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X-ray Standing Waves at the Diamond Light Source:  First beamtimes performed, first results published.

In August 2013 we have performed the first X-ray Standing Waves (XSW) experiment at the new beamline I09 of the Diamond Light Source (Diamond, Didcot, UK). The first results were published recently. This new beamline finally closes the gap left by closing beamline ID32 at ESRF in 2011.

Diamond AerialFig. 1: The Diamond Site at Harwell Campus.

The TeamFig. 2: The team, as seen by the electron analyser.



The first beamtime was a common project of the research groups of Prof. C. Kumpf, Prof. F.S. Tautz (both PGI-3) and Prof. M. Sokolowski (Univ. Bonn). In October 2013 the two PGI-3 groups were at I09 for a second experiment. Both experiments were very successful, so that we can already now conclude that finally there is a new, powerful endstation for XSW available in Europe. This closes the gap left by shutting down beamline ID32 at the European Synchrotron Radiation Facility (Grenoble, France) in 2011. ID32 was closed due to political decisions, shooting down all XSW activities in Europe and beyond for more than 2 years. (For details see here.)

In the first two experiments at Diamond we focused on the investigation of heteromolecular hybrid interfaces. With XSW we have measured the adsorption height (and hence the bonding distance) of different molecules (PTCDA, NTCDA, SnPc and CuPc) on Ag(111) in donor-acceptor heteromolecular films with a thickness of only one molecular layer. The results are presented in Phys. Rev. B 89, 161407(R) (2014), which represents the first publication containing XSW data measured at Diamond. The paper contains a systematic study of four different heteromolecular adsorbate structures, all well ordered and consisting of two different molecules each. The paper also demonstrates the progress in the performance of XSW beamlines because data recorded at ID32/ESRF in 2010 and at I09/Diamond in 2013 are compared in this paper.

CuPc+PTCDA_Ag111Fig. 3: Atomic model of the CuPc+PTCDA/Ag(111) mixed brickwall structure

The main finding reported in this paper is the alignment of the adsorption heights of the donor and acceptor molecules in all the heteromolecular films which can be explained by the interplay of charge delocalization from the donor to the acceptor molecule (via the metal substrate) and the push-back effect caused by the molecules approaching the surface. For one of the structures, the “Mixed Brick-Wall” structure of CuPc and PTCDA (see Fig. 2), all details of geometric and electronic properties were reported in a second paper recently: In Nature Communications 5, 3685 (2014) we present the results of a comprehensive investigation of this system based on LEED, XSW, STM, ARUPS, orbital tomography and DFT calculations.

 

Contact: C. Kumpf (Structure Determination of Interfaces and Nanoscale Systems)


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