Excited-State Charge Transfer in Covalently Tethered Cadmium Chalcogenide Quantum Dot Heterostructures
Wolfe II, Guy
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Semiconductor quantum dots (QDs) exhibit unusual properties, such as size-dependent molar absorptivities and bandgaps. In certain cases, QDs can allow for multiexcition generation and hot carrier extraction, making them prime candidates in solar energy conversion. The unusual properties of QDs are often exploited by incorporating them into QD-containing nanoscale assemblies, producing an energetic offset to promote charge separation. QDs often have a ligand shell, which is responsible for their colloidal stability and provides the possibility of tethering them to molecules or other materials, such as metal oxide films, nanowires, molecular acceptors, and other QDs. This dissertation focuses on the study of interfacial excited-state charge transfer within assemblies of cadmium chalcogenide QDs that are tethered together through their ligand shells by using a N,N'-dicyclohexylcarbodiimide-mediated coupling approach. In particular, excited-state charge transfer within 3 heterostructures was studied: (1) CdS-CdSe, a first generation heterostructure, (2) CdTe-CdSe, a second generation heterostructure, and (3) CdSe(lg)-CdSe(sm), a third generation heterostructure. CdSe-amide-CdS heterostructures were synthesized by using a carbodiimide-mediated formation of amide bonds between capping ligands on CdS QDs and CdSe QDs. The ligand shells of CdS and CdSe could be modified post-synthetically to contain an N-hydroxysuccinimide (NHS)-ester and primary amine moiety to promote the assembly of heterostructures. When CdS and CdSe were interfaced, trap-state emission from CdS was quenched significantly. Transient absorption studies of these heterostructures revealed the rapid growth (< 10-8 s) of a broad transient absorption band in the visible that was long-lived (> 10 5 s) and was not present in mixtures of non-interacting CdS and CdSe QDs. This transient absorption band was attributed to photogenerated holes that had transferred from CdS to CdSe. Time-resolved emission measurements revealed that hole transfer had occurred with a hole transfer rate constant of 9.6 × 107 s-1. This revealed that carbodiimide coupling can be used to selectively tether two colloidal QDs together from dispersion and promote interfacial excited-state charge transfer. Building on the carbodiimide-mediated coupling chemistry, heterostructures of CdTe and CdSe QDs were assembled from dispersions. The formation of amide bonds with 4-methlythio(aniline) (4-MTA) within the CdSe QD ligand shell had no effect on the emission decay kinetics of CdSe, which indicated that amide bond formation alone did not electronically affect the QD. When CdTe and CdSe QDs were interfaced, both the band-edge emission from CdTe and the trap-state emission from CdSe were quenched. Time-resolved emission studies revealed significant dynamic quenching of both CdTe band-edge and CdSe trap-state emission at all wavelengths, consistent with bidirectional charge transfer, a hallmark of a type-II energetic offset. We demonstrated that hole transfer from CdSe to CdTe was nearly 2-fold more efficient than electron transfer from CdTe to CdSe. The results therefore revealed that obtaining a type-II energetic offset is advantageous, since the same charge-separated state results, independently of which QD is excited.Lastly, heterostrucutures comprising larger CdSe QDs (CdSe(lg)) and smaller CdSe QDs (CdSe(sm)) were synthesized using the carbodiimide coupling chemistry. We showed that quantum confinement can be used to tune energetics and to drive charge separation. When CdSe(lg) and CdSe(sm) QDs were interfaced, significant quenching within the trap-state band of CdSe(sm) occurred. Time-resolved emission studies revealed significant dynamic quenching throughout the CdSe(sm) trap-state emission band, consistent with electron transfer from CdSe(sm) to CdSe(lg). The electron transfer efficiency was less than half, indicating that QD sizes will need to be tuned in order to promote more efficient charge separation.Taken together, the results reported in this dissertation suggest that carbodiimde-mediated coupling can be used to from heterostructures consisting of a range of different QDs, as long as the appropriate moieties are present within their ligand shells. The materials-assembly strategy is thus generalizable.