The formation of the single benzene molecule junction was investigated for Au and Ag electrodes by conductance measurements and inelastic tunneling electron spectroscopy at 10 K. While a single benzene molecule junction was hardly formed for the Au electrodes, a single benzene molecule junction was formed for the Ag electrodes. The single Ag/benzene/Ag junction showed a fixed conductance value of 0.24 G 0 (G 0 = 2e 2/h), indicating the formation of a well-defined single benzene molecule junction. By comparing with previously reported results for Pt electrodes, in which the single benzene molecule junction showed various conductance values, it was shown that the moderate metal−molecule interaction is favorable to obtain a well-defined single molecule junction.

We have investigated the conductance and atomic structure of the Co atomic contact in a hydrogen atmosphere. While clean Co did not form an atomic wire, the Co atomic contact could be stretched more than 0.4 nm in a hydrogen atmosphere, indicating the formation of the atomic wire. The interaction between hydrogen and the Co atomic wire was investigated with inelastic tunneling spectroscopy (IETS). The vibrational modes between hydrogen and the Co atomic wire were observed in IETS. The length histogram of the last conductance plateau, the shape of the dI/dV curves, the distribution of the vibrational energy, and previously reported theoretical calculation results suggested the formation of the Co atomic wire in which dissociated hydrogen atoms were adsorbed on the wire.

This study investigates the electron transport properties of Au, Ag and Cu atomic contacts in a hydrogen environment at 10 K, by using the mechanically controllable break junction (MCBJ) technique. A sharp 0.2 G 0 (2e 2 /h) peak was observed in the conductance histogram of H 2 /Cu contacts, whereas a low conductance tail (<1 G 0 ) was observed with H 2 /Au contacts. The conductance behavior of the Ag contacts was found not to change by the introduction of hydrogen, thus, indicating a weak interaction between hydrogen and Ag atomic contacts. The atomic configuration of the H 2 /Cu and H 2 /Au contacts was further investigated using differential conductance (dI/dV) spectra and statistical analysis of the length of the atomic contacts. This revealed that a single hydrogen molecule is bridged between the Au electrodes via hydrogen-decorated Au chains, while a single hydrogen molecule bridges the Cu electrodes directly. The atomic configuration of the contacts explained the appearance of a sharp 0.2 G 0 peak in the conductance histogram of the H 2 /Cu contacts.

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We have investigated the conductance and atomic structure of single ethylene and acetylene molecule junctions on the basis of the conductance measurement and vibration spectroscopy of the single molecule junction. Single molecule junctions have a conductance comparable to that of metal atomic junctions (around 0.9G 0 : G 0 = 2e 2 /h) due to effective hybridization between metal and the π molecular orbital. The ethylene molecules are bound to Pt electrodes via a di-σ bond, while the acetylene molecules are bound to Pt electrodes via di-σ and π bonds. By using the highly conductive single molecule junctions, we investigated the characteristics of vibration spectroscopy of the single molecule junction in an intermediate regime between tunneling and contact. The vibration modes that could modify the conduction orbital were excited for the ethylene and acetylene molecule junctions. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, whereas clear crossover was not observed for the acetylene molecule junction, reflecting the number of conduction orbitals in the single molecule junction.