Interaction of single pentacene molecules with monatomic Cu∕Cu(111) quantum wires

Abstract

An ultrahigh vacuum low-temperature scanning tunneling microscope operated at 7 K is used to assemble Cu adatom chains on a Cu(111) surface by atom manipulation. Cu atoms within the close-packed chain reside on nearest-neighbor fcc hollow sites (Cu–Cu spacing 2.55 Å) along the ⟨110⟩ in-plane directions. Spectroscopic measurements of the differential tunneling conductance dI∕dV reveal that the monatomic Cu chain exhibits unoccupied one-dimensional (1D) quantum well states trapped in the pseudogap of the ⟨111⟩-projected Cu bulk bands. These chain-localized states are described by a 1D energy band centered 3.2 eV above the Fermi level (total band width 3.6 eV) and derive from spz hybrid atomic orbitals associated with the single Cu∕Cu(111) adatom. Pentacene molecules (C22H14) deposited on Cu(111) by thermal evaporation adopt a planar adsorption geometry with their long molecular axis aligned with the ⟨110⟩ in-plane directions. The organic molecule can be laterally manipulated along different high-symmetry directions of the substrate via attractive tip/molecule interactions. Lateral manipulation is also capable to attach single pentacene molecules to the ends of assembled Cu chains with atomic-level precision. We find (i) an enhanced adsorptive binding of the attached molecule characterized by spatial overlap between its carbon framework and the outermost chain atoms, (ii) persistence of the chain-localized states for the molecule-chain hybrid structure, and (iii) a clear correspondence between the number of Cu chain atoms involved in the spatial overlap and the observed energetic upward shift of the chain-localized quantum levels.