Mechanisms of Interfacial Charge Transfer
People: Shichen Lian, Chen He, Kaitlyn Perez
Charge-Transfer through strongly-coupled ligands:
Certain molecules, upon adsorption to the quantum dot surface, become part of its electronic structure. QD-organic complexes formed from these molecules are true hybrid materials, and have special optical and electronic properties. Phenyldithiocarbamate, or PTC, causes unprecedented bathochromic shifts in the optical band gap of semiconductor quantum dots (QDs). This reduction in the optical band gap is caused by strong coupling between the HOMO of PTC and the valence band (VB) of the quantum dot, which creates new states that reduce the confinement potential experienced by the QD.
PTC relaxes the confinement of hole wavefunction of QDs and enhances the electronic coupling between quantum dots and their molecular redox partners. QD-PTC interfacial-states mediate ultrafast hole transfer from the QD core to molecular redox partners.
Exchange of native oleylamine ligands with 4-hexylphenyldithiocarbamate (C6-PTC), or 4-hexylthiophenol (C6-TP) on the surface of PbS quantum dots (QDs) decreases the confinement of the excitonic hole, causing a bathochromic shift in the absorption and emission spectra of the QDs. In this study, we probe the effect of delocalizing ligands such as these on the QD Fermi energy. We use photoluminescence spectroscopy to study hole transfer yield from the PbS QDs to a series of thiol-functionalized ferrocene derivatives with a range of HOMO energies. Addition of exciton-delocalizing ligands should increase the driving force to hole transfer, resulting in a greater yield of hole transfer for lower-HOMO ferrocene derivatives.
Tuning QD-molecule interactions through the ligand shell:
In order to best utilize QDs in analytical or photocatalytic applications, it is important to develop strategies to control desired interactions between the nanocrystal surface and molecules of interest, while suppressing nonspecific or unproductive interactions. The ligand shell that solubilizes semiconductor QDs behaves similarly to a self-assembled monolayer on a planar surface, and tuning the composition of the ligand shell can be used to change the permeability of this organic adlayer. The yield of charge transfer interactions between semiconductor QDs and molecular redox partners is an effective probe for ligand shell permeability, as the molecule must either pre-adsorb to the surface of the nanocrystal or diffuse through the organic adlayer within the lifetime of the QD’s exciton in order to extract an electron.
We have studied nanocrystals with fluorinated ligand layers in order to reduce nonspecific interactions with nonpolar molecules in solution. Wecan modify the the resistance of the QD ligand shell to permeation by nonpolar molecules through the addition of fully fluorinated ligands to the QD surface or by modifying the number of fluorine atoms per ligand.
By controlling the degree of fluorination of the ligand shell and the size of the QD core, we can regulate the permeability of the ligand shell to a small molecular electron acceptor. Using electron transfer as our probe for the ligand shell permeability of a molecular electron acceptor, we can investigate the interactions at the ligand shell/solvent interface.
We can also synthesize QDs capped with charged surfactants in order to tune the permeability of the ligand shell towards anionic redox partners in polar solvents. By changing the ratio of charged density on the QD surface we can modify to permeability of charged molecules through the ligand shell.
Additionally, we can modify to permeability of oppositely charged molecules through the ligand shell by the introduction of counterions that form ion pairs with the negatively charged carboxylates on the surface of the QD. By increasing the length of the alkyammonium salt introduced, we can increase the permeability of a negatively charged molecule through the ligand shell.