Measuring the Ultrafast Spectral Diffusion and Vibronic Coupling Dynamics in CdSe Colloidal Quantum Wells using Two-Dimensional Electronic Spectroscopy

We measure the ultrafast spectral diffusion, vibronic dynamics, and energy relaxation of a CdSe colloidal quantum wells (CQWs) system at room temperature using two-dimensional electronic spectroscopy (2DES). The energy relaxation of light-hole (LH) excitons and hot carriers to heavy-hole (HH) excitons is resolved with a time scale of ∼210 fs. We observe the equilibration dynamics between the spectroscopically accessible HH excitonic state and a dark state with a time scale of ∼160 fs. We use the center line slope analysis to quantify the spectral diffusion dynamics in HH excitons, which contains an apparent sub-200 fs decay together with oscillatory features resolved at 4 and 25 meV. These observations can be explained by the coupling to various lattice phonon modes. We further perform quantum calculations that can replicate and explain the observed dynamics. The 4 meV mode is observed to be in the near-critically damped regime and may be mediating the transition between the bright and dark HH excitons. These findings show that 2DES can provide a comprehensive and detailed characterization of the ultrafast spectral properties in CQWs and similar nanomaterials. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Submitted/Accepted version H.V.D. gratefully acknowledges the financial support in part from the Singapore Agency for Science, Technology and Research (A*STAR) SERC under Grant No. M21J9b0085, and the Singapore Ministry of Education Tier 1 grant (MOERG62/20). H.V.D. also gratefully acknowledges the support from TUBA. H.-S.T. gratefully acknowledges the financial support in part from the Singapore Ministry of Education Tier 1 grant (MOE-RG2/19 and MOE-RG14/20). O.V.P. acknowledges the financial support from the United States National Science Foundation under Grant No. CHE-2154367.