Single Impurity Effect on the Melting of Nanoclusters

We show by molecular dynamics simulations that the melting temperature of clusters can be tuned by selective doping. In fact, a single Ni or Cu impurity in Ag icosahedral clusters considerably increases the melting temperature even for sizes of more than a hundred atoms. The upward shift is correlated to the strain relaxation induced by a small central impurity in icosahedral clusters.

Figure 1

Snapshots from the melting simulations: pure ${\mathrm{Ag}}_{55}$ (top row), ${\mathrm{Ag}}_{54}{\mathrm{Ni}}_1$ (middle row), and ${\mathrm{Ag}}_{54}{\mathrm{Pd}}_{\mathbf{1}}$ (bottom row). At 600 K, ${\mathrm{Ag}}_{54}{\mathrm{Ni}}_1$ is still icosahedral, while ${\mathrm{Ag}}_{55}$ and ${\mathrm{Ag}}_{54}{\mathrm{Pd}}_1$ are already melted. In ${\mathrm{Ag}}_{54}{\mathrm{Pd}}_1$, the central impurity migrates to the intermediate icosahedral shell before melting. In ${\mathrm{Ag}}_{55}$ at 525 K, a rosette structure is shown, with a distorted hexagonal ring on its surface.

Figure 2

Caloric curves. The quantity plotted is $\delta E=E(T)-E(O)-3(N-1)k_{B}T$, where $E(T)$ is the total cluster energy, $E(0)$ is the minimum energy at 0 K, and $3(N-1)k_{B}T$ is the harmonic part of the energy. In the upper panel, size 55 is considered and all systems are shown. In the lower panel, size 147 is considered and only pure Ag, Ag-Cu, and Ag-Ni are shown. Crosses (+) refer to pure Ag, solid circles (●) to Ag-Ni, squares (□) to Ag-Cu, diamonds (◇) to Ag-Pd, and asterisks (*) to Ag-Au.

Figure 3

Melting temperatures and melting ranges for pure Ag clusters (crosses) and clusters with Ni impurity (dots). The error bars indicate the amplitudes of the melting ranges, and the symbols its center. A linear behavior of the melting temperature with $N^{-1/3}$ (namely, with the inverse cluster radius) is found, in qualitative agreement with Pawlow’s law.

Figure 4

Atomic stress on the central site ${\sigma }_{\mathrm{core}}$ and impurity solution energy $\Delta E_{\mathrm{imp}}$ for icosahedral clusters of different sizes. Symbols are the same as in Fig. 2. The impurity solution energy in bulk phases [34] is reported.