Role of core-shell energetics on anti-Mackay, chiral stacking in AgCu nanoalloys and thermally induced transition to chiral stacking

Abstract

Role of core-shell energetics on anti-Mackay, chiral stacking in Agcu nanoalloys and thermally induced transition to chiral stacking Manoj Settem & Anand K. Kanjarla in Agcu nanoalloys a size-dependent transition to the chiral stacking from the anti-Mackay stacking has been predicted previously. this trend is explained by considering the interplay between the core-shell energetics. Results indicate that the energy changes in the Ag shell alone is not sufficient to explain the stability of the chiral stacking and the energy changes in the cu core also need to be considered. in addition to this, thermally induced transition to chiral stacking was observed at sizes where anti-Mackay stacking is energetically favourable. on transition to the chiral stacking, the Ag-Ag, Ag-cu and Cu-Cu bond lengths change significantly. These observations are also applicable for AgCu nanoalloys with incomplete Ag shells. Bimetallic nanoalloy systems exhibit novel properties due to synergistic effects between the constituent metals. Bimetallic nanoclusters and nanoparticles have been extensively studied as they afford the opportunity to tune their chemical and physical properties through composition and chemical ordering 1-3. Along with the geometrical structure of a bimetallic nanocluster, due to the presence of two elements, different types of chemical ordering are possible: phase segregated and mixed. Depending on the bimetallic system, phase segregation can manifest as core-shell structure 4,5 where one of the constituent element covers the other completely or two phases joined at an interface, usually referred to as "Janus" type arrangemen 6-8. Similarly, mixed chemical ordering can be ordered (as is the case for bimetallic systems that form intermetallic phases) 9 or disordered 10. It is essential to know the optimal structures as a function of the size and the composition in order to understand the properties of bimetallic nanoclusters. Finding the optimal structures involves exploring the potential energy surface (PES) 11. Typically, methods based on genetic algorithm 12 or Monte Carlo combined with energy minimization commonly referred to as "basin hopping" Monte Carlo 13 are used to explore the PES and find the optimal structures using empirical atom-atom potentials such as the second moment approximation to tight binding (SMATB) 14-17 potential or the embedded atom method (EAM) 18 potential. Finally, the putative global minima and the lowest lying isomers belonging to the different structural motifs are singled out for relaxation using density functional theory (DFT). This provides a database of optimal structures which can be used to compare with the experimentally synthesized bimetallic nanoclusters. Depending on the constituent atoms in a bimetallic nanoalloy, specific structural motifs can become energetically favourable. The anti-Mackay icosahedron becomes stable in alloy systems which have a tendency to phase segregate and have atomic size mismatch combined with low surface energy for the larger atoms 19-21. AgCu, AgNi...

Decahedra and Icosahedra Everywhere: The Anomalous Crystallization of Au and Other Metals at the Nanoscale

“…In the case of icosahedral particles, an additional feature is observed in the layer stacking. As predicted by Settem and Kanjarla and Nelli and Ferrando [54,55] the surface is reconstructed showing anti-Mackay stacking. We have systematically observed that in icosahedral particles the last few layers show a different stacking as shown in Figure 8.…”

Simulation and analysis of melting behavior of local atomic structure of refractory metals vanadium

“…振动 [24] 及之后液滴的形成 [25] 等, 对其中团簇的研 究也主要侧重于结构稳定性 [26,27] 、化学序 [28,29] 、热 力学 [30,31] 与动力学行为 [32] 、团簇结构竞争 [33] 与团 簇相转变 [33,34] 、不同初始构型 [35] 及不同几何尺寸 [36,37] 对团簇熔点的影响 [38,39] 、金属小团簇熔点与其块体 材料熔点的异同 [40,41] (包括团簇原子数太少而难以 熔化的机理解释 [42] )、核-壳 [43] 二元反对称Mackay 二十面体与扶手型结构之间的转变 [44,45] , 以及近两 年新兴起的用机器学习 [46] 来研究和识别纳米颗粒 熔化过程中的原子分布与竞争关系等方面. 而对熔 化过程中特征原子团簇的结构演变、分布以及不同 熔化速率对微观原子结构的影响涉及较少, 一些具 体科学问题仍不清楚 [2,47] , 且针对原子团簇的相关 程序LAMMPS [48] 来模拟其熔化过程.…”