Spectroscopic signature of Cu on W(110) from scanning tunneling microscopy and inverse photoemission

Monolayer Cu stripes attached to the steps of W(110) are distinguished from W terraces by scanning tunneling microscopy using a Cu-induced empty state, which is characterized by inverse photoemission spectroscopy. While the contrast between Cu and W atoms is dominated by the difference of the atomic sizes at most biases, it can be reversed with a bias such that tunneling into the Cu state overwhelms the size difference. This effect provides an unambiguous and well-understood mechanism for identifying different metal atoms.