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

Abstract

The properties of Shockley-type interface states between π -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. The energetic position and wave function overlap of electronic states at the interface between layers of organic molecules and metals is of fundamental interest for the design of organic semiconductor devices and for future applications of molecular electronics. Previous studies, both experimentally and theoretically, concentrated on those electronic states that either result directly from the chemical bonding at the interface or from the shift and broadening of localized molecular orbitals upon interaction with the metal substrate.1-8 However, also states intrinsic to metal surfaces are affected by adsorption and can become an important factor for the electronic coupling between metal and organic molecules. This has recently become apparent for 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) on Ag(111), a structurally very well characterized model system for the interface between π -conjugated organic molecular layers and metals.4 A dispersing, free-electron-like electronic state located 0.6 eV above the Fermi level was observed by tunneling spectroscopy 9 and by two-photon photoemission (2PPE). 10,11 It originates from the Shockley state of the Ag(111) surface which is upshifted from below the metallic Fermi level by as much as 0.7 eV. The initially partially occupied state becomes unoccupied and approaches the conduction band of the organic semiconductor. [11][12][13] In this Rapid Communication we address the question which properties of the molecule-surface interaction determine the energetic position of such Shockley-type interface states (IS) and which factors facilitate a large overlap of the state with the molecular layer in order to tailor the degree of electronic coupling. For rare-gas adsorbates on noble metals it is well established that their adsorption also leads to an upshift of the Shockley state, albeit by much smaller values, and that the shift increases systematically from the weakly bound Ar to the more strongly interacting Xe.14,15 The comparison of our 2PPE experiments for 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111) with PTCDA/Ag(111) shows an analogous trend. The structurally similar, but smaller NTCDA molecules cause a smaller IS upshift than the larger PTCDA molecules. The time-resolved 2PPE data indicate a similar overlap of the wave function with the metal in both cases. Density functio...

Adsorption geometry and interface states: Relaxed and compressed phases of NTCDA/Ag(111)

“…[3] reveal that outside the surface, where STM and STS are measured, the IS indeed exhibits a density distribution that clearly reflects its hybridization with the molecular orbitals. Moreover, Marks et al [5] find that, in agreement with the original conjecture of ref. [1], the bending of the carboxylic oxygen atoms towards the Ag(111) surface enhances the molecular overlap with the up-shifted surface state.…”

“…The interface to the metallic leads is one of them, as it constitutes the link between two electronic sub-units with very different properties; discrete molecular levels, on the one hand, meet the continuum band structure of an inorganic solid, on the other. The layer in direct contact to the substrate (contact primer layer) thereby plays a decisive role, determining the electronic structure at these interfaces, and it has been found that distinct interface states (IS) may form as a result of the nearby molecular layer [1][2][3][4][5].…”

“…NTCDA adsorbed on Ag(111) has been studied by various experimental techniques in the past [5,[14][15][16][17][18][19][20][21]. Two long range ordered monolayer phases (relaxed and compressed monolayers -denoted as r-ML and c-ML in the following) have been identified [14,15,19,21].…”

Heteromolecular metal–organic interfaces: Electronic and structural fingerprints of chemical bonding

“…A similar effect has also been reported for pure PTCDA adsorbed on Ag(1 1 1) [61,71] and discussed in terms of a so called interface state (IS), originating from the well known Ag(1 1 1) surface state which becomes depopulated due to the adsorption of PTCDA [13,72]. For PTCDA/Ag(1 1 1) the IS is located at (0.570 ± 0.030) eV [72] and contains contributions of the LUMO+1 and LUMO+2 levels, as revealed by DFT [73]. Hence, the question arises whether the IS of the MBW structure also exhibits some density of states of the unoccupied molecular orbitals, apparently of both molecules.…”

Transformation of the Ag(111) surface state due to molecule-surface interaction with ordered organic molecular monolayers

“…13,14 As was observed by the 2PPES in Ref. 15, in the case of the so-called relaxed ordered NTCDA ML adsorbed on the Ag(111) surface, there is also an unoccupied Shockley-type interface state. The latter has smaller energy than that in the case of the PTCDA ML.…”