Allgemeines Physikalisches Kolloquium im Wintersemester 2008/2009

Ort:    48149 Münster, Wilhelm-Klemm-Str. 10, IG I, HS 2,
Zeit:    Donnerstag, 30.10.2008 16:00 Uhr c.t.
Kolloquiums-Kaffee ab 15:45 Uhr vor dem Hörsaal

Electrons at the organic-inorganic interfaces, surprising electronic and magnetic properties
Prof. Dr. R. Naaman, Department of Chemical Physics, Weizmann Institute, Israel

A common tacit assumption in the scientific literature is that the electronic properties of a close-packed organized organic monolayer adsorbed on solid substrate can be simply extracted based from the properties of the adsorbed single molecule.
The assumed weak coupling between the molecules in a monolayer seems to support this notion. This, then, is taken as a justification for using molecular-based calculations to predict the properties of the monolayer.
Our recent experimental results suggest that the above stated assumption can misguide our understanding in as far as the electronic and magnetic properties of the adsorbed layer are concerned. When considering the interface between the monolayer and the substrate, one has to take into account, of course, the equilibration of the electro-chemical potential in the two subsystems. In the case that the length of the dipole of the molecules is larger than the distance between them, the layer can be considered as a dipole layer.  A dipole layer is characterized by two important phenomena:
•    the field within the layer is very large, and
•    there is no field outside the layer. Hence, there is no image charge in the substrate.
It will be shown that a Cooperative Molecular Field Effect occurs even in the case of relatively moderate dipole moment strength. Minimization of the free energy requires, in this case, charge transfer between the layer and the substrate. This charge transfer is therefore a result of a cooperative effect and does not take place when a single molecule is adsorbed on the surface.
As will be shown, such charge transfer is indeed observed experimentally. It is the origin of new electric and magnetic properties. These properties can serve for the development of new sensors, as will be discussed in the talk.

D. Cahen, R. Naaman, Z. Vager, "Cooperative Molecular Field Effect", Adv. Funct. Mat., 15, 1571 (2005).