Odd–Even Effect in Molecular Electronic Transport
via an Aromatic Ring
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Abstract
A distinct odd–even effect
on the electrical properties,
induced by monolayers of alkyl-phenyl molecules directly bound to
Si(111), is reported. Monomers of H<sub>2</sub>CCH–(CH<sub>2</sub>)<sub><i>n</i></sub>–phenyl, with <i>n</i> = 2–5, were adsorbed onto Si–H and formed
high-quality monolayers with a binding density of 50–60% Si(111)
surface atoms. Molecular dynamics simulations suggest that the binding
proximity is close enough to allow efficient π–π
interactions and therefore distinctly different packing and ring orientations
for monomers with odd or even numbers of methylenes in their alkyl
spacers. The odd−even alternation in molecular tilt was experimentally
confirmed by contact angle, ellipsometry, FT-IR, and XPS with a close
quantitative match to the simulation results. The orientations of
both the ring plane and the long axis of the alkyl spacer are more
perpendicular to the substrate plane for molecules with an even number
of methylenes than for those with an odd number of methylenes. Interestingly,
those with an even number conduct better than the effectively thinner
monolayers of the molecules with the odd number of methylenes. We
attribute this to a change in the orientation of the electron density
on the aromatic rings with respect to the shortest tunneling path,
which increases the barrier for electron transport through the odd
monolayers. The high sensitivity of molecular charge transport to
the orientation of an aromatic moiety might be relevant to better
control over the electronic properties of interfaces in organic electronics