Probing Hole-State Splitting and Relaxation in Gradient Alloyed CdSe Core–Shell Quantum Dots Using Two-Dimensional Electronic Spectroscopy

Gradient alloyed Type-I CdSe quantum dots have been demonstrated to suppress Auger recombination, enabling highly efficient photoluminescence and enhanced optical gain. However, the influence of these gradient-alloyed shells on the band-edge structure and associated relaxation mechanisms remains poorly understood. In this work, we employed two-dimensional electronic spectroscopy to resolve the band-edge hole structure and the corresponding relaxation mechanism of continuously graded CdSe/Cd_xZn_1–xSe/ZnSe core–shell quantum dots (QDs). Analysis of an early-time two-dimensional electronic spectrum resolved three band-edge optical transitions, which are assigned as 1S_e–2S_hh, 1S_e–1S_lh, and 1S_e–1S_hh excitons. These findings suggest that the gradient alloyed shells on CdSe quantum dots cause symmetry breaking, which results in the light-hole and heavy-hole states splitting. Furthermore, ultrafast (<1 ps) relaxation processes via hole cooling across these three hole states are observed. The hole-state splitting provides a spectroscopic basis for the observed cooling dynamics and may serve as a contributing factor to the enhancement of photoluminescence and optical gain, offering a potential pathway for optimizing excitonic structure in next-generation QD-based optoelectronic devices.