ELECTRON-DELOCALIZATION AND EXCITON-PHONON COUPLING IN CdSe/CdS COLLOIDAL NANOCRYSTALS

Due to their unique optical properties such as a color-tunable and bright fluorescence, colloidal core-shell nanocrystals (NCs) made of CdSe/CdS currently realize their potential as highly robust phosphors in display and LED technologies. For these applications, it is crucial to minimize the spectral fluorescence linewidth to maximize the color purity of the device. The application relevant room temperature emission linewidth is determined by the exciton-phonon interaction in NCs. However, the strength and nature of exciton-phonon coupling in NCs remains poorly understood and has been subject to a multitude of discrepant studies in the past. In particular, it has proven difficult to reconcile the narrow (<1meV) low temperature (~4K) linewidths and weak phonon sidebands with the broad room temperature emission linewidth (60-100 meV) within the framework of a suitable model. Moreover, clear relationships between exciton-phonon coupling parameters and the architecture of NCs are not yet established. To elucidate the exciton-phonon interaction in NCs, we have studied a series of CdSe/CdS core-shell NCs with vastly different shell thicknesses. The quasi type-II band alignment in these materials allows for tuning of the excited state electron-hole separation with the shell-thickness. Using a combination of single NC emission spectroscopy over a wide temperature range and time dependent fluorescence spectroscopy, we have investigated the role of the shell-growth induced carrier separation on the exciton-phonon coupling parameters. We describe the temperature dependent emission lineshape with a unified model in terms of these exciton-phonon coupling parameters. Our study pinpoints the crucial structural parameters determining the linewidth and possibilities for synthetic improvements.