Temperature Dependence of Conductance and Plateau Length for

Article pubs.acs.org/JPCC

Temperature Dependence of Conductance and Plateau Length for Single-Molecule Junctions Formed with Silver Electrodes Pil Sun Yoo, Han Yeol Jo, and Taekyeong Kim* Department of Physics, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do, 449-791, Korea S Supporting Information *

ABSTRACT: The temperature dependence of the conductance and plateau length for the single-molecule junctions formed with Ag electrodes was investigated by the scanning tunneling microscope-based break-junction technique. The electrode−electrode gap distance (GD) was measured by varying temperature as soon as the Ag metal contact was ruptured. The compressed molecular junction (a tilted molecule) with a small GD at low temperature showed a higher conductance and longer plateau than the vertically stretched molecular junction with a large GD at high temperature. However, for the Au electrodes, the GD did not change by varying temperature, resulting in a constant conductance and plateau. These results are attributed to a larger diffusion constant of Ag compared to Au, yielding a relatively temperature-dependent GD for the Ag electrode. This study may advance the understanding of the electrical and mechanical properties in single-molecule-based devices by varying temperature.

1. INTRODUCTION Over the past decade, the field of thermoelectronics at the molecular scale has been explored with the various types of molecules and metal electrodes.1−10 Although such experiments have provided substantial insight into thermal charge transport through molecular junctions, the temperature effects on the metal−molecule junction formation and the evolution of the junctions upon elongation of the metal electrodes have not been studied. In recent years, Ag has been extensively used as a metal electrode in molecular electronics owing to the similarities of its atomic and electronic structure with Au. Typically, these experiments have focused on the measurements of the electrical properties such as conductance and current−voltage of the molecular junctions formed with Ag metal electrodes.11−13 However, the temperature effects on the physical deformations of the Ag metal−molecule junction as well as the electrical characteristics have been rarely reported. Herein, the conductance of a 4,4″-diamino-p-terphenyl formed with Ag electrodes was measured by the scanning tunneling microscope-based break-junction (STM-BJ) technique by varying temperature. We found that the gap distance (GD) between the top and bottom Ag electrodes after the Ag point contact is ruptured increases with increasing temperature. The tilted molecular junction with a small GD has a higher conductance and longer molecular plateau length than that of the vertically stretched molecular junction with a large GD, consistent with the previous results.11,14−16 For comparison, Au was also used to form molecular junctions; however, this did not show a temperature dependence of the conductance and plateau length owing to the fixed GD of Au electrodes with a temperature variation. These results were explained by considering the dynamics of Ag atoms, which are more movable with a larger diffusion constant than that of Au.11,17 © 2014 American Chemical Society

2. EXPERIMENTAL METHOD The conductance of the junction formed with 4,4″-diamino-pterphenyl for the Ag electrode was measured by repeatedly forming and breaking Ag point contacts in the presence of molecules with a STM-BJ setup as previously described in detail,18−20 where a freshly cut Ag wire tip (Alfa-Aesar, 99.9985% purity) and a mechanically polished Ag slug (AlfaAesar, 99.99% purity) were used as the top and bottom electrodes, respectively. The temperature of the Ag electrodes was controlled by a Peltier heater in the range ∼22−57 °C.2−4,21 4,4″-Diamino-p-terphenyl was deposited on the Ag substrate from its solution in acetone by allowing the solvent to evaporate. 3. RESULTS AND DISCUSSION Figure 1 shows the illustrative scenario of the formation and evolution of the Ag metal−molecule junction upon elongation at two different temperatures (∼22 and 57 °C). The conductance measurement starts by approaching the tip to the substrate to make the Ag metal point contact with a conductance of at least 5G0 (G0 = 2e2/h, the quantum of conductance). The Ag metal channel undergoes an initial relaxation opening up the GD of the top and bottom electrodes as soon as the Ag-metal contact (∼1 G0) is ruptured, within 10 μs time resolution. Then the molecule can be inserted into the gap, and the metal−molecule−metal junction is formed, where the amine (NH2) linker groups bind to the apex Ag atom of the tip and substrate.18−20,22 The binding energy of the amine Received: October 4, 2014 Revised: November 18, 2014 Published: November 25, 2014 29962

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Figure 1. Illustrative scenario of the molecular junction formation and evolution of the Ag electrodes at low (22 °C) and high (57 °C) temperature. The thick black arrows indicate the piezo movement. The blue and red boxes show the molecular junction elongations at low and high temperature conditions. Both side arrows (blue and red) are the gap distances (GD) at low and high temperatures.

group on undercoordinated Ag sites is ∼1.0−1.2 eV which is ∼0.2 eV lower than that of amine−Au binding.11 The initial GD at low temperature (∼22 °C) was smaller than that at high temperature (∼57 °C), resulting in a highly tilted molecular junction.11,14−16,23 This allows molecular junctions with a small GD electrode to extend an additional plateau length before the Ag−molecule is ruptured, as indicated by the blue and red boxes showing the junction elongation at low and high temperatures, respectively. The GD created after the Ag metal contact ruptures was measured by varying temperature, as shown in previous studies.11,14,15 Briefly, the metal contact was elongated until it ruptures at a speed of ∼500 nm/s, and then the electrodes were pushed back together to determine the net distance required for electrodes to move before a contact with a conductance of 1 G0 was formed without the molecules. The GD is the net distance measured for the Ag metal contacts. Figure 2 shows the histogram of the GD with over 2000 traces at three different temperatures of the Ag electrodes. The dashed curves are the Gaussian fits to the GD histogram peaks. The average GDs of ∼0.68, 0.82, and 1.05 nm were observed at 22 (blue), 38 (green), and 57 °C (red), respectively, from the Gaussian fitting. The inset is the average GD plot as a function of temperature, indicating that GD increases with increasing temperature. Individual conductance traces for the 4,4″-diamino-pterphenyl measured at three temperatures of the Ag electrodes are compared as shown in Figure 3a. These traces show the temperature-dependent conductance and plateaus for the molecular junctions with the highest conductance and the longest plateau at temperatures of 22 °C (blue), whereas those at 57 °C (red) have the lowest conductance and the shortest plateau. Figure 3b shows the corresponding normalized conductance histograms generated from over 5000 traces for each of the three temperatures, without any data selection. The

Figure 2. Histograms of GDs of the Ag electrodes at three different temperatures (22, 38, and 57 °C) shown in blue, green, and red, respectively. The dashed curves are the Gaussian fits to the histograms. The inset shows the GD peaks from the Gaussian fitting as a function of temperature.

Figure 3. (a) Sample conductance traces for the 4,4″-diamino-pterphenyl formed with the Ag electrodes measured at 22 °C (blue), 38 (green), and 57 °C (red). (b) Conductance histograms generated from over 5000 conductance traces using 100 bins/decade. The inset shows the conductance as a function of temperature.

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most probable junction conductances are ∼1.8 × 10−4, 1.5 × 10−4, and 0.85 × 10−4 G0 at 22 (blue), 38 (green), and 57 °C (red), respectively. The inset shows the conductance plots as a function of temperature, indicating that the conductance decreases with increasing temperature. This result indicates that the conductance of a highly tilted molecular junction (with the smallest electrode GD at 22 °C) is much higher than that of a stretched molecular junction (with the largest electrode GD at 57 °C), which is consistent with the previous experimental and theoretical results.11,15,24 To probe the variations in the molecular plateau with temperature for the substrate, a two-dimensional (2D) histogram including displacement information was generated. These histograms were created by overlaying all the conductance traces after aligning them along the displacement axis at a conductance of 0.7 G0, because conductance plateaus occur at random locations along the displacement axis.23 The 2D histograms are plotted using logarithmic bins along the conductance (y) axis and linear bins along the displacement (x) axis. The 2D histograms in Figure 4 show the molecular

the Au electrodes were measured in the same manner at two different temperatures. Figure 5a shows the conductance

Figure 5. (a) Conductance histograms for 4,4″-diamino-p-terphenyl formed with the Au electrodes at two different temperatures, 22 °C (blue) and 57 °C (red). The inset shows the GD of the Au electrodes at low (22 °C, blue) and high (57 °C, red) temperatures. (b) Normalized 2D conductance histograms for 4,4″-diamino-p-terphenyl formed with the Au electrodes at two temperatures. The dashed black arrows indicate the fully extended molecular junction just prior to rupture of the Au electrodes.

histograms with peaks of ∼2.0 × 10−4 G0 at 22 °C (blue) and ∼1.95 × 10−4 G0 at 57 °C (red) by the Gaussian fitting. The inset shows the GD histograms with peaks at ∼0.83 and 0.85 nm at 22 °C (blue) and 57 °C (red), respectively, by the Gaussian fitting. Figure 5b shows the 2D conductance− displacement histograms for the Au electrodes at two different temperatures. Both molecular junctions with plateau lengths extending out to ∼0.6 nm at two different temperatures were observed for the Au electrodes. It means that the GD, molecular conductance, and plateau length have no temperature dependence for the Au metal electrodes. These results were attributed to the different diffusion constant of each Ag and Au metal. Since Ag has a larger diffusion constant compared to Au, Ag atoms move more freely, and the Ag electrodes reorganize more quickly, opening up a larger GD after the rupture.11,17 The diffusion coefficient ranges from 3 × 10−7 cm2 s−1 around 200 K to 5 × 10−5 cm2 s−1 at 730 K for Au and from 5 × 10−6 cm2 s−1around 200 K to 7 × 10−5 cm2 s−1 at 610 K for Ag. However, this reorganization for the Au electrode is slower because of its smaller diffusion constant, yielding a relative temperature independence of GD, conductance, and plateau length for the measurements with Au electrodes.

Figure 4. Normalized two-dimensional (2D) conductance histograms for the 4,4″-diamino-p-terphenyl formed with the Ag electrodes at three different temperatures (22, 38, and 57 °C). The dashed black arrows indicate the fully extended molecular junction just prior to rupture of the Ag electrodes. The inset shows the molecular plateau length as a function of temperature.

4. CONCLUSIONS In conclusion, we measured the conductance and plateau length of the 4,4″-diamino-p-terphenyl molecule formed with the Ag electrodes by varying temperature. We found that the molecular conductance and plateau length decrease with increasing temperature of the Ag electrodes, attributed to a larger GD of the Ag electrodes at a high temperature compared to that at a low temperature. This was explained by investigating the dynamics of the Ag metal-contact rupture. However, there was no temperature dependence of the conductance and plateau length for the molecular junctions formed with the Au electrodes owing to the smaller diffusion constant of Au than that of Ag. Thus, our study provides an understanding of the electrical transport and mechanical evolution of the single molecular junction formed with the Ag electrodes by varying temperature.

conductance peak extending to ∼0.86, 0.75, and 0.48 nm along the x-axis at 22, 38, and 57 °C as indicated by the dashed black arrows. The inset shows the plateau length plots as a function of temperature, clearly indicating that the molecular plateau length decreases with increasing temperature. These results were explained using the initial GD difference by varying temperature. The initial GD of the electrodes at 22 °C is smaller by about 3−4 Å when compared with the GD of the electrodes at 57 °C. This allows molecular junctions to be extended an additional 3−4 Å before the rupture with the Ag electrodes. We now contrast these results to the measurements of 4,4″diamino-p-terphenyl molecule formed with the Au metal electrodes. The molecular conductance and plateau length of 29964

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-31-330-4373. Fax: +82-31-330-4374. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Hankuk University of Foreign Studies Research Fund. T. K. thanks Prof. L. Venkataraman in Columbia University for using STM-BJ setup and stimulating discussions. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2014R1A1A2053848).

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