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Paper IPM / P / 17146  


Abstract:  
Borophene has triggered a surge of interest due to its outstanding properties including mechanical flexibility, polymorphism, and optoelectrical anisotropy. Very recently, a novel semihydrogenated borophene, called $\alpha '$4H, was synthesized in largescale freestanding samples, exhibiting excellent airstability and semiconducting nature.
Herein, using the density functional theory (DFT) and manybody perturbation theory (MBPT), we investigate the electronic and excitonic optical properties of $\alpha '$4H borophene. The DFT results reveal that hydrogenation breaks the mirror symmetry and increases the buckling height of pure $\alpha '$ borophene, which results in an orbital hybridization and indirect band gap of 1.49 eV in $\alpha '$4H borophene. Moreover, the optical spectrum
achieved from solving the BetheSalpeter equation (i.e., GW+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)
$\times$10$^7$ m$^{1}$ for E$\parallel$x (E$\parallel$y) polarization, which shows a
linear dichroism for visible light. The effective mass and Bohr radius of the groundstate exciton are 0.78 m0 and
2.03 Ã?, 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 $\alpha '$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 $\alpha '$4H borophene and suggests it as a very
promising candidate for flexible optoelectronic applications.
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