STM study of single cobalt atoms and clusters adsorbed on Rh (111) and Pd (111)

We investigate the adsorption and electronic properties of single cobalt atoms and clusters adsorbed on Rh (111) and Pd (111) with scanning tunneling microscopy and scanning tunneling spectrum (STM/STS). It is found that there are two apparent heights for individual cobalt atoms on Rh (111), corresponding to Co atoms adsorbed hcp and fcc hollow sites. The Co atoms on both sites exhibit a notable peak near the Fermi energy, and the two peaks have a slight difference in peak shape. By fitting the dI/dV spectrum to the Fano lineshape, we find that the peak cannot be simply ascribed to the Kondo model. The peak position and full width at half maximum of the peak suggest that the magnetic impurity is in the mixed-valence regime rather than in the Kondo regime. And the peak can be interpreted as a combination of the Kondo resonance and bare d resonance. For Co dimers and trimers on Rh (111), there is no observable feature in their dI/dV spectra near the Fermi level. This is speculated to be due to the magnetic exchange interaction and orbital hybridization between Co atoms. For Co single atoms adsorbed on Pd (111) surface, only one apparent height is found, suggesting that it is due to a different interaction from Co on Rh (111) surface. We do not find notable feature near the Fermi level in the dI/dV spectra of all Co monomers, dimers and trimers.