Two-dimensional alloy of immiscible metals: Single and binary monolayer films of Pb and Sn on Rh(111)

The single and binary metal films of Pb and Sn on Rh(111) have been studied at room temperature by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. Both Pb and Sn are mobile at low coverage and form commensurate overlayers of (4×4)-Pb and c(2×4)-Sn, respectively. From atomically resolved STM images, the atomic arrangements of (4×4)-Pb and c(2×4)-Sn have been identified to be hexagonal and rectangular structures, respectively. With increasing coverage, both commensurate phases change into incommensurate phases followed by island formation (Stranski-Krastanov growth). This shows that Rh(111) is static and inert enough to support two-dimensional (2D) phases of Pb and/or Sn without alloying at room temperature. The Pb-Sn bimetallic film on Rh(111) forms an ordered 2D alloy of PbSn₃ with an incommensurate structure close to ((7)×(7)), contrary to the immiscibility of Pb and Sn in the bulk. From atomically resolved STM images, the atomic arrangement of this ((7)×(7))-(Pb,Sn) structure has been determined.