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      Revealing the molecular structure of single-molecule junctions in different conductance states by fishing-mode tip-enhanced Raman spectroscopy

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          Abstract

          The conductance of single-molecule junctions may be governed by the structure of the molecule in the gap or by the way it bonds with the leads, and the information contained in a Raman spectrum is ideal for examining both. Here we demonstrate that molecule-to-surface bonding may be characterized during electron transport by 'fishing-mode' tip-enhanced Raman spectroscopy (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature. Density functional theory calculations reveal that the most significant spectral change seen for a gold- 4,4′-bipyridine- gold junction results from the deformation of the pyridine ring in contact with the drain electrode at high voltage, and these calculations suggest that a stronger bonding interaction between the molecule and the drain may account for the nonlinear dependence of conductance on bias voltage. FM-TERS will lead to a better understanding of electron-transport processes in molecular junctions.

          Abstract

          The conductance of single-molecule junctions is affected by the structure of the molecule and how it is bound to the electrodes, which may be examined using Raman spectroscopy. Liu et al. have developed 'fishing-mode' tip-enhanced Raman spectroscopy, which allows the simultaneous determination of conductance and Raman spectra.

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          Most cited references17

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          Measurement of single-molecule resistance by repeated formation of molecular junctions.

          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 (betaN) for N-alkanedithiols was 1.0 +/- 0.1 per carbon atom and was weakly dependent on the applied bias. The resistance and betaN values are consistent with first-principles calculations.
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            Reproducible measurement of single-molecule conductivity.

            A reliable method has been developed for making through-bond electrical contacts to molecules. Current-voltage curves are quantized as integer multiples of one fundamental curve, an observation used to identify single-molecule contacts. The resistance of a single octanedithiol molecule was 900 +/- 50 megohms, based on measurements on more than 1000 single molecules. In contrast, nonbonded contacts to octanethiol monolayers were at least four orders of magnitude more resistive, less reproducible, and had a different voltage dependence, demonstrating that the measurement of intrinsic molecular properties requires chemically bonded contacts.
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              High-Resolution Near-Field Raman Microscopy of Single-Walled Carbon Nanotubes

              We present near-field Raman spectroscopy and imaging of single isolated single-walled carbon nanotubes with a spatial resolution of approximately 25 nm. The near-field origin of the image contrast is confirmed by the measured dependence of the Raman scattering signal on tip-sample distance and the unique polarization properties. The method is used to study local variations in the Raman spectrum along a single single-walled carbon nanotube.
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                Author and article information

                Journal
                Nat Commun
                Nature Communications
                Nature Publishing Group
                2041-1723
                May 2011
                10 May 2011
                : 2
                : 305
                Affiliations
                [1 ]simpleState Key Laboratory of Physical Chemistry of Solid Surfaces and Key Laboratory of Analytical Sciences, College of Chemistry and Chemical Engineering, Xiamen University , Xiamen 361005, China.
                Author notes
                Article
                ncomms1310
                10.1038/ncomms1310
                3112534
                21556059
                653bd5ef-cf2a-47ba-bd23-3984cb81b0cb
                Copyright © 2011, Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.

                This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/

                History
                : 23 November 2010
                : 13 April 2011
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