Melting of core-shell Ag-Ni and Ag-Co nanoclusters studied via molecular dynamics simulations Z. Kuntová, 1,2 G. Rossi, 1 and R. Ferrando 1 1 Dipartimento di Fisica, Università di Genova and CNR/INFM, Via Dodecaneso, 33, 16146 Genova, Italy 2 Institute of Physics, AS CR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic Received 18 March 2008; published 22 May 2008 The melting of binary metallic nanoclusters of Ag-Ni and Ag-Co is studied at magic sizes for the anti- Mackay icosahedron by means of molecular dynamics simulations within a many-body tight-binding potential model. This structure is especially stable for those compositions at which the external shell is completely made of silver, while the inner core is either made of Ni or Co. Our simulations clearly show that melting takes place in two steps. The external one-layer thick Ag shell melts first, while the inner core is still solid, then the whole cluster melts at a temperature that can be considerably higher than the melting temperature of the external shell. The width of the temperature interval in which the shell is melted while the core is still solid strongly depends on the system. DOI: 10.1103/PhysRevB.77.205431 PACS numbers: 36.40.c, 61.46.Bc I. INTRODUCTION Bimetallic nanoclusters, which are often referred to as nanoalloys, have recently received great attention in basic research and applications, 1 for example, in catalysis, optics, and magnetism. In the case of applications in optics and catalysis, core-shell nanoparticles can be of great interest. For example, a monolayer-thick external shell of metal A covering a core of metal B is likely to be highly strained in the case of large atomic size mismatch between species A and B. Strained overlayers can present unusual catalytic properties. 2 Thermal stability is a very important characteristic of nanosystems that aim to be used in applications. In this re- spect, the study of the melting of nanoparticles has recently received great attention, both from the point of view of ex- periment and of theory and/or simulation. 3 The melting of nanoparticles is a complex phenomenon, which presents some important differences with respect to the melting of bulk solids. First of all, the melting of nano- particles does not sharply occur at a precise temperature but somewhat smoothly in a finite temperature range. 4–6 More- over, the melting range is size dependent for a given mate- rial. As a general trend, the melting range of single- component nanoparticles decreases with their size. 5,7,8 For example, the melting temperature of bulk Ag is 1235 K; nevertheless, for small clusters, the melting point decreases. In fact, for octanethiol-capped Ag clusters with sizes of 2–4 nm, melting is in the range of 400 K. 9 The reason for this behavior consists in the increasing proportion of less coordi- nated surface atoms with decreasing size. This causes an average temperature decrease that is proportional to N -1/3 , where N is the number of atoms in the cluster. 7 For small sizes, however, the melting temperature does not depend smoothly on N but can present strong oscillations. A very nice example of this behavior has been found for Na clusters for N from 70 to 200 atoms. The experiments showed 10 that the melting temperatures of clusters such as Na 130 and Na 147 differ by 60 K, although their sizes are similar. The expla- nation is the difference in the structure of the clusters and the existence of superstable magic melters—in this case, the cluster that melts higher is the complete icosahedron Na 147 . The existence of magic melters with completed icosahedral shape is known also for metal clusters, 11 including Ni. 12,13 In the case of binary nanoparticles, the phenomenology is even richer since melting temperature can also depend on composition at fixed sizes. A striking example of this behav- ior was found in the simulation of the melting of icosahedral Ag clusters. 14 After substituting the central Ag atom in a 147-atom icosahedron with either Cu or Ni impurity, the melting temperature increases by 30 or 50 K, respectively. This behavior has to be attributed to the effective strain re- laxation, which takes place in the icosahedral structure when a small impurity occupies the central site. The dependence of the melting temperature on composition at fixed sizes de- pends on the specific features of the system under study, as shown by several simulation results. 1,15–19 The melting of core-shell and multishell clusters is especially interesting and has been studied in different systems by several groups. 15,17,20–24 A very important point is to investigate whether different shells melt at significantly different tem- peratures, 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 MD simulations. Sizes and compositions are chosen in such a way that the global minimum GM structures are highly symmetric: icosahedra with an external anti-Mackay 26 shell, which is completely made of silver, and with inner core ei- ther of Co or Ni. As we shall show in the following, perfect core-shell anti-Mackay icosahedra are the most stable struc- tures for these compositions: Ag 32 Ni 13 , Ag 32 Co 13 , Ag 72 Ni 55 , and Ag 72 Co 55 . The driving forces for obtaining these struc- tures are the lower surface energy of Ag with respect to Ni and Co and the size mismatch, which favors surface posi- tions for the large Ag atoms in icosahedral structures. 15 Size mismatch between Ag and Ni/Co is large: r Ag - r Ni  / r Ag = 0.138 and r Ag - r Co  / r Ag = 0.131. For this reason, the anti- Mackay external shell is favored over the Mackay shell be- cause the former is made of fewer atoms and can thus be better accommodated around the small Ni/Co core. Our simulations are aimed at understanding whether it is possible to produce a liquid layer of monatomic thickness in a signifi- cant temperature range. PHYSICAL REVIEW B 77, 205431 2008 1098-0121/2008/7720/2054318 ©2008 The American Physical Society 205431-1