| Metal-molecule-metal junctions have attracted wide attention for developing single molecule-based electronics. They are the key prerequisite for the ultimate realization of molecular electronic devices to take place of traditional silicon-based solid state electronics. Typically, the charge transport of single-molecule junctions can be influenced by molecular structure, contact geometry, anchoring groups and electrodes. Carboxylic acids are one of the famous target molecules that are widely studied in molecular conductanc measurement. In particular, systematic studies on the conductance measurements of homologous series of molecular junctions can contrubite to further understanding of charge transport at molecular level, which can propose an efficient guide and a feasible way to design high performable molecular electronics. Therefore, we apply electrochemical STM-BJ technique to measure the conductance of alkanedicarboxylic acids and its derivatives with more alkyl chains using Pd, Ag, and Cu electrodes. The main work is as follows:1. We measured the single-molecule conductance of alkanedicarboxylic acid (HOOC-(CH2)n-COOH, n=1-5) with Pd nanoclusters by electrochemical jump-to-contact STM-break junction. They gave out decay constant βN of 0.652 per (-CH2) unit and contact conductance of 97 nS. The single-molecule conductance of carboxylic acid binding to bulk metal electrode was also studied, and has almost the same value as that binding to Pd nanoclusters. The conductance of Pd-oxalic acid junctions (130 nS) shows that the electron transfers through the ultrashort molecular junctions is not only contributed by the "through-bond" (TB) mechanism but also "through-space" (TS) mechanism. Our work provides a new path to study the single-molecule conductance with Pd contacts.2. The systematic studies on different tunneling decay constant βN of alkanedicarboxylic acids depending on the metal electrodes (Pd, Ag, Cu) in various surroundings are discussed. The conductance of alkanedicarboxylic acids (HOOC-(CH2)n-COOH, n=2-4) contacting to Pd and Ag were systematically measured by the STM break junction (STM-BJ) in air. Similar tunneling decay constant βN about 1 per-CH2 was found for both metals, which might arise from the Fermi energy of electrodes pinning with the energy positions of molecular states under ambient atmosphere. However, the pin effect can be destroyed by potential control in electrochemistry, and the βN is determined by the alignment of the molecular energy levels relative to the Fermi energy level of the electrodes. According to-4.44 eV is experientially considered as the Fermi energy of the hydrogen reference electrode under standard conditions (SHE), the approximate barrier height Φ (Φ= EF-EHOMO) for Pd, Ag and Cu are 1.22 eV,1.32 eV and 1.63 eV, respectively, which well explains the order of βN,Pd<βN,Ag<βN,Cu in electrochemistry. The current work shows the important role of the surroundings in electron transport through molecular junction.3. We take 1,4-cyclohexanedicarboxylic acid and bicyclo[2.2.2]octane-1,4-dicarboxylic acid as target molecues, which naturally build different parallel tunnel paths inside the single molecular structure. The conductance measurement is carried out by electrochemical STM break junction approach with Ag, Cu electrodes. The conductance ratio between 1,4-cyclohexanedicarboxylic acid and 1,6-hexanedicarboxylic acid is 2.4 for Ag and 2.5 for Cu electrodes. This may be contributed to the constructive quantum interference in molecule with two parallel chains. However, the conductance value of bicyclo [2.2.2]octane-1,4-dicarboxylic acid with three parallel alkyl chains is smaller than that of 1,4-cyclohexanedicarboxylic acid, which may arise from the destructive quantum interference. The current work reveals the interference effect in the alkyl molecule, and proposes a feasible way to design high performable molecule wires.4. The conductance of f-TPE-PPy and l-TPE-PPy contacting to Au measured by traditional STM-BJ show almost the same value 1.5 nS; similarly, the conductance of f-TPE-PEEPy and l-TPE-PEPy are also about 0.5 nS. Those two folded molecules with a pair of biphenyls backbone can generate π-π stacking interaction. The experimental and calculation results demonstrate apparent through-space conjugation appearing in the inter-ring region of two π-stacked biphenyls which acts as an efficient conducting channel in a single-molecule junction and offsets the lower through-bond conjugation.The conductance of folded molecules, measured by the STM break-junction technique, is comparable to that of better conjugated linear counterparts. The folded molecules that combine through-bond and through-space conjugation are expected to be of a broad usage in novel single-molecule wires with multichannel conductance. |
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