Excited-State Charge Transfer within Covalently Linked Quantum Dot Heterostructures

Abstract

We synthesized quantum dot (QD) heterostructures via the N,N′-dicyclohexylcarbodiimide-mediated formation of amide bonds between capping ligands on CdS QDs and CdSe QDs. Products of ligand-exchange and coupling reactions were characterized by FTIR, 1H NMR, transmission electron micrscopy, and electronic absorption and emission spectroscopy. This cross-linking strategy yields exclusively heterostructures and prohibits the undesired formation of homostructures consisting of a single type of QD. The ground-state absorption spectra of the presynthesized colloidal QDs were unperturbed upon incorporation into heterostructures. Photoexcited CdS QDs transferred holes to molecularly tethered CdSe QDs, as evidenced by significant dynamic quenching of the trap-state emission from CdS QDs and the rapid (<10–8 s) growth of a broad and long-lived (>10–5 s) transient absorption band in the visible region. These spectral signatures were absent for mixed dispersions of noninteracting CdS and CdSe QDs. Our results reveal that carbodiimide coupling chemistry can be used to tether colloidal QDs selectively and covalently to each other and that the resulting heterostructures can undergo efficient photoinduced interfacial charge transfer.

Quaternary Alloy Quantum Dots as Fluorescence Probes for Total Acidity Detection of Paper-Based Relics

“…At the pH of 6, the resonance at 3.05 ppm is attributed to TMAC. Free pATP exhibited two quasi-doublets centered at 6.61 and 7.17 ppm, corresponding to the aromatic protons [ 47 , 48 ]. The molar ratio of free pATP to TMAC was 0.12:1, calculated from the relative integral intensity of the peaks.…”

Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2

“…Steady-state emission spectra were obtained with a Varian Cary Eclipse fluorimeter with excitation at 430 nm. Time-resolved emission data were obtained by time-correlated single-photon counting (TCSPC) using the system described previously and in Appendix S3 in the Supporting Information. , …”

“…Timeresolved emission data were obtained by time-correlated single-photon counting (TCSPC) using the system described previously and in Appendix S3 in the Supporting Information. 53,54 ■ RESULTS AND DISCUSSION Adsorption of Linkers to TiO 2 . The Immersion of TiO 2 films in solutions of ADA or MDA resulted in the growth of ν a (CH 2 ) and symmetric CH 2 stretching (ν s (CH 2 )) bands centered at approximately 2929 and 2857 cm −1 , respectively, in the IR spectra (Figure 1), indicating that the ligands adsorbed to TiO 2 .…”

Excited-State Charge Transfer and Extended Charge Separation within Covalently Tethered Type-II CdSe/CdTe Quantum Dot Heterostructures: Colloidal and Multilayered Systems

“…Semiconductor quantum dots (QDs) are prime candidates as light harvesters and donors of excited charge carriers for solar energy conversion, due to their unique physical and electronic properties. The potential to exploit size-dependent optical properties, tunable band-edge potentials, and rich surface chemistry and surface-localized electronic states, as well as the possibilities of multiexciton generation and hot carrier extraction make QDs an attractive candidate for solar cells and photocatalysts. − We aim to synthesize QD heterostructures with suitable energetic offsets, as determined by the sizes and compositions of constituent QDs, and appropriate interconnectivity to promote interfacial excited-state charge transfer while also optimizing the light-harvesting efficiency. − Extended charge separation in heterostructures enables charge extraction or subsequent redox processes to compete more efficiently with electron–hole recombination.…”

“…Carbodiimide-mediated coupling chemistry is a well-established method for covalently tethering QDs to secondary species such as carbon nanotubes, metallic nanoparticles, proteins, small molecules, antibodies, and more recently other QDs to form heterostructures. ,− Carbodiimide-mediated coupling between terminal functional groups of QD-adsorbed ligands selectively yields heterostructures and affords control over the rate and extent of reaction, which are distinct advantages in comparison to the formation of heterostructures via bifunctional linkers . Our group’s recent efforts have established the validity of utilizing carbodiimide-mediated coupling chemistry to selectively tether QDs through formation of an amide bond between terminal functional groups of respective capping ligands.…”