Abstract
The work function of metal substrates can be easily tuned, for instance, by adsorbing layers of molecular electron donors and acceptors. In this work, we discuss the possibility of changing the donor/acceptor mixing ratio reversibly after adsorption by choosing a donor/acceptor pair that is coupled via a redox reaction and that is in equilibrium with a surrounding gas phase. We discuss such a situation for the example of tetrafluoro-1,4-benzenediol (TFBD)/tetrafluoro-1,4-benzoquinone (TFBQ), adsorbed on Cu(111) and Ag(111) surfaces. We use density functional theory and ab initio thermodynamics to show that arbitrary TFBD/TFBQ mixing ratios can be set using hydrogen pressures attainable in low to ultrahigh vacuum. Adjusting the mixing ratio allows modifying the work function over a range of about 1 eV. Finally, we contrast single-species submonolayers with mixed layers to discuss why the resulting inhomogeneities in the electrostatic energy above the surface have different impacts on the interfacial level alignment and the work function.
Highlights
For many applications, ranging from catalysis to electronics, the work function (Φ) of metals plays an important role
We find that the reaction of H2 with TFBQ to TFBD is at least 1.4 eV more favorable than the adsorption of hydrogen on the metal surface
The planar structure of TFBQ is significantly distorted upon adsorption: On both substrates, the oxygen atoms are located below the carbon backbone, which is indicative of the formation of a bond between the carboxyl groups and the metal surface
Summary
For many applications, ranging from catalysis to electronics, the work function (Φ) of metals plays an important role. Particular interest comes from the field of organic electronics, where the performance of the devices is strongly affected by charge injection barriers built up at the interfaces between the metal electrodes and the organic semiconducters.[1] The electron and hole injection barriers, which exponentially affect the current density,[2] are (among other factors)[3] determined by the offset between the substrate Fermi energy and the charge-transport levels of the organic material that are typically associated with the lowest unoccupied and highest occupied molecular orbitals (LUMO and HOMO). Several strategies have been developed, including the deposition of thin layers of alkali halides,[4−6] alkali/alkaline earth metals,[7,8] ultrathin oxide films,[9,10] or dipolar self-assembled monolayers.[11−16] Another promising approach is the deposition of organic molecules undergoing charge-transfer reactions.[17,18] This bears the advantage of allowing for continuous Φ-tuning by varying the coverage of the adsorbed organic (sub)monolayer.[19,20] Upon employing this approach in organic light-emitting diodes, lower operating voltages as well as significant enhancements of electroluminescence and power efficiency were reported.[21]
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