Organic molecules as tools to control the growth, surface structure, and redox activity of colloidal quantum dots

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Abstract

In order to achieve efficient and reliable technology that can harness solar energy, the behavior of electrons and energy at interfaces between different types or phases of materials must be understood. Conversion of light to chemical or electrical potential in condensed phase systems requires gradients in free energy that allow the movement of energy or charge carriers and facilitate redox reactions and dissociation of photoexcited states (excitons) into free charge carriers. Such free energy gradients are present at interfaces between solid and liquid phases or between inorganic and organic materials. Nanostructured materials have a higher density of these interfaces than bulk materials. Nanostructured materials, however, have a structural and chemical complexity that does not exist in bulk materials, which presents a difficult challenge: To lower or eliminate energy barriers to electron and energy flux that inevitably result from forcing different materials to meet in a spatial region of atomic dimensions.Chemical functionalization of nanostructured materials is perhaps the most versatile and powerful strategy for controlling the potential energy landscape of their interfaces and for minimizing losses in energy conversion efficiency due to interfacial structural and electronic defects. Colloidal quantum dots are semiconductor nanocrystals synthesized with wet-chemical methods and coated in organic molecules. Chemists can use these model systems to study the effects of chemical functionalization of nanoscale organic/inorganic interfaces on the optical and electronic properties of a nanostructured material, and the behavior of electrons and energy at interfaces. The optical and electronic properties of colloidal quantum dots have an intense sensitivity to their surface chemistry, and their organic adlayers make them dispersible in solvent. This allows researchers to use high signal-to-noise solution-phase spectroscopy to study processes at interfaces.In this Account, I describe the varied roles of organic molecules in controlling the structure and properties of colloidal quantum dots. Molecules serve as surfactant that determines the mechanism and rate of nucleation and growth and the final size and surface structure of a quantum dot. Anionic surfactant in the reaction mixture allows precise control over the size of the quantum dot core but also drives cation enrichment and structural disordering of the quantum dot surface. Molecules serve as chemisorbed ligands that dictate the energetic distribution of surface states. These states can then serve as thermodynamic traps for excitonic charge carriers or couple to delocalized states of the quantum dot core to change the confinement energy of excitonic carriers. Ligands, therefore, in some cases, dramatically shift the ground state absorption and photoluminescence spectra of quantum dots. Molecules also act as protective layers that determine the probability of redox processes between quantum dots and other molecules. How much the ligand shell insulates the quantum dot from electron exchange with a molecular redox partner depends less on the length or degree of conjugation of the native ligand and more on the density and packing structure of the adlayer and the size and adsorption mode of the molecular redox partner.Control of quantum dot properties in these examples demonstrates that nanoscale interfaces, while complex, can be rationally designed to enhance or specify the functionality of a nanostructured system.

Original languageEnglish
Pages (from-to)2607-2615
Number of pages9
JournalAccounts of Chemical Research
Volume46
Issue number11
DOIs
Publication statusPublished - Nov 19 2013

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Surface structure
Semiconductor quantum dots
Molecules
Nanostructured materials
Charge carriers
Ligands
Electrons
Electronic properties
Free energy
Oxidation-Reduction
Optical properties
Anionic surfactants
Redox reactions
Energy barriers
Surface states
Laser modes
Potential energy
Surface chemistry
Energy conversion
Surface-Active Agents

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Organic molecules as tools to control the growth, surface structure, and redox activity of colloidal quantum dots. / Weiss, Emily A.

In: Accounts of Chemical Research, Vol. 46, No. 11, 19.11.2013, p. 2607-2615.

Research output: Contribution to journalArticle

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