Daniel Kaefer
The synchrotron-based X-ray absorption spectroscopy is a very powerful tool to unravel the orientation of organic molecules on surfaces or in thin films. This information on the alignment of – most often – highly anisotropic molecules can become crucial if an epitaxial or even crystalline organic growth is desired, if such thin film should serve as template or anchoring unit for further deposition/growth in a controlled bottom-up approach, if the molecules will be actively used as functional unit in spectroscopy or for charge transport, or if a precise in-depth analysis via ab initio calculations (yielding electronic structure, charge transport properties,…) is undertaken requiring perfect knowledge of input geometry. The broad applicability of NEXAFS for obtaining such information for nearly all kinds of different organic molecules (aliphatic, aromatic, functionalized rings, extended ring systems,…) will be shown using selected examples of thiol self-assembled monolayers on Au.
A case study of pentacene thin films on Ag(111) serves as example of analysis of the initial stage of multilayer film growth, revealing a flat lying monolayer while thicker films contain tilted molecules. Driving the resolution and detection capabilities to their limits, it was possible to use NEXAFS to prove that the flat lying monolayer does not reorient/reconstruct upon further film growth, thus creating a second interface (to the organic bulk), hampering all attempts of templated growth.
Finally, it will be shown that NEAXFS can ultimately even be used to unravel the molecular conformation in thin organic films of rubrene. Upon deposition on cold Au substrates, rubrene lands in its twisted and chiral conformer while hotter substrates cause the reorganization into the planar (bulk crystal) geometry. Such additional degree of freedom – the flexibility of the molecular entity - is only found in larger, soft organic molecules and not in the inorganic world, but can have significant impact on the overall growth behavior as the growth of crystallites in bulk structure can occur only at particular nucleation sites.
