Graphene-molecule-graphene single-molecule junctions to detect electronic reactions at the molecular scale

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.

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.

Toward Practical Single-Molecule/Atom Switches

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

Real-Time Direct Monitoring of Chirality Fixation and Recognition at the Single-Molecule Level

Chirality, a fundamental attribute of nature, significantly influences a wide range of phenomena related to physical properties, chemical reactions, biological pharmacology, and so on. As a pivotal aspect of chirality research, chirality recognition contributes to the synthesis of complex chiral products from simple chiral compounds and exhibits intricate interplay between chiral materials. However, macroscopic detection technologies cannot unveil the dynamic process and intrinsic mechanisms of single-molecule chirality recognition. Herein, we present a single-molecule detection platform based on graphene-molecule-graphene single-molecule junctions to measure the chirality recognition involving interactions between amines and chiral alcohols. This approach leads to the realization of in situ and real-time direct observation of chirality recognition at the single-molecule level, demonstrating that chiral alcohols exhibit compelling potential to induce the formation of the corresponding chiral configuration of molecules. The amalgamation of theoretical analyses with experimental findings reveals a synergistic action between electrostatic interactions and steric hindrance effects in the chirality recognition process, thus substantiating the microscopic mechanism governing the chiral structure-activity relationship. These studies open up a pathway for exploring novel chiral phenomena from the fundamental limits of chemistry, such as chiral origin and chiral amplification, and offer important insights into the precise synthesis of chiral materials.

Regulation of quantum spin conversions in a single molecular radical

Free radicals, generally formed through the cleavage of covalent electron-pair bonds, play an important role in diverse fields ranging from synthetic chemistry to spintronics and nonlinear optics. However, the characterization and regulation of the radical state at a single-molecule level face formidable challenges. Here we present the detection and sophisticated tuning of the open-shell character of individual diradicals with a donor-acceptor structure via a sensitive single-molecule electrical approach. The radical is sandwiched between nanogapped graphene electrodes via covalent amide bonds to construct stable graphene-molecule-graphene single-molecule junctions. We measure the electrical conductance as a function of temperature and track the evolution of the closed-shell and open-shell electronic structures in real time, the open-shell triplet state being stabilized with increasing temperature. Furthermore, we tune the spin states by external stimuli, such as electrical and magnetic fields, and extract thermodynamic and kinetic parameters of the transition between closed-shell and open-shell states. Our findings provide insights into the evolution of single-molecule radicals under external stimuli, which may proof instrumental for the development of functional quantum spin-based molecular devices.

Investigation of the Single-Particle Scale Structure-Activity Relationship Providing New Insights for the Development of High-Performance Batteries

As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure-activity relationship of active materials is key to advancing the research of high-performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure-activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure-activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single-particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high-performance batteries.

Single-Molecule Electrical Profiling of Peptides and Proteins

In recent decades, there has been a significant increase in the application of single-molecule electrical analysis platforms in studying proteins and peptides. These advanced analysis methods have the potential for deep investigation of enzymatic working mechanisms and accurate monitoring of dynamic changes in protein configurations, which are often challenging to achieve in ensemble measurements. In this work, the prominent research progress in peptide and protein-related studies are surveyed using electronic devices with single-molecule/single-event sensitivity, including single-molecule junctions, single-molecule field-effect transistors, and nanopores. In particular, the successful commercial application of nanopores in DNA sequencing has made it one of the most promising techniques in protein sequencing at the single-molecule level. From single peptides to protein complexes, the correlation between their electrical characteristics, structures, and biological functions is gradually being established. This enables to distinguish different molecular configurations of these biomacromolecules through real-time electrical monitoring of their life activities, significantly improving the understanding of the mechanisms underlying various life processes.

Oversaturating Liquid Interfaces with Nanoparticle-Surfactants

Assemblies of nanoparticles at liquid interfaces hold promise as dynamic "active" systems when there are convenient methods to drive the system out of equilibrium via crowding. To this end, we show that oversaturated assemblies of charged nanoparticles can be realized and held in that state with an external electric field. Upon removal of the field, strong interparticle repulsive forces cause a high in-plane electrostatic pressure that is released in an explosive emulsification. We quantify the packing of the assembly as it is driven into the oversaturated state under an applied electric field. Physiochemical conditions substantially affect the intensity of the induced explosive emulsification, underscoring the crucial role of interparticle electrostatic repulsion.

Effect of Bridging Manner on the Transport Behaviors of Dimethyldihydropyrene/Cyclophanediene Molecular Devices

A molecule-electrode interface with different coupling strengths is one of the greatest challenges in fabricating reliable molecular switches. In this paper, the effects of bridging manner on the transport behaviors of a dimethyldihydropyrene/cyclophanediene (DHP/CPD) molecule connected to two graphene nanoribbon (GNR) electrodes have been investigated by using the non-equilibrium Green's function combined with density functional theory. The results show that both current values and ON/OFF ratios can be modulated to more than three orders of magnitude by changing bridging manner. Bias-dependent transmission spectra and molecule-projected self-consistent Hamiltonians are used to illustrate the conductance and switching feature. Furthermore, we demonstrate that the bridging manner modulates the electron transport by changing the energy level alignment between the molecule and the GNR electrodes. This work highlights the ability to achieve distinct conductance and switching performance in single-molecular junctions by varying bridging manners between DHP/CPD molecules and GNR electrodes, thus offering practical insights for designing molecular switches.

Toward Quantitative Electrodeposition via In Situ Liquid Phase Transmission Electron Microscopy: Studying Electroplated Zinc Using Basic Image Processing and 4D STEM

High energy density electrochemical systems such as metal batteries suffer from uncontrollable dendrite growth on cycling, which can severely compromise battery safety and longevity. This originates from the thermodynamic preference of metal nucleation on electrode surfaces, where obtaining the crucial information on metal deposits in terms of crystal orientation, plated volume, and growth rate is very challenging. In situ liquid phase transmission electron microscopy (LPTEM) is a promising technique to visualize and understand electrodeposition processes, however a detailed quantification of which presents significant difficulties. Here by performing Zn electroplating and analyzing the data via basic image processing, this work not only sheds new light on the dendrite growth mechanism but also demonstrates a workflow showcasing how dendritic deposition can be visualized with volumetric and growth rate information. These results along with additionally corroborated 4D STEM analysis take steps to access information on the crystallographic orientation of the grown Zn nucleates and toward live quantification of in situ electrodeposition processes.

What Makes On-Chip Microdevices Stand Out in Electrocatalysis?

Clean and sustainable energy conversion and storage through electrochemistry shows great promise as an alternative to traditional fuel or fossil-consumption energy systems. With regards to practical and high-efficient electrochemistry application, the rational design of active sites and the accurate description of mechanism remain a challenge. Toward this end, in this Perspective, a unique on-chip micro/nano device coupling nanofabrication and low-dimensional electrochemical materials is presented, in which material structure analysis, field-effect regulation, in situ monitoring, and simulation modeling are highlighted. The critical mechanisms that influence electrochemical response are discussed, and how on-chip micro/nano device distinguishes itself is emphasized. The key challenges and opportunities of on-chip electrochemical platforms are also provided through the Perspective.

An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches.

The role of halogens in Au-S bond cleavage for energy-differentiated catalysis at the single-bond limit

The transformation from one compound to another involves the breaking and formation of chemical bonds at the single-bond level, especially during catalytic reactions that are of great significance in broad fields such as energy conversion, environmental science, life science and chemical synthesis. The study of the reaction process at the single-bond limit is the key to understanding the catalytic reaction mechanism and further rationally designing catalysts. Here, we develop a method to monitor the catalytic process from the perspective of the single-bond energy using high-resolution scanning tunneling microscopy single-molecule junctions. Experimental and theoretical studies consistently reveal that the attack of a halogen atom on an Au atom can reduce the breaking energy of Au-S bonds, thereby accelerating the bond cleavage reaction and shortening the plateau length during the single-molecule junction breaking. Furthermore, the distinction in catalytic activity between different halogen atoms can be compared as well. This study establishes the intrinsic relationship among the reaction activation energy, the chemical bond breaking energy and the single-molecule junction breaking process, strengthening our mastery of catalytic reactions towards precise chemistry.

In Situ Device-Level TEM Characterization Based on Ultra-Flexible Multilayer MoS2 Micro-Cantilever

Current state-of-the-art in situ transmission electron microscopy (TEM) characterization technology has been capable of statically or dynamically nanorobotic manipulating specimens, affording abundant atom-level material attributes. However, an insurmountable barrier between material attributes investigations and device-level application explorations exists due to immature in situ TEM manufacturing technology and sufficient external coupled stimulus. These limitations seriously prevent the development of in situ device-level TEM characterization. Herein, a representative in situ opto-electromechanical TEM characterization platform is put forward by integrating an ultra-flexible micro-cantilever chip with optical, mechanical, and electrical coupling fields for the first time. On this platform, static and dynamic in situ device-level TEM characterizations are implemented by utilizing molybdenum disulfide (MoS₂ ) nanoflake as channel material. E-beam modulation behavior in MoS₂  transistors is demonstrated at ultra-high e-beam acceleration voltage (300 kV), stemming from inelastic scattering electron doping into MoS₂  nanoflakes. Moreover, in situ dynamic bending MoS₂  nanodevices without/with laser irradiation reveals asymmetric piezoresistive properties based on electromechanical effects and secondary enhanced photocurrent based on opto-electromechanical coupling effects, accompanied by real-time monitoring atom-level characterization. This approach provides a step toward advanced in situ device-level TEM characterization technology with excellent perception ability and inspires in situ TEM characterization with ultra-sensitive force feedback and light sensing.