Measurement of Single Molecular Conductanceby “Point Contact Method” with Scanning Tunnelling Microscopy
Jiro Sasaki1, Satoshi Yasuda1, Shoji Yoshida1, Yoshitaka Okutsu1, Tohru Nakamura2, Osamu
Takeuchi1 and Hidemi Shigekawa1
1

Institute of Applied Physics, University of Tsukuba, 21stCOE, CREST-JST,Tsukuba Ibaraki 305-8573, Japan
2
NRI, AIST, Tsukuba Ibaraki 305-8565, Japan
http://dora.ims.tsukuba.ac.jp/

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frequency(b)Current (nA)(a)Current (nA)

Precise measurement of single molecule (a)(b)
conductivity has been one of the major focuses inmolecular electronics. To realize the single-moleculeconductivity measurement, reliable fabrication of
single molecular junction, in which a molecule ischemically connected with two electrodes to reduce
contact resistance, has been the key point in recentresearch. Here, we propose a new reliable fabrication Fig. 1. (a) STM image of isolated 3TS2
method of single molecular junction to form a low molecules (bright spots) embedded in acontact resistance by the combination of scanning C8 film (75nm×75nm). (b) Schematic oftunnelling microscopy (STM) with self–assembled the “point contact method” by STM.
monolayer (SAM) technique. As a demonstration, wemeasured the conductance of α, ω-bisacetylthio-terthiophene (3TS2) molecule, which has thiolgroups at both ends. By immersion of a Au(111) substrate into mixed solution containing 3TS2 andOctanethiol (C8) molecules, a C8 film, in which isolated 3TSe molecules were embedded, wasformed as shown Fig.1(a). For STM observation, we first placed a Au STM tip over a single 3TS2
molecule, and subsequently the tip was brought contact with the molecule. Since the thiol group of
3TS2 molecule is exposed toward the STM tip, a single molecular junction can be formed, by thechemical connection of the thiol group with the Au tip (Fig. 1(b)). After the connection being formed,
I-V measurement was performed several times to measure the conductance. All measurements wereperformed under UHV condition at room temperature. Figures. 2(a) and 2(b) show the I-Z curvesobtained on C8 and 3TS2 molecules, respectively. Tunnelling current increased exponentially when
the STM tip was approached to the C8 film area, in contrast, abrupt jump in tunnelling current wasobserved in the case of a single 3TS2 molecule. It is strongly suggested that the observed current
jump in the case of 3TS2 molecule is caused by the formation of a Au tip-S chemical bonding which
provides a single molecular junction. It was also found that the current observed for a 3TS2 molecule
was insensitive to further approach of the STM tip, about 0.2nm after the formation of the single
molecular junction, as shown in Fig.2 (b), indicating that the change of tip-molecule distance had little
influence on the conductivity measurement. Conductance histogram calculated from I-V curvemeasurements is shown in Fig.2(c), where one obvious peak is shown at 496 ± 256 nS, which is thevalue derived from single 3TS2 molecules. Since we can directly fabricate a single molecular junction
with confirmation of the target molecule by STM, we believe that our “point contact method” is thereliable and desired method to realize the measurement of single molecular conductance.
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Fig. 3. I-Z curves obtained for (a) C8 and (b) 3TS2 molecules, respectively. (c) Histogramof conductance from the ohmic region (±0.1V) of I-V curves obtained for 3TS2 molecules.
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