  Over the past thirty years, it has been consistently observed that surface
engineering of colloidal nanocrystals (NC) is key to their performance
parameters. In the case of lead chalcogenide NCs, for example, replacing thiols
with halide anion surface termination has been shown to increase power
conversion efficiency in NC-based solar cells. To gain insight into the origins
of these improvements, we perform ab initio molecular dynamics (AIMD) on
experimentally-relevant sized lead sulfide (PbS) NCs constructed with thiol or
Cl, Br, and I anion surfaces. The surface of both the thiol- and
halide-terminated NCs exhibit low and high-energy phonon modes with large
thermal displacements not present in bulk PbS; however, halide anion surface
termination reduces the overlap of the electronic wavefunctions with these
vibration modes. These findings suggest that electron-phonon interactions will
be reduced in the halide terminated NCs, a conclusion that is supported by
analyzing the time-dependent evolution of the electronic energies and
wavefunctions extracted from the AIMD. This work explains why electron-phonon
interactions are crucial to charge carrier dynamics in NCs and how surface
engineering can be applied to systematically control their electronic and
phononic properties. Furthermore, we propose that the computationally efficient
approach of gauging electron-phonon interaction implemented here can be used to
guide the design of application-specific surface terminations for arbitrary
nanomaterials.
