Electron transport in single molecules: from benzene to graphene

Electrochemical control of the single molecule conductance of a conjugated bis(pyrrolo)tetrathiafulvalene based molecular switch

The single molecule conductance of a conjugated molecular wire is electrochemically switched upon oxidising or reducing a central bispyrrolotetrathiafulvalene unit.

As the field of unimolecular electronics develops, there is growing interest in the development of functionalised molecular wires, such as switches, which will allow for more complex molecular-scale circuits. To this end, a three redox state single molecule switch, 1, based on bis(pyrrolo)tetrathiafulvalene (BPTTF) has been designed, synthesised and investigated using scanning tunnelling microscopy break junction (STM-BJ) studies and quantum transport calculations. Oxidising the BPTTF unit increases its conjugation, which was anticipated to increase the molecular conductance of 1. By changing the redox state of 1 electrochemically it was possible to vary the single molecule conductance by more than an order of magnitude (from 10^–5.2G₀ to 10^–3.8G₀). Simulations afforded a qualitatively similar trend. An additional, higher conductance feature is present in most traces at junction sizes of around 2.0 nm – further extension affords the switchable lower conductance feature at junction sizes closer to the molecular length (ca. 3.0 nm). Analysis of the conductance traces shows that these two conductance features occur sequentially in nearly all junctions. This behaviour is attributed to an alternative initial junction conformation in which one or more of the BPTTF sulfur atoms acts as an anchoring group. This hypothesis is supported by a computational study of binding conformations and STM-BJ studies on a model compound, 2, with only one thiol anchor. Our results indicate that the redox properties of BPTTF make it an excellent candidate for use in single molecule switches.

First principle electron transport modeling of Be-doped organic molecular junctions

The transport properties of beryllium doped anthracene molecular junction are investigated using density functional non-equillibrium Green's function method. The equilibrium conductance of anthracene Metal-molecule-Metal (MmM) junction increases by approximately 77% by adding beryllium impurity to it. The electronic transport characteristics under both zero bias as well as finite bias are explored of such molecular junction. We observe novel attributes such as molecular rectification and NDR behavior for the molecular junction under consideration. It is found that the doping effect of Be- atom significantly changes the transport properties of aromatic molecular junction. Our findings shed light on the electron transport metrics that affect the conductance of MmM junctions within appreciable transmission limits. We firmly believe that the results deduced in this paper can be generalized for other aromatic molecular junctions as well.

High conductance values in π-folded molecular junctions

Folding processes play a crucial role in the development of function in biomacromolecules. Recreating this feature on synthetic systems would not only allow understanding and reproducing biological functions but also developing new functions. This has inspired the development of conformationally ordered synthetic oligomers known as foldamers. Herein, a new family of foldamers, consisting of an increasing number of anthracene units that adopt a folded sigmoidal conformation by a combination of intramolecular hydrogen bonds and aromatic interactions, is reported. Such folding process opens up an efficient through-space charge transport channel across the interacting anthracene moieties. In fact, single-molecule conductance measurements carried out on this series of foldamers, using the scanning tunnelling microscopy-based break-junction technique, reveal exceptionally high conductance values in the order of 10-1 G0 and a low length decay constant of 0.02 Å-1 that exceed the values observed in molecular junctions that make use of through-space charge transport pathways.

Atomic structure of water/Au, Ag, Cu and Pt atomic junctions

Much progress has been made in understanding the transport properties of atomic-scale conductors. We prepared atomic-scale metal contacts of Cu, Ag, Au and Pt using a mechanically controllable break junction method at 10 K in a cryogenic vacuum. Water molecules were exposed to the metal atomic contacts and the effect of molecular adsorption was investigated by electronic conductance measurements. Statistical analysis of the electronic conductance showed that the water molecule(s) interacted with the surface of the inert Au contact and the reactive Cu ant Pt contacts, where molecular adsorption decreased the electronic conductance. A clear conductance signature of water adsorption was not apparent at the Ag contact. Detailed analysis of the conductance behaviour during a contact-stretching process indicated that metal atomic wires were formed for the Au and Pt contacts. The formation of an Au atomic wire consisting of low coordination number atoms leads to increased reactivity of the inert Au surface towards the adsorption of water.

Nitrogen-doped star-shaped polycyclic aromatic hydrocarbons based on fused triazatruxenes: synthesis and optoelectronic properties

A novel series of nitrogen-doped star-shaped polycyclic aromatic hydrocarbons based on naphthalenes-fused triazatruxenes with fine-tunable optoelectronic properties have been developed.

Extended O-Doped Polycyclic Aromatic Hydrocarbons

The synthesis of O‐doped benzorylenes, in which peripheral carbon atoms have been replaced by oxygen atoms, has been achieved for the first time. This includes key high‐yielding ring‐closure steps which, through intramolecular C−O bond formation, allow stepwise planarization of oligonaphthalenes. Single‐crystal X‐ray diffraction showed that the tetraoxa derivative forms remarkable face‐to‐face π–π stacks in the solid state, a favorable solid‐state arrangement for organic electronics.

Switchable DNA wire: deposition-stripping of copper nanoclusters as an "ON-OFF" nanoswitch

Today, a consensus that DNA working as a molecular wire shows promise in nanoscale electronics is reached. Considering that the “ON-OFF” switch is the basis of a logic circuit, the switch of DNA-mediated charge transport (DNA CT) should be conquered. Here, on the basis of chemical or electrochemical deposition and stripping of DNA-templated copper nanoclusters (CuNCs), we develop an “ON-OFF” nanoswitch for DNA CT. While CuNCs are deposited, the DNA CT is blocked, which can be also recovered after stripping the CuNCs. A switch cycle can be completed in a few seconds and can be repeated for many times. Moreover, by regulating the amount of reagents, deposition/stripping time, applied potential, etc., the switch is adjustable to make the wire at either an “ON-OFF” state or an intermediate state. We believe that this concept and the successful implementation will promote the practical application of DNA wire one step further.

Theoretical design of magnetic wires from acene and nanocorone derivatives

A theoretical design of molecular magnetic wires based on linear and cyclic oligoacene-bridged dicopper(ii) model complexes is highlighted in the present contribution.

Enhanced hydrogen production by carbon-doped TiO2 decorated with reduced graphene oxide (rGO) under visible light irradiation

Carbon doped TiO₂ anchored to reduced graphene oxide formed a hybrid nanocomposite (C-TiO₂/rGO) that exhibited greater photocatalytic activity and stability.

Molecular Electronics: History and Fundamentals

The increasing difficulties of meeting ‘Moore’s Law’ rates of progress in conventional semiconductor electronics, coupled with the advent of methods capable of measuring the electronic properties of single molecules in a laboratory setting, have seen a surge of activity in the field of molecular electronics over the last decade. However, the concepts of molecular electronics are far from new, and the basic premise and ideas of molecular electronics have been shadowing those of solid-state semiconductor electronics since the middle of the 20th century. In this Primer Review, we introduce the topic of molecular electronics, drawing on some of the earliest expressions of the fundamental concepts, and summarizing key concepts to provide the interested reader with an entry to this fascinating field of science and emerging technology.

Highly Resolved Nanostructured PEDOT on Large Areas by Nanosphere Lithography and Electrodeposition

Poly(ethylenedioxythiophene) (PEDOT) films were electrodeposited galvanostatically from an EDOT/sodium dodecyl sulfate solution in water, through a carboxylated polystyrene template monolayer self-assembled on ITO, after which the template was dissolved away in tetrahydrofuran. Analysis of the films by scanning electron microscopy and atomic force microscopy reveals large-area PEDOT honeycomb structures. The morphology of these structures was varied electrochemically, as the effective thickness and, surprisingly, the shape of the honeycomb arrangement depend on the polymerization time. Using nanospheres of 1 μm diameter and charge densities between 12 and 30 mC cm(-2) for electrodeposition generates PEDOT hexagons with very thin rectilinear walls 30-35 nm-thick and 800 nm-long, whereas at higher charge densities, circular bowls are created with 60 nm walls separating adjacent bowls; triangular areas as small as 0.02 μm(2) develop at the intersection of three nanospheres. These morphologies are specific to the use of carboxylated PS spheres and a water-based solution with a surfactant in the galvanostatic electrodeposition mode. Using smaller nanospheres, i.e. 500 nm in diameter, makes it possible to reach PEDOT hexagons with rectilinear walls as small as 11-17 nm-thick and 300 nm-long; circular bowls with 25-35 nm walls separating adjacent bowls and triangular areas as small as 0.003 μm(2) can also be generated. The wettabilities of the surfaces depend markedly on the pore depth of the PEDOT nanostructure, with contact angles going from 82° to 130° with increasing pore size. Finally these nanostructured PEDOT electrodes were used in Grätzel-type dye-sensitized solar cells (DSSCs) as Pt-free counter-electrodes, with an increase in the yield from 7.0 (bulk PEDOT) to 8.1%.

The spontaneous formation of single-molecule junctions via terminal alkynes

Herein, we report the spontaneous formation of single-molecule junctions via terminal alkyne contact groups. Self-assembled monolayers that form spontaneously from diluted solutions of 1, 4-diethynylbenzene (DEB) were used to build single-molecule contacts and assessed using the scanning tunneling microscopy-break junction technique (STM-BJ). The STM-BJ technique in both its dynamic and static approaches was used to characterize the lifetime (stability) and the conductivity of a single-DEB wire. It is demonstrated that single-molecule junctions form spontaneously with terminal alkynes and require no electrochemical control or chemical deprotonation. The alkyne anchoring group was compared against typical contact groups exploited in single-molecule studies, i.e. amine (benzenediamine) and thiol (benzendithiol) contact groups. The alkyne contact showed a conductance magnitude comparable to that observed with amine and thiol groups. The lifetime of the junctions formed from alkynes were only slightly less than that of thiols and greater than that observed for amines. These findings are important as (a) they extend the repertoire of chemical contacts used in single-molecule measurements to 1-alkynes, which are synthetically accessible and stable and (b) alkynes have a remarkable affinity toward silicon surfaces, hence opening the door for the study of single-molecule transport on a semiconducting electronic platform.

Searching the Hearts of Graphene-like Molecules for Simplicity, Sensitivity, and Logic

If quantum interference patterns in the hearts of polycyclic aromatic hydrocarbons could be isolated and manipulated, then a significant step toward realizing the potential of single-molecule electronics would be achieved. Here we demonstrate experimentally and theoretically that a simple, parameter-free, analytic theory of interference patterns evaluated at the mid-point of the HOMO-LUMO gap (referred to as M-functions) correctly predicts conductance ratios of molecules with pyrene, naphthalene, anthracene, anthanthrene, or azulene hearts. M-functions provide new design strategies for identifying molecules with phase-coherent logic functions and enhancing the sensitivity of molecular-scale interferometers.

Single-molecule conductance of a chemically modified, π-extended tetrathiafulvalene and its charge-transfer complex with F4TCNQ

We describe the synthesis and single-molecule electrical transport properties of a molecular wire containing a π-extended tetrathiafulvalene (exTTF) group and its charge-transfer complex with F4TCNQ. We form single-molecule junctions using the in situ break junction technique using a homebuilt scanning tunneling microscope with a range of conductance between 10 G0 down to 10(-7) G0. Within this range we do not observe a clear conductance signature of the neutral parent molecule, suggesting either that its conductance is too low or that it does not form a stable junction. Conversely, we do find a clear conductance signature in the experiments carried out on the charge-transfer complex. Due to the fact we expected this species to have a higher conductance than the neutral molecule, we believe this supports the idea that the conductance of the neutral molecule is very low, below our measurement sensitivity. This idea is further supported by theoretical calculations. To the best of our knowledge, these are the first reported single-molecule conductance measurements on a molecular charge-transfer species.

Fine-tuning of single-molecule conductance by tweaking both electronic structure and conformation of side substituents

Herein, we describe a method to fine-tune the conductivity of single-molecule wires by employing a combination of chemical composition and geometrical modifications of multiple phenyl side groups as conductance modulators embedded along the main axis of the electronic pathway. We have measured the single-molecule conductivity of a novel series of phenyl-substituted carotenoid wires whose conductivity can be tuned with high precision over an order of magnitude range by modulating both the electron-donating character of the phenyl substituent and its dihedral angle. It is demonstrated that the electronic communication between the phenyl side groups and the molecular wire is maximized when the phenyl groups are twisted closer to the plane of the conjugated molecular wire. These findings can be refined to a general technique for precisely tuning the conductivity of molecular wires.

A theoretical study of asymmetric electron transport through linearly aromatic molecules

Electron transport through a series of polyacene molecules connected via a π-conjugated bridge (an anthracene molecule) was investigated theoretically by using the nonequilibrium Green's function formalism combined with density functional theory. The results have shown that the asymmetric current-voltage characteristics can be achieved by tuning the position of a side π-bridge linked to the main conjugated backbone. The detailed analyses of the spatial distribution of molecular orbitals as well as the current density interpret how the location of π-bridge strongly affects the intramolecular electronic coupling. The rectification in the molecular junction arises from the localization of the molecular orbitals near the Fermi level and the asymmetric shift of molecule orbital energy levels under positive and negative bias. The rectification ratio decreases with increasing the length of the π-bridge which improves intramolecular electronic coupling between aromatic rings. Furthermore, the rectification properties of conjugated molecules are just slightly affected by the anchoring positions of thiol groups. These results demonstrated that the location and the length of π-bridge, which induce the asymmetric intramolecular coupling, play key roles in the rectification of the linearly aromatic molecules.

Transfer-free synthesis of doped and patterned graphene films

High-quality and wafer-scale graphene on insulating gate dielectrics is a prerequisite for graphene electronic applications. For such applications, graphene is typically synthesized and then transferred to a desirable substrate for subsequent device processing. Direct production of graphene on substrates without transfer is highly desirable for simplified device processing. However, graphene synthesis directly on substrates suitable for device applications, though highly demanded, remains unattainable and challenging. Here, we report a simple, transfer-free method capable of synthesizing graphene directly on dielectric substrates at temperatures as low as 600 °C using polycyclic aromatic hydrocarbons as the carbon source. Significantly, N-doping and patterning of graphene can be readily and concurrently achieved by this growth method. Remarkably, the graphene films directly grown on glass attained a small sheet resistance of 550 Ω/sq and a high transmittance of 91.2%. Organic light-emitting diodes (OLEDs) fabricated on N-doped graphene on glass achieved a current density of 4.0 mA/cm(2) at 8 V compared to 2.6 mA/cm(2) for OLEDs similarly fabricated on indium tin oxide (ITO)-coated glass, demonstrating that the graphene thus prepared may have potential to serve as a transparent electrode to replace ITO.

Hexaazatriphenylene (HAT) derivatives: from synthesis to molecular design, self-organization and device applications

The creativity and inventiveness of chemists working with the 1,4,5,8,9,12-hexaazatriphenylene (HAT) building block is highlighted in this review.

Multi-responsive photo- and chemo-electrical single-molecule switches

Incorporating molecular switches as the active components in nanoscale electrical devices represents a current challenge in molecular electronics. It demands key requirements that need to be simultaneously addressed including fast responses to external stimuli and stable attachment of the molecules to the electrodes while mimicking the operation of conventional electronic components. Here, we report a single-molecule switching device that responds electrically to optical and chemical stimuli. A light pointer or a chemical signal can rapidly and reversibly induce the isomerization of bifunctional spiropyran derivatives in the bulk reservoir and, consequently, switch the electrical conductivity of the single-molecule device between a low and a high level. The spiropyran derivatives employed are chemically functionalized such that they can respond in fast but practical time scales. The unique multistimuli response and the synthetic versatility to control the switching schemes of this single-molecule device suggest spiropyran derivatives as key candidates for molecular circuitry.

Self-assembly of decoupled borazines on metal surfaces: the role of the peripheral groups

Two borazine derivatives have been synthesised to investigate their self-assembly behaviour on Au(111) and Cu(111) surfaces by scanning tunnelling microscopy (STM) and theoretical simulations. Both borazines form extended 2D networks upon adsorption on both substrates at room temperature. Whereas the more compact triphenyl borazine 1 arranges into close-packed ordered molecular islands with an extremely low density of defects on both substrates, the tris(phenyl-4-phenylethynyl) derivative 2 assembles into porous molecular networks due to its longer lateral substituents. For both species, the steric hindrance between the phenyl and mesityl substituents results in an effective decoupling of the central borazine core from the surface. For borazine 1, this is enough to weaken the molecule-substrate interaction, so that the assemblies are only driven by attractive van der Waals intermolecular forces. For the longer and more flexible borazine 2, a stronger molecule-substrate interaction becomes possible through its peripheral substituents on the more reactive copper surface.

Highly conductive single-molecule wires with controlled orientation by coordination of metalloporphyrins

Porphyrin-based molecular wires are promising candidates for nanoelectronic and photovoltaic devices due to the porphyrin chemical stability and unique optoelectronic properties. An important aim toward exploiting single porphyrin molecules in nanoscale devices is to possess the ability to control the electrical pathways across them. Herein, we demonstrate a method to build single-molecule wires with metalloporphyrins via their central metal ion by chemically modifying both an STM tip and surface electrodes with pyridin-4-yl-methanethiol, a molecule that has strong affinity for coordination with the metal ion of the porphyrin. The new flat configuration resulted in single-molecule junctions of exceedingly high lifetime and of conductance 3 orders of magnitude larger than that obtained previously for similar porphyrin molecules but wired from either end of the porphyrin ring. This work presents a new concept of building highly efficient single-molecule electrical contacts by exploiting metal coordination chemistry.

Voltage-controlled fluorescence switching of a single redox protein

Heterogeneous electron transfer (ET) of the redox protein, wild-type azurin (wt-Az) from Pseudomonas aeruginosa, was monitored at the single-molecule (SM) level by fluorescence resonance energy transfer (FRET), one electron at a time. Azurin molecules were labeled with an organic fluorophore (Cy5), and the FRET-coupling between Cy5 and the redox center (copper) was used to study ET to a semi-transparent, 10nm thin gold electrode in an optical configuration. By using a confocal microscope and a bipotentiostat for control of the electrode potential, the oxidation and reduction processes of individual Az-Cy5 molecules were monitored. In the oxidized state of the redox center of the azurin molecule, the fluorescence emission of the covalently attached Cy5 was largely quenched by FRET ('off'-state), whereas the emission was recovered upon reduction ('on'-state). The work presented here, shows directly controlled single redox switching events of an individual redox protein and its thermodynamic dispersion. We show that the distribution of midpoint potentials (E0) of individual azurin molecules peaks at 45.7±0.5 mV with a full width at half maximum of 15 mV vs saturated calomel electrode (SCE).

Graphene mechanics: I. Efficient first principles based Morse potential

We present a computationally efficient pairwise potential for use in molecular dynamics simulations of large graphene or carbon nanotube systems, in particular, for those under mechanical deformation, and also for mixed systems including biomolecules. Based on the Morse potential, it is only slightly more complex and computationally expensive than a harmonic bond potential, allowing such large or mixed simulations to reach experimentally relevant time scales. By fitting to data obtained from quantum mechanics (QM) calculations to represent bond breaking in graphene patches, we obtain a dissociation energy of 805 kJ mol(-1) which reflects the steepness of the QM potential up to the inflection point. A distinctive feature of our potential is its truncation at the inflection point, allowing a realistic treatment of ruptured C-C bonds without relying on a bond order model. The results obtained from equilibrium MD simulations using our potential compare favorably with results obtained from experiments and from similar simulations with more complex and computationally expensive potentials.

Nanoscale methods for single-molecule electrochemistry

The development of experiments capable of probing individual molecules has led to major breakthroughs in fields ranging from molecular electronics to biophysics, allowing direct tests of knowledge derived from macroscopic measurements and enabling new assays that probe population heterogeneities and internal molecular dynamics. Although still somewhat in their infancy, such methods are also being developed for probing molecular systems in solution using electrochemical transduction mechanisms. Here we outline the present status of this emerging field, concentrating in particular on optical methods, metal-molecule-metal junctions, and electrochemical nanofluidic devices.

Quantitative studies of adsorbate dynamics at noble metal electrodes by in situ Video-STM

The surface diffusion of adsorbates at electrochemical interfaces is studied by in situ scanning tunneling microscopy with high temporal resolution, using sulfur and methyl thiolate on c(2 × 2) Cl covered Cu(100), Ag(100), and Au(100) electrode surfaces in 0.01 M HCl solution as an example. While on Au(100) quantitative studies were not possible because of the slow dynamics and high surface defect density, on Cu(100) and Ag(100) a pronounced exponential increase of the jump rates of isolated adsorbates toward more negative potentials was found, indicating a linear decrease of the tracer diffusion barriers with potential. The potential dependence is independent of the adsorbate species, but differs for Cu(100) and Ag(100) substrates. These trends can be explained by electrostatic contributions to the diffusion barrier, caused by the interaction of the adsorbates with the field of the electrochemical double layer, if the presence of the chloride coadsorbate layer is taken into account.