Rich Coverage-Dependent Carbon Phases Induced by Submonolayer Surface Segregation on Pt(111) at 78 K

We report on the systematical investigation of coverage-dependent carbon phases on Pt(111) at the submonolayer (ML) regime, utilizing low energy electron diffraction and scanning tunneling microscopy. At a coverage of ∼0.25 ML, carbon forms three domains with a relative rotation of 120° and each domain consists of rectangular structures, c (4 × 2)-2C. When the coverage reaches ∼0.3 ML, multiclusters with 7–17 atoms with an atomic arrangement of are found. At ∼0.5 ML, the structure changes back to a rectangular structure but with four carbon atoms in a one-unit cell, c (4 × 2)-4C. At ∼1.0 ML, the surface structure evolves into a close-packed (1 × 1) structure, that is, a three-fold structure again. Although the surface structure undergoes a series of transitions between two-fold and three-fold microscopically, the three-fold symmetry of Pt(111) is well maintained macroscopically as the rectangular structures are accompanied by three domains with a relative rotation of 120°. Our results reveal that the rich phase transitions are coverage-driven and induced by the surface segregation of carbon on Pt(111).