Spectroscopic STM studies of single pentacene molecules on $\mathrm{Cu}\left(110\right)-\mathrm{c}(6\times 2)\mathrm{O}$

Pentacene adsorbed on the $\mathrm{Cu}\left(110\right)-\mathrm{c}(6\times 2)\mathrm{O}$ surface has been investigated using scanning tunneling microscopy and spectroscopy and density functional theory calculations. The molecules lie almost flat on the surface and are anchored via the end carbon ring to a Cu atom of the topmost surface layer, leading to a break of the mirror symmetry of pentacene parallel to the short molecular axis. As a result the molecular electronic structure of the frontier orbitals exhibits a local and gentle modification around the anchoring site but is essentially unaffected in the remaining part of the molecule. The enhancement of the conductance peak related to the second highest occupied molecular orbital (HOMO-2) at the anchoring site, however, is suggestive of a resonance with a surface state or with the Cu $d$-band.

Figure 1

(a) Overview topographic STM image of the Cu(110)-c($6\times 2$)O surface after deposition of PEN molecules. Two equivalent configurations of adsorbed PEN molecules can be found, in which one end (the top or bottom end) of the molecule lies on top of a super Cu atom. (b) STM image of a single molecule with an overlay of the substrate lattice. (c) Proposed adsorption model showing the super Cu atoms (orange spheres) and one of the two equivalent adsorption geometries of the PEN molecules. Tunneling parameters: $I_{\mathrm{set}}=50\mathrm{pA}, V_{\mathrm{bias}}=-1\mathrm{V}$; (b) $I_{\mathrm{set}}=50\mathrm{pA}, V_{\mathrm{bias}}=-2\mathrm{V}$.

Figure 2

(a) $dI/dV$ spectra recorded at different tip positions as indicated in panel (b). The spectra are color coded and numbered according to the tip position at which they were acquired. The tip position 1 is directly above an uncovered super Cu atom, whereas the other three tip positions are above the PEN molecule. Four conductance peaks can be discerned, in the spectra taken above the PEN molecule, which can be attributed to resonances of the molecular orbitals: LUMO, HOMO, HOMO-1, and HOMO-2. (b) Topographic STM image of a single PEN molecule adsorbed on the Cu(110)-c($6\times 2$)O surface. (c) $dI/dV$ spectrum at tip position 3 from (a), normalized by $I/V$.

Figure 3

Left column: topographic STM images of a single adsorbed PEN molecule at different bias voltages. Each row corresponds to a bias voltage related to a specific molecular orbital. The second column shows the respective differential conductance ($dI/dV$) maps, and in the third column, again topographic STM images are shown, but with a PEN-modified tip (PEN molecule at the tip apex; see text for details). The last column depicts theoretically predicted STM images, obtained from DFT calculations. Tunneling parameters: $I_{\mathrm{set}}=1\mathrm{nA}$ in all images.

Figure 4

(a) Adsorption geometry of a PEN molecule on the Cu(110)-c($6\times 2$)O reconstructed surface as determined from DFT calculations. A bending of the PEN molecule above the super Cu atom (on the right) can be observed. Additionally, the charge density difference is depicted. Red regions correspond to a charge density accumulation, whereas blue ones correspond to a charge density depletion. (b) Calculated density of states (DOS) of PEN molecule and the substrate. The filled black curve depicts the total DOS of the substrate, whereas the orange dashed curve corresponds to the projections onto the super Cu atoms. The red curve denotes the DOS of the molecule. (c) Frontier molecular orbitals (LUMO and HOMO) of the adsorbed molecule in side and top views.