Energy shift and wave function overlap of metal-organic interface states

The properties of Shockley-type interface states between $\pi$-conjugated organic molecular layers and metal surfaces are investigated by time-resolved two-photon photoemission experiments and density functional theory. For perylene- and naphthalene-tetracarboxylic acid dianhydride (PTCDA and NTCDA) adsorbed on Ag(111), a common mechanism of formation of the interface state from the partly occupied surface state of the bare Ag(111) is revealed. The energy position is found to be strongly dependent on the distance of the molecular carbon rings from the metal and their surface density. Bending of the carboxyl groups enhances the molecular overlap of the interface state.

Figure 1

Energy-, angle- and time-resolved 2PPE data of monolayer films of NTCDA, PTCDA on Ag(111). (a) Normal emission (UV + UV) 2PPE spectra of commensurate PTCDA and NTCDA monolayers recorded at 300 and 90 K as indicated, and of the disordered LT-phase of PTCDA at 90 K. The insets sketch the molecular structure for PTCDA (top) and NTCDA (bottom). (b) Angular dependence of the peak maxima of the interface state of the ordered PTCDA (squares, red) and NTCDA (diamonds, green) phases measured at 300 K and the PTCDA low-temperature phase (dots, blue) measured at 90 K together with projected Ag(111) bulk bands (gray shaded area) and the Shockley state of the clean surface (black). (c) Excitation schemes employed to 2PPE spectroscopy (left) and time-resolved 2PPE (right). (d) Time-resolved 2PPE data recorded at the peak maxima of the interface states of the different PTCDA and NTCDA monolayers. Lifetimes were extracted from best fits of a rate equation model using Gaussian laser pulses that reproduce the measured (blue + UV) cross correlation on the clean surface (dashed line).

Figure 2

(a) Band structure of the NTCDA monolayer on the nine-layer Ag(111) film in the $\overline{\Gamma }\to {\overline{X}}_1$ direction of the surface Brillouin zone for the bend molecular geometry. Filled circles indicate wave vectors $\mathbf{k}$, at which the spatial distribution of the IS wave function was analyzed, the thick line represents the parabolic approximation of the corresponding dispersion, arrows indicate IS energies at $\overline{\Gamma }$ for different geometries (from top to bottom: red: $d=2.85$ Å flat, orange: $d=2.997$ Å flat, blue: less dense packing). (b) Spatial distribution of the IS probability density ${\rho }_{xy}$ at the $\overline{\Gamma }$ point in the vicinity of the molecular plane for the flat (top) and the bent-down geometry (bottom). (c) IS probability density ${\rho }_z$ perpendicular to the surface, $xy$ averaged over the unit cell (top: ${\rho }_z$ for the three considered NTCDA geometries; bottom: difference $\Delta {\rho }_z$ between the bent and the flat geometry at the distance of 2.997 Å).