Phonons in Core–Shell CdSe/CdS Nanoplatelets Studied by Vibrational Spectroscopies

In this work, the phonon spectra of core/shell CdSe/CdS nanoplatelets with different shell thicknesses were studied using IR and Raman spectroscopies. Nanoplatelets are rectangular plates, with lateral dimensions of tens of nanometers and thicknesses of core and shell layers of a few nanometers. Longitudinal optical (LO) and surface optical (SO) phonon modes in the CdSe core and the CdS shell dominate in the Raman spectra at frequencies of approximately 200–210 and 250–300 cm–1, respectively, while transverse optical (TO) modes in the nanoplatelets (NPLs) are active in the IR reflection/absorption spectra near 170 and 240 cm–1. The LO modes can be additionally activated in the IR spectra measured at an oblique angle of light incidence that provides a component of the electric field perpendicular to the surface. As the shell thickness increases, the phonon modes reveal frequency shifts of optical modes, which are explained in terms of the stress state in the core and shell, as well as the influence of phonon confinement. Low-frequency peaks of bare CdSe and core–shell CdSe/CdS platelets were observed in the Raman spectra, and their frequencies were compared with both literature data and the results of calculations within a two-phase model.