Chapter 9 - Metal-organic interfaces in organic and unimolecular electronics

Abstract

Organic layers, single molecules, and monolayers are used in the construction of electronic devices, such as diodes, transistors, etc., competing with inorganic semiconductors in manufacturing costs. Making good electrical contacts, for the electrons to cross from a metal, having continuous band structure, to organic molecules with discrete energy levels, requires interdisciplinary knowledge. The physicochemical state of the metal-organic interface determines the electron movement. When the movement of the electrons across the interface is hindered, the contact resistance is high and the device functions poorly. The aim is to design metal-organic interfaces such that electrons can move across as freely as possible. The chemical questions are concerned with the structure of the organic material, and attachment of molecules to metallic electrodes with appropriate functional groups/linkers. The electron movement from the metal to the molecule is determined by the binding energy, interaction of molecular orbitals with those of a metal, and conjugation ability of metal-linker orbitals with the delocalized orbitals on the molecular backbone. These chemical aspects are investigated in this chapter. The physics of the metal-organic interface is concerned with the development of electron transport theories, from the metal to molecule under an externally applied electric field. Basic notions of energy level alignment, electron tunneling, and working principles of some basic electronic devices are introduced.