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

We used N,N'-dicyclohexylcarbodiimide (DCC) coupling chemistry to synthesize (1) heterostructures of CdSe and CdTe quantum dots (QDs) in colloidal dispersions and (2) heterostructures of CdSe and CdTe QDs, as well as CdS and CdSe QDs, immobilized on metal oxide thin films. The DCC-mediated formation of amide bonds between terminal carboxylic acid and amine groups of ligands on different QDs drove the formation of heterostructures. This cross-linking mechanism selectively yields heterostructures and prohibits the undesired formation of homostructures consisting of just one type of QD. Products of adsorption, ligand-exchange, and covalent-coupling reactions were characterized by transmission electron microscopy and ATR-FTIR, H-1 NMR, electronic absorption, steady-state emission, and time-resolved emission spectroscopy. Ground-state absorption spectra of constituent QDs were unperturbed upon incorporation into heterostructures, enabling control over electronic properties. Heterostructures of CdSe and CdTe QDs exhibit type-II interfacial energetic offsets that promote charge separation following excitation of either QD. Indeed, photoexcited CdTe QDs transferred electrons to CdSe, and photoexcited CdSe QDs transferred holes to CdTe, on time scales of 10-100 ns, as evidenced by dynamic quenching of band-edge and trap-state emission. Mixed dispersions of noninteracting QDs did not undergo excited-state charge transfer. Constructing heterostructures on TiO2 thin films introduced an additional charge-transfer pathway, electron transfer from QDs to TiO2, which occurred on subnanosecond time scales and enabled extended spatial separation of photogenerated electrons and holes. Our results reveal that carbodiimide coupling chemistry can be used to tether colloidal QDs selectively and covalently to each other, yielding dispersed or immobilized heterostructures with programmable compositions and energetic offsets that can undergo efficient excited-state interfacial electron transfer.