Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies


Although an isolated individual molecule clearly has only one ionization potential, multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical π-conjugated organic compounds on metal substrates combined with first-principles calculations and electrostatic modelling reveal the existence of a surface dipole built into molecular layers. Conceptually different from the surface dipole at metal surfaces, its origin lies in details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. In addition to providing in-depth understanding of this phenomenon, our study offers design guidelines for improved organic–organic heterojunctions, hole- or electron-blocking layers and reduced barriers for charge-carrier injection in organic electronic devices.

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Figure 1: Experimental and theoretical photoelectron spectra of DH6T.
Figure 2: Experimental and theoretical photoelectron spectra of 6T.
Figure 3: Growth model for 6T–DH6T heterostructures on Ag(111).
Figure 4: Electrostatic potential for an isolated 6T molecule and 6T layers.
Figure 5: Electrostatic modelling of the orientation-dependent ionization potential.
Figure 6: Energy-level diagram for the 6T–DH6T heterostructures on Ag(111).


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The authors thank H. C. Starck GmbH for providing DH6T. N.K. acknowledges financial support by the Emmy Noether Program (DFG). G.H. is a Marie-Curie Fellow under the INSANE project (contract no. 021511). We thank L. Romaner for helpful discussions and the SFB 488 ‘Mesoscopically Organized Composites’ for financial support.

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S.D. and G.H. contributed equally to this work.

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Correspondence to Steffen Duhm or Georg Heimel.

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Duhm, S., Heimel, G., Salzmann, I. et al. Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies. Nature Mater 7, 326–332 (2008). https://doi.org/10.1038/nmat2119

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