Ultrathin Pb and Ge films deposited on Ag(111) surfaces have been investigated and compared. We found that at 1/3 ML, both films formed surface alloys, Ag 2 Pb and Ag 2 Ge, with √ 3 × √ 3R30 • and 19 20 √ 3 × 19 20 √ 3R30 • structures on Ag(111) but the surface electronic structures exhibit a most evident difference at the Ag(111) surface zone boundaryM Ag(111) , where the single band and the splitting ones were observed, respectively. Up to 1 ML, Ag 2 Ge subsequently develops into germanene with a striped phase and then a quasifreestanding phase, as previously reported [Lin et al., Phys. Rev. Mater. 2, 024003 (2018)], while Ag 2 Pb evolves to a dense Pb(111) phase that also reveals splitting bands atM Ag(111). We discover that the larger (smaller) atomic size of a Pb (Ge) atom with respect to an Ag atom causes the commensurate (incommensurate) interfaces and further demonstrate that the splitting bands of Ag 2 Ge surface alloy and 1-ML Pb film originated from the commonly incommensurate interface with Ag(111), which mediates umklapp scattering by inducing the mirror image of the pristine Ag 2 Ge and Pb(111) bands relative toM Ag(111) .

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