Optical Phonon Behaviors of Photocharged Nanocrystals: Effects of Free Charge Carriers

For semiconductor nanocrystals (NCs), the precise knowledge of phonons in the presence of free carriers is important for understanding their electronic and photonic properties in device applications. With Raman spectroscopy, this study investigates the effects of free charge carriers on optical phonon behaviors of NCs. The adoption of the photocharging method allows us to introduce free charge carriers into NCs without inducing other side effects. In the photocharged ZnO NCs, lower longitudinal optical (LO) phonon frequencies and weaker LO overtones relative to the fundamentals were found, which was explained by the screening and band-filling effects caused by the induced free carriers. The free carrier effects on optical phonon behaviors of NCs, usually neglected in previous studies, should be taken into consideration when discussing the electronic and photonic properties of NC-based devices.