Van der Waals Interactions Between Organic Adsorbates and at Organic/Inorganic Interfaces
- PDF / 328,013 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 35 Downloads / 168 Views
Interactions Between Organic Adsorbates and at Organic/Inorganic Interfaces
Alexandre Tkatchenko, Lorenz Romaner, Oliver T. Hofmann, Egbert Zojer, Claudia Ambrosch-Draxl, and Matthias Scheffler Abstract Van der Waals (vdW) interactions play a prominent role in the structure and function of organic/organic and organic/inorganic interfaces. Their accurate determination from first principles, however, is a notoriously difficult task. Recently, a surge of interest in modeling vdW interactions has led to promising theoretical developments. This article reviews the state-of-the-art of describing vdW interactions by density-functional theory with respect to accuracy and practicability. The performance of the different methods is demonstrated for simple systems, such as rare-gas dimers and small organic molecules. The nature of binding at organic/inorganic interfaces is then exemplified for the perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) molecule at surfaces of coinage metals. This fundamental system is the best-characterized organic molecule/metal interface in experiment and theory. We emphasize the crucial importance of a balanced description of both geometry and electronic structure in order to understand and model the properties of such systems. Finally, the relevance of vdW interactions to the function of actual devices based on interfaces is discussed.
Introduction Bonding at organic/organic and organic/inorganic interfaces results from interplay of several interactions, most notably (covalent) hybridization of wave functions, electron transfer processes, van der Waals (vdW) interaction, and Pauli repulsion. Typically, one distinguishes different bonding regimes: In the case of pure physisorption, the attraction between the adsorbate and the substrate is mainly due to vdW forces, which are, hence, essential for a correct description. Standard systems characterized by vdW bonding are molecules adsorbed on the basal plane of
graphite.1,2 An intermediate bonding regime arises for π-conjugated molecules adsorbed on metal surfaces where also electron transfer processes and covalent bonding come into play.3–9 In this context, we will discuss the adsorption of perylene-3,4,9,10-tetracarboxylic-3,4,9, 10-dianhydride (PTCDA) on Ag(111) as an example, as this is one of the bestcharacterized metal/organic interfaces both in experiment and in theory.36 For this and similar systems, vdW forces are found to be crucial for obtaining reliable geometries and energies. Interestingly, vdW inter-
MRS BULLETIN • VOLUME 35 • JUNE 2010 • www.mrs.org/bulletin
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 device
Data Loading...
