  Recently synthesized two-dimensional (2D) boron, borophene, exhibits a novel
metallic behavior rooted in the s-p orbital hybridization, distinctively
different from other 2D materials such as sulfides/selenides and semi-metallic
graphene. This unique feature of borophene implies new routes for charge
delocalization and band gap opening. Herein, using first-principles
calculations, we explore the routes to localize the carriers and open the band
gap of borophene via chemical functionalization, ribbon construction, and
defect engineering. The metallicity of borophene is found to be remarkably
robust against H- and F-functionalization and the presence of vacancies.
Interestingly, a strong odd-even oscillation of the electronic structure with
width is revealed for H-functionalized borophene nanoribbons, while an
ultra-high work function (~ 7.83 eV) is found for the F-functionalized
borophene due to its strong charge transfer to the atomic adsorbates.
