Cross-plane transport in a single-molecule two-dimensional van der Waals heterojunction

Photoconductivity Switching in Semiconducting Two-Dimensional Crystals via Molecular Tetris

Two-dimensional organic materials are mainly constructed by using orthogonal anisotropic connectivity of covalent bonding and π-π stacking. The noncovalent connectivity between building blocks is presumed to be too delicate to stabilize the two-dimensional (2D) layers. Contrary to this assumption, we constructed graphite-like 2D layered material by utilizing pure noncovalent connectivity, i.e., weak intermolecular and π-π interaction via a molecular Tetris strategy. We produce X-ray mountable single crystals comprising polycyclic aromatic heterocycles by employing a single-crystal-to-dissolution-to-single-crystal transformation methodology. The macromechanical analysis of this layered crystal shows shearing behavior, which is quantified using nanoindentation experiments. The 2D lattice's layer space allows reversible intercalation-deintercalation of iodine, which enhances the photoconductivity by 17 folds. Combined efforts of X-ray diffraction, solid-state spectroscopy, and electrochemical studies established the mechanism of intercalation and resulting photoconductivity enhancement.

Design of Self-Assembled Monolayer in Tungsten Diselenide Bilayer for Exciton Dissociation

Transition metal dichalcogenides (TMDs) have emerged as promising candidates for next-generation self-powered photodetectors due to their distinct optoelectronic properties, including strong light-matter interactions. However, their high exciton binding energies impede efficient exciton dissociation, hindering viable photodetector applications. This study, based on first-principles calculations, introduces a design approach featured by the asymmetrically enclosed structure of the TMD bilayer, i.e., two different self-assembled monolayers (SAMs) asymmetrically attached to each side of a tungsten diselenide bilayer by varying electron-donating and electron-withdrawing groups in SAMs. Compared to the electron-donating and electron-withdrawing tendencies, we demonstrate that the surface work function of the SAM is a crucial macroscopic parameter in fine-tuning the band offset without trap formation with a large degree of freedom. Optimizing the work function achieves trap-free exciton dissociation, establishing a type-II band alignment and a sufficient built-in electric field within the bilayer. This design approach offers not only a design strategy for two-dimensional (2D) self-powered photodetectors but also a guide to interface engineering of TMDs utilizing SAMs for integration into low-power 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.

Self-assembled hole-selective contact for efficient Sn-Pb perovskite solar cells and all-perovskite tandems

Self-assembled monolayers (SAMs) have displayed unpredictable potential in efficient perovskite solar cells (PSCs). Yet most of SAMs are largely suitable for pure Pb-based devices, precisely developing promising hole-selective contacts (HSCs) for Sn-based PSCs and exploring the underlying general mechanism are fundamentally desired. Here, based on the prototypical donor-acceptor SAM MPA-BT-BA (BT), oligoether side chains with different length (i.e., methoxy, 2-methoxyethoxy, 2-(2-methoxyethoxy)ethoxy group) were custom-introduced on the benzothiadiazole unit to produce the target SAMs with acronyms MPA-MBT-BA (MBT), MPA-EBT-BA (EBT), and MPA-MEBT-BA (MEBT), respectively, and acting as HSCs for efficient Sn-Pb PSCs and all-perovskite tandems. The introduction of oligoether side chains enables HSCs effectively accelerate hole extraction, regulate the crystal growth and passivate surface defects of Sn-Pb perovskites. In particular, benefiting from the enhanced Sn-Pb perovskite film quality and the suppressed interfacial non-radiative recombination losses, EBT-tailored LBG devices yield a champion efficiency of 23.54%, enabling 28.61% efficient monolithic all-perovskite tandems with an impressive VOC of 2.155 V and excellent operational stability as well as 28.22%-efficiency 4-T tandems.

From Molecular Electronics to Molecular Intelligence

Molecular electronics is a field that explores the ultimate limits of electronic device dimensions by using individual molecules as operable electronic devices. Over the past five decades since the proposal of a molecular rectifier by Aviram and Ratner in 1974 ( Chem. Phys. Lett.1974,29, 277-283), researchers have developed various fabrication and characterization techniques to explore the electrical properties of molecules. With the push of electrical characterizations and data analysis methodologies, the reproducibility issues of the single-molecule conductance measurement have been chiefly resolved, and the origins of conductance variation among different devices have been investigated. Numerous prototypical molecular electronic devices with external physical and chemical stimuli have been demonstrated based on the advances of instrumental and methodological developments. These devices enable functions such as switching, logic computing, and synaptic-like computing. However, as the goal of molecular electronics, how can molecular-based intelligence be achieved through single-molecule electronic devices? At the fiftieth anniversary of molecular electronics, we try to answer this question by summarizing recent progress and providing an outlook on single-molecule electronics. First, we review the fabrication methodologies for molecular junctions, which provide the foundation of molecular electronics. Second, the preliminary efforts of molecular logic devices toward integration circuits are discussed for future potential intelligent applications. Third, some molecular devices with sensing applications through physical and chemical stimuli are introduced, demonstrating phenomena at a single-molecule scale beyond conventional macroscopic devices. From this perspective, we summarize the current challenges and outlook prospects by describing the concepts of "AI for single-molecule electronics" and "single-molecule electronics for AI".

Single-Molecule Cross-Plane Conductance of Polycyclic Aromatic Hydrocarbon Derivatives

In the cross-plane single-molecule junctions, the correlation between molecular aromaticity and conductance remained puzzling. Cross-plane break junction (XPBJ) provides new insight into understanding the role of aromaticity and conjugation to molecules on charge transport through the planar molecules. In this work, we investigated the modulation of cross-plane charge transport in pyrene derivatives by hydrogenation and substituents based on the XPBJ method that differs from those used in-plane transport. We measured the electrical conductance of the hydrogenated derivatives of the pyrenes and found that hydrogenation reduces conductance, and the fully hydrogenated molecule has the lowest conductance. Conductance of pyrene derivatives increased after substitution by both electron-donating and electron-withdrawing groups. By calculating, the trend in decreased conductance of hydrogenated pyrene was found to be consistent with the change in aromaticity. Electron-withdrawing substituents reduce the aromaticity of the molecule and narrow the HOMO-LUMO gap, while electron-donating groups increase the aromaticity but also narrow the gap. Our work reveals the potential of fine-tuning the structure of the pyrene molecule to control the cross-plane charge transport through the single-molecule junctions.

Conductance Quantization in 2D Semi-Metallic Transition Metal Dichalcogenides

Conductance quantization of 2D materials is significant for understanding the charge transport at the atomic scale, which provides a platform to manipulate the quantum states, showing promising applications for nanoelectronics and memristors. However, the conventional methods for investigating conductance quantization are only applicable to materials consisting of one element, such as metal and graphene. The experimental observation of conductance quantization in transition metal dichalcogenides (TMDCs) with complex compositions and structures remains a challenge. To address this issue, an approach is proposed to characterize the charge transport across a single atom in TMDCs by integrating in situ synthesized 1T'-WTe2 electrodes with scanning tunneling microscope break junction (STM-BJ) technique. The quantized conductance of 1T'-WTe2 is measured for the first time, and the quantum states can be modulated by stretching speed and solvent. Combined with theoretical calculations, the evolution of quantized and corresponding configurations during the break junction process is demonstrated. This work provides a facile and reliable avenue to characterize and modulate conductance quantization of 2D materials, intensively expanding the research scope of quantum effects in diverse materials.

Recent Progress in Dopant-Free and Green Solvent-Processable Organic Hole Transport Materials for Efficient and Stable Perovskite Solar Cells

Dopant-free hole transport layers (HTLs) are crucial in enhancing perovskite solar cells (pero-SCs). Nevertheless, conventional processing of these HTL materials involves using toxic solvents, which gives rise to substantial environmental concerns and renders them unsuitable for large-scale industrial production. Consequently, there is a pressing need to develop dopant-free HTL materials processed using green solvents to facilitate the production of high-performance pero-SCs. Recently, several strategies have been developed to simultaneously improve the solubility of these materials and regulate molecular stacking for high hole mobility. In this review, a comprehensive overview of the methodologies utilized in developing dopant-free HTL materials processed from green solvents is provided. First, the study provides a brief overview of fundamental information about green solvents and Hansen solubility parameters, which can serve as a guideline for the molecular design of optimal HTL materials. Second, the intrinsic relationships between molecular structure, solubility in green solvents, molecular stacking, and device performance are discussed. Finally, conclusions and perspectives are presented along with the rational design of highly efficient, stable, and green solvent-processable dopant-free HTL materials.

Evolution of Single-Molecule Electronic Interfaces

Single-molecule electronics can fabricate single-molecule devices via the construction of molecule-electrode interfaces and also provide a unique tool to investigate single-molecule scale physicochemical processes at these interfaces. To investigate single-molecule electronic devices with desired functionalities, an understanding of the interface evolution processes in single-molecule devices is essential. In this review, we focus on the evolution of molecule-electrode interface properties, including the background of interface evolution in single-molecule electronics, the construction of different types of single-molecule interfaces, and the regulation methods. Finally, we discuss the perspective of future characterization techniques and applications for single-molecule electronic interfaces.

Atomically well-defined nitrogen doping for cross-plane transport through graphene heterojunctions

The nitrogen doping of graphene leads to graphene heterojunctions with a tunable bandgap, suitable for electronic, electrochemical, and sensing applications. However, the microscopic nature and charge transport properties of atomic-level nitrogen-doped graphene are still unknown, mainly due to the multiple doping sites with topological diversities. In this work, we fabricated atomically well-defined N-doped graphene heterojunctions and investigated the cross-plane transport through these heterojunctions to reveal the effects of doping on their electronic properties. We found that a different doping number of nitrogen atoms leads to a conductance difference of up to ∼288%, and the conductance of graphene heterojunctions with nitrogen-doping at different positions in the conjugated framework can also lead to a conductance difference of ∼170%. Combined ultraviolet photoelectron spectroscopy measurements and theoretical calculations reveal that the insertion of nitrogen atoms into the conjugation framework significantly stabilizes the frontier molecular orbitals, leading to a change in the relative positions of the HOMO and LUMO to the Fermi level of the electrodes. Our work provides a unique insight into the role of nitrogen doping in the charge transport through graphene heterojunctions and materials at the single atomic level.

Advances in single-molecule junctions as tools for chemical and biochemical analysis

The development of miniaturized electronics has led to the design and construction of powerful experimental platforms capable of measuring electronic properties to the level of single molecules, along with new theoretical concepts to aid in the interpretation of the data. A new area of activity is now emerging concerned with repurposing the tools of molecular electronics for applications in chemical and biological analysis. Single-molecule junction techniques, such as the scanning tunnelling microscope break junction and related single-molecule circuit approaches have a remarkable capacity to transduce chemical information from individual molecules, sampled in real time, to electrical signals. In this Review, we discuss single-molecule junction approaches as emerging analytical tools for the chemical and biological sciences. We demonstrate how these analytical techniques are being extended to systems capable of probing chemical reaction mechanisms. We also examine how molecular junctions enable the detection of RNA, DNA, and traces of proteins in solution with limits of detection at the zeptomole level.

Planar aromatic anchors control the electrical conductance of gold|molecule|graphene junctions

The synthesis of a family of alkanethiol molecules with planar aromatic head groups, designed to anchor molecules effectively to graphene electrodes, is reported. Characterisation of self-assembled monolayers of these molecules on a gold surface via conductive atomic force microscopy shows that when an aromatic head group is present, the conductance G graphene obtained using a graphene coated probe is higher than the conductance G Pt obtained using a platinum (Pt) probe. For Pt probe and graphene probe junctions, the tunnelling decay constant of benzyl ether derivatives with an alkanethiol molecular backbone is determined as β = 5.6 nm-1 and 3.5 nm-1, respectively. The conductance ratio G graphene/G Pt increases as the number of rings present in the aromatic head unit, n, increases. However, as the number of rings increases, the conductance path length increases because the planar head groups lie at an angle to the plane of the electrodes. This means that overall conductance decreases as n increases. Density functional theory-based charge transport calculations support these experimental findings. This study confirms that planar aromatic head groups can function as effective anchoring units for graphene electrodes in large area molecular junctions. However, the results also indicate that the size and geometry of these head groups must be considered in order to produce effective molecular designs.

Characterization and Application of Supramolecular Junctions

The convergence of supramolecular chemistry and single-molecule electronics offers a new perspective on supramolecular electronics, and provides a new avenue toward understanding and application of intermolecular charge transport at the molecular level. In this review, we will provide an overview of the advances in the characterization technique for the investigation of intermolecular charge transport, and summarize the experimental investigation of several non-covalent interactions, including π-π stacking interactions, hydrogen bonding, host-guest interactions and σ-σ interactions at the single-molecule level. We will also provide a perspective on supramolecular electronics and discuss the potential applications and future challenges.

A van der Waals heterojunction strategy to fabricate layer-by-layer single-molecule switch

Single-molecule electronics offer a unique strategy for the miniaturization of electronic devices. However, the existing experiments are limited to the conventional molecular junctions, where a molecule anchors to the electrode pair with linkers. With such a rod-like configuration, the minimum size of the device is defined by the length of the molecule. Here, by incorporating a single molecule with two single-layer graphene electrodes, we fabricated layer-by-layer single-molecule heterojunctions called single-molecule two-dimensional van der Waals heterojunctions (M-2D-vdWHs), of which the sizes are defined by the thickness of the molecule. We controlled the conformation of the M-2D-vdWHs and the cross-plane charge transport through them with the applied electric field and established that they can serve as reversible switches. Our results demonstrate that the M-2D-vdWHs, as stacked from single-layer 2D materials and a single molecule, can respond to electric field stimulus, which promises a diverse class of single-molecule devices with unprecedented size.

Advances of Various Heterogeneous Structure Types in Molecular Junction Systems and Their Charge Transport Properties

Molecular electronics that can produce functional electronic circuits using a single molecule or molecular ensemble remains an attractive research field because it not only represents an essential step toward realizing ultimate electronic device scaling but may also expand our understanding of the intrinsic quantum transports at the molecular level. Recently, in order to overcome the difficulties inherent in the conventional approach to studying molecular electronics and developing functional device applications, this field has attempted to diversify the electrical characteristics and device architectures using various types of heterogeneous structures in molecular junctions. This review summarizes recent efforts devoted to functional devices with molecular heterostructures. Diverse molecules and materials can be combined and incorporated in such two- and three-terminal heterojunction structures, to achieve desirable electronic functionalities. The heterojunction structures, charge transport mechanisms, and possible strategies for implementing electronic functions using various hetero unit materials are presented sequentially. In addition, the applicability and merits of molecular heterojunction structures, as well as the anticipated challenges associated with their implementation in device applications are discussed and summarized. This review will contribute to a deeper understanding of charge transport through molecular heterojunction, and it may pave the way toward desirable electronic functionalities in molecular electronics applications.

Quantum Interference-Controlled Conductance Enhancement in Stacked Graphene-like Dimers

Stacking interactions are of significant importance in the fields of chemistry, biology, and material optoelectronics because they determine the efficiency of charge transfer between molecules and their quantum states. Previous studies have proven that when two monomers are π-stacked in series to form a dimer, the electrical conductance of the dimer is significantly lower than that of the monomer. Here, we present a strong opposite case that when two anthanthrene monomers are π-stacked to form a dimer in a scanning tunneling microscopic break junction, the conductance increases by as much as 25 in comparison with a monomer, which originates from a room-temperature quantum interference. Remarkably, both theory and experiment consistently reveal that this effect can be reversed by changing the connectivity of external electrodes to the monomer core. These results demonstrate that synthetic control of connectivity to molecular cores can be combined with stacking interactions between their π systems to modify and optimize charge transfer between molecules, opening up a wide variety of potential applications ranging from organic optoelectronics and photovoltaics to nanoelectronics and single-molecule electronics.

Charge transport through single-molecule bilayer-graphene junctions with atomic thickness

The van der Waals interactions (vdW) between π-conjugated molecules offer new opportunities for fabricating heterojunction-based devices and investigating charge transport in heterojunctions with atomic thickness. In this work, we fabricate sandwiched single-molecule bilayer-graphene junctions via vdW interactions and characterize their electrical transport properties by employing the cross-plane break junction (XPBJ) technique. The experimental results show that the cross-plane charge transport through single-molecule junctions is determined by the size and layer number of molecular graphene in these junctions. Density functional theory (DFT) calculations reveal that the charge transport through molecular graphene in these molecular junctions is sensitive to the angles between the graphene flake and peripheral mesityl groups, and those rotated groups can be used to tune the electrical conductance. This study provides new insight into cross-plane charge transport in atomically thin junctions and highlights the role of through-space interactions in vdW heterojunctions at the molecular scale.

Charge transport through single-molecule bilayer-graphene junctions fabricated by a cross-plane break junction technique can be tuned at the atomic level.

Development of nano- and microdevices for the next generation of biotechnology, wearables and miniaturized instrumentation

This is a short communication based on recent high-impact publications related to how various chemical materials and substrate modifications could be tuned for nano- and microdevices, where their application for high point-of-care bioanalysis and further applications in life science is discussed. Hence, they have allowed different high-impact research topics in a variety of fields, from the control of nanoscale to functional microarchitectures embedded in various support materials to obtain a device for a given application or use. Thus, their incorporation in standard instrumentation is shown, as well as in new optical setups to record different classical and non-classical light, signaling, and energy modes at a variety of wavelengths and energy levels. Moreover, the development of miniaturized instrumentation was also contemplated. In order to develop these different levels of technology, the chemistry, physics and engineering of materials were discussed. In this manner, a number of subjects that allowed the design and manufacture of devices could be found. The following could be mentioned by way of example: (i) nanophotonics; (ii) design, synthesis and tuning of advanced nanomaterials; (iii) classical and non-classical light generation within the near field; (iv) microfluidics and nanofluidics; (v) signal waveguiding; (vi) quantum-, nano- and microcircuits; (vii) materials for nano- and microplatforms, and support substrates and their respective modifications for targeted functionalities. Moreover, nano-optics in in-flow devices and chips for biosensing were discussed, and perspectives on biosensing and single molecule detection (SMD) applications. In this perspective, new insights about precision nanomedicine based on genomics and drug delivery systems were obtained, incorporating new advanced diagnosis methods based on lab-on-particles, labs-on-a-chip, gene therapies, implantable devices, portable miniaturized instrumentation, single molecule detection for biophotonics, and neurophotonics. In this manner, this communication intends to highlight recent reports and developments of nano- and microdevices and further approaches towards the incorporation of developments in nanophotonics and biophotonics in the design of new materials based on different strategies and enhanced techniques and methods. Recent proofs of concept are discussed that could allow new substrates for device manufacturing. Thus, physical phenomena and materials chemistry with accurate control within the nanoscale were introduced into the discussion. In this manner, new potential sources of ideas and strategies for the next generation of technology in many research and development fields are showcased.

Tracking Confined Reaction Based on Host-Guest Interaction Using Single-Molecule Conductance Measurement

The host-guest interaction acts as an essential part of supramolecular chemistry, which can be applied in confined reaction. However, it is challenging to obtain the dynamic process during confined reactions below micromolar concentrations. In this work, a new method is provided to characterize the dimerization process of the guest 1,2-bis(4-pyridinyl) ethylene in host cucurbit[8]curil using scanning tunneling microscope-break junction (STM-BJ) technique. The guest reaction kinetics is quantitatively by nuclear magnetic resonance (NMR) and in situ single-molecule junctions. It is found that in the single-molecule conductance measurements, the electrical signals of the reactants with a concentration as low as 5 × 10^-6  m are clearly detected, and the reaction kinetics at micromolar concentrations are further obtained. However, in NMR measurements, the characteristic peak signal of the reactants is undetectable when the concentration of the reactants is lower than 0.5 × 10^-3  m and it cannot be quantified. In addition, the strong electric field from the nanogap accelerates the reaction. This work reveals that single-molecule STM-BJ techniques are more sensitive for tracking confined reactions than that by NMR techniques and can be used to study effect of extremely strong electric field on kinetics.

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes

Here, six phenanthrene (the smallest arm-chair graphene nanoribbon) derivatives with dithiomethyl substitutions at different positions as the anchoring groups were synthesized. Scanning tunneling microscopy break junction technique was used to measure their single molecule conductances between gold electrodes, which showed a difference as much as 20-fold in the range of ∼10^-2.82 G₀ to ∼10^-4.09 G₀ following the trend of G_2,7 > G_3,6 > G_2,6 > G_1,7 > G_1,6 > G_1,8. DFT calculations agree well with this measured trend and indicate that the single molecule conductances are a combination of energy alignment, electronic coupling, and quantum effects. This significant regio- and steric effect on the single molecule conductance of phenanthrene model molecules shows the complexity in the practice of graphene nanoribbons as building blocks for future carbon-based electronics in one hand but also provides good conductance tunability on the other hand.

Robust Giant Magnetoresistance in 2D Van der Waals Molecular Magnetic Tunnel Junctions

The spin transport across a zero-dimensional (0D) single-molecule sandwiched by two-dimensional (2D) van der Waals (vdW) ferromagnetic electrodes may open vast opportunities to create novel mixed-dimensional spintronics devices. However, this remains unexplored yet. Inspired by the recent discovery of 2D intrinsic ferromagnets Fe₃GeTe₂, using first-principles spin transport calculations, we show that single-molecule junctions based on Fe₃GeTe₂ can yield perfect spin filtering and a significant magnetoresistance (MR) of up to ∼6075%. This remarkable MR is more than 2 orders of magnitude higher than the MR obtained for the corresponding junctions with conventional ferromagnetic metals (e.g., Ni, Fe, and Co). We demonstrate the results of two representative examples that are feasible in the experiments: (i) A benzene or (ii) bezenedithiol (BDT) connected either through a scanning tunneling microscope or break-junction setups. We find that the conductance of BDT junctions is more than 10 times larger than that of the benzene junction due to a much stronger hybridization effect at the molecule-metal interfaces. The key mechanism of the perfect spin filtering and large MR in single-molecule junctions is mainly determined by the intrinsic properties of Fe₃GeTe₂ electrodes, while the actual conductance is determined by the hybridization strength of the majority spin channel at the molecule-metal interfaces. It is also predicted that the perfect spin filtering and the remarkably huge MR are highly insensitive to structural variations, interface defects, and stacking orders of the electrodes. Our results provide important insights for expanding molecular spintronics platforms from conventional ferromagnetic metals to new 2D vdw magnets.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.

Application of Micro/Nanofabrication Techniques to On-Chip Molecular Electronics

Molecular electronics is a promising subject to overcome the size limitation of silicon-based electronic devices. In the past decades, various micro/nanofabrication techniques have been developed for constructing molecular junctions, and a number of breakthroughs are made in the characterizations and applications of the single-molecule device. The history and progress are reviewed in this article, laying emphasis on the recent works on the combination of micro/nanofabrication techniques with other techniques such as electrochemical deposition and surface-enhanced Raman spectroscopy (SERS). Some prototypical single-molecule devices such as molecular transistors are presented. Finally, the challenges and prospects in the fabrication of single-molecule devices are discussed.

Preparation and Application of Microelectrodes at the Single-Molecule Scale

Molecular electronics offers a potential solution for the miniaturization of electronics beyond conventional silicon electronics. A key goal of molecular electronics is to fabricate the single-molecule junction with the functions of electronic units. The term "molecular junction" means a molecular cluster or a single molecule incorporated between two microelectrodes, and electrons are transported across it. The methods of constructing molecular junctions dynamically were developed, such as STM-BJ, AFM-BJ, and MCBJ, providing precise control of the gap and easy measurement of thousands of junctions. Electrodes based on these techniques are commonly called microelectrodes because at least one dimension is on the micron scale. In this manuscript, we summarize the preparation methods of microelectrodes and their application in single-molecule measurements. In addition, we discuss the electrode factor that influences the molecular electrical properties, such as material, curvature radius and cone angle, and further provide a brief prospect of molecular electronics.

Analytic expressions for the steady-state current with finite extended reservoirs

Open-system simulations of quantum transport provide a platform for the study of true steady states, Floquet states, and the role of temperature, time dynamics, and fluctuations, among other physical processes. They are rapidly gaining traction, especially techniques that revolve around "extended reservoirs," a collection of a finite number of degrees of freedom with relaxation that maintains a bias or temperature gradient, and have appeared under various guises (e.g., the extended or mesoscopic reservoir, auxiliary master equation, and driven Liouville-von Neumann approaches). Yet, there are still a number of open questions regarding the behavior and convergence of these techniques. Here, we derive general analytical solutions, and associated asymptotic analyses, for the steady-state current driven by finite reservoirs with proportional coupling to the system/junction. In doing so, we present a simplified and unified derivation of the non-interacting and many-body steady-state currents through arbitrary junctions, including outside of proportional coupling. We conjecture that the analytic solution for proportional coupling is the most general of its form for isomodal relaxation (i.e., relaxing proportional coupling will remove the ability to find compact, general analytical expressions for finite reservoirs). These results should be of broad utility in diagnosing the behavior and implementation of extended reservoir and related approaches, including the convergence to the Landauer limit (for non-interacting systems) and the Meir-Wingreen formula (for many-body systems).