Substrate-Induced Nanoscale Undulations of Borophene on Silver

Two-dimensional (2D) materials tend to be mechanically flexible yet planar, especially when adhered on metal substrates. Here, we show by first-principles calculations that periodic nanoscale one-dimensional undulations can be preferred in borophenes on concertedly reconstructed Ag(111). This "wavy" configuration is more stable than its planar form on flat Ag(111) due to anisotropic high bending flexibility of borophene that is also well described by a continuum model. Atomic-scale ultrahigh vacuum scanning tunneling microscopy characterization of borophene grown on Ag(111) reveals such undulations, which agree with theory in terms of topography, wavelength, Moiré pattern, and prevalence of vacancy defects. Although the lattice is coherent within a borophene island, the undulations nucleated from different sides of the island form a distinctive domain boundary when they are laterally misaligned. This structural model suggests that the transfer of undulated borophene onto an elastomeric substrate would allow for high levels of stretchability and compressibility with potential applications to emerging stretchable and foldable devices.