  Identification and design of defects in two-dimensional (2D) materials as
promising single photon emitters (SPE) requires a deep understanding of
underlying carrier recombination mechanisms. Yet, the dominant mechanism of
carrier recombination at defects in 2D materials has not been well understood,
and some outstanding questions remain: How do recombination processes at
defects differ between 2D and 3D systems? What factors determine defects in 2D
materials as excellent SPE at room temperature? In order to address these
questions, we developed first-principles methods to accurately calculate the
radiative and non-radiative recombination rates at defects in 2D materials,
using h-BN as a prototypical example. We reveal the carrier recombination
mechanism at defects in 2D materials being mostly dominated by defect-defect
state recombination in contrast to defect-bulk state recombination in most 3D
semiconductors. In particular, we disentangle the non-radiative recombination
mechanism into key physical quantities: zero-phonon line (ZPL) and Huang-Rhys
factor. At the end, we identified strain can effectively tune the
electron-phonon coupling at defect centers and drastically change non-radiative
recombination rates. Our theoretical development serves as a general platform
for understanding carrier recombination at defects in 2D materials, while
providing pathways for engineering of quantum efficiency of SPE.
