Dresden 2009 – scientific programme

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O: Fachverband Oberflächenphysik

O 44: Metal substrates: Adsorption of organic / bio molecules VI

O 44.3: Talk

Thursday, March 26, 2009, 11:00–11:15, SCH A118

Determining the diffusion coefficients of large organic molecules on a metal surface from tunneling current fluctuations — •Julian Ikonomov, Christoph H. Schmitz, and Moritz Sokolowski — Institut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstr. 12, 53115 Bonn, Deutschland

The diffusion of organic molecules on surfaces has so far been mostly investigated by molecule tracking using the video STM technique [1]. This method requires small molecule coverages and small diffusion coefficients. Here we present a new technique suitable for higher temperatures and arbitrarily coverages based on the temporal increase in the tunneling current that occurs when a molecule diffuses under the STM tip. For our measurements, we have chosen the model system perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) on Ag(100). The PTCDA molecules form highly ordered two-dimensional islands surrounded by gas-like phase [2]. In order to determine the diffusion coefficients, we used a continuum model to describe the diffusion of the molecules under the tip. Due to the finite size of the molecule it is even possible to obtain the spatial anisotropy of the diffusion coefficient. From the density of the peaks in the current-time spectra, one may in addition quantitatively estimate the tip-molecule interaction and the mean local coverage.

Funded by the DFG through SFB 624.

[1] M. Schunack et al., Phys. Rev. Lett., 88 (2002) 156102.

[2] J. Ikonomov et al., Surf. Sci., 602 (2008) 2061.