  The adsorption of molecules on surfaces affects the surface dipole and thus
changes in the work function may be expected. The effect in change of work
function is particularly strong if charge between substrate and adsorbate is
involved. Here we report the deposition of a strong electron acceptor molecule,
tetrafluorotetracyanoquinodimethane C$_{12}$F$_4$N$_4$ (F$_{4}$TCNQ) on a
monolayer of hexagonal boron nitride nanomesh ($h$-BN on Rh(111)). The work
function of the F$_{4}$TCNQ/$h$-BN/Rh system increases upon increasing
molecular coverage. The magnitude of the effect indicates electron transfer
from the substrate to the F$_{4}$TCNQ molecules. Density functional theory
calculations confirm the work function shift and predict doubly charged
F$_{4}$TCNQ$^{2-}$ in the nanomesh pores, where the $h$-BN is closest to the Rh
substrate, and to have the largest binding energy there. The preferred
adsorption in the pores is conjectured from a series of ultraviolet
photoelectron spectroscopy data, where the $\sigma$ bands in the pores are
first attenuated. Scanning tunneling microscopy measurements indicate that
F$_{4}$TCNQ molecules on the nanomesh are mobile at room temperature, as
"hopping" between neighboring pores is observed.
