The investigation of the structure of highly ordered organic thin films allows to study the nature of the interaction forces that govern the ordering processes on the molecular scale. While layers of medium sized molecules like perylene derivatives have already been analyzed in the past [1,2], we present a study on the almost twice as large quaterrylene derivatives, a class of molecules that only recently found attention [3].
When aiming at the potential future application of organics, generally referred to as 'molecular electronics', it is necessary to understand the influence of substituents on the adsorption and interaction behaviour of molecules.
We present the first comparison of the growth of three differently substituted quaterrylene derivatives: The unsubstituted quaterrylene (QT) [3], the quaterrylene tetracarboxylic diimide (QTCDI), and the asymmetric (and therefore dipolar) quaterrylene dicarboxylic monoimide (QDCMI). Highly ordered monolayers of each molecule on Au(111) are prepared by means of organic molecular beam epitaxy (OMBE).
Although all molecules contain the same large aromatic core and are of comparable size, they do not order in a similar fashion. We found a strong influence of the substituents - the imide end groups - as well on the growth as on the number of molecules in the unit cell. While all molecules grow in a point-on-line structure on Au(111), the QTCDI structure exhibits a brick-wall motif, while in the QT and QTCMI structures the molecules are aligned in parallel rows. The structures are characterized by two complementary techniques, namely STM and LEED.
We also discuss how the thermal stability of the molecules investigated limits the use of OMBE and how these limitations can partially be overcome.
References:
[1] S. Mannsfeld, M. Törker, T. Schmitz-Hübsch, F. Sellam, T. Fritz, und K. Leo, Organic Electronics 2, 121-134 (2001)
[2] H. Proehl, T. Dienel, R. Nitsche und T. Fritz, PRL 93, 097403 (2004)
[3] R. Franke, S. Franke, C. Wagner, T. Dienel, S.C.B. Mannsfeld and T. Fritz, APL 88, 161907 (2006)
|