Duffy Turner, NYU

Despite the many similarities between nanocrystals and atoms, it has remained unclear since the early 1990s why optical transitions in nanocrystals are so much broader than those of atoms. The spectral line width of an ensemble of nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use coherent two-dimensional electronic spectroscopy to study the line-broadening mechanisms of prototypical CdSe nanocrystals as a function of radii and crystal structure. The results allow us to distinguish contributions arising from excitonic fine structure, spectral diffusion, and exciton–phonon coupling.