Carbon Electrode–Molecule Junctions: A Reliable Platform for Molecular Electronics

Abstract

The development of reliable approaches to integrate individual or a small collection of molecules into electrical nanocircuits, often termed "molecular electronics", is currently a research focus because it can not only overcome the increasing difficulties and fundamental limitations of miniaturization of current silicon-based electronic devices, but can also enable us to probe and understand the intrinsic properties of materials at the atomic- and/or molecular-length scale. This development might also lead to direct observation of novel effects and fundamental discovery of physical phenomena that are not accessible by traditional materials or approaches. Therefore, researchers from a variety of backgrounds have been devoting great effort to this objective, which has started to move beyond simple descriptions of charge transport and branch out in different directions, reflecting the interdisciplinarity. This Account exemplifies our ongoing interest and great effort in developing efficient lithographic methodologies capable of creating molecular electronic devices through the combination of top-down micro/nanofabrication with bottom-up molecular assembly. These devices use nanogapped carbon nanomaterials (such as single-walled carbon nanotubes (SWCNTs) and graphene), with a particular focus on graphene, as point contacts formed by electron beam lithography and precise oxygen plasma etching. Through robust amide linkages, functional molecular bridges terminated with diamine moieties are covalently wired into the carboxylic acid-functionalized nanogaps to form stable carbon electrode-molecule junctions with desired functionalities. At the macroscopic level, to improve the contact interface between electrodes and organic semiconductors and lower Schottky barriers, we used SWCNTs and graphene as efficient electrodes to explore the intrinsic properties of organic thin films, and then build functional high-performance organic nanotransistors with ultrahigh responsivities. At the molecular level, to form robust covalent bonds between electrodes and molecules and improve device stability, we developed a reliable system to immobilize individual molecules within a nanoscale gap of either SWCNTs or graphene through covalent amide bond formation, thus affording two classes of carbon electrode-molecule single-molecule junctions. One unique feature of these devices is the fact that they contain only one or two molecules as conductive elements, thus forming the basis for building new classes of chemo/biosensors with ultrahigh sensitivity. We have used these approaches to reveal the dependence of the charge transport of individual metallo-DNA duplexes on π-stacking integrity, and fabricate molecular devices capable of realizing label-free, real-time electrical detection of biological interactions at the single-event level, or switching their molecular conductance upon exposure to external stimuli, such as ion, pH, and light. These investigations highlight the unique advantages and importance of these universal methodologies to pr...

Single‐Molecule Electrical Detection with Real‐Time Label‐Free Capability and Ultrasensitivity

“…Tunneling junctions formed by single molecules have also been widely used for monitoring molecular processes . In this case, a single‐molecule junction (SMJ) is used as a conducting channel and changes in the molecular states are directly transduced into a distinguishable electronic signal .…”

A Magic Ratio Rule for Beginners: A Chemist's Guide to Quantum Interference in Molecules

“…For instance, thiols, pyridines, amines, methyl sulfides and direct gold–carbon bonds have been utilised in metal–molecule–metal junctions. Amine‐terminated molecules can bridge nanogaps between carboxylic acid‐functionalized carbon nanotubes while aromatic planar anchor groups including anthracene and pyrene are of interest due to their binding ability to graphene electrodes via π–π stacking and van der Waals interactions. Having established stable anchors to the electrodes, the passage of electricity through single molecules requires making choices for the remainder of the molecule, which typically involves a central aromatic functional subunit attached to the anchor groups via spacers.…”

Complex formation dynamics in a single-molecule electronic device

“…This is because of (i) the unique properties of carbon electrodes (especially graphene), (ii) the ease of device fabrication, and (iii) the device reproducibility and stability. These CEM-SMJs hold great promise when it comes to realizing functional molecular devices that can convert intermolecular interactions or molecular behaviors, such as DNA hybridization, deoxyribozyme cleavage, DNA-protein interactions, and conformationally induced switching, into tangible electrical signals stemming from single-molecule recognition ( 15 , 20 , 21 ), thus rendering CEM-SMJs a reliable molecular electronics platform for practical applications ( 6 ). In this investigation, we demonstrate the capability of a graphene-molecule SMJ to probe the thermodynamic and kinetic parameters of single-event noncovalent bonding interactions between a crown ether and an electron-deficient guest.…”