Published on Web 05/27/2006

Bond Fluctuation of S/Se Anchoring Observed in Single-Molecule
Conductance Measurements using the Point Contact Method with Scanning
Tunneling Microscopy
Satoshi Yasuda,† Shoji Yoshida,† Jiro Sasaki,† Yoshitaka Okutsu,† Tohru Nakamura,‡
Atsushi Taninaka,† Osamu Takeuchi,† and Hidemi Shigekawa*,†
Institute of Applied Physics, CREST-JST, 21st Century COE, UniVersity of Tsukuba, Tsukuba 305-8573, Japan, andNational Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
Received March 26, 2006; E-mail: hidemi@ims.tsukuba.ac.jp

Understanding and controlling the characteristics of the electriccontact between a molecule and an electrode has become one of
the major objectives in recent research.1-3 For example, stochastic
switching of wired molecules exhibits interesting but puzzling
phenomena, which is a hotly debated issue.4-6 Additionally, theeffect of the different anchoring groups and their adsorption sites
on the electric contact is another urgent issue to be clarified.7 To
achieve further advances, precise measurement of the electronicproperties of a specific single molecule, which is wired to metalelectrodes, is extremely important.
Here, we show the results obtained using the point contact
method with scanning tunneling microscopy (STM) that enables
us to selectively perform a repeated analysis of a chosen target
molecule.
We used R,ω-bis(acetylthio)terthiophene (3TS2) and R,ω-bis(acetylseleno)terthiophene (3TSe2) molecules as illustrated in Figure
1a as samples because of their high potential for application inmolecular devices.8,9 Isolated 3TS2 or 3TSe2 molecules embedded
in a self-assembled monolayer (SAM) of octanethiol (C8) molecules
were prepared by immersing a Au(111) substrate into a solution
containing a predefined amount of 3TS2 (or 3TSe2) and C8molecules. (Free thiol and selenol groups were prepared by adding
pyrrolidine molecules.) All conductance measurements were performed in ultrahigh vacuum (UHV) at room temperature. In UHV,
contamination of the sample surface is prevented, and the STM tip
can be cleaned and fabricated by heat treatment.
Figure 1b shows an STM image of isolated 3TS2 molecules
embedded in a C8 SAM, where bright spots are 3TS2 molecules.
After we located target molecules, we placed a Au STM tip above
a 3TS2 molecule and moved it toward the chosen molecule until
an electric contact was achieved, where the sample bias voltage
was kept at 0.5 V during the process. Since the thiol group at one
end of the isolated 3TS2 molecule is exposed to the STM tip, achemical bond is formed between the thiol group and the Au tip(Figure 1c).
Graphs a and b of Figure 2 show plots of the I-Z curves obtainedfor a C8 molecule and a 3TS2 molecule, respectively. The
exponential change in the current exhibits two different slopes; seeinset in Figure 2a. This is explained by the change in the local
barrier height caused by the contact of the STM tip with the C8molecule. In contrast, an abrupt jump in the current was observed
for the 3TS2 molecule (Figure 2b) because of the formation of achemical bond between the Au tip and the 3TS2 molecule, as shownin Figure 1c. The current observed for the 3TS2 molecule wasinsensitive to further approach of the STM tip (∼0.2 nm) after theformation of the electric contact, indicating that a slight change in† University of Tsukuba.
‡ National Institute of Advanced Industrial Science and Technology.

7746 9 J. AM. CHEM. SOC. 2006, 128, 7746-7747

Figure 1. (a) Molecular structure. (b) An STM image of isolated single
3TS2 molecules embedded in a C8 SAM (sample bias +1.0 V, tunnelingcurrent 10 pA). (c) Measurement conformation by the “point contactmethod” using STM.

Figure 2. I-Z curves obtained for (a) C8 and (b) 3TS2 molecules. Inset in(a) is the logarithmic plot of the I-Z curve. Tip approach position is related
to the set point (sample bias +1.0 V, tunneling current 10 pA).

the molecular conformation has little influence on the subsequent
conductance measurement.1
After a junction was formed, the bias voltage was swept
repeatedly between -1 and +1 V, and the corresponding change
in the current for each sweep was measured. The I-V curves arealmost linear between -0.1 and +0.1 V, as shown in Figures 3and 4, and the conductance of each measurement was determinedfrom the gradient of each I-V curve in this voltage region.
In Figure 3a, all of the I-V curves obtained from more than
400 individual measurements are presented as a contour plot, whereseveral measurements were performed for each molecule. Contrastof the overlapped I-V curves in the low bias voltage region clearlyshows the existence of several groups of I-V curves. Since theI-V curves were observed over the different 3TS2 molecules, theobtained result represents the characteristic response of thismolecule, including the electric contact with the electrodes.
Figure 3b is a histogram consisting of all the values ofconductance determined from the I-V curves in Figure 3a. In
accordance with the groups in Figure 3a, the histogram consists ofseveral subpeaks, as indicated by arrows. To examine the details,
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Figure 3. Results obtained for the 3TS2 molecule. (a) All I-V curves
obtained for the single 3TS2 molecules. (b) Histogram consisting of allvalues of conductance obtained form all I-V curves in (a). Peaks indicatedby the arrows are at 70, 220, 440, and 620 nS. (c) I-V curves sequentiallyobtained for 20 cycles, and (d) conductance obtained from the I-V curves.
Flat area in (c) at high bias voltage region is due to saturation of the current.
(e) Two sequential I-V curves corresponding to the 10th and 11th
measurements in (d).

the off-states in the stochastic switching process that have been
observed for various molecules.4,5 The fluctuation occurs in boththe positive and negative bias voltage regions. Since conformationalchange, which, for example, is induced for the azo-molecule becauseof the interaction between the applied electric field and themolecular dipole moment,10 does not exist for this molecule, therole of the current in the production of such phenomena as inelastic
tunneling and electro-migration processes may be essential to thecase.6,11,12
A possible mechanism to explain the observed change in theconductance is the thiol-gold bond fluctuation among the adsorption sites, which has been discussed in recent papers.7,13 It is
important to note that the relative relationship among the observed
values of the conductance is comparable with that predicted by thetheoretical analysis.7
A similar experiment was performed for a single 3TSe2 molecule.
Figure 4a shows all the I-V curves obtained for the single 3TSe2molecules. Figure 4b is a histogram of all the values of conductanceobtained from the I-V curves in Figure 4a. In contrast to the caseof the 3TS2 (Figure 3b), two dominant peaks are apparent in thelow conductance region for the 3TSe molecule. This difference isconsidered to reflect the difference in the stability of the S/Se
anchoring on the Au surface.
In summary, we have shown the results of single-molecule
conductance measurements that enable us to selectively perform arepeated analysis of a chosen target molecule. Bond fluctuationsof the S/Se anchoring, which may explain the stochastic switchingobserved in previous works,4,5 were directly observed at the singlemolecule level. As mentioned earlier, understanding and controllingthe characteristics of the electric contact between a molecule andan electrode is the major challenge. The method outlined is expected
to contribute to the understanding of single-molecule electronics
and to the future development of molecular devices.
Acknowledgment. This work was supported by a Grant-in-Aidfor Scientific Research from the Ministry of Education, Culture,Sports, Science and Technology of Japan. We thank Ms. Rie
Yamashita for her help preparing this paper.
References

Figure 4. All I-V curves (a) and the conductance histogram (b) obtainedfor 3TSe2 molecules, respectively. Peaks indicated by the arrows are at 10and 120 nS.

I-V curves were sequentially measured for 20 cycles on the same
3TS2 molecule (Figure 3c), and the obtained values of conductanceare plotted as a function of the number of bias voltage sweeps(Figure 3d). Figure 3c shows smooth slopes in the low bias voltage
region, where stable measurement can be obtained. As seen inpanels c and d of Figure 3, there are three or four groups of different
conductance, the values of which correspond to the peaks in thehistogram of Figure 3b.
What causes the instability of the junction? Figure 3e shows twosequential I-V curves corresponding to the 10th and 11th measurements in Figure 3d. In the high bias voltage region, there appear tobe rapid changes in the current between the two envelopes of theI-V curves extrapolated from those in the low bias region. Thisfluctuation between the two states resulted in the observed change
in the conductance. In some cases, for a high voltage region, thecurrent measured was found to be zero, which can correspond to(1) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore,A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001,294, 571-574.
(2) Lindsay, S. M. J. Chem. Educ. 2005, 82, 727-733.
(3) Xu, B.; Tao, N. J. Science 2003, 301, 1221-1223.
(4) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell,J. D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.;
Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303-2307.
(5) Ramachandran, G. K.; Hopson, T. J.; Rawlett, A. M.; Nagahara, L. A.;
Primak, A.; Lindsay, S. M. Science 2003, 300, 1413-1416.
(6) Pitters, J. L.; Wolkow, R. A. Nano Lett. 2006, 6, 390-397.
(7) Ke, S.-H.; Baranger, H. U.; Yang, W. J. Am. Chem. Soc. 2004, 126,15897-15904.
(8) Zhitenev, N. B.; Meng, H.; Bao, Z. Phys. ReV. Lett. 2002, 88, 2268011-226801-4.
(9) Xu, B. Q.; Li, X. L.; Xiao, X. Y.; Sakaguchi, H.; Tao, N. J. Nano Lett.
2005, 5, 1491-1495.
(10) Yasuda, S.; Nakamura, T. Matsumoto, M.; Shigekawa, H. J. Am. Chem.
Soc. 2003, 125, 16430-16433.
(11) Komeda, T.; Kim, Y.; Kawai, M.; Persson, B. N. J.; Ueba, H. Science
2002, 295, 2055-2058.
(12) Park, H.; Lim, A. K. L.; Alivisatos, A. P.; Park, J.; McEuen, P. L. Appl.
Phys. Lett. 1999, 75, 301-303.
(13) Li, X.; He, J.; Hihath, J.; Xu, B.; Lindsay, S. M.; Tao, N. J. J. Am. Chem.
Soc. 2006, 128, 2135-2141.

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