Simultaneous Electrical and Mechanical Characterization of Single-Molecule Junctions Using AFM-BJ Technique

Quantitative Characterization and Influencing Factors for Electrode-Molecule-Electrode Junction Stability

Molecular electronics has made considerable progress in recent decades. The construction of a stable "electrode-molecule-electrode" junction is critical for the study of molecular electronics, as the stability can promote the exploration of the electrical properties of individual molecules and enable the prolonged observation of physical and chemical phenomena at the single-molecule scale. However, dispersed discussions and conflated concepts hinder our understanding of molecular junction stability. In this review, we systematically discuss the stability of molecular junctions from both thermodynamic and kinetic perspectives, summarize key quantitative parameters and their interrelationships, and provide an overview of the influencing factors at the molecule-electrode interface, as well as the experimental and theoretical analysis methods. We anticipate that this review will contribute to a thorough understanding of the stability of molecular junctions and offer valuable insights for the design of molecular devices based on molecular junctions.

Molecular Engineering for Future Thermoelectric Materials: The Role of Electrode and Metal Components in Molecular Junctions

As global temperatures increase due to climate change, the accumulation of excess heat on Earth presents a valuable resource that can be harnessed for electricity generation using thermoelectric materials. However, the intricate structures of bulk thermoelectric materials pose significant challenges to their comprehensive understanding and limit performance. Additionally, their relatively high production costs present practical obstacles. A promising solution to these issues lies in molecular control and the use of molecular junctions. Molecules are predicted to surpass the performance of existing bulk materials in energy conversion because they can be chemically tuned to achieve high thermoelectric efficiencies. This review identifies the thermoelectric parameters that affect the performance of molecular junctions. It also explores various experimental platforms for measuring thermoelectric performance from single molecules to assemblies of hundreds of molecules. Finally, it highlights recent advancements in thermoelectric molecular junctions, focusing on the crucial roles of electrodes and metal components within the molecules, such as Ru complexes, metalloporphyrins, metallocenes, conjugated silane wires, and endohedral metallofullerenes. Ultimately, our review provides a comprehensive analysis of strategies to enhance the thermoelectric efficiency of molecular junctions.

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.

Methods for the analysis, interpretation, and prediction of single-molecule junction conductance behaviour

This article offers a broad overview of measurement methods in the field of molecular electronics, with a particular focus on the most common single-molecule junction fabrication techniques, the challenges in data analysis and interpretation of single-molecule junction current-distance traces, and a summary of simulations and predictive models aimed at establishing robust structure-property relationships of use in the further development of molecular electronics.

Non-contact non-resonant atomic force microscopy method for measurements of highly mobile molecules and nanoparticles

Atomic force microscopy (AFM) is nowadays indispensable versatile scanning probe method widely employed for fundamental and applied research in physics, chemistry, biology as well as industrial metrology. Conventional AFM systems can operate in various environments such as ultra-high vacuum, electrolyte solutions, or controlled gas atmosphere. Measurements in ambient air are prevalent due to their technical simplicity; however, there are drawbacks such as formation of water meniscus that greatly increases attractive interaction (adhesion) between the tip and the sample, reduced spatial resolution, and too strong interactions leading to tip and/or sample modifications. Here we show how the attractive forces in AFM under ambient conditions can be used with advantage to probe surface properties in a very sensitive way even on highly mobile molecules and nanoparticles. We introduce a stable non-contact non-resonant (NCNR) AFM method which enables to reliably perform measurements in the attractive force regime even in air by controlling the tip position in the intimate surface vicinity without touching it. We demonstrate proof-of-concept results on helicene-based macrocycles, DNA on mica, and nanodiamonds on SiO2. We compare the results with other conventional AFM regimes, showing NCNR advantages such as higher spatial resolution, reduced tip contamination, and negligible sample modification. We analyze principle physical and chemical mechanisms influencing the measurements, discuss issues of stability and various possible method implementations. We explain how the NCNR method can be applied in any AFM system by a mere software modification. The method thus opens a new research field for measurements of highly sensitive and mobile nanoscale objects under air and other environments.

Formation of Molecular Junctions by Single-Entity Collision Electrochemistry

Controlling and understanding the chemistry of molecular junctions is one of the major themes in various fields ranging from chemistry and nanotechnology to biotechnology and biology. Stochastic single-entity collision electrochemistry (SECE) provides powerful tools to study a single entity, such as single cells, single particles, and even single molecules, in a nanoconfined space. Molecular junctions formed by SECE collision show various potential applications in monitoring molecular dynamics with high spatial resolution and high temporal resolution and in feasible combination with hybrid techniques. This Perspective highlights the new breakthroughs, seminal studies, and trends in the area that have been most recently reported. In addition, future challenges for the study of molecular junction dynamics with SECE are discussed.

Single-molecular protein-based bioelectronics via electronic transport: fundamentals, devices and applications

Biomolecular electronics is a rapidly growing multidisciplinary field that combines biology, nanoscience, and engineering to bridge the two important fields of life sciences and molecular electronics. Proteins are remarkable for their ability to recognize molecules and transport electrons, making the integration of proteins into electronic devices a long sought-after goal and leading to the emergence of the field of protein-based bioelectronics, also known as proteotronics. This field seeks to design and create new biomolecular electronic platforms that allow for the understanding and manipulation of protein-mediated electronic charge transport and related functional applications. In recent decades, there have been numerous reports on protein-based bioelectronics using a variety of nano-gapped electrical devices and techniques at the single molecular level, which are not achievable with conventional ensemble approaches. This review focuses on recent advances in physical electron transport mechanisms, device fabrication methodologies, and various applications in protein-based bioelectronics. We discuss the most recent progress of the single or few protein-bridged electrical junction fabrication strategies, summarise the work on fundamental and functional applications of protein bioelectronics that enable high and dynamic electron transport, and highlight future perspectives and challenges that still need to be addressed. We believe that this specific review will stimulate the interdisciplinary research of topics related to protein-related bioelectronics, and open up new possibilities for single-molecule biophysics and biomedicine.

Decoding the mechanical conductance switching behaviors of dipyridyl molecular junctions

Dipyridyl molecular junctions often show intriguing conductance switching behaviors with mechanical modulations, but the mechanisms are still not completely revealed. By applying the ab initio-based adiabatic simulation method, the configuration evolution and electron transport properties of dipyridyl molecular junctions in stretching and compressing processes are systematically investigated. The numerical results reveal that the dipyridyl molecular junctions tend to form specific contact configurations during formation processes. In small electrode gaps, the pyridyls almost vertically adsorb on the second Au layers of the tip electrodes by pushing the top Au atoms aside. These specific contact configurations result in stronger molecule-electrode couplings and larger electronic incident cross-sectional areas, which consequently lead to large breaking forces and high conductance. On further elongating the molecular junctions, the pyridyls shift to the top Au atoms of the tip electrodes. The additional scattering of the top Au atoms dramatically decreases the conductance and switches the molecular junctions to the lower conductive states. Perfect cyclical conductance switches are obtained as observed in the experiments by repeatedly stretching and compressing the molecular junctions. The O atom in the side-group tends to hinder the pyridyl from adsorbing on the second Au layer and further inhibits the conductance switch of the dipyridyl molecular junction.

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.

Measurements of single-molecule electromechanical properties based on atomic force microscopy fixed-junction technique

A hybrid technique combining atomic force microscopy and the fixed-junction technique is developed to simultaneously probe the electrical and mechanical characteristics of a single-molecule junction.

Single-Molecule Junction: A Reliable Platform for Monitoring Molecular Physical and Chemical Processes

Monitoring and manipulating the physical and chemical behavior of single molecules is an important development direction of molecular electronics that aids in understanding the molecular world at the single-molecule level. The electrical detection platform based on single-molecule junctions can monitor physical and chemical processes at the single-molecule level with a high temporal resolution, stability, and signal-to-noise ratio. Recently, the combination of single-molecule junctions with different multimodal control systems has been widely used to explore significant physical and chemical phenomena because of its powerful monitoring and control capabilities. In this review, we focus on the applications of single-molecule junctions in monitoring molecular physical and chemical processes. The methods developed for characterizing single-molecule charge transfer and spin characteristics as well as revealing the corresponding intrinsic mechanisms are introduced. Dynamic detection and regulation of single-molecule conformational isomerization, intermolecular interactions, and chemical reactions are also discussed in detail. In addition to these dynamic investigations, this review discusses the open challenges of single-molecule detection in the fields of physics and chemistry and proposes some potential applications in this field.

Mechanoresistive single-molecule junctions

Single-molecule junctions - devices fabricated by electrically connecting a single molecule to two electrodes - can respond to a variety of stimuli, that include electrostatic/electrochemical gating, light, other chemical species, and mechanical forces. When the latter is used, the device becomes mechanoresistive which means that its electrical resistance/conductance changes upon application of a mechanical stress. The mechanoresistive phenomenon can arise at the metal-molecule interface or it can be embedded in the molecular backbone, and several strategies to attain high reproducibility, high sensitivity and reversible behaviour have been developed over the years. These devices offer a unique insight on the process of charge transfer/transport at the metal/molecule interface, and have potential for applications as nanoelectromechanical systems, integrating electrical and mechanical functionality at the nanoscale. In this review, the status of the field is presented, with a focus on those systems that proved to have reversible behaviour, along with a discussion on the techniques used to fabricate and characterise mechanoresistive devices.