Commensurate Registry and Chemisorption at a Hetero-organic Interface

We present evidence for a partly chemisorptive bonding between single monolayers of copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) that are stacked on Ag(111). A commensurate registry between the two molecular layers and the substrate, i.e., a common crystallographic lattice for CuPc and PTCDA films as well as for the Ag(111) surface, indicates that the growth of the upper layer is dominated by the structure of the lower. Photoemission spectroscopy clearly reveals a gradual filling of the lowest unoccupied molecular orbital of PTCDA due to CuPc adsorption, which proves the chemisorptive character.

Figure 1

High-resolution LEED images ($E_{\mathrm{kin}}=27.2\mathrm{eV}$) for different CuPc coverages adsorbed on a closed layer PTCDA/Ag(111). Images in panels (a), (b), and (c) were recorded at RT; the image in panel (d) was recorded at 100 K. Red and blue circles mark calculated positions of diffraction spots from the PTCDA-monolayer structure and the CuPc/PTCDA stacked-bilayer (SB) structure, respectively.

Figure 2

Panels (a), (b), and (c): STM images of the SB phase at $T=10\mathrm{K}$ (0.75 V, 18 pA). All images were corrected for lateral distortions using the well-known PTCDA-on-Ag(111) unit cell. (a) A CuPc island (bright contrast) has condensed on a closed PTCDA monolayer on Ag(111). PTCDA and CuPc lattices are indicated by red and blue lines, respectively. (b) Close-up of the CuPc island. Six CuPc molecules (one unit cell) are shown as ball-and-stick models. (c) Dotted ellipses indicate the positions of PTCDA molecules under the CuPc layer. Bright STM contrast is found where CuPc wings lie above the PTCDA core. Panel (d) Model of the PTCDA ML-structure. Adsorption sites of CuPc molecules are marked by red circles, the size of which indicate statistical errors.

Figure 3

UPS data of CuPc and PTCDA on Ag(111) for different CuPc coverages at RT (overlapping black circles along the curves, $\hslash \omega =21.218\mathrm{eV}$, photoelectron acceptance angle 30°). The red curves represent least-square fits to the data. For the lowermost and uppermost curves, the fitting model consisting of up to four independent peaks is shown; PTCDA and CuPc states are displayed in green and blue, respectively. For fitting the background, we used a Fermi function which was broadened in order to account for the instrumental resolution, plus an exponential component. It is shown as a black solid line with the lowermost curve.