Recent work in the realm of photovoltaic devices has focused on quantum dot (QD) heterostructures, such as QDs attached to nanotubes or nanowires via an organic linker. The main utility of such systems is to capitalize on the efficient charge generation within the QD while relying on energetically favorable band offsets to efficiently separate charges. While organic ligands have been studied as single moieties, the nature of their bound-ligand states on the surface of the QD is not well understood. We have investigated the spectroscopic effects that a series of thiol and amine terminated, short-chain ligands have on CdSe QDs. The chosen ligands have highest-occupied-molecular-orbitals (HOMO) that lie near or above the top of the valence band in CdSe QD. The introduction of this mid-gap state leads to distinct changes in the QD photophysics, including the peak emission wavelength, intensity, and lifetime. With the goal of quantifying the energy offset as a function of binding site and chemical structure, we apply several optical probes that are sensitive to surface electronic structure. For example, we observe a distinct deviation between the optical extinction and photoluminescence excitation spectra of the ligand-exhanged QDs that reflects an excitation wavelength dependent quantum yield for emission. The assignment of these features to interfacial traps states is supported by the observation of wavelength dependent band-edge PL photofilling effects. We quantify the magnitude and kinetics of photofilling to understand the energy and quantity of surface states. These results are used to categorize not only the band offsets but to help deduce which ligand(s) are optimal for efficient charge separation.

Steady-state and time-resolved optical characterization of CdSe quantum dot � ligand interfacial states

247th National Meeting (2014)

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