Synthesis of Ultralow-Work-Function Bilayer Borophene on Iridium

Borophene research has predominantly focused on its monolayer forms due to the challenges associated with synthesizing multilayer boron sheets, despite the desirable properties predicted theoretically beyond the single-atomic-layer limit. Here, we present the successful synthesis of bilayer borophene on Ir(111) substrates, which possesses a distinct atomic structure compared to previously reported bilayer borophene phases. Using scanning tunneling microscopy and spectroscopy, a significantly reduced local work function is observed for bilayer borophene compared to monolayer borophene on iridium. Specifically, bilayer islands show strong peak position shifts toward lower energy in field emission resonance spectroscopy, which is further corroborated by density functional theory calculations. This ultralow-work-function bilayer phase presents expanded application opportunities for borophene including field emission sources, photoelectric devices, and quantum technologies.