Due to the ability to alter their electronic structure by controlling their size and shape, semiconductor nanocrystals hold great promise as active components for optoelectronic devices. However, these applications often require materials that readily transport either energy or charge, which is problematic for many nanocrystal-based solids as nanocrystal growth is commonly halted by capping their surfaces with long, insulating ligands. To address this challenge, we have explored the impact of replacing insulating ligands with exciton delocalizing ligands (EDLs), molecules with frontier molecular orbitals that have the proper symmetry and energetic alignment needed to strongly mix with states within a nanocrystal’s valence and conduction bands. In this presentation, we will discuss results measured on CdSe quantum dot solids that have been treated with the EDL phenyldithiolcarbamate (PDTC), which possesses a HOMO level well-situated to hybridize with the valence band of CdSe and relax quantum confinement of photoexcited holes. Using two-dimensional electronic spectroscopy (2D ES), we find PDTC motion plays a key role in modulating the bandgap of PDTC-functionalized nanocrystals, supporting the notion that nanocrystal valence band states contain significant ligand-centered character.  Using 2D ES in conjunction with transient absorption spectroscopy, we find excitons produced in PDTC-functionalized nanocrystal solids undergo rapid migration, achieving nanocrystal-to-nanocrystal energy transfer rates ~5 orders of magnitude faster than that of oleate-capped nanocrystals. However, we find the rate of exciton hopping to depend critically on the photon energy used to excite the nanocrystal, with higher excitation energies leading to faster exciton hopping. Electronic structure calculations provide a potential explanation for this observation as coupling between PDTC’s LUMO level and CdSe conduction band states significantly improves deeper into the conduction band. Together, these results suggest the dependence in transfer rate we observe with changing photon energy results from enhanced inter-nanocrystal electronic coupling involving high-energy states. The ultimate rate of exciton transport when exciting into these states represents a competition between exciton cooling and inter-nanocrystal exciton transfer.<br/>

Functional ligands for improved nanocrystal energy transport

2019 Fall National Meeting

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