Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton-Phonon Coupling and the Optimization of Spectral Linewidths

The optimization of photoluminescence spectral linewidths in semiconductor nanocrystal preparations involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Here, we use solution photon-correlation Fourier spectroscopy (S-PCFS) to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and composition of the shells. We find that the single-nanocrystal linewidth at room temperature is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal electric fields that affect exciton-phonon coupling. Our results explain the wide variations, both experimental and theoretical, in the magnitude and size dependence in previous reports on exciton-phonon coupling in CdSe nanocrystals. Moreover, our findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals.