In this Article, the single molecular junctions of thiophene-based molecules are measured by scanning tunneling microscopy break junction (STM-BJ) in ambient air. While the benzo­[1,2-b:4,5-b′]­dithiophene (1) and 2,6-dibromobenzo­[1,2-b:4,5-b′]­dithiophene (2) give out histograms with well-defined peaks corresponding to the molecular junction, thieno­[3,2-b]­thiophene (3) does not show a peak for the molecular junction. The disappearance of peak may contribute to the weak interaction between thiophene and Au. DFT calculation results demonstrate the weak interaction for 3 contacting Au as compared to 1 and 2. Interestingly, the step probability for molecular junction follows the order of binding interaction between thiophene and Au showing 1 > 2 > 3. The current results reveal the influence of molecular structure on contact interaction between anchoring group and electrode, which can help to understand the electron transport of the single molecular junction.

One key issue for the development of molecular electronic devices is to understand the electron transport of single-molecule junctions. In this work, we explore the electron transport of iodine-terminated alkane single molecular junctions using the scanning tunneling microscope-based break junction approach. The result shows that the conductance decreases exponentially with the increase of molecular length with a decay constant βN = 0.5 per –CH2 (or 4 nm−1). Importantly, the tunneling decay of those molecular junctions is much lower than that of alkane molecules with thiol, amine, and carboxylic acid as the anchoring groups and even comparable to that of the conjugated oligophenyl molecules. The low tunneling decay is attributed to the small barrier height between iodine-terminated alkane molecule and Au, which is well supported by DFT calculations. The work suggests that the tunneling decay can be effectively tuned by the anchoring group, which may guide the manufacturing of molecular wires.Electronic supplementary materialThe online version of this article (10.1186/s11671-018-2528-z) contains supplementary material, which is available to authorized users.

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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.

We have measured the alkane and benzene-based molecules with aldehyde and carboxylic acid as anchoring groups by using the electrochemical jump-to-contact scanning tunneling microscopy break junction (ECSTM-BJ) approach. The results show that molecule with benzene backbone has better peak shape and intensity than those with alkane backbone. Typically, high junction formation probability for same anchoring group (aldehyde and carboxylic acid) with benzene backbone is found, which contributes to the stronger attractive interaction between Cu and molecules with benzene backbone. The present work shows the import role of backbone in junction, which can guide the design molecule to form effective junction for studying molecular electronics.