Cooperative Electronic and Magnetic Properties of Self-Assembled Monolayers
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Electronic and Magnetic Properties of Self-Assembled Monolayers Ron Naaman and Zeev Vager
Abstract Self-assembled monolayers (SAMs) of organic dipolar molecules have new electronic and magnetic properties that result from their organization, despite the relatively weak interaction among the molecules themselves. Here we review the origin of this cooperative effect and summarize work performed on spin selective electron transmission through SAMs. The spin selectivity observed, in some cases, is consistent with a model in which a SAM containing chiral dipolar molecules behaves like a magnetic layer. The magnetic properties result in the SAMs behaving as spin filters, even without applying an external magnetic field to the layer.
Introduction We review results obtained in the last decade that describe interesting cooperative electronic and magnetic properties of organic molecules self-assembled as a monolayer (SAM) on a metal or semiconductor substrate. In the solid state, we can expect to observe new cooperative electronic properties upon the organization of many units, atoms, or molecules into a solid. This is due to the strong interaction between the units, which is of the order of magnitude of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Hence, the assembly of the units causes the states to mix, resulting in new properties. In comparison, the interaction between organic molecules organized in a monolayer or even in an organic solid is very weak and is typically on the order of a small fraction of 1 eV. As a result, if we take the interaction energy as the only measure of collectivity, it is not expected that in organized organic systems new cooperative properties will emerge. However, as will be reviewed, in two-dimensional self-assembled organized
systems of dipolar molecules, new and interesting properties have been detected. When dipolar organic molecules are self-assembled as a close-packed adsorbed monolayer on a conducting substrate, they form an electric dipole layer, with a thickness of a few nanometers, over a macroscopic area (see Figure 1). In such layers, the original dipole directions are packed parallel to each other at an angle almost perpendicular to the substrate. If each molecule retained its electric dipole moment, the electric field would exceed the dielectric breakdown potential (i.e., the electric field would be larger than that required to detach electrons from the molecules). In order to reduce the electric energy of the assembly, the molecules depolarize, either by charge reorganization within the molecule or by charge transfer between the substrate and the monolayer film.1 As a result, the average dipole moment per molecule in the layer is drastically reduced. The charge transfer or redistribution can be observed by monitoring the surface contact potential. These
MRS BULLETIN • VOLUME 35 • JUNE 2010 • www.mrs.org/bulletin
measurements indicate that even after the charge redistribution, the field across the organic monolayer is la
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