The performance of modern organic electronic thin film devices is strongly influenced by the electronic level alignment of the organic thin film with the conducting electrode. Energy level alignments at metal-organic interfaces sensitively depend on the atomic structure and the charge distribution at the interface. A common strategy to tailor the level alignment can be adsorption of strong electron acceptors and donors. We present a characterisation of a two-dimensional donor acceptor network formed by alkali atoms and the strong acceptor molecule TCNQ (7,7,8,8-tetracyanoquinodimethane) adsorbed at different facets of Ag surfaces. We characterise the adsorption structure with a combination of normal incidence x-ray standing wave (NIXSW) measurements, Scanning Tunnelling Microscopy, and dispersion-inclusive Density Functional Theory, which we find to be in excellent agreement with experiment. The adsorption structure sensitively depends on an interplay of molecule-metal charge transfer and long-range dispersion forces, which are influenced by the co-adsorption ratio between alkali atoms and TCNQ. In general, alkali atom co-adsorption reduces the molecule-metal interaction strength yet is energetically favoured compared to TNCQ co-adsorption with Ag adatoms. Upon establishing accurate structural models, we characterise the dependence of key electronic properties such as the work function and the level alignment at the interface as a function of surface facet, alkali atom, and TCNQ acceptor.

Effects of alkali atom doping on metal-organic frameworks: A joint experimental and computational study

ACS Spring 2022

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