Negative differential resistance and switching behavior of redox-mediated
tunnel contact
Alexander M. Kuznetsov
a͒
A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii Prospect 31, 119991 Moscow, Russia
͑Received 3 April 2007; accepted 17 July 2007; published online 31 August 2007͒
Theoretical description of various properties of redox-mediated tunnel contacts is presented. The
dependences of the current on the overpotential and bias voltage under the sweeping voltammetry
conditions are addressed. The effect of switching between two redox states on the shape of current/
voltage characteristics is discussed. The shot noise and telegraph noise of the bridged contacts
involving redox group are considered. Functional properties of the contact as a means for the
information processing are discussed. © 2007 American Institute of Physics.
͓DOI: 10.1063/1.2770725͔
I. INTRODUCTION
Much attention is paid during the last years to the study
of quantum-dot-mediated tunnel contacts both in solutions
and in solid-state environment.
1–12
These contacts provide a
possibility to study various physical phenomena related with
electron tunneling such as Coulomb interaction of
electrons,
13,14
interference effects,
15
Kondo effect,
16,17
recti-
fication and negative differential resistance ͑NDR͒
behavior,
18,19
the influence of the interaction with phonons,
20
switching,
10,21,22
etc. The electrochemical redox-mediated
tunnel contacts are of special importance since they demon-
strate an interesting behavior at room temperatures,
1,23,24
i.e.,
far above the cryogenic temperatures. An advantage of these
systems consists also of the fact that gating is a routine pro-
cedure typical for the electrochemical systems in solutions.
The bridged electrochemical tunnel contacts represent in fact
a sort of transistorlike arrangement with the voltage between
the electrodes as a source-drain ͑bias͒ voltage and the elec-
trode potential ͑overpotential͒ with respect to the reference
electrode as a gate voltage. Various bridge molecules are
under investigation as possible candidates for the role of the
elements of molecular electronic devices. These are, in par-
ticular, Fe-protoporphyrin IX, polymethylene chains with a
central, reversibly reducible bipyridinium moiety, osmium
and cobalt complexes, N-hexyl-N0-͑6-thiohexyl͒-4,40-
bipyridinium bromide ͑HS-6V6-H͒ and N ,N0-bis͑6-
thiohexyl͒-4,40-bipyridinium bromide ͑HS-6V6-SH͒, Ru-
terminated thiol, metalloproteins, etc.
Many works are devoted to the study of the phenomenon
which is called at present the redox-mediated electron
tunneling.
11,25,26
The electrochemical contacts with a redox
group confined in the tunnel gap are of special interest in
view of a number of effects related with a bistability of the
contact.
23
The bistability is due to the existence of two qua-
sistable valence states of the redox group, oxidized and re-
duced. The existence of these states at fixed values of the
electrode potentials is due to a strong interaction of the redox
group with the molecular environment, in particular, with the
solvent polarization and local vibrational modes. A similar
situation may take place for solid-state contacts of this sort in
the case of strong interaction of the electron state in the
quantum dot with phonons.
The theory of the electron transfer through such contacts
originates from the works in Refs. 27 and 28. The situation
with a weak interaction of the redox group with the elec-
trodes was considered in Refs. 27 and 28. The electron trans-
fer in this case has a sequential stepwise character. The re-
duction of the bridge group occurs at the first step due to
electron transfer from the source ͑S͒ electrode. Its reoxida-
tion with the electron transfer to the drain ͑D͒ electrode com-
pletes the cycle. The tunnel current was expressed through
the electrochemical rate constants ͑transition probabili-
ties͒.
27,28
The latter can be calculated with the use of the
Fermi golden rule both in classical and quantum limits for
the phonons.
29,30
Only classical limit was elaborated in detail
in Refs. 27 and 28 by two reasons: ͑1͒ the liquid electrolyte
solution does not exist at low temperatures and ͑2͒ the tran-
sition probabilities are strongly suppressed at cryogenic tem-
peratures. However, general starting expressions for the cur-
rent are valid both at high and low temperatures. This
approach was further elaborated in a number of works both
for electrochemical systems
1,3,23,26
and for solid-state
contacts.
7–9,31
A different situation arises when the interaction of the
bridge group with the source and drain electrodes is strong.
A flux of a large number of electrons passes through the
bridge group in each reduction and oxidation step.
23,32
The
current is much larger than that in the previous nonadiabatic
case and is of a pulselike character. A number of other effects
may be expected in this limit both at room and low tempera-
tures. Some of them were discussed in Refs. 1, 2, and 23.
The experimental studies in general supported the pre-
dictions of the theory.
5,33–36
However, there are some data
and details
5,37–40
which make necessary to return to the con-
sideration of these systems.
a͒
Author to whom correspondence should be addressed. Electronic mail:
theor@elchem.ac.ru
THE JOURNAL OF CHEMICAL PHYSICS 127, 084710 ͑2007͒
0021-9606/2007/127͑8͒/084710/8/$23.00 © 2007 American Institute of Physics127, 084710-1