  We perform photoluminescence excitation measurements on individual CdSe/ZnS
nanocrystal quantum dots (NCQDs) at room temperature to study optical
transition energies and broadening. The observed features in the spectra are
identified and compared to calculated transition energies using an effective
mass model. The observed broadening is attributed to phonon broadening,
spectral diffusion and size and shape inhomogeneity. The former two contribute
the broadening transitions in individual QDs while the latter contributes to
the QD-to-QD variation. We find that phonon broadening is often not the
dominant contribution to transition line widths, even at room temperature, and
that broadening does not necessarily increase with transition energy. This may
be explained by differing magnitude of spectral diffusion for different
quantum-confined states.
