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We used a range of scanning tunnelling microscopy (STM)-based methods to conduct a detailed study of single molecule junction conductance enhancement upon charge transfer complex formation, using bis(thiaalkyl)arene molecular wires as electron donors and tetracyanoethylene (TCNE) as an electron acceptor. Using the "hard" STM break junction (STM-BJ) method, in which a Au STM tip is pushed into a Au substrate and then withdrawn in the presence of molecules, we see a single, very broad, peak in the resulting conductance histogram when all data are used; the conductance enhancement is 25-fold for a terthiophene donor and 15-fold for a phenyl group. After rational data selection, in which only current-distance curves that contain a current plateau >0.2 nm long are used in the conductance histogram, three sharper peaks are resolved in the histograms for the charge transfer complexes; two substantially lower-conductance peaks are resolved for the uncomplexed molecules. Using the "soft" STM I(s) technique, in which initial contact between tip and substrate is avoided and the current limit is about an order of magnitude lower, we were able to resolve two peaks for the uncomplexed molecules depending upon the initial set point current (i.e., tip height), one at the same value as the lower of the two data-selected STM-BJ histogram peaks and an additional peak beyond the low-current limit for the STM-BJ experiment. For the terthiophene, the low, medium, and high conductance peaks for the TCNE complex are, respectively, ca. 70, 70, and 46 times higher in conductance than the corresponding peaks for the free molecule. [ABSTRACT FROM AUTHOR]

Using a first principles approach to electron transport, we calculate the electrical and thermoelectrical transport properties of a series of molecular wires containing benzo-difuran subunits. We demonstrate that the side groups introduce Fano resonances, the energy of which is changing with the electronegativity of selected atoms in it. We also study the relative effect of single, double, or triple bonds along the molecular backbone and find that single bonds yield the highest thermopower, approximately 22 μV/K at room temperature, which is comparable with the highest measured values for single-molecule thermopower reported to date. [ABSTRACT FROM AUTHOR]

As the field of molecular‐scale electronics matures and the prospect of devices incorporating molecular wires becomes more feasible, it is necessary to progress from the simple anchor groups used in fundamental conductance studies to more elaborate anchors designed with device stability in mind. This study presents a series of oligo(phenylene‐ethynylene) wires with one tetrapodal anchor and a phenyl or pyridyl head group. The new anchors are designed to bind strongly to gold surfaces without disrupting the conductance pathway of the wires. Conductive probe atomic force microscopy (cAFM) was used to determine the conductance of self‐assembled monolayers (SAMs) of the wires in Au–SAM–Pt and Au–SAM–graphene junctions, from which the conductance per molecule was derived. For tolane‐type wires, mean conductances per molecule of up to 10−4.37 G0 (Pt) and 10−3.78 G0 (graphene) were measured, despite limited electronic coupling to the Au electrode, demonstrating the potential of this approach. Computational studies of the surface binding geometry and transport properties rationalise and support the experimental results. [ABSTRACT FROM AUTHOR]

Single‐molecule junctions that are sensitive to compression or elongation are an emerging class of nanoelectromechanical systems (NEMS). Although the molecule–electrode interface can be engineered to impart such functionality, most studies to date rely on poorly defined interactions. We focused on this issue by synthesizing molecular wires designed to have chemically defined hemilabile contacts based on (methylthio)thiophene moieties. We measured their conductance as a function of junction size and observed conductance changes of up to two orders of magnitude as junctions were compressed and stretched. Localised interactions between weakly coordinating thienyl sulfurs and the electrodes are responsible for the observed effect and allow reversible monodentate⇄bidentate contact transitions as the junction is modulated in size. We observed an up to ≈100‐fold sensitivity boost of the (methylthio)thiophene‐terminated molecular wire compared with its non‐hemilabile (methylthio)benzene counterpart and demonstrate a previously unexplored application of hemilabile ligands to molecular electronics. [ABSTRACT FROM AUTHOR]

Cumulenes are sometimes described as "metallic" because an infinitely long cumulene would have the band structure of a metal. Herein, we report the single‐molecule conductance of a series of cumulenes and cumulene analogues, where the number of consecutive C=C bonds in the core is n=1, 2, 3, and 5. The [n]cumulenes with n=3 and 5 have almost the same conductance, and they are both more conductive than the alkene (n=1). This is remarkable because molecular conductance normally falls exponentially with length. The conductance of the allene (n=2) is much lower, because of its twisted geometry. Computational simulations predict a similar trend to the experimental results and indicate that the low conductance of the allene is a general feature of [n]cumulenes where n is even. The lack of length dependence in the conductance of [3] and [5]cumulenes is attributed to the strong decrease in the HOMO–LUMO gap with increasing length. [ABSTRACT FROM AUTHOR]

Abstract: The design of thermoelectric materials for the efficient conversion of waste heat into electricity requires simultaneous tuning of their electrical and thermal conductance. A comparative theoretical study of electron and phonon transport in thiophene and ethylenedioxythiophene (EDOT) based molecular wires is performed. It is shown that modifying thiophene by substituting ethylenedioxy enhances the thermoelectric figure of merit ZT for molecules of the same length. Furthermore, it is demonstrated that the electrical conductance of EDOT‐based wires decays more slowly with length than that of thiophene‐based wires and that their thermal conductance is lower. The room‐temperature ZT of undoped EDOT is found to be rather low. However, doping of EDOT by the electron acceptor tolunenesulfunate increases the Seebeck coefficient and electrical conductance, while decreasing the thermal conductance, leading to a thermoelectric figure of merit as high as ZT = 2.4. [ABSTRACT FROM AUTHOR]

The ancillary ligands 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine and 4′-(2,3-dihydrobenzo[ b]thiophene)-2,2′-6′,2'-terpyridine were used to synthesize two series of mono- and dinuclear ruthenium complexes differing in their lengths and anchoring groups. The electrochemical and single-molecular conductance properties of these two series of ruthenium complexes were studied experimentally by means of cyclic voltammetry and the scanning tunneling microscopy-break junction technique (STM-BJ) and theoretically by means of density functional theory (DFT). Cyclic voltammetry data showed clear redox peaks corresponding to both the metal- and ligand-related redox reactions. Single-molecular conductance demonstrated an exponential decay of the molecular conductance with the increase in molecular length for both the series of ruthenium complexes, with decay constants of βPY=2.07±0.1 nm−1 and βBT=2.16±0.1 nm−1, respectively. The contact resistance of complexes with 2,3-dihydrobenzo[ b]thiophene (BT) anchoring groups is found to be smaller than the contact resistance of ruthenium complexes with pyridine (PY) anchors. DFT calculations support the experimental results and provided additional information on the electronic structure and charge transport properties in those metal|ruthenium complex|metal junctions. [ABSTRACT FROM AUTHOR]

We demonstrate how redox control of intra-molecular quantum interference in phase-coherent molecular wires can be used to enhance the thermopower (Seebeck coefficient) S and thermoelectric figure of merit ZT of single molecules attached to nanogap electrodes. Using first principles theory, we study the thermoelectric properties of a family of nine molecules, which consist of dithiol-terminated oligo (phenylene-ethynylenes) (OPEs) containing various central units. Uniquely, one molecule of this family possesses a conjugated acene-based central backbone attached via triple bonds to terminal sulfur atoms bound to gold electrodes and incorporates a fully conjugated hydroquinonecentral unit. We demonstrate that both S and the electronic contribution ZelT to the figure of merit ZT can be dramatically enhanced by oxidizing the hydroquinone to yield a second molecule, which possesses a cross-conjugated anthraquinone central unit. This enhancement originates from the conversion of the pi-conjugation in the former to cross-conjugation in the latter, which promotes the appearance of a sharp anti-resonance at the Fermi energy. Comparison with thermoelectric properties of the remaining seven conjugated molecules demonstrates that such large values of S and ZelT are unprecedented. We also evaluate the phonon contribution to the thermal conductance, which allows us to compute the full figure of merit ZT�=�ZelT/(1�+ κp/κel), where κp is the phonon contribution to the thermal conductance and κel is the electronic contribution. For unstructured gold electrodes, κp/κel�≫⃒�1 and therefore strategies to reduce κp are needed to realize the highest possible figure of merit. [ABSTRACT FROM AUTHOR]

In the second column of this table, the single-molecule conductances for compounds B 3 b and B 5 b were incorrect (but the values plotted in Figure 3 g were correct). The authors of this Communication have noticed a mistake in Table 1. [Extracted from the article]

The authors of this Communication have noticed a mistake in Table 1. In the second column of this table, the single-molecule conductances for compounds B 3 b and B 5 b were incorrect (but the values plotted in Figure 3 g were correct). [Extracted from the article]

Zener interband tunneling through a potential profile induced by two top gates in the plane of the two-dimensional electron gas (2DEG) of a doped narrow-gap quantum well system is studied when the gates are biased differently. In contrast to the case of resonant transport through a potential profile created by a single gate these resonances exhibit only a weak dependence on temperature. We present results for a system based on an InSb quantum well and show that narrow resonances are maintained up to room temperature. Such narrow resonances may be exploited for voltage sensing at elevated temperatures. [ABSTRACT FROM AUTHOR]

A convenient two-step, one-pot synthesis of oligo(arylene–ethynylene) (OAE) type molecular wires in yields of up to 70% via in situ desilylation of protected bis(alkynes) Me3SiC≡CArC≡CSiMe3 (Ar = 2,5-thienyl, 1,4-naphthylene, 9,10-anthrylene) and subsequent Sonogashira cross-coupling with S -(4-iodophenyl) ethanethiolate, 4-iodothioanisole, or 5-bromo-3,3-dimethyl-2,3-dihydrobenzo[ b ]thiophene is described. The in situ desilylation avoids the manipulation of the sensitive terminal dialkynes (HC≡CArC≡CH), whilst the general approach presented has some advantages over alternative synthetic strategies based on coupling of aryl dihalides (XArX) by avoiding the multi-step preparation and purification of the terminal alkynes S -(4-ethynylphenyl) ethanethiolate, 4-ethynylthioanisole and 5-ethynyl 3,3-dimethyl-2,3-dihydrobenzo[ b ]thiophene. The molecular conductance of the resulting thiolate or thioether functionalised OAE molecular wires has been determined using scanning tunneling microscope break junction (STM-BJ) methods. The trends in molecular conductance do not track simply with the degree of aromaticity of the molecular core despite the rather similar molecular lengths. Rather, the STM-BJ data are better correlated with the nature of the anchor group, highlighting the important role of electrode–molecule coupling on electron transport in a molecular junction. The experimental conductance data are in good agreement with recently described quantum circuit rules, further highlighting the potential for these relationships to be used as predictive tools in molecular electronics research. A two-step, one-pot synthesis of oligo(arylene–ethynylene) molecular wires that avoids manipulation of sensitive terminal bis(alkynes) is described. Trends in molecular conductance do not track with the molecular core aromaticity, but correlate with the recently described quantum circuit rules and associated bridge and anchor parameters. [ABSTRACT FROM AUTHOR]

The charge transport characteristics of 11 tailor-made dithiol-terminated oligo(phenylene-ethynylene) (OPE)-type molecules attached to two gold electrodes were studied at a solid/liquid interface in a combined approach using an STM break junction (STM-BJ) and a mechanically controlled break junction (MCBJ) setup. We designed and characterized 11 structurally distinct dithiol-terminated OPE-type molecules with varied length and HOMO/LUMO energy. Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single-junction conductance of the linearly conjugate acenes. The experimental data and simulations suggest a nonresonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant β = 3.4 ± 0.1 nm-1 and a contact resistance Rc = 40 kΩ per Au-S bond. The introduction of a cross-conjugated anthraquinone or a dihydroanthracene central unit results in lower conductance values, which are attributed to a destructive quantum interference phenomenon for the former and a broken π-conjugation for the latter. The statistical analysis of conductance-distance and current-voltage traces revealed details of evolution and breaking of molecular junctions. In particular, we explored the effect of stretching rate and junction stability. We compare our experimental results with DFT calculations using the ab initio code SMEAGOL and discuss how the structure of the molecular wires affects the conductance values. [ABSTRACT FROM AUTHOR]

We report the electrical conductance at the single molecule level of the oligoyne molecular wires Py-(C≡C)n-Py (n = 1, 2 and 4; Py = 4-pyridyl) using STM-molecular break junction techniques in AulmoleculelAu configurations. The conductance histograms reveal multiple series of peaks attributed to differing contact geometries between the pyridyl head groups and the gold electrodes. Both experimental and theoretical evidence point to the higher conduction groups being related to adsorption of the pyridyl group at more highly coordinated sites such as step edges or alongside gold adatoms. All three conduction groups in the oligoyne series show a remarkably low β value of (0.06 ± 0.03) Å-1, that is, the conductance is almost independent of molecular length. 4,4'-Bipyridyl studied under the same conditions does not follow this exponential decay series. Theoretical calculations using a combination of density functional theory and nonequilibrium Green's function formalism support the experimental results. We conclude that oligoynes and polyynes are a very promising class of molecular wires for integration into electronic circuitry. [ABSTRACT FROM AUTHOR]