Ambipolar transport in an electrochemically gated single-molecule field-effect transistor

Toward Practical Single-Molecule/Atom Switches

Electronic switches have been considered to be one of the most important components of contemporary electronic circuits for processing and storing digital information. Fabricating functional devices with building blocks of atomic/molecular switches can greatly promote the minimization of the devices and meet the requirement of high integration. This review highlights key developments in the fabrication and application of molecular switching devices. This overview offers valuable insights into the switching mechanisms under various stimuli, emphasizing structural and energy state changes in the core molecules. Beyond the molecular switches, typical individual metal atomic switches are further introduced. A critical discussion of the main challenges for realizing and developing practical molecular/atomic switches is provided. These analyses and summaries will contribute to a comprehensive understanding of the switch mechanisms, providing guidance for the rational design of functional nanoswitch devices toward practical applications.

Supramolecular Transistors with Quantum Interference Effect

The charge transport through supramolecular junctions exhibits unique quantum interference (QI) effects, which provide an opportunity for the design of supramolecular transistors. Benefiting from the configuration dependence of QI, configuration control of the supramolecular assemblies to demonstrate the QI features is a key but challenging step. In this work, we fabricated the supramolecular transistors and investigated the charge transport through the conducting channel of the individual π-stacked thiophene/phenylene co-oligomers (TPCOs) using the electrochemically gated scanning tunneling microscope break junction technique. We controlled the configuration of the supramolecular channel and switched the QI features between the anti-resonance and resonance states of the supramolecular channels. We observed the supramolecular transistor with its on/off ratio above 103 (∼1300), a high gating efficiency of ∼165 mV/dec, a low off-state leakage current of ∼30 pA, and the channel length scaled down to <2.0 nm. Density functional theory calculations suggested that the QI features in π-stacked TPCOs vary depending on the supramolecular architecture and can be manipulated efficiently by fine-tuning the supramolecular configurations. This work reveals the potential of the supramolecular channels for molecular electronics and provides a fundamental understanding of intermolecular charge transport.

Supramolecular Radical Electronics

Supramolecular radical chemistry is an emerging area bridging supramolecular chemistry and radical chemistry, and the integration of radicals into the supramolecular architecture offers a new dimension for tuning their structures and functions. Although various efforts have been devoted to the fabrication of supramolecular junctions, the charge transport characterization through the supramolecular radicals remained unexplored due to the challenges in creating supramolecular radicals at the single-molecule level. Here, we demonstrate the fabrication and charge transport investigation of a supramolecular radical junction using the electrochemical scanning tunneling microscope-based break junction (EC-STM-BJ) technique. We found that the conductance of a supramolecular radical junction was more than 1 order of magnitude higher than that of a supramolecular junction without a radical and even higher than that of a fully conjugated oligophenylenediamine molecule with a similar length. The combined experimental and theoretical investigations revealed that the radical increased the binding energy and decreased the energy gap in the supramolecular radical junction, which leads to the near-resonant transport through the supramolecular radical. Our work demonstrated that the supramolecular radical can provide not only strong binding but also efficient electrical coupling between building blocks, which provides new insights into supramolecular radical chemistry and new materials with supramolecular radicals.

Single-Molecule Chemical Reactions Unveiled in Molecular Junctions

Understanding chemical processes at the single-molecule scale represents the ultimate limit of analytical chemistry. Single-molecule detection techniques allow one to reveal the detailed dynamics and kinetics of a chemical reaction with unprecedented accuracy. It has also enabled the discoveries of new reaction pathways or intermediates/transition states that are inaccessible in conventional ensemble experiments, which is critical to elucidating their intrinsic mechanisms. Thanks to the rapid development of single-molecule junction (SMJ) techniques, detecting chemical reactions via monitoring the electrical current through single molecules has received an increasing amount of attention and has witnessed tremendous advances in recent years. Research efforts in this direction have opened a new route for probing chemical and physical processes with single-molecule precision. This review presents detailed advancements in probing single-molecule chemical reactions using SMJ techniques. We specifically highlight recent progress in investigating electric-field-driven reactions, reaction dynamics and kinetics, host–guest interactions, and redox reactions of different molecular systems. Finally, we discuss the potential of single-molecule detection using SMJs across various future applications.

Single-molecule nano-optoelectronics: insights from physics

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.

Single-molecule field effect and conductance switching driven by electric field and proton transfer

Single-molecule junctions (SMJs) offer a novel strategy for miniaturization of electronic devices. In this work, we realize a graphene-porphyrin-graphene SMJ driven by electric field and proton transfer in two configurations. In the transistor configuration with ionic liquid gating, an unprecedented field-effect performance is achieved with a maximum on/off ratio of ~4800 and a gate efficiency as high as ~179 mV/decade in consistence with the theoretical prediction. In the other configuration, controllable proton transfer, tautomerization switching, is directly observed with bias dependence. Room temperature proton transfer leads to a two-state conductance switching, and more precise tautomerization is detected, showing a four-state conductance switching at high bias voltages and low temperatures. Such an SMJ in two configurations provides new insights into not only building multifunctional molecular nanocircuits toward real applications but also deciphering the intrinsic properties of matters at the molecular scale.

Electrochemical gating for single-molecule electronics with hybrid Au|graphene contacts

The single-molecular conductance of a redox active viologen molecular bridge between Au|graphene electrodes has been studied in an electrochemical gating configuration in an ionic liquid medium. A clear "off-on-off" conductance switching behaviour has been achieved through gating of the redox state when the electrochemical potential is swept. The Au|viologen|graphene junctions show single-molecule conductance maxima centred close to the equilibrium redox potentials for both reduction steps. The peak conductance of Au|viologen|graphene junctions during the first reduction is significantly higher than that of previously measured Au|viologen|Au junctions. This shows that even though the central viologen moiety is not directly linked to the enclosing electrodes, substituting one gold contact for a graphene one nevertheless has a significant impact on junction conductance values. The experimental data was compared against two theoretical models, namely a phase coherent tunnelling and an incoherent "hopping" model. The former is a simple gating monoelectronic model within density functional theory (DFT) which discloses the charge state evolution of the molecule with electrode potential. The latter model is the collective Kuznetsov Ulstrup model for 2-step sequential charge transport through the redox centre in the adiabatic limit. The comparison of both models to the experimental data is discussed for the first time. This work opens perspectives for graphene-based molecular transistors with more effective gating and fundamental understanding of electrochemical electron transfer at the single molecular level.

A single-molecule blueprint for synthesis

Chemical reactions that occur at nanostructured electrodes have garnered widespread interest because of their potential applications in fields including nanotechnology, green chemistry and fundamental physical organic chemistry. Much of our present understanding of these reactions comes from probes that interrogate ensembles of molecules undergoing various stages of the transformation concurrently. Exquisite control over single-molecule reactivity lets us construct new molecules and further our understanding of nanoscale chemical phenomena. We can study single molecules using instruments such as the scanning tunnelling microscope, which can additionally be part of a mechanically controlled break junction. These are unique tools that can offer a high level of detail. They probe the electronic conductance of individual molecules and catalyse chemical reactions by establishing environments with reactive metal sites on nanoscale electrodes. This Review describes how chemical reactions involving bond cleavage and formation can be triggered at nanoscale electrodes and studied one molecule at a time.

Molecular Imaging of Viologen Adlayers and In Situ Monitoring Structural Transformations at Electrode-Electrolyte Interfaces

The effects of temperature and molecular concentration on the ordering of two-dimensional (2D) nanostructures have been investigated at the well-defined Au(111)-electrolyte interface. In comparison to the assembly of thiolated alkanes or hydrogen-bonded nonthiolated molecules, fabricating large aromatic thiolated molecules into a highly ordered adlayer on a surface remained a challenge. In this study, we demonstrated the importance of controlling the assembly conditions and procedures for the formation of ordered adlayers of redox-active viologen derivatives. The assembly conditions that were explored include the variation of molar concentration of assembly solutions, assembly time, and thermal annealing. We report that the optimal assembly conditions for creating highly ordered thiolated viologen derivatives on a Au(111)-(1 × 1) electrode surface are to limit the time in which the electrode is immersed in a deoxygenated 0.05 mM ethanolic viologen solution (preheated to 70 °C) to 45 s, followed by thermal annealing in absolute ethanol for 12 h. Highly ordered molecular adlayers were imaged by electrochemical scanning tunneling microscopy (STM), revealing the molecular packing of low-coverage adlayers. Furthermore, in situ STM combined with cyclic voltammetry (CV) allowed for the exploration of the structural transformation and potential limit of reductive and "oxidative" desorption of adlayers within the electrochemical potential range of the sample potential (E_S) from -0.95 V to -0.10 V vs SCE.

Perylene Imide-Based Optical Chemosensors for Vapor Detection

Perylene imide (PI) molecules and materials have been extensively studied for optical chemical sensors, particularly those based on fluorescence and colorimetric mode, taking advantage of the unique features of PIs such as structure tunability, good thermal, optical and chemical stability, strong electron affinity, strong visible light absorption and high fluorescence quantum yield. PI-based optical chemosensors have now found broad applications in gas phase detection of chemicals, including explosives, biomarkers of some food and diseases (such as organic amines (alkylamines and aromatic amines)), benzene homologs, organic peroxides, phenols and nitroaromatics, etc. In this review, the recent research on PI-based fluorometric and colorimetric sensors, as well as array technology incorporating multiple sensors, is reviewed along with the discussion of potential applications in environment, health and public safety areas. Specifically, we discuss the molecular design and aggregate architecture of PIs in correlation with the corresponding sensor performances (including sensitivity, selectivity, response time, recovery time, reversibility, etc.). We also provide a perspective summary highlighting the great potential for future development of PIs optical chemosensors, especially in the sensor array format that will largely enhance the detection specificity in complexed environments.

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.

Moving Electrons Purposefully through Single Molecules and Nanostructures: A Tribute to the Science of Professor Nongjian Tao (1963-2020)

Electrochemistry intersected nanoscience 25 years ago when it became possible to control the flow of electrons through single molecules and nanostructures. Many surprises and a wealth of understanding were generated by these experiments. Professor Nongjian Tao was among the pioneering scientists who created the methods and technologies for advancing this new frontier. Achieving a deeper understanding of charge transport in molecules and low-dimensional materials was the first priority of his experiments, but he also succeeded in discovering applications in chemical sensing and biosensing for these novel nanoscopic systems. In parallel with this work, the investigation of a range of phenomena using novel optical microscopic methods was a passion of his and his students. This article is a review and an appreciation of some of his many contributions with a view to the future.

Electrostatic Gate Control in Molecular Transistors

Molecular transistors, in which single molecules serve as active channel components in a three-terminal device geometry, constitute the building blocks of molecular scale electronic circuits. To demonstrate such devices, a gate electrode has been incorporated in several test beds of molecular electronics. The frontier orbitals' alignments of a molecular transistor can be delicately tuned by modifying the molecular orbital energy with the gate electrode. In this review, we described electrostatic gate control of solid-state molecular transistors. In particular, we focus on recent experimental accomplishments in fabrication and characterization of molecular transistors.

Dynamical photo-induced electronic properties of molecular junctions

Nanoscale molecular-electronic devices and machines are emerging as promising functional elements, naturally flexible and efficient, for next-generation technologies. A deeper understanding of carrier dynamics in molecular junctions is expected to benefit many fields of nanoelectronics and power devices. We determine time-resolved charge current flowing at the donor-acceptor interface in molecular junctions connected to metallic electrodes by means of quantum transport simulations. The current is induced by the interaction of the donor with a Gaussian-shape femtosecond laser pulse. Effects of the molecular internal coupling, metal-molecule tunneling, and light-donor coupling on photocurrent are discussed. We then define the time-resolved local density of states which is proposed as an efficient tool to describe the absorbing molecule in contact with metallic electrodes. Non-equilibrium reorganization of hybridized molecular orbitals through the light-donor interaction gives rise to two phenomena: the dynamical Rabi shift and the appearance of Floquet-like states. Such insights into the dynamical photoelectronic structure of molecules are of strong interest for ultrafast spectroscopy and open avenues toward the possibility of analyzing and controlling the internal properties of quantum nanodevices with pump-push photocurrent spectroscopy.

Switching Effects in Molecular Electronic Devices

The creation of molecular electronic switches by using smart molecules is of great importance to the field of molecular electronics. This requires a fundamental understanding of the intrinsic electron transport mechanisms, which depend on several factors including the charge transport pathway, the molecule-electrode coupling strength, the energy of the molecular frontier orbitals, and the electron spin state. On the basis of significant progresses achieved in both experiments and theory over the past decade, in this review article we focus on new insights into the design and fabrication of different molecular switches and the corresponding switching effects, which is crucial to the development of molecular electronics. We summarize the strategies developed for single-molecule device fabrication and the mechanism of these switching effects. These analyses should be valuable for deeply understanding the switching effects in molecular electronic devices.

Supramolecular Systems and Chemical Reactions in Single-Molecule Break Junctions

The major challenges of molecular electronics are the understanding and manipulation of the electron transport through the single-molecule junction. With the single-molecule break junction techniques, including scanning tunneling microscope break junction technique and mechanically controllable break junction technique, the charge transport through various single-molecule and supramolecular junctions has been studied during the dynamic fabrication and continuous characterization of molecular junctions. This review starts from the charge transport characterization of supramolecular junctions through a variety of noncovalent interactions, such as hydrogen bond, π-π interaction, and electrostatic force. We further review the recent progress in constructing highly conductive molecular junctions via chemical reactions, the response of molecular junctions to external stimuli, as well as the application of break junction techniques in controlling and monitoring chemical reactions in situ. We suggest that beyond the measurement of single molecular conductance, the single-molecule break junction techniques provide a promising access to study molecular assembly and chemical reactions at the single-molecule scale.

Design of an efficient coherent multi-site single-molecule rectifier

We propose the design of a multi-site single-molecule diode with a rectification ratio exceeding a million.

A redox-active radical as an effective nanoelectronic component: stability and electrochemical tunnelling spectroscopy in ionic liquids

A redox-active persistent perchlorotriphenylmethyl (PTM) radical chemically linked to gold exhibits stable electrochemical activity in ionic liquids. Electrochemical tunnelling spectroscopy in this medium demonstrates that the PTM radical shows a highly effective redox-mediated current enhancement, demonstrating its applicability as an active nanometer-scale electronic component.

Single-Molecule Break Junctions Based on a Perylene-Diimide Cyano-Functionalized (PDI8-CN2) Derivative

In this letter, we report the single-molecule conductance properties of a cyano-functionalized perylene-diimide derivative (PDI8-CN2) investigated with gold nano-electrodes. This molecule is of large interest for the fabrication of high-performance and air-stable n-type organic field-effect transistors. Low-bias experiments performed on mechanically controllable break junctions reveal the presence of two different values of the single-molecule conductance, which differ by about two orders of magnitudes. Up to date, this feature was never observed for other perylene-diimide compounds having alternative chemical moieties attached to the basic aromatic core. Theoretical calculations suggest that the highest single-molecule conductance value here observed, comprised between 10(-2) and 10(-3) G0, is related to a charge transport path directly linking the two cyano groups.

Electronic transport in benzodifuran single-molecule transistors

Benzodifuran (BDF) single-molecule transistors have been fabricated in electromigration break junctions for electronic measurements. The inelastic electron tunneling spectrum validates that the BDF molecule is the pathway of charge transport. The gating effect is analyzed in the framework of a single-level tunneling model combined with transition voltage spectroscopy (TVS). The analysis reveals that the highest occupied molecular orbital (HOMO) of the thiol-terminated BDF molecule dominates the charge transport through Au-BDF-Au junctions. Moreover, the energy shift of the HOMO caused by the gate voltage is the main reason for conductance modulation. In contrast, the electronic coupling between the BDF molecule and the gold electrodes, which significantly affects the low-bias junction conductance, is only influenced slightly by the applied gate voltage. These findings will help in the design of future molecular electronic devices.

Surface engineering of reduced graphene oxide for controllable ambipolar flash memories

Tunable charge-trapping behaviors including unipolar charge trapping of one type of charge carrier and ambipolar trapping of both electrons and holes in a complementary manner is highly desirable for low power consumption multibit flash memory design. Here, we adopt a strategy of tuning the Fermi level of reduced graphene oxide (rGO) through self-assembled monolayer (SAM) functionalization and form p-type and n-type doped rGO with a wide range of manipulation on work function. The functionalized rGO can act as charge-trapping layer in ambipolar flash memories, and a dramatic transition of charging behavior from unipolar trapping of electrons to ambipolar trapping and eventually to unipolar trapping of holes was achieved. Adjustable hole/electron injection barriers induce controllable Vth shift in the memory transistor after programming operation. Finally, we transfer the ambipolar memory on flexible substrates and study their charge-trapping properties at various bending cycles. The SAM-functionalized rGO can be a promising candidate for next-generation nonvolatile memories.

Gating capacitive field-effect sensors by the charge of nanoparticle/molecule hybrids

The semiconductor field-effect platform is a powerful tool for chemical and biological sensing with direct electrical readout. In this work, the field-effect capacitive electrolyte-insulator-semiconductor (EIS) structure - the simplest field-effect (bio-)chemical sensor - modified with citrate-capped gold nanoparticles (AuNPs) has been applied for a label-free electrostatic detection of charged molecules by their intrinsic molecular charge. The EIS sensor detects the charge changes in AuNP/molecule inorganic/organic hybrids induced by the molecular adsorption or binding events. The feasibility of the proposed detection scheme has been exemplarily demonstrated by realizing capacitive EIS sensors consisting of an Al-p-Si-SiO2-silane-AuNP structure for the label-free detection of positively charged cytochrome c and poly-d-lysine molecules as well as for monitoring the layer-by-layer formation of polyelectrolyte multilayers of poly(allylamine hydrochloride)/poly(sodium 4-styrene sulfonate), representing typical model examples of detecting small proteins and macromolecules and the consecutive adsorption of positively/negatively charged polyelectrolytes, respectively. For comparison, EIS sensors without AuNPs have been investigated, too. The adsorption of molecules on the surface of AuNPs has been verified via the X-ray photoelectron spectroscopy method. In addition, a theoretical model of the functioning of the capacitive field-effect EIS sensor functionalized with AuNP/charged-molecule hybrids has been discussed.

Multi-responsive photo- and chemo-electrical single-molecule switches

Incorporating molecular switches as the active components in nanoscale electrical devices represents a current challenge in molecular electronics. It demands key requirements that need to be simultaneously addressed including fast responses to external stimuli and stable attachment of the molecules to the electrodes while mimicking the operation of conventional electronic components. Here, we report a single-molecule switching device that responds electrically to optical and chemical stimuli. A light pointer or a chemical signal can rapidly and reversibly induce the isomerization of bifunctional spiropyran derivatives in the bulk reservoir and, consequently, switch the electrical conductivity of the single-molecule device between a low and a high level. The spiropyran derivatives employed are chemically functionalized such that they can respond in fast but practical time scales. The unique multistimuli response and the synthetic versatility to control the switching schemes of this single-molecule device suggest spiropyran derivatives as key candidates for molecular circuitry.

Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS2(-)) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

Tunable charge transport in single-molecule junctions via electrolytic gating

We modulate the conductance of electrochemically inactive molecules in single-molecule junctions using an electrolytic gate to controllably tune the energy level alignment of the system. Molecular junctions that conduct through their highest occupied molecular orbital show a decrease in conductance when applying a positive electrochemical potential, and those that conduct though their lowest unoccupied molecular orbital show the opposite trend. We fit the experimentally measured conductance data as a function of gate voltage with a Lorentzian function and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations of transmission probability across single-molecule junctions. This work shows that electrochemical gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transport properties.

Orthogonally modulated molecular transport junctions for resettable electronic logic gates

Individual molecules have been demonstrated to exhibit promising applications as functional components in the fabrication of computing nanocircuits. Based on their advantage in chemical tailorability, many molecular devices with advanced electronic functions have been developed, which can be further modulated by the introduction of external stimuli. Here, orthogonally modulated molecular transport junctions are achieved via chemically fabricated nanogaps functionalized with dithienylethene units bearing organometallic ruthenium fragments. The addressable and stepwise control of molecular isomerization can be repeatedly and reversibly completed with a judicious use of the orthogonal optical and electrochemical stimuli to reach the controllable switching of conductivity between two distinct states. These photo-/electro-cooperative nanodevices can be applied as resettable electronic logic gates for Boolean computing, such as a two-input OR and a three-input AND-OR. The proof-of-concept of such logic gates demonstrates the possibility to develop multifunctional molecular devices by rational chemical design.