Single Molecular Conductance Studied
by “Point Contact Method” using Scanning Tunnelling Microscopy
Jiro Sasaki1, Yoshitaka Okutsu1, Shoji Yoshida1, Satoshi Yasuda2, Tohru Nakamura2,Osamu Takeuchi1 and Hidemi Shigekawa1
1Inst. of Appl. Phys., University of Tsukuba, 21stCOE, CREST, Tsukuba 305-8573, Japan
2
NRI, AIST, Tsukuba Ibaraki 305-8565, Japan

Precise measurement of single molecule (a)(b)
conductivity has been one of the major focuses inmolecular electronics. To realize a single moleculeconductivity measurement, reliable fabrications ofsingle molecular junction, in which a molecule ischemically connected with two electrodes to reduce
contact resistance, has been the key point in recent Fig. 1. (a) STM image of isolated 3TS2research. Here, we propose a new reliable molecules (bright spots) embedded in afabrication method of single molecular junction to C8 film (75nm? 75nm). (b) Schematic ofform a low contact resistance by the combination of the “point contact method” by STM.
scanning tunnelling microscopy (STM) withself–assembled monolayer (SAM) technique. As a demonstration, we measured theconductance of bisthiol- and biselenol-terthiophene (3TS2, 3TSe2) molecules. By immersion
of a Au(111) substrate into a mixed solution containing 3TS2 and Octanethiol (C8) molecules,a C8 film, in which isolated 3TS2 molecules were embedded, was formed as shown Fig .1(a).
For STM observation, we first placed a Au STM tip over a single 3TS2 molecule, andsubsequently the tip was brought to 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 bonding of the thiol group with the Au tip (Fig. 1(b)). After the bonidng being
formed, I-V measurement was performed several times to measure the conductance. All
measurements were performed under UHV condition at room temperature. Figure 2(a) showsan I-Z curve on a 3TS2 molecule. Abrupt jump in tunnelling current was observed in the caseof a single 3TS2 molecule. It is strongly suggested that the observed current jump in the caseof 3TS2 molecule is caused by the formation of a Au tip-S chemical bonding which provides
a single molecular junction. Fig. 2(b) shows all I-V curves of 3TS2 molecules obtained fromsingle molecular junctions. The four bundles in Fig. 2(b) correspond to the conductance peaks
shown in Fig. 2(c). Fig. 2(d) shows the histogram of 3TSe2 molecules, which shows distinct
two peaks. Each of the peaks is obtained even for a single molecule, suggesting that theAu-S/Se-bonding has several states. Since we can directly fabricate a single molecularjunction with confirmation of the characteristic of the target molecules by STM, we believethat our “point contact method” is the reliable and desired method to realize and analyze thesingle molecular conductance with bond fluctuation.
(http:/dora.bk.tsukuba.ac.jp)
(a)
(b)
(c)
(d)
1.0

300
200
10000.0

0.2

0.4

Relative ratio

1.0400Relative ratio

Current (nA)

500

0.5

0.0

0.0

0.6nm

0Z (nm)0.5

50100150Conductance (nS)0-50-100-150

Conductance (nS)-200

Fig. 2. (a) I-Z curve obtained from a 3TS2 molecule. (b) All I-V curves. (c), (d) Histograms ofconductance obtained from 3TS2, 3TSe2 molecules.
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