RESEARCH NEWS
Interactions of Molecules with Metallic
Quantum Wires**
By Huixin He and Nongjian Tao*
Can you imagine how to detect a single molecule? Techniques, such as fluorescent spectroscopy and scanning
probe microscopy have become available for us to probe single molecules. Described in this article is a simple
approach based on atomically thin metallic wires. The interactions of even a single molecule with the wire may
cause a large change in the conductance and mechanical properties of the wire and can thus be used to detect
single molecules.
1. Introduction
Molecules
(a)
The growing activity in the interdisciplinary of nanometer
scale science and technology is driven by the ever-increasing
quest toward miniaturization of devices. As devices shrink
down to the nanoscale, quantum phenomena become more
pronounced, which may lead to novel applications.[1,2] This
research news focuses on an important example of quantum
phenomenaÐconductance quantization in atomically thin
metallic wires. Upon molecular adsorption onto the wires,
both the conductance and the mechanical properties of the
wire change sharply,[3,4] which may be used to detect molecules (Fig. 1a).
The conductance of a classical (macroscopic) metallic wire
is proportional to the cross sectional area and inversely proportional to the length of the wire. When decreasing the
length below the electron mean free path, electrons transport
through the wire ballistically (i.e., without any collisions). If,
in addition, the diameter of the wire is shrunk to the order of
the electron wavelength, the conductance does not change
continuously as the diameter anymore; on the contrary, it is
quantized according to[5]
(1)
G (G0)
2. Basic Principle
G = NG0
(b)
3
2
1
0
Elongation
length
0
5
10
Stretching (Å)
±
Fig. 1. a) Adsorption of a molecule onto the quantum nanowire may cause a
large change in the conductance and the stability. b) Typical conductance vs.
stretching distance traces that show the quantized variation in the nanowire
conductance. An important quantity, elongation length, is defined as the distance over which a nanowire can be stretch before its conductance jumps to a
lower conductance step.
[**] Acknowledgment is made to AFSOR (F49620-99-1-0112) and NSF
(CHE-9818073) for financial support.
where N is the number of the quantum modes and G0 = 2e2/h
~12.9 kX±1 is one quantum unit of conductance (e is the electron charge and h is Planck's constant). Since the wavelength
of conduction electrons in a typical metal is only a few angstrom, comparable to the size of an atom, a metallic nanowire
[*] Prof. N. J. Tao, Dr. H. He
Department of Electrical Engineering and
Center for Solid-State Electronics Research
Arizona State University
Tempe, AZ 85287 (USA)
E-mail: [email protected]
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H. He, N. J. Tao/Interactions of Molecules with Metallic Quantum Wires
with conductance quantized at the lowest steps must be atomically thin. This conclusion has been directly confirmed by
high- resolution transmission electron microscopy images that
the narrowest portion of a quantum wire consists of a string of
atoms.[6] Conductance quantization was first clearly demonstrated in semiconductor devices containing a two-dimensional electron gas at low temperature.[7,8] The energy separation in a metallic quantum wire is much larger than room
temperature thermal energy, making it easy to observe the
quantum phenomenon at room temperature.
We note that the above simple picture holds nicely for alkali
metals, and approximately for monovalent transition metals,
such as Au. However, recent experimental evidence shows
that in general the conductance quantization behavior depends on the chemical valence of the metal and each quantum
step may significantly deviate from 1G0.[9]
trons impinge on the surface.[17] For a macroscopic metal wire,
molecule adsorption influences only over a distance of a few
angstrom from the surface. However, for an atomically thin
metal wire, adsorption of even a single molecule onto the wire
is expected to drastically change the conductance, which has
been recently studied experimentally by us[4,13] and theoretically by Landman et al.[18]
The quantum wires used in the experiment were fabricated
by the electrochemical method described above. The conductance was initially stabilized at a chosen quantum step and the
subsequent conductance was monitored after exposing the
wire to sample molecules. The inset of Figure 2 is an example
that shows a drastic decrease in the conductance a few seconds after exposing the wire to 1 mM mercaptopropionic acid
3. Fabrication of Metallic Quantum Wire
Such small metallic wires cannot be easily fabricated by
conventional fabrication techniques. Metallic wires with a diameter as small as 10 nm have been fabricated using etched
ion tracks in membranes or in thin mica sheets as templates.
Recently Penner's group has fabricated Mo nanowires using
sharp atomic steps on graphite as templates.[10] Conductance
quantization cannot be observed in these wires because a
10 nm diameter is still much greater than the electron Fermi
wavelength (a few Š) of typical metals. One method that has
been successfully used to create metallic wires with quantized
conductance is to separate two electrodes in contact mechanically with a scanning tunneling microscope (STM)[11] or
breaking junction setup.[12] During the separation process, a
metal neck is formed between the electrodes, which is
stretched into an atomically thin wire. The conductance during the stretching process changes in a stepwise fashion, as
shown in Figure 1b, which has been taken as strong evidence
of conductance quantization. Recently, we have developed an
electrochemical method to fabricate stable nanowires.[13±15]
The method started with pairs of Au microelectrodes supported on oxidized Si substrates. The two electrodes in each
pair were separated with a small gap (20±100 nm), which was
bridged by electrochemical deposition of Cu or Au into the
gap. The deposition was monitored continuously by measuring the conductance between the two electrodes and controlled until a desired conductance was reached with a feedback circuit. A similar approach was used to fabricate
electrodes separated with atomically scale gaps.[16]
4. Effects of Molecular Adsorption on the
Conductance of the Quantum Wires
It is well known that a conductor changes its conductance
upon molecular adsorption onto its surface, due to the scattering of the conduction electrons by the adsorbates as the elec-
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Fig. 2. MPA-, 2,2¢-bipyridine-, and dopamine-induced conductance changes of
Cu quantum wires with conductance at various quantum steps. The changes are
consistent with the relative binding strengths of the three molecules. Due to the
rapid decay in the changes, a logarithmic scale is used. Inset: Conductance
response of Cu nanowires with conductance quantized at 1G0 upon MPA
adsorption from the electrolyte.
(MPA). The conductance of quantum wires stabilized at higher quantum steps also decreases upon molecular adsorption,
but the relative change (DG/G) decreases as the step increases (Fig. 2). At very high steps, the ballistic transport is
eventually replaced by the classical diffusive transport. So it is
clear that quantum wire at the lowest quantum step gives the
highest sensitivity, which is not surprising because it is atomically thin. The adsorbate-induced conductance change
depends on the adsorption strengths of the molecules to the
quantum wires. For the three molecules (MPA, 2,2¢-bipyridine, and dopamine) studied in the literature,[4] MPA induces
the largest change, dopamine the least and 2,2¢-bipyridine in
between, which are consistent with the relative interaction
strengths of the three molecules to the metal wire.
Although molecular adsorption usually results in a decrease
in the conductance as shown in the inset of Figure 2, large
fluctuations in the conductance are sometimes observed upon
molecular adsorption, which may be contributed to the rearrangement of the atomic configuration of the quantum wires
induced by the adsorption.[4,13] It is known that strong binding
of a molecule with a surface atom can significantly weaken
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H. He, N. J. Tao/Interactions of Molecules with Metallic Quantum Wires
5. Effects of Molecular Adsorption on the Stability
of the Quantum Wires
An important quantity that provides information on the stability of the quantum wires is the elongation length, defined
as the distance over which a wire can be elongated before its
conductance jumps to another step (Fig. 1b). We determined
the elongation length using an electrochemical STM setup.
Although the setup cannot fabricate long-term stable quantum wires, it can quickly form (and destroy) thousands of
quantum wires, and is, therefore, ideal for a detailed statistical
analysis. The electrochemical potential allows us to control
the amount of adsorption in a flexible and reversible fashion.
We chose three molecules, 2,2¢-bipyridine, adenine, and MPA
for this study because they all adsorb on Au surfaces but with
different binding strengths.[19±21]
In the absence of molecular adsorbates, the elongation
length is about 1.2 Š, in good agreement with previous studies in controlled gas environment and in vacuum.[22,23] The
length is largely independent of the potential, so electrochemical charging of the metal wires has little effect on the elongation length. Figure 3 plots the average elongation length of
the wires with conductance at 1G0 in the presence of adenine.
At very low potentials, the elongation length is similar to that
adsorbed on might be even greater. The large increase in the
elongation length was actually also noticed during the experiment. We found that the nanowires were too ªstickyº to be
easily broken when raising the potentials above 0 V in concentrated adenine solutions (0.3 mM and 3 mM adenine).
Similar to adenine, the presence of MPA and 2,2¢-bipyridine
also increases the average elongation length. The amount of
the increase, however, varies according to the relative interaction strength of each molecule to Au electrode. The adsorbate-induced increase in the elongation length shows a strong
dependence of the mechanical stability of the metallic quantum wires on molecular adsorption. Computer simulations[24]
and force measurements[22] have shown that each stepwise
change in the conductance during elongation of a metallic
quantum wire is always correlated to a mechanical force relaxation associated with abrupt atomic rearrangements. The
atomic rearrangements have been attributed to the transitions
between the so-called stable magic atomic configurations,
originating from quantum confinement of the electrons in the
wires.[25,26] So the dependence of the elongation length can be
traced to the influence on magic atomic configuration of the
quantum wires by the molecules. This effect may be used to
fabricate wires with desired atomic configurations by introducing appropriate molecules.
In summary, the sensitive dependence of the quantum wire
conductance on molecular adsorptions suggests the possibility
of detecting even a single molecule with the quantum wires.
In addition to possible applications, metallic quantum wires
are also attractive systems for us to study various physical and
chemical phenomena on the nanometer scale because of the
following reasons. First, they can be fabricated either mechanically or electrochemically without sophisticated facilities. Each wire is naturally connected to thicker wires that
are ready to be attached to measuring instruments. This is important because many nanostructures fabricated chemically
or electrochemically need to be wired to an external circuit,
which has often been a difficult task. Finally, the conductance
quantization phenomenon in metallic quantum wires is robust. It can be observed and studied at room temperature, in
vacuum and in electrochemical cells, allowing a variety of
chemical processes on/in the quantum wires to be studied.
±
Fig. 3. Average elongation length of Au quantum wires with conductance at the
lowest quantum step as a function of potential in 0.1 M NaClO4, 0.3 mM adenine + 0.1 M NaClO4, and 3 mM adenine + 0.1 M NaClO4.
in the supporting NaClO4 electrolyte, which is expected since
no adenine adsorption occurs at low potentials. Increasing the
potential prompts adsorption of adenine onto the wires, and
the elongation length increases sharply. In 3.3 mM adenine,
the elongation length increases from ~1.2 to ~4.5 Š! We note
this number is averaged over many wires, including those that
have no chance to interact with molecules before breaking, so
the actual elongation lengths for those that do have molecules
Adv. Mater. 2002, 14, No. 2, January 16
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RESEARCH NEWS
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