Anomalously Large Polarization Effect Responsible for Excitonic Red Shifts in PbSe Quantum Dot Solids

Abstract

The formation of solid thin films from colloidal semiconductor quantum dots (QDs) is often accompanied by red shifts in excitonic transitions, but the mechanisms responsible for the red shifts are under debate. We quantitatively address this issue using optical absorption spectroscopy of two-dimensional (2D) and three-dimensional (3D) arrays of PbSe QDs with controlled inter-QD distance, which was determined by the length of alkanedithiol linking molecules. With decreasing inter-QD distance, the first and second exciton absorption peaks show increasing red shifts. Using thin films consisting of large and isolated QDs embedded in a matrix of small QDs, we determine that a dominant contribution to the observed red shift is due to changes in polarization of the dielectric environment surrounding each QD (∼88%), while electronic or transition dipole coupling plays a lesser role. However, the observed red shifts are more than 1 order of magnitude larger than theoretical predictions based on the dielectric polarization effect for spherical QDs. We attribute this anomalously large polarization effect to deviations of the exciton wave functions from eigenfunctions of the idealized spherical quantum well model.

Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

“…Traditional approaches to fabricate conductive nanocrystal films displace native surfactant ligands with pyridine, 4,91,105-110 primary alkylamines, 15,73,111-116 hydrazine 73,84,106,108,116-126 and alkane diamines, 3,4,73,84,108,113,114,118,127-133 and are therefore likely to significantly reduce the metal-enrichment. Chelating alkane diamines have been used to displace native ligands and link nanocrystals, resulting in films where the inter-nanocrystal spacing depends on the diamine chain length.…”

Epitaxially Connected PbSe Quantum-Dot Films: Controlled Neck Formation and Optoelectronic Properties

“…This behavior is as expected and has been reported several times in the literature. 3,15 The linear diamines on the other hand show a different behavior. In all cases the distance between the QDs is strongly reduced and the QDs exhibit significant necking.…”

A mirage study of CdSe colloidal quantum dot films, Urbach tail, and surface states

“…Larger than expected dielectric redshifts have been discussed for PbSe colloidal dots crosslinked with short dithiol molecules. 23 Possibly, the shift might be accounted for if one considers the permanent and excited dipole moments of the nanocrystals. Dielectric effects, however, cannot account for much larger redshifts nor the absorption tails beyond the gap seen with S 2− before and after annealing.…”