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

Abstract

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, 1 H NMR, electronic absorption, steady-state emission, and timeresolved 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 TiO 2 thin films introduced an additional charge-transfer pathway, electron transfer from QDs to TiO 2 , 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.

Interface-modulated kinetic differentials in electron and hole transfer rates as a key design principle for redox photocatalysis by Sb2VO5/QD heterostructures

“…This approach has the effect of shutting down oxidative degradation pathways and also affords long-lived excited-state electrons and holes separated in real space for use in redox catalysis. 9,23,24 Photoexcitation-driven separation of electrons and holes in semiconductor heterostructures is predicated on (i) the thermodynamic energy offsets between the staggered conduction and valence band edges of a pair of semiconductors (to localize photoexcited electrons and holes on different components) [25][26][27] and (ii) interfacial connectivity and separation between the semiconductors, which govern excited-state electron and hole transfer dynamics. 14,[28][29][30] In this article, we examine thermodynamic driving forces and excitedstate electron and hole transfer dynamics in SbV 2 O5/CdSe QD heterostructures, a promising new class of photocatalysts that leverage midgap states derived from the stereochemically active 5s 2 lone-pairs of Sb to effect hole extraction from photoexcited II-VI QDs.…”

“…13 A broad range of semiconductor heterostructures have been explored with staggered conduction and valence band edges that are amenable to charge separation and the generation of electron-hole pairs upon photoexcitation. 9,23,[25][26][27] A promising design strategy involves interfacing II-VI QDs with transition metal oxides exhibiting mid-gap states derived from the stereochemically active 5/6 s 2 electron lone pairs of p-block cations. 9,14,23,24,31,32 For instance, in MxV 2 O5 compounds shown in Fig.…”

Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution

“…Sunlight is abundant and universally available, and solar irradiance can be converted into electrical power or chemical potential. , One attractive, carbon-neutral solar-to-chemical energy conversion pathway is solar water splitting or the decomposition of H 2 O into H 2 and O 2 . − To split water, a photocatalyst system must promote a complex series of processes: (1) absorption of sunlight; (2) separation of photogenerated electrons and holes; (3) transport of charge carriers to catalytic sites; and (4) catalysis of multielectron, proton-coupled reduction and oxidation half-reactions. − It is unlikely that a single material can orchestrate this complex sequence in a synchronized manner. Instead, hybrid heterostructures, consisting of individual components that each mediates a subset of required processes, are promising photocatalyst candidates. ,− Semiconductor heterostructures with staggered conduction and valence band edges can enhance charge separation in real space by localizing photoexcited electrons and holes on different components. ,− Material components of heterostructures can be further optimized for solar water splitting, such as to lower the overpotential for proton reduction or to enhance light harvesting within the solar spectrum. , …”

Unlocking the electronic, optical and transport properties of semiconductor coupled quantum dots using first principles methods

“…15 Beyond these, there are several reports on II-VI semiconductor colloidal coupled dots, where the electronic and optical properties, for instance, absorbance, were tuned by controlling band offsets. 7,[16][17][18][19] Furthermore, the semiconductor coupled dots or heterodimers has similarity with molecules in molecular electronics 20,21 , where a single molecule with a donor part and an acceptor portion connected by a bridge acts as a molecular rectifier as shown by Aviram and Ratner. 21 Molecular rectifiers made from organic molecules are generally unstable and device fabrication is difficult, while colloidal nanostructures are expected to have an advantage from device fabrication viewpoint.…”