition. Here, we use cross-polarized pump-probe spectroscopy to measure the absorption coefficient spectrum of this transition and determine the biexciton oscillator strength (<i>f</i>_BX). We show that <i>f</i>_BX is independent of the NPL area and that the concomitant biexciton area (<i>S</i>_BX) agrees with predictions of a short-range interaction model. Moreover, we show that <i>f</i>_BX is comparable to the oscillator strength of forming localized excitons at room temperature while being unaffected itself by center-of-mass localization. These results confirm the relevance of biexcitons for light-matter interaction in NPLs. Moreover, the quantification of the exciton-biexciton transition introduced here will enable researchers to rank 2D materials by the strength of this transition and to compare experimental results with theoretical predictions.