Semiconducting Phase and Anisotropic Properties in Borophene via Chemical Surface Functionalization

In this work, employing first-principles calculations, the structures and electronic properties of three experimentally synthesized boron structures, i.e., δ₆, β₁₂, and χ₃, are comprehensively examined, both in pristine and chemically functionalized forms. In particular, we have studied 11 surface functionalization configurations made possible by adding H, Cl, and F atoms to the two-atom unit cell of δ₆-borophene. Our results show that the inherent metallicity of δ₆ is robust against the considered functionalization patterns except for the full-hydrogenation case that results in a metallic-to-Dirac band structure transformation. We have further demonstrated that for biaxial strains in |ϵ| ≤ 10 and perpendicular electric fields up to 10 V/nm, no band gap is induced in the hydrogenated δ₆. Next, we focused our study on the influence of hydrogen functionalization on β₁₂ and χ₃ structures. In particular, we have functionalized the χ₃ structure with H atoms and observed a sizable band gap of 0.88 eV in two of the consequent structures. We unfold a peculiar metal–semimetal transition in β₁₂ upon a specific hydrogenation pattern, giving rise to an anisotropic band structure around its conduction band minimum, with the ratio of the electron effective mass along the longitudinal direction (X-S) to that along the transverse direction being as high as 5.07. Interestingly, our calculated phonon dispersion curves confirm the dynamic stability of the aforementioned structures. It is also observed that the previously reported Kohn anomalies in δ₆ and χ₃ structures are eliminated upon hydrogenation.