Van der Waals and Electron Stimulated Covalent Hydrogenation of Borophene

The recent experimental realization of atomically thin, crystalline boron sheets has opened a new direction in the study of synthetic 2D materials. Chemical modification of borophene could further enable fine-tuning of its band structure for nanoelectronic applications. Here, we present the first experimental study of both the non-covalent and covalent hydrogenation of borophene. Adsorbed hydrogen molecules are observed to form a triangular lattice on both v_1/6 and v_1/5 borophene polymorphs. The rotational excitation of H₂ is detected at 36 meV, indicating the adsorption of hydrogen is dominated by its van der Waals interaction with borophene. The formation of this H₂-borophene vertical heterostructure increases the work function of borophene. Moreover, covalent modification of borophene is achieved by inducing a hydrogen-borophene reaction at elevated tip bias voltage. This reaction significantly modifies the electronic structure of borophene and further increases its work function. Overall, this work illustrates the sensitivity of the electronic properties of borophene to chemical modification, which is expected to stimulate further exploration of boron-based chemistry in the two-dimensional limit.