Charged and metallic molecular monolayers through surface-induced aromatic stabilization

Abstract

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

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Figure 1: Schematic of relevant energy levels and molecular structures.
Figure 2: Experimental results on structure, chemical nature and electronic structure.
Figure 3: Theoretical results on electronic structure and intramolecular distortions.
Figure 4: Evolution of metal–molecule interfacial electronic structure and mechanism of surface-induced aromatic stabilization.

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Acknowledgements

The authors thank S. Hecht and Q. Xin for critically reading the manuscript, E. Zojer for fruitful discussions and O. T. Hofmann for providing the van der Waals parameters of gold. This work was supported by the Global Centres of Excellence Program of the Ministry of Education, Culture, Sports, Science and Technology (G03: Advanced School for Organic Electronics, Chiba University), and by the German Research Foundation through projects FR2726/1, SFB658, SFB951, SPP1355 and SCHR700/14-1. T.H. gratefully acknowledges an Alexander von Humboldt fellowship for foreign researchers.

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G.H., S.D., I.S., C.B., S.W., A.W., R.S., J.F., B.B. and A.V. performed the XPS and UPS measurements and pre-processed the data. With A.G. leading the efforts, S.D., J.N., C.B., T.H. and B.D. performed NIXSW experiments and analysed the data. A.S., I.F-T. and G.S. performed the scanning tunnelling microscopy measurements under the guidance of K.J.F. and J.I.P. J.P. provided the purified materials. I.S. analysed the XPS and UPS data and prepared the figures. With significant input from S.K. and N.U., G.H. and S.D. interpreted the overall results. G.H. performed the calculations, coordinated the work and wrote the paper. A.G., F.S. and N.K. conceived the project and all the authors commented critically on the manuscript.

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Correspondence to G. Heimel or S. Duhm.

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Heimel, G., Duhm, S., Salzmann, I. et al. Charged and metallic molecular monolayers through surface-induced aromatic stabilization. Nature Chem 5, 187–194 (2013). https://doi.org/10.1038/nchem.1572

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