3 research outputs found
Controlling Formation of Single-Molecule Junctions by Electrochemical Reduction of Diazonium Terminal Groups
We report controlling the formation of single-molecule
junctions
by means of electrochemically reducing two axialdiazonium terminal
groups on a molecule, thereby producing direct AuāC covalent
bonds <i>in situ</i> between the molecule and gold electrodes.
We report a yield enhancement in molecular junction formation as the
electrochemical potential of both junction electrodes approach the
reduction potential of the diazonium terminal groups. Step length
analysis shows that the molecular junction is significantly more stable,
and can be pulled over a longer distance than a comparable junction
created with amine anchoring bonds. The stability of the junction
is explained by the calculated lower binding energy associated with
the direct AuāC bond compared with the AuāN bond
Non-exponential Length Dependence of Conductance in Iodide-Terminated Oligothiophene Single-Molecule Tunneling Junctions
An exponential decrease of molecular
conductance with length has
been observed in most molecular systems reported to date, and has
been taken as a signature of non-resonant tunneling as the conduction
mechanism. Surprisingly, the conductance of iodide-terminated oligothiophene
molecules presented herein does not follow the simple exponential
length dependence. The lack of temperature dependence in the conductance
indicates that tunneling still dominates the conduction mechanism
in the molecules. Transition voltage spectroscopy shows that the tunneling
barrier of the oligothiophene decreases with length, but the decrease
is insufficient to explain the non-exponential length dependence.
X-ray photoelectron spectroscopy, stretching length measurement, and
theoretical calculations show that the non-exponential length dependence
is due to a transition in the binding geometry of the molecule to
the electrodes in the molecular junctions as the length increases
The Orbital Selection Rule for Molecular Conductance as Manifested in Tetraphenyl-Based Molecular Junctions
Using
two tetraphenylbenzene isomers differing only by the anchoring points
to the gold electrodes, we investigate the influence of quantum interference
on the single molecule charge transport. The distinct anchor points
are realized by selective halogen-mediated binding to the electrodes
by formation of surface-stabilized isomers after iodine cleavage.
Both isomers are essentially chemically identical and only weakly
perturbed by the electrodes avoiding largely parasitic effects, which
allows us to focus solely on the relation between quantum interference
and the intrinsic molecular properties. The conductance of the two
isomers differs by over 1 order of magnitude and is attributed to
constructive and destructive interference. Our ab initio based transport
calculations compare very well with the accompanying scanning tunneling
microscope break junction measurements of the conductance. The findings
are rationalized using a two level model, which shows that the interorbital
coupling plays the decisive role for the interference effects