Quantum interference effect of single-molecule conductance influenced by insertion of different alkyl length

Phenol is a pH-activated linker to gold: a single molecule conductance study

Single molecule conductance measurements typically rely on functional linker groups to anchor the molecule to the conductive electrodes through a donor-acceptor or covalent bond. While many linking moieties, such as thiols, amines, thiothers and phosphines have been used among others, very few involve oxygen binding directly to gold electrodes. Here, we report successful single molecule conductance measurements using hydroxy (OH)-containing phenol linkers and show that the molecule-gold attachment and electron transport are mediated by a direct O-Au bond. We find that deprotonation of the hydroxy moiety is necessary for metal-molecule binding to proceed, so that junction formation can be activated through pH control. Electronic structure and DFT+Σ transport calculations confirm our experimental findings that phenolate-terminated alkanes can anchor on the gold and show charge transport trends consistent with prior observations of alkane conductance with other linker groups. Critically, the deprotonated O--Au binding shows features similar to the thiolate-Au bond, but without the junction disruption caused by intercalation of sulfur into electrode tips often observed with thiol-terminated molecules. By comparing the conductance and binding features of O-Au and S-Au bonds, this study provides insight into the aspects of Au-linker bonding that promote reproducible and robust single molecule junction measurements.

Substituent-mediated quantum interference toward a giant single-molecule conductance variation

Quantum interference (QI) in single molecular junctions shows a promising perspective for realizing conceptual nanoelectronics. However, controlling and modulating the QI remains a big challenge. Herein, two-type substituents at different positions ofmeta-linked benzene, namely electron-donating methoxy (-OMe) and electron-withdrawing nitryl (-NO₂), are designed and synthesized to investigate the substituent effects on QI. The calculated transmission coefficientsT(E) indicates that -OMe and -NO₂could remove the antiresonance and destructive quantum interference (DQI)-induced transmission dips at position 2. -OMe could raise the antiresonance energy at position 4 while -NO₂groups removes the DQI features. For substituents at position 5, both of them are nonactive for tuning QI. The conductance measurements by scanning tunneling microscopy break junction show a good agreement with the theoretical prediction. More than two order of magnitude single-molecule conductance on/off ratio could be achieved at the different positions of -NO₂substituent groups at room temperature. The present work proves chemical substituents can be used for tuning QI features in single molecular junctions, which provides a feasible way toward realization of high-performance molecular devices.

Controlling Contact Configuration of Carboxylic Acid-Based Molecular Junctions Through Side Group

In this paper, the contact configuration of single molecular junction is controlled through side group, which is explored by electrochemical jump-to-contact STM break junction. The conductance values of 2-methoxy-1,3-benzenedicarboxylic acid (2-M-1,3-BDC) is around 10-3.65 G0, which is different from that of 5-methoxy-1,3-benzenedicarboxylic acid (5-M-1,3-BDC) with 10-3.20 G0. Interestingly, the conductance value of 2-M-1,3-BDC is the same as that of 1,3-benzenedicarboxaldehyde (1,3-BDCA), while single molecular junctions of 5-M-1,3-BDC and 1,3-benzenedicarboxylic acid (1,3-BDC) give out similar conductance value. Since 1,3-BDCA binds to the Cu electrode through one oxygen atom, the dominated contact configuration for 1,3-BDC is through two oxygen atoms. The different conductance values between 2-M-1,3-BDC and 5-M-1,3-BDC can be attributed to the different contact configurations caused by the position of the side group. The current work provides a feasible way to control the contact configuration between the anchoring group and the electrode, which may be useful in designing future molecular electronics.

Precise tuning of single molecule conductance in an electrochemical environment

Tuning of molecular conductance in a liquid environment is a hot topic in molecular electronics. In this article, we explore a new concept where the Fermi level positions of the metallic ends are varied simply by modifying the electroactive salt concentration in solution. We rely on the electrochemical scanning tunneling microscope break junction method that allows the construction in solution of copper atomic contacts that can be then bridged by single molecules. The experimental conductance evolution is first confronted with an analytical formulation that allows the deduction of the molecule's LUMO position and electronic coupling factors. These parameters are in close agreement with those obtained by independent DFT calculations.

Gateway state-mediated, long-range tunnelling in molecular wires

If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of high-conductance molecular wires. For a wide variety of molecules conducting by phase coherent tunnelling, conductance G decays with length following the relationship G = Ae^-βL. It is widely accepted that the attenuation coefficient β is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety (phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to E_F through coupling to the bridge moiety. This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode-molecule interface properties.

The Role of Through-Space Interactions in Modulating Constructive and Destructive Interference Effects in Benzene

Quantum interference effects, whether constructive or destructive, are key to predicting and understanding the electrical conductance of single molecules. Here, through theory and experiment, we investigate a family of benzene-like molecules that exhibit both constructive and destructive interference effects arising due to more than one contact between the molecule and each electrode. In particular, we demonstrate that the π-system of meta-coupled benzene can exhibit constructive interference and its para-coupled analog can exhibit destructive interference, and vice versa, depending on the specific through-space interactions. As a peculiarity, this allows a meta-coupled benzene molecule to exhibit higher conductance than a para-coupled benzene. Our results provide design principles for molecular electronic components with high sensitivity to through-space interactions and demonstrate that increasing the number of contacts between the molecule and electrodes can both increase and decrease the conductance.

Detecting Electron Transport of Amino Acids by Using Conductance Measurement

The single molecular conductance of amino acids was measured by a scanning tunneling microscope (STM) break junction. Conductance measurement of alanine gives out two conductance values at 10^−1.85 G₀ (1095 nS) and 10^−3.7 G₀ (15.5 nS), while similar conductance values are also observed for aspartic acid and glutamic acid, which have one more carboxylic acid group compared with alanine. This may show that the backbone of NH₂–C–COOH is the primary means of electron transport in the molecular junction of aspartic acid and glutamic acid. However, NH₂–C–COOH is not the primary means of electron transport in the methionine junction, which may be caused by the strong interaction of the Au–SMe (methyl sulfide) bond for the methionine junction. The current work reveals the important role of the anchoring group in the electron transport in different amino acids junctions.