Adsorption of Organic Molecules on Metals

The specific properties of organic semiconductors - such as chemical tunability, relatively easy and cheap processing, compatibility with flexible substrates - allow to use these materials as active layers in different thin film applications [1]. Among the numerous factors that influence the overall efficiency of a device, the charge injection (respectively extraction) process is particularly important. In this context, we study the interaction of small conjugated molecules (COMs) with metallic substrates. While the interface between metals and semiconductors has been studied for many years, the resulting concepts (Schottky-Mott theory) have to be refined for organic semiconductors [1]. As shown by many studies in this field, the first molecular layer on the metal is of key importance. For example, the charge re-arrangement and the energy-level alignment at the metal-organic interface can be related to the adsorption geometry of the molecules [2]. The complexity of those effects, however, require sophisticated experiments and often quantum chemical calculations [3].

Depending on the molecule-metal combination the interaction strength varies considerably. Two extreme scenarios can be distinguished

• Weak physisorptive bonding due to long-range van der Waals forces.
• Strong chemisorptive bonding, where the electronic orbitals of the molecules form chemical bonds with the substrate atoms.

As the adsorption of π-conjugated molecules on metals is considered an intermediate situation, both contributions have to be included in the description of these systems. For strongly interacting systems the hybridization of molecular orbitals with the electronic states of the metal often involves a charge-transfer at the interface and distortions of the adsorbate [2]. In some cases, this may yield a metallic adsorbate layer with electronic states of the molecule lying at the Fermi level. In other cases the formation of an interface dipole (ID) pins the Fermi level, and thereby retains the semiconducting character of the organic layer [3,4].

Photoelectron spectroscopy is a powerful and well established technique to study the electronic properties of adsorbed molecules [1]. While X-ray photoelectron spectroscopy (XPS) excites core-level electrons and is sensitive to the chemical environment of the different atoms, ultraviolet photoelectron spectroscopy (UPS) probes the valence band region and is suitable to study charge-transfer phenomena as well as the emergence of interface states [2]. The molecule-substrate interaction, that influences the electronic properties of the system, also determines the adsorption geometry of the molecules. A planar molecule, for example, can be distorted on the surface if there are specific functional groups attached. In general, the element-specific adsorption distances of a molecule, which can be measured using the X-ray standing wave technique with unrivalled precision [1,5], do not only reflect the total interaction strength but also the subtleties of the bonding mechanism. Also, it has been shown that intermolecular forces cannot be neglected: Their influence goes beyond the lateral ordering (probed routinely by low energy electron diffraction) as the molecule-molecule interaction affects the bonding distances. Often, there are correlations between electronic and geometric changes, which can contribute to a better understanding of the organic-metal interface. Moreover, those observations provide an excellent test ground for state-of-the-art quantum chemical calculations.