Starting from single Co and Cu atoms on a Cu(111) surface, close-packed monatomic chains (interatomic spacing 255 pm) of various length and composition were assembled by atom manipulation in a low-temperature scanning tunneling microscope and characterized with respect to their electronic properties. We find that Co atoms can be attached to Cu chains to terminate the structure but also incorporated into the chain to occupy a predefined site. The topographic signature of an added Co atom corresponds to a height increase of 10 to 15 pm at the Co position for constant-current imaging in the regime of unoccupied sample states, providing a means to resolve the internal chain structure.
Spectroscopic measurements of the differential tunneling conductance dI/dV reveal that CoCu chains exhibit quantum states showing subtle downward shifts in energy (on the order of 100 meV) relative to the quantum states of pure Cu chains [1] comprised of the same number of atoms. The energy shifts sensitively depend on the internal structure of the CoCu chains. The experimental shifts are reproduced by a tight-binding parameterization which implies (I) significant electronic interatomic Co-Cu coupling within the chain and (II) an orbital binding energy associated with the Co adatom which is slightly below that of the sp-derived Cu/Cu(111) adatom state [2], leading to confined quantum states delocalized along the entire heteroatomic chain. Accordingly, dI/dV mapping of composite CoCu chains reveals modified quantum state densities with n lobes and n-1 nodes indicative for one-dimensional confinement. The modifications of the state densities induced by the Co incorporation are again well reproduced by the tight-binding model [3].
Our experimental observations in combination with the tight-binding analysis show that composite CoCu chains provide an interesting model case in which the quantum state of an atomic-scale host structure can be tuned by the controlled incorporation of foreign atoms.
[1] S. Fölsch et al., Phys. Rev. Lett. 92, 56803 (2004)
[2] F. E. Olsson et al., Phys. Rev. Lett. 93, 206803 (2004)
[3] J. Lagoute et al., Phys. Rev. Lett. (2007, accepted for publication) |