Recently, molecular electronic devices have attracted attention. Building the device with a single molecule demands an understanding of the electron transport properties through single molecule junctions connecting two metal electrodes. While electron transport properties have been investigated for various molecules, the Au-S bond is commonly used to connect molecules to metal electrodes. In the present study, the effect of anchoring group on the electrical conductance of a single molecule was studied using di-substituted benzene [1].
The measurements were carried out in tetraethyleneglycol dimethyl ether solution containing 1 mM of the molecules. A Au or Pt STM tip was repeatedly moved into and out of contact with Au (111) or Pt(111) substrates. After breaking the metal contact, each molecule was terminated with two isocyanide, thiol, or cyanide end groups bridging the tip and substrate electrodes via the respective anchoring groups.
In the solution containing 1,4-diisocyanobenzene, the conductance of the Au nano contact decreased in a stepwise fashion with each step occurring at an integer multiple of 2x10-3~3x10-3G0 (G0=2e2/h). The corresponding histogram showed a feature at G0. This experimental results indicated that the conductance of a single Au/1,4-diisocyanobenzene/Au junction was 3x10-3G0, which was comparable to 4x10-3G0 of a single Au/1,4-benzenedithiol/Au junction. By replacing the metal electrode from Au to Pt, the conductance of the single molecular junction increased by ten times. The conductance of a single molecule depends on the bonding strength between the molecule and metal, energy difference of the molecular and metal orbital, and density of states of the metal electrodes. The conductance of the single molecular junctions are discussed by considering these three factors.
Reference:
[1]M. Kiguch, S. Miura, K. Hara, M. Sawamura, K. Murakoshi, Appl. Phys. Lett. 89 (2006) 213104.
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