Atomic-Scale Friction Study by EC-AFM: Underpotential Deposition (UPD) of Ag on I-Modified Au(111) and Its Tip Penetration

The underpotential deposition (UPD) of Ag on an I-modified Au(111) surface is examined in perchloric acid solution by electrochemical lateral force microscopy (LFM). The transitions of ordered structures are observed as potential is swept and normal load changes. At low normal load, the structural changes with potential indicate that the iodine structure is very sensitive to the coverage of silver. At potentials positive of the 1st Ag UPD peak, the iodine forms a ( 3 × 3 R 30°) structure ( θ I = 0.33) on Au(111). The structural transition to a 1 × 1 structure observed as the normal load increases demonstrates that the AFM tip penetrates the iodine adlayer. As the potential passes the 1st Ag UPD peak, the iodine forms a p (3 × 3) ( θ I = 0.44) structure and also silver seems to form a p (3 × 3) ( θ Ag = 0.44) structure, as suggested by LFM images at high load. At even higher load, also this Ag layer is penetrated and the 1 × 1 structure of the substrate is observed. At potentials slightly negative of the 2nd Ag UPD peak, even though the iodine structure is maintained as a p (3 × 3) ( θ I = 0.44) structure, the atomic corrugations observed at high normal load indicate that silver completes a monolayer. The large-scale images clearly show that an Ag bilayer is formed at the 3rd Ag UPD peak. When the Ag bilayer completes, the iodine forms a ( 3 × 3 R 30°) ( θ I = 0.33).