Spatial confinement of charge carriers in nanosize semiconductor quantum dots (QDs) results in highly tunable, size-dependent optoelectronic properties that can be utilized in various commercial applications. Although in such nanostructures, non-stoichiometry is frequently encountered using conventional synthesis techniques, it is not often addressed or considered. Here, we perform ab initio molecular dynamics simulations on non-stoichiometric CdSe clusters to study the phonon-mediated charge carrier relaxation dynamics. We model cation-rich and anion-rich QDs passivated with monocharged neutralizing ligands of different sizes. Our studies confirm the presence of localized trap states at the valence band edge in only anion-rich QDs due to the presence of undercoordinated exposed surface Se atoms. Noteworthily, these localized states disappear when using bulkier ligands. Calculations reveal that the size of the ligands controls the crystal vibrations and electron–phonon coupling, while ligand coordination number affects the electronic structure. For a particular non-stoichiometric CdSe QD, a change of a ligand can either increase or decrease the total electron relaxation time compared to that of stoichiometric QDs. Our results emphasize the importance of ligand engineering in non-stoichiometric QDs for photoinduced dynamics and guide future work for the implementation of improved materials for optoelectronic devices.
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This content will become publicly available on January 9, 2025
Ground- and Excited-State Properties of Charged Non-Stoichiometric Quantum Dots
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 lowest- red-shifted charge-originated states.
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- Award ID(s):
- 2004197
- NSF-PAR ID:
- 10535971
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Chemistry of Materials
- Volume:
- 36
- Issue:
- 1
- ISSN:
- 0897-4756
- Page Range / eLocation ID:
- 146 to 156
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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