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Excited-State Charge Transfer within Covalently Linked Quantum Dot Heterostructures
We synthesized quantum dot (QD) heterostructures
via the <i>N</i>,<i>N</i>′-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, <sup>1</sup>H 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<sup>–8</sup> s) growth of a broad and long-lived
(>10<sup>–5</sup> 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