Understanding the Metal–Molecule Interface from First Principles

It is by now well-established that the molecule–metal interface can possess a range of surprising electronic properties that are of intense interest from both the basic science and applied research points of view. Indeed, this is emphasized throughout this book. Identifying and understanding these new properties is a very significant challenge not just for experiment, but also for theory. To a large extent, this is because considering the molecule–metal interface properties forces us to bridge two different “world views” – that of molecular orbital theory, which underlies much of organic chemistry, and that of delocalized electron waves, which underlies much of solid-state physics [1, 2]. One often encounters phenomena that are not welldescribed by either of the limiting textbook descriptions, and more elaborate theories need to be constructed. Bridging between the two limiting cases so as to form a coherent phenomenological framework is already difficult, but the challenge here is in fact significantly larger due to the possible emergence of “collective effects” at the interface. We define collective effects as phenomena that the individual components comprising the interface (say a single molecule or the isolated metallic substrate) do not exhibit [2, 3]. Perhaps the most striking example of such a collective effect is the emergence of magnetic phenomena at the interface between a nonmagnetic metal and a closed-shell molecular layer [4]. But there are many other examples. One such phenomenon is the emergence of qualitatively new electronic states at the interface [5–7], due to hybridization of metallic and molecular orbitals. Such hybridization can even result in the induction of magnetic phenomena frommetal to molecule, or vice versa [8–10]. Another phenomenon involves the appearance of interface-localized electron–hole pairs (sometimes referred to as “charge transfer” or “hybrid” excitons) [11–13]. Furthermore, by no means is direct chemical interaction necessary for collective phenomena to occur. For example, long-range electrostatic effects can drastically affect the static polarization in, and electric fields outside of, a molecular monolayer [14, 15]. Consequently, properties of the molec-