Pump-Power Dependence of Coherent Acoustic Phonon Frequencies in Colloidal CdSe/CdS Core/Shell Nanoplatelets

Femtosecond optical pump-probe spectroscopy resolves hitherto unobserved coherent acoustic phonons in colloidal CdSe/CdS core/shell nanoplatelets (NPLs). With increasing pump fluence, the frequency of the in-plane acoustic mode increases from 5.2 to 10.7 cm^-1, whereas the frequency of the out-of-plane mode remains at ∼20 cm^-1. Analysis of the oscillation phases suggests that the coherent acoustic phonon generation mechanism transitions from displacive excitation to subpicosecond Auger hole trapping with increasing pump fluence. The measurements yield Huang-Rhys parameters of ∼10^-2 for both acoustic modes. The weak electron-phonon coupling strengths favor the application of NPLs in optoelectronics.