Stacking up Electron-Rich and Electron-Deficient Monolayers to Achieve Extraordinary Mid- to Far-Infrared Excitonic Absorption: Interlayer Excitons in the C3B/<…

Our ability to efficiently detect and generate far-infrared (i.e., terahertz) radiation is vital in areas spanning from biomedical imaging to interstellar spectroscopy. Despite decades of intense research, bridging the terahertz gap between electronics and optics remains a major challenge due to the lack of robust materials that can efficiently operate in this frequency range, and two-dimensional (2D) type-II heterostructures may be ideal candidates to fill this gap. Herein, using highly accurate many-body perturbation theory within the GW plus Bethe-Salpeter equation approach, we predict that a type-II heterostructure consisting of an electron-rich ${\mathrm{C}}_{3}\mathrm{N}$ and an electron deficient ${\mathrm{C}}_{3}\mathrm{B}$ monolayers can give rise to extraordinary optical activities in the mid- to far-infrared range. ${\mathrm{C}}_{3}\mathrm{N}$ and ${\mathrm{C}}_{3}\mathrm{B}$ are two graphene-derived 2D materials that have attracted increasing research attention. Although both ${\mathrm{C}}_{3}\mathrm{N}$ and ${\mathrm{C}}_{3}\mathrm{B}$ monolayers are moderate-gap 2D materials, and they couple only through the rather weak van der Waals interactions, the bilayer heterostructure surprisingly supports extremely bright, low-energy interlayer excitons with large binding energies of 0.2--0.4 eV, offering an ideal material with interlayer excitonic states for mid- to far-infrared applications at room temperature. We also investigate in detail the properties and formation mechanism of the inter- and intralayer excitons.