Single-molecule Electronics Research Articles

Charge Transport through Single Molecules Connected to Semiconductor Electrodes

Single molecule electrical measurements have revolutionised our understanding of how charge flows through individual molecules and biomolecules in a wide variety of environments and are important for applications ranging from energy technology, DNA sequencing and interfacial electrochemistry. Such measurements have been typically made with metallic contacts but lately we have managed to extend this to the semiconductor gallium arsenide. This advance is important since future applications of molecular electronics will rely on integrating molecules alongside conventional semiconductor electronic technologies. In a recent publication in Nano Letters [1] we have shown that it is possible to make measurements of single molecules connected at one end to gallium arsenide and at the other end to a gold scanning tunnelling microscope tip. Using this methodology we can record current-voltage response of semiconductor – molecule – metal devices and measure the electrical conductance of single molecules in such junctions. For these measurements of charge transfer at the at the semiconductor-molecule interface the gallium arsenide surfaces are modified with organic self-assembled monolayers. Semiconductor / molecule(s) / metal junctions are formed by bringing a gold STM tip into contact with the molecular monolayer. Such junctions behave as a Schottky diode with the rectification ratio of the fabricated device being dependent on both the nature of the organic backbone and the electronic properties of the semiconductor. As well as recording I-V characteristics with the STM tip contacting the molecular monolayer, by using a time-dependent STM technique we were also able to detect the contribution of a single molecule to charge transport across the molecular junction. As well as showing that it is possible to form single molecule devices contacted to the semiconductor gallium arsenide we have also recently demonstrated that such single molecule devices show a strong photocurrent response.[2] Gallium arsenide is able to efficiently absorb visible light as it is a III-V direct bandgap (1.42 eV at room temperature) semiconductor and illumination of the molecular junction led to large changes in the current-voltage response. The junction configuration is schematically illustrated in the figure from reference [2] which shows a molecule bridging between the GaAs surface and the gold STM tip. The hemispheres in the GaAs substrate in this illustration depict the space charge layer (SCL) from which the charge carriers are collected. The resulting photo-generated charge carriers are transported to the metal electrode (STM tip) through the bridging organic dithiol giving the photo-current. Importantly, the photo-current response in these hybrid junctions can be controlled through the choice of molecular bridge as well as the doping density of the semiconductor amd the light intensity and wavelength.[2] In the dark high current rectification (> 103) is achieved with low-doping GaAs and saturated alkyl bridges and appreciable photocurrent is achieved upon illumination of these junctions. The response of the device is greatly influenced by molecular orbital alignment and this can be controlled by switching from saturated molecular bridges to conjugated ones with lower HOMO-LUMO gaps. Good photodiode functionality can be achieved with a space charge layer of only a few nanometres. Since the response of the molecular junctions is controlled by the combined properties of the semiconductor and the molecular wire this significantly extend of the single-molecule electronics “tool-box” as well as providing new methodologies for studying charge transfer at semiconductor-molecule interfaces. The presentation concludes with discussion of the perspective of single molecule photo-electrochemistry.

Voltage-Driven Conformational Switching with Distinct Raman Signature in a Single-Molecule Junction.

Precisely controlling well-defined, stable single-molecule junctions represents a pillar of single-molecule electronics. Early attempts to establish computing with molecular switching arrays were partly challenged by limitations in the direct chemical characterization of metal-molecule-metal junctions. While cryogenic scanning probe studies have advanced the mechanistic understanding of current- and voltage-induced conformational switching, metal-molecule-metal conformations are still largely inferred from indirect evidence. Hence, the development of robust, chemically sensitive techniques is instrumental for advancement in the field. Here we probe the conformation of a two-state molecular switch with vibrational spectroscopy, while simultaneously operating it by means of the applied voltage. Our study emphasizes measurements of single-molecule Raman spectra in a room-temperature stable single-molecule switch presenting a signal modulation of nearly 2 orders of magnitude.

Dual Control of Molecular Conductance through pH and Potential in Single-Molecule Devices.

One of the principal aims of single-molecule electronics is to create practical devices out of individual molecules. Such devices are expected to play a particularly important role as novel sensors thanks to their response to wide ranging external stimuli. Here we show that the conductance of a molecular junction can depend on two independent stimuli simultaneously. Using a scanning tunnelling microscope break-junction technique (STM-BJ), we found that the conductance of 4,4'-vinylenedipyridine (44VDP) molecular junctions with Ni contacts depends on both the electrochemically applied gate voltage and the pH of the environment. Hence, not only can the Ni|44VDP|Ni junction function as a pH-sensitive switch, but the value of the pH at which switching takes place can be tuned electrically. Furthermore, through the simultaneous control of pH and potential the STM-BJ technique delivers unique insight into the acid-base reaction, including the observation of discrete proton transfers to and from a single molecule.

DNA-Based Single-Molecule Electronics: From Concept to Function.

Beyond being the repository of genetic information, DNA is playing an increasingly important role as a building block for molecular electronics. Its inherent structural and molecular recognition properties render it a leading candidate for molecular electronics applications. The structural stability, diversity and programmability of DNA provide overwhelming freedom for the design and fabrication of molecular-scale devices. In the past two decades DNA has therefore attracted inordinate amounts of attention in molecular electronics. This review gives a brief survey of recent experimental progress in DNA-based single-molecule electronics with special focus on single-molecule conductance and I–V characteristics of individual DNA molecules. Existing challenges and exciting future opportunities are also discussed.

Electrochemical processes at the nanoscale

Summary Electrochemistry at the nanoscale is a very active area of research that sees the continuous emergence of new experiments, methods, tools and applications. Our ability to characterise processes at the nanoscale is unprecedented and there is an increasing ability to translate new insight to macroscopic scales, resulting in significant improvements in process efficiency and sensing capabilities. In this short review, we highlight three of these, quite recent developments, namely the evolution of advanced data analysis tools and Machine Learning in single-molecule electronics, studies on immobilised, individual nanoparticles, and nanoimpact electrochemistry, briefly review the directions they are taking, and finally how they might interact.

Side-Group-Mediated Mechanical Conductance Switching in Molecular Junctions.

A key target in molecular electronics has been molecules having switchable electrical properties. Switching between two electrical states has been demonstrated using such stimuli as light, electrochemical voltage, complexation and mechanical modulation. A classic example of the latter is the switching of 4,4'-bipyridine, leading to conductance modulation of around 1 order of magnitude. Here, we describe the use of side-group chemistry to control the properties of a single-molecule electromechanical switch, which can be cycled between two conductance states by repeated compression and elongation. While bulky alkyl substituents inhibit the switching behavior, π-conjugated side-groups reinstate it. DFT calculations show that weak interactions between aryl moieties and the metallic electrodes are responsible for the observed phenomenon. This represents a significant expansion of the single-molecule electronics "tool-box" for the design of junctions with electromechanical properties.

Unconventional Current Scaling and Edge Effects for Charge Transport through Molecular Clusters.

Metal–molecule–metal junctions are the key components of molecular electronics circuits. Gaining a microscopic understanding of their conducting properties is central to advancing the field. In the present contribution, we highlight the fundamental differences between single-molecule and ensemble junctions focusing on the fundamentals of transport through molecular clusters. In this way, we elucidate the collective behavior of parallel molecular wires, bridging the gap between single molecule and large-area monolayer electronics, where even in the latter case transport is usually dominated by finite-size islands. On the basis of first-principles charge-transport simulations, we explain why the scaling of the conductivity of a junction has to be distinctly nonlinear in the number of molecules it contains. Moreover, transport through molecular clusters is found to be highly inhomogeneous with pronounced edge effects determined by molecules in locally different electrostatic environments. These effects are most pronounced for comparably small clusters, but electrostatic considerations show that they prevail also for more extended systems.

Statistical approach to tunneling time in attosecond experiments

Tunneling, transport of particles through classically forbidden regions, is a pure quantum phenomenon. It governs numerous phenomena ranging from single-molecule electronics to donor–acceptor transition reactions. The main problem is the absence of a universal method to compute tunneling time. This problem has been attacked in various ways in the literature. Here, in the present work, we show that a statistical approach to the problem, motivated by the imaginary nature of time in the forbidden regions, lead to a novel tunneling time formula which is real and subluminal (in contrast to various known time definitions implying superluminal tunneling). In addition to this, we show explicitly that the entropic time formula is in good agreement with the tunneling time measurements in laser-driven He ionization. Moreover, it sets an accurate range for long-range electron transfer reactions. The entropic time formula is general enough to extend to the photon and phonon tunneling phenomena.

Radical‐Enhanced Charge Transport in Single‐Molecule Phenothiazine Electrical Junctions

We studied the single-molecule conductance through an acid oxidant triggered phenothiazine (PTZ-) based radical junction using the mechanically controllable break junction technique. The electrical conductance of the radical state was enhanced by up to 200 times compared to the neutral state, with high stability lasting for at least two months and high junction formation probability at room-temperature. Theoretical studies revealed that the conductance increase is due to a significant decrease of the HOMO-LUMO gap and also the enhanced transmission close to the HOMO orbital when the radical forms. The large conductance enhancement induced by the formation of the stable PTZ radical molecule will lead to promising applications in single-molecule electronics and spintronics.

Molecular-scale electronics: From device fabrication to functionality

By wiring molecules into circuits, “molecular electronics” aims at studying electronic properties of single molecules and their ensembles, on this basis exploiting their intrinsic functionalities, and eventually applying them as building blocks of electronic components for future electronic devices. Herein, fabricating reliable solid-state molecular devices and developing synthetic molecules endowed with desirable electronic properties, have been two major tasks since the dawn of molecular electronics. This review focuses on recent advances and efforts regarding the main challenges in this field, highlighting fabrication of nanogap electrodes for single-molecule junctions, and self-assembled-monolayers (SAMs) for functional devices. The prospect of molecular-scale electronics is also discussed.

Nonlinear and Nonsymmetric Single-Molecule Electronic Properties Towards Molecular Information Processing.

This review highlights molecular design for nonlinear and nonsymmetric single-molecule electronic properties such as rectification, negative differential resistance, and switching, which are important components of future single-molecule information processing devices. Perspectives on integrated "molecular circuits" are also provided. Nonlinear and nonsymmetric single-molecule electronics can be designed by utilizing (1) asymmetric molecular cores, (2) asymmetric anchoring groups, (3) an asymmetric junction environment, and (4) asymmetric electrode materials. This review mainly focuses on the design of molecular cores.

Influence of anchoring groups on single-molecular junction conductance: Theoretical comparative study of thiol and amine

Abstract Understanding the transport characteristics of single molecules bonded between metal electrodes is of fundamental importance for molecular scale electronics. By performing first-principle quantum transport calculations, we investigate the influence of anchoring groups on the conductance of a single molecular junction. The results indicated that the conductance of a single-molecule anchoring the Au electrode with thiol can be enhanced obviously by changing the anchoring group with amine. More importantly, the negative differential resistance behavior is found in the I-V characteristic of this single-molecule junction with thiol anchoring group, which also can be enlarged remarkably by replacing the anchoring group with amine. The results suggest that the diamine as anchoring group has a great potential in molecular electronics.

Covalent Co-O-V and Sb-N Bonds Enable Polyoxovanadate Charge Control.

The formation of [{CoII(teta)2}{CoII2(tren)(teta)2}VIV15SbIII6O42(H2O)]·ca.9H2O [teta = triethylenetetraamine; tren = tris(2-aminoethyl)amine] illustrates a strategy toward reducing the molecular charge of polyoxovanadates, a key challenge in their use as components in single-molecule electronics. Here, a V–O–Co bond to a binuclear Co2+-centered complex and a Sb–N bond to the terminal N atom of a teta ligand of a mononuclear Co2+ complex allow for full charge compensation of the archetypal molecular magnet [V15Sb6O42(H2O)]6–. Density functional theory based electron localization function analysis demonstrates that the Sb–N bond has an electron density similar to that of a Sb–O bond. Magnetic exchange coupling between the VIV and CoII spin centers mediated via the Sb–N bridge is comparably weakly antiferromagnetic.

Thermal Evolution of One-Dimensional Iodine Chains.

In one-dimensional (1D) systems, the definition of three common states of matter (solid, liquid, and gas) becomes obscure because it has been theoretically predicted that a 1D system has no phase transition. Due to technical difficulty in tracking 1D thermal evolution, hardly any experimental evidence has demonstrated whether there exist these three states. Here we report Raman experimental observation that 1D iodine molecular chains formed inside the nanosized channel undergo continuous transformation from chain structure to single molecules with increasing temperature, without having a sudden change as commonly observed in phase transition. At low temperatures, short-range order exists and manifests itself as long chains in structure, which gradually break into shorter chains with increasing temperature. The 1D system progressively gets more and more disordered, which is in agreement with the theoretical derivations. Our work may benefit the emerging molecular scale electronics.

Statistical I–V measurements of single-molecule junctions with an asymmetric anchoring group 1,4-aminobenzenethiol**Invited talk at 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2016), 8–12 November 2016, Ha Long City, Vietnam.

Molecular diodes are an interesting topic in the field of single-molecule electronics. Rectification of molecules such as 1,4-aminobenzenethiol (ABT) having different contact areas was reported. However, a more statistical approach is necessary to clarify the rectification of the ABT single-molecule junctions. In this research, we statistically measured the single molecular conductance and I–V characteristics of ABT single-molecule junctions using the scanning tunneling microscope break junction (STM-BJ) method. Two single molecular conductances caused to difference of bridging geometries were observed in the conductance measurements. Statistically significant rectification was not observed for ABT junctions. We concluded rectification does not appear only due to the difference of two anchoring groups in case of a small conjugate molecule such as ABT.

Syntheses and molecular structures of trans-bis(alkynyl) tetrakis-triethylphosphite ruthenium complexes

A simple synthesis of complexes trans-[Ru(C≡CC6H4-4-R)2{P(OEt)3}4] from trans-[RuCl2{P(OEt)3}4] and the terminal alkyne is described. Crystallographically determined molecular structures indicate a range of conformers, with the ethynyl arylene rings differing in their orientation with respect to each other across the Ru{P(OEt)3}4 fragment. Electrochemical analysis reveals a well-behaved one-electron oxidation process, which is only some 100 mV more positive that the analogous trans-[Ru(C≡CC6H4-4-R)2(dppe)2] complexes. Quantum chemical calculations reveal largely delocalised electronic structures and suggest that these compounds may find use as wire-like molecules within single molecule electronics.

Single-molecule electrical contacts on silicon electrodes under ambient conditions

The ultimate goal in molecular electronics is to use individual molecules as the active electronic component of a real-world sturdy device. For this concept to become reality, it will require the field of single-molecule electronics to shift towards the semiconducting platform of the current microelectronics industry. Here, we report silicon-based single-molecule contacts that are mechanically and electrically stable under ambient conditions. The single-molecule contacts are prepared on silicon electrodes using the scanning tunnelling microscopy break-junction approach using a top metallic probe. The molecular wires show remarkable current–voltage reproducibility, as compared to an open silicon/nano-gap/metal junction, with current rectification ratios exceeding 4,000 when a low-doped silicon is used. The extension of the single-molecule junction approach to a silicon substrate contributes to the next level of miniaturization of electronic components and it is anticipated it will pave the way to a new class of robust single-molecule circuits.

Preface: Special Topic on Frontiers in Molecular Scale Electronics

The electronic, mechanical, and thermoelectric properties of molecular scale devices have fascinated scientists across several disciplines in natural sciences and engineering. The interest is partially technological, driven by the fast miniaturization of integrated circuits that now have reached characteristic features at the nanometer scale. Equally important, a very strong incentive also exists to elucidate the fundamental aspects of structure-function relations for nanoscale devices, which utilize molecular building blocks as functional units. Thus motivated, a rich research field has established itself, broadly termed “Molecular Electronics,” that hosts a plethora of activities devoted to this goal in chemistry, physics, and electrical engineering. This Special Topic on Frontiers of Molecular Scale Electronics captures recent theoretical and experimental advances in the field.

Host-Guest Chemistry in Integrated Porous Space Formed by Molecular Self-Assembly at Liquid-Solid Interfaces.

Host-guest chemistry in two-dimensional (2D) space, that is, physisorbed monolayers of a single atom or a single molecular thickness on surfaces, has become a subject of intense current interest because of perspectives for various applications in molecular-scale electronics, selective sensors, and tailored catalysis. Scanning tunneling microscopy has been used as a powerful tool for the visualization of molecules in real space on a conducting substrate surface. For more than a decade, we have been investigating the self-assembly of a series of triangle-shaped phenylene-ethynylene macrocycles called dehydrobenzo[12]annulenes (DBAs). These molecules are substituted with six alkyl chains and are capable of forming hexagonal porous 2D molecular networks via van der Waals interactions between interdigitated alkyl chains at the interface of organic solvents and graphite. The dimension of the nanoporous space or nanowell formed by the self-assembly of DBAs can be controlled from 1.6 to 4.7 nm by simply changing the alkyl chain length from C6 to C20. Single molecules as well as homoclusters and heteroclusters are capable of coadsorbing within the host matrix using shape- and size-complementarity principles. Moreover, on the basis of the versatility of the DBA molecules that allows chemical modification of the alkyl chain terminals, we were able to decorate the interior space of the nanoporous networks with functional groups such as azobenzenedicarboxylic acid for photoresponsive guest adsorption/desorption or fluoroalkanes and tetraethylene glycol groups for selective guest binding by electrostatic interactions and zinc-porphyrin units for complexation with a guest by charge-transfer interactions. In this Feature Article, we describe the general aspects of molecular self-assembly at liquid/solid interfaces, followed by the formation of programmed porous molecular networks using rationally designed molecular building blocks. We focus on our own work involving host-guest chemistry in integrated nanoporous space that is modified for specific purposes.

Uncovering hierarchical data structure in single molecule transport

Interpretation of single molecule transport data is complicated by the fact that all such data are inherently highly stochastic in nature. Features are often broad, seemingly unstructured and distributed over more than an order of magnitude. However, the distribution contains information necessary for capturing the full variety of processes relevant in nanoscale transport, and a better understanding of its hierarchical structure is needed to gain deeper insight into the physics and chemistry of single molecule electronics. Here, we describe a novel data analysis approach based on hierarchical clustering to aid in the interpretation of single molecule conductance-displacement histograms. The primary purpose of statistically partitioning transport data is to provide avenues for unbiased hypothesis generation in single molecule break junction experiments by revealing otherwise potentially hidden aspects in the conductance data. Our approach is generalizable to the analysis of a wide variety of other single molecule experiments in molecular electronics, as well as in single molecule fluorescence spectroscopy, force microscopy, and ion-channel conductance measurements.