Effect of Mechanical Strain on Electric Conductance of Molecular

Article pubs.acs.org/JPCC

Effect of Mechanical Strain on Electric Conductance of Molecular Junctions Junichi Inatomi,† Shintaro Fujii,*,† Santiago Marqués-González,† Hiroshi Masai,‡ Yasushi Tsuji,‡ Jun Terao,*,‡ and Manabu Kiguchi*,† †

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 7, 2015 | http://pubs.acs.org Publication Date (Web): August 7, 2015 | doi: 10.1021/acs.jpcc.5b04386

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8511, Japan ‡ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Electromechanical properties of single molecular junctions are investigated using scanning tunneling microscopy based break junction method. Two types of molecular junctions consisting of π-conjugated backbones with and without coordinative bonding (i.e., Co((4-aniline)terpyridine)2 complex and oligo(phenylene-ethynylene) derivative) are prepared between two Au electrodes. Electronic transport measurements revealed molecular conductance of ca. 10−4 G0 (G0 = 2e2/h) for both of the molecular junctions. Then we assessed the electronic transport properties of the two types of molecular junctions under mechanical strain in their compression−elongation cycle. We found significant asymmetric electromechanical response for all covalent systems of the oligo(phenylene-ethynylene) derivative, while the Co complex with the coordinative bonding exhibits symmetric modulation of the electronic transport property in the compression−elongation cycle of the molecular junctions. The asymmetric and symmetric electromechanical behavior can be, respectively, ascribed to rigid covalent bonding in the π-conjugated backbone and flexible coordinative bonding at the metal center. This study demonstrates potential tunability of the molecular conductance under mechanical stimulus.

1. INTRODUCTION Molecular electronics reached maturity, based on an important effort from the scientific community and boosted by the quick development and availability of scanning probe microscopies. Single molecular junctions have attracted wide attention due to their potential applications to ultrasmall electronic devices.1−7 Large effort has been focused on the electronic side of the molecular junction,8−10 that in conjunction with theoretical models have greatly contributed to a better understanding of charge transport through single molecule junctions. A number of studies have highlighted the direct link between the structural details of molecular junctions and their electrical performance setting the bases for advanced molecular design (e.g., see ref 11). More recently, mechanical control over molecular conductance has been attracting increasing attention. Mechanical control has been induced through rearrangement of the molecular conformation or molecular contact engineering.12,13 In a pioneering work, Joachim et al. demonstrated a molecular electromechanical amplifier employing an STM tip to mechanically control the conductance of a single C60 molecule.14 Upon increasing tip pressure on C60, through tip−sample distance modulation ca. 0.1 nm, the conductance of C60 was found to increase by over 2 orders of magnitude. This © 2015 American Chemical Society

large conductance shift was attributed to the closure of the HOMO−LUMO gap as a result of the mechanical deformation induced on the C60 molecular cage. Furthermore, Parks et al. demonstrated mechanical control over the spin state and magnetic anisotropy of a Co terpyridine complex.15 The observed splitting of the Kondo peaks as a function of mechanical stretching was attributed to a distortion of the molecular symmetry induced by mechanical strain. Quek et al. reported mechanically induced conductance switching in 4,4′-bipyridine junctions through repeated compression−elongation of the molecular junction.16 This switching behavior was attributed to different contact geometries of the pyridine-Au bond leading to two distinct conductance states. More recently, Meisner et al.17 and DiezPerez et al.18 demonstrated examples of manipulating molecular conductance by increasing and decreasing π orbital coupling with the metal electrodes using scanning tunneling microscopybased break junction method (STM-BJ). In a closely related study, our group reported conductance switching between three discrete conductance states in quarterthiophene (QT) Received: May 7, 2015 Revised: July 13, 2015 Published: July 27, 2015 19452

DOI: 10.1021/acs.jpcc.5b04386 J. Phys. Chem. C 2015, 119, 19452−19457

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The Journal of Physical Chemistry C

smaller than previous ones.16−21 Molecular junctions were prepared by STM-BJ in a liquid environment. We compared electromechanical properties of two types of molecules with a similar molecular length consisting of rigid π-conjugated backbones with and without coordination bonds (i.e., Co((4aniline)-terpyridine)2 complex and oligo(phenylene-ethynylene) (OPE)). Both of the molecules have the same −NH2 anchoring groups at the termini and therefore bridge the gap between two Au electrodes to form molecular junctions via NH 2 −Au bonding. As reported previously mechanical perturbations modulate metal−molecule electronic couplings and affect the electromechanical properties of molecular junctions.16−21 We expect that additional effect of their molecular backbones on electromechanically properties can be elucidated by comparing electrochemical response of molecular junctions with the same anchoring group. The Co complex has coordination bonds at the center part while the OPE derivative is all covalent system bearing a permethylated α-cyclodextrin (PMα-CD) ring.22 The encapsulation strategy has been proven successful in many interesting aspects. It is now known that this coating can enhance the chemical stability, solubility, and optical properties of the frequently π-conjugated guest molecule. PMα-CD has been employed previously as a host able to generate an inert shielding on the guest molecule. It has been proven to be a successful method to inhibit π−π stacking interactions between OPE derivatives, resulting on a marked enhancement of the emission properties of the guest OPE. In the present STM-BJ study, the PMα-CD ring is employed to prevent the π−π stacking (bundling) of OPE to form multiple molecular junctions formation.

junctions, through dynamic control of the multiple molecular anchoring points available in QT.19 In previous studies on mechanical manipulation of molecular conductance in break junctions,16−21 external mechanical stimulus have been applied by compressing and elongating a molecular junction using a piezoactuator. The degree of mechanical stimulus can be approximately normalized to be 0.28−0.42,16 0.16,17 0.22,18 0.14−0.43,19 and 0.04−0.1120 where displacement of a molecular junction is divided by molecular length (i.e., interatomic distance between two binding groups at the molecular termini). In this work, we investigate the molecular conductance as a function of mechanical stretching. In contrast to the dynamic control over the molecular anchoring points,19 we pay attention to controlling the conformation of single molecular junctions. To avoid the drastic change in the molecular anchoring points we used relatively small mechanical perturbations with normalized mechanical stimulus of ca. 0.02 (i.e., 0.039 nm [displacement]/2 nm [molecular length], see the Experimental section and Figure 1 for a detail), which is 1 order of magnitude

2. EXPERIMENTAL AND THEORETICAL METHODS Conductance measurements of 1 and 2 (Figure 1) were carried out using the STM-BJ technique, introduced by Xu and Tao,10 employing an electrochemical STM (Pico-SPM, Molecular Imaging Co.) with a Nano Scope 3D controller (Digital Instruments Co.). A more detailed description of the setup typically employed by our group can be found elsewhere.19,23,24 Briefly, the STM tip was prepared from commercially available

Figure 1. Chemical structure of the Co complex 1 and oligo(phenylene-ethynylene) with permethylated α-cyclodextrin moiety 2. N to N distances at the two termini of 1 and 2 are indicated by arrows. The lengths are calculated by structural optimization based on density functional theory (for further detail, see the main text).

Figure 2. (a) Typical STM-BJ conductance traces obtained from tetraglyme (black/left), and tetraglyme solutions of 1 (blue/center) and 2 (red/ right). Traces are shifted along the x axis for clarity. All conductance data was referenced to the conductance quantum G0 = 2e2/h = 77.48 μS. (b) Logarithmically binned one-dimensional conductance histograms built using 111 representative conductance traces showing the most probable conductance value of 1 ((1.0 ± 0.2) × 10−4 G0) (blue/bottom) and 2 ((9.4 ± 1.0) × 10−5 G0) (red/top). The histograms are smoothed by adjacent averaging (see the Supporting Information). 19453


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Figure 3. Typical conductance profiles obtained during consecutive 16 ms compression−elongation cycles for the (a) Co complex 1 (blue/left) and for (b) OPE derivative 2 (red/right). Spectra were shifted along the y axis for clarity. (c) One example of conductance profiles of Co complex 1 (blue) and OPE derivative 2 (red). Spectra were not shifted for comparison.

Au wire (≥99%, ϕ ≈ 0.25 mm, Nilaco Co.). The gold substrates were flame-annealed and quenched in water prior to use. Thousands of junctions were produced by repeatedly bringing the STM tip in and out of contact with the Au substrate, until a statistically significant data set was obtained for each sample. Experiments were conducted in 0.5 mM tetraglyme solutions of the molecules under study, at a constant 20 mV bias voltage. The process was then repeated on distinct samples to ensure reproducibility. Electromechanical properties of molecular junctions were investigated by measuring the electronic transport properties under a compression−elongation cycle of the molecular junctions with the amplitude of 0.035 nm and the rate of 62.5 Hz (see Figure 4a). For details of the synthetic procedure of the molecules, see the Supporting Information. Density functional theory (DFT) calculations were carried out using the Gaussian 09 program.25 The B3LYP/3-21G* basis sets were used for C, N, and H, while the LanL2DZ is used for Co. The atomics positions of 1 and 2 (Figure 1) were relaxed to obtain initial equilibrium atomic structures. For the model of 2, the PMα-CD moiety was omitted. Potential energy of the molecules was calculated by performing structural optimization of the models with verifying N to N interatomic distances of ΔLN−N (where ΔLN−N is displacement from the equilibrium position). During the structural realization, the N to N interatomic distance was fixed.

3. RESULTS AND DISCUSSION Figure 2a shows typical conductance traces obtained when the STM-BJ measurements were performed in tetraglyme (black) and 0.5 mM tetraglyme solutions of 1 and 2 (blue and red, respectively). Most of the conductance traces showed a 1 G0 plateau. While a sharp exponential decay was typically observed for conductance measurements performed in tetraglyme, a number of well-defined conductance plateaus appeared for 1 and 2 ca. 10−4 G0. For the Co complex molecules, the conductance decreased in a stepwise fashion. The conductance value of the last step was around 0.0001 G0. In a few traces, steps appeared at conductance values whose conductance values were twice as that of the last step. Most of the steps showed either a positive or negative slope, which originated from a structural change in the molecular junction during the breaking of the junction. No features were observed below 5 × 10−5 G0 in molecular conductance histograms. In the blank solution, no peaks were observed below 1 G0 in conductance histograms. The conductance traces obtained for 1 and 2 were compiled into logarithmically binned histograms and plotted in a semilog scale (Figure 2b). The recurrent appearance of molecular plateaus ca. 10−4 G0 yielded a clear conductance peak corresponding to the most probable conductance values of 1 (1.0 ± 0.2) × 10−4 G0 (blue) and 2 (9.4 ± 1.0) × 10−5 G0 (red). The single molecular conductance value found for 2 is in good agreement with those for OPE without the α-CD moiety in the literature (1.3 × 10−4 G0 and 1.4 × 10−4 G0).26,27 As described in the Introduction, the PMα-CD ring is introduced 19454


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Figure 4. (a) Schematic representation of the bias pulse applied to the z-piezo element effectively changing the junction size by ±0.035 nm. Schematic representation of the points Ga − Gd defined to help the statistical analysis of the electromechanical response of 1 and 2. (b) Normal distribution obtained for ΔG = LGbackward − LGforward for 1 (ΔG1) and 2 (ΔG2) (scattered). Lorentz−Cauchy fitting of both distributions (solid line). Negative values correspond to those compression−elongation cycles were the conductance response is greater for the forward than for the backward directions (LGforward > LGbackward). Center position (ΔGcenter‑1) = −0.0197, Skew (ΔGskew‑1) = 1.33 for 1 and ΔGcenter‑2 = −0.0064, ΔGskew‑2 = 1.10 for 2. Skew was calculated on the raw data, not from the Lorentzian distribution.

Figure 5. (a) DFT calculated potential energy difference as a function of the interatomic distance between N atoms at the termini (LNN) of the Co complex 1 (blue) and the OPE derivative 2 (red). Calculated total energy of E and LNN at the equilibrium atomic positions are set to zero. Forward and backward directions (see Figure 4a) are indicated by arrows. In the forward direction, the junction is shrunk back to the original position. Conversely the junction is extended back to the zero position in the backward direction. (b) Molecular orbital levels with the highest energy as a function of LNN for the Co complex 1 (blue) and the OPE derivative 2 (red). (c) Visualization of the molecular orbital with the highest energy at LNN = 0 for 1 (left) and 2 (right). The isovalues are |0.02| e/A3. At the molecular center, 1 displays nonbonding character while 2 is characterized by bonding character along the molecular axis along the charge transport direction. White, gray, and blue balls correspond to H, C, and N atoms, respectively. Isosurface is visualized by red and green colors depending on the phase signs.

to prevent bundling of OPE to form multiple molecular junctions-formation. Interestingly no clear effect of the PMαCD ring on the electronic transport property was apparent. Additionally, a series of experiments were performed in order to obtain the electromechanical response of single molecular junctions of 1 and 2. To that end, single molecules were addressed with the STM tip and the junction conductance was monitored while the tip−substrate distance was subject of a series of reversible subnanometer variations effectively leading

to compression−elongation of the molecular junction. Once the single molecule junction was formed, a triangular bias pulse was applied to the z-piezo element resulting on the modulation of the tip−substrate gap by ±0.035 nm in 16 ms compression− elongation cycles (see Figure 4a). The typical electromechanical response of single molecule junctions of 1 (blue/ left) and 2 (red/right) is shown in Figure 3. Remarkable differences can be observed between the electromechanical response of 1 and 2. First the variation in 19455


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occupied molecular orbitals with the highest energy that can contribute charge transport though the molecular junctions. The energy levels are plotted as a function of N to N interatomic distance. The Co complex 1 exhibits a nonbonding π state, which has dominant contribution on the charge transport. A previous DFT-based charge transport study on the OPE derivative 2 revealed that the charge transport channel originating from the highest occupied molecular orbital (HOMO) has larger contribution on the charge transport (i.e., The HOMO channel is energetically closest to the Fermi level of the Au electrodes).29 We found linear shift of the molecular orbital for the Co complex 1, while the OPE derivative 2 displays a nonlinear shift of the molecular orbital at negative and positive deformation within a small scale (ΔLNN = ± 0.1 nm). The negative and positive shifts of the energy level of the occupied molecular orbital-channel induce decrease and increase in the charge transport efficiently. Therefore, the OPE derivative 2 with the nonlinear shift of the molecular orbital can exhibit a higher degree of asymmetric response of the charge transport efficient against negative and positive deformation. To obtain intuitive understanding of the observed symmetric and asymmetric shift of the molecular orbital we visualized the molecular orbitals as show in Figure 5c. The molecular orbital of the Co complex 1, is characterized by nonbonding character at the molecular center. The interaction energy of such nonbonding state is essentially insensitive against small deformation of the molecular backbone, which leads to the linear shift of the molecular orbital (blue line Figure 5b). On the contrary, the molecular orbital of the OPE derivative indicates unidirectional bonding character of the molecular axis along the charge transport direction. At the negative deformation, the bonding character stabilizes the electronic energy of the system, which causes nonlinear shift of the molecular orbital (red line in Figure 5c). We intuitively attribute the symmetric (weakly asymmetric) electrochemical response of the Co complex 1 to the flexibility of the coordinative bonding relative to covalent bonding. The external mechanical strain could be effectively relaxed at the metal center of the Co complex while the unidirectional fully covalent bonding system of OPE exhibits sensitive response against the deformation. From a molecular orbital perspective, the conduction orbital of the Co complex 1 possesses nonbonding character at the molecular center. The energy level of the nonbonding state (i.e., energy level alignment between the conduction orbital and the Fermi level of the Au electrodes) is insensitive against deformation, which leads to a weak and relatively symmetric electrochemical response. The conduction orbital of the OPE derivative 2 is characterized by unidirectional bonding character along the charge transport direction, which causes “bonding character-induced nonlinear energetic stabilization” and higher degrees of asymmetric electromechanical response.

conductance (amplitude) of 2 was found to be larger than that of 1, which indicates higher degree of electromechanical response of the OPE junctions. Importantly, a marked asymmetry can be observed in many of the compression− elongation cycles collected for 2. In order to quantitatively show this finding, the steepness of the electrical response to mechanical strain of both cases was statistically analyzed as follows. The conductance of the molecular junction in the forward (Ga,Gb) and backward (Gc,Gd) directions was taken into account (Figure 4a). In the forward direction, the junction is shrunk back to the position at z = 0. Conversely the junction is extended back to the zero-position in the backward direction. The steepness in the conductance response was then calculated for the forward (LGforward = log Gb − log Ga) and backward directions (LGbackward = log Gc − log Gd). In order to obtain a statistically relevant data set, a large number of compression−elongation cycles were analyzed, i.e., 10 157 cycles for 1 and 13 219 cycles for 2. From this analysis, the averaged LGforward and LGbackward were obtained, exposing the marked asymmetry of the compression−elongation cycles collected for 2 (LGforward = 0.089 LGbackward = 0.10) in comparison with those collected for 1 (LGforward = LGbackward = 0.10). Since the stretch length is 0.0175 nm, LG without molecular junction formation is expected to be log{(exp(−0.392)} = 0.17 assuming the tunneling transport (conductance ≈ exp(−βL), where L and β are the distance between electrodes and the decay constant) and typical β value of 22.4 nm−1 for vacuum tunneling.28 In order to help with the visualization of this finding the difference between the forward and backward directions was calculated for each cycle as ΔG = LGbackward − LGforward, and it is shown in Figure 4b. This study confirms the almost symmetric response of 1 in comparison with 2. Results were fitted to a Lorentz−Cauchy distribution. Distribution center and skewness were found to be ΔGcenter‑1 = −0.0064 and ΔGskew‑1 = 1.10 for 1 and ΔGcenter‑2 = −0.0197 and ΔGskew‑2 = 1.33 for 2. The asymmetry of the compression−elongation cycles results in the center of the distribution being more negative for 2. In addition, the marked asymmetry of the distribution obtained for the OPE junctions, a marked positive skewness of 1.33 (i.e., higher possibility of negative ΔG values), is also observed for the compression elongation cycles of 2. The OPE derivative 2 exhibits larger electromechanical response in the forward direction than that in the backward direction, which leads to the remarkable asymmetry of ΔGcenter‑2 = −0.0197 and ΔGskew‑2 = 1.33 that is a higher possibility of negative ΔG values (Figure 4b). The symmetric (or weakly asymmetric) behavior with the AF of 1.0 is responsible for the observed weak asymmetry of ΔGcenter‑1 = −0.0064 and ΔGskew‑1 = 1.10 for the Co complex 1 (Figure 4b). Both the molecular junctions of 1 and 2 have the same Au− NH2 metal−molecule contacts; therefore, the difference in the electromechanical properties should be caused by their molecular backbones themselves. To get information on the modulated electronic transport properties we performed electronic calculations based on density function theory (DFT). Figure 5a shows DFT-calculated potential energy as a function of N to N interatomic distance, in which significant and weak asymmetries are, respectively, apparent for the OPE derivative 2 and the Co complex 1. For example, asymmetric factors (AFs) at LNN= ± 0.1 nm are found to be 1.0 and 1.7 for 1 and 2 where the AF is defined as AF = (ΔE at ΔLNN= −0.1)/ (ΔE at ΔLNN= + 0.1). Figure 5b shows energy levels of

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CONCLUSIONS We have demonstrated tunability of the transport properties of molecular junctions under mechanical perturbation on the single molecular scale (i.e., ∼0.1 nm displacement). STM-BJ experiments reveled the single molecular conductance of (1.0 ± 0.2) × 10−4 and (9.4 ± 1.0) × 10−5 G0 for the Co((4-aniline)terpyridine)2 complex and oligo(phenylene-ethynylene) derivative sandwiched between two Au electrodes. The molecular junctions with rigid π-conjugated backbones (oligo(phenyleneethynylene) derivative) exhibited substantially asymmetric 19456


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The Journal of Physical Chemistry C electromechanical behavior with ΔGcenter‑2 = −0.0197 and ΔGskew‑2 = 1.33 in response to the mechanical stimulus. In contrast, the Co((4-aniline)-terpyridine)2 complex with a flexible metal center displayed a symmetric response of ΔGcenter‑1 = −0.0064 and ΔGskew‑1 = 1.10. These results demonstrate potential tunability of the molecular conductance under mechanical stimulus on the atomic scale, which could provide fundamental guidance toward electronic modulation of the molecular junctions 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.5b04386. Conductance histograms for the molecular junctions of 1 and 2 and synthetic methods for 1 and 2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *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 and 26102013) from MEXT and Asahi Glass Foundation.

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REFERENCES

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Effect of Mechanical Strain on Electric Conductance of Molecular

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