ScienceWeek

CHEMICAL PHYSICS: ELECTRICAL RESISTANCE OF SINGLE MOLECULES

The following points are made by B. Xu and N.J. Tao (Science 2003 301:1221):

1) Wiring individual molecules into an electronic circuit is an exciting idea that has been pursued actively by many groups (1). Although recent advances have been impressive (2–5), a basic question that remains a subject of debate is what is the resistance of a simple molecule, such as an alkane chain, covalently attached to two electrodes? Large disparities have been found between different experiments, which reflects the difficulty of forming identical molecular junctions. Even if the resistance of a molecular junction is reproducibly measured, ensuring that the resistance is really due to a single molecule is another substantial challenge.

2) For molecules with a metal redox center, a single-electron charging effect has been used as a signature of single-molecule measurement (5). For many other molecules, a different signature is required. Cui et al (2001) reported a conducting atomic force microscope (AFM) method to measure the resistance of octanedithiol that has one end of the molecule anchored to a gold substrate and the other end attached to a gold nanoparticle. In that work, the molecular junction was measured hundreds of times so that statistical analysis could be performed. However, the procedure involves several elaborate assembly steps, and the measured resistance is complicated by a Coulomb blockade effect due to finite contact resistance between the AFM probe and the gold nanoparticle.

3) The authors report a simple and unambiguous measurement of single-molecule resistance, achieved by repeatedly forming thousands of molecular junctions in which molecules are directly connected to two electrodes. The authors created individual molecular junctions by repeatedly moving a gold scanning tunneling microscope (STM) tip into and out of contact with a gold substrate in a solution containing the sample molecules (4,4' bipyridine and N-alkanedithiols). 4,4' bipyridine, a heterocyclic molecule, has two nitrogen atoms on its two ends that can bind strongly to gold electrodes to form a molecular junction. During the initial stage of pulling the tip out of contact with the substrate, the conductance decreased in a stepwise fashion, with each step occurring preferentially at an integer multiple of conductance quantum G(sub0) = 2e^(2)/h, where (e) is the electron charge and (h) is Planck's constant.

4) In summary: The conductance of a single molecule connected to two gold electrodes was determined by repeatedly forming thousands of gold-molecule-gold junctions. Conductance histograms revealed well-defined peaks at integer multiples of a fundamental conductance value, which was used to identify the conductance of a single molecule. The resistances near zero bias were 10.5 +-0.5, 51 +- 5, 630 +- 50, and 1.3 +- 0.1 megohms for hexanedithiol, octanedithiol, decanedithiol, and 4,4' bipyridine, respectively. The tunneling decay constant (beta-N) for N-alkanedithiols was 1.0 +- 0.1 per carbon atom and was weakly dependent on the applied bias. The resistance and beta-N values are consistent with first-principles calculations.

References (abridged):

1. A. Aviram, M. Ratner, Chem. Phys. Lett. 29, 277 (1974)

2. J. Chen, M. A. Reed, A. M. Rawlett, J. M. Tour, Science 286, 1550 (1999)

3. D. I. Gittins, D. Bethell, D. J. Schiffrin, R. J. Nichols, Nature 408, 67 (2000)

4. C. P. Collier et al., Science 285, 391 (1999)

5. J. Park et al., Nature 417, 722 (2002)

Science http://www.sciencemag.org

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KONDO RESONANCE IN A SINGLE-MOLECULE TRANSISTOR

The following points are made by W. Liang et al (Nature 2002 417:725):

1) When an individual molecule(1), nanocrystal(2-4), nanotube(5), or lithographically defined quantum dot is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization. As the coupling to the electrodes increases, higher-order tunneling and correlated electron motion give rise to new phenomena, including the Kondo resonance. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry.

2) The authors prepared devices by an extension of the methods previously used in constructing single-C60 (ref. 1) and single-nanocrystal transistors(3). Using electron-beam lithography, a narrow gold bridge was fabricated on an aluminum pad with a 3-nm oxide layer serving as a gate electrode. The electromigration-induced break-junction technique(1,3) was then used to create two closely spaced gold electrodes. Scanning electron microscope imaging and tunnel current measurements reveal that the narrowest gap between the two electrodes is consistently 1 nm.

3) In summary: The authors report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule serves as a spin impurity. The authors find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV. The authors suggest the present study demonstrates that molecules can provide a Kondo system where critical parameters of Kondo physics, such as the spin and orbital degrees of freedom, are defined by chemical synthesis. With the recent advances of synthetic methodology, the preparation of molecular clusters possessing adjustable magnetic properties is becoming feasible. Future investigations of such species are expected to provide detailed insight into electron transport through a molecular system where the spin and orbital degeneracies are precisely controlled.

References (abridged):

1. Park, H. et al. Nano-mechanical oscillations in a single-C60 transistor. Nature 407, 57-60 (2000)

2. Klein, D. L. et al. A single-electron transistor made from a cadmium selenide nanocrystal. Nature 389, 699-701 (1997)

3. Park, H. et al. Fabrication of metallic electrodes with nanometer separation by electromigration. Appl. Phys. Lett. 75, 301-303 (1999)

4. Banin, U., Cao, Y., Katz, D. & Millo, O. Identification of atomic-like electronic states in indium arsenide nanocrystal quantum dots. Nature 400, 542-544 (1999)

5. Tans, S. J. et al. Individual single-wall carbon nanotubes as quantum wires. Nature 386, 474-476 (1997)

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