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Free-electron-like dispersion in an organic monolayer film on a metal substrate

Abstract

Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively1,2,3, particularly the degree of electron delocalization that can be achieved2,4. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available5,6,7,8, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47me, where me is the bare electron mass. This dispersion is far stronger than expected for the organic film alone7, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.

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Acknowledgements

The work was financially supported by the Deutsche Forschungsgemeinschaft. Author Contributions R.T. and S.S. conducted the experiments and prepared the figures. A.L., in the context of her B.Sc. thesis, participated in the analysis of the data on the delocalized state. F.S.T. and R.T. wrote the paper. R.T., S.S. and F.S.T. discussed the experiments, the data analysis, and the manuscript intensively at all stages of the work.

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Competing interests

Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

Correspondence to F. S. Tautz.

Supplementary information

Supplementary Notes

This file contains background information on the structure of the PTCDA/Ag(111) interface (section a), additional data supporting an important conclusion of the main text (section b), and simulated wave functions of two-dimensional confined states for comparison with our data. (PDF 186 kb)

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Further reading

Figure 1: Structure and electronic properties of PTCDA/Ag(111).
Figure 2: Free-electron-like two-dimensional band state of PTCDA/Ag(111).
Figure 3: Wavefunctions and energy level scheme of P1 and P2.
Figure 4: Band state in herringbone and square phase islands.

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