Interfacial Stereoelectronic Effect Induced by Anchoring Orientation.

Heterogeneous interfaces in most devices play a key role in the material performance. Exploring the atomic structure and electronic properties of metal-molecule interfaces is critical for various potential applications, such as surface sensing, molecular recognition, and molecular electronic devices. This study unveils a ubiquitous interfacial stereoelectronic effect in conjugated molecular junctions by combining first-principles simulation and scanning tunneling microscopy break junction technology. Single-molecule junctions with same-side interfacial anchoring (cis configuration) exhibit higher conductance than those with opposite-side interfacial anchoring (trans configuration). The cis and trans configurations can undergo reversible conversions, resulting in a conductance switching. The stability of these configurations can be adjusted by an electric field, achieving precise regulation of conductance states. Our findings provide important insights for designing high-quality materials and enhancing the device performance.

In Situ Detection of Interfacial Ions at the Single-Bond Level.

Detecting the ionic state at the solid-liquid interface is essential to reveal the various chemical and physical processes that occur at the interface. In this study, the adsorption states of the highly electronegative ions F- and OH- at the solid-liquid interface are detected by using the scanning tunneling microscopy break junction technique. With the active hydrogen atom of the amino group as a probe, the formed ionic hydrogen bonds are successfully detected, thereby enabling in situ monitoring of the ionic state at the solid-liquid interface. Through noise power spectral density analysis and theoretical simulations, we reveal the mechanism by which ionic hydrogen bonds at the interface affect the charge transport properties. In addition, we discover that the ionic state at the solid-liquid interface can be effectively manipulated by electric fields. Under high electric fields, the concentration of the anion near the electrode is higher, and the proportion of hydrogen bonds formed is greater than that under low electric fields. This study of the interfacial ionic state at the single-bond level provides guidance for the design of high-performance materials for energy conversion and environmental purification.

Tracking Noncovalent Interactions of π, π-Hole, and Ion in Molecular Complexes at the Single-Molecule Level.

Noncovalent interactions involving aromatic rings, such as π-stacking and π-ion interactions, play an essential role in molecular recognition, assembly, catalysis, and electronics. However, the inherently weak and complex nature of these interactions has made it challenging to study them experimentally, especially with regard to elucidating their properties in solution. Herein, the noncovalent interactions between π and π-hole, π and cation, and π-hole and anion in molecular complexes in nonpolar solution are investigated in situ through single-molecule electrical measurements in combination with theoretical calculations. Specifically, phenyl and pentafluorobenzyl groups serve as π and π-hole sites, respectively, while Li+ and Cl- are employed as the cation and anion. Our findings reveal that, in comparison with homogeneous π···π interactions, heterogeneous π···π-hole and π···cation interactions exhibit greater binding energies, resulting in a longer binding lifetime of the molecular junctions. Meanwhile, π···Li+ and π-hole···Cl- interactions present significantly distinct binding characteristics, with the former being stronger but more flexible than the latter. Furthermore, by changing the molecular components, similar conductivity can be achieved in both molecular dimers or sandwich complexes. These results provide new insights into π- and π-hole-involved noncovalent interactions, offering novel strategies for precise manipulation of molecular assembly, recognition, and molecular device.

Electronic Devices Based on Heterostructures of 2D Materials and Self-Assembled Monolayers.

2D materials (2DMs), known for their atomically ultrathin structure, exhibit remarkable electrical and optical properties. Similarly, molecular self-assembled monolayers (SAMs) with comparable atomic thickness show an abundance of designable structures and properties. The strategy of constructing electronic devices through unique heterostructures formed by van der Waals assembly between 2DMs and molecular SAMs not only enables device miniaturization, but also allows for convenient adjustment of their structures and functions. In this review, the fundamental structures and fabrication methods of three different types of electronic devices dominated by 2DM-SAM heterojunctions with varying architectures are timely elaborated. Based on these heterojunctions, their fundamental functionalities and characteristics, as well as the regulation of their performance by external stimuli, are further discussed.

Unveiling the Properties of Sulfhydryl Groups in a Single-Molecule Junction.

The metal-thiol interface is ubiquitous in nanotechnology and surface chemistry. It is not only used to construct nanocomposites but also plays a decisive role in the properties of these materials. When organothiol molecules bind to the gold surface, there is still controversy over whether sulfhydryl groups can form disulfide bonds and whether these disulfide bonds can remain stable on the gold surface. Here, we investigate the intrinsic properties of sulfhydryl groups on the gold surface at the single-molecule level using a scanning tunneling microscope break junction technique. Our findings indicate that sulfhydryl groups can react with each other to form disulfide bonds on the gold surface, and the electric field can promote the sulfhydryl coupling reaction. In addition to these findings, ultraviolet irradiation is used to effectively regulate the coupling between sulfhydryl groups, leading to the formation and cleavage of disulfide bonds. These results unveil the intrinsic properties of sulfhydryl groups on the gold surface, therefore facilitating the accurate construction of broad nanocomposites with the desired functionalities.

Solution-processable 2D semiconductors for high-performance large-area electronics.

Two-dimensional (2D) materials, consisting of atomically thin crystal layers bound by the van der Waals force, have attracted much interest because of their potential in diverse technologies, including electronics, optoelectronics and catalysis1-10. In particular, solution-processable 2D semiconductor (such as MoS2) nanosheets are attractive building blocks for large-area thin-film electronics. In contrast to conventional zero- and one-dimensional nanostructures (quantum dots and nanowires, respectively), which are typically plagued by surface dangling bonds and associated trapping states, 2D nanosheets have dangling-bond-free surfaces. Thin films created by stacking multiple nanosheets have atomically clean van der Waals interfaces and thus promise excellent charge transport11-15. However, preparing high-quality solution-processable 2D semiconductor nanosheets remains a challenge. For example, MoS2 nanosheets and thin films produced using lithium intercalation and exfoliation are plagued by the presence of the metallic 1T phase and poor electrical performance (mobilities of about 0.3 square centimetres per volt per second and on/off ratios of less than 10)2,12, and materials produced by liquid exfoliation exhibit an intrinsically broad thickness distribution, which leads to poor film quality and unsatisfactory thin-film electrical performance (mobilities of about 0.4 square centimetres per volt per second and on/off ratios of about 100)14,16,17. Here we report a general approach to preparing highly uniform, solution-processable, phase-pure semiconducting nanosheets, which involves the electrochemical intercalation of quaternary ammonium molecules (such as tetraheptylammonium bromide) into 2D crystals, followed by a mild sonication and exfoliation process. By precisely controlling the intercalation chemistry, we obtained phase-pure, semiconducting 2H-MoS2 nanosheets with a narrow thickness distribution. These nanosheets were then further processed into high-performance thin-film transistors, with room-temperature mobilities of about 10 square centimetres per volt per second and on/off ratios of 106 that greatly exceed those obtained for previous solution-processed MoS2 thin-film transistors. The scalable fabrication of large-area arrays of thin-film transistors enabled the construction of functional logic gates and computational circuits, including an inverter, NAND, NOR, AND and XOR gates, and a logic half-adder. We also applied our approach to other 2D materials, including WSe2, Bi2Se3, NbSe2, In2Se3, Sb2Te3 and black phosphorus, demonstrating its potential for generating versatile solution-processable 2D materials.

Dipole-Modulated Charge Transport through PNP-Type Single-Molecule Junctions.

The PNP structure realized by energy band engineering is widely used in various electronic and optoelectronic devices. In this work, we succeed in constructing a PNP-type single-molecule junction and explore the intrinsic characteristics of the PNP structure at the single-molecule level. A back-to-back azulene molecule is designed with opposite ∼1.7 D dipole moments to create PNP-type single-molecule junctions. In combination with theoretical and experimental studies, it is found that the intrinsic dipole can effectively adjust single-molecule charge transport and the corresponding potential barriers. This energy band control and charge transport regulation at the single-molecule level improve deep understanding of molecular charge transport mechanisms and provide important insights into the development of high-performance functional molecular nanocircuits toward practical applications.

Accurate Single-Molecule Kinetic Isotope Effects.

An accurate single-molecule kinetic isotope effect (sm-KIE) was applied to circumvent the inherent limitation of conventional ensemble KIE by using graphene-molecule-graphene single-molecule junctions. In situ monitoring of the single-molecule reaction trajectories in real time with high temporal resolution has the capability to characterize the deeper information brought by KIE. The C-O bond cleavage and the C-C bond formation of the transition state (TS) were observed in the Claisen rearrangement through the secondary kinetic isotope effect, demonstrating the high detection sensitivity and accuracy of this method. More interestingly, this sm-KIE can be used to determine TS structures under different electric fields, revealing the multidimensional regulation of the TS. The detection and manipulation of the TS provide a broad perspective to understand and optimize chemical reactions and biomimetic progress.

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.

Single-molecule nano-optoelectronics: insights from physics.

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Recent Advances in Photochemical Reactions on Single-Molecule Electrical Platforms.

The photochemical reaction is a very important type of chemical reaction. Visualizing and controlling photo-mediated reactions is a long-standing goal and challenge. In this regard, single-molecule electrical detection with label-free, real-time, and in situ characteristics has unique advantages in monitoring the dynamic process of photoreactions at the single-molecule level. In this review, a valuable summary of the latest process of single-molecule photochemical reactions based on single-molecule electrical platforms is provided. The single-molecule electrical detection platforms for monitoring photoreactions are displayed, including their fundamental principles, construction methods, and practical applications. The single-molecule studies of two different types of light-mediated reactions are summarized as below: i) photo-induced reactions, including reversible cyclization, conformational isomerization, and other photo-related reactions; ii) plasmon-mediated photoreactions, including reaction mechanisms and concrete examples, such as plasmon-induced photolysis of SS/OO bonds and tautomerization of porphycene. In addition, the prospects for future research directions and challenges in this field are also discussed.

Large-Area Synthesis and Patterning of All-Inorganic Lead Halide Perovskite Thin Films and Heterostructures.

All-inorganic lead halide perovskites have attracted tremendous interest for their excellent stability when compared with hybrid perovskites. Here we report a large-area growth of monocrystalline all-inorganic perovskite thin films and further patterning them into heterostructure arrays. We show that highly oriented CsPbBr3 microcrystal domains can be readily grown on muscovite mica substrates with a well-defined epitaxial relationship, which can further expand and eventually merge into large-area monocrystalline CsPbBr3 thin films with an excellent optical quality. Taking a step further, we show the large-area CsPbBr3 thin film can be further patterned and selectively transformed into CsPbI3 using a selective anion-exchange process to produce CsPbBr3-CsPbI3 lateral heterostructure arrays with spatially modulated photoluminescence emission and an apparent current rectification behavior. The capability to grow large-area CsPbBr3 monocrystalline thin films and heterostructure arrays defines a robust material platform for both the fundamental investigations and potential applications in optoelectronics.

Vibration-Assisted Charge Transport through Positively Charged Dimer Junctions.

Intermolecular charge transport plays a vital role in the fields of electronics, as well as biochemical systems. Here, we design supramolecular dimer junctions and investigate the effects of charge state and energy level alignment on charge transport under nanoconfinement. Incoherent tunneling caused by thermally-induced vibrations is enhanced in positively charged systems. The transition between coherent and incoherent tunneling is associated with specific molecular vibration modes. Positively charged systems with smaller torsional barriers and vibrational frequencies result in lower transition temperatures. Multiple thermal effects have a great impact on the conductance in the off-resonant tunneling, while thermally-induced vibron-assisted tunneling contributes more to the transport in the resonant tunneling. These investigations offer a deep mechanism understanding of intermolecular charge transport and facilitate the development of practical functional molecular devices.

Tunable Symmetry-Breaking-Induced Dual Functions in Stable and Photoswitched Single-Molecule Junctions.

The aim of molecular electronics is to miniaturize active electronic devices and ultimately construct single-molecule nanocircuits using molecules with diverse structures featuring various functions, which is extremely challenging. Here, we realize a gate-controlled rectifying function (the on/off ratio reaches ∼60) and a high-performance field effect (maximum on/off ratio >100) simultaneously in an initially symmetric single-molecule photoswitch comprising a dinuclear ruthenium-diarylethene (Ru-DAE) complex sandwiched covalently between graphene electrodes. Both experimental and theoretical results consistently demonstrate that the initially degenerated frontier molecular orbitals localized at each Ru fragment in the open-ring Ru-DAE molecule can be tuned separately and shift asymmetrically under gate electric fields. This symmetric orbital shifting (AOS) lifts the degeneracy and breaks the molecular symmetry, which is not only essential to achieve a diode-like behavior with tunable rectification ratio and controlled polarity, but also enhances the field-effect on/off ratio at the rectification direction. In addition, this gate-controlled symmetry-breaking effect can be switched on/off by isomerizing the DAE unit between its open-ring and closed-ring forms with light stimulus. This new scheme offers a general and efficient strategy to build high-performance multifunctional molecular nanocircuits.

Molecule-Based Transistors: From Macroscale to Single Molecule.

Molecule-based field-effect transistors (FETs) are of great significance as they have a wide range of application prospects, such as logic operations, information storage and sensor monitoring. This account mainly introduces and reviews our recent work in molecular FETs. Specifically, through molecular and device design, we have systematically investigated the construction and performance of FETs from macroscale to nanoscale and even single molecule. In particular, we have proposed the broad concept of molecular FETs, whose functions can be achieved through various external controls, such as light stimulation, and other physical, chemical or biological interactions. In the end, we tend to focus the discussion on the development challenges of single-molecule FETs, and propose prospects for further breakthroughs in this field.

Nanowire Electronics: From Nanoscale to Macroscale.

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Label-Free Dynamic Detection of Single-Molecule Nucleophilic-Substitution Reactions.

The mechanisms of chemical reactions, including the transformation pathways of the electronic and geometric structures of molecules, are crucial for comprehending the essence and developing new chemistry. However, it is extremely difficult to realize at the single-molecule level. Here, we report a single-molecule approach capable of electrically probing stochastic fluctuations under equilibrium conditions and elucidating time trajectories of single species in non-equilibrated systems. Through molecular engineering, a single molecular wire containing a functional center of 9-phenyl-9-fluorenol was covalently wired into nanogapped graphene electrodes to form stable single-molecule junctions. Both experimental and theoretical studies consistently demonstrate and interpret the direct measurement of the formation dynamics of individual carbocation intermediates with a strong solvent dependence in a nucleophilic-substitution reaction. We also show the kinetic process of competitive transitions between acetate and bromide species, which is inevitable through a carbocation intermediate, confirming the classical mechanism. This unique method creates plenty of opportunities for carrying out single-molecule dynamics or biophysics investigations in broad fields beyond reaction chemistry through molecular design and engineering.

Molecular-Scale Electronics: From Concept to Function.

Creating functional electrical circuits using individual or ensemble molecules, often termed as "molecular-scale electronics", not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level. This Review covers the major advances with the most general applicability and emphasizes new insights into the development of efficient platform methodologies for building reliable molecular electronic devices with desired functionalities through the combination of programmed bottom-up self-assembly and sophisticated top-down device fabrication. First, we summarize a number of different approaches of forming molecular-scale junctions and discuss various experimental techniques for examining these nanoscale circuits in details. We then give a full introduction of characterization techniques and theoretical simulations for molecular electronics. Third, we highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits. Finally, we provide a critical discussion of limitations and main challenges that still exist for the development of molecular electronics. These analyses should be valuable for deeply understanding charge transport through molecular junctions, the device fabrication process, and the roadmap for future practical molecular electronics.

Stereoelectronic Effect-Induced Conductance Switching in Aromatic Chain Single-Molecule Junctions.

Biphenyl, as the elementary unit of organic functional materials, has been widely used in electronic and optoelectronic devices. However, over decades little has been fundamentally understood regarding how the intramolecular conformation of biphenyl dynamically affects its transport properties at the single-molecule level. Here, we establish the stereoelectronic effect of biphenyl on its electrical conductance based on the platform of graphene-molecule single-molecule junctions, where a specifically designed hexaphenyl aromatic chain molecule is covalently sandwiched between nanogapped graphene point contacts to create stable single-molecule junctions. Both theoretical and temperature-dependent experimental results consistently demonstrate that phenyl twisting in the aromatic chain molecule produces different microstates with different degrees of conjugation, thus leading to stochastic switching between high- and low-conductance states. These investigations offer new molecular design insights into building functional single-molecule electrical devices.

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

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