Exciton–Ligand Interactions in PbS Quantum Dots Capped with Metal Chalcogenides

Abstract

Colloidal quantum dots (CQDs) are typically decorated with organic molecules that provide surface passivation and colloidal solubility. An alternate but less studied surface functionalization approach via inorganic complexes can produce stable CQDs with attractive transport and optical properties. Further development of such all-inorganic CQD solids is dependent on the deeper understanding of the energetic and dynamic interactions of the new ligands with the CQD excitons. Herein, a series of four metal chalcogenide (MCC) ligands of the K z XS 4 type were attached to PbS CQDs. Out of the four MCC complexes studied, we find that only K 4 GeS 4 ligands yield robust PbS CQD films with bright photoluminescence (PL) in the solid state. A systematic spectroscopic investigation of the K 4 GeS 4 -capped CQD films provides evidence of the temperature-dependent ligand-mediated exciton delocalization and trapping processes. At low temperatures, efficient trapping at ligand-induced states is found to occur within ∼6 ns after photoexcitation, followed by a considerably slower exciton back transfer to the CQD core. At elevated temperatures, the CQD films become photoconductive, providing evidence of exciton dissociation via carrier transfer within adjacent dots. The addition of a thin CdS shell suppresses the delocalization and trapping of excitons, resulting in brighter emission and significantly slower transient absorption and PL dynamics.

Defect Passivation in Lead‐Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

“…Thea uthors suggest that the excitons can be trapped by the inter-dot ligands,which quench the photoluminescence from the corelevel excitons. [62] Additionally,R ossi et al reported that Fçrster resonance energy transfer occurs between CsPbBr 3 nanocrystals and ligands based on perylene diimide,w hich causes emission from the ligands. [63] Based on previous studies of traditional quantum dot systems,s uch as PbS and CdS, [64] we expected to have ligand-exciton interactions or trapping phenomena.…”

Defect Passivation in Lead‐Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

“…The authors suggest that the excitons can be trapped by the inter‐dot ligands, which quench the photoluminescence from the core‐level excitons. [62] Additionally, Rossi et al. reported that Förster resonance energy transfer occurs between CsPbBr 3 nanocrystals and ligands based on perylene diimide, which causes emission from the ligands.…”

“…K 4 GeS 4 ). The authors suggest that the excitons can be trapped by the inter‐dot ligands, which quench the photoluminescence from the core‐level excitons [62] . Additionally, Rossi et al.…”

The impact of chemical composition of halide surface ligands on the electronic structure and stability of lead sulfide quantum dot materials

“…[23][24][25][26] In the study of Brown et al, ultraviolet photoelectron spectroscopy and density functional theory calculations were used to show that the ligands can significantly influence the position of the Fermi level in relation to valence and conduction bands. 27 Moreover, the ligand choice also influences the stability of the solid film as well as the formation of trap states, as shown in the time-resolved photoluminescence study of Papagiorgis et al 28 Even small surface modifications can drastically enhance the efficiency of absorption as shown in the study of Giansante et al where they were able to enhance the absorption by 300%. 29 Eventually, the search for most optimal ligands moved the research towards inorganic halide ligands.…”