  The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS
nanoparticles, embedded in a PMMA layer is studied at room temperature. We
observe a strong spectral jitter of up to 55 meV, which is correlated with a
change in the observed linewidth. We evaluate this correlation effect using a
simple model, based on the quantum confined Stark effect induced by a diffusing
charge in the vicinity of the nanoparticle. This allows us to derive a mean
distance between the center of the particle and the diffusing charge of
approximately 3.3 nm on average, as well as a mean charge carrier displacement
within the integration time. The distances are larger than the combined radius
of particle core and shell of about 3 nm, but smaller than the overall radius
of 5 nm including ligands. These results are reproducible, even for particles
which exhibit strong blueing, with shifts of up to 150 meV. Both the statistics
and its independence of core-shell alterations lead us to conclude that the
charge causing the spectral jitter is situated in the ligands.
