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Establishing the Molecular Basis for Molecular Electronics.

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Molecular Electronics
Establishing the Molecular Basis for Molecular
Ronald A. Wassel and Christopher B. Gorman*
electron transport · molecular devices · molecular
electronics · single-molecule studies · structure–
activity relationships
Performing logic and memory operations with one or a very small collection
of molecules would be the ultimate in
electronic-device miniaturization. For
this reason, the field of molecular electronics has received attention that ranges from scientific curiosity to the generation of intellectual property and
venture capital. While new paradigms
and financial rewards in nanotechnology
will probably come (although perhaps
not as fast as an investor would like),
answers to several key questions are a
necessary first step in this evolution. In
this regard, chemists (who might be
regarded as molecular engineers) have
an exciting task ahead of them—sorting
out the fundamental precepts that will
govern this field. A number of central
questions have emerged. Some loom
large and will probably require substantial shifts in our approaches for working
with molecules. For example, what mix
of lithography (top-down engineering)
and self-assembly (bottom-up manufacturing) will be required to achieve the
dense integration of components that
allow us to truly exploit the size scale of
single-molecule devices? How will
nanotubes be used in these regards?[1]
To date, no realistic approach has addressed this issue. Other questions have
proven to be more manageable and are
equally important. They require us to
question fundamentally how molecular
science will work in nanometer-scale
[*] R. A. Wassel, Prof. C. B. Gorman
Department of Chemistry
North Carolina State University
Raleigh, NC 27695-8204 (USA)
Fax: (+ 1) 919-515-8920
collections. For example, how does one
make contact with a molecule? What is
the electronic structure of a molecule
when it is in contact with “wires”? Can
molecular structure–property relationships be derived that relate the structure
of a molecule to nonlinear current–voltage behaviors, switching, and, ultimately, gating? These latter questions have
been addressed with some recent, plausible approaches. Such work is highlighted herein.
In performing nanoscale electronic
measurements, the issue at hand, first and
foremost, is how to make electrical contact to these elements. In doing so, one
must confront the issue that this contact
is going to perturb the molecules under
study. The first strategies for contact to
small collections of molecules began with
the mechanical break junction.[2] A break
junction is formed by attaching a metal
wire onto a flexible substrate and then
bending the substrate just until the wire
has broken. The gap produced is then
exposed to molecules designed to bind
across it. Resistances are measured that
are determined to be consistent with the
resistance of a single molecule.[3]
A second top contact can be made to
a collection of molecules (e.g., a selfassembled monolayer (SAM) or Langmuir–Blodgett (LB) film) by metal
evaporation. In a nanopore configuration[4] the area of the nanopore is
designed to be smaller than the domain
size of the SAM and the evaporated
metal accumulates only on the top of the
SAM. By using nanopores, Reed, Tour,
et al. showed current–voltage measurements in molecules containing a nitroamine redox center that exhibited negative differential resistance.
As these metal–molecule–metal assemblies must be made one at a time, it
can be difficult to get a sense of how
variable their behavior is. Furthermore,
although evaporating a top contact
makes a metal–molecule–metal sandwich that most naturally resembles a
device, metals are strong reducing
agents. Reduced molecules are typically
quite chemically reactive. Thus, the
molecule that is placed into the sandwich may not be the structure that is
ultimately measured. This concern is
exacerbated by the fact that the geometry of the sandwich precludes any
spectroscopic characterization of the
molecules in that device.
To address this issue, a number of
investigators have employed the tip of a
scanning tunneling microscope (STM,
or conducting atomic force microscope,
AFM) as a second contact to a molecule
(often organized into a self-assembled
monolayer).[5–21] Several examples are
noted. Hipps and co-workers reported
orbital-mediated tunneling through
phthalocyanins and porphyrins that contain metal centers.[5–8] Tour, Bard, and
co-workers displayed peak shaped I–V
curves in phenylene ethynylene oligomers (OPEs) by using a tuning-fork
STM.[9, 10] We have studied negative
differential resistance in patterned, electroactive SAMs by using STM.[16] Weiss
and co-workers inserted individual
OPEs into an insulating n-alkanethiolate SAM background and determined
that these molecules were more conducting than the background.[11–14] By
visualizing individual molecules over
time, they observed changes in conductance. These variations in conductance
(stochastic switching) were attributed to
DOI: 10.1002/anie.200301735
Angew. Chem. Int. Ed. 2004, 43, 5120 –5123
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
conformational variations in the molecules rather than the electrostatic effects
of charge transfer. Similar stochastic
switching has been observed by Lindsay
and co-workers on carotenoid molecules
with a gold nanoparticle on top[20, 21]and
by our group on ferrocenyl-terminated
alkanethiols inserted into an n-alkanethiolate SAM.[17]
Although these approaches that
make use of scanning probe microscopy
address several key aspects of molecular
contacts, they still leave an important
issue largely open. How do metallic
contacts influence the electronic structure of the molecule between them? A
recent report by Nazin et al. has elegantly illustrated how the electronic
structure of a molecule evolves as it is
contacted with larger and larger metal
bridges.[22] Specifically, a low-temperature ultrahigh-vacuum (UHV) STM was
used to make nanostructures composed
of a copper phthalocyanine (CuPc)
molecule bonded to chains of various
numbers of gold atoms. These nanostructures were assembled on a
NiAl(110) surface by thermally evaporating single Au atoms and CuPc molecules onto the surface (Figure 1).
When the STM tip was close enough
to the surface, it could be employed to
pull the Au atoms along the direction of
the moving tip. In this way, chains of
gold atoms were built along Ni troughs.
The authors made two chains of gold
Figure 1. CuPc@2 Au3 hybrid structures
a) Bare 2 Au3 junctions before the molecules
were added (imaging conditions: Vbias = 1 V,
I = 1 nA; image size is 47 ? by 47 ?).
b) Assembled hybrid structures (Vbias = 0.5 V,
I = 1 nA; these imaging conditions emphasize
the molecular adsorption configuration).
c) Representation of Au–CuPc–Au (metal–
molecule–metal) assembled on a NiAl(110)
surface. Adapted from reference [22] .
Angew. Chem. Int. Ed. 2004, 43, 5120 –5123
atoms separated by 5 Ni Ni lattice constants and then moved a CuPc molecule
into the space between them. When the
CuPc molecule was contacted to two Au
atoms, peaks in the dI/dV curves that
were ascribed to the CuPc were shifted
and split. When more Au atoms were
added on to each chain, the molecular
peaks did not shift further when compared to the peaks with just two Au
atoms. The Au atoms in contact with the
molecule can now be considered to be
part of an extended CuPc molecule.
Peaks in the dI/dV curves taken over the
last Au atom in the chain, shifted to
higher energies when compared to Au
chains that are not in contact with a
CuPc molecule. The Au atoms that were
in direct contact with the molecule
shifted out of resonance with the rest
of the Au chain. From these data, it was
illustrated to what extent the degree of
coupling between the CuPc and the
various Au chains could vary. This significant variation will have an important
role on the conductivity that will be
measured across these junctions. Thus,
from a chemistDs perspective, metal–
molecule–metal junctions should be regarded as extended molecular systems,
and the nature and type of contact is
going to be a dominant factor in the
rational design of molecular-scale devices.
The effects of contact to a molecule
have also been illustrated by Kubatkin
et al., in which electronic transport was
measured at 4 K through a single paraphenylene vinylene oligomer deposited
into a small (2 nm) gap on a semiconducting substrate that acts as a gate
electrode.[23] The gate voltage (Vg) of the
single-electron transistor (SET) with a
single molecule in the nanogap changed
in small steps from 4.3 V to + 4.3 V.
Source–drain I–V measurements were
taken at each step. Eight different open
states in the molecule were probed,
which is consistent with a molecule of
this type having a large number of
discrete states. However, the energies
of these electronic states obtained from
the data on this SET were strongly
perturbed compared to those obtained
from measurements on the molecule in
solution. It was suggested that image
charges in the source and drain electrodes could account for this perturbation.
Again, contact between a molecule and
a metal is not just a simple connection.
There are many other factors involved
when electronic behavior is investigated.
A second key issue in molecular
electronics is to relate electronic behavior to the structure of the molecule(s)
being probed. Chemists have always
sought to systematically vary properties
with structure to illustrate an optimal
molecular structure for a given behavior. However, in a still very-young field,
establishing structure–property relationships has an even more important
role. This variation can help support the
hypothesis that the behavior under observation is influenced by the structure
of the molecule and thus, the behavior is
attributable to the molecule.
A number of papers have reported
the systematic study of the conductance
of molecules of various lengths. These
studies almost always took place on
simple n-alkanethiolate SAMs[11, 19] or
on SAMs composed of various conjugated
molecules.[2, 10, 12, 14, 18, 21, 24–34] These studies established that the conductance varies
with molecular length when scaled with
the relative rate of a discrete electron
transfer through a similar molecular
bridge (e.g., the electron-transfer coefficient (b) value for a given molecular
architecture in which ln ket = br and r is
the distance between a donor and acceptor group).
The phenomenon of Kondo resonance has been exploited in two key
papers that credibly relate a unique
structural feature of a given molecule
to an electronic behavior that relies on
that feature. In the Kondo effect, the
spin state of an “impurity” (in this case,
a molecule) in a wire affects its conductance. Liang et al. demonstrated
Kondo resonance in a single-molecule
transistor by using a single divanadium
complex (“V2”) that could be tuned by
changing the gate voltage to alter the
spin and charge state of the molecule.[35]
A sharp dI/dV peak was observed when
the gate voltage was modulated to set
the V2+ molecules to an S = 1/2 spin
state. The peak was not observed when
the molecules were set to an S = 0 spin
state. Another feature of Kondo resonance is the splitting of dI/dV peaks by
an applied magnetic field when S = 1/2.
Park et al. reported Kondo resonance by
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
changing the degree of coupling of a
Co2+/3+ ion to the electrodes.[36] The
Co2+/3+ ion is bonded in an octahedral
environment to two terpyridinyl linker
molecules with thiol end groups, which
are placed between gold electrodes in a
break junction. A peak in the dI/dV
curves was observed in the Co2+ complex, which was also split by applying a
magnetic field. Thus, by selecting molecules that could exist in different spin
states under the influence of different
gate voltages, a link between a molecular property and an electronic property
of those molecules in a junction could be
Most recently, Mayor et al. have
documented how electronic transport
through a molecule is affected by its
structure.[37–40] Mechanical break junctions that contain two 9,10-bis(phenylethynyl)anthracene molecules were compared; the position of the thiol anchor
group was varied from para to meta
(Figure 2).
Figure 2. Representation of molecules containing a bis-9,10-phenyl-ethynylanthracene
core with a thiol linker in the meta (top) and
para (bottom) position inserted into a break
The lack of a fully conjugated pathway in the meta-linked molecule significantly reduces the electronic communication between the metal electrodes and
the molecule when compared to analogous data obtained with the molecule
linked by para-thiol groups. Specifically,
the immobilization of the molecule with
the thiol linker in the meta position
afforded I–V curves with currents that
were almost two orders of magnitude
smaller than the values measured for the
molecule in the para position. In addition, I–V curves measured on the metalinked molecule at 30 K showed steps in
the current curves, which were attributed to resonant tunneling through the
HOMO of the molecule.
There are many challenges that need
to be addressed before a viable molecular device can be fabricated. While this
task may seem daunting, the work
presented herein is revealing as to how
the structure of a molecule relates to its
electronic properties and how the binding of molecules to bulk metal electrodes influences their electronic properties. These results provide an intriguing step in the evolution of this field.
Published Online: September 16, 2004
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