Phonon Behaviors of Charged Nanocrystals: Effects of Free Charge Carriers

The precise knowledge of phonons is important to understand the electronic and photonic properties of nanocrystals (NCs) and NC-based devices. It is common to introduce free charge carriers into NCs for device applications, highlighting the significance of studying the free carrier effects on phonon behaviors. In this study, we introduced the free carriers into a series of NCs (ZnO, CdSe, PbSe) and investigated the carrier-dependent phonon behaviors by using Raman spectroscopy. For charged NCs, the change of phonon frequencies and the overtone to fundamental intensity ratios were explained by the screening and band-filling effects caused by the accumulated free carriers. By using a time-dependent model, the overtone to fundamental intensity ratios were further correlated with the electron-phonon coupling strength of the involved excited states. The effects of free carriers on phonon behaviors, usually neglected in previous studies, should be carefully considered when analyzing the properties of charged NCs. Moreover, the relationship between the free carrier densities and Raman frequencies/relative intensities are potentially useful for nondestructive characterization of free carriers in NCs.