Postadsorption Work Function Tuning via Hydrogen Pressure Control

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.

The Impact of Dipolar Layers on the Electronic Properties of Organic/Inorganic Hybrid Interfaces

“…Conversely, at the position of the adsorbed molecule, the local dipole has a much larger impact on the electrostatic energy, reminiscent of the situation of an isolated point dipole. This is illustrated in Figure 3c by calculations for tetrafluorobenzoquinone on Ag(111): [92] For full monolayer coverage (red line), the electrostatic energy experiences the abrupt step discussed above for densely packed dipoles. It is essentially the same at the position of the molecule, z TFBQ,C and far from the interface.…”

“…This occurs, for example, for very weakly coupled interfaces (see the integer charge transfer scenario discussed in Section 5.2.4), or at interfaces at which a fraction of the molecules is chemically modified, e.g., through reactions with a residual gas. [92]…”

“…A quantitative discussion is given in ref. [92]. Reproduced under the terms of the Creative Commons CC-BY license.…”

“…Reproduced under the terms of the Creative Commons CC-BY license. [92] Copyright 2015, American Chemical Society. microscopy measurements.…”

Adsorption Behavior of Nonplanar Phthalocyanines: Competition of Different Adsorption Conformations

“… 34 , 49 − 54 Then, it is essentially the energy of the “molecular pinning level” that determines the work function, at least as long as (i) all molecular dipoles are located spatially between the metal substrate and the molecular states at which the Fermi level is pinned 54 and (ii) the packing density is sufficiently high. 55 In such a case, ΔΦ bond has to compensate for differences in ΔΦ dip . This is exactly what happens here for Cl-down and Cl-diss.…”