Configuration-specific electronic structure of strongly interacting interfaces: TiOPc on Cu(110)

We use low-temperature scanning tunneling microscopy in combination with angle-resolved ultraviolet and two-photon photoemission spectroscopy to investigate the interfacial electronic structure of titanyl phthalocyanine (TiOPc) on Cu(110). We show that the presence of two unique molecular adsorption configurations is crucial for a molecular-level analysis of the hybridized interfacial electronic structure. Specifically, thermally induced self-assembly exposes marked adsorbate-configuration-specific contributions to the interfacial electronic structure. The results of this work demonstrate an avenue towards understanding and controlling interfacial electronic structure in chemisorbed films even for the case of complex film structure.

Figure 1

(a) Surface-normal UPS spectra for clean Cu(110) (black) and 0.5-ML TiOPc (blue). Blue windows indicate spectral regions with molecular DOS, and the brown window indicates the spectral region for emission from Cu $d$ bands. The energy scale is referenced to the Fermi level (binding energy $E\text{−}{E}_F$). See Fig. 5 for a close-up of the frontier orbital region. (b) STM image of 0.5-ML TiOPc on Cu(110) (Sample bias $V_S=5\mathrm{mV}$; tunneling current $I_T=0.20\mathrm{nA}$).

Figure 2

TiOPc adsorption configurations. Molecules with the oxygen atom directed toward the surface are labeled “O-down” while those with the oxygen atom directed away from the surface are labeled “O-up.” STM image taken from Fig. 1. Molecular structure models visualized in Avogadro [58].

Figure 3

(a) Surface-normal UPS spectra for clean Cu(110) (black) and 1-ML TiOPc (blue). See Fig. 5 for a close-up of the frontier orbital region. (b) STM image of near 1-ML TiOPc on Cu(110) $(V_S=10\mathrm{mV},I_T=0.10\mathrm{nA})$.

Figure 4

(a) Room temperature grown film of near 1-ML TiOPc $(V_S=50\mathrm{mV},I_T=0.10\mathrm{nA})$. (b) The same film after UHV annealing to 190 °C $(V_S=10\mathrm{mV},I_T=0.10\mathrm{nA})$. The surface is coarsened by cooperative self-assembly of TiOPc with Cu adatoms.

Figure 5

(a) Survey ultraviolet photoemission spectra for 1-ML TiOPc on Cu(110) before (blue) and after (red) self-assembly $(V_S=-5\mathrm{V}$). (b) High-resolution spectra for the M1–M3 molecular regions. Features in M1 and M2 change significantly upon self-assembly, while those in region M3 largely retain their original profile. (c) High-resolution spectra for the frontier orbital region near $E_F$. In (b,c), the spectra were taken with a restricted analyzer acceptance angle of $\pm 1.5^{\circ }$ and $V_S=-3\mathrm{V}$.

Figure 6

(a) 2PPE spectra for 0.87-ML TiOPc on Cu(110) before (blue) and after (red) thermal annealing and self-assembly. Analyzer acceptance $\mathrm{angle}=\pm 1.5^{\circ }$ and $V_S=-1.2\mathrm{V}$. (b) Fit of the $f$-LUMO region before self-assembly with two Gaussians modified by Fermi-Dirac functions (thick lines). The full Gaussian DOS extending into the unoccupied region is also shown (thin lines). The background is derived from clean Cu(110) data (gray) and the model sum is shown by the shaded area. (c) Fit of the $f$-LUMO region after self-assembly, as in (b).

Figure 7

Cartoon of local contributions to the electrostatic potential above the surface, $V\left(x,y,z\right)$, in a simplified point dipole picture. A cut through $V\left(x,y,z\right)$ at constant coordinate $y$ and constant height $z$ is shown above each cartoon, and associated image dipoles are shown beneath the surface. (a) Before self-assembly, free “O-down” molecules show an apparent height of $\sim 0.9\AA$ in STM images. (b) In self-assembly, adatoms coordinate with “O-down” molecules to form nanoribbons and reorient the “O-down” dipole moment. “O-down” molecules in these nanoribbons show an apparent height of $\sim 2.0\AA$.

Figure 8

(a) Energy-level diagram for Cu(110) and free TiOPc. (b) Upon adsorption, the work function change and energy renormalization bring the LUMO DOS below $E_F$ ($f$-LUMO). (c) After configuration-selective self-assembly, the $f$-LUMO DOS for “O-down” TiOPc shifts above $E_F$ while the $f$-LUMO DOS for “O-up” changes only slightly.