Article pubs.acs.org/JPCC
Tunneling Decay Constant of Alkanedicarboxylic Acids: Different Dependence on the Metal Electrodes between Air and Electrochemistry Ya-Hao Wang, Ze-Wen Hong, Yan-Yan Sun, Dong-Fang Li, Di Han, Ju-Fang Zheng, Zhen-Jiang Niu, and Xiao-Shun Zhou* Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China S Supporting Information *
ABSTRACT: In this paper, the different tunneling decay constants βN of alkanedicarboxylic acids depending on the metal electrodes in various surroundings are discussed. The conductance of alkanedicarboxylic acids (HOOC−(CH2)n−COOH, n = 2−4) contacting to Pd and Ag were systematically measured by the scanning tunneling microscopy (STM) break junction (STM-BJ) in air. A similar tunneling decay constant βN of about 1 per −CH2 was found for both metals, which might arise from the Fermi energy of electrodes pinning with the energy positions of molecular states under ambient atmosphere. However, the pinning effect can be destroyed by potential control in electrochemistry, and the βN is determined by the alignment of the molecular energy levels relative to the Fermi energy level of the electrodes, which well explains the order of βN,Pd < βN,Ag < βN,Cu in electrochemistry. The current work shows the important role of the surroundings in electron transport through molecular junctions.
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INTRODUCTION Single molecular junctions have attracted wide attention for developing single-molecule-based electronics,1−5 such as molecular wires,6−8 molecular switches,9−11 molecular rectification,12−14 and molecular transistors.15−17 The quantitative conductance measurements of single molecular junctions contribute significantly to fundamentally understand chargetransport characteristics. Typically, the conductance of metal− molecule−metal junctions can be influenced by the molecular structure,18−20 contact geometry,21,22 anchoring groups,23−25 and electrode materials.25−30 Therefore, a variety of molecules with different metal electrodes have been investigated, among which saturated n-alkanes with varying chain lengths and anchoring groups have attracted lots of attention. Many literatures have reported that such n-alkanes with anchoring groups of −NH2, −SH, −NCS, and −CN in nonconductive media or air give out the similar tunneling decay constant βN value (∼1 per −CH2) for different metal electrodes.25,31−33 The almost same βN might indicate that the efficiency of electron transport through the alkanes is independent of the electrode materials. However, it is reported that the tunneling decay constant βN depends on the barrier height between the Fermi energy of electrodes and molecular energy levels.22,32,34 Thus, the same βN of n-alkanes molecules intuitively seems impossible for the different Fermi energy levels of metal electrodes. One of the © 2014 American Chemical Society
proposed reasons was that the Fermi level was pinned with the energy positions of molecular states,32,35 which caused the approximately same barrier height for different metal contacts. Surprisingly, our previous experiment showed that the alkanedicarboxylic acids binding to Cu, Ag, and Pd possess different tunneling decay constants βN following the trend of Pd (0.65) < Ag (0.71) < Cu (0.95) in electrochemical environment.28,29 Different tunneling decay constant dependence was found between our experiment with potential control and literatures in air. However, the clear reasons for the different βN dependence on the electrodes between air and potential control, and the order of βN,Pd < βN,Ag < βN,Cu, are still unknown. In order to study the different tunneling decay constant dependent on the metal electrodes between air and electrochemistry, it is better to perform those experiments in the same group. Thus, we carry out the conductance measurements of HOOC−(CH2)n− COOH (n = 2−4) binding to Pd and Ag electrodes by scanning tunneling microscopy break junction (STM-BJ) under ambient atmosphere. Then the tunneling decay constants βN for Pd and Ag are compared with those obtained in electrochemical environment. The further details of the different βN between air and electrochemistry are also discussed. Received: May 31, 2014 Revised: July 15, 2014 Published: July 16, 2014 18756
dx.doi.org/10.1021/jp505374v | J. Phys. Chem. C 2014, 118, 18756−18761
The Journal of Physical Chemistry C
Article
Figure 1. (a) Typical conductance traces and (b) one-dimensional histograms of a Pd−(succinic acid)−Pd junction.
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EXPERIMENTAL SECTION Pd(111) surfaces were naturally formed on single-crystal beads by Clavilier’s method.36 Prior to each experiment, the Pd(111) was electrochemically polished and then carefully annealed in a hydrogen flame, while the Ag substrate was freshly prepared by electrodeposition of Ag on a clean and flat Au(111) substrate in solution containing 50 mM AgNO3. Mechanically cut Pd and Ag wires were used as the STM tips for the conductance measurement; Ag wire was especially immersed into a 0.5 mM H2SO4 solution to remove the oxides before used. The Pd(111) and Ag substrates were immersed into a freshly prepared aqueous solution containing 2 mM alkanedicarboxylic acids for 5 min and washed by the ultrapure water, then dried with nitrogen. The conductance measurement was carried out on the modified Nanoscope IIIa STM (Veeco, U.S.A.) using the STM break junction,17,37,38 which simply measures the conductance of single-molecule junctions formed by in situ repeating moving the tip into and out of contact with substrate at constant speed. During the process, the molecules could anchor between the two metal electrodes and form single molecular junctions. At the same time the current versus distance curve (I−d) was recorded by a DA−AD card at a sampling frequency of 20 kHz, and thousands of such curves were collected for statistical analysis. All the experiments were performed with a fix bias voltage of 50 mV.
Compared with the conductance value (23 nS) for succinic acid binding to Pd in electrochemistry,28 the conductance of 9 nS in the current work is much smaller. Usually, conductance of metal−molecule−metal junctions is influenced by the energy coupling between the Fermi levels of the electrode and the frontier molecular orbital.25,30,39 The Fermi level of the electrode can be altered by potential control in electrochemistry; thus, the single-molecule conductance measured in an electrochemical environment might be different from that in the nonconductive media or air. Besides, another proposal reason is the different dominated contact configuration between the anchoring group and electrode in air and electrochemical surroundings. On the other hand, the influence of solvent on the conductance was also reported to influence the conductance value.40 We also take the Ag electrode to measure the conductance of succinic acid under ambient atmosphere. The bulk Ag deposited on Au(111) was used as the substrate allowing molecular selfassembly of succinic acids. We observed two sets of conductance values in the conductance histogram; one is approximately at 16.5 nS denoted as high conductance (HC), and another is around 1.6 nS denoted as low conductance (LC). Typical conductance traces are shown in Figure 2. We assign the two types conductances to different bonding geometries, which is similar to alkanedicarboxylic acids contacting to Au electrode.23 Adversely, only one conductance value of 13.2 nS was observed in the conductance measurement in electrochemistry.28 Thus, conductance of succinic acids contacting to Pd and Ag both show different values compared to that measured in electrochemistry; it is reasonable to conclude that the charge transfer through alkanedicarboxylic acids could be affected by the surroundings with electrochemical potential control. However, that the orientation/binding anchoring group depends on the electrode potential cannot be excluded. The conductance measurement of Ag−succinic acid junctions was also carried out under N2 atmosphere. Compared with conductance values (16.5 and 1.6 nS) measured under ambient atmosphere with humidity (
