Tunable electrochemical switches based on ultrathin organic films

Article abstract of DOI:10.1021/ja068143u

We have performed electroreduction of bisthienyl aminobenzene and thienyl aminobenzene diazonium salts on glassy carbon and polycrystalline gold and generated ultrathin organic films. This method allows covalent bonding between the corresponding aromatic

Full text of DOI:10.1021/ja068143u

Published on Web 01/25/2007
Tunable Electrochemical Switches Based on Ultrathin Organic Films
Claire Fave, Yann Leroux, Gaelle Tripp e´ , Hyacinte Randriamahazaka, Vincent Noel,* and
Jean-Christophe Lacroix*
Interfaces, Traitements, Organisation et Dynamique des Syst e` mes, UniVersit e´ Paris 7-Denis Diderot, UMR 7086,
1
rue Guy de la Brosse, 75005 Paris, France
Received November 20, 2006; E-mail: noel@itodys.jussieu.fr; lacroix@paris7.jussieu.fr
During the last two decades, there has been considerable interest
in replacing conventional inorganic semiconductors with organic
materials in optical and electronic devices. Among organic materi-
1
als, since the seminal work of Aviram, conjugated polymers (CPs)
have been proposed and used as basic building blocks in molecular
electronics. Thus intense research efforts are being devoted to
understanding electron transport and electron transfer on the
nanoscale within such molecules or through metal/oligomer/metal
junctions.2 One of the most important properties of CPs is their
ability to switch upon chemical or electrochemical doping between
two states with different electronic characteristics. Their electro-
chemical switching has been demonstrated to be an easy means of
,3
Figure 1. (a) Formation of organic film by electroreduction. (b) Lines
profile through a scratch of a BTB film. Inset: 2 × 2 µm image showing
a 500 × 500 nm scratch formed with contact mode AFM.
4
controlling the properties of grafted molecules and of metallic
5
nanoparticles. To realize efficient organic electronic devices based
on a few electron transfers, progress in triggering the properties of
grafted groups must be made in order to control the interface
between CP units and metallic electrodes. A monolayer junction
based on conjugated oligomers, with a well-defined metal/oligomer
interface retaining reversible on/off switching capabilities controlled
by the redox state of the oligomer would be of interest for molecular
electronics.
We report here the properties of ultrathin organic films on glassy
carbon (GC) or polycrystalline gold obtained by electroreduction
of diazonium salts. This method allows the preparation of very thin
organic layers with covalent bonding to the electrode6 (see Figure
Figure 2. Cyclic voltammetry in ACN + 0.01 M Fc + 0.1 M LiClO4
solution of (1) bare GC, (2) BTB, and (3) TB electrodes. Inset: (red) first
scan and (black) five successive scans, scan rate 0.1 V/s.
,7
1
a).
a threshold around 0.5 V/SCE is observed in the cyclic voltammetry
of a BTB-modified GC electrode. Multiple cycles show no
degradation of this signal when the inversion potential is kept below
1 V.
The organic film was obtained by cycling a GC or gold electrode
in a solution containing the diazonium salt of 1-(2-bisthienyl)-4-
aminobenzene (BTAB). Grafted molecules will be noted BTB.
Supporting Information describes this electrochemical process. On
the basis of the area of the reduction peak during the modification
of the GC electrode surface with the diazonium salt, the coverage
Fresh BTB-modified GC electrodes were then plunged into a
ferrocene (Fc) solution to probe their electrochemical behavior
toward reversible outer-sphere redox species. Figure 2 compares
the Fc electrochemical response on bare and BTB-modified GC
electrodes. No current is observed on the BTB-modified electrode
in the potential range where Fc redox reactions usually occur on
bare electrodes. The organic layer totally blocks the electrode in
such a potential window. Above 0.5 V/SCE, the current dramatically
increases and an “irreversible” wave is observed with an oxidation
peak at 0.65 V as compared to 0.4 V on the bare electrode. Such
an electrochemical signal suggests at first sight an EC mechanism,
but a ring-disk experiment, where the disk is a BTB-modified GC
electrode, shows that the species generated in this wave is
ferrocenium and can be collected on the ring (at a potential
-
9
of the BTB moieties was calculated to be around 2 × 10
-
2
mol‚cm (note that this procedure does not yield to a precise value
since grafting efficiency will not be of 100%). The reported surface
8
-10
coverage on GC substrates varies in the range of 4 to 12 × 10
-
2
mol‚cm with the theoretical maximum surface coverage for a
-
9
-2
monolayer on GC surfaces being around 10 mol‚cm . The
-
9
-2
surface coverage of 2 × 10 mol‚cm clearly indicates that the
GC electrode is modified with a few monomolecular layers rather
than with a thick film of BTB molecules. Similar results are
obtained on gold. Surface probe microscopy has been used to probe
the organic layer thickness (see Figure 1b). Since the GC substrates
show high surface roughness, we performed AFM scratching
experiments (see Supporting Information) on gold substrates and
obtained an average depth of 5.8((0.4) nm for the BTB film
thickness. If the size of the grafted BTB is estimated to be 12.6 Å,
the layer is not monomolecular, in agreement with previous
reports.9
+
characteristic of the Fc /Fc redox couple). The “irreversible” wave
shows a diffusion-controlled profile with I
p
scaling linearly with
1/2
V
and the Fc concentration within the investigated sweep rate
(and concentration) as expected for the oxidation peak of a simple
reversible system. The peak shape is also identical on both the bare
and modified GC electrodes, suggesting that the heterogeneous rate
constant for Fc oxidation is similar on both substrates. This behavior
is in marked contrast with results already published by several
,10
Electroactivity of the film was then studied in a solution of
LiClO
4
in acetonitrile (see Supporting Information). A wave with
1890
9
J. AM. CHEM. SOC. 2007, 129, 1890-1891
10.1021/ja068143u CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
groups where strong peak shape deformation is associated with a
significant decrease in the heterogeneous rate constant. However,
in these previous studies, both anodic and cathodic Fc peaks were
seen, which makes it possible to evaluate the heterogeneous rate
whether or not transport process within such low band gap tunneling
barrier is coherent or incoherent is a separate question which will
be addressed elsewhere.)15
A BTB electrode behaves as a reversible and reproducible
electrochemical switch when the applied potential reaches a value
close to the grafted BTB oxidation potential. Hence it ought to be
possible to tune the threshold potential of the conductance switch
by changing the number of thiophene units. In order to check this
proposal, we have synthesized 1-(2-thienyl)-4-aminobenzene (TAB)
and characterized the resulting organic electrodes. Gold and GC
electrodes were modified via diazonium salt reduction and studied
in the same manner as with BTB. AFM scratching measurements
yield a layer thickness of 4.5((0.7) nm. Figure 2 (curve 3) shows
the electrochemical activity of Fc probes on the modified electrode.
Once again, no current is observed near the normal potential of
Fc, and a diffusion-controlled anodic peak is obtained at 0.86 V.
As previously, the current increase is observed around the organic
layer switching potential and above that for BTB layers. The anodic
potential shift of the Fc oxidation peak (from BTB to TB layers)
indicates that the switching potential depends, as expected, on the
nature of the organic film.
We have shown that ultrathin organic layers constituting a
reversible conductance switch can be obtained by electrografting
diazonium salts onto a GC electrode. The organic electrodes switch
reversibly between conducting and totally blocking states with a
threshold voltage which can be tuned by carefully choosing the
molecule grafted onto the electrode. Such junctions based on
conjugated oligomers with a well-defined metal/oligomer interface
retaining reversible on/off switching capabilities controlled by the
redox state of the oligomer will be of interest as active interconnects
for molecular electronics or in redox molecular actuators.
constant decrease by measuring ∆E
P
. In the present case, no peak
+
for Fc reduction is observed even at very cathodic polarization
during the reverse potential scan. Another feature is the stability
of the electrochemical response in successive scans (cf. Figure 2,
inset).
To the best of our knowledge, no examples exist in the literature
describing such ultrathin organic layers displaying this electro-
chemical behavior. McCreery et al.11 have studied biphenyl
molecular layers. Initially, the film is described in an “off”, low
conductivity state, and Fc electroactivity is slower than on bare
GC. However, after cathodic polarization of the modified electrode,
they observe that the Fc electrochemical response is modified and
is now similar to that on bare GC. They interpret these results in
terms of organic layer conductance switch, and in their scheme, an
“on” of high conductivity state is reached under cathodic polariza-
tion. However, no reversibility of the on/off switch was observed.
Charge transfer from one electrode to a redox group separated
by an organic linker assembled in the form of a dense monolayer
has been extensively studied.12 For an ideal monolayer with no
pinholes or defects, electron transfer occurs through the layer.
Electron tunneling is usually used to describe the barrier properties
of such organic layers. The tunneling current is a function of the
intrinsic properties of the bridge (HOMO/LUMO gap) and the
electron tunneling distance. For film thicknesses above a few
nanometers, coherent tunneling is not efficient. The value of 5 nm
found for BTB layers is too high for tunneling through molecules
with a large HOMO-LUMO gap to be the main electron transfer
mechanism.
Acknowledgment. This work was supported by the Nano-
sciences ACI administered through the French Research Ministry.
In the case of electron transfer through pinholes or defects, the
peak current would be smaller than that on a bare electrode,
Supporting Information Available: Electrochemical and AFM
experimental details, BTB and TAB synthetic methods. This material
is available free of charge via the Internet at http://pubs.acs.org.
especially in the case of nonspherical diffusion.7
,13,14
Thus, electron
transfer is likely to occur through the BTB layer by a sequential
mechanism for a potential above a certain threshold. This threshold
must be related to BTB oxidation potential as suggested by the
apparent correspondence between BTB electroactivity in Fc free
solution and the potential for Fc oxidation on BTB-modified GC.
Following this idea, an ECcatalytic mechanism consisting of two
successive reactions could be first envisaged:
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