This Review describes emerging techniques for characterizing the fundamental properties of molecular junctions besides electronic transport. The idea of using individual molecules as active electronic components provided the impetus to develop a variety of experimental platforms to probe their electronic transport properties. Among these, single-molecule junctions in a metal–molecule–metal motif have contributed significantly to our fundamental understanding of the principles required to realize molecular-scale electronic components from resistive wires to reversible switches. The success of these techniques and the growing interest of other disciplines in single-molecule-level characterization are prompting new approaches to investigate metal–molecule–metal junctions with multiple probes. Going beyond electronic transport characterization, these new studies are highlighting both the fundamental and applied aspects of mechanical, optical and thermoelectric properties at the atomic and molecular scales. Furthermore, experimental demonstrations of quantum interference and manipulation of electronic and nuclear spins in single-molecule circuits are heralding new device concepts with no classical analogues. In this Review, we present the emerging methods being used to interrogate multiple properties in single molecule-based devices, detail how these measurements have advanced our understanding of the structure–function relationships in molecular junctions, and discuss the potential for future research and applications.

/pdf/single-molecule-junctions-beyond-electronic-transport-8vw7kj5quk.pdf

Single-molecule junctions beyond electronic transport

The use of single molecules in electronics represents the next limit of miniaturisation of electronic devices, which would enable us to continue the trend of aggressive downscaling of silicon-based electronic devices. More significantly, the fabrication, understanding and control of fully functional circuits at the single-molecule level could also open up the possibility of using molecules as devices with novel, not-foreseen functionalities beyond complementary metal-oxide semiconductor technology (CMOS). This review aims at highlighting the chemical design and synthesis of single molecule devices as well as their electrical and structural characterization, including a historical overview and the developments during the last 5 years. We discuss experimental techniques for fabrication of single-molecule junctions, the potential application of single-molecule junctions as molecular switches, and general physical phenomena in single-molecule electronic devices.

/pdf/single-molecule-electronics-from-chemical-design-to-2xqzaddscw.pdf

Single-molecule electronics: from chemical design to functional devices

This tutorial outlines the basic theoretical concepts and tools which underpin the fundamentals of phase-coherent electron transport through single molecules. The key quantity of interest is the transmission coefficient T(E), which yields the electrical conductance, current-voltage relations, the thermopower S and the thermoelectric figure of merit ZT of single-molecule devices. Since T(E) is strongly affected by quantum interference (QI), three manifestations of QI in single-molecules are discussed, namely Mach-Zehnder interferometry, Breit-Wigner resonances and Fano resonances. A simple MATLAB code is provided, which allows the novice reader to explore QI in multi-branched structures described by a tight-binding (Huckel) Hamiltonian. More generally, the strengths and limitations of materials-specific transport modelling based on density functional theory are discussed.

Basic concepts of quantum interference and electron transport in single-molecule electronics

Over the past two decades, various techniques for fabricating nano-gapped electrodes have emerged, promoting rapid development in the field of single-molecule electronics, on both the experimental and theoretical sides. To investigate intrinsic quantum phenomena and achieve desired functionalities, it is important to fully understand the charge transport characteristics of single-molecule devices. In this Review, we present the principles that have been developed for fabricating reliable molecular junctions and tuning their intrinsic properties from an engineering perspective. Through holistic consideration of the device structure, we divide single-molecule junctions into three intercorrelated components: the electrode, the contact (spacer–linker) interface and the molecular backbone or functional centre. We systematically discuss the selection of the electrode material and the design of the molecular components from the point of view of the materials, the interface and molecular engineering. The influence of the properties of these elements on the molecule–electrode interface coupling and on the relative energy gap between the Fermi level of the electrode and the orbital energy levels of the molecule, which directly influence the charge transport behaviour of single-molecule devices, is also a focus of our analysis. On the basis of these considerations, we examine various functionalities demonstrated in molecular junctions through molecular design and engineering. In this Review, the principles developed for fabricating reliable molecular junctions and tuning their intrinsic properties are examined from the point of view of the electrode, the interface and molecular engineering. The various functionalities demonstrated in molecular junctions through molecular design are discussed, along with the open challenges in the field.

Concepts in the design and engineering of single-molecule electronic devices

Kirchhoff's conductance superposition law is investigated in single-molecule circuits. A single-molecule junction with two backbones in a parallel configuration can exhibit more than twice the conductance of a single-molecule junction with one backbone, a demonstration of constructive quantum interference.

/pdf/probing-the-conductance-superposition-law-in-single-molecule-2cu6rpg72z.pdf

Probing the conductance superposition law in single-molecule circuits with parallel paths

Electronic factors in molecules such as quantum interference and cross-conjugation can lead to dramatic modulation and suppression of conductance in single-molecule junctions. Probing such effects ...

/pdf/dissecting-contact-mechanics-from-quantum-interference-in-4ujwqrw925.pdf

Dissecting contact mechanics from quantum interference in single-molecule junctions of stilbene derivatives.

We measure the conductance of single Au-molecule-Au junctions with a series of air-stable diphenylphosphine-terminated molecules using the scanning tunneling microscope-based break junction technique. Thousands of conductance versus displacement traces collected for each molecule are used to statistically analyze junction conductance and evolution upon elongation. Mea- sured conductances for a series of alkane-based molecules exhibit an exponential decrease with increasing length, as expected for saturated molecules, with a tunneling decay constant of 0.98 ( 0.04. Measurements of junction elongation indicate strong metal-molecule binding, with a length that increases with the number of methylene groups in the backbone. Measured conductance histograms for four molecules with short, unsaturated backbones (e.g., benzene) are much broader with less well-defined peaks. These measurements are supported by density function theory calculations. The phosphine binds selectively to under- coordinated gold atoms through a donor-acceptor bond with a binding energy of about 1 eV. The calculated tunnel coupling correlates very well with experiment. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

/pdf/reliable-formation-of-single-molecule-junctions-with-air-43tjtny1w7.pdf

Reliable Formation of Single Molecule Junctions with Air-Stable Diphenylphosphine Linkers

We study the effects of molecular structure on the electronic transport and mechanical stability of single-molecule junctions formed with Au point contacts. Two types of linear conjugated molecular wires are compared: those functionalized with methylsulfide or amine aurophilic groups at (1) both or (2) only one of its phenyl termini. Using scanning tunneling and atomic force microscope break-junction techniques, the conductance of mono- and difunctionalized molecular wires and its dependence on junction elongation and rupture forces were studied. Charge transport through monofunctionalized wires is observed when the molecular bridge is coupled through a S−Au donor− acceptor bond on one end and a relatively weak Au−π interaction on the other end. For monofunctionalized molecular wires, junctions can be mechanically stabilized by installing a second aurophilic group at the meta position that, however, does not in itself contribute to a new conduction pathway. These results reveal the important interplay between electronic coupling through metal−π interactions and quantum mechanical effects introduced by chemical substitution on the conjugated system. This study affords a strategy to deterministically tune the electrical and mechanical properties through molecular wires.

/pdf/importance-of-direct-metal-p-coupling-in-electronic-4iqdfpe2tq.pdf

Importance of Direct Metal−π Coupling in Electronic Transport Through Conjugated Single-Molecule Junctions

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Dissecting contact mechanics from quantum interference in single-molecule junctions of stilbene derivatives.

Reliable Formation of Single Molecule Junctions with Air-Stable Diphenylphosphine Linkers

Importance of Direct Metal−π Coupling in Electronic Transport Through Conjugated Single-Molecule Junctions