Efficient
charge transfer across metal–organic interfaces
is a key physical process in modern organic electronics devices, and
characterization of the energy level alignment at the interface is
crucial to enable a rational device design. We show that the insertion
of alkali atoms can significantly change the structure and electronic
properties of a metal–organic interface. Coadsorption of tetracyanoquinodimethane
(TCNQ) and potassium on a Ag(111) surface leads to the formation of
a two-dimensional charge transfer salt, with properties quite different
from those of the two-dimensional Ag adatom TCNQ metal–organic
framework formed in the absence of K doping. We establish a highly
accurate structural model by combination of quantitative X-ray standing
wave measurements, scanning tunnelling microscopy, and density-functional
theory (DFT) calculations. Full agreement between the experimental
data and the computational prediction of the structure is only achieved
by inclusion of a charge-transfer-scaled dispersion correction in
the DFT, which correctly accounts for the effects of strong charge
transfer on the atomic polarizability of potassium. The commensurate
surface layer formed by TCNQ and K is dominated by strong charge transfer
and ionic bonding and is accompanied by a structural and electronic
decoupling from the underlying metal substrate. The consequence is
a significant change in energy level alignment and work function compared
to TCNQ on Ag(111). Possible implications of charge-transfer salt
formation at metal–organic interfaces for organic thin-film
devices are discussed.
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