S. Duhm et al., Nature Materials, acceptedWhile 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 modeling 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. S. Duhm et al., Nature Materials, acceptedIt is well established that the work function (Φ) of metals depends on the crystal face 1-3 .Φ is defined as the energy difference between the Fermi level (E F ) and the electrostatic potential above the sample, the vacuum level (V vac ). For, e.g., copper, Φs of the (100), (110), and (111) surfaces are spread over a range of 0.5 eV 1,2 . As E F is constant, this observation has been explained by the difference in the intrinsic "surface dipole": Differences in the geometric and, consequently, electronic structure cause a different amount of the electronic cloud to spill out of the bulk into the vacuum 3, 4 . The resulting dipole raises V vac to a larger or smaller extent and thus impacts Φ 4, 5 . Note that this effect can only be observed for laterally extended surfaces, as the spatial region above the sample where V vac is raised reaches farther away from the surface with increasing sample size (i.e., area of the exposed surface) 6, 7 . Small metal clusters with multiple facets of different crystal orientations have only one well-defined work function 8, 9 .For van der Waals (i.e., non-covalent) crystals of non-dipolar molecules, surface dipoles and work-function anisotropy have not yet been explored 6 . While variations of the ionization potential (IP; the molecular equivalent of the work function) depending on the molecular orientation on a substrate have been reported before [10][11][12][13][14][15][16] , the prevalent interpretation in terms of variable photo-hole screening could never be satisfactorily quantified. Here, we propose a qualitatively different and novel explanation for the intriguing observation that one and the same molecule can have different -still welldefined -IPs if part of an ordered supramolecular structure. S. Duhm et al., Nature Materials, acceptedWe performed X-ray photoelectron spectroscopy (XPS) and u...

The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules and Cu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distances for PEN and PFP differ strongly, that is, 2.34 A for PEN versus 2.98 A for PFP. An adsorption-induced nonplanar conformation of PFP is suggested by XSW (F atoms 0.1 A above the carbon plane), which causes an intramolecular dipole of approximately 0.5 D. These observations explain why the hole injection barriers at both molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas the molecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results show that the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionization energy of molecules is not always readily applicable.