Thermal and vibronic effects on the absorption spectra of II–VI quantum dots: Atomistic origins of the Urbach tail

Explicit treatments of exciton-phonon coupling and ensemble averaging are often neglected in computational studies of nanocrystal optical properties. Here, we present a simple harmonic approximation framework for predicting temperature-dependent UV-Vis absorption spectra of II-VI quantum dots from first principles, without empirical parameters. Two variants are introduced: the equilibrium harmonic approximation, which captures homogeneous broadening, and the averaged variant, which additionally accounts for configurational sampling and bandgap renormalization. Application to II-VI quantum dot model systems demonstrates nearly quantitative agreement with experiment and clarifies the microscopic origin of the Urbach tail. Atomistic analysis reveals that surface-to-bulk excitations associated with hole trap states exhibit greater sensitivity to thermal fluctuations, leading to broader excitations that form an extended low-energy absorption tail. These findings underscore the need for ab initio treatments of exciton-phonon interactions in predictive models of nanocrystal optoelectronic properties. The harmonic approximation framework provides a practical pathway toward the explicit inclusion of thermal and ensemble effects in future computational studies.