Direct observation of adsorption-induced electronic states by low-temperature scanning tunneling microscopy

We have studied local density states of cobalt-phthalocyanine (CoPc) molecules on a Cu (1 0 0) surface by low-temperature scanning tunneling microscopy. Differential conductance (dI/dV) spectra of CoPc molecules had two peaks at 0.45 eV below and 0.1 eV above the Fermi level (EF). The dI/dV image revealed that Co atoms had large contribution to the state around 0.45 eV. The peak at 0.1 eV above EF was assigned to the states induced by the adsorption of molecules on surfaces. The porphyrin part of the CoPc molecules was found to contribute to this state. The lowest unoccupied molecular orbital was thought to hybridize with the electronic states of Cu (1 0 0) to generate the adsorption-induced state.