Symmetry of Single Hydrogen Molecular Junction ... - ACS Publications

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The Journal of Physical Chemistry

Symmetry of Single Hydrogen Molecular Junction with Au, Ag and Cu Electrodes

Yu Li, Satoshi Kaneko*, Shintaro Fujii, Manabu Kiguchi*

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

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ABSTRACT: We present a charge transport study on a H2 molecule wired into Au, Ag and Cu electrodes using mechanically controllable break junction techniques at 10 K. Formation of molecular junctions with a H2 molecule bound between the metal electrodes was identified by measuring vibrational modes of the H2 molecule using inelastic electron tunneling spectroscopy. The H2 molecule exhibited a preferential conductance value of 0.3 G0 (G0 = 2e2/h) for Cu electrodes while displayed variable conductance values below 1 G0 for Au and Ag electrodes. Geometrical symmetry of the molecular junctions was characterized by current versus bias voltage (I-V) characteristics of the junctions, in which H2/Cu junctions and H2/Au, Ag junctions, respectively, showed asymmetric and symmetric shapes at the positive and negative bias voltages. The shape of the I-V characteristics reflects geometrical symmetry of metal electrodes (i.e., symmetry of metal-molecule contacts in molecular junctions). We investigated breaking process of the metal contacts to get evidence of geometrical symmetry of the electrodes. The Au and Ag contacts were characterized by formation and rupture of atomic wires, while the Cu contact was suddenly ruptured without the atomic wire-formation. The abrupt break of the metal contact causes geometrical asymmetry of the Cu electrodes that is asymmetry of metal-molecule contacts in molecular junctions.

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1. Introduction The theoretical proposal of a molecular device by Aviram and Ratner1 and its subsequent realization of single molecular junctions2,3 have opened a new frontier in nanoscience research.4-10 Among various single molecular junctions, the single hydrogen molecular junctions have been studied as a model system of the single molecular junctions. The single hydrogen molecular junctions have been studied with Pt, Pd, Fe, Co, Ni, Au, Ag, Cu electrodes.11-18 The H2/Pt junction has been most extensively studied with the conductance, point contact spectroscopy, inelastic tunneling spectroscopy (IETS), conductance fluctuation, and shot noise measurement, and theoretical calculations.11,12,19 The detailed studies revealed that the single hydrogen molecule bridges between Pt electrodes with its molecular axis parallel to the junction axis, and that electron transport through a single channel with the transmission probability close to one. In the case of Pd and Co electrodes, the formation of the single hydrogen molecular wire was revealed by the conductance measurement.14,15 Here, it is noted that clean Pd and Co do not form atomic wire. The introduction of hydrogen to the metal atomic contact stabilizes the single molecular wire. There have been growing interests in the single hydrogen molecular junctions with coinage metals (Au, Ag, and Cu), because their application to the molecular electronics

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is easier compared to more reactive transition metals.16-18 However, there are little systematic studies for the single hydrogen molecular junctions with coinage metals, in contrast with the transition metal electrodes. The two-dimensional (2D) conductance histograms are not reported for coinage metals, and the IETS are not reported for the H2/Ag junction. Recent conductance and IETS measurements of the H2/Cu junctions suggested the formation of the asymmetric single hydrogen molecular junction.18 The structural symmetry of the single molecular junction is an interesting topic. The appearance of the diode properties is predicted for the asymmetric molecular junction by theoretical calculation.20 However, there are little experimental investigation on the symmetry of the single molecular junction, including H2/Cu junctions. Here, we pay attention to the current-voltage (I-V) characteristic as a tool to study the symmetry of the single molecular junction. Since the symmetry of the single molecular junction affects its electron transport property, the current-voltage characteristic should give us information about the symmetry of the single molecular junction. We systematically investigated the H2/Au, Ag, and Cu junctions using mechanically controllable break junction (MCBJ) technique at low temperature. The conductance, IETS, I-V characteristic measurements and length analysis of the single molecular junction were performed. The formation of the single hydrogen molecular junctions was confirmed by

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the conductance and IETS measurements. The I-V characteristics reveled that H2/Au, and H2/Ag junctions were with symmetric metal-molecule contacts, while H2/Cu junctions possessed asymmetric metal-molecule contacts. The structural symmetry of the single hydrogen molecular junction was discussed based on the chain length analysis of the single hydrogen molecular junctions.

2. Experimental Experiments were performed in an ultrahigh vacuum (UHV) using a MCBJ setup at about 10 K following previous research.18 Briefly, a notched Au, Ag, or Cu wire (0.10 mm in diameter) was fixed on top of an electrically insulated phosphor bronze substrate with polyimide tape. The substrate was mounted in a three-point bending configuration in a custom-made vacuum pot. The metal wire was broken at the notch, by bending of the substrate. A single atomic contact could be formed just prior to breaking the wire. Hydrogen gas was introduced to the metal contacts via a capillary. The DC two-point voltage-biased conductance measurements were performed during the breaking process under an applied bias voltage of 100 mV. The differential conductance was measured using a standard lock-in technique with AC modulation at 1 mV and 7.777 kHz. The conductance was monitored for a fixed contact configuration during the DC bias

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between −100 to +100 mV. The I-V characteristics were measured by sweeping the bias voltage. The bias voltage started from 50 mV or 100 mV to +1.0V and went down to −1.0 V, and got back to the initial bias voltage. The sweep rate of I-V characteristics measurement was 100 kHz. The experiments were performed for 7, 6 and 5 independent samples for Au, Ag and Cu electrodes respectively.

3. Results and Discussion Figure 1 shows the typical conductance traces and conductance histograms of Au, Ag and Cu contacts before and after introduction of hydrogen. The stretch length was defined as the displacement between the stem parts of the metal electrodes, which were fixed with an epoxy adhesive (see Supporting information). The conductance histograms were constructed from over 1000 conductance traces during the breaking process. For clean metal contacts, steps appeared at 1 G0 (2e2/h) in the conductance traces, and conductance histograms showed prominent peaks at 1 G0, corresponding to clean metal atomic contacts.17 After introduction of H2, additional steps appeared below 1 G0. The conductance value of steps fluctuated with the conductance traces for Au and Ag electrodes, leading to the broad feature in the conductance histograms. For Cu electrodes, clear steps appeared at 0.3 G0 in the conductance traces, and a sharp 0.3 G0

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peak appeared in the conductance histogram. The appearance of the broad feature for Au electrode, and a sharp 0.3 G0 peak for Cu electrode agreed with the previously reported results.18,21 In addition to the appearance of features below 1 G0, the 1 G0 peak in the conductance histogram changed after introduction of hydrogen for Au and Ag electrodes. The intensity of the 1 G0 peak decreased for Au and Ag electrodes, and the conductance value of the 1 G0 peak decreased for Ag electrodes. We discuss these conductance behaviors in the latter section.

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Figure 1. Typical conductance traces and conductance histograms of Au (a, d), Ag (b, e) and Cu (c, f) before (black line) and after (red line) introduction of hydrogen. The conductance histograms were constructed without data selection from more than 1000 conductance traces during a breaking process of metal contacts. The intensity of the conductance histograms was normalized with the number of the conductance traces used for constructing the histogram. The bin size was 0.004 G0.

The differential conductance (dI/dV) curves were investigated for H2/Au, Ag and Cu junctions showing conductance values of 0.01~1 G0 while keeping the electrodes

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separation fixed. Figure 2 displays the dI/dV curves and their derivatives for H2/Au, Ag and Cu junctions. The dotted lines denote shallow symmetric upward or downward features in differential conductance. Symmetric peaks in the derivatives are found around ±33 and ±42 meV for H2/Au and Ag junctions, respectively. The conductance enhancement and reduction in dI/dV curves are explained by the excitation of a vibration mode (see detail discussion in Supporting information).5 For H2/Cu junctions, symmetrical peaks were observed in dI/dV curves at around ±35 meV, which was explained by the abrupt switching between two slightly different local geometric configurations induced by the phonon excitation.22 The energy of the peaks in dI/dV curves provides the vibration energy of the single molecular junction, and has been utilized as the vibration spectroscopy of the single molecular junctions.22 More than 15 dI/dV spectra were collected for each metal. The vibrational modes were observed for all systems around 30~40meV. Since the phonon modes of clean metal are below 20 meV,23 the observed features corresponded to vibrational modes between a hydrogen molecule and metal electrodes, which confirmed that the hydrogen molecule bridged the gap between Au, Ag, and Cu electrodes.

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Figure 2. Examples of dI/dV spectra and their differential spectra for H2/Au (a, d), Ag (b,e) and Cu (c,f) junctions.

In order to discuss the symmetry of the single hydrogen molecular junction, we have measured the I-V characteristics for H2/Au, H2/Ag, and H2/Cu junctions. Figure 3 shows the typical I-V characteristics for clean Au, Ag, and Cu atomic contacts, and H2/Au, H2/Ag, and H2/Cu junctions. Linear symmetric I-V characteristics were obtained for clean metal atomic contacts. The single hydrogen molecular junctions provided non-linear and asymmetric I-V characteristics.24,25 An increase in the applied bias voltage brings the chemical potential of the electrode closer to the level of the molecular orbital leading to the steep increase in the charge flow across the junction. Now, we focus on the symmetry of the I-V characteristics. Symmetric I-V characteristics were

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observed for H2/Au and H2/Ag junctions, while asymmetric I-V characteristics were observed for H2/Cu junctions. Similar I-V characteristics were observed for more than three samples for each junction (Fig. S1). In the case of the single molecular junction with a symmetric molecule, the electric field is uniform in the molecule, when the molecule symmetrically binds to the metal electrodes and the metal-molecule couplings are the same at both ends of the molecule. The symmetric I-V characteristics are obtained for the symmetric single molecular junctions. Meanwhile, asymmetric I-V characteristic is obtained, when the molecule asymmetrically binds to the metal electrodes. The previously reported theoretical studies have revealed that the difference in the potential drops and/or metal-molecule couplings cause the asymmetry of the I-V characteristic of the single molecular junction.20,26 When the molecule asymmetrically binds to metal electrodes, the potential drop at the molecule-metal interface is larger and metal-molecule coupling is weaker for one of the electrodes, where the interaction between molecule and metal is weaker. Therefore, the asymmetric I-V characteristics for H2/Cu junctions suggest that the interaction between metal and molecule should be asymmetric, which means the formation of the asymmetric single molecular junction.

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Figure 3. Typical I-V characteristics for clean Au (a), Ag (b) and Cu (c) atomic contacts, and H2/Au (d), H2/Ag (e), and H2/Cu (f) junctions.

The length of the single hydrogen molecular junction was evaluated by the statistical analysis of the conductance traces. Figure 4(a-c) show the 2D conductance histograms of the Au, Ag, and Cu contacts after introduction of hydrogen. These histograms were generated by identifying the first data point that had a conductance value lower than 1.2 G0 and assigning it as a relative zero distance z = 0, for each trace, then overlapping all of the individual traces in 2D space. Large counts were observed in the region of 1 G0 for all cases. For Au and Ag contacts, the features extended more than 1 nm, together with the decrease in the conductance values. For Cu contacts, 1 G0 feature extended within 0.3 nm, and small counts appeared around 0.3 G0,

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corresponding to the single molecular junctions. Figure 4(d-f) show the length histogram of the single atom or molecular junctions for Au, Ag, Cu contacts before and after introduction of hydrogen. The length of the single atom or molecular junctions was defined as the distance between the points at which the conductance dropped to the range of 0.04 to 1.1 G0. Before introduction of hydrogen, Au atomic wire extended up to 1.5 nm and a sequence of peaks was observed in the length histogram. The interval of the peaks was 0.255 nm, which corresponds to the Au-Au distance of a clean Au atomic wire.10 In contrast, the Ag and Cu atomic contacts broke within 0.5 nm, indicating that the metal atomic wires were not formed for the Ag and Cu contacts. These results agree with previously reported results.27,28 After introduction of hydrogen, the length of the single atomic or molecular junctions extended further compared to clean one for Au and Ag contacts. On the other hand, the length of the single atomic or molecular junction did not change before and after the introduction of hydrogen for Cu contacts. The atomic wire was not formed for Cu contacts after introduction of hydrogen. The average lengths were 0.58 nm, 0.45 nm, and 0.13 nm for clean Au, Ag, Cu contacts, and 0.82, 1.60 and 0.24 nm for H2/Au, H2/Ag, and H2/Cu junctions, respectively.

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Figure 4. The 2D conductance histogram for Au (a), Ag (b) and Cu (c) contacts after introduction of hydrogen. The length histogram for Au (d), Ag (e), Cu (f) contacts before (black lines) and after (red lines) introduction of hydrogen The inset in (f) shows the typical conductance trace for Cu contacts after introduction of hydrogen.

Here, we discuss the origin of the formation of the symmetric H2/Au and H2/Ag junctions and asymmetric H2/Cu junctions based on the experimental results of the length histogram (Fig. 4). The single hydrogen molecular junction is formed as the following process. 1) a metal atomic wire or contact breaks, 2) hydrogen molecules diffuse on the surface of the metal electrodes, and 3) a single hydrogen molecule is trapped between the metal electrodes, forming the single hydrogen molecular junction. 14


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In the case of H2/Au, H2/Ag junctions, metal atomic wires were formed (left figure of Fig. 5a). After breaking the metal atomic wire, protruded single metal atoms were remained on the surface of the both metal electrodes. Then, the single hydrogen molecule bound to the protruded Au (Ag) atoms on metal electrodes, leading to the locally symmetric metal-molecule contacts (i.e., symmetric single hydrogen molecular junction), as shown in right figure of Fig. 5a. In the case of H2/Cu junction, metal atomic wire was not formed. After breaking the metal contact (not a wire), one Cu atom remained on the surface at either side of the metal electrodes, while the surface of the other side of the metal electrode was rather smooth without protruded single metal atom as shown in Fig. 5b. The hydrogen adsorbed on an atop site on the protruded surface, while it adsorbed on various adsorption sites (atop, bridge, hollow) on the rather flat surface. Therefore, the locally asymmetric single hydrogen molecular junctions were formed in the H2/Cu junctions.

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Figure 5. Formation process of single hydrogen molecular junction. a: symmetric H2/Au and Ag junctions are formed after breaking the metal atomic wires, b: asymmetric H2/Cu junction is formed after breaking the metal atomic contact.

Finally, we briefly comment on the conductance behaviors of the single hydrogen molecular junctions. The H2/Au junctions provided the broad feature below 1 G0 in the conductance histogram. The H2/Ag junctions also provided the broad feature below 1 G0, together with the decrease in the conductance value of the 1 G0 peak. For H2/Cu junctions, a sharp peak of the H2 molecular junction appeared at 0.3 G0 in the conductance histogram, but the intensity of 1 G0 peak did not change in contrast with the H2/Au and Ag junctions. When the interaction between the metal contacts and molecule is strong, the single molecular junction can take various atomic configurations, while only most energetically favorable structure can take when the metal-molecule 16


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interaction is moderate.29 Since the conductance of the single molecular junction depends on its atomic configuration, the strong interaction provides the broad feature in the conductance histogram. The appearance of broad features below 1 G0 for H2/Au and Ag junctions and a sharp peak at 0.3 G0 for H2/Cu junctions indicated that the interaction between hydrogen molecule and metal contacts was large for Au and Ag contacts and moderate for Cu contacts. By comparing the 1 G0 features in the conductance histogram of H2/Au, Ag and Cu junctions, it was suggested that the interaction between the hydrogen molecule and Ag, Au, Cu contacts decreased in this order. The large interaction of the Ag contacts can be explained by the decrease in the coordination number of the atoms of the contacts. Ag contacts formed atomic wires as verified the length analysis (Fig. 4e), and the coordination number of the atoms in the metal wire was small. Since the reactivity of metal increased with a decrease in the coordination number, the reactivity increased for the Ag atomic wires. The strongly interacted hydrogen molecules act as the scattering centers for conduction electrons. The conductance of the Ag atomic contact (1 G0 peak) decreased by this scattering process. Similar decrease in the conductance value is observed for Au atomic contacts decorated with hydrogen atoms.17

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4. Conclusion We have investigated the single hydrogen molecular junction with Au, Ag and Cu electrodes using mechanically controllable break junction techniques at 10 K. The measurements of conductance and differential conductance spectroscopy showed the bridging of hydrogen molecule between metal electrodes. The current-voltage characteristics of the single hydrogen molecular junction showed the formation of the asymmetric H2/Cu junctions, and symmetric H2/Au and Ag junctions. The symmetry of the single hydrogen molecular junction is discussed based on the length of the single molecular junction. The statistical analysis of the length of the single molecular junction showed the formation of single atomic or molecular wire for H2/Au and Ag junctions, and H2/Cu junctions did not form wires. By breaking Au and Ag atomic wires, protruded single metal atoms were prepared on the surface of the both metal electrodes. The single hydrogen molecule bounded to the protruded Au (Ag) atoms to form single molecular junctions with symmetric metal-molecule contacts at both ends. The Cu contact broke without a metal wire formation, which causes formation structurally asymmetric electrodes with protruded and flat surfaces. The hydrogen adsorbed on an atop site on the protruded electrode and various sites (atop, bridge, hollow) on the flat electrode to forms single molecular junctions with asymmetric metal-molecule contacts.

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AUTHOR INFORMATION

Corresponding Author

[email protected] [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by Grants-in-Aid for Scientific Research in Innovative Areas (26102013) and a Grant-in-Aid for Scientific Research (A) (No. 21340074), Young Scientists (B) (15K17842) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Asahi Glass Foundation.

Supporting Information Available The calibration of the stretching length, shape of dI/dV curves and I-V curves. The material is available free of charge via the Internet at http://pubs.acs.org.

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