Confined Acoustic Phonons in Colloidal Nanorod Heterostructures Investigated by Nonresonant Raman Spectroscopy and Finite Elements Simulations

Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials. A prominent material for light emitting applications are CdSe/CdS core shell dot-in-rods. Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by non resonant Raman spectroscopy with laser excitation energy lower than their bandgap. With high signal-to-noise ratio in the frequency range from 5-50 cm(-1), we reveal distinct Raman bands that can be related to confined extensional and radial breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architecture leads to a reduction of the sound core region of the rod, which results in a redshift of the rod RBM frequency and localization of the phonon in vicinity of the core where optical transitions occur. Such localized effects potentially can be exploited as a tool to phonon coupling in nanocrystal heterostructures.