Prof. Emily Weiss, Northwestern University

A Molecule to Detect and Perturb the Confinement of Charge Carriers in Colloidal Quantum Dots

School Colloquium

This talk describes unprecedented bathochromic shifts (up to 970 meV) of the optical bandgaps of CdS, CdSe, and PbS quantum dots (QDs) upon adsorption of an organic ligand, phenyldithiocarbamate (PTC), and the use of PTC to map the quantum confinement of specific charge carriers within the QDs as a function of their radius. For a given QD material and physical radius, R, the magnitude of the increase in apparent excitonic radius (ΔR) upon delocalization by PTC directly reflects the degree of quantum confinement of one or both charge carriers. The plots of ΔR vs. R for CdSe and CdS show that exciton delocalization by PTC occurs specifically through the excitonic hole. Furthermore, the plot for CdSe, which spans a range of R over multiple confinement regimes for the hole, identifies the radius (R ~ 1.9 nm) at which the hole transitions between regimes of strong and intermediate confinement. Substituting the PTC molecule at the para position tunes the resonance between the PTC HOMO and the valence band levels of the QD.  This demonstration of ligand-induced delocalization of a specific charge carrier is a first step toward eliminating current-limiting resistive interfaces at organic-inorganic junctions within solid-state hybrid devices.

For more information contact Prof. Seth Marder (404-385-6048).