Atomic-scale defect-mediated electronic modulation in monolayer Cu2Pb on Cu(111)

Defects are not merely structural imperfections but can serve as atomic-scale regulators of electronic properties in low-dimensional materials. However, how defect-mediated electronic modulation emerges in metallic two-dimensional surface alloys remains largely unexplored. Here, using low-temperature scanning tunneling microscopy/spectroscopy combined with first-principles calculations, we establish an atomic-scale picture of intrinsic defects in a Cu2Pb monolayer grown on Cu(111), a metallic surface alloy predicted to host nodal-line semimetal states. STM measurements reveal an atomically ordered √3 × √3 surface-alloy structure, providing a well-defined platform for resolving intrinsic defects and their local consequences. We identify intrinsic one-dimensional mirror twin boundaries and multiple point defects, determine their atomic configurations and formation mechanisms, and elucidate their relative energetics. Spectroscopy measurements and electronic-structure calculations reveal localized electronic modulation at mirror twin boundaries, while point defects are characterized in terms of their atomic configurations and formation energetics. These results provide an atomic-scale understanding of intrinsic defects in metallic two-dimensional surface alloys and offer microscopic insight into defect-dependent electronic properties in low-dimensional quantum materials.