Nanoscale molecular rectifiers

A Robust Single-Molecule Diode with High Rectification Ratio and Integrability

Advancements in molecular electronics focus on single molecules as key components to create stable and functional devices that meet the requirements of device miniaturization and molecular function exploration. However, as the pioneering concept of a molecular diode, all single-molecule rectifiers reported previously are limited by their modest rectification ratios, owing to electron transmission in the off-state, highlighting the imperative for performance enhancements. Here, we demonstrate a unique method capable of realizing a stable and reproducible high-performance single-molecule rectifier through the strategic application of an electric-field-catalyzed Fries rearrangement. This flexible reaction enables the exquisite control of reversible conductance switching between a structure with constructive quantum interference and a structure with destructive quantum interference, therefore leading to an exceptional rectification ratio of up to 5000 at a bias of 1.0 V, which ranks the highest among the rectifiers constructed by only one individual molecule. The stable operation of nearly 100 devices at high temperatures demonstrates reproducibility. Moreover, on-chip integration of different single-molecule rectifiers succeeds in achieving half-wave and bridge rectifications, thus facilitating efficient alternating current-to-direct current conversions. This convenient strategy of electric-field-catalyzed quantum interference switching potentially revolutionizes device efficiency and miniaturization in nanotechnology, laying an actual step toward future practical integrated molecular-scale electronic nanocircuits.

Explore the Structural and Electronic Properties at the Organic/Organic Interfaces of Thiophene-Based Supramolecular Architectures

The structural and electronic properties at organic/organic interfaces determine the functionality of organic electronics. Here, we investigated the structural and electronic properties at interfaces between methyl-substituted dicyanovinyl-quinquethiophenes (DCV5T-Me2) and other electron acceptor molecules, namely fullerene (C60) and tetracyanoquinodimethane (TCNQ), by using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). Upon adsorption on Au(111), DCV5T-Me2 molecules self-assemble into compact islands at sub-monolayer coverage through hydrogen bonding and electrostatic interactions. A similar bonding configuration dominates in the second layer of a bilayer film, where DCV5T-Me2 possesses higher-lying LUMO (lowest unoccupied molecular orbital) and LUMO+1 in energy due to a decoupling effect. The co-deposition of DCV5T-Me2 and C60 does not result in ordered hybrid assemblies at the sub-monolayer coverage on Au(111). On the other hand, C60 molecules can self-assemble into ordered islands on top of the DCV5T-Me2 monolayer. The dI/dV spectra reveal that the LUMO of decoupled C60 is 400 mV lower in energy than the LUMO of decoupled DCV5T-Me2. This energy difference facilitates electron transfer from DCV5T-Me2 to C60. The co-deposition of DCV5T-Me2 and TCNQ leads to the formation of hybrid nanostructures. A tip-induced electric field can manipulate the charging and discharging of TCNQ by surrounding DCV5T-Me2, manifested as sharp peaks and dips in dI/dV spectra recorded over TCNQ.

Single-Molecule Junctions Formed Using Different Electrode Metals Under an Inert Atmosphere

The properties of single-molecule junctions, electronic devices approaching the limits of miniaturization, have typically been probed using inert gold electrodes. However, a complete understanding and the ultimate technological exploitation of molecular devices may only be realized if they can be readily evaluated using non-gold electrode metals - a task broadly impeded by the rapid oxidation of such materials in the air. This study demonstrates that single-molecule junctions can be formed using seven metals (gold, silver, copper, platinum, zinc, nickel, and cobalt) under an inert atmosphere inside a glovebox. The characteristic conductance features of atomic-sized junctions are first identified for each metal at room-temperature and ambient pressure, a guiding signature of nanoscale electrode formation. It is then shown that the conductance of single-molecule junctions comprising four different components does not strongly correlate with electrode work function. Snapback measurements reveal that the size of the nanogap opened upon breaking atomic point contacts exponentially correlates with the material's melting point, a proxy for the metal diffusion constant. Together, this work exposes exciting new opportunities to experimentally probe the influence of electrode metal on the formation, stability, and function of these nanoscale structures, a critical step toward the practical utilization of molecule-based nanoelectronic circuitry.

Structural and Electronic Properties of Thiophene-Based Supramolecular Architectures: Influence of the Underlying Metal Surfaces

Dicyanovinyl (DCV)-substituted oligothiophenes consist of both electron donor and acceptor ligands, which makes them promising materials for organic electronics. Here, we studied the structural and electronic properties of methyl-substituted dicyanovinyl-quinquethiophenes (DCV5T-Me2) adsorbed on different metal surfaces, namely Au(111), Ag(111), and Cu(111), by using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). It is found that the assembled structures of DCV5T-Me2 and the corresponding electronic properties vary depending on the underlying substrates. On Au(111) and Ag(111), compact organic islands are formed through intermolecular hydrogen bonding and electrostatic interactions. The lowest unoccupied molecular orbital (LUMO) and LUMO+1 of DCV5T-Me2 are lower in energy on Ag(111) than those on Au(111), due to the stronger molecule-surface interaction when adsorbed on Ag(111). Moreover, orbital distributions of the LUMO and LUMO+1 in dI/dV maps on Au(111) and Ag(111) are the same as the DFT-calculated orbital distributions in gas phase, which indicates physisorption. In contrast, chemisorption dominates on Cu(111), where no ordered assemblies of DCV5T-Me2 could be formed and resonances from the LUMO and LUMO+1 vanish. The present study highlights the key role of molecule-substrate interactions in determining the properties of organic nanostructures and provides valuable insights for designing next-generation organic electronics.

Harnessing carbon electrodes in molecular junctions: progress and challenges in device engineering

The relentless pursuit of miniaturization and enhanced functionality in electronic devices has driven researchers to explore innovative approaches. Carbon electrode-based molecular junctions (MJs) have emerged as a promising frontier in the quest for next-generation electronics. This review provides a comprehensive overview of the current state of research on carbon-based MJs for practical devices, focusing on their unique properties, such as charge transport phenomena, fabrication methods, and potential applications in revolutionizing electronic components. The inherent quantum nature of molecules introduces distinct electronic properties, enabling functionalities beyond those achievable with traditional semiconductor-based devices. The diverse range of molecules employed in creating these junctions highlights their tailored electronic characteristics and, consequently, device performance. The fabrication techniques for MJs are discussed in detail. The charge transport mechanisms in such junctions are also discussed, along with temperature effects. Additionally, the review addresses the integration of MJs into electronic circuits, considering scalability, reproducibility, and compatibility with existing manufacturing technologies. The potential applications of MJs in electronic devices, such as temperature-independent robust practical photosensors, photoswitches, charge storage devices, sensors and LEDs, are elucidated. However, challenges, such as stability, variability, and large-scale integration, are also addressed to realize the full potential of MJs in practical applications.

Interface Phenomena in Molecular Junctions through Noncovalent Interactions

Noncovalent interactions, both between molecules and at the molecule-electrode interfaces, play essential roles in enabling dynamic and reversible molecular behaviors, including self-assembly, recognition, and various functional properties. In macroscopic ensemble systems, these interfacial phenomena often exhibit emergent properties that arise from the synergistic interplay of multiple noncovalent interactions. However, at the single-molecule scale, precisely distinguishing, characterizing, and controlling individual noncovalent interactions remains a significant challenge. Molecular electronics offers a unique platform for constructing and characterizing both intermolecular and molecule-electrode interfaces governed by noncovalent interactions, enabling the isolated study of these fundamental interactions. Furthermore, precise control over these interfaces through noncovalent interactions facilitates the development of enhanced molecular devices. This review examines the characterization of interfacial phenomena arising from noncovalent interactions through single-molecule electrical measurements and explores their applications in molecular devices. We begin by discussing the construction of stable molecular junctions through intermolecular and molecule-electrode interfaces, followed by an analysis of electron tunneling mechanisms mediated by key noncovalent interactions and their modulation methods. We then investigate how noncovalent interactions enhance device sensitivity, stability, and functionality, establishing design principles for next-generation molecular electronics. We have also explored the potential of noncovalent interactions for bottom-up self-assembled molecular devices. The review concludes by addressing the opportunities and challenges in scaling up molecular electronics through noncovalent interactions.

A Robust Molecular Rectifier Based on Ferrocene-Functionalized Bis(diarylcarbene) on Gold

While thiol-based adsorbates have achieved significant success in surface modification and molecular electronics, their thermal and storage instability has hindered long-term commercial viability. Carbene-based thin films offer a promising alternative due to their enhanced stability; however, their molecular electronic properties after postfunctionalization remain to be investigated. In this study, by attaching a ferrocene (Fc) unit to a bis(diarylcarbene)-modified gold surface via carbodiimide coupling, the system exhibits diode behavior with a current rectification ratio of ∼100. This diode behavior is highly temperature-dependent, indicating that hopping is the dominant mechanism for current rectification. Notably, the system demonstrated excellent electrical stability under ambient storage conditions for over 6 months. Our work highlights the potential for designing carbene-based molecular junctions through postmodification and for developing durable functional molecular electronic devices.

Supramolecular Diodes with Donor-Acceptor Interactions

Inspired by the initial proposal of σ-bridged donor-acceptor (D-σ-A) single-molecule diodes in 1974, extensive studies over the past 50 years have explored various designs for π-conjugated D-π-A single-molecule diodes due to their feasible chemical synthesis and effective charge transfer. However, the rectification ratio of π-conjugated single-molecule diodes has been long-term limited by the challenge of asymmetric electronic coupling to induce the rectification effect. Here, we present a supramolecular diode constructed through an intramolecular π-π interaction-driven assembly strategy. The asymmetric transmission in this system is tunable via subangström mechanical control, resulting in a rectification ratio of up to 16. Electron transport studies reveal that this through-space D-π-π-A system constructed by the π-π stacking between pyrene (Py) and naphthalenediimide (NDI) is crucial for achieving asymmetric currents under different bias polarization. Theoretical calculations suggest that the intermolecular destructive quantum interference not only enables a sharp variation in electron transmission but also facilitates asymmetric electronic energy shifts through mechanical stretching, significantly improving the rectification ratio. Our work provides a general approach to fabricating and modulating asymmetric molecular architectures through noncovalent supramolecular interactions, showcasing the potential of high-performance single-molecule rectifiers.

Electrochemically grafted molecular layers as on-chip energy storage molecular junctions

Molecular junctions (MJs) are celebrated nanoelectronic devices for mimicking conventional electronic functions, including rectifiers, sensors, wires, switches, transistors, negative differential resistance, and memory, following an understanding of charge transport mechanisms. However, capacitive nanoscale molecular junctions are rarely seen. The present work describes electrochemically (E-Chem) grown covalently attached molecular thin films of 10, 14.3, and 18.6 nm thickness using benzimidazole (BENZ) diazonium salts on ITO electrodes on a quartz substrate upon which 50 nm of aluminum (Al) top contact was deposited to fabricate large-scale (area = 500 × 500 μm2) molecular junctions. The capacitance of the molecular junctions decreases with increasing thickness of molecular layers, a behavior attributed to a classical dielectric role in which the geometric capacitance of the device within a uniform dielectric component is expected to decrease with increasing thickness. An electrical dipole moment in BENZ oligomers enhances polarizability; hence, the dielectric constant of the medium leads to an increase in the capacitance of MJs, which reaches a maximum value of ∼53 μF cm-2 for a junction of 10 nm molecular film thickness. In addition to direct-current (DC) electrical measurements, and computational studies, we performed alternating current (AC)-based electrical measurements to understand the frequency response of molecular junctions. Our present study demonstrates that BENZ-based molecular junctions behave as classical organic capacitors and could be a suitable building block for nanoscale on-chip energy storage devices.

Molecular-scale in-operando reconfigurable electronic hardware

It is challenging to reconfigure devices at molecular length scales. Here we report molecular junctions based on molecular switches that toggle stably and reliably between multiple operations to reconfigure electronic devices at molecular length scales. Rather than static on/off switches that always revert to the same state, our voltage-driven molecular device dynamically switches between high and low conduction states during six consecutive proton-coupled electron transfer steps. By changing the applied voltage, different states are accessed resulting in in operando reconfigurable electronic functionalities of variable resistor, diode, memory, and NDR (negative differential conductance). The switching behavior is voltage driven but also has time-dependent features making it possible to access different memory states. This multi-functional switch represents molecular scale hardware operable in solid-state devices (in the form of electrode-monolayer-electrode junctions) that are interesting for areas of research where it is important to have access to time-dependent changes such as brain-inspired (or neuromorphic) electronics.

Electrosynthesis of Ru (II)-Polypyridyl Oligomeric Films on ITO Electrode for Two Terminal Non-Volatile Memory Devices and Neuromorphic Computing

Molecular electronics exhibiting resistive-switching memory features hold great promise for the next generation of digital technology. In this work, electrosynthesis of ruthenium polypyridyl nanoscale oligomeric films is demonstrated on an indium tin oxide (ITO) electrode followed by an ITO top contact deposition yielding large-scale (junction area = 0.7 × 0.7 cm2) two terminal molecular junctions. The molecular junctions exhibit non-volatile resistive switching at a relatively lower operational voltage, ±1 V, high ON/OFF electrical current ratio (≈103), low-energy consumption (SET/RESET = 27.94/14400 nJ), good cyclic stability (>300 cycles), and switching speed (SET/RESET = 25 ms/20 ms). A computational study suggests that accessible frontier molecular orbitals of metal-complex to the Fermi level of ITO electrodes facilitate charge transport at a relatively lower bias followed by a filamentformation. An extensive analysis is performed of the performance of binary neural networks exploiting the current-voltage features of the devices as binary synaptic weights and exploring their potential for neuromorphic logic-in-memory implementation of IMPLICATION (IMPLY) operation which can realize universal gates. The comprehensive analysis indicates that the proposed redox-active complex-based memory device may be a promising candidate for high-density data storage, energy-efficient implementation of neuromorphic networks with software-level accuracy, and logic-in-memory implementations.

Boron-Doped Single-Molecule van der Waals Diode

Single-molecule diode was the first proposed device in molecular electronics. Despite the great efforts and advances over 50 years, the reported rectification ratios, the most critical parameter of a diode, remain moderate for the single-molecule diode. Herein, we report an approach to achieve a larger rectification ratio by adopting the combined strategies of p-type boron doping, the single-layer graphene nodes, and the van der Waals layer-by-layer architecture. Measured current-voltage curves showed one of the as-fabricated single-molecule diodes hit an unprecedented large rectification ratio of 457 at ±1 V. Break junction operations and spectroscopic measurements revealed the three-atom-thick configuration of the single-molecule diodes. With the experimental and theoretical calculation results, we demonstrated the doped boron atoms induced holes to redistribute the electron density, making the asymmetric coupling at positive and negative biases, and the van der Waals interaction promoted asymmetric coupling and significantly boosted diode performance.

Highly Strained Polymeric Monolayer Stacked for Wafer-Scale and Transferable Nanodielectrics

As the keystones of molecular electronics, high-quality nanodielectric layers are challenging to assemble due to the strictest criteria for their reliability and uniformity over a large area. Here, we report a strained poly(4-vinylphenol) monolayer, ready to be stacked to form defect-free wafer-scale nanodielectrics. The thickness of the nanodielectrics can be precisely adjusted in integral multiples of the 1.2 nm thick PVP monolayer. By employing a double cross-linking strategy, an exceptional dielectric performance is achieved with a leakage current of 10-7-10-8 A/cm2 at 2 MV/cm across the low-k PVP layers as thin as 3.6 nm. Furthermore, the obtained nanodielectric layers could be laminated onto various substrates on demand via polydimethylsiloxane soft stamps, enabling its application in organic field-effect transistors of both bottom-gate and top-gate configurations. This work represents a pivotal development in (opto-)electronic molecular materials and heralds an emerging avenue for the exploration of functional nanodielectrics in the field of nanoelectronics.

Revealing Single-Molecule Photocurrent Generation Mechanisms under On- and Off-Resonance Excitation

We investigate photocurrent generation mechanisms in a pentacene single-molecule junction using subnanometer resolved photocurrent imaging under both on- and off-resonance laser excitation. By employing a wavelength-tunable laser combined with a lock-in technique, net photocurrent signals are extracted to elucidate photoinduced electron tunneling processes. Under off-resonance excitation, photocurrents are found to arise from photon-assisted tunneling, with contributions from three distinct frontier molecular orbitals at different bias voltages. In contrast, under resonance excitation, photocurrents are found to be significantly enhanced at negative bias voltages, exhibiting a spatial distribution linked to molecular electronic transitions and associated transition dipoles. This study reveals the contributions of different frontier molecular orbitals in the photon-assisted tunneling processes under off-resonance excitation, while molecular optical transitions are also found to be important at negative bias under resonance excitation. These findings could be instructive to the design and optimization of advanced optoelectronic devices.

Gating the Rectifying Direction of Tunneling Current through Single-Molecule Junctions

In electronic functional chips, one of the most crucial components is the field-effect transistor (FET). To meet the urgent demands for further miniaturization of electronic devices, solid-state single-molecule transistors by molecular orbital gating have been extensively reported. However, under negative bias and positive bias, achieving a distinct gating effect is extremely challenging because molecular orbital gating is independent of the bias polarity. Here, we demonstrated that rectifiers can be realized in single-molecule junctions with a symmetric molecular structure and an electrode material by simply breaking the symmetry of the electrode's chemical potential via ionic adsorption. We further demonstrated that the tunneling current can be gated with opposite change tendencies under negative and positive bias by applying an ionic gating voltage, which eventually results in a reversal of the rectifying direction. Our experiments elucidate that, unlike the classical mechanism for solid molecular FET, the modulation of the electrode's chemical potential, rather than the regulation of molecular orbitals, might dominate the electron transport in the ionic liquid environment upon a gating voltage. Our study gains deeper insights into the mechanism of ionic liquid gating and opens a window for designing high-performance electrochemical-based functional devices.

Flexible Crossbar Molecular Devices with Patterned EGaIn Top Electrodes for Integrated All-Molecule-Circuit Implementation

Here, a unique crossbar architecture is designed and fabricated, incorporating vertically integrated self-assembled monolayers in electronic devices. This architecture is used to showcase 100 individual vertical molecular junctions on a single chip with a high yield of working junctions and high device uniformity. The study introduces a transfer approach for patterned liquid-metal eutectic alloy of gallium and indium top electrodes, enabling the creation of fully flexible molecular devices with electrical functionalities. The devices exhibit excellent charge transport performance, sustain a high rectification ratio (>103), and stable endurance and retention properties, even when the devices are significantly bent. Furthermore, Boolean logic gates, including OR and AND gates, as well as half-wave and full-wave rectifying circuits, are successfully implemented. The unique design of the flexible molecular device represents a significant step in harnessing the potential of molecular devices for high-density integration and possible molecule-based computing.

Programmed Heterostructures for Enhanced Electrical Conductivity

Interfacial electron transport in multicomponent systems plays a crucial role in controlling electrical conductivity. Organic-inorganic heterostructures electronic devices where all the entities are covalently bonded to each other can reduce interfacial electrical resistance, thus suitable for low-power consumption electronic operations. Programmed heterostructures of covalently bonded interfaces between ITO-ethynylbenzene (EB) and EB-zinc ferrite (ZF) nanoparticles, a programmed structure showing 67 978-fold enhancement of electrical current as compared to pristine NPs-based two terminal devices are created. An electrochemical approach is adopted to prepare nearly π-conjugated EB oligomer films of thickness ≈26 nm on ITO-electrode on which ZF NPs are chemically attached. A "flip-chip" method is employed to combine two EB-ZnFe2O4 NPs-ITO to probe electrical conductivity and charge conduction mechanism. The EB-ZnFe2O4 NPs exhibit strong electronic coupling at ITO-EB and EB-NPs with an energy barrier of 0.13 eV between the ITO Fermi level and the LUMO of EB-ZF NPs for efficient charge transport. Both the DC and AC-based electrical measurements manifest a low resistance at ITO-EB and EB-ZF NPs, revealing enhanced electrical current at ± 1.5 V. The programmed heterostructure devices can meet a strategy to create well-controlled molecular layers for electronic applications toward miniaturized components that shorten charge carrier distance, and interfacial resistance.

Efficient Molecular Rectification in Metal-Molecules-Semimetal Junctions

Molecular rectification is expected to be observed in metal-molecule-metal tunnel junctions in which the resonance levels responsible for their transport properties are spatially localized asymmetrically with respect to the leads. Yet, effects such as electrostatic screening and formation of metal induced gap states reduce the magnitude of rectification that can be realized in such junctions. Here we suggest that junctions of the form metal-molecule(s)-semimetal mitigate these interfacial effects. We report current rectification in junctions based on the semimetal bismuth (Bi) with high rectification ratios (>102) at 1.0 V using alkanethiols, molecules for which rectification has never been observed. In addition to the alleviation of screening and surface states, the efficient rectification is argued to be related to symmetry breaking of the applied bias in these junctions because of a built-in potential within the Bi lead. The significance of this built-in potential and its implications for the future and other applications are discussed.

Molecular Electronic Junctions Achieved High Thermal Switch Ratios in Atomistic Simulations

The development of devices that improve thermal energy management requires thermal regulation with efficiency comparable to the ratios R ∼ 105 in electric regulation. Unfortunately, current materials and devices in thermal regulators have only been reported to achieve R ∼ 10. We use atomistic simulations to demonstrate that Ferrocenyl (Fc) molecules under applied external electric fields can alter charge states and achieve high thermal switch ratios R = Gq/G0, where Gq and G0 are the high and low limiting conductances. When an electric field is applied, Fc molecules are positively charged, and the SAM-Au interfacial interaction is strong, leading to high heat conductance Gq. On the other hand, with no electric field, the Fc molecules are charge neutral and the SAM-Au interfacial interaction is weak, leading to low heat conductance G0. We optimized various design parameters for the device performance, including the Au-to-Au gap distance L, the system operation temperature T, the net charge on Fc molecules q, the Au surface charge number Z, and the SAM number N. We find that Gq can be very large and increases with increasing q, Z, or N, while G0 is near 0 at L > 3.0 nm. As a result, R > 100 was achieved for selected parameter ranges reported here.

Gradual Change between Coherent and Incoherent Tunneling Regimes Induced by Polarizable Halide Substituents in Molecular Tunnel Junctions

This paper describes a gradual transition of charge transport across molecular junctions from coherent to incoherent tunneling by increasing the number and polarizability of halide substituents of phenyl-terminated aliphatic monolayers of the form S(CH2)10OPhXn, X = F, Cl, Br, or I; n = 0, 1, 2, 3, or 5. In contrast to earlier work where incoherent tunneling was induced by introducing redox-active groups or increasing the molecular length, we show that increasing the polarizability, while keeping the organization of the monolayer structure unaltered, results in a gradual change in the mechanism of tunneling of charge carriers where the activation energy increased from 23 meV for n = 0 (associated with coherent tunneling) to 257 meV for n = 5 with X = Br (associated with incoherent tunneling). Interestingly, this increase in incoherent tunneling rate with polarizability resulted in an improved molecular diode performance. Our findings suggest an avenue to improve the electronic function of molecular devices by introducing polarizable atoms.

Many-Body Quantum Interference Route to the Two-Channel Kondo Effect: Inverse Design for Molecular Junctions and Quantum Dot Devices

Molecular junctions-whether actual single molecules in nanowire break junctions or artificial molecules realized in coupled quantum dot devices-offer unique functionality due to their orbital complexity, strong electron interactions, gate control, and many-body effects from hybridization with the external electronic circuit. Inverse design involves finding candidate structures that perform a desired function optimally. Here we develop an inverse design strategy for generalized quantum impurity models describing molecular junctions, and as an example, use it to demonstrate that many-body quantum interference can be leveraged to realize the two-channel Kondo critical point in simple 4- or 5-site molecular moieties. We show that remarkably high Kondo temperatures can be achieved, meaning that entropy and transport signatures should be experimentally accessible.

Electron-Beam-Induced Modification of N-Heterocyclic Carbenes: Carbon Nanomembrane Formation

Electron irradiation of self-assembled monolayers (SAMs) is a versatile tool for lithographic methods and the formation of new 2D materials such as carbon nanomembranes (CNMs). While the interaction between the electron beam and standard thiolate SAMs has been well studied, the effect of electron irradiation for chemically and thermally ultrastable N-heterocyclic carbenes (NHCs) remains unknown. Here we analyze electron irradiation of NHC SAMs featuring different numbers of benzene moieties and different sizes of the nitrogen side groups to modify their structure. Our results provide design rules to optimize NHC SAMs for effective electron-beam modification that includes the formation of sulfur-free CNMs, which are more suitable for ultrafiltration applications. Considering that NHC monolayers exhibit up to 100 times higher stability of their bonding with the metal substrate toward electron-irradiation compared to standard SAMs, they offer a new alternative for chemical lithography where structural modification of SAMs should be limited to the functional group.

Dynamically blocking leakage current in molecular tunneling junctions

Molecular tunneling junctions based on self-assembled monolayers (SAMs) have demonstrated rectifying characteristics at the nanoscale that can hardly be achieved using traditional approaches. However, defects in SAMs result in high leakage when applying bias. The poor performance of molecular diodes compared to silicon or thin-film devices limits their further development. In this study, we show that incorporating "mixed backbones" with flexible-rigid structures into molecular junctions can dynamically block tunneling currents, which is difficult to realize using non-molecular technology. Our idea is achieved by the interaction between interfacial dipole moments and electric field, triggering structured packing in SAMs. Efficient blocking of leakage by more than an order of magnitude leads to a significant enhancement of the rectification ratio to the initial value. The rearrangement of supramolecular structures has also been verified through electrochemistry and electroluminescence measurements. Moreover, the enhanced rectification is extended to various challenging environments, including endurance measurements, bending of electrodes, and rough electrodes, thus demonstrating the feasibility of the dynamic behavior of molecules for practical electronic applications.

Self-formed asymmetric Schottky contacts between graphene and WSiGeN4

In this paper, the electronic properties and transport characteristics of WSiGeN4/graphene heterostructures were explored by combining the quantum transport method with first-principle calculations. The band structures indicate that the heterostructures can form either p-type or n-type Schottky contacts, depending on the stacking mode. Due to the self-formed asymmetric Schottky contacts, we design an asymmetric van der Waals (vdW) metal-semiconductor-metal (MSM) structure, which exhibits a pronounced asymmetric current-voltage (I-V) curve. The corresponding physical mechanisms are attributed to carrier transport mechanisms, which are primarily governed by thermionic excitation at positive bias voltages and tunneling effects at negative bias voltages. Our study offers a viable strategy for integrating asymmetric Schottky barriers into MSM configurations, laying the groundwork for a wider range of applications in a range of Janus two-dimensional semiconductors.

The thermodynamics of self-assembled monolayer formation: a computational and experimental study of thiols on a flat gold surface

A methodology based on molecular dynamics simulations is presented to determine the chemical potential of thiol self-assembled monolayers on a gold surface. The thiol de-solvation and then the monolayer formation are described by thermodynamic integration with a gradual decoupling of one molecule from the environment, with the necessary corrections to account for standard state changes. The procedure is applied both to physisorbed undissociated thiol molecules and to chemisorbed dissociated thiyl radicals, considering in the latter case the possible chemical potential of the produced hydrogen. We considered monolayers formed by either 7-mercapto-4-methylcoumarin (MMC) or 3-mercapto-propanoic acid (MPA) on a flat gold surface: the free energy profiles with respect to the monolayer density are consistent with a transition from a very stable lying-down phase at low densities to a standing-up phase at higher densities, as expected. The maximum densities of thermodynamically stable monolayers are compared to experimental measures performed with reference-free grazing-incidence X-ray fluorescence (RF-GIXRF) on the same systems, finding a better agreement in the case of chemisorbed thiyl radicals.

Tuneable Current Rectification Through a Designer Graphene Nanoribbon

Unimolecular current rectifiers are fundamental building blocks in organic electronics. Rectifying behavior has been identified in numerous organic systems due to electron-hole asymmetries of orbital levels interfaced by a metal electrode. As a consequence, the rectifying ratio (RR) determining the diode efficiency remains fixed for a chosen molecule-metal interface. Here, a mechanically tunable molecular diode exhibiting an exceptionally large rectification ratio (>105) and reversible direction is presented. The molecular system comprises a seven-armchair graphene nanoribbon (GNR) doped with a single unit of substitutional diboron within its structure, synthesized with atomic precision on a gold substrate by on-surface synthesis. The diboron unit creates half-populated in-gap bound states and splits the GNR frontier bands into two segments, localizing the bound state in a double barrier configuration. By suspending these GNRs freely between the tip of a low-temperature scanning tunneling microscope and the substrate, unipolar hole transport is demonstrated through the boron in-gap state's resonance. Strong current rectification is observed, associated with the varying widths of the two barriers, which can be tuned by altering the distance between tip and substrate. This study introduces an innovative approach for the precise manipulation of molecular electronic functionalities, opening new avenues for advanced applications in organic electronics.

Metal-free platforms for molecular thin films as high-performance supercapacitors

Controlling chemical functionalization and achieving stable electrode-molecule interfaces for high-performance electrochemical energy storage applications remain challenging tasks. Herein, we present a simple, controllable, scalable, and versatile electrochemical modification approach of graphite rods (GRs) extracted from low-cost Eveready cells that were covalently modified with anthracene oligomers. The anthracene oligomers with a total layer thickness of ∼24 nm on the GR electrode yield a remarkable specific capacitance of ∼670 F g-1 with good galvanostatic charge-discharge cycling stability (10 000) recorded in 1 M H2SO4 electrolyte. Such a boost in capacitance is attributed mainly to two contributions: (i) an electrical double-layer at the anthracene oligomer/GR/electrolyte interfaces, and (ii) the proton-coupled electron transfer (PCET) reaction, which ensures a substantial faradaic contribution to the total capacitance. Due to the higher conductivity of the anthracene films, it possesses more azo groups (-N[double bond, length as m-dash]N-) during the electrochemical growth of the oligomer films compared to pyrene and naphthalene oligomers, which is key to PCET reactions. AC-based electrical studies unravel the in-depth charge interfacial electrical behavior of anthracene-grafted electrodes. Asymmetrical solid-state supercapacitor devices were made using anthracene-modified biomass-derived porous carbon, which showed improved performance with a specific capacitance of ∼155 F g-1 at 2 A g-1 with an energy density of 5.8 W h kg-1 at a high-power density of 2010 W kg-1 and powered LED lighting for a longer period. The present work provides a promising metal-free approach in developing organic thin-film hybrid capacitors.

Phosphonic acid anchored tripodal molecular films on indium tin oxide

Whereas monopodal self-assembling monolayers (SAMs) are most frequently used for surface and interface engineering, tripodal SAMs are less popular due to the difficulty in achieving a reliable and homogeneous bonding configuration. In this context, in the present study, the potential of phosphonic acid (PA) decorated triptycene (TripPA) for formation of SAMs on oxide substrates was studied, using indium tin oxide (ITO) as a representative and application-relevant test support. A combination of several complementary experimental techniques was applied and a suitable monopodal reference system, benzylphosphonic acid (PPA), was used. The resulting data consistently show that TripPA forms well-defined, densely packed, and nearly contamination-free tripodal SAMs on ITO, with the similar parameters and properties as the monopodal reference system. Modification of wetting properties and work function of ITO by non-substituted and cyano-decorated TripPA SAMs was demonstrated, showing a potential of this tripodal system for surface engineering of oxide substrates.

Metal-organic Frameworks in Semiconductor Devices

Metal-organic frameworks (MOFs) are a specific class of hybrid, crystalline, nano-porous materials made of metal-ion-based 'nodes' and organic linkers. Most of the studies on MOFs largely focused on porosity, chemical and structural diversity, gas sorption, sensing, drug delivery, catalysis, and separation applications. In contrast, much less reports paid attention to understanding and tuning the electrical properties of MOFs. Poor electrical conductivity of MOFs (~10-7-10-10 S cm-1), reported in earlier studies, impeded their applications in electronics, optoelectronics, and renewable energy storage. To overcome this drawback, the MOF community has adopted several intriguing strategies for electronic applications. The present review focuses on creatively designed bulk MOFs and surface-anchored MOFs (SURMOFs) with different metal nodes (from transition metals to lanthanides), ligand functionalities, and doping entities, allowing tuning and enhancement of electrical conductivity. Diverse platforms for MOFs-based electronic device fabrications, conductivity measurements, and underlying charge transport mechanisms are also addressed. Overall, the review highlights the pros and cons of MOFs-based electronics (MOFtronics), followed by an analysis of the future directions of research, including optimization of the MOF compositions, heterostructures, electrical contacts, device stacking, and further relevant options which can be of interest for MOF researchers and result in improved devices performance.

Gaining insight into molecular tunnel junctions with a pocket calculator without I-V data fitting. Five-thirds protocol

The protocol put forward in the present paper is an attempt to meet the experimentalists' legitimate desire of reliably and easily extracting microscopic parameters from current-voltage measurements on molecular junctions. It applies to junctions wherein charge transport dominated by a single level (molecular orbital, MO) occurs via off-resonant tunneling. The recipe is simple. The measured current-voltage curve I = I(V) should be recast as a curve of V5/3/I versus V. This curve exhibits two maxima: one at positive bias (V = Vp+), another at negative bias (V = Vp-). The values Vp+ > 0 and Vp- < 0 at the two peaks of the curve for V5/3/I at positive and negative bias and the corresponding values Ip+ = I(Vp+) > 0 and Ip- = I(Vp-) < 0 of the current is all information needed as input. The arithmetic average of Vp+ and |Vp-| in volt provides the value in electronvolt of the MO energy offset ε0 = EMO - EF relative to the electrode Fermi level (|ε0| = e(Vp+ + |Vp-|)/2). The value of the (Stark) strength of the bias-driven MO shift is obtained as γ = (4/5)(Vp+ - |Vp-|)/(Vp+ + |Vp-|) sign (ε0). Even the low-bias conductance estimate, G = (3/8)(Ip+/Vp+ + Ip-/Vp-), can be a preferable alternative to that deduced from fitting the I-V slope in situations of noisy curves at low bias. To demonstrate the reliability and the generality of this "five-thirds" protocol, I illustrate its wide applicability for molecular tunnel junctions fabricated using metallic and nonmetallic electrodes, molecular species possessing localized σ and delocalized π electrons, and various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).

Tuning the polarity of charge carriers in N-heterocyclic carbene-based single-molecule junctions via atomic manipulation

Tuning the polarity of charge carriers at a single-molecular level is essential for designing complementary logic circuits in the field of molecular electronics. Herein, the transport properties of N-heterocyclic carbene (NHC)-linked single-molecule junctions are investigated using the ab initio quantum transport approach. The results reveal that the hydrogen atoms in NHCs function as a switch for regulating the polarity of charge carriers. Dehydrogenation changes the chemical nature of NHC anchors, thereby rendering holes as the major charge carriers rather than electrons. Essentially, dehydrogenation changes the anchoring group from electron-rich to electron-deficient. The electrons transferred to molecules from the electrodes raise the molecular level closer to the Fermi level, thus resulting in charge carrier polarity conversion. This conversion is influenced by the position and number of hydrogen atoms in the NHC anchors. To efficiently and decisively alter charge carrier polarity via atomic manipulation, a methyl substitution approach is developed and verified. These results confirm that atomic manipulation is a significant method for modulating the polarity of charge carriers in NHC-based single-molecule devices.

Extensive photochemical restructuring of molecule-metal surfaces under room light

The molecule-metal interface is of paramount importance for many devices and processes, and directly involved in photocatalysis, molecular electronics, nanophotonics, and molecular (bio-)sensing. Here the photostability of this interface is shown to be sensitive even to room light levels for specific molecules and metals. Optical spectroscopy is used to track photoinduced migration of gold atoms when functionalised with different thiolated molecules that form uniform monolayers on Au. Nucleation and growth of characteristic surface metal nanostructures is observed from the light-driven adatoms. By watching the spectral shifts of optical modes from nanoparticles used to precoat these surfaces, we identify processes involved in the photo-migration mechanism and the chemical groups that facilitate it. This photosensitivity of the molecule-metal interface highlights the significance of optically induced surface reconstruction. In some catalytic contexts this can enhance activity, especially utilising atomically dispersed gold. Conversely, in electronic device applications such reconstructions introduce problematic aging effects.

Odd-even effects in aryl-substituted alkanethiolate SAMs: nonsymmetrical attachment of aryl unit and its impact on the SAM structure

Aryl-substituted alkanethiolate (AT) self-assembled monolayers (SAMs) exhibit typically so-called odd-even effects, viz. systematic variations in the film structure, packing density, and molecular inclination depending on the parity of the number of the methylene units in the alkyl linker, n. As an exception to this rule, ATs carrying an anthracen-2-yl group (Ant-n) as tail group were reported to have different behavior due the non-symmetric attachment of the anthracene unit to the AT linker, providing additional degree of freedom for the molecular organization and allowing for partial compensation of the odd-even effects. In this context, the structure of SAMs formed by adsorption of anthracene-substituted ATs (Ant-n; n = 1-6) at room temperature on Au(111) substrate was investigated by high-resolution scanning tunnelling microscopy (STM). Most of these SAMs exhibit a coexistence of two different ordered phases, some of which are common for either n = odd or n = even while other vary over the series, showing a broad variety of different structures. The average packing density of the Ant-n SAMs, derived from the analysis of the STM data, varies by 7.5-10% depending on the parity of n, being, as expected, higher for n = odd. The respective extent of the odd-even effects is noticeably lower than that usually observed for other aryl-substituted monolayers (∼25%), which goes in line with the previous findings and emphasizes the impact of the non-symmetric attachment of the aromatic unit.

Assessing the importance of multireference correlation in predicting reversed conductance decay

In a classical electronic resistor, conductance decays as the device length increases according to Ohm's Law. While most molecular series display a comparable exponential decay in conductance with increasing molecular length, a class of single-molecule device series exists where conductance instead increases with molecular/device length, a phenomenon called reversed conductance decay. While reversals of conductance decay have been repeatedly theoretically predicted, they have been far more difficult to demonstrate experimentally. Previous studies have suggested that theoretical multi-reference(static) correlation errors may be a major cause of this discrepancy, yet most single-molecule transport methods are unable to treat multireference correlation. Using our unique multireference transport method based on non-equilibrium Green's function and multiconfigurational pair-density functional theory (NEGF-MCPDFT), we examined a previously predicted case of reversed conductance decay in systems of linear chains of phenyl rings with varying lengths and electrode designs. We compare our NEGF-MCPDFT results to those of non-multireference NEGF methods to quantify the exact role of static correlation in conductance decay reversals and clarify their relative importance to geometric and electrode design/coupling considerations.

Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopy to remotely control and probe the interfacial charge transfer dynamics with sub-10 nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

Dynamic Variation of Rectification Observed in Supramolecular Mixed Mercaptoalkanoic Acid

Functionality in molecular electronics relies on inclusion of molecular orbital energy level within a transmission window. This can be achieved by designing the active molecule with accessible energy levels or by widening the window. While many studies have adopted the first approach, the latter is challenging because defects in the active molecular component cause low breakdown voltages. Here, it is shown that control over the packing structure of monolayer via supramolecular mixing transforms an inert molecule into a highly tunable rectifier. Binary mixed monolayer composed of alkanethiolates with and without carboxylic acid head group as a proof of concept is formed via a surface-exchange reaction. The monolayer withstands high voltages up to |4.5 V| and shows a dynamic rectification-external bias relationship in magnitude and polarity. Sub-highest occupied molecular orbital (HOMO) levels activated by the widened transmission window account for these observations. This work demonstrates that simple supramolecular mixing can imbue new electrical properties in electro-inactive organic molecules.

Thickness-Dependent Charge Transport in Three Dimensional Ru(II)- Tris(phenanthroline)-Based Molecular Assemblies

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)-tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4-16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current-voltage (j-V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm-1). Such a lower β value is attributed to the accessibility of Ru(Phen)3 molecular conduction channels to Fermi levels of both the electrodes and a strong electronic coupling at ITO-molecules interfaces. The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10-16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole-Frenkel charge conduction, showing a clear "molecular signature" in the nanometric junctions.

Coexistent, Competing Tunnelling, and Hopping Charge Transport in Compressed Metal Nanocluster Crystals

Understanding charge transport among metal particles with sizes of approximately 1 nm poses a great challenge due to the ultrasmall nanosize, yet it holds great significance in the development of innovative materials as substitutes for traditional semiconductors, which are insulative and unstable in less than ∼10 nm thickness. Herein, atomically precise gold nanoclusters with well-defined compositions and structures were investigated to establish a mathematical relation between conductivity and interparticle distance. This was accomplished using high-pressure in situ resistance characterizations, synchrotron X-ray diffraction (XRD), and the Murnaghan equation of state. Based on this precise correlation, it was predicted that the conductivity of Au25(SNap)18 (SNap: 1-naphthalenethiolate) solid is comparable to that of bulk silver when the interparticle distance is reduced to approximately 3.6 Å. Furthermore, the study revealed the coexisting, competing tunneling, and incoherent hopping charge transport mechanisms, which differed from those previously reported. The introduction of conjugation-structured ligands, tuning of the structures of metal nanoclusters, and use of high-pressure techniques contributed to enhanced conductivity, and thus, the charge carrier types were determined using Hall measurements. Overall, this study provides valuable insight into the charge transport in gold nanocluster solids and represents an important advancement in metal nanocluster semiconductor research.

Coexistence of Electrochromism and Bipolar Nonvolatile Memory in a Single Viologen

Viologens are fascinating redox-active organic compounds that have been widely explored in electrochromic devices (ECDs). However, the combination of electrochromic and resistive random-access memory in a single viologen remains unexplored. We report the coexistence of bistate electrochromic and single-resistor (1R) memory functions in a novel viologen. A high-performance electrochromic function is achieved by combining viologen (BzV2+2PF6) with polythiophene (P3HT), enabling a "push-pull" electronic effect due to the efficient intermolecular charge transfer in response to an applied bias. The ECDs show high coloration efficiency (ca. 1150 ± 10 cm2 C-1), subsecond switching time, good cycle stability (>103 switching cycles), and low-bias operation (±1.5 V). The ECDs require low power for switching the color states (55 μW cm-2 for magenta and 141 μW cm-2 for blue color). The random-access memory devices (p+2-Si/BzV2+2PF6/Al) exhibit distinct low and high resistive states with an ON/OFF ratio of ∼103, bipolar and nonvolatile characteristics that manifest good performances, and "Write"-"Read"-"Erase" (WRE) functions. The charge conduction mechanism of the RRAM device is elucidated by the Poole-Frenkel model where SET and RESET states arise at a low transition voltage (VT = ±1.7 V). Device statistics and performance parameters for both electrochromic and memory devices are compared with the literature data. Our findings on electrochromism and nonvolatile memory originated in the same viologen could boost the development of multifunctional, smart, wearable, flexible, and low-cost optoelectronic devices.

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films

The injection of pure spin current into the non-magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (≈20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80 Fe20 ) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin-mixing conductance of ≈1.02×1018  m-2 , a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy-pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.

Fine-Tuning the Molecular Design for High-Performance Molecular Diodes Based on Pyridyl Isomers

Better control of molecule-electrode coupling (Γ) to minimize leakage current is an effective method to optimize the functionality of molecular diodes. Herein we embedded 5 isomers of phenypyridyl derivatives, each with an N atom placed at a different position, in two electrodes to fine-tune Γ between self-assembled monolayers (SAMs) and the top electrode of EGaIn (eutectic Ga-In terminating in Ga2 O3 ). Combined with electrical tunnelling results, characterizations of electronic structures, single-level model fittings, and DFT calculations, we found that the values of Γ of SAMs formed by these isomers could be regulated by nearly 10 times, thereby contributing to the leakage current changing over about two orders of magnitude and switching the isomers from resistors to diodes with a rectification ratio (r+ =|J(+1.5 V)/J(-1.5 V)|) exceeding 200. We demonstrated that the N atom placement can be chemically engineered to tune the resistive and rectifying properties of the molecular junctions, making it possible to convert molecular resistors into rectifiers. Our study provides fundamental insights into the role of isomerism in molecular electronics and offers a new avenue for designing functional molecular devices.

Tuning the conductance of carbon rings with impurities and electric fields

We explore two mechanisms to tune the electronic conductance of carbon atom rings, namely, substitutional impurities and in-plane external electric fields. First-principles calculations and a tight-binding approach are used to model the systems. Two bond configurations are studied, cumulenic and polyynic, which can be relevant depending on the number of carbon atoms in the ring. We find that both impurity substitution and electric field mechanisms allow for modifying the electronic spectrum and transport characteristics. Interestingly, cumulenic and polyynic carbon rings present a different response to these perturbations, which can also be a way to elucidate the bond nature of these structures.

Fabrication and characterization of a flexible and disposable impedance-type humidity sensor based on polyaniline (PAni)

This work presents a highly sensitive, economical, flexible, and disposable humidity sensor developed with a facile fabrication process. The sensor was fabricated on cellulose paper using polyemaraldine salt, a form of polyaniline (PAni), via the drop coating method. A three-electrode configuration was employed to ensure high accuracy and precision. The PAni film was characterized using various techniques including ultraviolet-visible (UV-vis) absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The humidity sensing properties were evaluated through electrochemical impedance spectroscopy (EIS) in a controlled environment. The sensor exhibits a linear response with R 2 = 0.990 for impedance over a wide range of (0%-97%) relative humidity (RH). Further, it displayed consistent responsiveness, a sensitivity of 1.1701 Ω/%RH, acceptable response (≤220 s)/recovery (≤150 s), excellent repeatability, low hysteresis (≤2.1%) and long-term stability at room temperature. The temperature dependence of the sensing material was also studied. Due to its unique features, cellulose paper was found to be an effective alternative to conventional sensor substrates according to several factors including compatibility with the PAni layer, flexibility and low cost. These unique characteristics make this sensor a promising option for use in specific healthcare monitoring, research activities, and industrial settings as a flexible and disposable humidity measurement tool.