We present here a comprehensive study of the sin
gle molecule conductance of alkanes with carboxylic-acidterminal
groups, investi
gated with the
I(
s) and
I(
t) scannin
g tunnelin
g microscopy (STM) methods. Thesemethods enable the formation of sin
gle molecular brid
ges between the
gold substrate surface and
gold STMtip without the need to first form a metallic break junction. The
I(
s) and
I(
t) methods have up until this studybeen used primarily with thiol head
groups, and this current study shows that they can be extended to othercontact
groups. The sin
gle molecule conductance of a series of dicarboxylic acids, HOOC-(CH
2)
n-COOH,has been investi
gated as a function of len
gth of the alkane chain (
N = 4-12), temperature, pH, electrochemicalpotential, and contact
gap separation. The latter parameter, which has been often ne
glected in sin
gle moleculeconductance studies, is particularly noteworthy
given the stron
g dependence of molecular conductance onthe separation between the STM tip and the surface. In this study, particular emphasis is put on determinin
gthe contact
gap spacin
g correspondin
g to the measured conductance values. Both molecular conductance andthe temperature dependence of molecular conductance are shown to depend si
gnificantly on contact
gapseparation for these dicarboxylic acid-terminated molecular wires. Possible ori
gins for this behavior arediscussed. Values for the tunnelin
g decay constant,
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N, and the contact resistance,
A, have been determinedfor the
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![](/ima<font color=)
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ga.
gif" BORDER=0 >-dicarboxylic acid-terminated alkanes fully stretched in the contact junction (all-trans confi
guration).
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N values determined for this stretched confi
guration a
gree well with literature expectations, and the hi
ghvalues for the contact resistance attest to a relatively poor couplin
g between the carboxylate end
groups andthe Au contacts.