Transition from metal to semiconductor by semi-hydrogenation of borophene

Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and optoelectrical anisotropy. Very recently, a novel semi-hydrogenated borophene, called ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H, was synthesized in large-scale freestanding samples, exhibiting excellent air-stability and semiconducting nature. Herein, using the density functional theory (DFT) and many-body perturbation theory (MBPT), we investigate the electronic and excitonic optical properties of ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene. The DFT results reveal that hydrogenation breaks the mirror symmetry and increases the buckling height of pure ${\ensuremath{\alpha}}^{\ensuremath{'}}$ borophene, which results in an orbital hybridization and indirect band gap of 1.49 eV in ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene. Moreover, the optical spectrum achieved from solving the Bethe-Salpeter equation (i.e., $\mathrm{GW}+\mathrm{BSE}$) shows an optical band gap of 2.40 eV, which corresponds to a strongly bound and stable bright exciton with a binding energy of 1.18 eV. The mean value of absorption within the visible area is 1.68 (1.13)$\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ for $E\ensuremath{\parallel}x$ ($E\ensuremath{\parallel}y$) polarization, which shows a linear dichroism for visible light. The effective mass and Bohr radius of the ground-state exciton are 0.78 ${m}_{0}$ and 2.03 \AA{}, respectively, which demonstrates the characteristics of Frenkel exciton. The excitonic states are robust against tension up to 10%, under which the monolayer is dynamically stable. We also study the bilayer ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene with different stackings. For the weak van der Waals interactions between the layers, the bilayer preserves most of the structural and electronic properties of the monolayer. Our study exposes the underlying physics behind the structural, electronic, and optical properties of ${\ensuremath{\alpha}}^{\ensuremath{'}}$-4H borophene and suggests it as a very promising candidate for flexible optoelectronic applications.