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 self-assembly.9
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 Jain
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 room temperature.13
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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9. Jain, T., Westerlund, F., Johnson, E., Moth-Poulsen, K. and 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 Electronics" Nanotechnology 20 (24), 245205, (2009).
12. Danilov, A.V. et al. Electronic transport in single molecule junctions: Control of the molecule-electrode coupling through intramolecular tunneling barriers Nano Lett. 8, 1-5 (2008).
13. Martin, C.A. et al. Fullerene based anchoring groups for molecular electronics J. Am. Chem. Soc. 130, 13198-13199 (2008).
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