  The measured low frequency vibrational energies of some quantum dots (QDs)
deviate from the predictions of traditional elastic continuum models. Recent
experiments have revealed that these deviations can be tuned by changing the
ligands that passivate the QD surface. This observation has led to speculation
that these deviations are due to a mass-loading effect of the surface ligands.
In this article, we address this speculation by formulating a continuum elastic
theory that includes the mass-loading effects of the surface ligands. We
demonstrate that this model is capable of accurately reproducing the $l = 0$
phonon energy across variety of different QD samples, including cores with
different ligand identities and epitaxially grown CdSe/CdS core/shell
heterostructures. We highlight that our model performs well even in the small
QD regime, where traditional elastic continuum models are especially prone to
failure. Furthermore, we show that our model combined with Raman measurements
can be used to infer the elastic properties of surface bound ligands, such as
sound velocities and elastic moduli, that are otherwise challenging.
