The interface energetics of self-assembled monolayers on metals

Self-assembled monolayers (SAMS) of organic molecules generally modify the surface properties when covalently linked to substrates. In organic electronics, SAMS are used to fine-tune the work functions of inorganic electrodes, thereby minimizing the energy barriers for injection or extraction of charge carriers into or out of an active organic layer; a detailed understanding of the interface energetics on an atomistic scale is required to design improved interfaces. In the field of molecular electronics, the SAM itself (or, in some cases, one or a few molecules) carries the entire device functionality; the interface then essentially becomes the device and the alignment of the molecular energy levels with those of the electrodes defines the overall charge-transport characteristics. This Account provides a review of recent theoretical studies of the interface energetics for SAMS of pi-conjugated molecules covalently linked to noble metal surfaces. After a brief description of the electrostatics of dipole layers at metal/ molecule interfaces, the results of density functional theory calculations are discussed for SAMS of representative conjugated thiols on Au(111). Particular emphasis is placed on the modification of the work function of the clean metal surface upon SAM formation, the alignment of the energy levels within the SAM with the metal Fermi level, and the connection between these two quantities. To simplify the discussion, we partition the description of the metal/SAM system into two parts by considering first an isolated free-standing layer of molecules and then the system obtained after molecule-metal bond formation. From an electrostatic standpoint, both the isolated monolayer and the metal-molecule bonds can be cast in the form of dipole layers, which lead to steps in the electrostatic potential energy at the interface. While the step due to the isolated molecular layer impacts only the work function of the SAM-covered surface, the step arising from the bond formation influences both the work function and the alignment of the electronic levels in the SAM with respect to the metal Fermi energy. Interestingly, headgroup substitutions at the far ends of the molecules forming the SAM are electrostatically decoupled from the metal-thiol interface in densely packed SAMS; as a result, the nature of these substituents and the binding chemistry between the metal and the molecules are two largely unrelated handles with which to independently tune the work function and the level alignment. The establishment of a comprehensive atomistic picture regarding the impact of the individual components of a SAM on the interface energetics at metal/organic junctions paves the way for clear guidelines to design improved functional interfaces in organic and molecular electronics.