The Melting And Breaking Of Nanobridges

We have also investigated the properties of nanobridges as the temperature is raised. The initial structures of the nanobridges were the structures obtained from Oscillations of Nanobridges and were equilibrated at 300 K. Each system was heated by uniformly scaling the atomic velocities. The MD runs of 20,000 time steps were performed with each temperature step from 300 K. Therefore the average temperature ascent rate is 0.1 K/psec. We monitored the internal energy as a function of temperature.

The caloric curves for some of the ultrathin Cu nanobridges in Table 1 are shown in Fig. 4. To compare nanobridges with semi-infinite nanowires, Fig. 4 includes the caloric curve of the CMS 16-11-6-1 applied to the PBC from Ref.[35]. The caloric curve of the CMS 16-11-6-1 nanowire close to the one-dimensional system is in the regime of the pseudofirst-order transitions and the jump in the caloric curve is apparent. The slope of the caloric curve of the CMS 16-11-6-1 corresponds to the Dulong-Petit specific heat.[35] The Mpoint and Bpoint indicate the temperatures of pronounced upward and downward points in the caloric curve, which are related to the melting and the breaking of the nanowire. The downward curvature in the caloric curve has not been observed until now. The caloric curve of the CMS 16-11-6-1 nano-wires is divided into five regions. The first region ranges below the Mpoint, where the ultrathin nanowire is solid. The second region is the Mpoint at which the caloric curve exhibits an upward curvature, where the specific heat markedly increases at the beginning of

Fig. 4 Caloric curves of some of nanobridges and the CMS 16-11-6-1 nanowire. Mpoint and Bpoint indicate temperatures of upward and downward points in the caloric curve, respectively.

the surface melting. This upward curvature is associated with the loss of the solid rigidity of the nanowire. The third region is between the Mpoint and the Bpoint, where the ultrathin nanowire is in the melting state, and the slope is the same as that in the first region. The fourth region is the Bpoint at which the ultrathin nanowire is broken and then the spherical cluster is formed. In the last region, a spherical cluster has been maintained in the MD simulation applied to the PBC. The slope in the caloric curve of the CMS 16-11-6-1 nanowire is always the same as those in the first, third, and fifth regions. Thus properties of the caloric curve of nanowires are in excellent agreement with previous works on nanowires[22,23,35] and nanoclusters.[36] The melting temperatures of the ultrathin Cu nanowires and nanobridges are much lower than the bulk melting temperature.

The properties of the caloric curves of nanobridges are generally similar to those of nanowires or nanoclusters.[22,23,35,36] However, the caloric curves of nanobridges are different from those of nanowires for the slopes of the caloric curves. The caloric curves of nanobridges make it difficult to define both the Mpoint and the Bpoint. The difference of the slopes in the caloric curves is because of the supporting layers. As investigated in Oscillations of Nanobridges, the attractive force between the supporting layer and the nanobridge could maintain the nanobridges. Therefore the caloric curves of the nanobridges include effects of structure dependence related to the tension on the nanobridge as well as temperature dependence. In the

Table 3 The Mpoint and Bpoint of Nanobridges Mpoint (K)

Bpoint (K)

Table 3 The Mpoint and Bpoint of Nanobridges Mpoint (K)

Bpoint (K)

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