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

Abstract

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.