Conductance Measurement Of Metal Atomic-Size Wires And Non-Gold Molecular Junctions And Their Gating By Electrochemical Approach

Metal atomic-size wires and metal-molecule-metal junctions exhibit novel quantum transport properties, such as the conductance of metal atomic-size wires displaying quantum effects, and metal-molecule-metal junctions exhibiting electrical properties of amplification, rectification, switch and negative differential resistance, they may be used in future nano-electronics and molecular electronic devices. The conductance of the metal atomic-size wires and single molecular junction depends not only on the intrinsic properties of the metal, but also on the surrounding environment, and the intrinsic properties of the molecular and contact geometries of the molecule-electrode contact can also influence the conductance of molecular junctions. The traditional scanning tunneling microscope break junction (STM-BJ) methods are usually only suitable to construct and study the atomic-size wire of Au, which has single channel with stable transmission at ambient temperature and pressure, and single molecular junctions with Au electrode. The limitation of the methods restricts the investigation of conductance of transitional metal atomic-size wire and single molecular junction with non-Au electrode. Here, the single molecule conductance of alkanedicarboxylic acid with different length binding to Cu and Ag electrodes was sytematically studied, and the conductance of Cd atomic-size wire has been measured and can be changed by the applying different potentials of electrodes, then the electrochemical gating of Cd-terephthalic acid-Cd was also realized. The main works are as follows:1. The single molecule conductance of alkanedicarboxylic acid (HOOC-(CH2)n-COOH,n=1-5) binding to Cu and Ag electrodes is systematically studied by using the electrochemical jump-to-contact scanning tunneling microscopy break junction approach (ECSTM-BJ). The results show that the conductance depends on molecular length and the electrode materials, which give a decay constant βN of0.95±0.02per (-CH2) unit for Cu electrodes and0.71±0.03for Ag electrodes. The contact conductance shows the order of Gn=0,Cu>Gn=0,Ag-These differences can be attributed to the different electronic coupling efficiencies between molecules and electrodes. The conductance of ultrashort molecular junctions is also studied using oxalic acid as the target molecule, the results revealing that the through-space mechanism (TS) should be considered when the distance between two electrodes is very short. The present work demonstrates that electrode materials play an important role on the molecular conductance, contact conductance, and also the tunneling decay constant.2. The conductance of the soft metal Cd was measured and tuned under the electrochemical conditions. Firstly, the conductance of Cd atomic-size wires can be changed from0.4Go to0.89Go, while the substrate potential varying from15mV to60mV with fixed bias; secondly, with the fixed tip potentials, the conductance of Cd atomic-size wires is0.45Go at the bias of25mV, while it changes to0.94Go at the bias of225mV. The change conductance of atomic-size wires may caused by the different eigenchannels and transmission probability at different potentials.3. The conductance of Cd-molecule junctions were studied in electrochemical environment. The single molecule conductance of Cd-terephthalic acid-Cd molecule depends on applying potential:the high conductance value changes from220nS to340nS while the low conductance value changes from55nS to120nS through increasing potential of40mV (From35mV to75mV). However, the dependendence between the conductance of Cd-succinic acid-Cd and electrode potential is not foud which indicates that the change of coupling between anchoring group and electrodes uponing the increasing potential can be exclude; Similarly, the single molecule conductance of Au-terephthalic acid-Au almost keeps stable when the substrate potential changes from-400mV to400mV. The above show that the Cd plays an important role in the electrochemical gating of conductance, which may be explained by the change of electronic coupling efficiencies between molecules and electrodes.