Dresden 2011 – scientific programme

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O: Fachverband Oberflächenphysik

O 90: Electronic structure III

O 90.3: Talk

Thursday, March 17, 2011, 17:45–18:00, WIL C107

Work-Function Modification beyond Pinning: When Do Molecular Dipoles Count? — •Oliver T. Hofmann, David A. Egger, Gerold M. Rangger, and Egbert Zojer — Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria

Control over the work function (Φ) of metals can be exerted by creating a sheet of dipoles above the surface, e.g., via depositing strong electron donors or acceptors onto the surface. For systems with strong charge-transfer character, one observes pinning of the Fermi level at the molecular level that donates or accepts electrons; i.e., Φ of the combined system is almost independent on the underlying metal and only governed by properties of the adsorbate. At the same time, molecules with intrinsic dipoles are also well known to also modify Φ. This raises the question how these effects interfere in systems inducing charge transfer and simultaneously bearing a dipole. Using density functional theory, we find [1] that intrinsic molecular dipole moments are compensated by adjustments in the amount of the charge transferred, as long as they are located spatially between the metal and the region of the pinning level of the organic molecule. In this case, the intrinsic dipoles are essentially inconsequential for the net work function. In contrast, dipole moments located outside the immediate metal/molecule junction region leave the charge-transfer unscathed. Clever design of the molecules, therefore, allows going beyond Fermi level*pinning. Along these lines, we suggest strategies for work-function changes of unprecedented magnitude. [1] Hofmann et al.,Nano Lett.,2010,10(11),pp4369