Physics Letters A 374 (2010) 1769–1772

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Physics Letters A

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Controllable irregular melting induced by atomic segregation in bimetallic clusterswith fabricating different initial configurations

Guojian Li, Tie Liu, Qiang Wang ∗, Xiao Lü, Kai Wang, Jicheng He

Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 November 2009Received in revised form 6 February 2010Accepted 11 February 2010Available online 13 February 2010Communicated by R. Wu

Keywords:MeltingBimetallic clusterMolecular dynamics

The melting process of Co, Co–Cu and Co–Ni clusters with different initial configurations is studiedin molecular dynamics by a general embedded atom method. An irregular melting, at which energydecreases as the temperature increase near the melting point, is found in the onion-like Co–Cu–Coclusters, but not in the mixed Co–Cu and onion-like Co–Ni–Co clusters. From the analysis of atomicdistributions and energy variation, the results indicate the irregular melting is induced by Cu atomicsegregation. Furthermore, this melting can be controlled by doping hetero atoms with different surfaceenergies and controlling their distributions.

© 2010 Elsevier B.V. All rights reserved.

Recently, the phenomena of metallic clusters melting has at-tracted considerable experimental [1,2] and theoretical [3] interestsdue to their extensive applications in catalysts, sensors, micro-electronic and optoelectronic devices [4]. It is more important forbimetallic clusters as their melting maybe tuned by varying theirsizes, compositions and structures [5]. A particular example is thatthe melting temperature of bimetallic clusters has been changed byselectively doping with a single impurity [6]. The melting was de-pendent on the concentrations and sizes of clusters [7]. It was alsofound that the solid phase changes without diffusion induced bythe variation of atomic distribution will result in a sharp decline ofthe temperature-dependent energy curve [8–10]. In addition, at thetemperatures near the melting point of cluster, premelting and dif-fusion become obvious, except for structural evolutions. This mayincrease the possibility to induce a special melting phenomenon.Therefore, a controllable melting of the clusters may be designedby doping hetero atoms with different physical parameters andcontrolling their distributions. It plays a key role in synthesizingnew materials with fascinating potential applications. However, lit-tle attention has been paid to this up to now.

In this Letter, the cuboctahedral clusters, including all 309atoms, were set-up as the objectives. They were truncated froma 30a0 × 30a0 × 30a0 large bulk. This can make the cluster have asimilar lattice structure with its bulk. On the other hand, the simu-lation can be continued without considering the periodic boundarycondition. However, the icosahedral cluster with the good low-

* Corresponding author. Tel.: +86 24 83681726; fax: +86 24 83681758.E-mail address: wangq@mail.neu.edu.cn (Q. Wang).

0375-9601/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.physleta.2010.02.027

energy structure cannot be truncated directly from the bulk. But,in our previous study [10], we found that the cuboctahedral clus-ter can transform to icosahedra at low temperature. Therefore, inorder to obtain the low-energy icosahedral cluster, cuboctahedralclusters were selected as the initial objectives due to the exis-tence of cuboctahedra–icosahedra transformation at low temper-ature. The effect of composition and atomic distribution on themelting of bimetallic clusters was studied. Firstly, in order to definethe influence of compositions on melting, Co, Cu, and Ni elementswere selected to form onion-like Co–Cu–Co and Co–Ni–Co clus-ters. These clusters were constructed with 55 Co atoms in thecore (the 3rd-layer), 92 Cu or Ni atoms in the middle (the 2nd-layer) and 162 Co atoms in the surface (the 1st-layer). Co, Cu,and Ni were chosen on account of the following reasons: (1) Thesurface energy of Cu (1592 mJ m−2) is much lower than that ofCo (2197 mJ m−2) and the energy of Ni (2104 mJ m−2) is sim-ilar to that of Co [11]. From the previous study, we know thatthe surface energy is strongly related to surface orientation. How-ever, any change of atomic situation will induce the distortion oflattice for small clusters. This will lead to difficulties in defin-ing the segregated position during heating process. Therefore, theorientation-dependent surface energy is not distinguished and av-eraged surface energy is used to define the influence of segregationon melting process. Because most of the Cu atoms in the 2nd-layerwill segregate to the Co–Cu surface during the heating process,while only a few Ni atoms will segregate to the Co–Ni surface.This will lead to the difference of energy variations during themelting of clusters and can be used to explore the composition-dependent melting. (2) The small differences in atomic radii be-tween Co (0.1385 nm), Cu (0.1412 nm), and Ni (0.1378 nm) can

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avoid the formation of polyicosahedral structures in the clustersdue to the large lattice mismatch [12,13]. Secondly, the meltingof mixed Co–Cu cluster (Co–Cu), which was obtained by randomlysubstituting 92 Co atoms with 92 Cu atoms, was simulated. Then,its melting was compared with that of Co–Cu–Co to reveal therelationship between the atomic distribution and melting. Finally,the molecular dynamics with a general embedded atom method(EAM) was used to study the melting of all the clusters mentionedabove. The construction and accuracy of this model were describedin our previous papers [7,14]. The melting processes were simu-lated as follows: the clusters were heated up to 1800 K from 100 Kwith 0.2 ns equilibrated simulation time for each temperature step(20 K) and a time step of 1 fs.

In this study, the melting points were defined by using the heatcapacity C p(T ), which was obtained as C p(T ) = d(Energy)/dT [15].The premelting occurs before melting, which could affect the clus-ter surface and lead to the change of heat capacity. The energy

Fig. 1. Curves of the heat capacity C p , the temperature-dependent energy for mixedCo–Cu, the onion-like Co–Cu–Co and Co–Ni–Co clusters, and the fitted energy ofCo–Cu–Co during the heating process.

and heat capacity curves during the heating processes were shownin Fig. 1. There is a sharp increase in the heat capacity curve ofCo–Cu–Co (at 120 K) existing due to the structural transformationfrom cuboctahedron to icosahedron [9]. However, similar cases didnot appear in the clusters of Co–Cu because its structural trans-formations occurred during the relaxation process at 100 K. Thisalso leads to its heat capacity curves being kept unchanged un-til the premelting occurred. The melting points of the Co–Cu–Coand Co–Cu clusters can be obtained from the maximum apparentheat capacity, which values are 1240 K and 1120 K, respectively.Although the same Cu concentrations were used in both clusters,their melting points are different due to the different distributionsof the Cu atoms. Furthermore, their temperature-dependent en-ergy curves were also different and two phenomena can be found:(1) For Co–Cu, the energy increases with the increase in tempera-ture and there is a sharp energy increase at 1120 K which indicatesthat the cluster melts completely. (2) For Co–Cu–Co, after the pre-melting at 1140 K, the energy decreases with the increase in tem-perature until 1220 K and the cluster melts completely at 1240 K.However, by comparing the fitted energy curve of Co–Cu–Co with-out considering premelting and segregation (obtained by fittingthe temperature-dependent energy curve of Co–Cu–Co from 100–1020 K with a third order exponential decay), we found that, theenergy at 1240 K increases as expected, while it was still lessthan the fitted energy at the same temperature. This melting isdifferent from those of general clusters [8,16] and indicates thatan irregular melting occurred in Co–Cu–Co. In the meantime, itcan also be found that the energy of solid Co–Cu is lower thanthat of solid Co–Cu–Co at the same temperature, but that afterthe cluster melting, the energy of Co–Cu is larger than that ofCo–Cu–Co at the same temperature. Since both the concentrationsof Cu in the clusters and the heating processes are same, it is rea-sonable to conclude that the atomic distribution plays a key role inthis melting phenomenon. However, for the Co–Ni–Co cluster, thesharp energy increase is obvious at 1300 K and no similar irreg-ular melting appeared. Since only the composition changes duringthe heating processes of Co–Cu–Co and Co–Ni–Co, the variation of

Fig. 2. Atomic numbers of Co and Cu in different layers of the onion-like Co–Cu–Co and mixed Co–Cu clusters at different temperatures near the melting points. The layer nin the figure indicates to which layer the atom belongs to.

G.J. Li et al. / Physics Letters A 374 (2010) 1769–1772 1771

Table 1Variation of the atomic energy at 0 K when one Cu atom in different positions in the 2nd-layer segregated to different positions in the 1st-layer.

meV (111)-facet (2nd-layer) Edge (2nd-layer) Corner (2nd-layer)

(111)-facet (1st-layer) 0.881 0.911 1.126Edge (1st-layer) 1.229 1.259 1.474Corner (1st-layer) 1.777 1.807 2.022

composition in the 2nd-layer also contributes a lot to this irregularmelting.

Then, the atomic numbers in different layers of the clusterswere used to define the atomic segregation. Since segregationmainly occurs in Co–Cu bimetallic clusters, only Co and Cu wereconsidered here. Fig. 2 shows the atomic numbers of Co and Cu indifferent layers at different temperatures near their melting points.In the cases of both Co–Cu and Co–Cu–Co, the atomic numberof Cu in the 1st-layer increases while that of Co decreases withthe increase in temperature. In the 2nd-layer, the atomic numberof Cu decreases while that of Co increases with the increase intemperature. In addition, only a few Cu atoms migrated into the3rd-layer of Co–Cu–Cu, while no change occurred for the atomicnumber of Co and Cu in the 3rd-layer of Co–Cu. These indicatethat the segregation of Cu atoms was mainly controlled by theatomic migration between the 1st- and 2nd-layers. But the seg-regated atomic numbers of Cu in Co–Cu–Co is much larger thanthose in Co–Cu. This indicates that the irregular melting is stronglyrelated to the atomic segregation. For the Co–Ni–Co cluster, onlyseven atoms segregated to the surface before the cluster melted.And the segregated atomic number was almost unchanged withincreasing temperature.

Generally, the melting can be influenced by the energy varia-tion induced by the atomic segregation. When one Cu or Ni atomin different positions in the 2nd-layer of the cluster segregatedto different positions in the 1st-layer at 0 K, the energy variationper atom will be changed. Therefore, it was used to describe thesegregated energy and explore the relationship between the melt-ing and segregation. The icosahedral clusters with one Cu or Niatom embedded at the positions of the corner, edge and (111)-facet of the 1st- and 2nd-layers were constructed. Then, the av-erage energy per atom of these clusters was obtained by relaxingthe clusters for 0.2 ns at 0 K. The atomic energy of Co–Cu wasgiven in Table 1. It can be seen that the difference in segregatedpositions resulted in different segregated energy. The segregatedenergy is strongly related to the nearest-neighbor atomic numbersbefore/after segregation. However, it is difficult to define the segre-gated position during the heating process in this study. Therefore,it is useful to assume that the chances for segregating to differ-ent positions are similar. The energy variation of segregation peratom can be calculated by averaging the segregated energy. In thisLetter, the calculated segregated energy, viz., the energy variationwhen one Cu atom segregated from the 2nd-layer to the 1st-layer,is 1.387 meV. For Ni, this value is 0.282 meV, which is much lowerthan that of Cu segregation. Therefore, when only a few Ni atomssegregated to the surface, the energy variation can almost be ne-glected.

If the energy of bimetallic cluster without segregation was de-termined, the temperature-dependent segregated energy of clustercan be calculated by using the values mentioned above. Further-more, the relationship between melting and segregation can alsobe obtained. The fitted temperature-dependent energy of clusterwithout segregation and premelting was shown in Fig. 3 (the blacksolid line). Only the near-melting cases were shown because nosegregation occurred at lower temperature. According to the fit-ted energy curve, the segregated energy curves of clusters withdifferent atomic numbers segregating to the 1st-layer at each tem-

Fig. 3. Temperature-dependent energy obtained at different conditions. The atomicnumber, segregated from the 2nd-layer to the 1st-layer at each temperature step,was shown by using the number in the figure. The line with squares was the simu-lated results and the line with circles was derived by calculating.

perature step can also be fitted, as shown in Fig. 3. The numbers inthe figure indicate the atomic ones are segregating from the 2nd-layer to the 1st-layer at each temperature step. Clearly, the energyof cluster with segregation obviously decreases more than thatwithout segregation. Furthermore, the larger the segregated atomicnumber is, the more obvious the decrease of energy. If the segre-gation is from three atoms, the energy almost remains unchangedwith the increase in temperature. When the segregated atomicnumbers continue to increase, the energy per atom will decreasewith the increase in temperature. Here, the irregular melting ofCo–Cu–Co (the energy decreases with the increase in temperature)occurs. It is reasonably concluded from above: (1) The segregatedenergy for different elements is different when one atom segre-gated from the 2nd-layer to the 1st-layer. Therefore, this irregularmelting can be controlled by doping the atoms with different sur-face energies. (2) The difference in atomic position before and aftersegregation will induce different segregated energy. This indicatesthe irregular melting can also be controlled by the atomic distri-bution. From these viewpoints, it is obvious to see this irregularmelting is controllable. Additionally, the segregated energy can becalculated by using the segregated atomic numbers at each tem-perature combined with segregated energy. This calculated resultwas shown in Fig. 3 (the line with circles) and compared with thesimulated one (the line with squares in Fig. 3). It can be found thattheir trends of the energy variations with the increase in temper-ature are similar. But there is a little difference. Since the latticemismatch induced by segregation and premelting increases the in-terface energy. At the same time, the diffusions of Cu to the innerof the cluster also increase the energy. However, both were notconsidered during the calculation of the segregated energy. Andthe increase becomes more obvious with the increase in temper-ature. These results lead to the difference between the calculatedand simulated results.

In conclusion, it is found a special melting in Co–Cu–Co clus-ters occurs at the temperatures near their melting points in this

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study. This irregular melting is strongly related to the compositionand atomic distribution. It maybe provide a new method to obtaincontrollable melting of clusters by designing nanoparticles and canbe designed by doping different hetero atoms and tuning their dis-tributions.

Acknowledgements

This work is supported by the Fundamental Research Funds forthe Central Universities (Grant Nos. N090309003 and N090209001),the Program for New Century Excellent Talents in University (GrantNo. NCET-06-0289) and the 111 project (Grant No. B07015).

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