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Metal nanowires (NWs) are at present receiving a huge attention due to their quantum conductance behavior. NWs may be generated by stretching metal contacts; during the elongation and just before rupture, the conductance shows flat plateaus and abrupt jumps of approximately a conductance quantum. Although, the simplicity of the experimental procedure, the conductance measurements are rather difficult to interpret because both atomic and electronic structure change simultaneously. Also, experiments are frequently performed for a wide range of temperatures (4-300 K) and environment conditions (ambient-UHV); this has lead to the suggestion of rather conflicting models. In this work, we aim to perform a more comprehensive analysis, by studying both the atomistic processes of nanowire elongation and, their influence on the conductance measurements. We present a study of the atomic arrangement of NWs by means of time-resolved atomic resolution transmission electron microscope (HRTEM) and, molecular dynamics simulations; electrical properties were studied with an UHV mechanically controllable break junction (MCBJ). These experiments were performed at room and liquid nitrogen temperatures. HRTEM video recordings have revealed that, at room temperature, metal (Au, Ag, Cu, etc.) junctions generated by tensile deformation are crystalline and free of defects. The neck structure is strongly dependent on the surface properties of the analyzed metal. On the basis of atomic arrangements derived form HRTEM, it can be demonstrated that the quantum conductance behavior is defined by the preferred atomic structures at the narrowest constriction. At low temperatures, significant changes of the NW structural behavior is observed, with the generation of extended defects, mainly stacking faults and twins inside the NWs. This induces important modifications of the NW conductance behavior as a function of stretching. We have also studied the atomistic mechanisms associated with the elongation and rupture of bimetallic (Au-Ag) nanocontacts at room temperature. The narrowest neck of the stretched alloy NWs become gold enriched during the elongation even when gold is present only in small amounts. Suspended atomic chains containing atom of different species can be generated.
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