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

Abstract

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...

Interplay of local and global interfacial electronic structure of a strongly coupled dipolar organic semiconductor

“…Its lack of a photon-energy dependence of E final (∆ 0) identifies it as a discrete final state located above the global vacuum level, as was also reported e.g. for the unoccupied π* e 2u state of benzene on Cu(111) [12].…”

“…Structure is often inferred spectroscopically [11][12][13], an approach that works well for highly ordered organic thin films such as e.g. C 60 on Cu (111) [14] or PTCDA on Ag (111) [9].…”

Energy level alignment and interactive spin polarization at organic/ferromagnetic metal interfaces for organic spintronics

“…Previous research has shown that the work function of organic semiconductor molecules chemisorbed on metals is closely correlated to the orientation of the chemisorbed molecules. 34 In view of the relatively high work function of TCNQ/Fe interface (~5.5 eV) in present studies, it is believed that the molecular plane of adsorbed TCNQ is parallel to Fe substrate, i.e., TNCQ molecules "lay" on the substrate in the interface formation. However, as TCNQ deposition goes on, the work function decreases, by about 0.2 eV.…”

Charge equilibration and potential steps in organic semiconductor multilayers

“…A change in the static environment can change the ionization potential considerably, as is demonstrated convincingly on molecular layers with different orientations [25]. The pinning level would be affected in a similar way.…”