Symmetry of Single Hydrogen Molecular Junction with Au, Ag, and Cu Electrodes

We present a charge-transport study on a H-2 molecule wired to Au, Ag, and Cu electrodes using the mechanically controllable break junction technique at 10 K. Formation of molecular junctions with a H-2 molecule bound between the metal electrodes was identified by measuring the vibrational modes of the H-2 molecule using inelastic electron tunneling spectroscopy. The H-2 molecule exhibited a preferential conductance value of 0.3G(0) (G(0) = 2e(2)/h) for Cu electrodes, whereas it displayed variable conductance values below 1G(0) for Au and Ag electrodes. The geometrical symmetry of the molecular junctions was characterized in terms of the current versus bias voltage (I-V) characteristics of the junctions, in which H-2/Cu junctions and H-2/Au, H-2/Ag junctions showed asymmetric and symmetric shapes, respectively, at positive and negative bias voltages. The shape of the I-V curve reflects the geometrical symmetry of the metal electrodes (i.e., symmetry of metal-molecule contacts in molecular junctions). We investigated the breaking process of the metal contacts to obtain information about the geometrical symmetry of the electrodes. The Au and Ag contacts were characterized by the formation and rupture of atomic wires, whereas the Cu contact was suddenly ruptured without atomic-wire formation. The abrupt breaking of the metal contact caused geometrical asymmetry of the Cu electrodes resulting in the asymmetry of the metal-molecule contacts in the molecular junctions.