PTCDA induced reconstruction on Sn/Si(111)-$2\sqrt{3}\times 2\sqrt{3}$

The electronic structures of the Sn/Si(111)-$2\sqrt{3}\times 2\sqrt{3}$ surface and deposited 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) have been studied by use of high-resolution photoelectron spectroscopy and scanning tunneling microscopy. On deposition, PTCDA molecules form a $4\sqrt{3}\times 2\sqrt{3}$ periodicity superposed on the substrate. The new reconstruction is caused by a charge transfer between the Sn/Si(111)-$2\sqrt{3}\times 2\sqrt{3}$ surface and the molecules, as indicated by a new component of the Sn 4$d$ core level that is shifted toward higher binding energy. In contrast to earlier reports, the charge provided by Sn is given to carbonyl C atoms instead of O atoms. This is evidenced by a new component in the C $1s$ core-level spectra, which is shifted toward lower binding energy. The charge transfer also induces a splitting in the highest occupied molecular orbital level of PTCDA seen in the valence band structure.

Figure 1

LEED patterns. (a) A clean Sn/Si(111)$2\sqrt{3}\times 2\sqrt{3}$ surface, 46 eV. (b) A PTCDA-induced $4\times 2\sqrt{3}$ reconstruction, 0.3 ML, 25 eV.

Figure 2

STM images of PTCDA on Sn-$2\sqrt{3}$ with a tunneling current of 10 pA. (a) Filled states, 0.15-ML PTCDA, $200\AA \times 200\AA$, $U=-1.5$ V. A $2\sqrt{3}\times 2\sqrt{3}$ unit cell is indicated by a parallelogram. (b) Diagrams A and B show two adsorption configurations for PTCDA. (c) Filled states, 0.3-ML PTCDA, $179\AA \times 179\AA$, $U=-1.5$ V. (d) Filled states, 0.6-ML PTCDA, $200\AA \times 200\AA$, $U=-2.0$ V. A $4\times 2\sqrt{3}$ unit cell is indicated by a parallelogram.

Figure 3

Band structure of the clean and the adsorbed surfaces. Intensity in the color map indicates features in ARPES data ($h\nu =40$ eV) at RT. Vertical dotted lines indicate every $\overline{\Gamma }$ points in the SBZ. (a) The Sn/Si(111)-$2\sqrt{3}\times 2\sqrt{3}$ surface. (b) The 0.6 ML PTCDA-induced $4\sqrt{3}\times 2\sqrt{3}$ surface.

Figure 4

Normal emission core-level spectra at an incidence angle of 55${}^{\circ }$ with different PTCDA coverages on Sn-$2\sqrt{3}$. (a) Sn 4$d$, $h\nu =80$ eV. (b) C 1$s$, $h\nu =350$ eV.