Adsorbates and at Organic/Inorganic Interfaces

Alexandre Tkatchenko, Lorenz Romaner, Oliver T. Hofmann, Egbert Zojer, Claudia Ambrosch-Draxl, and Matthias Scheffler actions play a noticeable role even for chemisorption when strong chemical bonds are formed between docking groups of a molecule and the substrate (e.g., for self-assembled monolayers [SAMs] bonded to gold by thiolates).10 Here they can decisively influence the molecular tilting angle, molecular distortions, or isomerization.3 Not surprisingly, the interface geometry is crucial for its electronic properties, which, in turn, control the function of organic devices. In particular, the interface dipole, and hence the alignment of the molecular electronic states with respect to the Fermi level of the electrode, are sensitive to the atomic positions. Generally, the term “vdW forces” can refer to different types of intermolecular interactions, such as electrostatics (permanent multipole–permanent multipole interaction), induction (permanent multipole–induced multipole interaction), and dispersion (induced multipole–induced multipole interaction). Dispersion usually is the strongest attractive term for molecules physisorbed or weakly chemisorbed on surfaces. Hence, the term “dispersion energy” is frequently used interchangeably with “vdW energy,” and we follow this convention in this article. The dispersion energy arises from the correlated motion of electrons and must be described by many-electron quantum mechanics. Unfortunately, accurate quantum-chemical calculations are only feasible in the near future for finite systems up to ∼100 light atoms. Thus, for most systems of interest in interface modeling, fully first-principles calculations of vdW forces are not realistic at present. However, for many situations, alternative approaches exist that have comparable accuracy, and we will discuss these approaches in the next section, particularly concentrating on Kohn-Sham density-functional theory (DFT). Our main focus is on methods that currently show the most promise, contain few empirical parameters, and have already been applied to the modeling of inter faces. Next, the influence of vdW interactions for the geometry and electronic structure of an organic/inorganic model system—PTCDA on Ag(111)—will be discussed. Lastly, it will be shown that vdW interactions also play a prominent role in determining the structure and function of real interfaces, such as SAMs on metal surfaces.