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This paper benchmarks the accuracy of perturbative quantum master equation (QME) and mixed quantum-classical (MQC) methods against fully quantum mechanical dynamics for modeling hot-exciton relaxation in CdSe and CdSe/CdS nanocrystals. They find that ultrafast initial decay in bare CdSe arises from rapid diabatic state mixing driven by low-frequency phonon fluctuations, rather than energy relaxation. The mapping approach to surface hopping (MASH) provides the most consistent agreement with benchmark dynamics and equilibrium populations.
Widely used approximations for modeling hot-exciton relaxation in semiconductor nanocrystals can fail, but mapping surface hopping (MASH) offers a more reliable alternative.
Hot-exciton relaxation in semiconductor nanocrystals (NCs) is often described using perturbative theories, but their accuracy is difficult to assess for realistic exciton--phonon Hamiltonians. Here, we benchmark the perturbative quantum master equation (QME) and several mixed quantum--classical (MQC) methods against fully quantum mechanical dynamics. Using atomistically parameterized models for CdSe core and CdSe/CdS core--shell NCs, we find that bare CdSe exhibits an ultrafast initial decay followed by slower cooling, whereas the core--shell system is dominated by the slower component. Analysis of reduced models shows that the ultrafast component arises from rapid diabatic state mixing driven by thermal fluctuations of low-frequency phonons, rather than from nuclear-assisted energy relaxation. The QME captures the initial fast decay but can fail for the slower relaxation in the diabatic representation, while the mapping approach to surface hopping (MASH) gives the most consistent agreement with both benchmark dynamics and equilibrium populations. These results establish a benchmark for exciton-cooling dynamics in NCs and clarify the physical regimes in which widely used approximate methods are reliable.
BAIR