Interface dipoles of organic molecules on Ag(111) in hybrid density-functional theory

We investigate the molecular acceptors 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA), 2,3,5,6-tetra uoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), and 4,5,9,10-pyrenetetraone (PYTON) on Ag(111) using densityfunctional theory. For two groups of the HSE(\alpha, \omega) family of exchange-correlation functionals (\omega = 0 and \omega = 0.2\AA) we study the isolated components as well as the combined systems as a function of the amount of exact-exchange (\alpha). We find that hybrid functionals favour electron transfer to the adsorbate. Comparing to experimental work-function data, we report for (\alpha) ca. 0.25 a notable but small improvement over (semi)local functionals for the interface dipole. Although Kohn-Sham eigenvalues are only approximate representations of ionization energies, incidentally, at this value also the density of states agrees well with the photoelectron spectra. However, increasing (\alpha) to values for which the energy of the lowest unoccupied molecular orbital matches the experimental electron affinity in the gas phase worsens both the interface dipole and the density of states. Our results imply that semi-local DFT calculations may often be adequate for conjugated organic molecules on metal surfaces and that the much more computationally demanding hybrid functionals yield only small improvements.Comment: submitted to New Journal of Physics (2013). More information can be found at http://th.fhi-berlin.mpg.de/site/index.php?n=Publications.Publication

Distinguishing between Charge-Transfer Mechanisms at Organic/Inorganic Interfaces Employing Hybrid Functionals

When modeling inorganic/organic interfaces with density functional theory (DFT), the outcome often depends on the chosen functional. Hybrid functionals, which employ a fraction of Hartree–Fock exchange α, tend to give better results than the more commonly applied semilocal functionals, because they remove or at least mitigate the unphysical electron self-interaction. However, the choice of α is not straightforward, as its effect on observables depends on the physical properties of the investigated system, such as the size of the molecule and the polarizability of the substrate. In this contribution, we demonstrate this impact exemplarily for tetrafluoro-1,4-benzoquinone on semiconducting (copper-I-oxide Cu₂O) and metallic (Cu) substrates and explore how the simulated charge transfer depends on α. We determine the value α* that marks the transition point between spurious over-localization and over-delocalization of charges. This allows us to shed light on the interplay between the value of α* and the physical properties of the interface. We find that on the inert semiconducting substrate, α* strongly depends on surface screening. Furthermore, α has a significant impact on the amount of charge transfer and, in particular, the charge localization. Conversely, for the adsorption on Cu, α affects only the amount of transferred charge, but not its localization, which is a consequence of strong hybridization. Finally, we discuss limitations to the predictive power of DFT for modeling charge transfer at inorganic/organic interfaces and explain why the choice of a “correct” amount of Hartree–Fock exchange is difficult, if not impossible. However, we argue why simulations still provide valuable insights into the charge-transfer mechanism at organic/inorganic interfaces and describe how α can be chosen sensibly to simulate any given system

Interplay of Adsorption Geometry and Work Function Evolution at the TCNE/Cu(111) Interface

The adsorption of organic electron acceptors on metal surfaces is a powerful way to change the effective work function of the substrate through the formation of charge-transfer-induced dipoles. The work function of the interfaces is hence controlled by the redistribution of charges upon adsorption of the organic layer, which depends not only on the electron affinity of the organic material but also on the adsorption geometry. As shown in this work, the latter dependence controls the work function also in the case of adsorbate layers exhibiting a mixture of various adsorption geometries. Based on a combined experimental (core-level and infrared spectroscopy) and theoretical (density functional theory) study for tetracyanoethylene (TCNE) on Cu(111), we find that TCNE adsorbs in at least three different orientations, depending on TCNE coverage. At low coverage, flat lying TCNE dominates, as it possesses the highest adsorption energy. At a higher coverage, additionally, two different standing orientations are found. This is accompanied by a large increase in the work function of almost 3 eV at full monolayer coverage. Our results suggest that the large increase in work function is mainly due to the surface dipole of the free CN groups of the standing molecules and less dependent on the charge-transfer dipole of the differently oriented and charged molecules. This, in turn, opens new opportunities to control the work function of interfaces, e.g., by synthetic modification of the adsorbates, which may allow one to alter the adsorption geometries of the molecules as well as their contributions to the interface dipoles and, hence, the work function