Low-Frequency Vibrations of Semiconductor Nanoplatelets

Colloidal semiconductor nanoplatelets have recently emerged as exciting materials for optoelectronic and biological applications. Their vibrational excitations, or phonon modes, are responsible for energy relaxation of charge carriers, excitons, trions, and other electronic excitations and can participate in their spin relaxation. In the present study, theory of elastically isotropic thin plates is generalized to account for crystalline anisotropy of nanoplatelets. Dispersion relations for acoustic and flexural vibrations of CdSe nanoplatelets with a zinc-blende crystal lattice grown along the [001] crystallographic axis are found. The rest of the low-frequency modes is approached from the model of isotropic elastic medium confined between two planes and assuming the free-standing boundary conditions at the planes. This provides a systematic way to classify low-frequency vibrational modes of a nanoplatelet.