Recent Progress of Break Junction Technique in Single-Molecule Reaction Chemistry

Abstract

Molecular electronics has been the subject of increasing interest since 1974. Although it describes the utilization of single molecules as active components of electrical devices, molecular electronics remains a fundamental subject to date. Considering that the length of a single molecule is typically several nanometers, the electrical characterization of a probe molecule is a significant experimental challenge. A metal/molecule/metal junction can bridge the gap between nanometer-sized molecules and the macroscopic measuring circuit and is, thus, generally considered as the most common prototype in molecular electronics. For the fabrication and characterization of single-molecule junctions, break junction methods, which include the mechanically controllable break junction (MCBJ) technique and the scanning tunneling microscopy-break junction (STM-BJ) technique, were proposed at the turn of the century and have been developed rapidly in recent years. These methods are widely employed in the experimental study of charge transport through single-molecule junctions and provide a platform to investigate the physical and chemical processes at the singlemolecule level. In this review, we mainly focus on MCBJ and STM-BJ techniques applicable for single-molecule conductance measurement and highlight the progress of these techniques in the context of identification and modulation of chemical reactions and evaluation of their reaction kinetics at the single-molecule level. We begin by presenting the operation principles of MCBJ and STM-BJ and stating their brief comparison. Subsequently, we summarize the recent advances in modulating single-molecule chemical reactions. In this regard, we introduce several examples that involve changing the environmental solution, applying an external electrical field, and resorting to electrochemical gating. Next, we overview the application of the break junction techniques in the investigation of reaction kinetics at the single-molecule level. In this section, we also present a brief introduction to studies on single-molecule reaction kinetics using graphenebased nanogaps, wherein conventional metallic electrodes were replaced by graphene electrodes. Furthermore, we discuss the combination of break junction techniques and surface-enhanced Raman spectroscopy for detecting singlemolecule reactions occurring at nanometer-scale separation. We discuss the historical development of this combined method and present the latest advancement explaining the origin of the low conductance of 1,4-benzenedithiol, which is a topic of significant concern in single-molecule electronics. Finally, we discuss some future issues in molecular electronics, including the expansion from simple molecules to complex molecular systems and the introduction of multi-physical fields into single-molecule junctions. Moreover, we provide a list of critical characterization tools in molecular electronics and discuss their potential applications.

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

“…Techniques such as atomic force microscopy (AFM), optical tweezer, and magnetic tweezer can detect single-molecule information through force control. , The electrical techniques used for monitoring single-molecule behaviors include scanning probe microscope, single-molecule junctions, field-effect transistors based on nanowire or nanotube, nanopore technology, and other electrical techniques. − Among them, the electrical single-molecule junction platform uses a single molecule as a conductive channel to directly convert the changed molecular states into resolvable electronic signals. − Therefore, single-molecule junctions can be used for real-time monitoring of molecular physical and chemical processes. Single-molecule junction techniques include mechanically controllable break junctions (MCBJs), scanning tunneling microscopy break junctions (STM-BJ), conductive AFM junctions, electromigration junctions, graphene–molecule–graphene single-molecule junctions (GMG-SMJs), and others. − Moreover, the success of the connection of target molecules can also be characterized by a variety of detection methods, such as IETS , and AFM. , In addition, the recently developed optical and electrical synchronous detection technology also strongly demonstrates the reliability of single-molecule junctions. , These single-molecule junctions can be used to detect the corresponding physical or chemical states of single molecules by monitoring their charge transport. Therefore, single-molecule junctions enable the discovery of fundamental physical and chemical phenomena at the single-molecule level.…”

Increased Surface Density of States at the Fermi Level for Electron Transport Across Single‐Molecule Junctions

“…To configure single-molecule junctions for the conductance measurements, the most prevalent approach is termed break-junction which involves two electrodes repeatedly joined together and pulled apart at the nanometer level. [3,4,[34][35][36][37][38][39] To preserve the SLRR monolayer, this present study employs a c-AFM equipped with a tactile-feedback controller [31] to monitor the tip-substrate force and to freely navigate the tip movement which avoids the imposing of excessive loads that might damage the Pd or Pt adlayer. The experimental design [31] enabled synchronously acquired traces of conductance and force during the retraction of the tip from the substrate (see Figure S2).…”

Single-Molecule Chemical Reactions Unveiled in Molecular Junctions

“…Reacting chemicals at the single-molecule level has offered novel insights into fundamental reaction mechanisms [22][23][24][25][26][27]. By analyzing the interaction of reactants at the atomic scale, a unique perspective is provided on the mechanisms involved in product formation [28].…”