Excited State Phononic Processes in Semiconductor Nanocrystals Revealed by Excitonic State-Resolved Pump/Probe Spectroscopy

Semiconductor nanocrystals are being developed with increasingly complex shapes and geometries, often featuring complex shell structures. One aims to characterize these structures by different probes, beyond electronic spectroscopies. Vibrational spectroscopy is a useful tool to probe the phononic structure, but the commonly used frequency-domain methods can be plagued by artifacts due to charge-trapping dynamics. To circumvent these issues, coherent phonons may be measured in the time domain via excitonic state-resolved pump/probe spectroscopy. These measurements reveal several new observations on phononic processes, focusing on model systems of radially graded alloys of core/shell nanocrystals: CdSeCdxZn1-xS. The main new observation is frequency changes to the longitudinal optical phonon at high energy due to electronic mixing. This new, softened phonon mode appears via previously unobserved biexcitonic signals. The state-resolved measurements reveal insights into how the shelling process controls excitonic polarization, carrier trapping, and perturbations to sphericity.