Exciton-phonon coupling (EXPC) plays a key role in the optoelectronic properties of semiconductor nanocrystals (NCs). Despite the importance of EXPC for determining emission linewidths and hot carrier cooling rates, a microscopic picture of EXPC is still lacking, particularly regarding the magnitude and scaling with NC size, the dependence on phonon frequency, and the role of the NC surface. Due to the computational complexity associated with obtaining accurate descriptions of excitons and phonons, previous theoretical studies of EXPC have been limited to small NCs, noninteracting electron-hole models, and/or a small number of phonon modes. Here, we develop a microscopic, atomistic approach for describing EXPC in NCs of experimentally relevant sizes. We validate our approach by comparing the calculated reorganization energies, a measure of EXPC, for a series of CdSe and CdSe-CdS core-shell NCs, finding good agreement with experimental measurements. We demonstrate that exciton formation distorts the NC lattice primarily along the coordinates of low-frequency acoustic modes that are delocalized over the entire NC. Modes at the NC surface play a significant role in smaller NCs while coupling to interior modes dominates for larger systems.

Interplay of surface and interior modes in exciton-phonon coupling at the nanoscale

ACS Spring 2022

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