Because of the ability to tune several of their properties by altering their size, shape, and composition, quantum dots (QD) have emerged as a promising material for the creation of optoelectronic devices. In QD solids, controlling QD surface structure is key to achieving films that can readily transport energy or charge. In prior work from our group, we showed that replacing native oleate ligands used to cap CdSe QDs during their synthesis with phenyl dithiocarbamate (PDTC) ligands, can facilitate the transfer of energy between neighboring QDs on a sub-picosecond timescale. This rapid energy transfer rate was ascribed to the ability of PDTC to act as an exciton-delocalizing ligand that relaxed the quantum confinement of CdSe QDs and facilitated improved wavefunction overlap between neighboring QDs. However, PDTC has also been suggested to decompose under certain ligand exchange conditions, suggesting that such decomposition could play a potential role in energy transport in QD solids. This has motivated us to undertake a detailed study of how the conditions used for solid-state ligand exchange impact energy transfer in QD solids. We indeed find that as QD films are exposed to PDTC dissolved in methanol the surface concentration of bound PDTC molecules varies nonmonotonically. Following a short ligand exchange period, we find that PDTC acts to replace a substantial fraction of bound oleate ligands. However, as the film is exposed further to methanol, the PDTC surface concentration decreases, suggesting either decomposition or desorption of bound PDTC molecules. Interestingly, the loss of PDTC acts to decrease the average spacing between QDs, which we find leads to improved exciton migration. We conclude the detachment or decomposition of PDTC plays a key role in improving exciton migration in QD solids, suggesting that the close association of particles, in addition to exciton delocalization, impacts energy transport.

How solid-state ligand exchange with an exciton delocalizing ligand impacts energy migration in quantum dot solids

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