Interaction of single pentacene molecules with monatomic quantum wires
Sequence of constant-current STM images ( , 1 nA, 1 V) demonstrating the assembly of a monatomic Cu chain on Cu(111) by lateral manipulation at 7 K. Starting from an irregular arrangement of discrete Cu adatoms in (a), a Cu dimer is formed by joining two adatoms in (b), and straight monatomic chains are built up by repeated incorporation of single Cu adatoms via lateral manipulation along the in-plane direction. The unstable and round-shaped appearance of the dimer shown in the inset in (b) [ , 1 nA, 1 V] is due to the characteristic intracell diffusion of the dimer at temperatures . 10
Left panel: Constant-current STM images ( , 1 nA, 1 V) of a Cu monomer, a dimer, and straight chains three to seven atoms in length (from bottom to top). Right panel: spectra taken at a constant tip height of in the regime of unoccupied sample states; for clarity the spectra are offset by increments of relative to the bottom curve showing the spectrum of the bare Cu(111) surface. While a single resonance is observed at for the single Cu adatom, a series of chain-localized levels develops with increasing chain length. The location of the tip during the spectroscopic measurement is indicated by crosses in the STM images.
maps measured at constant tip height showing the spatial variation of the local density of states of a chain. The indicated sample biases correspond to the respective eigenstate energies of the chain-localized states (tunneling parameters prior to opening the feedback loop: 1 nA, 1 V). The quantum number indicates the number of lobes of the confined squared wave function.
Constant-current images of an extended surface area [(a), , 1 nA, 5 mV] after depositing at room temperature and cooling down to the measuring temperature of 7 K and of a smaller area [(b), , 4.4 nA, 65 mV]; the long molecular symmetry axis is always parallel to the close-packed Cu rows along , cf. atomically resolved surface corrugation of bare Cu(111) in (c) [ , 65 mV]. Single molecules exhibit an enhanced tunneling conductance at the ends, cf. constant-current profile in (d) measured across a pentacene molecule at 1 nA and 460 mV; the separation between the lobes at the molecular ends is close to the separation of the outermost benzene units of the free molecule.
Constant-current STM images ( , 1 nA, 1 V) of a pentacene molecule in proximity to a chain prior to (a) and after (b) attachment by lateral manipulation. Points A and B in (a) mark the initial and final tip position during the lateral manipulation event at a reduced tip height of . Diagram (c) shows corresponding constant-current profiles measured across the initial structure (dashed line) and the molecule-chain hybrid structure (full line) indicating an enhanced tunneling conductance in the contact area; the origin of the axis refers to the center atom position of the chain. The structure model in (d) as derived from the data in (a)–(c) depicts the registry of the attached molecule with respect to the chain and the substrate and indicates that the carbon framework of the molecule has overlap with the two outermost Cu chain atoms.
spectra taken for a bare chain (a), on top of the chain extremity of the molecule-chain hybrid structure (b) discussed in connection with Fig. 5 , and a bare chain (c); the tip position during the measurement is indicated in the respective STM inset images. Comparison of the spectra in (a)-(c) shows that the molecule attachment in (b) shifts the ground state and the first exited state to higher binding energies close to the energetic positions found for the bare chain [cf. (c)].
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