Influence of the Local Adsorption Environment on the Intra-Molecular Contrast of Organic Molecules in Non-Contact Atomic Force Microscopy
(Result of the month 08/2007)

Organic molecules have fascinating electrical and optical properties making them promising candidates as fundamental building blocks for miniaturized and high-capacity electronic devices. In the effort to exploit the opportunities offered by organic molecules, however, one has to take into account that the local chemical environment of the molecule may strongly influence its properties. In particular the substrate, on which the molecules are adsorbed, may lead to drastic changes in the electronic configuration inside the molecule. A typical representative of a large, pi-conjugated molecule is the semi-conducting molecule 3,4,9,10-perylenetetracarboxylic dianhydrite (PTCDA), which has received widespread interest, motivated by its favorable epitaxial growth properties. Here we present high-resolution non-contact atomic force microscopy (nc-AFM) experiments in ultrahigh vacuum on thin epitaxial PTCDA layers adsorbed on a Cu(111) surface revealing the intra-molecular structure of the individual molecules. We find that this contrast depends critically on the local adsorption environment. The molecules in the first layer, which are in direct contact to the metallic substrate, show no detectable internal structure within the molecular unit. In contrast, the second layer molecules reveal distinct intra-molecular features characteristic of the internal electronic structure. The internal charge distribution of PTCDA is dominated by two areas of high potential located at the center of anhydrite groups at both ends. These characteristic features corroborate well with the apparent image corrugation of the second layer molecules in the AFM images. The differences in the apparent contrast are believed to be due to the presence of the metallic substrate for the first layer molecules, which demonstrates the strong influence of the local adsorption environment on the internal electronic properties of organic molecules.

Epitaxial PTCDA film grown on a Cu(111) surface imaged by noncontact-AFM under ultrahigh vacuum conditions at room temperature. Left: High resolution image of a monolayer (bottom) and double layer (top) of PTCDA imaged in one scan. Right: Enlarged images of the monolayer (bottom) and double layer (top) molecules from the left image showing the different intra-molecular image contrast. Here, the grayscale corresponds to 100 pm. The tip bias was +0.2 V for all images, while the sample was kept at ground potential (frequency shift = -107 Hz).

Contour plot of the PTCDA molecules in the double layer from Image 1 (top) clearly visualizing the sub-molecular double peak structure of the observed by nc-AFM on the individual molecules. (image size 3.75 x 3.75 nm, frequency shift = -107 Hz)