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Highly Efficient Long-Range Electron Transport in Viologen-Based Molecular Junction Quyen Van Nguyen, Pascal Martin, Denis Frath, Maria Luisa Della Rocca, Frederic Lafolet, Sebastien Bellynck, Philippe Lafarge, and Jean-Christophe Lacroix J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05589 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018
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Journal of the American Chemical Society
Highly Efficient LongLong-Range Electron Transport in ViologenViologen-Based MoMolecular Junction Quyen van Nguyen,#, ! Pascal Martin,#,* Denis Frath,# Maria Luisa Della Rocca‡, Frederic Lafolet#, Sebastien Bellinck, Philippe Lafarge‡, Jean-Christophe Lacroix.#, !,* #Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France. ! Department of Advanced Materials Science and Nanotechnology, University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam. ‡ Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot, Sorbonne Paris Cité, 75205 Paris Cedex 13, France. Supporting Information Placeholder ABSTRACT: Thin layers of viologen-based oligomers with thicknesses between 3 and 14 nm were deposited on gold electrodes by electrochemical reduction of a diazonium salt, then a Ti /Au top contact was applied to complete a solidstate molecular junction (MJ). MJs show symmetric J(V) curves and highly efficient long-range transport with an -1 attenuation factor as small as 0.25 nm . This is attributed to the fact that the viologen LUMO energy lies between the energies of the Fermi levels of the two contacts and to strong electronic coupling between molecules and contacts. As a consequence, resonant tunneling is likely to be the dominant transport mechanism within these MJs, but the temperature dependence of the transport properties suggests that activated redox hopping plays a role at high temperature.
A molecular junction (MJ) consists of a single molecule or 1-3 an ensemble of many molecules between two electrodes. When the distance d between these electrodes is below 5 nm, direct off-resonant tunneling through the molecule acting as a barrier is the dominant transport mechanism. The rate of electron transport is independent of temperature, decreases exponentially with d, and is given in its simplest form by = o × where J and Jo are the current density and the pre-exponential factor; β is an attenuation factor and 1-8 has units of inverse nanometers. Values of β depend strongly on the molecular type and it is generally agreed that -1 6-11 -1 β is 2-3 nm for conjugated molecules and 8-10 nm for 3-4, 9 saturated molecules. When the thickness of the layer increases, transport by direct tunneling between the contacts becomes inefficient and the long-range transport observed in many MJs is no longer consistent with this mechanism. A change in β, often observed in the Ln J versus distance plot, indicates the critical distance where the dominant transport mechanism changes. In most cases, above this distance acti5-7, 11 vated redox “hopping” occurs. However, for bisthienylbenzene (BTB)-based molecular layers up to d = 22 -1 nm, activation-less transport was observed with a β ≈ 1 nm 10 at temperatures below 100 K. These results indicate an
unusual transport mechanism for conjugated MJs with d 9-10, 12-13 This stimulated conabove the direct tunneling limit. sideration of molecular layers with thicknesses above 5 nm, with a view to developing reliable MJs showing efficient long-range transport. In this thickness range a strong “molecular signature” for the electronic response of the devices can be revealed, in contrast to that observed when direct tunneling between the contacts is the dominant transport mechanism. The present report describes MJs made by electrolreduc9-10, 12, 14-17 tion of diazonium reagents. Advances made when this method is used to attach a molecular layer on a bottom 18 electrode and to generate MJs have been recently reviewed. The contacts were Au and Ti/Au, so structural asymmetry from contacts with different work functions was always pre14-17 sent in the devices. The molecular layer used is based on acceptor molecules, namely N-methyl-viologen oligomers, with low-energy LUMOs. It is compared with a BTB layer, a molecule whose HOMO energy is close to, but below, the 14-15 Fermi level of the gold bottom electrode. We have synthesized 1-(4-aminophenyl)-1’-methyl-4,4’bipyridinium bis-hexafluorophosphate using a known proce19 dure. This molecule (Figure 1a) incorporates a viologen core. It bears an aniline group used for in situ diazonium generation and, when grafted onto a surface, will be named VIO-C1, as it is terminated by a methyl group. Figure 1b shows the electroactivity of VIO-C1 in solution. Two reversible redox signals at -0.4 and -0.8 V/SCE are observed. They 2+ +. +. are assigned to the reduction of VIO to VIO and of VIO 20-21 to VIO. These values indicate that VIO-C1 has a lowenergy LUMO between -5 and -4 eV, i.e. between the Fermi levels of gold and titanium. Next we grafted VIO-C1 onto Glassy Carbon electrodes using in situ diazonium genera17, 19, 22-23 tion. Details on the electrochemical deposition conditions are given in Figure S1 and Table S1. The electroactivity of the VIO-C1-modified substrate is displayed in Figure 1b. The modified electrode shows two reversible signals at-0.2 and -0.7 V/SCE respectively. These values are significantly higher than that of VIO-C1 in solution.
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These observations indicate that VIO-C1 can be grafted and suggest that there is a strong electronic coupling between the electrode and the graft in agreement with DFT calculation of the LUMO of VIO-C1 which is partially delocalized into the phenyl ring (Figure 1c). Next, the thickness 14-17 of layers deposited on gold was estimated by AFM (Figure S2). The thicknesses were successfully varied from 3 nm to 14 nm by changing the electrochemical conditions (Table S1) for film deposition. The root mean square roughness of the molecular layers was also estimated from AFM measurements and is around 0.8 nm, which is close to that of a bare Au stripe electrode. Molecular junctions were fabricated by established meth14-17 ods, all with the same Au/VIO-C1/Ti/Au structure (Figure S3). Junction designations include subscripts indicating layer thicknesses, with all devices using the same bottom and top electrodes. The fabrication yield is high (above 90%; 45 out of 48 junctions worked for thicknesses above 3 nm) which confirms that diazonium-based MJs are suitable for commercial applications and are compatible with massively parallel 10, 24 fabrication techniques. The room-temperature JV curves of several MJs for each thickness are presented in Figure 2. They show low standard deviation from one device to another, which signifies that the molecular films are homogeneous and the fabrication process reproducible. Moreover, Figure 2d overlays the JV 5 curves after fabrication and after 10 potential scans for a VIO-C15nm device The two JV curves are identical, which underlines their stability. These devices also show identical JV curves after two-year storage at ambient condition in a desiccator. A first important result is that the JV curves of VIO-C1 MJs are symmetric with no preferred current direction flow. This is in marked contrast with results obtained with BTB-based MJs using the same contacts; these show pronounced rectifi14-15, 17 cation properties. Besides, rectification in BTB-based devices markedly depend on layer thickness, whereas in VIO-C1 MJs rectification is not observed at any thickness.
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Figure 1: a) Chemical structure of VIO-C1. b) Electroactivity of VIO-C1-modified carbon electrode (green) compared to -1 that in solution (red), TBAPF6 0.1 M, scan rate 0.1 V.s . c) LUMO of VIO-C1 calculated by DFT (B3LYP/6-31G*)
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Figure 2. JV curves of several VIO-C1 MJs of different thicknesses (a) 3 nm, (b) 7 nm, (c) 10 nm. (d) Overlay of JV curves 5 of VIO-C15nm before and after 10 cycles in air. Figure 3a overlays representative JV curves for each thickness of the VIO-C1 layer. It reveals a clear influence of the thickness on the electrical properties. This dependence is better seen in Figure 3b which presents the plot of Ln J versus the thickness of the VIO-C1 layer. Ln J varies linearly with thickness from 3 to 14 nm, which suggests that = o applies on such a large distance range. The β value of VIO-C1 -1 MJs is 0.25 nm at 1 V. This is among the smallest values obtained in large-area MJs based on π-conjugated organic molecules and demonstrate highly efficient long-range transport in these MJs. Figure 3b compares Ln J versus thickness at 1 V for VIO-C1 and BTB MJs. The marked differences clearly indicate a strong molecular signature on transport. Firstly, the current density in VIO-C1 MJs is much higher than in BTB MJs. Secondly, we observe that in the 1 to 5 nm thickness range, the attenuation value for VIO-C1 MJs is much smaller (almost -1 ten times lower) than in BTB MJs, which is ~ 1.8 nm and is 1, 5-10, 25-26 consistent with previous studies. This value generally indicates that transport occurs by direct tunneling be8-9, 25-26 tween the electrodes. The results shown here are -1 clearly different, as β in VIO-C1 MJs is as low as 0.25 nm for all the thicknesses investigated. Thirdly, no variation of the slope in the attenuation plot is evidenced in VIO-C1 Mjs whereas it is in BTB MJs with Au and Ti/Au contacts. As a consequence, a transition between two transport regimes is not observed in VIO-C1 MJs. On the basis of these evidences, we can exclude direct off-resonant tunneling as the dominant transport mechanism in VIO-C1 MJs with layers 3 to 14 nm-thick. This mechanism may only apply below 3 nm. Two different, but not mutually exclusive, mechanisms can be considered to explain the highly efficient long-range 27-29 transport in these MJs, namely, on-resonant tunneling 10 and intrachain hopping based on redox events.
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Journal of the American Chemical Society of the electrodes. It was estimated from electrochemical data to be around -4.5 eV while the Fermi levels of Au and Ti electrodes are -5.1 eV and -4.1 eV, respectively and, consequently, VIO-C1 MJs are one of the first devices reported to date in which the LUMO level of the molecule in vacuum lies between the Fermi level of the electrodes (Scheme 1a). Moreover, due to the formation of covalent bonds, we expect strong electronic coupling at both interfaces and that the LUMO of the VIO-C1 layer is pinned to the electrodes and significantly broadened. In the connected state and at zero bias, the LUMO cannot remain 0.4 eV below the Fermi level of the Ti electrode and an electrochemical equilibrium must occur, so that the VIO-C1 LUMO and Fermi level in Ti are nearly aligned (Scheme 1b).
Figure 3. (a) JV curves for Au/VIO-C1/Ti/Au MJs with different thicknesses. (b) Ln J vs. thickness (nm) plot taken at 1 V for VIO-C1 (red) and BTB (green) MJs. (c) JV curves for VIOC15nm with temperature from 9.5 to 302 K. (d) Arrhenius plot: Ln J at 1 V (blue) and at 1.5 V (red) vs. 1000/T. 29
Kolivoska et al. reported that in single-molecule MJs based on extended viologen compounds, the β measured by -1 STM-BJ is 0.06 nm for thicknesses from 2 to 11 nm, which they attributed to phase-coherent tunneling. Moreover, the LUMO of VIO-C1 lies between the Fermi level of gold and titanium, a situation which favors on-resonant tunneling and could explain the low β. However, intrachain hopping is also known to give low β values and we note that VIO-C1-based MJs incorporate a large amount of ionic species, probably associated to solvent molecules despite the compactness of the film. This suggests that a counter-ion effect on the electrostatic field inside the junction may make hopping the dominant mechanism at unusually low thicknesses. In order to investigate further the mechanism of longrange-transport in these MJs, temperature dependence measurements were performed (Figure S4). Figure 3c shows JV curves obtained from 9.5 to 302 K for VIO-C15 nm junctions. Arrhenius plots derived from these JV curves at two different bias values (Figure 3d) provide activation barrier energies (Ea). Obviously, the JV curves depend on temperature but there are two separate regions: At high temperature (between 150 and 300 K) Ea is around 80 meV while below 150 K it is less than 1 meV. These values indicate that at room temperature, an activated mechanism is involved and demonstrate that on-resonant tunneling may not be the only transport mechanism, whereas it probably is at low tempera-1 ture. A low attenuation factor (β = 0.4 nm ) associated with a dependence on temperature (Ea above 80 meV at 300 K) have 27-28 been reported for porphyrin oligomers. Despite this activation energy, conventionally taken as indicating transport by activated hopping, the temperature and length dependence were shown to be also consistent with phase-coherent tunneling. More recently, efficient long-range transport attributed to resonant charge transport in single porphyrin 30 molecular wires was also reported. Scheme 1 presents a plausible energy diagram for VIO-C1 MJs explaining high current, low β and the lack of rectification despite asymmetric contacts. We assume here that charge transport mainly involves electron injection, since the LUMO energy of the VIO-C1 layer is close to the Fermi level
Scheme 1. Energy diagram of Au/VIO-C1/Ti/Au MJs in (a) free state, (b) connected state and zero bias. Number of orbital energy levels in the VIO-C1 layer is arbitrary. With this representation, when a bias is applied, and independently of its polarization, it will always be easy to inject electrons into the layer and there will always be a molecular level in resonance with the Fermi level of one electrode, so that there will be a high current density between the electrodes with no preferred direction (no rectification). In this situation the dominant transport mechanism depends little on thickness and intrachain activated hopping between VIO center can participate in transport above 150 K as a mixed 31 valence situation is generated in the layer as a result of the electrochemical equilibrium at the Ti/VIO-C1 interface. To summarize, Au/VIO-C1/Ti/Au MJs show highly efficient long-range transport, unique β values for π-conjugated-based large-area MJs and no rectification at any thickness investigated (from 3 to 14 nm). Transport is activated above 150 K but is activation-less below. We attribute these results to the strong electronic coupling at both interfaces, the VIO-C1 LUMO level between the Fermi levels of the electrodes and the amount of counter-ion in the film which makes redox events possible inside these solid-state junctions. As a consequence, transport is close to resonant tunneling at all T but activated hopping between VIO centers may also occur at higher temperature.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. It includes electrochemical deposition, AFM analysis and fabrication details.
AUTHOR INFORMATION Corresponding Author *
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ACKNOWLEDGMENT This work was supported by the Agence Nationale de la Recherche ANR (France) is gratefully acknowledged for its financial support of this work (ANR-15-CE09 0001-01). We thank Dr. John Lomas for editing our manuscript
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