  Atomically thin semiconductors, encompassing both 2D materials and quantum
wells, exhibit a pronounced enhancement of excitonic effects due to geometric
confinement. Consequently, these materials have become foundational platforms
for the exploration and utilization of excitons. Recent ab initio studies have
demonstrated that phonons can substantially screen electron-hole interactions
in bulk semiconductors and strongly modify the properties of excitons. While
excitonic properties of atomically thin semiconductors have been the subject of
extensive theoretical investigations, the role of phonon screening on excitons
in atomically thin structures remains unexplored. In this work, we demonstrate
via ab initio GW-Bethe-Salpeter equation calculations that phonon screening can
have a significant impact on optical excitations in atomically thin
semiconductors. We further show that the degree of phonon screening can be
tuned by structural engineering. We focus on atomically thin GaN quantum wells
embedded in AlN and identify specific phonons in the surrounding material, AlN,
that dramatically alter the lowest-lying exciton in monolayer GaN via
screening. Our studies provide new intuition beyond standard models into the
interplay among structural properties, phonon characteristics, and exciton
properties in atomically thin semiconductors, and have implications for future
experiments.
