Bimodal electronic structure of isolated Co atoms on Pt(111)

We have used low-temperature scanning tunneling microscopy and spectroscopy to probe the local electronic structure of Co adatoms on the Pt(111) surface. Two varieties of Co adatoms that differ in their dI/dV(V) spectra are identified. We find that this contrast in spectral density is due to the adatom binding site on the surface. Atoms at different surface lattice sites (i.e., fcc versus hcp sites) exhibit different local densities of states (LDOS). Manipulation of a Co atom from one kind of lattice site to the other results in the expected change in its LDOS. dI/dV(V) spectra measured in this study were normalized using a method that compensates for differences in tunneling current that occur at different points on the surface. Such normalized spectra are shown to be useful in predicting the spatial contrast of dI/dV maps.