The origins of Stokes shift in PbS nanocrystals

The Stokes shifts in lead chalcogenides family of QDs are anomalously large and its origin has not been fully resolved. In this work we show, using ab initio calculations that the presence of intrinsic defects can cause substantial electron density localization of the bandedge states, causing excessive Franck-Condon (FC) shifts. In addition, the energetics and size disorders of a polydisperse QD film were found to contribute a 20 to 50 meV increase in Stokes shift compared to isolated QDs. FC shifts increase as the electronegativities of the of passivating ligands increase. These effects are small, and mainly due to the stronger bindings between surface Pb atoms and the ligands, resulting in smaller effective QD sizes. Our findings are significant to improving the understanding of the optical properties of PbS QDs and are important to designing future optoelectronics and photovoltaics devices.