Organic electronics have become increasingly popular for the various semiconductor applications that utilise conjugated organic materials. Tailoring the electronic and optical properties of such organic heterostructures, however, requires a detailed understanding of the interfaces between organic molecules and metal substrates [1]. A detailed characterisation of the first molecular layer on the substrate is crucial because this layer serves as a nucleus for the growth of the organic thin film, and thus largely determines the charge transfer into the organic layer. We have studied the adsorption behaviour of pentacene (PEN, C22H14) and perfluorinated pentacene (PFP, C22F14), two prototypical and widely studied organic semiconductor materials that are used in organic field-effect transistors with high hole (PEN) and electron (PFP) mobility.

Fig. 90: An X-ray standing wave, with the periodicity of the substrate lattice dhkl, is generated by Bragg reflection from a single crystal. By scanning the photon energy of the incident wave through the Bragg condition the phase of the interference field changes by p and as a consequence the nodal planes of the standing wave field are shifting by dhkl/2. Photoelectrons are used as element-specific signal. Depending on the site of the emitting atoms relative to the Bragg planes the photoelectron yield Yp(E) exhibits a characteristic shape which reveals the adsorption site d0 of a given element.

The X-ray standing wave (XSW) data recorded at beamline ID32 reveal a surprisingly different bonding behaviour of PEN and PFP molecules on Cu(111). Like previous studies, which demonstrated the unique potential of this technique for organic systems [2,3], the new results are based on the high spatial resolution, the element specific information and the sensitivity to the chemical environment of the atoms. The bonding distances d0 (Figure 90) derived from the photoelectron signals of PEN and PFP (Figure 91a) give direct evidence of their different interaction mechanism with the substrate. The analysis of the XSW data (Figure 91b) shows that PEN interacts strongly with the Cu(111) substrate and adsorbs with an average bonding distance of only 2.34 ± 0.02 Å, whereas the carbon core of PFP is located at d0 = 2.98 ± 0.07 Å. Despite the relatively large distance from the surface, the PFP molecules exhibit an adsorption geometry that does not coincide with their planar gas phase structure (Figure 91c).

Fig. 91: a) Core-level spectra with the corresponding analysis for submonolayers of PEN and PFP on Cu(111) which were used for the XSW measurements. b) X-ray standing wave scans obtained on submonolayers of PEN and PFP on Cu(111). The symbols represent the photoelectron yield (circles) and substrate reflectivity (triangles) data measured for both adsorbate systems. The solid lines show least-squares fits based on dynamical diffraction theory which provide the coherent position PH (d0 / d111 = PH , modulo n) and coherent fraction fH for each element [2,3]. The coherent position thus reveals the adsorption distance d0 of PEN and PFP, whereas the coherent fraction (,fH< 1) with fH = 0.55 for PEN/C1s and fH = 0.41 for PFP/C1s and PFP/F1s, is related to disorder in the adsorbate system. c) Schematic conformation of PEN (top) and PFP (bottom) on Cu(111) derived from XSW measurements, indicating the different average positions of the F and C atoms.

The considerable difference of 0.64 Å found for the positions of the carbon cores of PFP and PEN demonstrates that the later one interacts much stronger with the Cu(111) substrate. Unlike its perfluorinated counterpart PEN molecules obviously undergo a strong chemisorptive binding. This observation is fully supported by complementary photoemission studies of the valence band structure. The particular hole injection barriers for PEN and PFP on Cu(111) can be understood by taking into account that the non-planar conformation of PFP results in an adsorption-induced intramolecular dipole of ~0.5 Debye perpendicular to the surface. Overall the example illustrates how XSW measurements at ID32 can be employed to study the subtle interplay between structural and electronic properties at organic/metal interfaces.

 

Principal publication and authors

N. Koch (a), A. Gerlach (b), S. Duhm (a), H. Glowatzki (a), G. Heimel (a), A. Vollmer (c), Y. Sakamoto (d), T. Suzuki (d), J. Zegenhagen (e), J.P. Rabe (a), F. Schreiber (b), J. Am. Chem. Soc. 130, 7300 (2008).
(a) Humboldt-Universität zu Berlin (Germany)
(b) Universität Tübingen (Germany)
(c) BESSY (Germany)
(d) Institute for Molecular Science (Japan)
(e) ESRF (France)

References

[1] N. Koch, A. Vollmer, S. Duhm, Y. Sakamoto, T. Suzuki. Adv. Mater. 19, 112 (2007).
[2] L. Romaner, G. Heimel, J.L. Bredas, A. Gerlach, F. Schreiber, R.L. Johnson, J. Zegenhagen, S. Duhm, N. Koch, E. Zojer. Phys. Rev. Lett. 99, 256801 (2007).
[3] A. Gerlach, F. Schreiber, S. Sellner, H. Dosch, I.A. Vartanyants, B.C.C. Cowie, T.-L. Lee, J. Zegenhagen. Phys. Rev. B 71, 205425 (2005).