Home
Search
Collections
Journals
About
Contact us
My IOPscience
Conductance measurement of pyridyl-based single molecule junctions with Cu and Au contacts
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 465204 (http://iopscience.iop.org/0957-4484/24/46/465204) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 93.180.53.211 This content was downloaded on 13/11/2013 at 07:11
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 465204 (9pp)
doi:10.1088/0957-4484/24/46/465204
Conductance measurement of pyridylbased single molecule junctions with Cu and Au contacts Xiao-Yi Zhou, Zheng-Lian Peng, Yan-Yan Sun, Li-Na Wang, Zhen-Jiang Niu and Xiao-Shun Zhou Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, People’s Republic of China E-mail: [email protected]
Received 9 March 2013, in final form 6 July 2013 Published 28 October 2013 Online at stacks.iop.org/Nano/24/465204 Abstract We studied the conductance of pyridyl-based single molecule junctions with Cu contacts by using an electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach. The single molecule junctions of 4,40 -bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE) and 1,2-di(pyridin-4-yl)ethane (BPY-EA) bridged with Cu clusters show three sets of conductance values. These values are smaller than the conductance values of single molecule junctions with Au electrodes measured by the traditional scanning tunneling microscopy break junction in acidic or neutral solutions, which can be attributed to the different electronic coupling efficiencies between molecules and electrodes. The consistent conductance of pyridyl-based molecules in acidic and neutral solutions may show that the protonated pyridyl group contacts to the electrode through the deprotonated form. (Some figures may appear in colour only in the online journal)
1. Introduction
configuration [28, 30, 32, 33] contacting to Au electrodes have received a lot of attention, the influence of electrode materials other than Au on single molecule junctions is less often reported [34–36]. In particular, the electrode materials have an effect on the electronic coupling efficiency between the molecule and electrode in the junction, and thus play an important role in the charge transport [5, 34, 37]. We have developed an electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach for conductance measurement of atomic-size nanowires [38, 39] and single molecule junctions [36, 37] with different metallic electrodes; in particular, the electrodes are made of metal clusters. Target metal clusters can be formed by in situ deposition from solutions containing metal ions. By using such an approach, systematic research on the alkanedicarboxylic acid binding to Cu and Ag clusters has been carried out [36, 37]. While the pyridyl anchoring group contacting to Au electrodes has been received a great deal of investigation [1, 2, 13, 15, 33, 40–44], there is no report on
An understanding of the charge transport through single molecule junctions is fundamentally important in molecular electronics [1–12]. The conductance of single molecule junctions can be measured by the STM break junction (STM-BJ) [1, 3, 13–16], mechanically controllable break junction (MCBJ) [13, 17–19], conducting atomic force microscopy (C-AFM) [20–22], STM trapping method [23] and electromigration [24–27]. Typically, good contact between the molecule and the electrode is required in these methods. While the intrinsic nature of the molecular structure is demonstrated to influence the conductance of single molecule junctions, the molecule–electrode contact also plays a very important role in the electron transport [3, 5]. The factors of molecule–electrode contact effects include the anchoring groups, electrodes and contact configuration between anchoring groups and electrodes. Although the anchoring groups, such as thiol [1, 28], pyridyl [1, 2], amine [3, 29] and carboxylic acid [30, 31], and the contact 0957-4484/13/465204+09$33.00
1
c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
formed by pulling the tip out of the contact with a speed of 20 nm s−1 . Finally, molecular junctions with deposited metal as electrodes can be formed after breaking of the atomic-sized metal wires (figure 2(d)). The conductance curves are recorded at the same time with a sampling frequency of 20 kHz. By repeating the whole process at different positions on the surface, well-defined deposited metal contacts can be realized. A large number of conductance traces are collected, and only the traces with clear stepwise features are selected to construct the conductance histogram (normally, hundreds of curves are selected from thousands of curves). All experiments were carried out at a fixed bias voltage of 50 mV unless otherwise noted.
Figure 1. Molecular structures of the compounds 4, 40 -bipyridine (BPY), 1,2-di-(pyridin-4-yl)ethene (BPY-EE) and 1,2-di(pyridin4-yl)ethane (BPY-EA) used in this study.
the conductance measurement of single molecule junctions through contact between pyridyl and non-Au electrodes. Here, we extend the ECSTM-BJ approach to investigate the conductance of molecules with a pyridyl anchoring group contacting to Cu clusters (as shown in figure 1), and a comparison is also carried out between molecule junctions contacting to Cu clusters and Au electrodes. Cu is chosen as the model system for the following reasons. First, Cu is an important electrode for the investigation of molecular electronics [45–48]. Second, the pyridyl anchoring group can bind to Cu, while the Cu cluster can be formed very easily by the ECSTM-BJ approach [37, 38, 49–51]. Third, Cu has similar electronic structure to Au, which facilitates comparison between them. Furthermore, the form of the interaction between the protonated pyridyl group and the metal electrode in an acidic solution is also discussed in this work.
3. Results and discussion 3.1. Conductance of BPY, BPY-EE and BPY-EA contacting with Cu The conductance measurement of Cu–BPY–Cu junctions was carried out in aqueous solution containing 0.05 M H2 SO4 , 1 mM CuSO4 and 0.5 mM BPY by using the ECSTM-BJ approach. The acidic supporting electrolyte was chosen because there is a strong interaction between the pyridyl group and Cu2+ in a neutral solution, where deposition can be formed immediately after the mixture of BPY and Cu2+ . Due to the pK1 = 3.5 and pK2 = 4.6 for BPY, the pyridyl group should be in the protonated form in 0.05 M H2 SO4 solution [52–55]. The Pt–Ir tip and Au(111) substrate were controlled at −5 mV and 45 mV versus the Cu wire, respectively. Figure 3(a) shows typical conductance curves exhibiting the conductance plateaus of BPY. The plateaus are caused by the formation of molecular junctions, as can be seen by comparing with the smooth decay of the curve (gray line in figure 3(a)) that is recorded in the same solution but without BPY molecules. Such a smooth decay curve corresponds to a tunneling process between the tip and the substrate. A two-dimensional (2D) conductance histogram was constructed from curves with stepwise features, and reveals a single conductance value of 130 nS as shown in figure 3(b). The conductance value is the same as the value using the one-dimensional (1D) histogram method (figure 3(d)). One of the corresponding arrays of Cu clusters (figure 3(c)), which was formed simultaneously by breaking off of the junction with the conductance curves, demonstrates the Cu atoms transferred from the tip to the substrate. These clusters with ∼1 nm height and ∼6 nm full width at half-maximum can be removed at a certain positive potential [37]. Comparing with the single set of conductance values for the –COOH ending group contacting with Cu by using the same approach [36, 37], we found two more sets of conductance values at 13 nS (figure 3(e)) and 5.5 nS (figure 3(f)) in the BPY–Cu contact with the pyridyl ending group. This may be caused by the different interaction between carboxylic acid–Cu and pyridyl–Cu. Three sets of conductance values, 130 nS, 13 nS and 5.5 nS, can be defined
2. Experimental section Naturally formed Au(111) from single crystal bead was used as the substrate, and was annealed in a hydrogen flame before each experiment. Mechanically cut Pt–Ir or electrochemically etched Au wires were used as the tips, which were insulated by thermosetting polyethylene glue. Pt and Cu wires were used as the counter and reference electrodes, respectively. The counter-electrode is used as the current circuit to pass the current in order to balance the current at the working electrode, while the reference electrode with a constant electrochemical potential is to act as reference in measuring and controlling the working electrode’s potential. 4, 40 -bipyridine (BPY) and CuSO4 were purchased from Alfa Aesar, while 1,2-di(pyridin-4-yl)ethene (BPY-EE), 1,2-di(pyridin-4-yl)ethane (BPY-EA) and NaClO4 were purchased from Sigma-Aldrich. H2 SO4 was purchased from Sinopharm Chemical Reagent Co. Ltd. All of them were used as received. Conductance measurement was carried out by using an ECSTM-BJ approach performed on a modified Nanoscope IIIa STM (Veeco, US) as in our previous reports at room temperature (around 25 ◦ C) [36–38]. Briefly, the metal is continuously electrodeposited onto the STM tip (figure 2(a)). Then, the deposited tip is pulled far away from the substrate with the STM feedback disabled (figure 2(b)). Next, the tip is driven towards the surface, and transfer of atoms of deposited metal from the tip to the substrate can occur upon the application of a pulse to the z-piezo of the STM, which is called a jump-to-contact process (figure 2(c)). Then, an atomic-sized wire of the deposited metal can be 2
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 2. Schematic diagram of the ECSTM-BJ approach for conductance measurement of single molecular junctions with in situ formed electrodes.
Figure 3. (a) Typical conductance curves of Cu–BPY–Cu junctions measured at a bias of 50 mV. A smooth decay curve (gray line) was recorded in the same solution without BPY molecules. (b) 2D conductance histogram constructed from the curves shown in (a). (c) The STM image (200 × 200 nm2 ) of a 10 ×10 array of Cu clusters simultaneously generated with the conductance curves. (d)–(f) 1D conductance histograms of single molecule junctions of (d) high conductance (0.89 nS bin size), (e) medium conductance (0.089 nS bin size) and (f) low conductance values (0.089 nS bin size).
3
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 4. (a) High conductance (0.89 nS bin size), (b) medium conductance (0.089 nS bin size) and (c) low conductance (0.089 nS bin size) histograms of BPY-EE junctions with contact of Cu. (d) High conductance (0.089 nS bin size), (e) medium conductance (0.0055 nS bin size) and (f) low conductance (0.0028 nS bin size) histograms of BPY-EA junctions with contact of Cu.
Figure 5. Conductance histograms of (a) BPY (0.89 nS bin size), (b) BPY-EE (0.89 nS bin size) and (c) BPY-EA (0.089 nS bin size) constructed by the current traces without data selection. The gray lines are the control experiments performed in the same solution but without sample molecules. All conductance histograms are normalized to one curve.
as high conductance (HC), medium conductance (MC) and low conductance (LC) values, respectively. The HC is about 10 times the MC, and is about 24 times the LC. Multiple sets of conductance values can be attributed to different contact geometries between the pyridyl anchoring group and the electrodes [2, 13, 41], and a change of the coupling between the electrode and the π system of the BPY molecule was also proposed recently [33, 40]. The conductance values of BPY-EE and BPY-EA contacting with Cu were also measured by the same approach in aqueous solution containing 0.05 M H2 SO4 , 1 mM CuSO4 and 0.5 mM target molecules. The conductance values for BPY-EE and BPY-EA are shown in figure 4. The HC, MC and LC values for BPY-EE are 45 nS, 4.5 nS and 1.1 nS, while those for BPY-EA are 5.5 nS, 0.5 nS and 0.1 nS, respectively. Three sets of conductance values are found for all three molecules (BPY, BPY-EE and BPY-EA). Here, the LC of BPY-EA was measured at a bias of 200 mV, because it is difficult to obtain well-defined data at a bias of 50 mV with the current scanner amplifier. Conductance histograms were also constructed for all current traces without data selection. Taking the high value for
pyridyl-based molecules as an example, similar conductance positions can be found in figure 5. These results appear to be similar to the histograms of –COOH, –NH2 and –SH terminated alkanes without data selection [30]. Comparing with the conductance histograms with data selection (figures 3 and 4), these histograms have very broad and weak peaks due to the increase in the background counts. The control experiments were also carried out in the same solution but without sample molecules; the histogram constructed from such curves (the gray line of figure 3(a)) is completely featureless. This feature shows the property of tunneling current between the tip and the substrate in electrolyte without sample molecules. The obviously different histogram shapes are found in solutions with and without molecules. 3.2. Conductance of BPY, BPY-EE and BPY-EA contacting with Au electrodes in acidic and neutral solutions The conductance of pyridyl-based single molecule junctions contacting to Au electrodes has been investigated in various conditions, such as neutral aqueous solution, organic solution and atmosphere [1, 2, 13, 15, 33, 40–43]. However, there is 4
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 6. (a) High conductance (0.89 nS bin size), (b) medium conductance (0.089 nS bin size) and (c) low conductance (0.089 nS bin size) histograms of BPY contacting to Au in 0.05 M H2 SO4 . (d) High conductance (0.89 nS bin size), (e) medium conductance (0.089 nS bin size) and (f) low conductance (0.089 nS bin size) histograms of BPY contacting to Au in 0.1 M NaClO4 .
Table 1. Summary of single molecule conductance (nS) with contact of Au and Cu in NaClO4 and H2 SO4 solutions. BPY Au (NaClO4 ) Au (H2 SO4 ) Cu (H2 SO4 )
BPY-EE
BPY-EA
HC
MC
LC
HC
MC
LC
HC
MC
LC
350 350 135
45 46 13
14 16 5.5
163 165 45
20 18 4.5
6.0 6.0 1.1
32 32 5.5
3.1 3.2 0.5
0.42 0.40 0.10
the protonated pyridyl group might contact to the electrode through the deprotonated form in the molecular junctions. Futamata also reported that the protonated form of pyridine and 2,20 -bipyridine are deprotonated before adsorption to the Au surface in a strong acidic solution [55, 58]. The BPY-EE and BPY-EA molecules were also studied in a similar way in acidic and neutral solutions, and also show three sets of conductance values. These data together with those for BPY are shown in table 1. Again, the similar conductance of BPY-EE and BPY-EA in the acidic and neutral solutions may demonstrate that the pyridyl-based molecule contacts to the electrode through the pyridyl group, although the pyridyl is in protonated form in acidic solution. Taking the HC values as an example, the conductance of BPY is about two times as large as that of BPY-EE, and is about one order of magnitude larger than that of BPY-EA. The small difference in conductance value between BPY and BPY-EE is attributed to the conjugated backbone structure of BPY-EE. However, the conjugation is broken for BPY-EA by the insertion of the saturated CH2 CH2 unit [2, 33]. The conductance measurement of Au–(BPY-EE)–Au junctions was also carried out in air with a self-assembled monolayer on Au(111), and gave the same value as measured in the aqueous solution. The result shows that the dominant means of electron transport is through single molecule junctions rather than through the electrolyte in the aqueous solution.
no report on the conductance measurement of pyridyl-based molecular junctions in acidic solution. We know that the existence of H+ can cause protonation of the pyridyl group in acidic solution [53–55], which might have an influence on the charge transport of molecular junctions. The single molecule conductance of a protonated pyridylbased molecule contacting to Au electrodes was studied in an aqueous solution containing 0.05 M H2 SO4 and 0.5 mM target molecules by using a traditional scanning tunneling microscopy break junction (STM-BJ) [1, 2, 13, 56, 57]. This approach is based on the repeated formation and breaking of molecular junctions between the Au tip and the Au(111) substrate, and the conductance curves are recorded during the process by applying a constant bias voltage between the tip and the substrate. As shown in figures 6(a)–(c), a single molecule junction of BPY with a protonated pyridyl group has three sets of conductance values, 350 nS, 46 nS and 16 nS, which can be defined as HC, MC and LC, respectively. These values are almost the same as those found in aqueous solution containing 0.05 M NaClO4 and 0.5 mM BPY shown in figures 5(d)–(f). Moreover, these data are similar to the conductance values reported by us (HC: 364 nS, MC: 46 nS) [2], Wang et al (HC: 584 nS, MC: 42 nS, LC: 10 nS) [41], Quek et al and Kamenetska et al (MC: 46 nS, LC: 12 nS) [33, 40]. The consistent single molecule conductance of the BPY molecule is found in acidic and neutral solutions, and shows that 5
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 7. Natural logarithmic plots of single molecule conductance versus (a) number of CH=CH units and (b) molecular length for molecular junctions formed with Cu clusters.
constant which describes the efficiency of electron transport along the molecules. Obviously, the different conductance of molecular junctions with various electrodes should be caused by the different molecule–electrode coupling strengths and efficiencies of electron transport along the molecules. The binding interactions of pyridyl–Cu and pyridyl–Au follow the order of pyridyl–Cu slightly larger than pyridyl–Au by theoretical calculation [49, 62]. On the other hand, the single molecule conductance of Cu–BPY–Cu is smaller than that of Au–BPY–Au (table 1), which is opposite to the order of the binding interaction. This may indicate the important contribution of the efficiency of electron transport along the molecule for Cu and Au electrodes to the single molecule conductance. Theoretical calculations show that the electron transport is dominated by the LUMO of these molecules binding to the Au electrode, as the LUMO is much closer to the Fermi level of the Au electrode [13, 33, 41]. No result has been reported for pyridyl–Cu contact, thus further theoretical and experimental investigations on pyridyl–Cu are needed. Now, we take the HC value as an example to discuss the efficiency of electron transport along the molecules with Cu and Au contacts. The N–N lengths of BPY, BPY-EE and ˚ 9.5 A ˚ and 9.5 A, ˚ BPY-EA are approximately equal to 7.2 A, respectively. The formula G = Gn=0 exp(−βN N) can also be expressed as ln G = ln Gn=0 − βN N. The linear fit slope of all points gives the βN . Taking the molecules of BPY and BPY-EE contacting to Cu as an example, the single molecule conductance values of BPY and BPY-EE are 135 nS and 45 nS, respectively (table 1). If we use the number of CH=CH as the unit, a slope of −1.099 can be obtained from figure 7(a), then the decay constant is 1.099 per CH=CH. Of course ˚ as the unit (the formula is now we can use the length A G = A exp(−βL L), ln G = ln A−βL L), and the decay constant ˚ −1 as shown in figure 7(b). Similarly, the decay is 0.478 A constant upon insertion of CH=CH is estimated to be 0.624 ˚ −1 ) from BPY to BPY-EE with per CH=CH (or ∼0.271 A Au electrodes. These decay constants are comparable with previously measured values for conjugated molecules [33, 61, 63]. Meanwhile, the decay constants upon the insertion of ˚ −1 ) and 3.201 CH2 CH2 are 2.392 per CH2 CH2 (or ∼1.04 A
3.3. The influence of the metal electrode Now we focus on the influence of the different electrodes on the single molecule junctions. Typically, the single molecule conductance is determined by the alignment of the molecular energy levels relative to the Fermi energy level of the electrode, and the electronic coupling efficiency between the anchoring group and the metal electrode [36]. Table 1 summarizes single molecule conductance with contact of Au in neutral and acidic solutions, and contact of Cu in acidic solution. For example, the conductance values of BPY–Cu, 135 nS (HC), 13 nS (MC) and 5.5 nS (LC), are smaller than those of BPY–Au, 350 nS (HC), 46 nS (MC) and 16 nS (LC), in H2 SO4 solution. Besides, the conductance values of BPY-EE and BPY-EA follow the same order for Cu and Au contacts. In particular, one end of the Cu–molecule–Cu junction is made of Cu clusters on Au(111) substrates. Such clusters might raise the question as to whether the conductance value of pyridyl–Cu is lower than that of pyridyl–Au due to the additional tunneling barrier from the cluster. This possibility can be excluded by the fact that the conductance of Cu–(succinic acid)–Cu is higher than that of Au–(succinic acid)–Au, as we previously reported using the same ECSTM-BJ approach [37]. Thus, the different conductance values between contacts with Cu clusters and Au electrodes can be attributed (or at least mainly attributed) to the different electrode material in the junctions. Interestingly, metal clusters created by the ECSTM-BJ approach have well-defined crystallographic structure [39, 59]. BPY, BPY-EE and BPY-EA have short backbones (N–N distance of 0.72–0.95 nm), and the conductance of such short molecular junctions follows the superexchange mechanism [33, 60, 61]. At low bias, the conductance can be expressed as G = Gn=0 exp(−βN N), where G is the molecular conductance, N is the number of CH=CH (or CH2 CH2 ) units, Gn=0 is a constant determined by the molecule–electrode coupling strength and responds to the contact conductance, and βN is the tunneling decay 6
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
˚ −1 ) from BPY to BPY-EA with per CH2 CH2 (or ∼1.392 A the Au and Cu contacts, respectively. Normally, smaller decay constants with saturated alkane have been reported by many groups [28, 30, 36], and the departure can be attributed to the decay constant estimated from the insertion of CH2 CH2 on conjugated BPY in this work. We state that these decay constants only reflect the degree of electron decoupling upon insertion of one CH2 CH2 or CH=CH to BPY, and are not expected to predict the conductance of the molecules upon further insertion of CH2 CH2 or CH=CH groups. To sum up, the decay constants for insertion of CH=CH or CH2 CH2 with Cu are larger than those for Au, which shows that the efficiency of electron transport along the pyridyl-based molecules for Cu is worse than for Au. The decay constant is sensitive to the alignment of the Fermi level of the metal in the molecular junction with the molecular energy levels [4, 34, 64]. Correspondingly, we may attribute the lower conductance value of the Cu–molecule–Cu junction to the lower efficiency of electron transport along the molecules than that of the Au–molecule–Au junction.
[3] Venkataraman L, Klare J E, Nuckolls C, Hybertsen M S and Steigerwald M L 2006 Dependence of single-molecule junction conductance on molecular conformation Nature 442 904–7 [4] Salomon A, Cahen D, Lindsay S, Tomfohr J, Engelkes V B and Frisbie C D 2003 Comparison of electronic transport measurements on organic molecules Adv. Mater. 15 1881–90 [5] Chen F, Hihath J, Huang Z F, Li X L and Tao N J 2007 Measurement of single-molecule conductance Annu. Rev. Phys. Chem. 58 535–64 [6] Heath J R 2009 Molecular electronics Annu. Rev. Mater. Sci. 39 1–23 [7] McCreery R L and Bergren A J 2009 Progress with molecular electronic junctions: meeting experimental challenges in design and fabrication Adv. Mater. 21 4303–22 [8] Song H, Reed M A and Lee T 2011 Single molecule electronic devices Adv. Mater. 23 1583–608 [9] Lu Q, Liu K, Zhang H M, Du Z B, Wang X H and Wang F S 2009 From tunneling to hopping: a comprehensive investigation of charge transport mechanism in molecular junctions based on oligo(p-phenylene ethynylene)s ACS Nano 3 3861–8 [10] Liu H M, Wang N, Zhao J W, Guo Y, Yin X, Boey F Y C and Zhang H 2008 Length-dependent conductance of molecular wires and contact resistance in metal-molecule-metal junctions ChemPhysChem 9 1416–24 [11] Bruot C, Hihath J and Tao N J 2012 Mechanically controlled molecular orbital alignment in single molecule junctions Nature Nanotechnol. 7 35–40 [12] Ma G H, Shen X, Sun L L, Zhang R X, Wei P, Sanvito S and Hou S M 2010 Low-bias conductance of single benzene molecules contacted by direct Au–C and Pt–C bonds Nanotechnology 21 495202 [13] Hong W, Manrique D Z, Moreno-Garc´ıa P, Gulcur M, Mishchenko A, Lambert C J, Bryce M R and Wandlowski T 2012 Single molecular conductance of tolanes: experimental and theoretical study on the junction evolution dependent on the anchoring group J. Am. Chem. Soc. 134 2292–304 [14] Zhou X S, Liu L, Fortgang P, Lefevre A S, Serra-Muns A, Raouafi N, Amatore C, Mao B W, Maisonhaute E and Sch¨ollhorn B 2011 Do molecular conductances correlate with electrochemical rate constants? Experimental insights J. Am. Chem. Soc. 133 7509–16 [15] Ie Y, Hirose T, Nakamura H, Kiguchi M, Takagi N, Kawai M and Aso Y 2011 Nature of electron transport by pyridinebased tripodal anchors: potential for robust and conductive single-molecule junctions with gold electrodes J. Am. Chem. Soc. 133 3014–22 [16] Sedghi G et al 2011 Long-range electron tunnelling in oligo-porphyrin molecular wires Nature Nanotechnol. 6 517–23 [17] Martin C A, Ding D, Sorensen J K, Bjornholm T, van Ruitenbeek J M and van der Zant H S J 2008 Fullerene-based anchoring groups for molecular electronics J. Am. Chem. Soc. 130 13198–9 [18] Tian J H et al 2010 The fabrication and characterization of adjustable nanogaps between gold electrodes on chip for electrical measurement of single molecules Nanotechnology 21 274012 [19] Taniguchi M, Tsutsui M, Yokota K and Kawai T 2009 Inelastic electron tunneling spectroscopy of singlemolecule junctions using a mechanically controllable break junction Nanotechnology 20 434008 [20] Xu B Q, Xiao X Y and Tao N J 2003 Measurements of single-molecule electromechanical properties J. Am. Chem. Soc. 125 16164–5
4. Conclusions The single molecule conductance of pyridyl-based molecules binding to Cu and Au was investigated. The conductance values of pyridyl–Cu contacts show the order BPY > BPY-EE > BPY-EA, which is the same as that of pyridyl–Au contacts. Three sets of conductance values are found for all three molecules (BPY, BPY-EE and BPY-EA) contacting to Cu clusters. These values are smaller than those of molecules contacting to Au electrodes, which can be attributed to the different electronic coupling efficiencies between the molecules and the electrodes. The same conductance values for the pyridyl-based molecules are found for both neutral and acidic solutions, which shows that the protonated pyridyl group contacts to the metal electrode through the deprotonated form in the acidic solution.
Acknowledgments We gratefully acknowledge support from Professor Bing-Wei Mao and Mr Zhao-Bin Chen of Xiamen University, and the reviewer’s kind suggestion for improvement of the discussion. This work is supported by the National Natural Science Foundation of China (Nos 21003110, 21273204, 21211130097) and the Planned Science and Technology Project of Zhejiang Province (No. 2011C37052).
References [1] Xu B Q and Tao N J 2003 Measurement of single-molecule resistance by repeated formation of molecular junctions Science 301 1221–3 [2] Zhou X S, Chen Z B, Liu S H, Jin S, Liu L, Zhang H M, Xie Z X, Jiang Y B and Mao B W 2008 Single molecule conductance of dipyridines with conjugated ethene and nonconjugated ethane bridging group J. Phys. Chem. C 112 3935–40 7
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
[21] Cui X D, Primak A, Zarate X, Tomfohr J, Sankey O F, Moore A L, Moore T A, Gust D, Harris G and Lindsay S M 2001 Reproducible measurement of single-molecule conductivity Science 294 571–4 [22] Nef C, Frederix P L T M, Brunner J, Sch¨onenberger C and Calame M 2012 Force–conductance correlation in individual molecular junctions Nanotechnology 23 365201 [23] Haiss W, Wang C S, Grace I, Batsanov A S, Schiffrin D J, Higgins S J, Bryce M R, Lambert C J and Nichols R J 2006 Precision control of single-molecule electrical junctions Nature Mater. 5 995–1002 [24] Yu L H, Keane Z K, Ciszek J W, Cheng L, Stewart M P, Tour J M and Natelson D 2004 Inelastic electron tunneling via molecular vibrations in single-molecule transistors Phys. Rev. Lett. 93 266802 [25] Taychatanapat T, Bolotin K I, Kuemmeth F and Ralph D C 2007 Imaging electromigration during the formation of break junctions Nano Lett. 7 652–6 [26] de Leon N P, Liang W, Gu Q and Park H 2008 Vibrational excitation in single-molecule transistors: deviation from the simple Franck–Condon prediction Nano Lett. 8 2963–7 [27] Park H, Park J, Lim A K L, Anderson E H, Alivisatos A P and McEuen P L 2000 Nanomechanical oscillations in a single-C60 transistor Nature 407 57–60 [28] Li C, Pobelov I, Wandlowski T, Bagrets A, Arnold A and Evers F 2008 Charge transport in single Au/alkanedithiol/ Au junctions: coordination geometries and conformational degrees of freedom J. Am. Chem. Soc. 130 318–26 [29] Widawsky J R, Kamenetska M, Klare J, Nuckolls C, Steigerwald M L, Hybertsen M S and Venkataraman L 2009 Measurement of voltage-dependent electronic transport across amine-linked single-molecular-wire junctions Nanotechnology 20 434009 [30] Chen F, Li X L, Hihath J, Huang Z F and Tao N J 2006 Effect of anchoring groups on single-molecule conductance: comparative study of thiol-, amine-, and carboxylic-acidterminated molecules J. Am. Chem. Soc. 128 15874–81 [31] Martin S, Haiss W, Higgins S, Cea P, Lopez M C and Nichols R J 2008 A comprehensive study of the single molecule conductance of α, ω-dicarboxylic acid-terminated alkanes J. Phys. Chem. C 112 3941–8 [32] Li X L, He J, Hihath J, Xu B Q, Lindsay S M and Tao N J 2006 Conductance of single alkanedithiols: conduction mechanism and effect of molecule–electrode contacts J. Am. Chem. Soc. 128 2135–41 [33] Kamenetska M, Quek S Y, Whalley A C, Steigerwald M L, Choi H J, Louie S G, Nuckolls C, Hybertsen M S, Neaton J B and Venkataraman L 2010 Conductance and geometry of pyridine-linked single-molecule junctions J. Am. Chem. Soc. 132 6817–21 [34] Ko C H, Huang M J, Fu M D and Chen C H 2010 Superior contact for single-molecule conductance: electronic coupling of thiolate and isothiocyanate on Pt, Pd, and Au J. Am. Chem. Soc. 132 756–64 [35] Kiguchi M 2009 Electrical conductance of single C60 and benzene molecules bridging between Pt electrode Appl. Phys. Lett. 95 073301 [36] Peng Z L, Chen Z B, Zhou X Y, Sun Y Y, Liang J H, Niu Z J, Zhou X S and Mao B W 2012 Single molecule conductance of carboxylic acids contacting Ag and Cu electrodes J. Phys. Chem. C 116 21699–705 [37] Zhou X S, Liang J H, Chen Z B and Mao B W 2011 An electrochemical jump-to-contact STM-break junction approach to construct single molecular junctions with different metallic electrodes Electrochem. Commun. 13 407–10 [38] Zhou X S, Wei Y M, Liu L, Chen Z B, Tang J and Mao B W 2008 Extending the capability of STM break junction for conductance measurement of atomic-size nanowires:
[39]
[40]
[41]
[42]
[43]
[44]
[45] [46] [47]
[48] [49] [50]
[51] [52] [53]
[54] [55]
[56]
8
an electrochemical strategy J. Am. Chem. Soc. 130 13228–30 Liang J H, Liu L, Gao Y Y, Wei Y M, Chen Z B, Zhou X S, Zhao J W and Mao B W 2013 Correlating conductance and structure of silver nano-contacts created by jump-to-contact STM break junction J. Electroanal. Chem. 688 257–61 Quek S Y, Kamenetska M, Steigerwald M L, Choi H J, Louie S G, Hybertsen M S, Neaton J B and Venkataraman L 2009 Mechanically controlled binary conductance switching of a single-molecule junction Nature Nanotechnol. 4 230–4 Wang C, Batsanov A S, Bryce M R, Martin S, Nichols R J, Higgins S J, Garcia-Suarez V M and Lambert C J 2009 Oligoyne single molecule wires J. Am. Chem. Soc. 131 15647–54 Tam E S, Parks J J, Shum W W, Zhong Y-W, Santiago-Berr´ıos M B, Zheng X, Yang W, Chan G K L, Abru˜na H D and Ralph D C 2011 Single-molecule conductance of pyridine-terminated dithienylethene switch molecules ACS Nano 5 5115–23 Li Z and Borguet E 2012 Determining charge transport pathways through single porphyrin molecules using scanning tunneling microscopy break junctions J. Am. Chem. Soc. 134 63–6 Pobelov I V, M´esz´aros G, Yoshida K, Mishchenko A, Gulcur M, Bryce M R and Wandlowski T 2012 An approach to measure electromechanical properties of atomic and molecular junctions J. Phys.: Condens. Matter 24 164210 Schull G, Frederiksen T, Arnau A, Sanchez-Portal D and Berndt R 2011 Atomic-scale engineering of electrodes for single-molecule contacts Nature Nanotechnol. 6 23–7 Lennartz M C, Caciuc V, Atodiresei N, Karth¨auser S and Bl¨ugel S 2010 Electronic mapping of molecular orbitals at the molecule–metal interface Phys. Rev. Lett. 105 066801 McGovern W R, Anariba F and McCreery R L 2005 Importance of oxides in carbon/molecule/metal molecular junctions with titanium and copper top contacts J. Electrochem. Soc. 152 E176–83 Bonifas A P and McCreery R L 2010 ‘Soft’ Au, Pt and Cu contacts for molecular junctions through surface-diffusionmediated deposition Nature Nanotechnol. 5 612–7 Wu D Y, Li J F, Ren B and Tian Z Q 2008 Electrochemical surface-enhanced Raman spectroscopy of nanostructures Chem. Soc. Rev. 37 1025–41 Wang D, Xu Q-M, Wan L-J, Wang C and Bai C-L 2002 Adlayer structures of pyridine, pyrazine and triazine on Cu(111): an in situ scanning tunneling microscopy study Langmuir 18 5133–8 Gao J S and Tian Z Q 1996 Surface enhanced Raman scattering of pyridine at copper electrodes excited with a 514.5 nm line Chem. Phys. Lett. 262 151–4 Musgrave T R and Mattson C E 1968 Coordination chemistry of 4,40 -bipyridine Inorg. Chem. 7 1433–6 Diao Y X, Han M J, Wan L J, Itaya K, Uchida T, Miyake H, Yamakata A and Osawa M 2006 Adsorbed structures of 4,40 -bipyridine on Cu(111) in acid studied by STM and IR Langmuir 22 3640–6 Wandlowski T, Ataka K and Mayer D 2002 In situ infrared study of 4,40 -bipyridine adsorption on thin gold films Langmuir 18 4331–41 Futamata M 2000 Unique adsorbed state of 4,40 -BiPy and -BiPyH2+ 2 on Au(111) electrode Chem. Phys. Lett. 332 421–7 Kiguchi M, Takahashi T, Takahashi Y, Yamauchi Y, Murase T, Fujita M, Tada T and Watanabe S 2011 Electron transport through single molecules comprising aromatic stacks enclosed in self-assembled cages Angew. Chem. Int. Edn 50 5708–11
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
[57] Aradhya S V, Frei M, Hybertsen M S and Venkataraman L 2012 Van der Waals interactions at metal/organic interfaces at the single-molecule level Nature Mater. 11 872–6 [58] Futamata M 2001 Adsorbed state of 4,40 -BiPy and BiPyH2 2+ on Au(111) electrode J. Phys. Chem. B 105 6933–42 [59] Wei Y M, Liang J H, Chen Z B, Zhou X S, Mao B W, Oviedo O A and Leiva E P M 2013 Stretching single atom contacts at multiple subatomic step-length Phys. Chem. Chem. Phys. 15 12459–65 [60] Davis W B, Svec W A, Ratner M A and Wasielewski M R 1998 Molecular-wire behaviour in p-phenylenevinylene oligomers Nature 396 60–3 [61] Ho Choi S, Kim B and Frisbie C D 2008 Electrical resistance of long conjugated molecular wires Science 320 1482–6
[62] Wu D Y, Ren B, Xu X, Liu G K, Yang Z L and Tian Z Q 2003 Periodic trends in the bonding and vibrational coupling: pyridine interacting with transition metals and noble metals studied by surface-enhanced Raman spectroscopy and density-functional theory J. Chem. Phys. 119 1701–9 [63] He J, Chen F, Li J, Sankey O F, Terazono Y, Herrero C, Gust D, Moore T A, Moore A L and Lindsay S M 2005 Electronic decay constant of carotenoid polyenes from single-molecule measurements J. Am. Chem. Soc. 127 1384–5 [64] Engelkes V B, Beebe J M and Frisbie C D 2004 Lengthdependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance J. Am. Chem. Soc. 126 14287–96
9
Search
Collections
Journals
About
Contact us
My IOPscience
Conductance measurement of pyridyl-based single molecule junctions with Cu and Au contacts
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 465204 (http://iopscience.iop.org/0957-4484/24/46/465204) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 93.180.53.211 This content was downloaded on 13/11/2013 at 07:11
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 465204 (9pp)
doi:10.1088/0957-4484/24/46/465204
Conductance measurement of pyridylbased single molecule junctions with Cu and Au contacts Xiao-Yi Zhou, Zheng-Lian Peng, Yan-Yan Sun, Li-Na Wang, Zhen-Jiang Niu and Xiao-Shun Zhou Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, People’s Republic of China E-mail: [email protected]
Received 9 March 2013, in final form 6 July 2013 Published 28 October 2013 Online at stacks.iop.org/Nano/24/465204 Abstract We studied the conductance of pyridyl-based single molecule junctions with Cu contacts by using an electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach. The single molecule junctions of 4,40 -bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE) and 1,2-di(pyridin-4-yl)ethane (BPY-EA) bridged with Cu clusters show three sets of conductance values. These values are smaller than the conductance values of single molecule junctions with Au electrodes measured by the traditional scanning tunneling microscopy break junction in acidic or neutral solutions, which can be attributed to the different electronic coupling efficiencies between molecules and electrodes. The consistent conductance of pyridyl-based molecules in acidic and neutral solutions may show that the protonated pyridyl group contacts to the electrode through the deprotonated form. (Some figures may appear in colour only in the online journal)
1. Introduction
configuration [28, 30, 32, 33] contacting to Au electrodes have received a lot of attention, the influence of electrode materials other than Au on single molecule junctions is less often reported [34–36]. In particular, the electrode materials have an effect on the electronic coupling efficiency between the molecule and electrode in the junction, and thus play an important role in the charge transport [5, 34, 37]. We have developed an electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach for conductance measurement of atomic-size nanowires [38, 39] and single molecule junctions [36, 37] with different metallic electrodes; in particular, the electrodes are made of metal clusters. Target metal clusters can be formed by in situ deposition from solutions containing metal ions. By using such an approach, systematic research on the alkanedicarboxylic acid binding to Cu and Ag clusters has been carried out [36, 37]. While the pyridyl anchoring group contacting to Au electrodes has been received a great deal of investigation [1, 2, 13, 15, 33, 40–44], there is no report on
An understanding of the charge transport through single molecule junctions is fundamentally important in molecular electronics [1–12]. The conductance of single molecule junctions can be measured by the STM break junction (STM-BJ) [1, 3, 13–16], mechanically controllable break junction (MCBJ) [13, 17–19], conducting atomic force microscopy (C-AFM) [20–22], STM trapping method [23] and electromigration [24–27]. Typically, good contact between the molecule and the electrode is required in these methods. While the intrinsic nature of the molecular structure is demonstrated to influence the conductance of single molecule junctions, the molecule–electrode contact also plays a very important role in the electron transport [3, 5]. The factors of molecule–electrode contact effects include the anchoring groups, electrodes and contact configuration between anchoring groups and electrodes. Although the anchoring groups, such as thiol [1, 28], pyridyl [1, 2], amine [3, 29] and carboxylic acid [30, 31], and the contact 0957-4484/13/465204+09$33.00
1
c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
formed by pulling the tip out of the contact with a speed of 20 nm s−1 . Finally, molecular junctions with deposited metal as electrodes can be formed after breaking of the atomic-sized metal wires (figure 2(d)). The conductance curves are recorded at the same time with a sampling frequency of 20 kHz. By repeating the whole process at different positions on the surface, well-defined deposited metal contacts can be realized. A large number of conductance traces are collected, and only the traces with clear stepwise features are selected to construct the conductance histogram (normally, hundreds of curves are selected from thousands of curves). All experiments were carried out at a fixed bias voltage of 50 mV unless otherwise noted.
Figure 1. Molecular structures of the compounds 4, 40 -bipyridine (BPY), 1,2-di-(pyridin-4-yl)ethene (BPY-EE) and 1,2-di(pyridin4-yl)ethane (BPY-EA) used in this study.
the conductance measurement of single molecule junctions through contact between pyridyl and non-Au electrodes. Here, we extend the ECSTM-BJ approach to investigate the conductance of molecules with a pyridyl anchoring group contacting to Cu clusters (as shown in figure 1), and a comparison is also carried out between molecule junctions contacting to Cu clusters and Au electrodes. Cu is chosen as the model system for the following reasons. First, Cu is an important electrode for the investigation of molecular electronics [45–48]. Second, the pyridyl anchoring group can bind to Cu, while the Cu cluster can be formed very easily by the ECSTM-BJ approach [37, 38, 49–51]. Third, Cu has similar electronic structure to Au, which facilitates comparison between them. Furthermore, the form of the interaction between the protonated pyridyl group and the metal electrode in an acidic solution is also discussed in this work.
3. Results and discussion 3.1. Conductance of BPY, BPY-EE and BPY-EA contacting with Cu The conductance measurement of Cu–BPY–Cu junctions was carried out in aqueous solution containing 0.05 M H2 SO4 , 1 mM CuSO4 and 0.5 mM BPY by using the ECSTM-BJ approach. The acidic supporting electrolyte was chosen because there is a strong interaction between the pyridyl group and Cu2+ in a neutral solution, where deposition can be formed immediately after the mixture of BPY and Cu2+ . Due to the pK1 = 3.5 and pK2 = 4.6 for BPY, the pyridyl group should be in the protonated form in 0.05 M H2 SO4 solution [52–55]. The Pt–Ir tip and Au(111) substrate were controlled at −5 mV and 45 mV versus the Cu wire, respectively. Figure 3(a) shows typical conductance curves exhibiting the conductance plateaus of BPY. The plateaus are caused by the formation of molecular junctions, as can be seen by comparing with the smooth decay of the curve (gray line in figure 3(a)) that is recorded in the same solution but without BPY molecules. Such a smooth decay curve corresponds to a tunneling process between the tip and the substrate. A two-dimensional (2D) conductance histogram was constructed from curves with stepwise features, and reveals a single conductance value of 130 nS as shown in figure 3(b). The conductance value is the same as the value using the one-dimensional (1D) histogram method (figure 3(d)). One of the corresponding arrays of Cu clusters (figure 3(c)), which was formed simultaneously by breaking off of the junction with the conductance curves, demonstrates the Cu atoms transferred from the tip to the substrate. These clusters with ∼1 nm height and ∼6 nm full width at half-maximum can be removed at a certain positive potential [37]. Comparing with the single set of conductance values for the –COOH ending group contacting with Cu by using the same approach [36, 37], we found two more sets of conductance values at 13 nS (figure 3(e)) and 5.5 nS (figure 3(f)) in the BPY–Cu contact with the pyridyl ending group. This may be caused by the different interaction between carboxylic acid–Cu and pyridyl–Cu. Three sets of conductance values, 130 nS, 13 nS and 5.5 nS, can be defined
2. Experimental section Naturally formed Au(111) from single crystal bead was used as the substrate, and was annealed in a hydrogen flame before each experiment. Mechanically cut Pt–Ir or electrochemically etched Au wires were used as the tips, which were insulated by thermosetting polyethylene glue. Pt and Cu wires were used as the counter and reference electrodes, respectively. The counter-electrode is used as the current circuit to pass the current in order to balance the current at the working electrode, while the reference electrode with a constant electrochemical potential is to act as reference in measuring and controlling the working electrode’s potential. 4, 40 -bipyridine (BPY) and CuSO4 were purchased from Alfa Aesar, while 1,2-di(pyridin-4-yl)ethene (BPY-EE), 1,2-di(pyridin-4-yl)ethane (BPY-EA) and NaClO4 were purchased from Sigma-Aldrich. H2 SO4 was purchased from Sinopharm Chemical Reagent Co. Ltd. All of them were used as received. Conductance measurement was carried out by using an ECSTM-BJ approach performed on a modified Nanoscope IIIa STM (Veeco, US) as in our previous reports at room temperature (around 25 ◦ C) [36–38]. Briefly, the metal is continuously electrodeposited onto the STM tip (figure 2(a)). Then, the deposited tip is pulled far away from the substrate with the STM feedback disabled (figure 2(b)). Next, the tip is driven towards the surface, and transfer of atoms of deposited metal from the tip to the substrate can occur upon the application of a pulse to the z-piezo of the STM, which is called a jump-to-contact process (figure 2(c)). Then, an atomic-sized wire of the deposited metal can be 2
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 2. Schematic diagram of the ECSTM-BJ approach for conductance measurement of single molecular junctions with in situ formed electrodes.
Figure 3. (a) Typical conductance curves of Cu–BPY–Cu junctions measured at a bias of 50 mV. A smooth decay curve (gray line) was recorded in the same solution without BPY molecules. (b) 2D conductance histogram constructed from the curves shown in (a). (c) The STM image (200 × 200 nm2 ) of a 10 ×10 array of Cu clusters simultaneously generated with the conductance curves. (d)–(f) 1D conductance histograms of single molecule junctions of (d) high conductance (0.89 nS bin size), (e) medium conductance (0.089 nS bin size) and (f) low conductance values (0.089 nS bin size).
3
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 4. (a) High conductance (0.89 nS bin size), (b) medium conductance (0.089 nS bin size) and (c) low conductance (0.089 nS bin size) histograms of BPY-EE junctions with contact of Cu. (d) High conductance (0.089 nS bin size), (e) medium conductance (0.0055 nS bin size) and (f) low conductance (0.0028 nS bin size) histograms of BPY-EA junctions with contact of Cu.
Figure 5. Conductance histograms of (a) BPY (0.89 nS bin size), (b) BPY-EE (0.89 nS bin size) and (c) BPY-EA (0.089 nS bin size) constructed by the current traces without data selection. The gray lines are the control experiments performed in the same solution but without sample molecules. All conductance histograms are normalized to one curve.
as high conductance (HC), medium conductance (MC) and low conductance (LC) values, respectively. The HC is about 10 times the MC, and is about 24 times the LC. Multiple sets of conductance values can be attributed to different contact geometries between the pyridyl anchoring group and the electrodes [2, 13, 41], and a change of the coupling between the electrode and the π system of the BPY molecule was also proposed recently [33, 40]. The conductance values of BPY-EE and BPY-EA contacting with Cu were also measured by the same approach in aqueous solution containing 0.05 M H2 SO4 , 1 mM CuSO4 and 0.5 mM target molecules. The conductance values for BPY-EE and BPY-EA are shown in figure 4. The HC, MC and LC values for BPY-EE are 45 nS, 4.5 nS and 1.1 nS, while those for BPY-EA are 5.5 nS, 0.5 nS and 0.1 nS, respectively. Three sets of conductance values are found for all three molecules (BPY, BPY-EE and BPY-EA). Here, the LC of BPY-EA was measured at a bias of 200 mV, because it is difficult to obtain well-defined data at a bias of 50 mV with the current scanner amplifier. Conductance histograms were also constructed for all current traces without data selection. Taking the high value for
pyridyl-based molecules as an example, similar conductance positions can be found in figure 5. These results appear to be similar to the histograms of –COOH, –NH2 and –SH terminated alkanes without data selection [30]. Comparing with the conductance histograms with data selection (figures 3 and 4), these histograms have very broad and weak peaks due to the increase in the background counts. The control experiments were also carried out in the same solution but without sample molecules; the histogram constructed from such curves (the gray line of figure 3(a)) is completely featureless. This feature shows the property of tunneling current between the tip and the substrate in electrolyte without sample molecules. The obviously different histogram shapes are found in solutions with and without molecules. 3.2. Conductance of BPY, BPY-EE and BPY-EA contacting with Au electrodes in acidic and neutral solutions The conductance of pyridyl-based single molecule junctions contacting to Au electrodes has been investigated in various conditions, such as neutral aqueous solution, organic solution and atmosphere [1, 2, 13, 15, 33, 40–43]. However, there is 4
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 6. (a) High conductance (0.89 nS bin size), (b) medium conductance (0.089 nS bin size) and (c) low conductance (0.089 nS bin size) histograms of BPY contacting to Au in 0.05 M H2 SO4 . (d) High conductance (0.89 nS bin size), (e) medium conductance (0.089 nS bin size) and (f) low conductance (0.089 nS bin size) histograms of BPY contacting to Au in 0.1 M NaClO4 .
Table 1. Summary of single molecule conductance (nS) with contact of Au and Cu in NaClO4 and H2 SO4 solutions. BPY Au (NaClO4 ) Au (H2 SO4 ) Cu (H2 SO4 )
BPY-EE
BPY-EA
HC
MC
LC
HC
MC
LC
HC
MC
LC
350 350 135
45 46 13
14 16 5.5
163 165 45
20 18 4.5
6.0 6.0 1.1
32 32 5.5
3.1 3.2 0.5
0.42 0.40 0.10
the protonated pyridyl group might contact to the electrode through the deprotonated form in the molecular junctions. Futamata also reported that the protonated form of pyridine and 2,20 -bipyridine are deprotonated before adsorption to the Au surface in a strong acidic solution [55, 58]. The BPY-EE and BPY-EA molecules were also studied in a similar way in acidic and neutral solutions, and also show three sets of conductance values. These data together with those for BPY are shown in table 1. Again, the similar conductance of BPY-EE and BPY-EA in the acidic and neutral solutions may demonstrate that the pyridyl-based molecule contacts to the electrode through the pyridyl group, although the pyridyl is in protonated form in acidic solution. Taking the HC values as an example, the conductance of BPY is about two times as large as that of BPY-EE, and is about one order of magnitude larger than that of BPY-EA. The small difference in conductance value between BPY and BPY-EE is attributed to the conjugated backbone structure of BPY-EE. However, the conjugation is broken for BPY-EA by the insertion of the saturated CH2 CH2 unit [2, 33]. The conductance measurement of Au–(BPY-EE)–Au junctions was also carried out in air with a self-assembled monolayer on Au(111), and gave the same value as measured in the aqueous solution. The result shows that the dominant means of electron transport is through single molecule junctions rather than through the electrolyte in the aqueous solution.
no report on the conductance measurement of pyridyl-based molecular junctions in acidic solution. We know that the existence of H+ can cause protonation of the pyridyl group in acidic solution [53–55], which might have an influence on the charge transport of molecular junctions. The single molecule conductance of a protonated pyridylbased molecule contacting to Au electrodes was studied in an aqueous solution containing 0.05 M H2 SO4 and 0.5 mM target molecules by using a traditional scanning tunneling microscopy break junction (STM-BJ) [1, 2, 13, 56, 57]. This approach is based on the repeated formation and breaking of molecular junctions between the Au tip and the Au(111) substrate, and the conductance curves are recorded during the process by applying a constant bias voltage between the tip and the substrate. As shown in figures 6(a)–(c), a single molecule junction of BPY with a protonated pyridyl group has three sets of conductance values, 350 nS, 46 nS and 16 nS, which can be defined as HC, MC and LC, respectively. These values are almost the same as those found in aqueous solution containing 0.05 M NaClO4 and 0.5 mM BPY shown in figures 5(d)–(f). Moreover, these data are similar to the conductance values reported by us (HC: 364 nS, MC: 46 nS) [2], Wang et al (HC: 584 nS, MC: 42 nS, LC: 10 nS) [41], Quek et al and Kamenetska et al (MC: 46 nS, LC: 12 nS) [33, 40]. The consistent single molecule conductance of the BPY molecule is found in acidic and neutral solutions, and shows that 5
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
Figure 7. Natural logarithmic plots of single molecule conductance versus (a) number of CH=CH units and (b) molecular length for molecular junctions formed with Cu clusters.
constant which describes the efficiency of electron transport along the molecules. Obviously, the different conductance of molecular junctions with various electrodes should be caused by the different molecule–electrode coupling strengths and efficiencies of electron transport along the molecules. The binding interactions of pyridyl–Cu and pyridyl–Au follow the order of pyridyl–Cu slightly larger than pyridyl–Au by theoretical calculation [49, 62]. On the other hand, the single molecule conductance of Cu–BPY–Cu is smaller than that of Au–BPY–Au (table 1), which is opposite to the order of the binding interaction. This may indicate the important contribution of the efficiency of electron transport along the molecule for Cu and Au electrodes to the single molecule conductance. Theoretical calculations show that the electron transport is dominated by the LUMO of these molecules binding to the Au electrode, as the LUMO is much closer to the Fermi level of the Au electrode [13, 33, 41]. No result has been reported for pyridyl–Cu contact, thus further theoretical and experimental investigations on pyridyl–Cu are needed. Now, we take the HC value as an example to discuss the efficiency of electron transport along the molecules with Cu and Au contacts. The N–N lengths of BPY, BPY-EE and ˚ 9.5 A ˚ and 9.5 A, ˚ BPY-EA are approximately equal to 7.2 A, respectively. The formula G = Gn=0 exp(−βN N) can also be expressed as ln G = ln Gn=0 − βN N. The linear fit slope of all points gives the βN . Taking the molecules of BPY and BPY-EE contacting to Cu as an example, the single molecule conductance values of BPY and BPY-EE are 135 nS and 45 nS, respectively (table 1). If we use the number of CH=CH as the unit, a slope of −1.099 can be obtained from figure 7(a), then the decay constant is 1.099 per CH=CH. Of course ˚ as the unit (the formula is now we can use the length A G = A exp(−βL L), ln G = ln A−βL L), and the decay constant ˚ −1 as shown in figure 7(b). Similarly, the decay is 0.478 A constant upon insertion of CH=CH is estimated to be 0.624 ˚ −1 ) from BPY to BPY-EE with per CH=CH (or ∼0.271 A Au electrodes. These decay constants are comparable with previously measured values for conjugated molecules [33, 61, 63]. Meanwhile, the decay constants upon the insertion of ˚ −1 ) and 3.201 CH2 CH2 are 2.392 per CH2 CH2 (or ∼1.04 A
3.3. The influence of the metal electrode Now we focus on the influence of the different electrodes on the single molecule junctions. Typically, the single molecule conductance is determined by the alignment of the molecular energy levels relative to the Fermi energy level of the electrode, and the electronic coupling efficiency between the anchoring group and the metal electrode [36]. Table 1 summarizes single molecule conductance with contact of Au in neutral and acidic solutions, and contact of Cu in acidic solution. For example, the conductance values of BPY–Cu, 135 nS (HC), 13 nS (MC) and 5.5 nS (LC), are smaller than those of BPY–Au, 350 nS (HC), 46 nS (MC) and 16 nS (LC), in H2 SO4 solution. Besides, the conductance values of BPY-EE and BPY-EA follow the same order for Cu and Au contacts. In particular, one end of the Cu–molecule–Cu junction is made of Cu clusters on Au(111) substrates. Such clusters might raise the question as to whether the conductance value of pyridyl–Cu is lower than that of pyridyl–Au due to the additional tunneling barrier from the cluster. This possibility can be excluded by the fact that the conductance of Cu–(succinic acid)–Cu is higher than that of Au–(succinic acid)–Au, as we previously reported using the same ECSTM-BJ approach [37]. Thus, the different conductance values between contacts with Cu clusters and Au electrodes can be attributed (or at least mainly attributed) to the different electrode material in the junctions. Interestingly, metal clusters created by the ECSTM-BJ approach have well-defined crystallographic structure [39, 59]. BPY, BPY-EE and BPY-EA have short backbones (N–N distance of 0.72–0.95 nm), and the conductance of such short molecular junctions follows the superexchange mechanism [33, 60, 61]. At low bias, the conductance can be expressed as G = Gn=0 exp(−βN N), where G is the molecular conductance, N is the number of CH=CH (or CH2 CH2 ) units, Gn=0 is a constant determined by the molecule–electrode coupling strength and responds to the contact conductance, and βN is the tunneling decay 6
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
˚ −1 ) from BPY to BPY-EA with per CH2 CH2 (or ∼1.392 A the Au and Cu contacts, respectively. Normally, smaller decay constants with saturated alkane have been reported by many groups [28, 30, 36], and the departure can be attributed to the decay constant estimated from the insertion of CH2 CH2 on conjugated BPY in this work. We state that these decay constants only reflect the degree of electron decoupling upon insertion of one CH2 CH2 or CH=CH to BPY, and are not expected to predict the conductance of the molecules upon further insertion of CH2 CH2 or CH=CH groups. To sum up, the decay constants for insertion of CH=CH or CH2 CH2 with Cu are larger than those for Au, which shows that the efficiency of electron transport along the pyridyl-based molecules for Cu is worse than for Au. The decay constant is sensitive to the alignment of the Fermi level of the metal in the molecular junction with the molecular energy levels [4, 34, 64]. Correspondingly, we may attribute the lower conductance value of the Cu–molecule–Cu junction to the lower efficiency of electron transport along the molecules than that of the Au–molecule–Au junction.
[3] Venkataraman L, Klare J E, Nuckolls C, Hybertsen M S and Steigerwald M L 2006 Dependence of single-molecule junction conductance on molecular conformation Nature 442 904–7 [4] Salomon A, Cahen D, Lindsay S, Tomfohr J, Engelkes V B and Frisbie C D 2003 Comparison of electronic transport measurements on organic molecules Adv. Mater. 15 1881–90 [5] Chen F, Hihath J, Huang Z F, Li X L and Tao N J 2007 Measurement of single-molecule conductance Annu. Rev. Phys. Chem. 58 535–64 [6] Heath J R 2009 Molecular electronics Annu. Rev. Mater. Sci. 39 1–23 [7] McCreery R L and Bergren A J 2009 Progress with molecular electronic junctions: meeting experimental challenges in design and fabrication Adv. Mater. 21 4303–22 [8] Song H, Reed M A and Lee T 2011 Single molecule electronic devices Adv. Mater. 23 1583–608 [9] Lu Q, Liu K, Zhang H M, Du Z B, Wang X H and Wang F S 2009 From tunneling to hopping: a comprehensive investigation of charge transport mechanism in molecular junctions based on oligo(p-phenylene ethynylene)s ACS Nano 3 3861–8 [10] Liu H M, Wang N, Zhao J W, Guo Y, Yin X, Boey F Y C and Zhang H 2008 Length-dependent conductance of molecular wires and contact resistance in metal-molecule-metal junctions ChemPhysChem 9 1416–24 [11] Bruot C, Hihath J and Tao N J 2012 Mechanically controlled molecular orbital alignment in single molecule junctions Nature Nanotechnol. 7 35–40 [12] Ma G H, Shen X, Sun L L, Zhang R X, Wei P, Sanvito S and Hou S M 2010 Low-bias conductance of single benzene molecules contacted by direct Au–C and Pt–C bonds Nanotechnology 21 495202 [13] Hong W, Manrique D Z, Moreno-Garc´ıa P, Gulcur M, Mishchenko A, Lambert C J, Bryce M R and Wandlowski T 2012 Single molecular conductance of tolanes: experimental and theoretical study on the junction evolution dependent on the anchoring group J. Am. Chem. Soc. 134 2292–304 [14] Zhou X S, Liu L, Fortgang P, Lefevre A S, Serra-Muns A, Raouafi N, Amatore C, Mao B W, Maisonhaute E and Sch¨ollhorn B 2011 Do molecular conductances correlate with electrochemical rate constants? Experimental insights J. Am. Chem. Soc. 133 7509–16 [15] Ie Y, Hirose T, Nakamura H, Kiguchi M, Takagi N, Kawai M and Aso Y 2011 Nature of electron transport by pyridinebased tripodal anchors: potential for robust and conductive single-molecule junctions with gold electrodes J. Am. Chem. Soc. 133 3014–22 [16] Sedghi G et al 2011 Long-range electron tunnelling in oligo-porphyrin molecular wires Nature Nanotechnol. 6 517–23 [17] Martin C A, Ding D, Sorensen J K, Bjornholm T, van Ruitenbeek J M and van der Zant H S J 2008 Fullerene-based anchoring groups for molecular electronics J. Am. Chem. Soc. 130 13198–9 [18] Tian J H et al 2010 The fabrication and characterization of adjustable nanogaps between gold electrodes on chip for electrical measurement of single molecules Nanotechnology 21 274012 [19] Taniguchi M, Tsutsui M, Yokota K and Kawai T 2009 Inelastic electron tunneling spectroscopy of singlemolecule junctions using a mechanically controllable break junction Nanotechnology 20 434008 [20] Xu B Q, Xiao X Y and Tao N J 2003 Measurements of single-molecule electromechanical properties J. Am. Chem. Soc. 125 16164–5
4. Conclusions The single molecule conductance of pyridyl-based molecules binding to Cu and Au was investigated. The conductance values of pyridyl–Cu contacts show the order BPY > BPY-EE > BPY-EA, which is the same as that of pyridyl–Au contacts. Three sets of conductance values are found for all three molecules (BPY, BPY-EE and BPY-EA) contacting to Cu clusters. These values are smaller than those of molecules contacting to Au electrodes, which can be attributed to the different electronic coupling efficiencies between the molecules and the electrodes. The same conductance values for the pyridyl-based molecules are found for both neutral and acidic solutions, which shows that the protonated pyridyl group contacts to the metal electrode through the deprotonated form in the acidic solution.
Acknowledgments We gratefully acknowledge support from Professor Bing-Wei Mao and Mr Zhao-Bin Chen of Xiamen University, and the reviewer’s kind suggestion for improvement of the discussion. This work is supported by the National Natural Science Foundation of China (Nos 21003110, 21273204, 21211130097) and the Planned Science and Technology Project of Zhejiang Province (No. 2011C37052).
References [1] Xu B Q and Tao N J 2003 Measurement of single-molecule resistance by repeated formation of molecular junctions Science 301 1221–3 [2] Zhou X S, Chen Z B, Liu S H, Jin S, Liu L, Zhang H M, Xie Z X, Jiang Y B and Mao B W 2008 Single molecule conductance of dipyridines with conjugated ethene and nonconjugated ethane bridging group J. Phys. Chem. C 112 3935–40 7
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
[21] Cui X D, Primak A, Zarate X, Tomfohr J, Sankey O F, Moore A L, Moore T A, Gust D, Harris G and Lindsay S M 2001 Reproducible measurement of single-molecule conductivity Science 294 571–4 [22] Nef C, Frederix P L T M, Brunner J, Sch¨onenberger C and Calame M 2012 Force–conductance correlation in individual molecular junctions Nanotechnology 23 365201 [23] Haiss W, Wang C S, Grace I, Batsanov A S, Schiffrin D J, Higgins S J, Bryce M R, Lambert C J and Nichols R J 2006 Precision control of single-molecule electrical junctions Nature Mater. 5 995–1002 [24] Yu L H, Keane Z K, Ciszek J W, Cheng L, Stewart M P, Tour J M and Natelson D 2004 Inelastic electron tunneling via molecular vibrations in single-molecule transistors Phys. Rev. Lett. 93 266802 [25] Taychatanapat T, Bolotin K I, Kuemmeth F and Ralph D C 2007 Imaging electromigration during the formation of break junctions Nano Lett. 7 652–6 [26] de Leon N P, Liang W, Gu Q and Park H 2008 Vibrational excitation in single-molecule transistors: deviation from the simple Franck–Condon prediction Nano Lett. 8 2963–7 [27] Park H, Park J, Lim A K L, Anderson E H, Alivisatos A P and McEuen P L 2000 Nanomechanical oscillations in a single-C60 transistor Nature 407 57–60 [28] Li C, Pobelov I, Wandlowski T, Bagrets A, Arnold A and Evers F 2008 Charge transport in single Au/alkanedithiol/ Au junctions: coordination geometries and conformational degrees of freedom J. Am. Chem. Soc. 130 318–26 [29] Widawsky J R, Kamenetska M, Klare J, Nuckolls C, Steigerwald M L, Hybertsen M S and Venkataraman L 2009 Measurement of voltage-dependent electronic transport across amine-linked single-molecular-wire junctions Nanotechnology 20 434009 [30] Chen F, Li X L, Hihath J, Huang Z F and Tao N J 2006 Effect of anchoring groups on single-molecule conductance: comparative study of thiol-, amine-, and carboxylic-acidterminated molecules J. Am. Chem. Soc. 128 15874–81 [31] Martin S, Haiss W, Higgins S, Cea P, Lopez M C and Nichols R J 2008 A comprehensive study of the single molecule conductance of α, ω-dicarboxylic acid-terminated alkanes J. Phys. Chem. C 112 3941–8 [32] Li X L, He J, Hihath J, Xu B Q, Lindsay S M and Tao N J 2006 Conductance of single alkanedithiols: conduction mechanism and effect of molecule–electrode contacts J. Am. Chem. Soc. 128 2135–41 [33] Kamenetska M, Quek S Y, Whalley A C, Steigerwald M L, Choi H J, Louie S G, Nuckolls C, Hybertsen M S, Neaton J B and Venkataraman L 2010 Conductance and geometry of pyridine-linked single-molecule junctions J. Am. Chem. Soc. 132 6817–21 [34] Ko C H, Huang M J, Fu M D and Chen C H 2010 Superior contact for single-molecule conductance: electronic coupling of thiolate and isothiocyanate on Pt, Pd, and Au J. Am. Chem. Soc. 132 756–64 [35] Kiguchi M 2009 Electrical conductance of single C60 and benzene molecules bridging between Pt electrode Appl. Phys. Lett. 95 073301 [36] Peng Z L, Chen Z B, Zhou X Y, Sun Y Y, Liang J H, Niu Z J, Zhou X S and Mao B W 2012 Single molecule conductance of carboxylic acids contacting Ag and Cu electrodes J. Phys. Chem. C 116 21699–705 [37] Zhou X S, Liang J H, Chen Z B and Mao B W 2011 An electrochemical jump-to-contact STM-break junction approach to construct single molecular junctions with different metallic electrodes Electrochem. Commun. 13 407–10 [38] Zhou X S, Wei Y M, Liu L, Chen Z B, Tang J and Mao B W 2008 Extending the capability of STM break junction for conductance measurement of atomic-size nanowires:
[39]
[40]
[41]
[42]
[43]
[44]
[45] [46] [47]
[48] [49] [50]
[51] [52] [53]
[54] [55]
[56]
8
an electrochemical strategy J. Am. Chem. Soc. 130 13228–30 Liang J H, Liu L, Gao Y Y, Wei Y M, Chen Z B, Zhou X S, Zhao J W and Mao B W 2013 Correlating conductance and structure of silver nano-contacts created by jump-to-contact STM break junction J. Electroanal. Chem. 688 257–61 Quek S Y, Kamenetska M, Steigerwald M L, Choi H J, Louie S G, Hybertsen M S, Neaton J B and Venkataraman L 2009 Mechanically controlled binary conductance switching of a single-molecule junction Nature Nanotechnol. 4 230–4 Wang C, Batsanov A S, Bryce M R, Martin S, Nichols R J, Higgins S J, Garcia-Suarez V M and Lambert C J 2009 Oligoyne single molecule wires J. Am. Chem. Soc. 131 15647–54 Tam E S, Parks J J, Shum W W, Zhong Y-W, Santiago-Berr´ıos M B, Zheng X, Yang W, Chan G K L, Abru˜na H D and Ralph D C 2011 Single-molecule conductance of pyridine-terminated dithienylethene switch molecules ACS Nano 5 5115–23 Li Z and Borguet E 2012 Determining charge transport pathways through single porphyrin molecules using scanning tunneling microscopy break junctions J. Am. Chem. Soc. 134 63–6 Pobelov I V, M´esz´aros G, Yoshida K, Mishchenko A, Gulcur M, Bryce M R and Wandlowski T 2012 An approach to measure electromechanical properties of atomic and molecular junctions J. Phys.: Condens. Matter 24 164210 Schull G, Frederiksen T, Arnau A, Sanchez-Portal D and Berndt R 2011 Atomic-scale engineering of electrodes for single-molecule contacts Nature Nanotechnol. 6 23–7 Lennartz M C, Caciuc V, Atodiresei N, Karth¨auser S and Bl¨ugel S 2010 Electronic mapping of molecular orbitals at the molecule–metal interface Phys. Rev. Lett. 105 066801 McGovern W R, Anariba F and McCreery R L 2005 Importance of oxides in carbon/molecule/metal molecular junctions with titanium and copper top contacts J. Electrochem. Soc. 152 E176–83 Bonifas A P and McCreery R L 2010 ‘Soft’ Au, Pt and Cu contacts for molecular junctions through surface-diffusionmediated deposition Nature Nanotechnol. 5 612–7 Wu D Y, Li J F, Ren B and Tian Z Q 2008 Electrochemical surface-enhanced Raman spectroscopy of nanostructures Chem. Soc. Rev. 37 1025–41 Wang D, Xu Q-M, Wan L-J, Wang C and Bai C-L 2002 Adlayer structures of pyridine, pyrazine and triazine on Cu(111): an in situ scanning tunneling microscopy study Langmuir 18 5133–8 Gao J S and Tian Z Q 1996 Surface enhanced Raman scattering of pyridine at copper electrodes excited with a 514.5 nm line Chem. Phys. Lett. 262 151–4 Musgrave T R and Mattson C E 1968 Coordination chemistry of 4,40 -bipyridine Inorg. Chem. 7 1433–6 Diao Y X, Han M J, Wan L J, Itaya K, Uchida T, Miyake H, Yamakata A and Osawa M 2006 Adsorbed structures of 4,40 -bipyridine on Cu(111) in acid studied by STM and IR Langmuir 22 3640–6 Wandlowski T, Ataka K and Mayer D 2002 In situ infrared study of 4,40 -bipyridine adsorption on thin gold films Langmuir 18 4331–41 Futamata M 2000 Unique adsorbed state of 4,40 -BiPy and -BiPyH2+ 2 on Au(111) electrode Chem. Phys. Lett. 332 421–7 Kiguchi M, Takahashi T, Takahashi Y, Yamauchi Y, Murase T, Fujita M, Tada T and Watanabe S 2011 Electron transport through single molecules comprising aromatic stacks enclosed in self-assembled cages Angew. Chem. Int. Edn 50 5708–11
Nanotechnology 24 (2013) 465204
X-Y Zhou et al
[57] Aradhya S V, Frei M, Hybertsen M S and Venkataraman L 2012 Van der Waals interactions at metal/organic interfaces at the single-molecule level Nature Mater. 11 872–6 [58] Futamata M 2001 Adsorbed state of 4,40 -BiPy and BiPyH2 2+ on Au(111) electrode J. Phys. Chem. B 105 6933–42 [59] Wei Y M, Liang J H, Chen Z B, Zhou X S, Mao B W, Oviedo O A and Leiva E P M 2013 Stretching single atom contacts at multiple subatomic step-length Phys. Chem. Chem. Phys. 15 12459–65 [60] Davis W B, Svec W A, Ratner M A and Wasielewski M R 1998 Molecular-wire behaviour in p-phenylenevinylene oligomers Nature 396 60–3 [61] Ho Choi S, Kim B and Frisbie C D 2008 Electrical resistance of long conjugated molecular wires Science 320 1482–6
[62] Wu D Y, Ren B, Xu X, Liu G K, Yang Z L and Tian Z Q 2003 Periodic trends in the bonding and vibrational coupling: pyridine interacting with transition metals and noble metals studied by surface-enhanced Raman spectroscopy and density-functional theory J. Chem. Phys. 119 1701–9 [63] He J, Chen F, Li J, Sankey O F, Terazono Y, Herrero C, Gust D, Moore T A, Moore A L and Lindsay S M 2005 Electronic decay constant of carotenoid polyenes from single-molecule measurements J. Am. Chem. Soc. 127 1384–5 [64] Engelkes V B, Beebe J M and Frisbie C D 2004 Lengthdependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance J. Am. Chem. Soc. 126 14287–96
9
