Single Tripyridyl–Triazine Molecular Junction with Multiple Binding

Apr 9, 2016 - We demonstrated that the single TPTZ molecular junctions exhibit highly conductive character up to 10–1 ... moiety/metal] junction can...
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Single Tripyridyl−Triazine Molecular Junction with Multiple Binding Sites Madoka Iwane, Shintaro Fujii,* Tomoaki Nishino, and Manabu Kiguchi* Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan S Supporting Information *

ABSTRACT: We present an electronic characterization of a single molecular junction of 2,4,6-tris(2′,2″,2‴-pyridyl)-1,3,5-triazine (TPTZ) with multiple metal− molecule binding sites using scanning tunneling microscopy-based break junction method under ambient conditions. The TPTZ molecule consists of a centered triazine moiety and surrounding three 2-pyridyl groups. The benzene rings containing a N atom in TPTZ act as molecular binding sites for bridging a gap between two Au electrodes to form a single molecular junction. Because the N atom at the ortho-position in the 2-pyridyl groups is spatially hidden from the electrode surfaces, the single molecular junction forms via direct metal−π couplings. We demonstrated that the single TPTZ molecular junctions exhibit highly conductive character up to 10−1 G0 (G0 = 2e2/h), which is due to the effect of the direct metal−π coupling. We found three preferential conductance states of ca. 10−1, 10−2, and 10−4 G0, which suggests that the single TPTZ molecular junctions have three charge transport paths depending on the molecular anchoring sites on the Au electrodes. Analysis of electrode−gap distance in the molecular junction revealed that effective gap length is 0.5, 0.9, and 1.2 nm for the high, medium, and low conductance states, respectively. By combining the results of the measured conductance and the estimated electrode−gap distance, we proposed models of junction-structures for the observed three conductance states. This study demonstrates that a molecular junction consisting of multiple metal−π binding sites provides high and tunable conductance behavior based on the multiple charge transport paths within a molecule.

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been investigated for quaterthiophene derivatives,9 fullerene derivatives,10 and heterocycle-based oligo (phenylene ethynylene) derivative.11 Our group also reported the conductance switch for a quaterthiophene-based molecular junction with multiple thiophene anchoring sites.12 By tuning the distance between metal electrodes and changing the anchoring sites in the single molecular junction, electronic conductance switched among three states of 0.05 G0 (G0 = 2e2/h), 0.005 G0, and 0.0005 G0. Ie et al. have demonstrated formation of a single molecular junction by making multiple metal−molecule contacts and resultant increases in the structural stability and electronic conductance for a tripodal bipyridine derivative.13 While there is an increasing number of investigations on the single molecule junctions with multiple anchoring groups (i.e., multiple charge transport paths), the conductance value of the single molecular junction is still lower than 0.01 G0. Our group have reported that a highly conductive single molecular junction can be prepared by direct binding of π-molecules (e.g., benzene and C60) to metal electrodes.14−16 The conductance of the single benzene molecular junction with Pt is 1 G0, which is close to the conductance of the metal atomic contacts. In this study, we focus on the 2,4,6-tris(2′,2″,2‴-

INTRODUCTION An understanding of the charge transport through single molecular junctions is of considerable fundamental interest in molecular electronics.1,2 In recent years, the single molecule junction where a single molecule is trapped between two metal electrodes has been routinely prepared by break junction methods, and its electronic conductance has been measured using scanning tunneling microscopy-based break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques.3−7 While the intrinsic structural nature of the molecular backbone has been demonstrated to affect the electronic conductance of single molecular junctions, the molecule−electrode coupling also plays a significant role in the charge transport.2,6−8 In most single molecular junctions, chemical functional groups (i.e., anchoring groups) are present at both of the molecular termini to bind the single molecule to the metal electrodes. Besides the two-terminal molecular junctions, multiterminal molecular junctions have attracted increasing interest because the multiple anchoring groups offer higher junction stability and higher electronic conductance and provide an opportunity to investigate (i) conductance modulation due to a change in the molecular anchoring groups to be contacted to the metal electrodes and (ii) the quantum interference effect in a multiterminal electronic transport system. The conductance modulation due to changes in available molecular anchoring sites within a molecule has © XXXX American Chemical Society

Received: December 30, 2015 Revised: April 9, 2016

A

DOI: 10.1021/acs.jpcc.5b12728 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C pyridyl)-1,3,5-triazine (TPTZ) molecule (Figure 1) for a πmolecule with multiple π-binding sites. The TPTZ molecule is

Figure 2. STM image of the TPTZ molecules on Au(111). Imaging area = 60 × 60 nm; scale bar = 20 nm; tunneling set point (It) = 0.64 nA; sample bias voltage (Vs) = 100 mV.

Figure 1. (a) Molecular structure of 2,4,6-tris(2′,2″,2‴-pyridyl)-1,3,5triazine (TPTZ). (b) Schematic view of the STM-BJ setup.

molecules on Au(111) have molecular orientations with its πplane parallel to the substrate. Two-dimensional (2D) conductance histograms are presented in Figure 3 and Figure S2. To construct the 2D histogram, the first identified data point at a threshold value was set to distance = 0 to overlap all individual traces (Figure S1) in the 2D space. The threshold values were 1 G0 for Figure 3a,c and 0.026 G0 for Figure 3b,d. The large intensity around 1 G0 in Figure 3a,c corresponds to the formation of a Au atomic contact, and breakage of the Au atomic contact leads to the formation of nanosized Au electrodes. The TPTZ molecule then stochastically bridges the gap between the Au electrodes to form the single molecular junctions. In the molecular 2D histograms (Figure 3c,d), we found three main conductance distributions around 10−1 G0 (high (H) state), 10−2 G0 (medium (M) state), and 10−4 G0 (low (L) state). In the blank sample (Figure 3a,b), no preferential conductance is apparent in the conductance range below 1 G0. The three H, M, and L conductance states could originate from the formation of the single TPTZ molecular junction with three different charge transport paths. The 2D conductance histograms also show that the H, M, and L conductance states appear in that order during the stretching process of the junction. The conductance values of the H, M, and L states were determined by the peak positions in the 1D conductance histogram (Figure 4). The conductance values are determined to be 0.087 (H state), 0.7 × 10−2 G0 (M state), and 0.4 × 10−4 G0 (L state), respectively. We successfully fabricated the single molecular junction with multiple conductance states, where the highest conductance values are close to metal atomic contact. To get information on TPTZ junction-structures, we estimated the electrode−gap distance in the single molecular junction by analyzing the length of the conductance traces during the breaking process. Just after breakage of the Au atomic contact, a nanogap is formed between the resultant Au electrodes. At this breaking point, the gap size is about 0 nm. The gap size increases along with stretching of the molecular junction, and it is approximately equal to the stretch length from this breaking point. In the present study, the gap distance was calculated as the distance between points (set points) at which the conductance drops below 0.3 G0 and 0.026 G0 for the H state. The set points are 0.026 G0 and 0.8 × 10−3 G0 for the M state, and 0.026 G0 and 1.0 × 10−5 G0 for the L state, respectively. Figure 5 shows the distribution of the gap distance for the H, M, and L states. The peak−gap distance is 0.01, 0.30, and 0.67 nm for the H, M, and L states, respectively. By considering the effect of the elastic response of the Au electrodes and the offset distance for the M and L states, the

a planar π-molecule, in which a centered triazine moiety has three 2-pyridine functional groups. When the central triazine binds to two metal electrodes with its molecular plane perpendicular to junction axis, resultant direct π-binding in the [metal/triazine moiety/metal] junction can cause highly conductive character. Because the side pyridyl groups can also be anchoring sites, the single TPTZ molecular junction can display several conductance states depending on the anchoring sites to be bonded to the two metal electrodes. Here, we have investigated the electronic conductance behavior of single TPTZ molecular junctions using the STM-BJ at ambient conditions. The STM-BJ measurements revealed that the single TPTZ molecular junction exhibited three conductance states. The highest conductance value is ca. 0.1 G0, which is much larger than those of the conventional single molecular junctions.4,17 On the basis of conductance change during the stretch process of the molecular junctions, structural models of three conductance states are proposed.

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EXPERIMENTAL SECTION STM imaging and STM-BJ measurements were performed using a commercially available STM system (Digital Instruments Nanoscope V). The STM tip was prepared from a Au wire (diameter ≈ 0.3 mm; Nilaco). The Au(111) substrate was prepared by thermal deposition of Au on mica at elevated temperature in high vacuum. TPTZ (Figure 1a) was deposited on Au(111) by immersing the substrate in a 1 mM TPTZ ethanol solution at room temperature for 1−3 days. In the STM-BJ experiments, molecular junctions were typically formed by repeated formation and breaking of a Au point contact between the Au tip and the TPTZ-deposited Au(111) substrate (Figure 1b). During the cycles, the molecules stochastically bridge the gap between two electrodes of the Au(111) substrate and Au tip. The sample bias voltage was fixed at 50 or 100 mV. We used two current amplifiers to cover the wide conductance regime (1 μA/V for (1.3−1.0) × 10−3 G0, 10 nA/V for (0.026−1.0) × 10−5 G0). The details for this procedure are described elsewhere.12,13,18,19

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RESULTS AND DISCUSSION Figure 2 shows a STM image of the TPTZ molecule on flat Au(111) substrate, where the substrate is covered by nanosized spots. The bright spots are as small as 1 nm and are attributable to the individual molecules. The apparent height of the molecule is 0.14 nm, which is much smaller than the diameter of the TPTZ molecule (∼1.2 nm). Therefore, the TPTZ B

DOI: 10.1021/acs.jpcc.5b12728 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 3. 2D conductance histograms for Au contacts (a,b) without and (c,d) with TPTZ molecules for (a,c) high ((1.3−3) × 10−3 G0, Vs = 100 mV) and (b,d) low ((0.026−1) × 10−5 G0, Vs = 50 mV) conductance regimes using different amplifiers. A linear bin-size of 0.01 nm and a logarithmic Y-bin-size for Δ log(G/G0) of 0.01 were used for construction of conductance histograms. The histograms were constructed from 1000 traces without data selection. The H, M, and L conductance states are indicated by arrows in the histogram.

electrode surfaces, it is difficult to form the Au−N couplings. Therefore, the binding modes in the models (Figure 6) are restricted to the direct π−Au couplings, which leads to the perpendicular molecular orientations along the charge transport direction. Similar perpendicular orientations have been reported for tolane derivatives with meta-substituting pyridyl terminal groups.23 In the TPTZ molecule, the pyridyl groups connect to the meta-positions of the central triazine. The previous theoretical study revealed that the molecular π-state cannot effectively contribute to the charge transport by quantum interference effect.23−25 A very recent study on the tolane derivatives with two pyridyl terminal groups demonstrated that a tolane derivative with two meta-substituting pyridyl terminal groups featured considerably lower conductance in contrast to a conventional one with two para-substituting pyridyl terminal groups. On the basis of the effective length of the molecular backbone, we analyzed the effective length dependence of single molecular conductance. In a tunneling transport regime, molecular conductance (G) can be described by G ∝ exp(−βL), where β is a decay constant and L is effective junction-length. The obtained β value of 1.1 Å−1 is larger than those of the π-conjugated molecule (carotenoid; ∼0.5 Å−1) and saturated ones (alkane and peptides; ∼1 Å−1).2,17,18

actual gap distance is estimated to be 0.5, 0.9, and 1.2 nm for the H, M, and L states. For details, see the main text and section 2 in the Supporting Information. Now, we discuss the structure of the single TPTZ molecular junction based on the experimentally obtained conductance and gap distance. Considering that the H state features a short gap distance of 0.01 nm, the H state is formed just after the breakage of a Au atomic contact where the TPTZ molecule can bind to Au electrodes with its molecular plane perpendicular to the junction axis (Figure 6a). A similar molecular orientation can be found in the STM image, in which the TPTZ molecules have more like a parallel orientation on Au(111) (i.e., a perpendicular orientation along the charge transport direction). A highly conductive single molecular junction is formed by the direct binds of a π-plane of the triazine moiety to metal electrodes, which agrees with the previously reported results of benzene/Pt, Ag; ethylene/Pt; C60/Au, Pt;14−16 and pyradine/ Pt.20 The gap distances of the single TPTZ molecular junction of the M and L states (L = 0.5 nm, M = 0.9 nm, L = 1.2 nm) are in reasonable agreement with the estimated length (L = 0.60 nm, M = 1.01 nm, L = 1.32 nm) based on the interplane distance between the planar TPTZ molecule and a Au electrode (ca. 0.3 nm) and the previous X-ray crystallographic study on a metal complex of the TPTZ molecules, wherein the inter-ring distances between triazine−pyridine and pyridine−pyridine are found to be 0.41 and 0.72 nm21 (see sections 3 and 4 in the Supporting Information). It should be noted that the pyridyl groups can bind to Au electrodes via two binding modes of π− Au or N−Au couplings.22,23 Because the ortho-N atom in the pyridyl group in TPTZ is spatially hidden (Figure S6) from the

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CONCLUSION We have successfully fabricated the single TPTZ molecular junction with the multiple conductance states using STM-BJ under ambient conditions. We found three preferential single C

DOI: 10.1021/acs.jpcc.5b12728 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 6. Schematic illustration of the single TPTZ molecular junctions in (a) H, (b) M, and (c) L conductance states. Yellow, white, gray, and blue balls represent Au, H, C, and N atoms.

the molecular conductance based on the multiple metal−π binding sites, which could provide a fundamental guide toward modulation of the molecular conductance in the fields of molecular electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12728. Additional details regarding (1) individual conductance traces, (2) blank measurements, (3) gap distance analysis, (4) DFT calculations, and (5) STM-BJ experiments for molecular junctions of 4,4′-bipyridine (PDF)

Figure 4. 1D conductance histograms for Au contacts (a) without and (b) with TPTZ (Vs = 50−100 mV). The histograms are constructed from 1000 traces without data selection. A logarithmic bin-size for Δ log(G/G0) of 0.01 was used for conductance histograms. The conductance peaks are denoted by arrows (H, (0.087 ± 0.006) G0; M, (0.70 ± 0.09) × 10−2 G0; L, (0.40 ± 0.17) × 10−4 G0). The peak positions are obtained by Gaussian fitting (see solid lines in (b)).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (nos. 24245027, 26102013) from MEXT, and by the Asahi Glass Foundation.

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Figure 5. Distribution of the gap distance of the H, M, and L conductance states for the single TPTZ molecular junctions. The distribution was constructed without data selection from 1000 traces recorded at a bias voltage of 50−100 mV. A logarithmic bin-size for Δ log(X/nm) of 0.033 was used. Solid lines are obtained by Gaussian fittings (peak positions are 0.01, 0.30, and 0.67 nm for the H, M, and L states, respectively.). Arithmetic mean values are 0.02, 0.24, 0.57 nm for the H, M, and L states.

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DOI: 10.1021/acs.jpcc.5b12728 J. Phys. Chem. C XXXX, XXX, XXX−XXX