Single molecule electronics is a research field focusing on the study of
electron transfer through single molecules.
One of the long-term goals is to develop devices with functional units
defined by the single molecule. This is the ultimate limit for miniaturization
of molecular electronics.1
During the last 10 -15 years the field has been focused on basic
understanding of electron transport through single molecules and the development
of appropriate test-beds.2-7
The current challenges include development of methods for integration of
multiple single molecule components in a reproducible way. Control, with atom
precision, of the interface geometries between molecule and electrode is a key
to success in this area.8
Self-Assembled Electrodes with a Single Molecule Incorporated?
During the last decades, top-down lithographic techniques has been enormously
refined - but despite this - the methods are still far away from mass
fabrication of identical copies of electrode nanogaps on the 1-2 nm length
scale. Gaps with single molecules in them are even harder to fabricate.
In contrast to top-down fabrication techniques, self- assembly methods rely
on the use of intermolecular forces that typically operate on the sub-nanometer
length scale. In an attempt to bridge the gap between the molecular length scale
and the capabilities of top down lithography -scientist at the Nano-Science Center at the
University of Copenhagen have developed a method where gold electrodes are
grown from preassembled gold nanoparticle seeds by solution based
In brief, the method involves a two-step process where first a single
molecule is used to link two nanoparticle seeds together to form dimers. In a
second step, the dimers are exposed to a gold salt, a surfactant and a mild
reduction agent. At the right reaction conditions the gold nanoparticle seed
will grow to form single crystal gold nanorods (Figure 1). By tuning the
reaction conditions, the length of the rods can be controlled from 20 to 500 nm
length10 -a length scale that is much easier to
contact with top -down lithographic techniques11.
An attractive aspect of this method is that it may be possible to fabricate
multiple single molecule devices.9
|Figure 1. Chemically
grown gold nanorods are used to contact single molecule.9 Image Courtesy of Titoo
Well-Defined Contacts from Chemical Design
The interface between metal electrodes and molecule is of paramount
importance for the nature of the electron transport through single molecules. If
the coupling between molecule and electrode is strong -the electrons tunnel
directly through the molecule. If the coupling is weak, the electron transport
is a two-step process where the electron will reside on the molecule as a part
of the electron transport from source to drain electrode. The weak coupled
transport is called Coulomb blockade and can be used to construct single
electron transistors.8, 12
In an attempt to develop a better control the interface geometry -the
Scientists at the Nano-Science
Center at the University of Copenhagen have designed molecules that
incorporate fullerene (C60 molecules) in the contact region between
electrode and the molecule of interest. The size and electronic structure of
fullerene allows for a larger contact area and stable chemical contact between
molecule and electrode and thus allowing for stable device measurements -even at
Figure 2. Interface between molecule
and electrodes by the use of fullerene anchoring groups.13
During the last decades the field of molecular electronics has been focused
on basic understanding of electron transport through single molecules and the
development of appropriate test-beds. These experiments have promoted an
understanding of the intriguing interplay between molecular structure, molecular
energy levels and interface geometry - all factors that determine the electron
transport through single molecules.8
The technology is still far away from being able to fabricate multiple
devices with single molecule components. The development of new molecular
structures with better defined contact between molecule and electrode13 together
with new self-assembly methods are important steps towards future development of
integrated devices with multiple single molecule components.9, 11
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et al. Single-electron transistor of a single organic molecule with access to
several redox states Nature 425, 698-701 (2003).
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state three-terminal devices: Status and challenges for molecular electronics
with single molecules" Nature Nanotech. 4 (9), 551-556, (2009).
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Bjørnholm, T. "Self-assembled Nanogaps for Single-Molecule Electronics" ACS
Nano, , 3 (4), 828-834, (2009).
10. Gao, J.; Bender, C. M.;
Murphy, C. J., Dependence of the Gold Nanorod Aspect Ratio on the Nature of the
Directing Surfactant in Aqueous Solution. Langmuir, 19, 9065-9070 (2003).
11. Tang, Q., Tong, Y., Jain, T., Hassenkam,T., Wan, Q.,
Moth-Poulsen, K. and Bjørnholm, T. "Self-assembled Nanogaps for Single-Molecule
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based anchoring groups for molecular electronics J. Am. Chem. Soc. 130,
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Thomas Bjørnholm (Nano-Science Center, University of Copenhagen)