Molecule-electrode interfaces in molecular electronic devices

Recent Advances of Nanospheres Lithography in Organic Electronics

Nanospheres lithography (NSL) is an economical technique, which makes use of highly monodispersed nanospheres such as deposition or etch masks for generating patterns with nanoscale features. Embedding nanostructures into organic electronic devices can endow them with unique capabilities and enhanced performance, which have greatly advanced the development of organic electronics. In this review, a brief summary of the methods for the preparation of monodispersed nanospheres is presented. Afterward, the authors highlight the recent advances of a wide variety of applications of nanospheres lithography in organic electronic devices. Finally, the challenges in this field are pointed out, and the future development of this field is discussed.

A single atom change turns insulating saturated wires into molecular conductors

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH₂)_(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å⁻¹. Impedance measurements show tripled dielectric constants (ε_r) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH₂)_nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in ε_r. Here, we demonstrate experimentally that \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$\end{document}β∝1/εr, suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

In molecular junctions, where a molecule is placed between two electrodes, the current passed decays exponentially as a function of length. Here, Chen et al. show that this exponentially attenuation can be controlled by changing a single atom at the end of the molecular wire.

Tuning surface d bands with bimetallic electrodes to facilitate electron transport across molecular junctions

Understanding chemical bonding and conductivity at the electrode-molecule interface is key for the operation of single-molecule junctions. Here we apply the d-band theory that describes interfacial interactions between adsorbates and transition metal surfaces to study electron transport across these devices. We realized bimetallic Au electrodes modified with a monoatomic Ag adlayer to connect α,ω-alkanoic acids (HO₂C(CH₂)_nCO₂H). The force required to break the molecule-electrode binding and the contact conductance G_n=0 are 1.1 nN and 0.29 G₀ (the conductance quantum, 1 G₀ = 2e²/h ≈ 77.5 μS), which makes these junctions, respectively, 1.3-1.8 times stronger and 40-60-fold more conductive than junctions with bare Au or Ag electrodes. A similar performance was found for Au electrodes modified by Cu monolayers. By integrating the Newns-Anderson model with the Hammer-Nørskov d-band model, we explain how the surface d bands strengthen the adsorption and promote interfacial electron transport, which provides an alternative avenue for the optimization of molecular electronic devices.

Molecular Structure-(Thermo)electric Property Relationships in Single-Molecule Junctions and Comparisons with Single- and Multiple-Parameter Models

The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO-LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer-Büttiker model.

The Characterization of Electronic Noise in the Charge Transport through Single-Molecule Junctions

With the goal of creating single-molecule devices and integrating them into circuits, the emergence of single-molecule electronics provides various techniques for the fabrication of single-molecule junctions and the investigation of charge transport through such junctions. Among the techniques for characterization of charge transport through molecular junctions, electronic noise characterization is an effective strategy with which issues from molecule-electrode interfaces, mechanisms of charge transport, and changes in junction configurations are studied. Electronic noise analysis in single-molecule junctions can be used to identify molecular conformations and even monitor reaction kinetics. This review summarizes the various types of electronic noise that have been characterized during single-molecule electrical detection, including the functions of random telegraph signal (RTS) noise, flicker noise, shot noise, and their corresponding applications, which provide some guidelines for the future application of these techniques to problems of charge transport through single-molecule junctions.

Fine-tuning the DNA conductance by intercalation of drug molecules

In this work we study the structure-transport property relationships of small ligand intercalated DNA molecules using a multiscale modeling approach where extensive ab initio calculations are performed on numerous MD-simulated configurations of dsDNA and dsDNA intercalated with two different intercalators, ethidium and daunomycin. DNA conductance is found to increase by one order of magnitude upon drug intercalation due to the local unwinding of the DNA base pairs adjacent to the intercalated sites, which leads to modifications of the density of states in the near-Fermi-energy region of the ligand-DNA complex. Our study suggests that the intercalators can be used to enhance or tune the DNA conductance, which opens new possibilities for their potential applications in nanoelectronics.

Modulation of charge transport through single-molecule bilactam junctions by tuning hydrogen bonds

Bilactam derivatives with different side groups were synthesized and the twisting angle tuning effect induced by the intramolecular hydrogen bond on the charge transport through their single-molecule junctions was investigated. Molecules with strong intramolecular hydrogen bonds exhibited twice higher conductance because of the reduced dihedral twisting, which was reversible with the addition of hydrogen bond destroying solvent. Our findings reveal that the presence of intramolecular hydrogen bonds promotes the planarization of the molecular structure without additional transmission channels, offering a new strategy for controlling molecular switches via tuning the molecular twisting.

Single molecule electronic devices with carbon-based materials: status and opportunity

The field of single molecule electronics has progressed remarkably in the past decades by allowing for more versatile molecular functions and improving device fabrication techniques. In particular, electrodes made from carbon-based materials such as graphene and carbon nanotubes (CNTs) may enable parallel fabrication of multiple single molecule devices. In this perspective, we review the recent progress in the field of single molecule electronics, with a focus on devices that utilizes carbon-based electrodes. The paper is structured in three main sections: (i) controlling the molecule/graphene electrode interface using covalent and non-covalent approaches, (ii) using CNTs as electrodes for fabricating single molecule devices, and (iii) a discussion of possible future directions employing new or emerging 2D materials.

Construction of Ultrathin Layer-by-Layer Films of Oligothiophene Derivatives on an Electrode

Oligothiophene derivatives, which are known as p-type materials, have been synthesized, and their ultrathin layer-by-layer films have been constructed on an electrode using a simple and convenient dipping method. The stepwise deposition behavior of quaterthiophene and sexithiophene derivatives on the electrode via hydrogen bonding was monitored by electronic spectra measurement, and the constructed films were evaluated by X-ray photoelectron spectroscopy, grazing-incidence small-angle X-ray scattering, and cyclic voltammetry. It has been clarified that the constructed layer-by-layer films were electroactive and photoelectroactive.

The energy level alignment of the ferrocene-EGaIn interface studied with photoelectron spectroscopy

The energy level alignment after the formation of a molecular tunnel junction is often poorly understood because spectroscopy inside junctions is not possible, which hampers the rational design of functional molecular junctions and complicates the interpretation of the data generated by molecular junctions. In molecular junction platforms where the top electrode-molecule interaction is weak; one may argue that the energy level alignment can be deduced from measurements with the molecules supported by the bottom electrode (sometimes referred to as "half junctions"). This approach, however, still relies on a series of assumptions, which are challenging to address experimentally due to difficulties in studying the molecule-top electrode interaction. Herein, we describe top electrode-molecule junctions with a liquid metal alloy top electrode of EGaIn (which stands for eutectic alloy of Ga and In) interacting with well-characterised ferrocene (Fc) moieties. We deposited a ferrocene derivative on films of EGaIn, coated with its native GaOx layer, and studied the energy level alignment with photoelectron spectroscopy. Our results reveal that the electronic interaction between the Fc and GaOx/EGaIn is very weak, resembling physisorption. Therefore, investigations of "half junctions" for this system can provide valuable information regarding the energy level alignment of complete EGaIn junctions. Our results help to improve our understanding of the energy landscape in weakly coupled molecular junctions and aid to the rational design of molecular electronic devices.

Selective Fabrication of Single-Molecule Junctions by Interface Engineering

Recent progress in addressing electrically driven single-molecule behaviors has opened up a path toward the controllable fabrication of molecular devices. Herein, the selective fabrication of single-molecule junctions is achieved by employing the external electric field. For molecular junctions with methylthio (-SMe), thioacetate (-SAc), amine (-NH₂ ), and pyridyl (-PY), the evolution of their formation probabilities along with the electric field is extracted from the plateau analysis of individual single-molecule break junction traces. With the increase of the electric field, the SMe-anchored molecules show a different trend in the formation probability compared to the other molecular junctions, which is consistent with the density functional theory calculations. Furthermore, switching from an SMe-anchored junction to an SAc-anchored junction is realized by altering the electric field in a mixed solution. The results in this work provide a new approach to the controllable fabrication and modulation of single-molecule junctions and other bottom-up nanodevices at molecular scales.

Functional Redox-Active Molecular Tunnel Junctions

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.

Folding a Single-Molecule Junction

Stimuli-responsive molecular junctions, where the conductance can be altered by an external perturbation, are an important class of nanoelectronic devices. These have recently attracted interest as large effects can be introduced through exploitation of quantum phenomena. We show here that significant changes in conductance can be attained as a molecule is repeatedly compressed and relaxed, resulting in molecular folding along a flexible fragment and cycling between an anti and a syn conformation. Power spectral density analysis and DFT transport calculations show that through-space tunneling between two phenyl fragments is responsible for the conductance increase as the molecule is mechanically folded to the syn conformation. This phenomenon represents a novel class of mechanoresistive molecular devices, where the functional moiety is embedded in the conductive backbone and exploits intramolecular nonbonding interactions, in contrast to most studies where mechanoresistivity arises from changes in the molecule-electrode interface.

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

The societal impact of the electronics industry is enormous—not to mention how this industry impinges on the global economy. The foreseen limits of the current technology—technical, economic, and sustainability issues—open the door to the search for successor technologies. In this context, molecular electronics has emerged as a promising candidate that, at least in the short-term, will not likely replace our silicon-based electronics, but improve its performance through a nascent hybrid technology. Such technology will take advantage of both the small dimensions of the molecules and new functionalities resulting from the quantum effects that govern the properties at the molecular scale. An optimization of interface engineering and integration of molecules to form densely integrated individually addressable arrays of molecules are two crucial aspects in the molecular electronics field. These challenges should be met to establish the bridge between organic functional materials and hard electronics required for the incorporation of such hybrid technology in the market. In this review, the most advanced methods for fabricating large-area molecular electronic devices are presented, highlighting their advantages and limitations. Special emphasis is focused on bottom-up methodologies for the fabrication of well-ordered and tightly-packed monolayers onto the bottom electrode, followed by a description of the top-contact deposition methods so far used.

A quantum chemical study of substituent effects on CN bonds in aryl isocyanide molecules adsorbed on the Pt surface

A periodicity implemented scheme of natural bond orbital (NBO) theory and normal mode analysis has been employed to investigate the tendency of the chemical bond strength of aryl isocyanide molecules with different para-substituted groups adsorbed on the Pt(111) surface. The NC bond order shows a clear correspondence with the NC stretching frequency; both of them exhibit a "volcano-like" profile as a function of the Hammett constant of the para-substituted groups for isolated molecules. When a molecule is adsorbed on the Pt(111) surface, the NC stretching frequency variations are determined by the resultant effect of σ donation and π back-donation between the molecule and the surface. The present comprehensive and systematic computations clarify the electron donating and withdrawing effects of the substituted groups on the interaction between the aryl isocyanide molecule and the transition metal substrate.

Thermally activated charge transport in carbon atom chains

Charge transport through single molecules is at the heart of molecular electronics for realizing the practical use of the rich quantum characteristics of electrode-molecule-electrode systems. Despite the extensive studies reported in the past, little experimental efforts have been focused on the electron transport mechanism at a temperature higher than the ambient temperature. In this work, we have reported the observation of the subtle interplay between electron tunneling and charge hopping in carbon chains connected to two Au electrodes at elevated temperatures. We measured the single-molecule conductance of Au-alkanedithiol-Au molecular junctions at various temperatures from 300 K to 420 K in vacuum. The temperature dependence of conductance suggested substantial roles of superexchange with inter-chain charge hopping under elevated temperatures for alkane chains longer than heptane. This finding provides a guide to design functional molecular junctions under practical conditions.

Systematic Modulation of Charge Transport in Molecular Devices through Facile Control of Molecule-Electrode Coupling Using a Double Self-Assembled Monolayer Nanowire Junction

We report a novel solid-state molecular device structure based on double self-assembled monolayers (D-SAM) incorporated into the suspended nanowire architecture to form a "Au|SAM-1||SAM-2|Au" junction. Using commercially available thiol molecules that are devoid of synthetic difficulty, we constructed a "Au|S-(CH2)6-ferrocene||SAM-2|Au" junction with various lengths and chemical structures of SAM-2 to tune the coupling between the ferrocene conductive molecular orbital and electrode of the junction. Combining low noise and a wide temperature range measurement, we demonstrated systematically modulated conduction depending on the length and chemical nature of SAM-2. Meanwhile, the transport mechanism transition from tunneling to hopping and the intermediate state accompanied by the current fluctuation due to the coexistence of the hopping and tunneling transport channels were observed. Considering the versatility of this solid-state D-SAM in modulating the electrode-molecule interface and electroactive groups, this strategy thus provides a novel facile strategy for tailorable nanoscale charge transport studies and functional molecular devices.

Scale-Up of Room-Temperature Constructive Quantum Interference from Single Molecules to Self-Assembled Molecular-Electronic Films

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.

Fabrication and functions of graphene-molecule-graphene single-molecule junctions

The past two decades have witnessed increasingly rapid advances in the field of single-molecule electronics, which are expected to overcome the limitation of the miniaturization of silicon-based microdevices, thus promoting the development of device manufacturing technologies and characterization means. In addition to this, they can enable us to investigate the intrinsic properties of materials at the atomic- or molecular-length scale and probe new phenomena that are inaccessible in ensemble experiments. In this perspective, we start from a brief introduction on the manufacturing method of graphene-molecule-graphene single-molecule junctions (GMG-SMJs). Then, we make a description on the remarkable functions of GMG-SMJs, especially on the investigation of single-molecule charge transport and dynamics. Finally, we conclude by discussing the main challenges and future research directions of molecular electronics.

First principle approach to elucidate transport properties through L-glutamic acid-based molecular devices using symmetrical electrodes

Protein-based electronics is one of the emerging technology in which inventive electronic devices are being adduced and developed based on the selective actions of specific proteins. The explicit actions can be predicted if the building blocks of proteins (i.e., amino acids) are studied decorously. We emphasize our work on electronic transport properties of L-glutamic acid (i.e., L-amino acid) stringed to gold, silver, and copper electrodes, respectively, to form three distinct devices. For our calculations, we employ NEGF-DFT approach using self-consistent function. Electronic coupling and tunneling barriers between the molecule and the electrodes have been emphasized with an inception of delocalization of molecular orbitals within the device. We observe strong correlation between tunneling barrier and Mulliken charge transfer between molecule and electrodes. The asymmetrical carbon chain (-CH₂) within the molecule exhibits negative differential resistance (NDR) and rectification ratio. The device using molecule with copper electrodes exhibits the highest peak to valley current ratio of 1.84. The rectification ratio of the device with gold, silver, and copper electrodes is 2.35, 2.25, and 15.62, respectively, at finite bias. These results yield fresh insight on the potential of L-glutamic acid like bio-molecule in the emerging field of proteotronics.

Interface Engineering in Organic Field-Effect Transistors: Principles, Applications, and Perspectives

Heterogeneous interfaces that are ubiquitous in optoelectronic devices play a key role in the device performance and have led to the prosperity of today's microelectronics. Interface engineering provides an effective and promising approach to enhancing the device performance of organic field-effect transistors (OFETs) and even developing new functions. In fact, researchers from different disciplines have devoted considerable attention to this concept, which has started to evolve from simple improvement of the device performance to sophisticated construction of novel functionalities, indicating great potential for further applications in broad areas ranging from integrated circuits and energy conversion to catalysis and chemical/biological sensors. In this review article, we provide a timely and comprehensive overview of current efficient approaches developed for building various delicate functional interfaces in OFETs, including interfaces within the semiconductor layers, semiconductor/electrode interfaces, semiconductor/dielectric interfaces, and semiconductor/environment interfaces. We also highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits, which have been neglected in most previous reviews. This review will provide a fundamental understanding of the interplay between the molecular structure, assembly, and emergent functions at the molecular level and consequently offer novel insights into designing a new generation of multifunctional integrated circuits and sensors toward practical applications.

Biocompatible alkyne arms containing Schiff base fluorescence indicator for dual detection of CdII and PbII at physiological pH and its application to live cell imaging

An alkyne arms containing salen-type Schiff base ligand L acts as a dual sensor for Cd^II and Pb^II with well-separated excitation and emission wavelengths. The ligand L has been utilized in cell imaging studies for both metal ions.

Intramolecular and Metal-to-Molecule Charge Transfer Electronic Resonances in the Surface-Enhanced Raman Scattering of 1,4-Bis((E)-2-(pyridin-4-yl)vinyl)naphthalene

Electrochemical surface-enhanced Raman scattering (SERS) of the cruciform system 1,4-bis((E)-2-(pyridin-4-yl)vinyl)naphthalene (bpyvn) was recorded on nanostructured silver surfaces at different electrode potentials by using excitation laser lines of 785 and 514.5 nm. SERS relative intensities were analyzed on the basis of the resonance Raman vibronic theory with the help of DFT calculations. The comparison between the experimental and the computed resonance Raman spectra calculated for the first five electronic states of the Ag2-bpyvn surface complex model points out that the selective enhancement of the SERS band recorded at about 1600 cm-1, under 785 nm excitation, is due to a resonant Raman process involving a photoexcited metal-to-molecule charge transfer state of the complex, while the enhancement of the 1570 cm-1 band using 514.5 nm excitation is due to an intramolecular π→π* electronic transition localized in the naphthalenyl framework, resulting in a case of surface-enhanced resonance Raman spectrum (SERRS). Thus, the enhancement of the SERS bands of bpyvn is controlled by a general chemical enhancement mechanism in which different resonance processes of the overall electronic structure of the metal-molecule system are involved.

Exploiting the versatile alkyne-based chemistry for expanding the applications of a stable triphenylmethyl organic radical on surfaces

The incorporation of terminal alkynes into the chemical structure of persistent organic perchlorotriphenylmethyl (PTM) radicals provides new chemical tools to expand their potential applications. In this work, this is demonstrated by the chemical functionalization of two types of substrates, hydrogenated SiO₂-free silicon (Si-H) and gold, and, by exploiting the click chemistry, scarcely used with organic radicals, to synthesise multifunctional systems. On one hand, the one-step functionalization of Si-H allows a light-triggered capacitance switch to be successfully achieved under electrochemical conditions. On the other hand, the click reaction between the alkyne-terminated PTM radical and a ferrocene azide derivative, used here as a model azide system, leads to a multistate electrochemical switch. The successful post-surface modification makes the self-assembled monolayers reported here an appealing platform to synthesise multifunctional systems grafted on surfaces.

Metal-Single-Molecule-Semiconductor Junctions Formed by a Radical Reaction Bridging Gold and Silicon Electrodes

Here we report molecular films terminated with diazonium salts moieties at both ends which enables single-molecule contacts between gold and silicon electrodes at open circuit via a radical reaction. We show that the kinetics of film grafting is crystal-facet dependent, being more favorable on ⟨111⟩ than on ⟨100⟩, a finding that adds control over surface chemistry during the device fabrication. The impact of this spontaneous chemistry in single-molecule electronics is demonstrated using STM-break junction approaches by forming metal-single-molecule-semiconductor junctions between silicon and gold source and drain, electrodes. Au-C and Si-C molecule-electrode contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 1.1 s, which is 30-400% higher than that reported for conventional molecular junctions formed between gold electrodes using thiol and amine contact groups. The high stability enabled measuring current-voltage properties during the lifetime of the molecular junction. We show that current rectification, which is intrinsic to metal-semiconductor junctions, can be controlled when a single-molecule bridges the gap in the junction. The system changes from being a current rectifier in the absence of a molecular bridge to an ohmic contact when a single molecule is covalently bonded to both silicon and gold electrodes. This study paves the way for the merging of the fields of single-molecule and silicon electronics.

Single molecule vs. large area design of molecular electronic devices incorporating an efficient 2-aminepyridine double anchoring group

When a molecule is bound to external electrodes by terminal anchor groups, the latter are of paramount importance in determining the electrical conductance of the resulting molecular junction. Here we explore the electrical properties of a molecule with bidentate anchor groups, namely 4,4'-(1,4-phenylenebis(ethyne-2,1-diyl))bis(pyridin-2-amine), in both large area devices and at the single molecule level. We find an electrical conductance of 0.6 × 10-4G0 and 1.2 × 10-4G0 for the monolayer and for the single molecule, respectively. These values are approximately one order of magnitude higher than those reported for monodentate materials having the same molecular skeleton. A combination of theory and experiments is employed to understand the conductance of monolayer and single molecule electrical junctions featuring this new multidentate anchor group. Our results demonstrate that the molecule has a tilt angle of 30° with respect to the normal to the surface in the monolayer, while the break-off length in the single molecule junction occurs for molecules having a tilt angle estimated as 40°, which would account for the difference in their conductance values per molecule. The bidentate 2-aminepyridine anchor is of general interest as a contact group, since this terminal functionalized aromatic ring favours binding of the adsorbate to the metal contact resulting in enhanced conductance values.

Modularized Tuning of Charge Transport through Highly Twisted and Localized Single-Molecule Junctions

Although most molecular electronic devices and materials consist of a backbone with a planar structure, twisted molecular wires with reduced inter-ring π-orbital overlap offer a unique opportunity for the modularized fabrication of molecular electronic devices. Herein we investigate the modularized tuning of the charge transport through the localized molecules by designing highly twisted molecules and investigating their single-molecule charge transport using the scanning tunneling microscopy break junction technique. We find that the anthracenediyl-core molecule with a 90° inter-ring twist angle shows an unexpectedly high conductance value, which is five times higher than that of the phenylene-core molecule with a similar configuration, whereas the conductance of the delocalized planar molecule with an anthracenediyl core or a phenylene core is almost the same. Theoretical calculations revealed that highly twisted angles result in weak interactions between molecular building blocks, for which molecule orbitals are separated into localized blocks, which offers the chance for the modularized tuning of every single block. Our findings offer a new strategy for the design of future molecular devices with a localized electronic structure.

Electrically transmissive alkyne-anchored monolayers on gold

Well-ordered, tightly-packed (surface coverage 0.97 × 10-9 mol cm-2) monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene (1) on gold are prepared via a simple self-assembly process, taking advantage of the ready formation of alkynyl C-Au σ-bonds. Electrochemical measurements using [Ru(NH3)6]3+, [Fe(CN)6]3-, and ferrocenylmethanol [Fe(η5-C5H4CH2OH)(η5-C5H5)] redox probes indicate that the alkynyl C-Au contacted monolayer of 1 presents a relatively low barrier for electron transfer. This contrasts with monolayer films on gold of other oligo(phenylene ethynylene) derivatives of comparable length and surface coverage, but with different contacting groups. Additionally, a low voltage transition (Vtrans = 0.51 V) from direct tunneling (rectangular barrier) to field emission (triangular barrier) is observed. This low transition voltage points to a low tunneling barrier, which is consistent with the facile electron transport observed through the C-Au contacted self-assembled monolayer of 1.

The Rate of Charge Tunneling in EGaIn Junctions Is Not Sensitive to Halogen Substituents at the Self-Assembled Monolayer//Ga2O3 Interface

This paper describes experiments that are designed to test the influence of terminal groups incorporating carbon-halogen bonds on the current density (by hole tunneling) across self-assembled monolayer (SAM)-based junctions of the form M^TS/S(CH₂)₉NHCOCH _nX_{3- n}//Ga₂O₃/EGaIn (where M = Ag and Au and X = CH₃, F, Cl, Br, I). Within the limits of statistical significance, these rates of tunneling are insensitive to the nature of the terminal group at the interface between the SAM and the Ga₂O₃. The results are relevant to the origin of an apparent inconsistency in the literature concerning the influence of halogen atoms at the SAM//electrode interface on the tunneling current density.

Tuning Charge Transport in Aromatic-Ring Single-Molecule Junctions via Ionic-Liquid Gating

Achieving gate control with atomic precision, which is crucial to the transistor performance on the smallest scale, remains a challenge. Herein we report a new class of aromatic-ring molecular nanotransistors based on graphene-molecule-graphene single-molecule junctions by using an ionic-liquid gate. Experimental phenomena and theoretical calculations confirm that this ionic-liquid gate can effectively modulate the alignment between molecular frontier orbitals and the Fermi energy level of graphene electrodes, thus tuning the charge-transport properties of the junctions. In addition, with a small gate voltage (|V_G |≤1.5 V) ambipolar charge transport in electrochemically inactive molecular systems (E_G >3.5 eV) is realized. These results offer a useful way to build high-performance single-molecule transistors, thus promoting the prospects for molecularly engineered electronic devices.

Tuning the Coupling in Single-Molecule Heterostructures: DNA-Programmed and Reconfigurable Carbon Nanotube-Based Nanohybrids

Herein a strategy is presented for the assembly of both static and stimuli-responsive single-molecule heterostructures, where the distance and electronic coupling between an individual functional nanomoiety and a carbon nanostructure are tuned via the use of DNA linkers. As proof of concept, the formation of 1:1 nanohybrids is controlled, where single quantum dots (QDs) are tethered to the ends of individual carbon nanotubes (CNTs) in solution with DNA interconnects of different lengths. Photoluminescence investigations-both in solution and at the single-hybrid level-demonstrate the electronic coupling between the two nanostructures; notably this is observed to progressively scale, with charge transfer becoming the dominant process as the linkers length is reduced. Additionally, stimuli-responsive CNT-QD nanohybrids are assembled, where the distance and hence the electronic coupling between an individual CNT and a single QD are dynamically modulated via the addition and removal of potassium (K+) cations; the system is further found to be sensitive to K+ concentrations from 1 pM to 25 × 10-3 m. The level of control demonstrated here in modulating the electronic coupling of reconfigurable single-molecule heterostructures, comprising an individual functional nanomoiety and a carbon nanoelectrode, is of importance for the development of tunable molecular optoelectronic systems and devices.

Molecular signature of polyoxometalates in electron transport of silicon-based molecular junctions

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic-inorganic polyoxometalate hybrids [PM11O39{Sn(C6H4)C[triple bond, length as m-dash]C(C6H4)N2}]3- (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current-voltage curves with the Simmon's equation gives a mean tunnel energy barrier ΦPOM of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity.

Unconventional Single-Molecule Conductance Behavior for a New Heterocyclic Anchoring Group: Pyrazolyl

Electrical conductance across a molecular junction is strongly determined by the anchoring group of the molecule. Here we highlight the unusual behavior of 1,4-bis(1H-pyrazol-4-ylethynyl)benzene that exhibits unconventional junction current versus junction-stretching distance curves, which are peak-shaped and feature two conducting states of 2.3 × 10^-4 G₀ and 3.4 × 10^-4 G₀. A combination of theory and experiments is used to understand the conductance of single-molecule junctions featuring this new anchoring group, i.e., pyrazolyl. These results demonstrate that the pyrazolyl moiety changes its protonation state and contact binding during junction evolution and that it also binds in either end-on or facial geometries with gold contacts. The pyrazolyl moiety holds general interest as a contacting group, because this linkage leads to a strong double anchoring of the molecule to the gold electrode, resulting in enhanced conductance values.

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

Charge transfer (CT) and charge transport (CTr) are at the core of life-sustaining biological processes and of processes that govern the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes and suppressing undesired ones. The notion about dipole-generated local fields affecting CT has evolved since the middle of the 20th century. In the 1990s, the first reports demonstrating the dipole effects on the kinetics of long-range electron transfer appeared. Concurrently, the development of molecular-level designs of electric junctions has led the exploration of dipole effects on CTr. Biomimetic molecular electrets such as polypeptide helices are often the dipole sources in CT systems. Conversely, surface-charge electrets and self-assembled monolayers of small polar conjugates are the preferred sources for modifying interfacial electric fields for controlling CTr. The multifaceted complexity of such effects on CT and CTr testifies for the challenges and the wealth of this field that still remains largely unexplored. This review outlines the basic concepts about dipole effects on CT and CTr, discusses their evolution, and provides accounts for their future developments and impacts.

Electrical and SERS detection of disulfide-mediated dimerization in single-molecule benzene-1,4-dithiol junctions

Electrical and in situ SERS characterization of the benzene-1,4-dithiol (BDT) junction suggested that dimerization of BDT contributed to the low conductance.

We applied a combination of mechanically controllable break junction (MCBJ) and in situ surface enhanced Raman spectroscopy (SERS) methods to investigate the long-standing single-molecule conductance discrepancy of prototypical benzene-1,4-dithiol (BDT) junctions. Single-molecule conductance characterization, together with configuration analysis of the molecular junction, suggested that disulfide-mediated dimerization of BDT contributed to the low conductance feature, which was further verified by the detection of S–S bond formation through in situ SERS characterization. Control experiments demonstrated that the disulfide-mediated dimerization could be tuned via the chemical inhibitor. Our findings suggest that a combined electrical and SERS method is capable of probing chemical reactions at the single-molecule level.

Understanding the charge transport properties of redox active metal-organic conjugated wires

For Rh₂-organic molecular wires, we found that weaker coupling systems built using longer bridging ligands exhibit better electrical conductance.

Layer-by-layer assembly of the dirhodium complex [Rh₂(O₂CCH₃)₄] (Rh₂) with linear N,N′-bidentate ligands pyrazine (L_S) or 1,2-bis(4-pyridyl)ethene (L_L) on a gold substrate has developed two series of redox active molecular wires, (Rh₂L_S)_n@Au and (Rh₂L_L)_n@Au (n = 1–6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current–voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh₂L_L)_n@Au wires exhibit smaller distance attenuation factors (β) in both the tunneling (β = 0.044 Å^–1) and hopping (β = 0.003 Å^–1) regimes, although in (Rh₂L_S)_n@Au the electronic coupling between the adjacent Rh₂ centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh₂)–π(L) orbital interactions, and (Rh₂L_L)_n@Au has a smaller energy gap between the filled π*(Rh₂) and the empty π*(L) orbitals. Thus, for (Rh₂L_L)_n@Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh₂L_S)_n@Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion. On this basis, we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter (n ≤ 4) and longer (n > 4) wires, respectively, and the Rh₂L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.

Gateway state-mediated, long-range tunnelling in molecular wires

If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of high-conductance molecular wires. For a wide variety of molecules conducting by phase coherent tunnelling, conductance G decays with length following the relationship G = Ae^-βL. It is widely accepted that the attenuation coefficient β is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety (phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to E_F through coupling to the bridge moiety. This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode-molecule interface properties.

DNA-Based Single-Molecule Electronics: From Concept to Function

Beyond being the repository of genetic information, DNA is playing an increasingly important role as a building block for molecular electronics. Its inherent structural and molecular recognition properties render it a leading candidate for molecular electronics applications. The structural stability, diversity and programmability of DNA provide overwhelming freedom for the design and fabrication of molecular-scale devices. In the past two decades DNA has therefore attracted inordinate amounts of attention in molecular electronics. This review gives a brief survey of recent experimental progress in DNA-based single-molecule electronics with special focus on single-molecule conductance and I–V characteristics of individual DNA molecules. Existing challenges and exciting future opportunities are also discussed.

Understanding interface (odd–even) effects in charge tunneling using a polished EGaIn electrode

Charge transport across large area molecular tunneling junctions is widely studied due to its potential in the development of quantum electronic devices.

Interface-Engineered Charge-Transport Properties in Benzenedithiol Molecular Electronic Junctions via Chemically p-Doped Graphene Electrodes

In this study, we fabricated and characterized vertical molecular junctions consisting of self-assembled monolayers of benzenedithiol (BDT) with a p-doped multilayer graphene electrode. The p-type doping of a graphene film was performed by treating pristine graphene (work function of ∼4.40 eV) with trifluoromethanesulfonic (TFMS) acid, producing a significantly increased work function (∼5.23 eV). The p-doped graphene-electrode molecular junctions statistically showed an order of magnitude higher current density and a lower charge injection barrier height than those of the pristine graphene-electrode molecular junctions, as a result of interface engineering. This enhancement is due to the increased work function of the TFMS-treated p-doped graphene electrode in the highest occupied molecular orbital-mediated tunneling molecular junctions. The validity of these results was proven by a theoretical analysis based on a coherent transport model that considers asymmetric couplings at the electrode-molecule interfaces.

Can Molecular Quantum Interference Effect Transistors Survive Vibration?

Quantum interference in cross-conjugated molecules can be utilized to construct molecular quantum interference effect transistors. However, whether its application can be achieved depends on the survivability of the quantum interference under real conditions such as nuclear vibration. We use two simulation methods to investigate the effects of nuclear vibration on quantum interference in a meta-linked benzene system. The simulation results suggest that the quantum interference is robust against nuclear vibration not only in the steady state but also in its transient dynamics, and thus the molecular quantum interference effect transistors can be realized.

High surface coverage of a self-assembled monolayer by in situ synthesis of palladium nanodeposits

Nascent metal|monolayer|metal devices have been fabricated by depositing palladium, produced through a CO-confined growth method, onto a self-assembled monolayer of an amine-terminated oligo(phenylene ethynylene) derivative on a gold bottom electrode. The high surface area coverage (85%) of the organic monolayer by densely packed palladium particles was confirmed by X-ray photoemission spectroscopy (XPS) and atomic force microscopy (AFM). The electrical properties of these nascent Au|monolayer|Pd assemblies were determined from the I-V curves recorded with a conductive-AFM using the Peak Force Tunneling AFM (PF-TUNA™) mode. The I-V curves together with the electrochemical experiments performed rule out the formation of short-circuits due to palladium penetration through the monolayer, suggesting that the palladium deposition strategy is an effective method for the fabrication of molecular junctions without damaging the organic layer.