The authors have studied the conductance of a 1,4-disubstituted isocyanide͑-NC͒ or thiol͑-SH͒ benzene molecule anchored to two Pt electrodes. A single molecular junction showing a well-defined conductance value ͑ϳ3 ϫ 10 −2 G 0 , G 0 =2e 2 / h͒ was fabricated with the Pt electrodes. The conductance of the molecular junction was one order higher than the previously documented value using Au electrodes. These observations could be explained by differences in the local density of states of the contact metal atom at the Fermi level and the extent of the hybridization and energy difference between the molecular and metal orbitals. Further insight into the binding strengths of the metal-anchoring group bond was obtained by statistically analyzing the stretching length of the molecular junction.
We investigated the single 1,4-benzenediamine molecule bridging between Au or Pt electrodes. The conductances of the molecular junctions with the Au−NH2 and Pt−NH2 bonds (Au−NH2 and Pt−NH2 molecular junctions) were 1 × 10−2
G
0 (2e
2/h) and 5 × 10−3
G
0, respectively. The stretching lengths of the Au−NH2 and Pt−NH2 molecular junctions were 0.03 and 0.07 nm, respectively. The conductance value of the Au−NH2 molecular junction was unexpectedly larger than the value evaluated with the density of states of the metal electrodes and the molecule−metal bond strength, which have been discussed before. The large conductance value could be explained by the small energy difference between metal and molecular orbitals (ΔE) and the high degree of π-conjugation (P) of the Au−NH2 molecular junction, which would be unique characteristics of the Au−NH2 bond. The present study showed the importance of these two factors (ΔE, P) in studying the conductance of the single molecular junction.
We have mechanically fabricated Au, Ag, and Cu nanocontacts in solution under electrochemical potential control. At the hydrogen evolution potential, fractional conductance peaks appeared near 0.5 G(0) (G(0) = 2e(2)/h) in the conductance histogram of Au and Cu. This fractional conductance peak was not observed in the conductance histogram of Ag. In the case of Au nanocontacts in 50 mM H(2)SO(4) solution, a 0.1 G(0) peak appeared in the conductance histogram, as well as the 0.5 G(0) peak. The origin of the fractional conductance peak and its metal dependence are discussed based on previously reported values of metal-hydrogen binding energy, which was estimated by the exchange current density for the hydrogen evolution reaction.
We have mechanically fabricated Cu nanowires in solutions with and without thiourea. In both solutions, the conductance was quantized in units of G 0 ( ). A well-defined 1 G h e / 2 2 0 peak was observed in the conductance histogram. While the conductance value was not changed, the stability of the mono atomic contact was improved by adding thiourea. The effect of thiourea on the stability of the atomic contact was investigated by measuring the stretched length of the atomic contact. The average length of the last plateau was 0.044 nm in the solution without thiourea. The length increased to 0.076 nm in the solution with thiourea. The stabilization could be explained by the decrease in the surface energy caused by adsorption of thiourea molecules on the Cu nanowires.
We have mechanically fabricated Ni and Cu nano constrictions in solution, and studied their electrical conductance under the electrochemical potential control. Conductance quantization can be observed with both metals. This is the first observation of the conductance quantization behavior for non-gold metal nano constrictions, which are mechanically fabricated in solution at room temperature. The conductance of Cu was quantized in units of G0 (=2e2/h), and a sharp 1 G0 peak is observed in the conductance histogram. For Ni, a conductance plateau showed a slightly negative slope, and a broad peak at 1~1.5 G0 was observed in the histogram. The conductance quantization behavior was discussed by comparing the result of nano constrictions fabricated in non-solution condition, with those fabricated by an electrochemical method. It is suggested that a certain atomic configuration was stabilized in solution under the electrochemical potential control
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