Theory of Photoluminescence Spectral Line Shapes of Semiconductor Nanocrystals

Single-moleculephotoluminescence (PL) spectroscopy ofsemiconductornanocrystals (NCs) reveals the nature of exciton-phonon interactionsin NCs. Understanding the homogeneous spectral line shapes and theirtemperature dependence remains an open problem. Here, we develop anatomistic model to describe the PL spectrum of NCs, accounting forexcitonic effects, phonon dispersion relations, and exciton-phononcouplings. We validate our model using single-NC measurements on CdSe/CdSNCs from T = 4 to 290 K, and we find that the slightlyasymmetric main peak at low temperatures is comprised of a narrowzero-phonon line (ZPL) and acoustic phonon sidebands. Furthermore,we identify the specific phonon modes that give rise to the opticalphonon sidebands. At temperatures above 200 K, the spectral line widthshows a stronger dependence upon the temperature, which we demonstrateto be correlated with higher order exciton-phonon couplings.We also identify the line width dependence upon reorganization energy,NC core sizes, and shell thicknesses.

Automated summary

Single-molecule photoluminescence spectroscopy is used to study the exciton-phonon interactions in semiconductor nanocrystals. A detailed model is developed to explain the PL spectrum, taking into account excitonic effects, phonon dispersion relations, and exciton-phonon couplings. The model is validated with experimental measurements on CdSe/CdS nanocrystals, revealing the presence of a narrow zero-phonon line and acoustic phonon sidebands in the slightly asymmetric main peak at low temperatures. The dependence of the spectral line width on temperature is shown to be correlated with higher order exciton-phonon couplings.