<p>Detailed knowledge of metal-carbon interactions is indispensable for generating graphene-based nanostructures for application in catalysis, electrodes, and magnetic storage[1,2]. Graphene has been grown on many transition metal surfaces by chemical vapor deposition, where the layer forms a moiré superstructure due to the mismatch between the lattice constants of graphene and those of the underlying metal surface[3]. Consequently, the structure and electronic states of graphene are modulated depending on the region in the moiré unit cell. Such moiré patterns have been used as templates to grow uniformly sized metal clusters. During cluster formation, metal atoms impinge to the surface, followed by thermal diffusion and nucleation. In this process, the metal atoms prefer to interact with specific sites in the unit cell, which was predicted to be responsible for the formation of uniform cluster arrays. In this work, we studied the interaction of Ag atoms with graphene supported on Rh(111) by using a scanning tunneling microscope (STM)[4]. Because of their unique localized plasmon response, Ag nanoclusters have been studied as optical materials. Therefore, we also investigated the correlation between the structure of Ag clusters and their optical properties.</p><p></p><p>Upon adsorption at 15 K, the Ag atoms existed mainly as monomers at specific sites in the moiré unit cell because thermal diffusion was suppressed. The Ag atoms exhibited a dI/dV peak at ∼0.5 V above the Fermi level, which was assigned to Ag(5s)-C(2p_z) anti-bonding state. At elevated temperature to 78 K, the Ag adatoms were thermally activated to diffuse and form clusters, whose sizes were smaller than ~5 nm. At room temperature, the clusters did not exist on graphene terrace and they grew preferentially at the steps. In the corresponding extinction spectra, surface plasmon responses due to Ag clusters were observed, and their peak energies changed depending on the growth temperature.</p><p></p><p>[1] X. Liu et al., Progress in Surface Science 90, 397-443 (2015).</p><p>[2] A.T. N’Diaye et al., Phys. Rev. Lett. 97, 215501 (2006).</p><p>[3] J. Wintterlin and M.-L. Bocquet, Surface Science 603, 1841-1852 (2009).</p><p>[4] H. Okuyama, D. Yamamoto, S. Hatta, T. Aruga, Carbon 210, 118032 (2023).</p>