Structure of excited-state transitions of individual semiconductor nanocrystals probed by photoluminescence excitation spectroscopy

We perform for the first time photoluminescence excitation (PLE) studies of individual nanocrystals (NCs) that reveal the structure of excited-state transitions not obscured by ensemble averaging. Single-NC PLE spectra strongly deviate from a traditional idealized picture of sharp, quasiatomic resonances. We detect only a few relatively narrow transitions (3-4 meV) at the band edge, while at higher spectral energies, we observe a broad structureless feature separated from the band-edge peaks by a >50 meV "minigap." These observations can be rationalized by analyzing hole intraband relaxation behavior.