Electronic structure of PTCDA on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$

The electronic structures of PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$ have been studied by high-resolution photoelectron spectroscopy (PES) and near-edge x-ray absorption fine structure (NEXAFS). There is a clear chemical interaction between PTCDA molecules and the Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$ surface, as indicated by new components in Sn 4$d$ core levels that are shifted to higher binding energies. The chemical interaction is also evidenced by new components in the O-1$s$ and C-1$s$ core-level spectra. The new components are shifted to lower binding energies and have their origins from two reactions between Sn dangling bonds and the PTCDA molecules. One reaction is located at the anhydride O atom and the other is at the carbonyl C atom. These reactions also induce modifications in the HOMO and LUMO levels seen in the valence band and NEXAFS spectra. For thin films, the NEXAFS results suggest a tilting molecular configuration with respect to the substrate, while for the thicker films, there is an improved ordering of the molecular orientation to the substrate.

Figure 1

Normal emission Sn-4$d$ core-level spectra from PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$ at different coverages. All spectra were recorded with a photon energy of 80 eV at an incidence angle of 40${}^{\circ }$.

Figure 2

Normal emission C-1$s$ core-level spectra from PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$ at different coverages. All spectra were obtained with a photon energy of 350 eV at an incidence angle of 40${}^{\circ }$. Note that the 1.0-Å spectrum has been rescaled in order to visualize the fitting details.

Figure 3

Normal emission O-1$s$ core-level spectra from PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$ at different coverages. All spectra were obtained with a photon energy of 600 eV at an incidence angle of 40${}^{\circ }$.

Figure 4

C K-edge NEXAFS spectra recorded at 70${}^{\circ }$ incidence from PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$. Note that the peak A is shifted at different PTCDA coverages.

Figure 5

C K-edge NEXAFS spectra recorded from two PTCDA coverages (3.2 and 32 Å) on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$. The spectra were obtained at three incidence angles of 0${}^{\circ }$, 40${}^{\circ }$, and 70${}^{\circ }$.

Figure 6

O K-edge NEXAFS spectra recorded from PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$. The spectra were obtained at various incidence angles. It is obvious that the peaks, a and c, are shifted at different coverages.

Figure 7

Normal emission valence-band spectra from PTCDA films on Sn/Si(111)-$\sqrt{3}\times \sqrt{3}$ at different coverages. The spectra were recorded with a photon energy of 51 eV at an incidence angle of 40${}^{\circ }$.