Firuz Demir, George Kirczenow
We examine theoretically the effects of the bonding geometries at the
gold-thiol interfaces on the inelastic tunneling spectra of propanedithiolate
(PDT) molecules bridging gold electrodes and show that inelastic tunneling
spectroscopy combined with theory can be used to determine these bonding
geometries experimentally. With the help of density functional theory, we
calculate the relaxed geometries and vibrational modes of extended molecules
each consisting of one or two PDT molecules connecting two gold nanoclusters.
We formulate a perturbative theory of inelastic tunneling through molecules
bridging metal contacts in terms of elastic transmission amplitudes, and use
this theory to calculate the inelastic tunneling spectra of the gold-PDT-gold
extended molecules. We consider PDT molecules with both trans and gauche
conformations bound to the gold clusters at top, bridge and hollow bonding
sites. Comparing our results with the experimental data of Hihath et al. [Nano
Lett. 8, 1673 (2008)], we identify the most frequently realized conformation in
the experiment as that of trans molecules top-site bonded to both electrodes.
We find the switching from the 42 meV vibrational mode to the 46 meV mode
observed in the experiment to be due to the transition of trans molecules from
mixed top-bridge to pure top-site bonding geometries. Our results also indicate
that gauche molecular conformations and hollow site bonding did not contribute
significantly to the experimental inelastic tunneling spectra. For pairs of PDT
molecules connecting the gold electrodes in parallel we find total elastic
conductances close to twice those of single molecules bridging the contacts
with similar bonding conformations and small splittings of the vibrational mode
energies for the modes that are the most sensitive to the molecule-electrode
bonding geometries.
View original:
http://arxiv.org/abs/1202.1821
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