Structural and thermal behavior of compact core-shell nanoparticles: Core instabilities and dynamic contributions to surface thermal stability

Abstract

An orbital-free density-functional-theory molecular dynamics technique is applied to investigate the minimum-energy structure and meltinglike transition of Cs 55 , Li 13 Na 32 Cs 42 , and Li 55 Cs 42 nanoparticles. Icosahedral packing is found to be optimal for homogeneous Cs 55 , as expected. Heterogeneous particles show a complete segregation of Cs atoms to the cluster surface, and form perfect core-shell structures, that is, structures where each atomic species occupies and completes a different concentric atomic shell. For Li 13 Na 32 Cs 42 , the size mismatch between atomic species forming different shells leads to polyicosahedral packing. For Li 55 Cs 42 , however, the size mismatch is huge and perfect polyicosahedral ordering is frustrated, resulting in more complex structural behavior. The three clusters investigated share the same surface shell, formed by 42 Cs atoms, and comparison of their melting behaviors helps to rationalize the increased thermal stability of the cluster surface upon alloying. Cs 55 melts homogeneously at approximately 85 K. Both Li 13 Na 32 Cs 42 and Li 55 Cs 42 show a substantial thermal stability, compared to Cs 55 and other alloy compositions where a perfect core-shell structure does not appear. We demonstrate that an important contribution to this increased thermal stability in the nanoalloys comes from the large difference in the atomic masses of the constituent particles, which results in a poor coupling of atomic vibrations along the radial direction. We also give arguments to show that the meltinglike transition in these clusters is triggered by the thermal instability of interior rather than surface atoms. Segregation of Cs atoms to the cluster surface is fully maintained in the liquid state, so that core and surface shells form two inmiscible liquid layers.

Structure determination in 55-atom Li–Na and Na–K nanoalloys

“…Hristova et al 24 reported the GM structures of equiatomic K-Cs and Rb-Cs nanoalloys at the EP level. They observed a preference for p-Ih packing and core-shell segregation, in agreement with results by Aguado et al [12][13][14][15] for other alkali nanoalloys. A goal of the present study is to show that EP results are not quantitative for these systems as quantum electron shell effects play an important role.…”

Thermodynamics of nanoalloys

“…Although scarce in comparison with semiempirical approaches, first-principle modelings with an explicit account for electronic structure have been achieved by Aguado and coworkers, notably for aluminum-doped clusters 35 but also mixed alkali clusters. 86 Likewise Ojha et al have evaluated the influence of an aluminum impurity on the melting of gallium clusters, 87 which in pure form exhibit higher-than-bulk melting temperature due to covalent contributions to bonding. 36 The first-principles approach is undoubtly more realistic, but also much more demanding than simulations based on explicit potentials.…”

Melting of core-shell Ag-Ni and Ag-Co nanoclusters studied via molecular dynamics simulations

“…1, [15][16][17][18][19] The melting of core-shell and multishell clusters is especially interesting and has been studied in different systems by several groups. 15,17,[20][21][22][23][24] A very important point is to investigate whether different shells melt at significantly different temperatures, as observed also in pure clusters. 3,25 In this paper, we study the melting process of Ag shell Ni core and Ag shell Co core nanoparticles by molecular dynamics ͑MD͒ simulations.…”