Spectroscopy of neutral and charged exciton states in single CdSe/ZnS nanocrystals

We report a low temperature spectroscopic study of the lowest-energy exciton states in single CdSe/ZnS colloidal nanocrystals (NCs). In their neutral charge state, the zero-phonon lines of the lowest two levels of the band-edge exciton fine structure are directly observed in the photoluminescence (PL) spectrum. The attribution of these lines is confirmed by polarization-resolved studies of the PL spectrum and decay. We also identified the emission from charged NCs which displays a single zero-phonon line ∼17 meV redshifted with respect to the neutral NC emission lines. This bright trion state has a lifetime which ranges from 1.5 to 4.5 ns.