Structures, Mechanics, and Electronics of Borophanes

Motivated by the very recent experimental synthesis of hydrogenated borophenesborophanes, we perform a systematic first-principles study of structures, mechanics, and electronics of freestanding hydrogenated v_1/6 sheets. We identify two distinctly stable borophane structures at 20% hydrogen coverage and two at 40% hydrogen coverage. Despite significant structural modification by formed B–H single bonds and three-center–two-electron B–H–B bridge bonds, borophanes inherit high in-plane elasticity and structural flexibility from pristine borophenes. Yet, the structural buckling of borophanes induced by hydrogenation gives rise to peculiar mechanical behaviors, such as nearly zero Poisson’s ratios and extremely low bending stiffness of 0.37 eV, compared to 1.46 eV of graphene. Borophanes exhibit similar structural fluxionality to the v_1/6 sheet that allows for structural phase transitions under tension as well as exotic fracture behaviors at critical strains. Furthermore, all borophanes possess modified work functions to form devisible Volta potentials across atomically smooth interfaces with the v_1/6 sheet. Meanwhile, most borophanes show attenuated anisotropy of electronic states and host Dirac fermions. These results illustrate the concept of borophene as a robust atomic-scale scaffold for realizing tailored mechanical and electronic properties by chemical modification and suggest the potential of utilizing borophanes for nanoelectronic devices.