Article

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

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Abstract

Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

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... Traditionally, the asymmetric molecular structure or the different interactions between the top/bottom electrode and the corresponding molecular terminations will induce the current rectification behavior, operating as a diode. By the control of molecular switches with external stimuli, the molecular tunneling device can be set to high resistance state (OFF) or low resistance state (ON), 525 Because of the partial electrical transparency and selective permeability, graphene allowed the in situ control of the redox states and the molecule/electrode coupling via chemical and electrochemical reactions. As shown in Figure 30a, an ultraflat Au film on conducting silicon was used as the source electrode. ...
... A monolayer of 6-ferrocenylhexanethiol (FcC6S, Fc-SAM) was then self-assembled on the surface of Au film and CVD-grown monolayer graphene was transferred on the top of Fc-SAM. 525 A small amount of reactive solution or electrolyte solution was dropped on the top of tunneling channel. Since the graphene layer with its high impermeability can prevent direct contact between the Fc-SAM and the solution, the chemical redox reactions between the oxidant/reductant and Fc groups across graphene layer could be investigated. ...
... After adding oxidizing agent, the current density in J D −V D curve of the Fc + -SAM junction decreased markedly and dropped by over 2 orders of magnitude at the negative bias. 525 After the reduction treatment, the J D −V DD characteristics of the device almost recovered to its initial state. In response to alternate chemical oxidation and reduction treatments, the |J D | at V D = −0.5 V changed between low and high conductance states with an I on / I off ratio of ∼120. ...
Article
Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal–oxide–semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More–than–Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond–CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.
... SAMs tunnel junction using Au and single layer graphene electrodes (Fig. 29a). [108] The vertical tunnel junction thus created allowed in situ control over redox properties of ferrocene unit. Redox reactions of ferrocene groups present beneath the graphene layer could be controlled by applying suitable oxidant, reductant or electrochemical stimuli at the top. ...
... (b) current density vs source-drain voltage for the device upon treatment with both oxidising and reducing solutions. Reproduced with permission from Ref.[108], Copyright 2020, Elsevier. ...
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Ferrocene, since its discovery in 1951, has been extensively exploited as a redox probe in a variety of processes ranging from solution chemistry, medicinal chemistry, supramolecular chemistry, surface chemistry to solid-state molecular electronic and spintronic circuit elements to unravel electrochemical charge-transfer dynamics. Ferrocene represents an extremely chemically and thermally stable, and highly reproducible redox probe that undergoes reversible one-electron oxidation and reduction occurring at the interfaces of electrode/ferrocene solution in response to applied anodic and cathodic potentials, respectively. It has been almost 70 years after its discovery and has become one of the most widely studied and model organometallic compounds not only for probing electrochemical charge-transfer process but also as molecular building blocks for the synthesis of chiral organometallic catalysts, potential drug candidates, polymeric compounds, electrochemical sensors, to name a few. Ferrocene and its derivatives have been a breakthrough in many aspects due to its versatile reactivity, fascinating chemical structures, unconventional metal-ligand coordination, and the magic number of electrons (18 e-). The present review discusses the recent progress made towards ferrocene-containing molecular systems exploited for redox reactions, surface attachment, spin-dependent electrochemical process to probe spin polarization, photo-electrochemistry, and integration into prototype molecular electronic devices. Overall, the present reviews demonstrate a piece of collective information about the recent advancements made towards the ferrocene and its derivatives that have been utilized as iconic redox markers.
... This design makes the exploitation of intrinsic molecular property of SAMs inaccessible due to the presence of two bulk electrodes causing the isolation of the SAMs from the external environment. Duan and co-workers prepared a ferrocene-based SAMs tunnel junction using Au and single-layer graphene electrodes (Fig. 28a) [139]. The vertical tunnel junction thus created allowed in situ control over the redox properties of the ferrocene unit. ...
... (b) Current density vs source-drain voltage for the device upon treatment with both oxidising and reducing solutions. Reproduced with permission from Ref.[139], Copyright 2020, Elsevier. ...
Article
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Ferrocene since its inception in the year 1951, has been extensively exploited as a crucial redox probe to unravel electrochemical charge-transfer dynamics in a variety of platforms ranging from solution-based systems and molecular thin-films to solid-state molecular electronics and spintronic devices. Having completed almost 71 years of its existence, ferrocene has now become one of the most widely studied organometallic compounds. Several experimental and theoretical frameworks are made to understand ferrocene's electronic and electrochemical properties. Ferrocene is an 18-electron metallocene that shows interesting metal–ligand coordination and has led to the preparation of a great number of important molecules. Ferrocene and its numerous derivatives have brought a breakthrough in metallocene chemistry. Ferrocene represents a chemically and thermally stable system that undergoes reversible electrochemical oxidation and reduction processes. Ferrocene-based self-assembled monolayers (SAMs) are considered as model system for performing on-surface redox reactions and have been applied to create nanoelectronic devices for molecular switching, rectification, and low-voltage operational memory devices. The present review discusses the recent progress made toward a ferrocene-containing molecular system that have been utilized in redox reactions, surface attachment, spin-dependent electrochemical processes to understand spin polarization, photo-electrochemistry, and molecular electronic devices. This review provides an excellent platform for understanding the electrochemical properties and the rational design of ferrocene-based molecular systems for optoelectronic applications.
... This proofof-concept work yielded uselessly low conductance ratios, but demonstrated stable, two-terminal redox switching. Jia and colleagues demonstrated that the reversible switching between conductance states can also be operated in a threeterminal, large-area junction comprising redox-active self-assembled monolayers ( Figure 1B) [5]. The junctions are constructed in a vertical geometry, in which the monolayers of ferrocenyl alkanethiols are sandwiched between an Au bottom electrode and a graphene top electrode, while a bias is applied through a third electrode (gate) and an electrolyte to enable oxidation/reduction of the molecules. ...
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The development of high-speed, nonvolatile memory devices with low power consumption remains a significant challenge for next-generation computing. A recent study reported molecular switches operating at low voltages in large-area junctions by coupling supramolecular structural changes and counterion migration to bias-dependent redox, culminating in proof-of-concept memory comprising self-assembled monolayers.
... Large area molecular junctions typically lack a gate electrode, but recently a large-area junction of the form of AuÀ SAM//graphene/electrolyte with a derivative of compound 13 has been reported where the redox-state of Fc could be controlled. [62] Here, the oxidation state of the Fc units can be controlled via electrochemical gating through the graphene top electrode. In this device configuration, the rectification of the junction could be turned on and off by controlling the redox state Fc moieties (Figure 11c). ...
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Redox‐active molecular junctions have attracted considerable attention because redox‐active molecules provide accessible energy levels enabling electronic function at the molecular length scales, such as, rectification, conductance switching, or molecular transistors. Unlike charge transfer in wet electrochemical environments, it is still challenging to understand how redox‐processes proceed in solid‐state molecular junctions which lack counterions and solvent molecules to stabilize the charge on the molecules. In this minireview, we first introduce molecular junctions based on redox‐active molecules and discuss their properties from both a chemistry and nanoelectronics point of view, and then discuss briefly the mechanisms of charge transport in solid‐state redox‐junctions followed by examples where redox‐molecules generate new electronic function. We conclude with challenges that need to be addressed and interesting future directions from a chemical engineering and molecular design perspectives. In recent years, redox‐active molecules have contributed significantly to the development of functional molecular junctions including molecular diodes, molecular memory and molecular transistors. This minireview gives an overview of recent progresses in functional redox‐active molecular junctions and introduces the underlying operating mechanisms of the different types of functional molecular junctions.
... Furthermore, the electric field and ion migration driven effects enable the molecular devices to be preprogrammed in a single-molecule layer to achieve diode and memory functions (figure 13(d)) [13]. Furthermore, the regulation of the charged terminal redox state can be realized through a specially designed device with a vertical graphene/SAM/Au structure [139], in which the redox regulation of molecules can be realized through the top single-layer graphene electrode (figure 13(e)). The principle of rectification is due to the charged terminal groups, as they can electrostatically interact with the electrode. ...
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Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.
... In particular, a growing number of studies have identified quantum interference effects in the thermoelectric properties of single molecules . More recently, it has been demonstrated that these single-molecule QI effects can be translated into self-assembled monolayers [146][147][148][149][150][151][152][153][154][155][156][157][158], thereby creating two-dimensional materials, whose electronic and thermoelectric properties are controlled by room-temperature quantum interference. Interest in molecular-scale thermoelectricity has also stimulated studies of thermal transport through single molecules [159][160][161][162][163][164][165][166][167][168][169][170][171], where room-temperature phonon interference may provide a route to suppressing thermal conductance and increasing the thermoelectric performance of molecular-scale devices and materials. ...
... Due to their relative simplicity, their reproducibility, and the versatility of organic and organometallic compounds, SAMs of thiols have found applications across many scientific and engineering disciplines, e.g., anti-fouling surfaces, 75 lubrication, 76,77 corrosion resistance, 78,79 protein binding, 80,81 DNA assemblies, 82 cellular signaling and interactions, 83 photovoltaics, [84][85][86] and transistors. 68,87 In MEJs, thiols (and/or disulfides) serve as anchoring groups to form molecular ...
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Molecular electronics aims to miniaturize electronic devices by using subnanometre-scale active components. A single-molecule diode, a circuit element that directs current flow, was first proposed more than 40 years ago and consisted of an asymmetric molecule comprising a donor-bridge-acceptor architecture to mimic a semiconductor p-n junction. Several single-molecule diodes have since been realized in junctions featuring asymmetric molecular backbones, molecule-electrode linkers or electrode materials. Despite these advances, molecular diodes have had limited potential for applications due to their low conductance, low rectification ratios, extreme sensitivity to the junction structure and high operating voltages. Here, we demonstrate a powerful approach to induce current rectification in symmetric single-molecule junctions using two electrodes of the same metal, but breaking symmetry by exposing considerably different electrode areas to an ionic solution. This allows us to control the junction's electrostatic environment in an asymmetric fashion by simply changing the bias polarity. With this method, we reliably and reproducibly achieve rectification ratios in excess of 200 at voltages as low as 370 mV using a symmetric oligomer of thiophene-1,1-dioxide. By taking advantage of the changes in the junction environment induced by the presence of an ionic solution, this method provides a general route for tuning nonlinear nanoscale device phenomena, which could potentially be applied in systems beyond single-molecule junctions.
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This tutorial outlines the basic theoretical concepts and tools which underpin the fundamentals of phase-coherent electron transport through single molecules. The key quantity of interest is the transmission coefficient T(E), which yields the electrical conductance, current-voltage relations, the thermopower S and the thermoelectric figure of merit ZT of single-molecule devices. Since T(E) is strongly affected by quantum interference (QI), three manifestations of QI in single-molecules are discussed, namely Mach-Zehnder interferometry, Breit-Wigner resonances and Fano resonances. A simple MATLAB code is provided, which allows the novice reader to explore QI in multi-branched structures described by a tight-binding (Hückel) Hamiltonian. More generally, the strengths and limitations of materials-specific transport modelling based on density functional theory are discussed.
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35 pages, 8 figures.-- Printed version published on Mar 25, 2002.-- ArXiv pre-print available at: http://arxiv.org/abs/cond-mat/0111138 We have developed and implemented a self-consistent density functional method using standard norm-conserving pseudopotentials and a flexible, numerical linear combination of atomic orbitals basis set, which includes multiple-zeta and polarization orbitals. Exchange and correlation are treated with the local spin density or generalized gradient approximations. The basis functions and the electron density are projected on a real-space grid, in order to calculate the Hartree and exchange-correlation potentials and matrix elements, with a number of operations that scales linearly with the size of the system. We use a modified energy functional, whose minimization produces orthogonal wavefunctions and the same energy and density as the Kohn-Sham energy functional, without the need for an explicit orthogonalization. Additionally, using localized Wannier-like electron wavefunctions allows the computation time and memory required to minimize the energy to also scale linearly with the size of the system. Forces and stresses are also calculated efficiently and accurately, thus allowing structural relaxation and molecular dynamics simulations. This work was supported by the Fundación Ramón Areces and by Spain’s MCyT grant BFM2000-1312. JDG would like to thank the Royal Society for a University Research Fellowship and EPSRC for the provision of computer facilities. DSP acknowledges support from the Basque Government (Programa de Formación de Investigadores). Peer reviewed
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Methods for forming single- and multiple-molecule junctions are key to the development of molecular electronics and the further study of allied electronic and electrical properties of molecules arising from through-molecule charge transport. The organometallic complex trans-Ru(CC-3-C4H3S)(CC-1,4-C6H4CCAuPPh3)(dppe)2 forms well-ordered, densely packed self-assembled monolayers on gold and silver substrates, contacted through the sulfur atoms of the thiophenyl groups. Upon mild thermal treatment (150–200 °C, two hours) the gold moiety decomposes to liberate PPh3 and form quite uniform, disc-shaped gold nanoparticles on top of the organometallic monolayer. The resulting molecular junctions give rise to sigmoidal shaped I–V curves characteristic of through-molecule conductance, rather than linear, ohmic traces associated with metallic contacts (i.e. short circuits). This work therefore demonstrates the feasibility of thermal processing routes to form good quality molecular junctions from organometallic complexes of relatively complex molecular structure capped with uniformly-shaped nanoparticles formed in situ.
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One of the simplest molecular-scale electronic devices is the molecular rectifier. In spite of considerable efforts aimed at understanding structure-property relationships in these systems, however, devices with predictable and stable electronic properties are yet to be developed. Here, we demonstrate highly efficient current rectification in a new class of compounds that form self-assembled monolayers on silicon. We achieve this by exploiting the coupling of the molecules with the top electrode which, in turn, controls the position of the relevant molecular orbitals. The molecules consist of a silane anchoring group and a nitrogen-substituted benzene ring, separated by a propyl group and imine linkage, and result from a simple, robust, and high-yield synthetic procedure. We find that when incorporated in molecular diodes, these compounds can rectify current by as much as three orders of magnitude, depending on their structure, with a maximum rectification ratio of 2635 being obtained in (E)-1-(4-cyanophenyl)-N-(3-(triethoxysilyl) propyl)methanimine (average Ravg = 1683 ± 458, at an applied voltage of 2V). This performance is on par with that of the best molecular rectifiers obtained on metallic electrodes, but it has the advantage of lower cost and more efficient integration with current silicon technologies. The development of molecular rectifiers on silicon may yield hybrid systems that can expand the use of silicon towards novel functionalities governed by the molecular species grafted onto its surface.
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Over the past two decades, various techniques for fabricating nano-gapped electrodes have emerged, promoting rapid development in the field of single-molecule electronics, on both the experimental and theoretical sides. To investigate intrinsic quantum phenomena and achieve desired functionalities, it is important to fully understand the charge transport characteristics of single-molecule devices. In this Review, we present the principles that have been developed for fabricating reliable molecular junctions and tuning their intrinsic properties from an engineering perspective. Through holistic consideration of the device structure, we divide single-molecule junctions into three intercorrelated components: the electrode, the contact (spacer–linker) interface and the molecular backbone or functional centre. We systematically discuss the selection of the electrode material and the design of the molecular components from the point of view of the materials, the interface and molecular engineering. The influence of the properties of these elements on the molecule–electrode interface coupling and on the relative energy gap between the Fermi level of the electrode and the orbital energy levels of the molecule, which directly influence the charge transport behaviour of single-molecule devices, is also a focus of our analysis. On the basis of these considerations, we examine various functionalities demonstrated in molecular junctions through molecular design and engineering.
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If single-molecule, room-temperature, quantum interference (QI) effects could be translated into massively parallel arrays of molecules located between planar electrodes, QI-controlled molecular transistors would become available as buildingblocks for future electronic devices. Here, we demonstrate unequivocal signatures of room-temperature QI in vertical tunneling transistors, formed from self-assembled monolayers (SAMs), with stable room-temperature switch- ing operations. As a result of constructive QI effects, the conductances of the junctions formed from anthanthrene-based molecules with two different connectivities differ by a factor of 34, which can further increase to 173 by controlling the molecule-electrode interface with different terminal groups. Field-effect control is achieved using an ionic liquid gate, whose strong vertical electric field penetrates through the graphene layer and tunes the energy levels of the SAMs. The resulting room-temperature on-off current ratio of the lowest-conductance SAMs can reach up to 306, about one order of magnitude higher than that of the highest-conductance SAMs.
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ConspectusQuantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in single-molecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural-electronic relationships in molecular devices and materials at the nanoscale.In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the single-molecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.
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The functionalities offered by single-molecule electrical junctions have yet to be translated into monolayer or few-layer molecular films, where effective and reproducible electrical contact represents one of the challenging bottlenecks. Here we take a significant step in this direction by demonstrating that excellent electrical contact can be made to a monolayer biphenyl-4, 4’-dithiol (BPDT) molecular film, sandwiched between gold and graphene electrodes. This sandwich device structure is advantageous, because the current flows through the molecules to the gold substrate in a ‘cross-plane’ manner, perpendicular to the plane of the graphene, yielding high-conductance devices. We elucidate the nature of cross-plane graphene/molecule/Au transport using quantum transport calculations and introduce a simple analytical model, which captures generic features of the current-voltage characteristic. Asymmetry in junction properties results from the disparity in electrode electrical properties, the alignment of the BPDT HOMO-LUMO energy levels and the specific characteristics of the graphene electrode. The experimental observation of scalability of junction properties within the junction area, in combination with a theoretical description of the transmission probability of the thiol-graphene contact, demonstrate that between 10%-100% of the molecules are contacted to the electrodes, which is several orders of magnitude greater than achieved to date in the literature.
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In molecular electronics, it is important to control the strength of the molecule–electrode interaction to balance the trade-off between electronic coupling strength and broadening of the molecular frontier orbitals: too strong coupling results in severe broadening of the molecular orbitals while the molecular orbitals cannot follow the changes in the Fermi levels under applied bias when the coupling is too weak. Here, a platform based on graphene bottom electrodes to which molecules can bind via π–π interactions is reported. These interactions are strong enough to induce electronic function (rectification) while minimizing broadening of the molecular frontier orbitals. Molecular tunnel junctions are fabricated based on self-assembled monolayers (SAMs) of Fc(CH2)11X (Fc = ferrocenyl, X = NH2, Br, or H) on graphene bottom electrodes contacted to eutectic alloy of gallium and indium top electrodes. The Fc units interact more strongly with graphene than the X units resulting in SAMs with the Fc at the bottom of the SAM. The molecular diodes perform well with rectification ratios of 30–40, and they are stable against bias stressing under ambient conditions. Thus, tunnel junctions based on graphene with π–π molecule–electrode coupling are promising platforms to fabricate stable and well-performing molecular diodes.
Article
Graphene is a promising candidate for an "ideal membrane material". Its ultralow (one-atomic) thickness potentially provides high permeation and at the same time high selectivity. In the presented work, it is demonstrated that these properties can be used to create a confined, two-dimensional electrochemical environment between a graphene layer and a single crystal Pt(111) surface. The well-defined "fingerprint" voltammetric characteristics of Pt(111) provide an immediate information about the penetration and intercalation of ions into the confined space. These processes are shown to be highly selective.
Article
Molecular diodes operating in the tunnelling regime are intrinsically limited to a maximum rectification ratio R of ∼10³. To enhance this rectification ratio to values comparable to those of conventional diodes (R ≥ 10⁵) an alternative mechanism of rectification is therefore required. Here, we report a molecular diode with R = 6.3 × 10⁵ based on self-assembled monolayers with Fc–C≡C–Fc (Fc, ferrocenyl) termini. The number of molecules (n(V)) involved in the charge transport changes with the polarity of the applied bias. More specifically, n(V) increases at forward bias because of an attractive electrostatic force between the positively charged Fc units and the negatively charged top electrode, but remains constant at reverse bias when the Fc units are neutral and interact weakly with the positively charged electrode. We successfully model this mechanism using molecular dynamics calculations.
Article
Single-molecule detection based on electricity can realize direct, real-time, and label-free monitoring of the dynamic processes of either chemical reactions or biological functions at the single-molecule/single-event level. This provides a fascinating platform to probe detailed information of chemical and biological reactions, including intermediates/transient states and stochastic processes that are usually hidden in ensemble-averaged experiments, which is of crucial importance to chemical, biological, and medical sciences. Here, the focus is on a valuable survey of the state-of-art progress in single-molecule dynamics studies that are based on electrical nanocircuits formed from one-dimensional nanoarchitectures and molecular-tunneling junctions. Further interesting applications, useful statistical-analysis methods, and future promising directions toward the study of chemical-reaction dynamics and biomolecular activities are also discussed.
Article
We review charge transport across molecular monolayers, which is central to molecular electronics (MoE) using large-area junctions (NmJ). We strive to provide a wide conceptual overview of three main sub-topics. First, a broad introduction places NmJ in perspective to related fields of research, and to single molecule junctions (1mJ), in addition to a brief historical account. As charge transport presents an ultra sensitive probe for the electronic perfection of interfaces, in the second part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across molecular junctions. Notwithstanding the original motivation of MoE, I-V traces are often not very sensitive to molecular details and then provide a poor probe for chemical information. Instead we focus on how to analyse the net electrical performance of molecular junctions, from a functional device perspective. Finally, we shortly point to creation of a built-in electric field as a key to achieve functionality, including non-linear current-voltage characteristics that originate in the molecules or their contacts to the electrodes.
Article
This Account describes a body of research in atomic level design, synthesis, physicochemical characterization, and macroscopic electrical testing of molecular devices made from ferrocene-functionalized alkanethiol molecules, which are molecular diodes, with the aim to identify, and resolve, the failure modes that cause leakage currents. The mismatch in size between the ferrocene headgroup and alkane rod makes waxlike highly dynamic self-assembled monolayers (SAMs) on coinage metals that show remarkable atomic-scale sensitivity in their electrical properties. Our results make clear that molecular tunnel junction devices provide an excellent testbed to probe the electronic and supramolecular structures of SAMs on inorganic substrates. Contacting these SAMs to a eutectic "EGaIn" alloy top-electrode, we designed highly stable long-lived molecular switches of the form electrode-SAM-electrode with robust rectification ratios of up to 3 orders of magnitude. The graphic that accompanies this conspectus displays a computed SAM packing structure, illustrating the lollipop shape of the molecules that gives dynamic SAM supramolecular structures and also the molecule-electrode van der Waals (vdW) contacts that must be controlled to form good SAM-based devices. In this Account, we first trace the evolution of SAM-based electronic devices and rationalize their operation using energy level diagrams. We describe the measurement of device properties using near edge X-ray absorption fine structure spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy complemented by molecular dynamics and electronic structure calculations together with large numbers of electrical measurements. We discuss how data obtained from these combined experimental/simulation codesign studies demonstrate control over the supramolecular and electronic structure of the devices, tuning odd-even effects to optimize inherent packing tendencies of the molecules in order to minimize leakage currents in the junctions. It is now possible, but still very costly to create atomically smooth electrodes and we discuss progress toward masking electrode imperfections using cooperative molecule-electrode contacts that are only accessible by dynamic SAM structures. Finally, the unique ability of SAM devices to achieve simultaneously high and atom-sensitive electrical switching is summarized and discussed. While putting these structures to work as real world electronic devices remains very challenging, we speculate on the scientific and technological advances that are required to further improve electronic and supramolecular structure, toward the creation of high yields of long-lived molecular devices with (very) large, reproducible rectification ratios.
Article
Cyclic oligomers comprising strongly interacting redox-active monomer units represent an unknown, yet highly desirable class of nanoscale materials. Here we describe the synthesis and properties of the first family of molecules belonging to this compound category - differently sized rings comprising only 1,1′-disubstituted ferrocene units (cyclo[n], n = 5-7, 9). Due to the close proximity and connectivity of centres (covalent Cp-Cp linkages; Cp = cyclopentadienyl) solution voltammograms exhibit well-resolved, separated 1e - waves. Theoretical interrogations into correlations based on ring size and charge state are facilitated using values of the equilibrium potentials of these transitions, as well as their relative spacing. As the interaction free energies between the redox centres scale linearly with overall ring charge and in conjunction with fast intramolecular electron transfer (1/410 7 €...s '1), these molecules can be considered as uniformly charged nanorings (diameter 1/41-2 €...nm).
Article
The measurement of thermopower in molecular junctions offers complementary information to conductance measurements and is becoming essential for the understanding of transport processes at the nanoscale. In this review, we discuss the recent advances in the study of the thermoelectric properties of molecular junctions. After presenting the theoretical background for thermoelectricity at the nanoscale, we review the experimental techniques for measuring the thermopower in these systems and discuss the main results. Finally, we consider the challenges in the application of molecular junctions in viable thermoelectric devices.
Article
The electrical properties of ferrocene-alkanethiolate self-assembled monolayers (SAMs) on a high yield solid-state device structure are investigated. The devices are fabricated using a conductive polymer interlayer between the top electrode and the SAM on both silicon-based rigid substrates and plastic-based flexible substrates. Asymmetric electrical transport characteristics that originate from the ferrocene moieties are observed. In particular, a distinctive temperature dependence of the current (i.e., a decrease in current density as temperature increases) at a large reverse bias, which is associated with the redox reaction of ferrocene groups in the molecular junction, is found. It is further demonstrated that the molecular devices can function on flexible substrates under various mechanical stress configurations with consistent electrical characteristics. This study enhances the understanding of asymmetric molecules and may lead to the development of functional molecular electronic devices on both rigid and flexible substrates.
Article
One of the main goals of organic and molecular electronics is to relate the performance and electronic function of devices to the chemical structure and intermolecular interactions of the organic component inside them, which can take the form of an organic thin film, a self-assembled monolayer or a single molecule. This goal is difficult to achieve because organic and molecular electronic devices are complex physical-organic systems that consist of at least two electrodes, an organic component and two (different) organic/inorganic interfaces. Singling out the contribution of each of these components remains challenging. So far, strong π-π interactions have mainly been considered for the rational design and optimization of the performances of organic electronic devices, and weaker intermolecular interactions have largely been ignored. Here, we show experimentally that subtle changes in the intermolecular van der Waals interactions in the active component of a molecular diode dramatically impact the performance of the device. In particular, we observe an odd-even effect as the number of alkyl units is varied in a ferrocene-alkanethiolate self-assembled monolayer. As a result of a more favourable van der Waals interaction, junctions made from an odd number of alkyl units have a lower packing energy (by ∼0.4-0.6 kcal mol(-1)), rectify currents 10 times more efficiently, give a 10% higher yield in working devices, and can be made two to three times more reproducibly than junctions made from an even number of alkyl units.
Article
Aqueous solutions of 0.1 M NaC104 (pH 1.5, 10 mM KHzPO4) show potential-dependent wetting on self-assembled monolayers (SAMs) formed from 15-(ferroceny1carbonyl)pentadecanethiol (FcCQ(CH&s- SH; Fc = (~~-CSH~IF~(~~~- CSH~I) adsorbed on Au surfaces. Contact angles (8) decreased from 71° to 43O (A cos 8 = -0.40) when the electrical potential of the SAM was increased from 0.3 to 0.5 V (vs a Ag wire reference electrode) and then increased from 43O to 58O when the potential of the SAM was returned to 0.2 V. Repeated cycling between these values of the potential leads to a progressively decreasing response, as the Fc groups were destroyed by side reactions. Contact angles of aqueous solutions on SAMs formed from CH&H&SH decrease by only 2' (from -115' to 113O, A cos 8 = 0.05) over the same range of potentials (Sontag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560-2566). The contrast between the wettability of SAMs terminated with Fc and CH3 groups suggests that potential-dependent wetting of the former is caused primarily by the electrochemical oxidation of the electrically neutral, surface-confined Fc to the more polar and plausibly more wettable Fc+ cation. Linear sweep cyclic voltammetric measurements support this hypothesis. Surfaces of gold patterned with SAMs formed from FCCO(CHZ)~SSH and CH~(CHZ)I~SH were used to construct a micrometer-scale 'gate" that controls the flow of liquid down a surface.