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
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