Single-molecule Electronic Devices Research Articles

Zero-Bias Anti-Ohmic Behaviour in Diradicaloid Molecular Wires.

Open-shell materials bearing multiple spin centres provide a key route to efficient charge transport in single-molecule electronic devices. They have narrow energy gaps, and their molecular orbitals align closely to the Fermi level of the metallic electrodes, thus allowing efficient electronic transport and higher conductance. Maintaining and stabilising multiple open-shell states - especially in contact with metallic electrodes - is however very challenging, generally requiring a continuous chemical or electrochemical potential to avoid self-immolation of the open-shell character. To overcome this issue, we designed, synthesised, and measured the conductance of a series of bis(indeno) fused acenes, where stability is imparted by a close-shell quinoidal conformation in resonance with the diradical electronic configuration. We show here that these compounds have anti-ohmic behaviour, with conductance increasing with increasing molecular length, at an unprecedented rate and across the entire bias window ([[EQUATION]]). Density Functional Theory (DFT) calculations support our findings, showing the rapidly narrowing HOMO-LUMO gap, unique to these diradicaloid structures, is responsible for the observed behaviour. Our results provide a framework for achieving efficient transport in neutral compounds and demonstrate the promise that diradicaloid materials have in single-molecule electronics, owing to their great stability and unique electronic structure.

Evolution of Single-Molecule Electronic Interfaces.

Single-molecule electronics can fabricate single-molecule devices via the construction of molecule-electrode interfaces and also provide a unique tool to investigate single-molecule scale physicochemical processes at these interfaces. To investigate single-molecule electronic devices with desired functionalities, an understanding of the interface evolution processes in single-molecule devices is essential. In this review, we focus on the evolution of molecule-electrode interface properties, including the background of interface evolution in single-molecule electronics, the construction of different types of single-molecule interfaces, and the regulation methods. Finally, we discuss the perspective of future characterization techniques and applications for single-molecule electronic interfaces.

The effect of weak π-π interactions on single-molecule electron transport properties of the tetraphenylethene molecule and its derivatives: a first-principles study.

Intramolecular π-π interactions are a significant research focus in fields such as chemistry, biology, and materials science. Different configurations of benzene-benzene moieties within a molecule can affect the magnitude of their π-π interactions, consequently influencing the electronic transport capabilities of the molecule. In this study, we designed three π-conjugated molecules, TPEM, TPEEM, and TEEPM, based on tetraphenylethene (TPE). These three molecules exhibit three distinct π-conjugated structures: linear cis-π-conjugation, linear trans-π-conjugation, and cross-π-conjugation. Thereinto, TPEM and TPEEM molecules share the same TPE core, with identical π-π interaction distances, while the TEEPM molecule has acetylene groups between the TPE units, thereby increasing the π-π interaction distances between the benzene moieties. Using density functional theory calculations combined with non-equilibrium Green's function (DFT+NEGF), our results reveal that the conductance order of different π-conjugated structures in TPEM and TPEEM molecules is as follows: cis > cross ≈ trans. Through analysis of transmission spectra, transmission pathways, and the innermost π orbitals, we find that in TPEM and TPEEM molecules, the cis- and cross-π-conjugated structures exhibit π-π interactions between benzene moieties and provide special through-space electron transport pathways, enhancing their electronic transport capabilities in coordination with the bonded molecular framework, whereas their trans-conjugated structures only allow electron transport along the molecular backbone. In contrast, in TEEPM molecule, due to the absence of π-π interactions, the conductance of different π-conjugated structures is primarily determined by the molecular backbone and follows the order: trans > cis > cross. These findings provide a theoretical basis for designing single-molecule electronic devices with multiple electron channels based on intramolecular π-π interactions.

Dependence of Charge Transfer on the Adsorption Site at Metal–Molecule Interfaces: Implications for Single-Molecule Electronic Devices

Dependence of Charge Transfer on the Adsorption Site at Metal–Molecule Interfaces: Implications for Single-Molecule Electronic Devices

Switching Quantum Interference in Single-Molecule Junctions by Mechanical Tuning.

The charge transport through single-molecule electronic devices can be controlled mechanically by changing the molecular geometrical configuration in situ, but the tunable conductance range is typically less than two orders of magnitude. Herein, we proposed a new mechanical tuning strategy to control the charge transport through the single-molecule junctions via switching quantum interference patterns. By designing molecules with multiple anchoring groups, we switched the electron transport between the constructive quantum interference (CQI) pathway and the destructive quantum interference (DQI) pathway, and more than four orders of magnitude conductance variation can be achieved by shifting the electrodes in a range of about 0.6 nm, which is the highest conductance range ever achieved using mechanical tuning.

Do quantum interference effects manifest in acyclic aliphatic molecules with anchoring groups?

The ability to control single molecule electronic conductance is imperative for achieving functional molecular electronics applications such as insulation, switching, and energy conversion. Quantum interference (QI) effects are generally used to control electronic transmission through single molecular junctions by tuning the molecular structure or the position of the anchoring group(s) in the molecule. While previous studies focussed on the QI between σ and/or π channels of the molecular backbone, here, we show that single molecule electronic devices can be designed based on QI effects originating from the interactions of anchoring groups. Furthermore, while previous studies have concentrated on the QI mostly in conjugated/cyclic systems, our study showcases that QI effects can be harnessed even in the simplest acyclic aliphatic systems-alkanedithiols, alkanediamines, and alkanediselenols. We identify band gap state resonances in the transmission spectrum of these molecules whose positions and intensities depend on the chain length, and anchoring group sensitive QI between the nearly degenerate molecular orbitals localized on the anchoring groups. We predict that these QI features can be harnessed through an external mechanical stimulus to tune the charge transport properties of single molecules in the break-junction experiments.

Structure and single-molecule conductance of two endohedral metallofullerenes with large C88 cage.

Endohedral metallofullerenes are capable of holding peculiar metal clusters inside the carbon cage. Additionally, these display many chemical and physical properties originating from the complexation between the metal clusters and carbon cages, which could be acquired for wide applications. In this study, two metallofullerenes (Ce2O@C88 and Ce3N@C88) with an identical large C88-D2(35) cage, and their molecular structures and single-molecule conductance properties were investigated comparatively. Characterizations of UV-vis-NIR absorption spectroscopy, Raman spectroscopy, and DFT calculations were employed to determine the geometries and electronic structures of Ce2O@C88 and Ce3N@C88. These molecules revealed varied energy gaps, structural parameters, vibrational modes, and molecular frontier orbitals. Although the two metallofullerenes have an identical cage isomer of C88-D2(35), their different endohedral clusters can influence their structures and physicochemical properties. Furthermore, the single-molecule conductance properties were measured using the scanning tunneling microscopy break junction technique (STM-BJ). The experimental results revealed that Ce2O@C88 has a higher conductance than Ce3N@C88 and C60. This revealed the cluster-dependent electron transportation as well as the significant research value of metallofullerenes with large carbon cages. These results provide guidance for fabricating single-molecule electronic devices.

Switchable single-molecule electronic and thermoelectric device induced by light in a designed diarylethene molecule

Tremendous efforts have been devoted to studying ordinary and novel properties in molecular-scale. Especially the reversible and controlled electrical and thermoelectric switches, which have been extensively studied experimentally and theoretically. They possess two distinct electronic and thermoelectric transport states actuated by external triggers, such as light, temperature, and electric field. Here, a photochromic diarylethene, linked to a nontriazatriangulene platform covalently, is studied by first-principles calculation. By means of density functional theory, we calculate the density of states and scanning tunneling microscope (STM) images of the designed diarylethene adsorbed on a Au(111) surface. Moreover, the thermoelectric transport properties of diarylethene-based molecular junction with a Au(111) electrode are studied systematically by using nonequilibrium Green's function transport formalism. We illustrate that due to the structure chang e of the designed diarylethene induced by light, the electronic and thermoelectric transport properties of the junction are completely different. Those differences can be visualized by the calculated electronic transmission eigenchannel and transport pathway. Furthermore, we demonstrate that the structure change of the designed diarylethene can be detected by STM experiments. The present study shows that the designed diarylethene could be a desirable candidate for electronic and thermoelectric molecular switch driven by light.

A Metallofullertube of Ce2 @C100 with a Carbon Nanotube Segment: Synthesis, Single-Molecule Conductance and Supramolecular Assembly.

Tubular fullerenes can be considered as end-capped carbon nanotubes with accurate structure, which are promising nanocarbon materials for advanced single-molecule electronic devices. Herein, we report the synthesis and characterization of a metallofullertube Ce2 @D5 (450)-C100 , which has a tubular C100 cage with a carbon nanotube segment and two fullerene end-caps. As there are structure correlations between tubular Ce2 @D5 (450)-C100 and spherical Ce2 @Ih -C80 , their structure-property relationship has been compared by means of experimental and theoretical methods. Notably, single-molecule conductance measurement determined that the conductivity of Ce2 @D5 (450)-C100 was up to eight times larger than that of Ce2 @Ih -C80 . Furthermore, supramolecular assembly of Ce2 @D5 (450)-C100 and a [12]CPP nanohoop was investigated, and theoretical calculations revealed that metallofullertube Ce2 @D5 (450)-C100 adopted a "standing" configuration in the cavity of [12]CPP. These results demonstrate the special nature of this kind of metallofullertube.

Design and modulation of electrode–molecule interfaces in single-molecule electronic devices

<p indent="0mm">The interface between molecules and electrodes plays a crucial role in determining charge transport properties of single-molecule electronic devices. The electrode–molecule interfacial properties can be manipulated by tuning the structures of anchor groups and electrodes. In this review, we first introduce the techniques employed for fabricating and characterizing single-molecule devices, and then describe the design principles of the anchor, the electrode and their interactions for forming electrode–molecule contact interface. Finally, we examine the recent progress of modulating the interfacial properties through external stimuli such as mechanical force, electrochemical potential and electrical field.

(Keynote) Conduction Under Potential Control: From Molecules to Proteins

My brilliant former student NJ Tao, to whose memory this symposium is dedicated, showed in a remarkable single-author paper1 that the conductance of a molecule containing a redox center was strongly dependent on surface potential, being a maximum at the redox potential. Some years later, we showed2 that defined contacts and statistical analysis were the keys to measuring molecular conductance quantitatively. NJ Tao came up with a simpler way of implementing our method, now the gold standard in single molecule conductance measurements.3 Gating of single-molecule conductance has now been reliably demonstrated.4 Despite many advances, there have been few real applications for single-molecule electronic devices. Proteins make excellent electronic conductors, as demonstrated by measurements under potential control that eliminate Faradaic contributions to the current.5 A remarkably long electron mean-free path and means for self-assembly give proteins advantages over conventional molecular wires.6 Furthermore, enzyme activity may be followed directly via conductance fluctuations, a possible basis for a new DNA sequencing technology.7 This talk will look at the physics behind these phenomena. Tao, N. J., Probing Potential-Tuned Resonant Tunneling through Redox Molecules with Scanning Tunneling Microscopy. Physical Review Letters 1996, 76 (21), 4066-4069.Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M., Reproducible Measurement of Single-Molecule Conductivity. Science (New York, N.Y.) 2001, 294 (5542), 571.Xu, B.; Tao, N. J., Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions. Science (New York, N.Y.) 2003, 301 (5637), 1221.Song, H.; Kim, Y.; Jang, Y. H.; Jeong, H.; Reed, M. A.; Lee, T., Observation of molecular orbital gating. Nature 2009, 462 (7276), 1039-1043.Zhang, B.; Song, W.; Pang, P.; Lai, H.; Chen, Q.; Zhang, P.; Lindsay, S., The Role of Contacts in Long-Range Protein Conductance. Proc Natl Acad Sci U S A 2019, 116, 5886-5891.Lindsay, S., Ubiquitous Electron Transport in non-Electron Transfer Proteins. Life 2020, 10, 72Zhang, B.; Deng, H.; Mukherjee, S.; Song, W.; Wang, X.; Lindsay, S., Engineering an Enzyme for Direct Electrical Monitoring of Activity. ACS Nano 2020, 14, 1630-1638.

Single-molecule junction spontaneously restored by DNA zipper

The electrical properties of DNA have been extensively investigated within the field of molecular electronics. Previous studies on this topic primarily focused on the transport phenomena in the static structure at thermodynamic equilibria. Consequently, the properties of higher-order structures of DNA and their structural changes associated with the design of single-molecule electronic devices have not been fully studied so far. This stems from the limitation that only extremely short DNA is available for electrical measurements, since the single-molecule conductance decreases sharply with the increase in the molecular length. Here, we report a DNA zipper configuration to form a single-molecule junction. The duplex is accommodated in a nanogap between metal electrodes in a configuration where the duplex is perpendicular to the nanogap axis. Electrical measurements reveal that the single-molecule junction of the 90-mer DNA zipper exhibits high conductance due to the delocalized π system. Moreover, we find an attractive self-restoring capability that the single-molecule junction can be repeatedly formed without full structural breakdown even after electrical failure. The DNA zipping strategy presented here provides a basis for novel designs of single-molecule junctions.

High temporal resolution electrical characterization technology of single-molecule devices

<p indent=0mm>Single-molecule electronics is a cross-discipline which aims at employing the individual molecule as the functional element for further miniaturization of the silicon-based microelectronics, and also focuses on the physical and chemical phenomena at single-molecule level. With the development of the single-molecule electronics, numerous novel physical and chemical effects such as Kondo physics, superconductivity and chemical reactions catalyzed by local strong electric fields, and single-molecule electronic devices including filed-effect transistors, diodes, and photo-switches have been widely explored. However, most of the abovementioned investigations are implemented at the millisecond level owing to the low temporal resolution of the measurement instrument, which makes it difficult to monitor the reaction events at faster time scales as well as meet the requirement of the high operating speed of the single-molecule devices. As a result, more attentions have been paid to develop the high temporal resolution electrical characterization techniques. In conventional single-molecule electrical measurement techniques, there is a big challenge that the extremely weak current signal through the single-molecule junction puts higher demands on the current amplification system of the instrument. However, the response speed of such amplifying circuits is relatively low, which limits the temporal resolution for single-molecule electrical measurements. In order to break through the limitation of temporal resolution, much efforts have been made in recent years. On the one hand, the low response speed caused by DC circuits can be improved through increasing the response speed of the current amplifying circuits. On the other hand, AC signal provides a promising approach to improve the temporal resolution, considering that the AC signal has already proven its application potential in the study of other micro-scale high-speed processes. This method requires the addition of AC signal measurement equipment in the circuit, including AC-DC separation circuits, RF amplifiers and lock-in amplifiers, etc., in addition, how to conduct effective data analysis is also a challenge worth considering. In this article, we will give a comprehensive review on the high temporal resolution electrical characterization techniques in the field of single-molecule electronics. This review will describe the challenges, developments, and applications of high temporal resolution electrical characterization techniques. We will briefly explain the significance of the research on the high temporal resolution electrical characterization techniques in the first part, the second part will describe the technical bottleneck of conventional single-molecule electrical measurement techniques, the third part will focus on the applications of the high temporal resolution electrical characterization techniques, and we will draw some expectations and conclusions in the fourth part. Although the high temporal resolution electrical characterization technology of single-molecule devices is still staying at the primary stage for its high requirements for circuit technology and measurement equipment, but this characterization technology can provide us with effective means to study higher-speed processes and more essential properties at the single-molecule scale, which includes the rearrangement of atoms, the structural transformation of molecules, and even the charge or energy transfer at the molecule-electrode interface. Those properties are also the guarantee for the manufacture of high-performance single-molecule electronic devices in the future. Therefore, we believe that the high temporal resolution electrical characterization techniques of single-molecule devices is full of development prospects and will be employed widely in the future.

Single molecule electronic devices with carbon-based materials: status and opportunity.

The field of single molecule electronics has progressed remarkably in the past decades by allowing for more versatile molecular functions and improving device fabrication techniques. In particular, electrodes made from carbon-based materials such as graphene and carbon nanotubes (CNTs) may enable parallel fabrication of multiple single molecule devices. In this perspective, we review the recent progress in the field of single molecule electronics, with a focus on devices that utilizes carbon-based electrodes. The paper is structured in three main sections: (i) controlling the molecule/graphene electrode interface using covalent and non-covalent approaches, (ii) using CNTs as electrodes for fabricating single molecule devices, and (iii) a discussion of possible future directions employing new or emerging 2D materials.

Single-molecule conductance variations of up to four orders of magnitude via contacting electrodes with different anchoring sites

Control of conductance through a single molecule via alternating anchoring points provides a unique perspective to design single-molecule electronic devices.

Sharp Negative Differential Resistance from Vibrational Mode Softening in Molecular Junctions.

We unravel the critical role of vibrational mode softening in single-molecule electronic devices at high bias. Our theoretical analysis is carried out with a minimal model for molecular junctions, with mode softening arising due to quadratic electron-vibration couplings, and by developing a mean-field approach. We discover that the negative sign of the quadratic electron-vibration coupling coefficient can realize, at high voltage, a sharp negative differential resistance (NDR) effect with a large peak-to-valley ratio. Calculated current-voltage characteristics, obtained based on physical parameters for a nitro-substituted oligo(phenylene ethynylene) junction, agree very well with the measurements. Our results establish that vibrational mode softening is a crucial effect at high voltage, underlying NDR, a substantial diode effect, and the breakdown of current-carrying molecular junctions.

Single Molecule-Based Electronic Devices: A Review

In the face of the fact that the development of traditional silicon-based electronic devices is increasingly limited, single molecule electronic device, which has been attracting more and more attention, is considered as one of the most hopeful candidates to realize the miniaturization of conventional electronic devices. In this paper, an overview of single molecule electronic devices is provided, including molecular electronic devices and electrode types. First, several molecular electronic devices are presented, including molecular diodes, molecular memories, molecular wires, molecular field effect transistors (FET) and molecular switches. Then the influence of different electrode types of the transport characteristics is introduced, showing that graphene is a promising electrode material for single molecule electronic devices. Moreover, other excellent characteristics of molecular devices are briefly introduced, such as potential thermoelectric effects, new thermally induced spin transport phenomena and negative differential resistance (NDR) behavior. Finally, the future challenges to the development of electronic devices based on single molecules are described.

Single-Molecule Electrical Detection: A Promising Route toward the Fundamental Limits of Chemistry and Life Science.

The ultimate limit of analytical chemistry is single-molecule detection, which allows one to visualize the dynamic processes of chemical/biological interactions with single-molecule or single-event sensitivity and hence enables the study of stochastic fluctuations under equilibrium conditions and the observation of time trajectories and reaction pathways of individual species in nonequilibrated systems. In addition, such studies may also allow the direct observation of novel microscopic quantum effects and fundamental discoveries of underlying molecular mechanisms in organic reactions and biological processes that are not accessible in ensemble experiments, thus providing unique opportunities to solve the key problems of physical, chemical, and life sciences. Consequently, the field of single-molecule detection has received considerable attention and has witnessed tremendous advances in different directions in combination with other disciplines. This Account describes our ongoing work on the development of groundbreaking methods (termed "single-molecule electrical approaches") of translating the detailed processes of chemical reactions or biological functions into detectable electrical signals at the single-event level on the platform of single-molecule electronic devices, with a particular focus on graphene-molecule-graphene single-molecule junctions (GMG-SMJs) and silicon-nanowire-based single-molecule electrical nanocircuits. These nanocircuit-based architectures are complementary to conventional optical or mechanical techniques but exhibit obvious advantages such as the absence of problems associated with bleaching and fluorescent labeling. Dash-line lithography (DLL) is an efficient lithographic method of cutting graphene and forming carboxylic-acid-functionalized nanogapped graphene point contact arrays developed to address the formidable challenges of molecular device fabrication difficulty and poor stability. Molecules of interest terminated by amines on both ends can be covalently sandwiched between graphene point contacts to create high-throughput robust GMG-SMJs containing only one molecule as the conductive element. In conjunction with the ease of device fabrication and device stability, this feature distinguishes GMG-SMJs as a new testbed platform for single-molecule analysis characterized by high temporal resolution and superior signal-to-noise ratios. By exploiting the DLL method, we have fabricated molecular devices that are sensitive to external stimuli and are capable of transducing chemical/biochemical events into electrical signals at the single-molecule level, with notable examples including host-guest interaction, hydrogen bond dynamics, DNA intercalation, photoinduced conformational transition, carbocation formation, nucleophilic addition, and stereoelectronic effect. In addition to GMG-SMJs and considering compatibility with the silicon-based industry, we have also developed a reliable method of point-functionalizing silicon-nanowire-based nanotransistors to afford single-molecule electrical nanocircuits. This approach proved to be a robust platform for single-molecule electrical analysis capable of probing fast dynamic processes such as single-protein detection, DNA hybridization/polymorphism, and motor rotation dynamics. The above systematic investigations emphasize the importance and unique advantages of universal single-molecule electrical approaches for realizing direct, label-free, real-time electrical measurements of reaction dynamics with single-event sensitivity. These approaches promise a fascinating mainstream platform to explore the dynamics of stochastic processes in chemical/biological systems as well as gain information in fields ranging from reaction chemistry for elucidating the intrinsic mechanisms to genomics or proteomics for accurate molecular and even point-of-care clinical diagnoses.

Side-group chemical gating via reversible optical and electric control in a single molecule transistor

By taking advantage of large changes in geometric and electronic structure during the reversible trans–cis isomerisation, azobenzene derivatives have been widely studied for potential applications in information processing and digital storage devices. Here we report an unusual discovery of unambiguous conductance switching upon light and electric field-induced isomerisation of azobenzene in a robust single-molecule electronic device for the first time. Both experimental and theoretical data consistently demonstrate that the azobenzene sidegroup serves as a viable chemical gate controlled by electric field, which efficiently modulates the energy difference of trans and cis forms as well as the energy barrier of isomerisation. In conjunction with photoinduced switching at low biases, these results afford a chemically-gateable, fully-reversible, two-mode, single-molecule transistor, offering a fresh perspective for creating future multifunctional single-molecule optoelectronic devices in a practical way.

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

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.