Can fluorine and chlorine functionalization stabilize the graphene like borophene?

Abstract

During the last couple of years, borophene, novel two-dimensional materials made from boron atoms have attracted remarkable attentions. As an exciting experimental achievement, most recently graphene like borophene has been realized (arXiv:1712.08770), which however suffers from dynamical instability because of its electron-deficiency. Nevertheless, as it was theoretically confirmed, hydrogenated graphene like borophene structures are stable which were surprisingly very recently experimentally synthesized. Halogenated graphene like borophene is also expected to form B−Halogen bonds and thus form energetically favorable structures. The effects of halogenations for varying levels of coverage (B1X1, B2X1 and B4X1; X is F and Cl) on the properties of graphene like borophene such as the geometric structures, thermal stability, mechanical response, electronic structure and optical properties are systematically investigated by the first-principles methods. The stability of these structures is tested by phonon spectrum analyses and ab initio molecular dynamics (AIMD) simulations. The investigation of mechanical properties demonstrates their high in-plane stiffness and outstanding stretchability as well. Notably, B1F1 and B1Cl1 were found to present direct band gaps of 2.15 and 2.47 eV, respectively, according to the HSE06 functional results, which can be further finely tuned by applying mechanical strains. In contrast, B2F1 and B2Cl1 have metallic character. The optical calculations illustrate that applying different tensile strains on these monolayers result in blue and red shift in optical spectra, suggesting that they are potentially promising for the applications in optoelectronics and nanoelectronics. Interestingly, the in-plane optical anisotropy of these novel 2D materials is highly desirable for the design of polarization-sensitive photodetectors.