Postadsorption Work Function Tuning via Hydrogen Pressure Control

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 <i>after</i> 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 <i>ab initio</i> 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

Impact of Static Distortion Waves on Superlubricity

Friction is a major source of energy loss in mechanical devices. This energy loss may be minimized by creating interfaces with extremely reduced friction, i.e., superlubricity. Conventional wisdom holds that incommensurate interface structures facilitate superlubricity. Accurately describing friction necessitates the precise modeling of the interface structure. This, in turn, requires the use of accurate first-principles electronic structure methods, especially when studying organic/metal interfaces, which are highly relevant due to their tunability and propensity to form incommensurate structures. However, the system size required to calculate incommensurate structures renders such calculations intractable. As a result, studies of incommensurate interfaces have been limited to very simple model systems or strongly simplified methodology. We overcome this limitation by developing a machine-learned interatomic potential that is able to determine energies and forces for structures containing thousands to tens of thousands of atoms with an accuracy comparable to conventional first-principles methods but at a fraction of the cost. Using this approach, we quantify the breakdown of superlubricity in incommensurate structures due to the formation of static distortion waves. Moreover, we extract design principles to engineer incommensurate interface systems where the formation of static distortion waves is suppressed, which facilitates low friction coefficients

Reducing the Metal Work Function beyond Pauli Pushback: A Computational Investigation of Tetrathiafulvalene and Viologen on Coinage Metal Surfaces

Application of (sub)monolayers of organic molecules on metal electrodes in order to tune the effective work function has become a field of significant interest. Due to its low ionization potential and quinoidal structure, viologen (1H,1′H-[4,4′]bipyridinylidene) is proposed as a particularly potent work function reducing molecule. In the present contribution, its interaction with Au(111), Ag(111), and Cu(111) is compared to that of the prototypical electron donor tetrathiafulvalene (TTF) using density functional theory based band-structure calculations. The work function modification in both systems is found to be determined by a subtle interplay between effects due to adsorption induced geometric distortions and the donation of electrons from the respective molecular HOMO to the metal. The interfacial charge transfer is investigated in real space as well as in terms of changes in the occupation of the molecular orbitals. Overall, viologen is found to be an excellent choice for decreasing the substrate work function. On gold, a reduction by up to −1.6 eV is predicted, resulting in the viologen covered Au surface having a work function equivalent to that of pristine magnesium

Can We Predict Interface Dipoles Based on Molecular Properties?

We apply high-throughput density functional theory calculations and symbolic regression to hybrid inorganic/organic interfaces with the intent to extract physically meaningful correlations between the adsorption-induced work function modifications and the properties of the constituents. We separately investigate two cases: (1) hypothetical, free-standing self-assembled monolayers with a large intrinsic dipole moment and (2) metal–organic interfaces with a large charge-transfer-induced dipole. For the former, we find, without notable prior assumptions, the Topping model, as expected from the literature. For the latter, highly accurate correlations are found, which are, however, clearly unphysical

Work-Function Modification beyond Pinning: When Do Molecular Dipoles Count?

Deposition of monolayers of strong electron donors or acceptors on metal surfaces in many cases results in a metal-independent work function as a consequence of Fermi-level pinning. This raises the question whether in such a situation molecular dipoles, which are also frequently used to tune the interface energetics, still can have any impact. We use density functional theory to show that the spatial position of the dipoles is the determining factor and that only dipoles outside the immediate metal−molecule interface allow work-function changes beyond the pinning limit

Role of Adatoms for the Adsorption of F4TCNQ on Au(111)

Organic adlayers on inorganic substrates often contain adatoms, which can be incorporated within the adsorbed molecular species, forming two-dimensional metal–organic frameworks at the substrate surface. The interplay between native adatoms and adsorbed molecules significantly changes various adlayer properties such as the adsorption geometry, the bond strength between the substrate and the adsorbed species, or the work function at the interface. Here, we use dispersion-corrected density functional theory to gain insight into the energetics that drive the incorporation of native adatoms within molecular adlayers based on the prototypical, experimentally well-characterized system of F4TCNQ on Au(111). We explain the adatom-induced modifications in the adsorption geometry and the adsorption energy based on the electronic structure and charge transfer at the interface

Electronic Properties of Biphenylthiolates on Au(111): The Impact of Coverage Revisited

We study the impact of coverage on the electronic structure of substituted biphenylthiolate-based self-assembled monolayers (SAMs) on Au(111) surfaces with a particular focus on SAM-induced work-function changes, ΔΦ. This is done using density functional theory accounting also for van der Waals interactions. We find that the tilt angle of the molecules increases significantly when reducing the coverage, which results in a marked decrease of the perpendicular component of the molecular dipole moment. However, ΔΦ does not follow the trend that one would expect on purely geometrical grounds. While for donor-substituted SAMs, ΔΦ decreases much more slowly than anticipated, for acceptor-substituted SAMs the coverage-induced reduction of ΔΦ is clearly more pronounced than expected. In fact, in that case ΔΦ already vanishes around half coverage. This is in part associated with the (coverage-dependent) bond dipole originating from the “Au–S–C” bond. Especially for low coverages, however, the relevance of the “Au–S–C” dipole diminishes, and we observe a significant contribution of Pauli-Pushback (also known as “cushion effect”) to the interfacial charge rearrangements, an effect that hitherto received only minor attention in the discussion of covalently bonded SAMs