Ground- and Excited-State Properties of Charged Non-Stoichiometric Quantum Dots

Abstract

Charged excited states can accumulate on the surface of colloidal quantum dots (QDs), affecting their optoelectronic properties. In experimental samples, QDs often have non-stoichiometric structures, giving rise to cation-rich and anion-rich nanostructures. We explore the effect of charge on the ground-and excited-state properties of CdSe non-stoichiometric QDs (NS-QDs) of ∼1.5 nm in size using density functional theory calculations. We compare two cases: (i) NS-QDs with a charge introduced by direct hole or electron injection and (ii) neutral NS-QDs with one removed surface ligand (with a dangling bond). Our calculations reveal that a neutral dangling bond has an effect on the electronic structure similar to that of the electron injection for the Cd-rich NS-QDs or hole injection for the Se-rich NS-QDs. In Cd-rich structures, either the injection of an electron or the removal of a passivating ligand results in the surface-localized half-filled trap state inside the energy gap. For Se-rich structures, either the injection of a hole or the removal of a ligand introduces surface-localized unoccupied trap states inside the energy gap. As a result, the charge localization formed by these two approaches leads to an appearance of low-energy electronic transitions strongly red-shifted from the main excitonic band of NS-QDs. These transitions related to a negative charge or a dangling bond exhibit weak optical activity in Cd-rich NS-QDs. Transitions related to a positive charge or a dangling bond are optically forbidden in Se-rich NS-QDs. In contrast, electron injection in Se-rich NS-QDs strongly increases the optical activity of the lowestred-shifted charge-originated states.

ZnSeTe quantum dots modified with zinc chloride toward bright trap-state emission

“…33 Chlorides were commonly employed as neutral ligands in non-stoichiometric colloidal QDs, where they have been considered as beneficial additives for reducing surface defects and enhancing photocurrent and photocatalytic activity. 34 So we focused solely on the changes in surface states induced by ZnCl 2 modification, rather than the effects of ZnCl 2 itself on the QDs. Combining our experimental results, we proposed a mechanism by which ZnCl 2 modification led to changes in the emission spectra of QDs.…”