Domain-Dependent Electronic Properties of 2D Copper Boride Synthesized from Reversible Subsurface Diffusion on Cu(111)

Emerging boron-based 2D materials hold properties with potential applications ranging from electronic devices to catalysis and energy storage. Using physical vapor deposition, 2D copper borides are formed on Cu(111) at elevated temperatures. However, the as-prepared surfaces often contain boron clusters that compromise structural homogeneity, and the dynamics of their formation is not yet fully understood. Through in situ low-energy electron microscopy (LEEM), Auger electron spectroscopy (AES), and scanning tunneling microscopy/spectroscopy (STM/STS), we show that cluster-free 2D CuB_<i>x</i> is synthesized via reversible subsurface diffusion of boron atoms. The resolved regular atomic structure aligns with the previously calculated Cu₈B₁₄ model. In a rare case, possibly due to surface strain and a change of orientation, a second domain of CuB_<i>x</i> was observed with a pronounced change in electronic properties, yet electronic states remain delocalized in both domains. These findings demonstrate the sensitivity of electronic properties to domain structure and highlight a potential opportunity to tune 2D CuB_<i>x</i> for diverse applications.