  Coherent spin dynamics in colloidal CdSe quantum dots (QDs) typically show
two spin components with different Larmor frequencies, whose origin is an open
question. We exploit the photocharging approach to identify their origin and
find that surface states play a key role in the appearance of the spin signals.
By controlling the photocharging with electron or hole acceptors, we show that
the specific spin component can be enhanced by the choice of acceptor type. In
core/shell CdSe/ZnS QDs, the spin signals are significantly weaker. Our results
exclude the neutral exciton as the spin origin and suggest that both Larmor
frequencies are related to the coherent spin precession of electrons in
photocharged QDs. The lower frequency is due to the electron confined in the
middle of the QD, and the higher frequency to the electron additionally
localized in the vicinity of the surface.
