Excited-State Charge Transfer within Covalently Linked Quantum Dot Heterostructures

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.