Modulation of Low-Frequency Acoustic Vibrations in Semiconductor Nanocrystals through Choice of Surface Ligand.

Recent experimental and theoretical results have highlighted the surprisingly dominant role of acoustic phonons in regulating dynamic processes in nanocrystals. While it has been known for many years that acoustic phonon frequencies in nanocrystals depend on their size, strategies for tuning acoustic phonon energy at a given fixed size were not available. Here, we show that acoustic phonon frequencies in colloidal quantum dots (QDs) can be tuned through the choice of the surface ligand. Using low-frequency Raman spectroscopy, we explore the dependence of the l = 0 acoustic phonon resonance in CdSe QDs on ligand size, molecular weight, and chemical functionality. On the basis of these aggregated observations, we conclude that the primary mechanism for this effect is mass loading of the QD surface and that interactions between ligands and with the surrounding environment play a comparatively minor yet non-negligible role.