Fabrication of a Well-Defined Single Benzene Molecule Junction

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Fabrication of a Well-Defined Single Benzene Molecule Junction Using Ag Electrodes Satoshi Kaneko, Tomoka Nakazumi, and Manabu Kiguchi* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

ABSTRACT The formation of the single benzene molecule junction was investigated for Au and Ag electrodes by conductance measurements and inelastic tunneling electron spectroscopy at 10 K. While a single benzene molecule junction was hardly formed for the Au electrodes, a single benzene molecule junction was formed for the Ag electrodes. The single Ag/benzene/Ag junction showed a fixed conductance value of 0.24 G0 (G0 = 2e2/h), indicating the formation of a welldefined single benzene molecule junction. By comparing with previously reported results for Pt electrodes, in which the single benzene molecule junction showed various conductance values, it was shown that the moderate metal-molecule interaction is favorable to obtain a well-defined single molecule junction. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

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here is a growing interest in electron transport properties through single molecules for the purpose of developing molecular electronic devices.1-9 In particular, understanding of the effect of the molecule-metal contact on electron transport is an important issue, because the contact plays a decisive role on the electron transport property and stability of the single molecule junction. The effect of the metal-molecule contact on the conductance of the single molecule junction has been systematically investigated for 1,4-disubstituted benzene molecules (-SH, NH2, and -NC).3-5,8,9 For example, it was revealed that the conductance of 1,4-disubstituted (-NC and -SH) benzene molecules bridging between Pt electrodes was 1 order larger than those bridging between Au electrodes. While stable single molecule junctions can be prepared by using the anchoring groups, the anchoring groups could act as the resistive spacers between the molecule and metal. The conductance of the single molecule junction is below 0.01 G0 (G0 = 2e2/h). Recently, we found that a highly conductive single benzene molecule junction can be prepared by direct binding of the benzene molecule to the Pt electrodes without using the anchoring group.10,11 The conductance of the Pt/benzene/Pt junction was 0.1-1 G0, which was comparable to that of the metal atomic contact. The direct binding of a π-conjugated molecule to metal electrodes is a promising technique to obtain a conducting molecule wire in molecular electronic devices. Although the single Pt/benzene/Pt junction is highly conductive, it does not show a fixed conductance value. The variance of the conductance of the Pt/benzene/Pt junction was a factor of 10. In order to develop the single molecular electronics, it is strongly desired to fabricate well-defined single molecule junctions showing high and fixed conductance values. In the case of the Pt/benzene/Pt junction, the junction can take various atomic configurations during stretching as a

r 2010 American Chemical Society

result of the strong metal-molecule interaction. In the present study, we have observed less reactive metals, Au and Ag. The conductance of the Au and Ag contacts in a benzene atmosphere was investigated at 10 K. A single benzene molecule junction was formed for the Ag electrode, but a benzene molecule junction was hardly formed for the Au electrodes. The single Ag/benzene/Ag junction was characterized by inelastic electron tunneling spectroscopy (IETS).10,12 IETS provided information on the vibration mode between the benzene molecule and the Ag electrode. Figure 1(b,c) shows typical conductance traces and conductance histograms for Au contacts before and after the introduction of benzene. Before the introduction of benzene, the conductance decreased in a stepwise fashion, with each step occurring at integer multiples of G0. The corresponding conductance histograms showed a peak at 1 G0, which corresponded to clean Au atomic contacts.2 After the introduction of benzene, the conductance trace and conductance histogram did not change from those for the clean Au contacts. The conductance trace rarely showed steps below 1 G0, and no feature was observed below 1 G0 in the conductance histogram. These results suggested that the benzene molecule hardly bridged between Au electrodes. Figure 2 shows typical conductance traces and conductance histograms for Ag contacts before and after the introduction of benzene. Before the introduction of benzene, the conductance traces showed 1 G0 steps, and the conductance histogram showed a peak at 1 G0. After the introduction of benzene, the conductance trace and conductance histogram Received Date: November 6, 2010 Accepted Date: December 1, 2010 Published on Web Date: December 06, 2010

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DOI: 10.1021/jz101506u |J. Phys. Chem. Lett. 2010, 1, 3520–3523

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Figure 1. (a) Schematic view of the experimental setup. (b) Conductance trace and (c) conductance histogram of Au contacts before (black) and after (red) introduction of benzene at a bias voltage of 100 mV. The intensity of the conductance histograms was normalized with the number of conductance traces. The conductance histogram was obtained from 1000 conductance traces of breaking Au contacts. The bin size was 0.004 G0.

Ag/benzene/Ag junctions was precisely determined to be 0.24 ( 0.08 G0. In order to characterize the single molecule junction, IETS was measured for the single Ag/benzene/Ag junction taken at a conductance of around 0.1-0.5 G0. Figure 3 shows the example of differential conductance and its derivative (IETS) as a function of the bias voltage for a single Ag/benzene/Ag junction. A symmetric upward step was observed in differential conductance around 40 meV, and clear symmetric peaks were observed in its derivative. The conductance enhancement was explained by the opening of an additional tunneling channel for electrons that lost energy to a vibration mode. For the clean Ag contacts, vibration mode is observed around 10 and 20 meV in point contact spectroscopy.13 The energy of the internal vibration mode is higher than 50 meV for the benzene molecule.14 Therefore, the vibration modes around 40 meV observed in the present study could be assigned to the vibration mode between benzene and Ag. The formation of the Ag/benzene/Ag junction was revealed by IETS. We then discuss the appearance of a clear peak in the conductance histogram for the Ag contacts in the presence of benzene by comparing it with that for the Pt contacts. During breaking the Ag contact, the Ag contact showed conductance steps around 0.2-0.3 G0 without showing additional low conductance steps. The conductance histogram showed a clear peak at 0.24 G0. In contrast, the conductance of the Pt contact gradually decreased, showing various conductance steps below 1 G0, leading to the low conductance tail in the conductance histogram. The difference in the conductance behavior between Ag and Pt contacts is discussed on the basis of the binding energy of the benzene molecule on a flat metal surface. Although the binding energy of the benzene molecule to the sharp tip would be different from that to the flat surface, there is some relationship between the two binding energies.2,15,16 We could, thus, discuss the relative strength of the binding energy of the molecule to metal tip based on that to the flat surface. The theoretical calculation revealed that the binding energy of the benzene molecule was 1.2 and 0.08 eV on Pt(111) and Ag(111), respectively.17,18 The benzene-Pt interaction is stronger than the benzene-Ag interaction.

Figure 2. (a) Conductance trace and (b) conductance histogram of Ag contacts before (black) and after (red) introduction of benzene at a bias voltage of 100 mV. The intensity of the conductance histograms was normalized with the number of conductance traces. The conductance histogram was obtained from 1000 conductance traces of breaking Ag contacts. The bin size was 0.004 G0.

drastically changed. The conductance trace showed steps at integer multiples of 0.2 G0 (arrow in Figure 2a) after showing step at 1 G0, and the corresponding conductance histograms showed a peak at 0.2 G0 (arrow in Figure 2b). Neither steps nor peaks were observed below 0.1 G0 in the conductance trace and conductance histogram. The steps in the conductance trace showing values of 1 and 2 0.2 G0 could be ascribed to one and two benzene molecules, respectively. Two benzene molecules bridge between Ag electrodes in parallel when the junction showed a conductance of 2 0.2 G0. In the present work, the procedures to determine the conductance values and standard deviations is as follows: (I) a conductance histogram was constructed from 1000 conductance traces (one data set), and the first-peak position of conductance was determined; (II) we repeated procedure I for three data sets for six samples (total 18 data sets for the first-peak positions); and (III) from 18 data sets for the first peak, a conductance value was determined as the averaged position for the first peaks (standard deviation was also determined through this procedure). The conductance of the single


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gap size is comparable to the length of the Au atomic chain.2 Therefore, the benzene molecule could not bridge between the Au electrodes due to the large nano gap. In contrast, Ag does not form a monoatomic chain during breaking of the Ag contact. The gap size is not as large after breaking the Ag contact. The benzene molecule could, thus, bridge between Ag electrodes. In conclusion, the conductance behavior of the Au and Ag contacts were investigated in a benzene atmosphere at 10 K. While a single Ag/benzene/Ag junction was formed, a single Au/benzene/Au junction was hardly formed. The Ag/benzene/ Ag junction showed a fixed conductance value of 0.24 G0, indicating the formation of a well-defined single molecule junction. The junction was characterized by IETS, which showed the vibration mode between the benzene molecule and the Ag electrodes. The IETS confirmed the formation of a single Ag/benzene/Ag junction. The present study revealed that the moderate metal-molecule interaction is favorable to obtain a well-defined single molecule junction.

Figure 3. Example of dI/dV (top) and d2I/dV2 (bottom) spectra for Ag contact after the introduction of benzene taken at a zero bias conductance of 0.2 G0.

The atomic configuration of the single Pt/benzene/Pt junction during the junction stretching was investigated by the previous theoretical calculation.10 The benzene molecule initially sat on the Pt electrodes with its molecular plane perpendicular to the Pt junction axis. The conductance of the Pt/benzene/Pt junction with this configuration was about 1.2 G0. With stretching the junction, the benzene molecule was progressively tilted with respect to the Pt junction axis, and the conductance of the junction decreased to 0.1 G0. The single Pt/benzene/Pt junction could show conductance values ranging from 1.2 G0 to 0.1 G0, depending on the atomic configuration of the junction. In the case of the Ag/benzene/ Ag junction, the benzene molecule would initially sit on the Ag electrodes with its molecular plane perpendicular to the Ag junction axis. The conductance of the Ag/benzene/Ag junction with this configuration would be 0.24 G0.When the Ag/benzene/Ag junction is stretched, the junction breaks as a result of its weak interaction between the benzene molecule and the Ag electrodes. Therefore, a single Ag/benzene/Ag junction with a fixed atomic configuration could be formed during stretching the Ag contact in the presence of benzene. In developing the single molecular electronics, single molecule junctions with high and fixed conductance values are strongly desired. The moderate metal-molecule interaction is appropriate to obtain such a single molecule junction. Here, the stability of the single molecule junction is commented. The stability of the single Ag/benzene/Ag junction prepared in the present study was lower than that of the Pt/benzene/Pt junction. The stability of the single molecule junction can be increased by using highly stable metal electrodes. The highly stable metal electrodes could be prepared with the lithographic technique.2 The metal electrode could be also stabilized by the molecular or atomic adsorption.19 The molecular adsorption decreases the surface energy of the metal electrode, which leads to the stabilization of the metal electrodes. Finally, we discuss the reason why the single benzene molecule junction was not formed for the Au electrode based on the breaking process of the metal electrodes.2 In the case of Au, a monoatomic chain is formed during the breaking of the Au contact at low temperature. After breaking the contact, a nano gap is formed between the metal electrodes. Here, the


EXPERIMENTAL METHODS The measurements were performed using the mechanically controllable break junction (MCBJ) technique (see ref. 7 for a detailed description). Notched Au and Ag wires (0.1 mm in diameter, 10 mm in length) were fixed with epoxy adhesive (Stycast 2850FT) on top of a bending beam and mounted in a three-point bending configuration inside a vacuum chamber. In ultrahigh vacuum at 10 K, the metal wire was broken by mechanical bending of the substrate, and clean fracture surfaces were exposed. The bending could be relaxed to form atomic-sized contacts between the wire ends using a piezo element for fine adjustment. Benzene was admitted to the contacts via a capillary with a heater. Direct current (DC) twopoint voltage-biased conductance measurements were performed by applying a bias voltage in the range from 100 to 300 mV. Alternating current (AC) voltage bias conductance measurements were performed using a standard lock-in technique. The conductance was recorded for the fixed contact configuration using an AC modulation of 1 mV amplitude and a frequency of 7.777 kHz, while slowly ramping the DC bias between -100 and þ100 mV. The experiments were performed for six independent samples.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail address: [email protected].

ACKNOWLEDGMENT This work was partially supported by a

Grant-in-Aid for Scientific Research on Priority Areas “Electron transport through a linked molecule in nano-scale” from MEXT.

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Fabrication of a Well-Defined Single Benzene Molecule Junction

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