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Formation of a Chain-like Water Single Molecule Junction with Pd Electrodes Risa Fukuzumi,† Satoshi Kaneko,*,† Shintaro Fujii,† Tomoaki Nishino,† and Manabu Kiguchi*,† †

Department of Chemistry, Tokyo Institute of Technology, 2-12-1 W4-10 Ookayama, Meguro-ku, 152-8551 Tokyo, Japan S Supporting Information *

ABSTRACT: Atomic scale interaction between the water molecule and the Pd electrodes was investigated by the mechanically controllable break junction technique at cryogenic temperature. The interaction between the water molecule and the atomic scale Pd electrodes and the resultant formation of the single-molecule junction of the water molecule bridging the gap between the Pd electrodes were confirmed by vibrational spectroscopy where the water−Pd vibrational mode of 70 meV was identified. We found that no water dissociation occurred on the atomic scale Pd electrodes. The electronic transport measurement revealed that the water single molecule junction carried the electronic current in the ballistic transport regime and the conductance was determined to be 1 G0 where G0 is the conductance quantum. The length analysis and current-bias voltage measurement of the junction suggest that the single water molecule is connected to Pd atomic chain. dissociation reaction does not occur on the clean flat Pd surface, it proceeds on the oxygen covered Pd surface and Pd nano particles.12−14 Gladys et al. reported that water molecules react with oxygen to form OH resulting in a mixed H2O/OH layer with a local √3 × √3R30° super structure.12 Zheng et al. reported the water dissociation on Pd nanoparticles deposited upon MgO(100).13 The water dissociation on the nanoparticles is explained by the finite size effect, support effect, and surface effect.13 With the decrease in the coordination number of metal atoms of atomic steps on the surface, and the surface of nanoparticles and atomic wires, the catalytic reactivity generally increases. In the case of the hydrogen/Au atomic junction, the dissociated hydrogen atoms have been detected by the electrical conductance measurement, whereas the dissociated hydrogen atoms have never been directly observed on the flat surface by experimental techniques.15,16 On the basis of these interests, we focus on the interaction between the water molecule and Pd atomic junction in this study. Although there is little work on this system, the water/ Au, Ag, Cu, and Pt junctions have been investigated.17−20 The water/Cu junction shows a fixed conductance value, whereas the water/Pt and Au junctions do not show fixed conductance values. Because the conductance of the single molecule junction sensitively depends on its atomic configuration,21 these conductance behaviors indicate that a certain atomic configuration is preferentially formed for the water/Cu junction,

1. INTRODUCTION The interaction of a water molecule with a solid surface is important in many fields including environmental chemistry, electrochemistry, corrosion, biochemistry, and so on.1 Especially, the adsorption/desorption and dissociation/formation of water molecules on metal surfaces have attracted attention, because they are involved in industrial catalytic processes, such as steam re-forming of natural gas, water gas shift, and reaction in fuel cells, which is the hottest topic in current industry.2−5 Water adsorption and dissociation have been extensively studied on well-defined atomically flat surfaces using various experimental techniques (e.g., X-ray photo emission spectroscopy (XPS), high resolution electron energy loss spectroscopy (HREELS)).6 Through extensive studies, it is revealed that the water dissociation occurs on Cu and Ni surfaces, whereas the water dissociation does not occur on Au, Pd, and Pt surfaces.7 The interaction between a water molecule and Pd surface has received less attention compared with other transition metals, because a water molecule does not dissociate on a flat Pd surface, and Pd is not normally used as a catalyst in fuel cells.8,9 On Pd(111), water molecules adsorb at low temperature with their molecular plane parallel to the surface.10,11 When the Pd surface is heated, water molecules desorb around 150 K without dissociation. However, it is well-known that Pd can absorb and storage large amount of hydrogen gas in the bulk phase. Therefore, once the water dissociation occurs on the Pd surface, Pd can storage hydrogen which is the product of the water dissociation. This can open door for the single-system of hydrogen generation and storage. Although the water © XXXX American Chemical Society

Received: December 22, 2017 Revised: January 23, 2018 Published: February 12, 2018 A

DOI: 10.1021/acs.jpcc.7b12605 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Scheme of experiment. The Pd wire was fixed on the bending beam, and the Pd wire was mechanically broken by bending the beam. Water molecules were introduced into the Pd contact through a heated capillary. (b) Conductance traces of Pd junction before (black) and after (red) the introduction of water. The bias voltage was 0.1 V. The conductance traces are offset for clarity. (c) The corresponding conductance histograms constructed from 1000 conductance traces. The bin size of the conductance histogram is 0.004 G0.

3. RESULTS AND DISCUSSION Figure 1b shows the conductance traces of the Pd junctions before and after the introduction of water. For the clean Pd junctions, steps appear around 1.7 G0, and a clear 1.7 G0 peak is observed in the corresponding conductance histogram (Figure 1c). There is a little step below 1.5 G0 in the conductance traces, and no feature below 1.5 G0 in the conductance histogram. The 1.7 G0 steps and 1.7 G0 peak correspond to the Pd atomic junction.24 As for the Pd atomic junction, several partially open channels contribute to the electron transport, and thus, the conductance of the Pd atomic junction is more than a quantum unit of 1 G0.24 After introduction of water, additional steps appear below 1.5 G0 in the conductance traces, and a clear peak appears around 1 G0 in the conductance histogram. Sometimes we observed peaks below 1 G0 (e.g., 0.5 G0 and 0.7 G0: Figure S3) in the conductance histograms for the H2O/Pd junctions. The appearance of the clear 1 G0 peak in the conductance histogram indicates the formation of a nanojunction consisting of Pd atoms and/or water molecule(s) with a well-defined atomic configuration. The atomic structure of the H2O/Pd junction was discussed on the basis of the statistical analysis of the conductance traces. Figure 2 shows the two-dimensional (2D) conductance versus stretch length histogram for the H2O/Pd junctions. The histogram is generated by identifying the first data point that has a conductance value smaller than 1.3 G0, and assigning it as z = 0 for each trace, then overlapping individual traces in 2D

whereas various atomic configurations are allowed for the water/Au and Pt junctions. The shape of dI/dV curves have been investigated for the water/Pt junction. When the junction conductance is larger than 0.5 G0 (G0 = 2e2/h), the conductance is decreased by the phonon excitation, and dips appear in the d2I/dV2 curves. Meanwhile, the conductance is enhanced by the phonon excitation, and peaks appear in the d2I/dV2 curves, when the junction conductance is smaller than 0.5 G017.22 Although there are several studies for the water/ metal junction,17−21 the atomic configuration is not clear up to now. We have investigated the atomic configuration of the water/Pd junction with a mechanically controllable break junction (MCBJ) technique at low temperature (∼10 K).23 The water/Pd atomic junction was characterized by the conductance, point contact spectroscopy (PCS), conductance trace analysis, and I−V characteristic measurements. The conductance measurement shows the formation of the junction showing the conductance value of 1 G0. The length analysis of the conductance traces reveals the formation of the atomic chain. The PCS indicates the bridging of the water molecule between Pd electrodes. The water molecule does not dissociate on the Pd atomic junction.

2. EXPERIMENTAL SECTION Experiments were performed in the ultrahigh vacuum chamber (UHV) at low temperature (10 K) using the MCBJ setup (Figure 1a, Figure S1). The notched Pd wire (diameter ∼0.10 mm) was fixed with an epoxy adhesive (Stycast 2850FT) on top of the insulated phosphor bronze substrate (thickness ∼1 mm). When the substrate was bent using a three-point bending configuration, the Pd wire was mechanically broken at the notched part. The Pd wire was repeatedly broken and made contact by controlling the bending of the substrate using the piezo element. Water molecules were introduced into the Pd contact through a heated capillary. The water was purified by repeated freezing-pumping-thawing cycles. The conductance measurements were performed under a bias voltage of 100 mV. The differential conductance was measured using a standard lock-in technique with ac modulation of 1 mV and 7.777 kHz. The differential conductance was monitored by sweeping the dc bias voltage from −100 to +100 mV, while the contact configuration was fixed. The I−V characteristics were measured by sweeping the bias voltage, which started from 100 mV, increased to +1.0 V, decreased to −1.0 V, and returned to the initial value. The sweep rate of the I−V characteristics measurements was 100 kHz.

Figure 2. Two dimensional log-conductance versus stretch length histogram of Pd junctions after the introduction of water. The histogram was generated by identifying the first data point that had a conductance value smaller than 1.3 G0, and assigning it as z = 0 for each trace. A linear bin-size of 0.02 nm and a logarithmic Y-bin-size for Δlog(G/G0) of 0.01 were used for construction of 2D conductancetrace histograms. B

DOI: 10.1021/acs.jpcc.7b12605 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C space.25 Large counts are observed around 1 G0, and they extend up to 0.2 nm. Figure 3 shows the length histogram for

stem part of the electrodes, the average return length is approximately proportional to the plateau length, indicating that a fragile structure is formed with a length corresponding to that of the last plateau.26,27 The long plateau of over 0.4 nm and one by one relationship between the plateau length and return length confirms the formation of atomic chain for the H2O/Pd junction. An atomic chain is formed when the bond strength in the chain is much stronger than in the bulk. It is typical for a metal bond that the bond strength increases as the coordination number is reduced. The increase in the bond strength is significant for 5d metals (e.g., Pt, Ir, and Au), and atomic chains are formed for 5d metals.28,29 In the case of 3d and 4d metals including Pd, this effect is not significant, and thus, atomic chains are not formed for 3d and 4d metals.28,29 Bahn et al.28 calculated the ratio of the break force for the chains to the bulk crystal for Ni, Pd, Pt, Cu, Ag, and Au with density functional theory. The chain/bulk ratio is larger than 3 for Au and Pt, whereas it is below 2.5 for other metals. The large stabilization is caused by the relativistic effect of the valence electrons. Atomic chains are not formed for 3d and 4d metals. The adsorption of water molecules on the surface of the metal atomic chain changes this situation. The surface energy of the atomic chain would be decreased by the adsorption of water molecules.24,30 An atomic chain is unstable when its surface energy is large. Thanks to a decrease in the surface energy, the atomic chains could be formed for the H2O/Pd junction. Similar stabilization of atomic chains have been reported for the O2/Ag30 and H2/Pd junctions.24 To determine the substance bridging Pd electrodes, we have investigated the PCS for the H2O/Pd junctions. Figure 4 shows the examples of dI/dV curves taken at a zero bias conductance of 0.7 G0 and 1.2 G0. A symmetric downward steps in the differential conductance is observed around 70 mV, which can be determined more accurately by the corresponding dips in the derivative (d2I/dV2). The nonsymmetric features could be explained by the quantum interference effect.31 In the ballistic transport region (conductance ∼1 G0), electrons moving to the forward direction occupy higher states than electrons moving to the backward direction, when the bias voltage is applied to the junction.17,22 Above a threshold voltage, electrons can excite vibrational modes of the junction. By excitation of the vibrational mode, electrons around the top of the states of forward direction lose energy. In this process, electrons should

Figure 3. (a) Length histograms for Pd (black) and H2O/Pd (red) junctions. The plateau length was defined as the distance between the points where conductance dropped below 1.2 G0 and 2.6 G0 for the Pd junction, and the boundaries were 0.5 G0 and 2.0 G0 for the H2O/Pd junction. The histograms were constructed from 1000 conductance traces. (b) Average return lengths for the H2O/Pd junction as a function of the plateau length. The return length was defined as the distance over which the two electrodes were moved back to make a contact after the junction breaks.

the Pd and H2O/Pd junction. The plateau length is defined as the distance between the points where conductance drops below 1.2 G0 and 2.6 G0 for the Pd junction, and the boundaries are 0.5 G0 and 2.0 G0 for the H2O/Pd junction. The length histogram show that the Pd/H2O junction could be stretched as long as 1 nm, whereas the clean Pd junction breaks within 0.4 nm. The average plateau length is 0.2 nm for the clean Pd junction, and 0.4 nm for the H2O/Pd junction. Because the Pd−Pd distance is 0.27 nm for bulk Pd and molecular size of H2O is 0.2 nm, the 1 nm long plateau indicates the formation of an atomic chain of Pd atoms. Although the experimental results mentioned above do not fix whether the pure Pd atomic chain bridges Pd electrodes or H2O bridges the Pd atomic chain connected to the Pd electrodes, it is revealed that the atomic Pd chain is stabilized in the presence of water molecules. The formation of the Pd atomic chain is supported by the analysis of the return conductance traces. Figure 3b shows the average return length as a function of plateau length (chain length). The return length is defined as the distance over which the two electrodes are moved back to make a contact after the junction breaks. The return length is averaged 1000 breaking-return cycles. Apart from an offset of 0.1 nm due to the elastic response of the

Figure 4. Examples of differential conductance (dI/dV) and its derivative (d2I/dV2) for the H2O/Pd junctions taken at a zero bias conductance of 0.7 G0 (black curves) and 1.2 G0 (red curves). The d2I/dV2 curves are offset for clarity. A symmetric downward step is observed in the dI/dV curves around 70 mV (arrows in Figure 4a). The peaks and dips (arrows in Figure 4b) are observed in the d2I/dV2 curves at negative and positive bias voltage, respectively. C

DOI: 10.1021/acs.jpcc.7b12605 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C scatter backward, because the states of the reverse direction are occupied at lower energy. This backscattering leads to the decrease in the differential conductance. Therefore, the energy of the steps in dI/dV curves and dips in the d2I/dV2 curves provides the energy of the vibrational mode. The present PCS results show that the substance bridging Pd electrodes has the vibrational mode of 70 meV. We assign the vibrational mode of 70 meV as the vibrational mode of water−Pd bond, as the following reasons. The phonon modes observed in PCS of Pd contact is around 20 meV,32 which is much smaller than 70 meV. When a water molecule or dissociation product adsorbs on the bridging Pd atom, conduction electrons transport through the bridging Pd atom, leading to the appearance of the Pd phonon mode in PCS, which is different from the present experimental results. The appearance of 70 meV mode indicates the bridging of substance other than Pd. The previous study for the H2/Pd junction reports the vibrational mode of 30 meV in PCS.24 The absence of 30 meV mode and appearance of 70 meV mode indicates that the water molecule does not dissociate into hydrogen in the present study. The energy of vibrational modes is 453, 198, and 466 meV for a free water molecule. The 70 meV mode does not correspond to the any intramolecular vibrational modes of the water molecule. We assign the 70 meV mode as the mixed Pd−H2O translation and rotation mode. On Pd(111) or Pd(110), this mode is observed at 480 cm−1 (60 meV) in HREELS. Because the circumstance of a water molecule in the single molecule junction is different from that on the flat surface, we should not simply compare them. Meanwhile, the previously reported results for ethylene and acetylene single molecule junctions with Pt electrodes show that the difference in the vibrational energy between the single molecule junction and the flat surface is as small as 5 meV.33 Therefore, we can refer the vibrational energy for molecules adsorbed on the flat surface, to discuss the vibrational mode of the single molecule junction. The PCS results clearly show that the water molecule does not dissociate on the Pd atomic junction and show the bridging of the water molecule. The previous theoretical calculation reveals that 6 eV is required to cleave a water O−H bond in the gas phase, and the barrier decreases to 1 eV on the flat Pd surface.14 The barrier would still be high for the Pd atomic junction, or the dissociation step is endothermic on the Pd atomic junction. Here, we compare the present result of the H2O/Pd junction with others obtained at similar experimental conditions of temperature and vacuum. The interaction between molecules and metal atomic junctions have been investigated for H2O/Au, Ag, Cu, H2/Pt, and H2/Pd junctions, and so on.17−20,24,27,29−33 In the case of the H2/Pd junction, a hydrogen molecule dissociates on the Pd atomic junction, and the hydrogen incorporated Pd atomic contact is formed. The water molecule does not dissociate on the Cu atomic junction. The dissociation energy of the hydrogen molecule is 4.5 eV,34 which is smaller than that of the water molecule. Therefore, it is reasonable that the water molecule does not dissociate on the Pd atomic junction, whereas the hydrogen molecule does dissociate on the Pd atomic junction (Supporting Information 4). We then investigate the current−voltage characteristic of the water single molecule junction. Figure 5 shows an example of the I−V curve taken at a zero bias conductance of 1.0 G0. The linear I−V curves are obtained for the water single molecule junction. When electrons transport through a molecule with small HOMO−LUMO gap, and the energy of the conduction orbital (HOMO or LUMO) is different from the Fermi level of

Figure 5. Example of the I−V curve of the H2O/Pd junction taken at a zero bias conductance of 1 G0. Inset: Proposed structure model of the single water molecule junction. A single water molecule bridges between Pd atomic chains. Gray, red, and white spheres correspond to Pd, O, and H atoms, respectively.

the metal electrodes, a nonlinear I−V curve is obtained. The current rapidly increases when the conduction orbital enters within the bias window. The liner I−V curve indicates the ballistic electron transport through the water single molecule junction. The ballistic electron transport is observed for the H2/Pt junction.29,35 In the case of the H2/Pt junction, electrons are carried by single fully transparent conductance channel.29,35 The antibonding LUMO shifts to the Fermi level by the hybridization with the Pt metal states, which realizes the ballistic electron transport. Similarly, the molecular orbital of the water molecule would effectively hybridize with the Pd metal states, and the hybridized orbital would be formed around the Fermi level, which realizes the high conductivity of the water single molecule junction. The electron transport mechanism can be revealed by the theoretical calculation and/ or shot noise measurement. The I−V curve shown in Figure 5 is symmetric one (see more examples in Figure S4), indicating the symmetric atomic configuration of the water single molecule junction. The symmetry of the I−V curve depends on the contact condition of the single molecule junction.36 When a 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 curves are obtained. Meanwhile, asymmetric I−V curves are obtained, when the molecule asymmetrically binds to metal electrodes. Therefore, the symmetric I−V curve for the water single molecule junction suggests that the interaction between metal and molecule is symmetric, which means the formation of the symmetric single molecule junction. Finally, we discuss the atomic configuration of the H2O/ junction based on the experimental results obtained in the present study. The conductance measurement reveals the formation of a nano junction with a well-defined atomic configuration showing conductance of 1 G0. The formation of the Pd atomic chain is confirmed by the length analysis of the conductance traces. The PCS shows the bridging of the water molecule between Pd electrodes. The symmetric molecule− metal coupling is found by the I−V curve. On a Pd surface, water molecules adsorb in an atop site of the Pd surface with their molecular plane parallel to the plane of the Pd surface.14 D

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(6) Michaelides, A.; Alavi, A.; King, D. A. Different Surface Chemistries of Water on Ru{0001}: From Monomer Adsorption to Partially Dissociated Bilayers. J. Am. Chem. Soc. 2003, 125, 2746−2755. (7) Phatak, A. A.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F. Density Functional Theory Comparison of Water Dissociation Steps on Cu, Au, Ni, Pd, and Pt. J. Phys. Chem. C 2009, 113, 7269−7276. (8) Brosseau, R.; Ellis, T. H.; Morin, M.; Wang, H. Strong AdsorbateAdsorbate Interactions in the H2O/Pd(110) System. J. Electron Spectrosc. Relat. Phenom. 1990, 54, 659−666. (9) Shimizu, K.; Kubo, T.; Satsuma, A. Surface Oxygen-Assisted Pd Nanoparticle Catalysis for Selective Oxidation of Silanes to Silanols. Chem. - Eur. J. 2012, 18, 2226−2229. (10) Brosseau, R.; Brustein, M. R.; Ellis, T. H. The Chemisorption of Water onto Ni(100) and Pd(110): A HREELS Study. Surf. Sci. 1993, 280, 23−37. (11) Zhu, X. Y.; White, J. M.; Wolf, M.; Hasselbrink, E.; Ertl, G. Photochemical Pathways of Water on Pd(111) at 6.4 eV. J. Phys. Chem. 1991, 95, 8393−8402. (12) Gladys, M. J.; El Zein, A. A.; Mikkelsen, A.; Andersen, J. N.; Held, G. Chemical Composition and Reactivity of Water on Clean and Oxygen-Covered Pd{111}. Surf. Sci. 2008, 602, 3540−3549. (13) Zheng, K.; Yu, Y.; Guo, Q.; Liu, S.; Wang, E. G.; Xu, F.; Møller, P. J. Coverage-Dependent Dissociation of H2O on Pd/MgO(100)/ Mo(100). J. Phys.: Condens. Matter 2005, 17, 3073−3081. (14) Papas, B. N.; Whitten, J. L. Dissociation of Water on a Palladium Nanoparticle. Int. J. Quantum Chem. 2010, 110, 3072−3079. (15) Kiguchi, M.; Konishi, T.; Murakoshi, K. Conductance Bistability of Gold Nanowires at Room Temperature. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 125406. (16) Kiguchi, M.; Konishi, T.; Hasegawa, K.; Shidara, S.; Murakoshi, K. Three Reversible States Controlled on a Gold Monoatomic Contact by the Electrochemical Potential. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 245421. (17) Tal, O.; Krieger, M.; Leerink, B.; van Ruitenbeek, J. M. ElectronVibration Interaction in Single-Molecule Junctions: From Contact to Tunneling Regimes. Phys. Rev. Lett. 2008, 100, 196804. (18) Tal, O.; Kiguchi, M.; Thijssen, W. H. A.; Djukic, D.; Untiedt, C.; Smit, R. H. M.; van Ruitenbeek, J. M. Molecular Signature of Highly Conductive Metal-Molecule-Metal Junctions. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 085427. (19) Li, Y.; Kaneko, S.; Fujii, S.; Nishino, T.; Kiguchi, M. Atomic Structure of Water/Au, Ag, Cu and Pt Atomic Junctions. Phys. Chem. Chem. Phys. 2017, 19, 4673−4677. (20) Li, Y.; Demir, F.; Kaneko, S.; Fujii, S.; Nishino, T.; Saffarzadeh, A.; Kirczenow, G.; Kiguchi, M. Electrical Conductance and Structure of Copper Atomic Junctions in the Presence of Water Molecules. Phys. Chem. Chem. Phys. 2015, 17, 32436−32442. (21) Kiguchi, M.; Ohto, T.; Fujii, S.; Sugiyasu, K.; Nakajima, S.; Takeuchi, M.; Nakamura, H. Single Molecular Resistive Switch Obtained via Sliding Multiple Anchoring Points and Varying Effective Wire Length. J. Am. Chem. Soc. 2014, 136, 7327−7332. (22) Paulsson, M.; Frederiksen, T.; Ueba, H.; Lorente, N.; Brandbyge, M. Unified Description of Inelastic Propensity Rules for Electron Transport through Nanoscale Junctions. Phys. Rev. Lett. 2008, 100, 226604. (23) Kaneko, S.; Murai, D.; Marques-Gonzalez, S.; Nakamura, H.; Komoto, Y.; Fujii, S.; Nishino, T.; Ikeda, K.; Tsukagoshi, K.; Kiguchi, M. Site-Selection in Single-Molecule Junction for Highly Reproducible Molecular Electronics. J. Am. Chem. Soc. 2016, 138, 1294−1300. (24) Kiguchi, M.; Hashimoto, K.; Ono, Y.; Taketsugu, T.; Murakoshi, K. Formation of a Pd Atomic Chain in a Hydrogen Atmosphere. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 195401. (25) Fujii, S.; Marques-Gonzalez, S.; Shin, J. Y.; Shinokubo, H.; Masuda, T.; Nishino, T.; Arasu, N. P.; Vazquez, H.; Kiguchi, M. Highly-Conducting Molecular Circuits Based on Antiaromaticity. Nat. Commun. 2017, 8, 15984. (26) Yanson, A. I.; Bollinger, G. R.; van den Brom, H. E.; Agrait, N.; van Ruitenbeek, J. M. Formation and Manipulation of a Metallic Wire of Single Gold Atoms. Nature 1998, 395, 783−785.

So, the single water molecule would be connected with the Pd atomic chain where its molecular plane is perpendicular to the junction axis. The inset figure shows the structural model.

4. CONCLUSION We have investigated the atomic configuration of the water/Pd junction at low temperature by measurement of conductance, PCS, I−V curve, and statistical analysis of conductance traces. The water/Pd junction was fabricated by breaking the Pd contact in the presence of water with MCBJ. The conductance measurement revealed the formation of a nano junction with a well-defined atomic configuration showing a conductance of 1 G0. The formation of the Pd atomic chain was confirmed by the length analysis of the conductance traces. The PCS showed that a water molecule bridged Pd electrodes. The symmetric atomic configuration was found by the I−V curve. The single water molecule was connected to Pd atomic chain, and the water dissociation did not occur on the Pd atomic junction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12605. Calibration of the stretching length, 2D conductance histogram of Pd junctions, local heating, example of I−V curves of H2O/Pd junctions PDF)

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AUTHOR INFORMATION

Corresponding Authors

*S. Kaneko. E-mail: [email protected]. *M. Kiguchi. E-mail: [email protected]. ORCID

Shintaro Fujii: 0000-0003-2869-7674 Manabu Kiguchi: 0000-0002-8179-7466 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Grants-in-Aids for Scientific Research (No. 26102013, 17K19100) from the MEXT, and Tokuyama, Kato, Precise measurement technology, Hitachi metals foundation, and research foundation for optscience and technology.

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b12605 J. Phys. Chem. C XXXX, XXX, XXX−XXX