Geometrical, electronic, and magnetic properties of CunFe (n=1–12) clusters: A density functional study

Journal of Physics and Chemistry of Solids 76 (2015) 10–16

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Geometrical, electronic, and magnetic properties of CunFe (n ¼ 1–12) clusters: A density functional study Wang Ling a, Die Dong a,b,n, Wang Shi-Jian a, Zhao Zheng-Quan a a b

School of Physics and Chemistry, Xihua University, Chengdu 610039, China Key Laboratory of Advanced Scientific Computation, Xihua University, Chengdu 610039, China

art ic l e i nf o

a b s t r a c t

Article history: Received 25 April 2014 Received in revised form 21 July 2014 Accepted 31 July 2014 Available online 6 August 2014

The geometrical, electronic, and magnetic properties of small CunFe (n ¼1–12) clusters have been investigated by using density functional method B3LYP and LanL2DZ basis set. The structural search reveals that Fe atoms in low-energy CunFe isomers tend to occupy the position with the maximum coordination number. The ground state CunFe clusters possess planar structure for n ¼2–5 and threedimensional (3D) structure for n ¼6–12. The electronic properties of CunFe clusters are analyzed through the averaged binding energy, the second-order energy difference and HOMO–LUMO energy gap. It is found that the magic numbers of stability are 1, 3, 7 and 9 for the ground state CunFe clusters. The energy gap of Fe-encapsulated cage clusters is smaller than that of other configurations. The Cu5Fe and Cu7Fe clusters have a very large energy gap (4 2.4 eV). The vertical ionization potential (VIP), electron affinity (EA) and photoelectron spectra are also calculated and simulated theoretically for all the ground-state clusters. The magnetic moment analyses for the ground-state CunFe clusters show that Fe atom can enhance the magnetic moment of the host cluster and carries most of the total magnetic moment. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Fe-doped copper clusters Structural, electronic and magnetic properties Density-functional method

1. Introduction Over the past decade, the binary clusters have received considerable attention [1–42]. Experimental and theoretical research has manifested that the nature of cluster can be changed dramatically with the addition of single impurity atom [9–32]. Doping of copper clusters with different atoms is expected to tailor the desired structural, electronic, catalytic, and magnetic properties for potential applications in materials science, solid state chemistry, microelectronics and nanotechnology [27–36]. For instance, the Ni-doping in copper clusters introduces a remarkable modulation of the electronic structures of the host clusters. The chemical activities of CunNi clusters toward CO2 adsorption are found to be d-band position dependent and can be modified by varying cluster size [33]. The magnetic properties of small CunM (n¼3–7, 12; M¼Ru, Rh, Pd) clusters are mainly affected by coordination number of dopant atom and symmetry of the cluster [34]. The melting behavior of Cu–Co bimetallic clusters strongly depends on the component materials, stoichiometries and local structure [35]. Nucleation energy and chemical reactivity of bimetallic CunM (M¼Ni, Pd, Pt; n¼1–4) present an odd–even oscillations in the case of gas phase [36]. The (CuS)n clusters are suitable for n Corresponding author at: Xihua University, School of Physics and Chemistry, Chengdu 610039, China. E-mail address: [email protected] (D. Dong).

http://dx.doi.org/10.1016/j.jpcs.2014.07.022 0022-3697/& 2014 Elsevier Ltd. All rights reserved.

candidates in the current search of novel nanomaterials for renewable energy sources, specifically in the photocatalysis field [37]. Recently, the pure copper doped with iron has been investigated due to the unique magnetic properties. It was shown that the Kondo effect is reduced in 0.3 at% Fe-doped Cu particles and this phenomenon is ascribed to the strong diminution of the Kondo screening cloud, which cannot be bigger than the particle size [43]. The 3d magnetic moments of Cu in dilute Cu54Fe embedded cluster are aligned parallel to the 3d magnetic moments of Fe, whereas the 4p magnetic moments of Cu are aligned antiparallel [44]. The behavior of local magnetism for Fe impurity in Cu12 clusters is different from that in bulk as well as from that in Al12 clusters [45]. The Cu6Fe compound is a novel singlemolecule paramagnet and its magnetic centers are very weakly coupled within the cluster [46]. Moreover, most of these investigations focused on the Fe-doped Cu particles or great-size clusters. To the best of our knowledge, however, there is a lack of theoretical work on small Fe-doped copper clusters. On the other hand, it is well known that small clusters often have special properties, which should be very different from those of bulk materials or the atom, on account of the so-called size and surface effects. Consequently, in this paper, the geometrical, electronic, and magnetic properties of small CunFe (n¼1–12) clusters will be studied by means of density functional theory (DFT). It is hoped that our work would be informative to understand the influence of material structure on its properties and could provide a guideline for future experiments.

W. Ling et al. / Journal of Physics and Chemistry of Solids 76 (2015) 10–16

2. Computational method To search the ground state structures of CunFe clusters, many initial isomers, which include one-, two- and three-dimensional configurations, had been taken into account in geometry optimizations. Geometry optimizations and vibrational frequency analyses were carried out based on DFT with the B3LYP exchange correlation functional and an effective core potential LanL2DZ basis set [47–50], as implemented in GAUSSIAN09 program package [51]. The convergence thresholds were set to a constant value in all calculations, 1.5  10  5 hartree/Bohr for the forces, 6.0  10  5 Å for the displacement, and 10  6 hartree for the energy change. Owing to the spin polarization, every initial configuration was optimized at different spin states. The validity of current computational method has been tested by calculations on copper dimer and iron dimer. The results, which are summarized in Table 1, show the reliability of current computational scheme to describe small CunFe clusters.

3. Results and discussion 3.1. Geometrical structures The structure optimizations for CuFe show that the quartet spin state is lower in energy than the doublet and sextet spin states by 1.14 and 0.75 eV, respectively. Accordingly, the quartet CuFe dimer with electronic state of 4 Σ g is the ground-state structure and its bond length is 2.35 Å. For each CunFe (n ¼2–12) clusters, Fig. 1 shows the ground state structure and three low-lying isomers. According to the energies from low to high, these isomers are denoted by na, nb, nc and nd, where n represents the number of Cu atoms in the CunFe clusters. Meanwhile, their symmetry, spin multiplicity, and energy difference compared to each of the ground state structures are also indicated in the figure. The geometric features of the ground state CunFe (n¼ 1–12) clusters are listed in Table 2. The ground state configurations of CunFe (n ¼2–5) clusters evidently favor planar structures, as shown in Fig. 1. The 2a, 3a and 4a isomers, which resemble the lowest energy structures of Cu3, Cu4 and Cu5 clusters [52,53], are the ground state structures of Cu2Fe, Cu3Fe and Cu4Fe clusters, respectively. The 2d isomer with C1v symmetry is obtained only in singlet spin states. The 3d isomer is the first three-dimensional (3D) structure. The 5a isomer is the ground state structure of Cu5Fe cluster and is lower in energy than 5b isomer which is similar to the lowest energy structure of Cu6 clusters [52,53]. Starting from n ¼6, the low energy isomers of CunFe clusters prefer 3D structures. A pentagonal bipyramid structure with the Fe atom on the top is found to be the most stable structure of Cu6Fe cluster. Due to the Jahn–Teller effect, the 6a and 6b isomers with Cs symmetry have a slight deviation from C5v and C3v symmetries, respectively. The 6c isomer, where Fe atom occupies a low coordination position, has the same configuration and spin

Table 1 The bond lengths and electronic properties of Cu2 and Fe2 dimers. System

Cu2 Fe2 a b

r (Å)

De (eV)

VIP (eV)

EA (eV)

11

multiplicity as the 6b isomer. The 6d isomer with Fe atom at the center of Cu6 ring is a high-symmetry planar structure and is more stable than other isomers. With regard to the Cu7Fe and Cu8Fe clusters, 40-odd isomers were attained by optimizing various structures. The 7a and 7b isomers correspond to the most stable structures of Cu8 cluster reported by Ramirez et al. [52]. The former with C3v symmetry is energetically lower than the latter by 0.18 eV and the ground-state structure of Cu7Fe cluster. The 7c and 7d isomers are obtained by adding one Cu atom to the 6b isomer. The coordination number of Fe atom is larger in 7c isomer than in 7d isomer. The 8a and 8b isomers can be viewed as a face-bicapped pentagonal bipyramid and the former, which relates to the lowest energy Au9 cluster reported by Baishya et al. [53], is the global minimum for Cu8Fe. The quintet 8c isomer is almost degenerate with the triplet 8d isomer. In addition, some other isomers with Fe possessing less coordination are significantly higher in total energy and do not appear in Fig. 1. In the case of CunFe (n ¼9–12) clusters, the search for the most stable structures becomes more difficult because of the increasing number of possible isomers. Hence, the strategy of substituting one Cu atom by a Fe atom from the Cun þ 1 cluster [52,53] or adding Cu atom(s) to former CunFe clusters is also used in geometry optimizations. For the Cu9Fe, Cu10Fe and Cu11Fe clusters, the ground state structures, namely 9a, 10a and 11a isomers, can be looked upon as a substitution of Fe atom for central Cu atom in the lowest energy Cu10, Cu11 and Cu12 clusters [52,53]. The low-lying 9b, 9d, 10d, 11b and 11d isomers are generated by adding a Cu atom to 8a, 9c, 10d and 10c isomers. The 9c isomer with C3v symmetry, which was expected to be the lowest energy structure of Cu9Fe cluster, is 0.09 eV less stable than the 9a isomer. The 10c isomer is obtained by distorting the geometry from D5d to C2h symmetry. The Cu12Fe isomer with Fe atom at the center is more stable as compared to the vertex position. The icosahedral 12a isomer with D3d symmetry is the most stable structure of Cu12Fe cluster. The 12b isomer with D5d symmetry is above 12a by 0.02 eV. As for the configuration of the lowest energy Cu13 cluster [52,53], we considered all different sites for a Fe atom. Nevertheless, these isomers, such as 11c isomer, are energetically higher than 12a. From the above discussions, it is obvious that the ground-state structures of CunFe (n ¼ 3–11) clusters exhibit a similar structure as that of the Cun þ 1 clusters [52,53]. The Fe atom in the ground state CunFe clusters, which possesses a planar structure for n ¼2–5 and a 3D structure for n ¼6–12, tends to occupy the most highly coordinated position. This may be attributed to the principle of maximum overlap in molecular orbital theory. If the orbital overlap between Fe and Cu atoms increases, the energy of CunFe cluster will decrease. 3.2. Electronic properties In this section, the electronic properties of the ground state CunFe (n ¼ 1–12) clusters are analyzed by means of the atomic averaged binding energies, second-order difference of energies, energy gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), VIP, EA and photoelectron spectroscopy (PES). The atomic averaged binding energies (Eb) of the CunFe clusters are calculated by the following formula:

Calc.

Expt.

Calc.

Expt.

Calc.

Expt.

Calc.

Expt.

Eb ¼ ðnE½Cu þ E½Fe  E½Cun FeÞ=ðn þ 1Þ

2.26 2.15

2.22a 2.02b

2.02 1.28

2.01a 1.30b

7.99 6.60

7.90a –

0.63 0.81

0.83a –

where E½Cu, E½Fe and E½Cun Fe are the energies of a Cu atom, a Fe atom, and the CunFe cluster. The calculated binding energies per atom for the most stable CunFe clusters are shown in Fig. 2. As can be seen from this figure, the averaged binding energy is a monotonically

Refs. [52,56,57]. Refs. [58,59].

ð1Þ

12

W. Ling et al. / Journal of Physics and Chemistry of Solids 76 (2015) 10–16

C2v, 5, 0 eV

Cs, 5, 0.08 eV

C2v, 3, 0.26 eV

C∞v, 1, 3.38 eV

2a

2b

2c

2d

C2v, 4, 0 eV

C2v, 4, 0.04 eV

C2v, 4, 0.16 eV

C3v, 4, 1.00 eV

C2v, 5, 0 eV

Cs, 5, 0.11 eV

Cs, 5, 0.25 eV

C2v, 5, 0.41 eV

3a

3b

3c

3d

4a

4b

4c

4d

C2v, 4, 0 eV

C2v, 4, 0.21 eV

Cs, 4, 0.29 eV

C5v, 4, 0.62 eV

Cs, 5, 0 eV

Cs, 5, 0.11 eV

Cs, 5, 0.25 eV

D6h, 3, 0.28 eV

5a

5b

5c

5d

6a

6b

6c

6d

C3v, 4, 0 eV

Cs, 4, 0.18 eV

Cs, 4, 0.20 eV

Cs, 4, 0.24 eV

C2v, 3, 0 eV

Cs, 3, 0.18 eV

7a

7b

7c

7d

8a

8b

8c

8d

Cs, 4, 0 eV

Cs, 4, 0.06 eV

C3v, 2, 0.09 eV

C1, 4, 0.10 eV

C1, 5, 0 eV

C2v, 3, 0.09eV

C2h, 3, 0.13 eV

Cs, 3, 0.14 eV

9a

9b

9c

9d

10a

10b

10c

10d

Cs, 4, 0 eV

C1, 4, 0.15 eV

C1, 4, 0.25 eV

C5v, 4, 0.25 eV

D3d,, 3, 0 eV

D5d, 3, 0.02 eV

C1, 3, 0.09 eV

Cs, 3, 0.11 eV

11a

11b

11c

11d

12a

12b

12c

12d

C1, 5, 0.337 eV Cs, 3, 0.343 eV

Fig. 1. The ground-state structures of CunFe (n¼ 2–12) clusters, and three low-lying isomers for CunFe clusters. The point group, spin multiplicity, and energy difference compared to each ground state structure are given below them. The red and black balls represent Cu and Fe atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

increasing function of atoms in the clusters. Especially, Eb increase rapidly for the planar structures and gradually for the 3D structures. This hints that the clusters can continue to gain energy during the growth process. From the ground state structures in Fig. 1, we find that the atomic averaged binding energies are closely related to the number of chemical bond in the cluster. The chemical bond per atom increases in direct proportion to the atomic averaged binding energy. Therefore, Eb can offer information about chemical bonding of cluster. The second-order difference of energies (Δ2 E) is extremely sensitive to the relative stability of clusters and can be compared with the relative abundances determined in mass spectroscope experiment. For the ground state CunFe clusters, Δ2 E is defined as

follows: Δ2 E ¼ EðCun þ 1 FeÞ þ EðCun  1 FeÞ  2EðCun FeÞ

ð2Þ

where E is the total energy of the ground state cluster. Fig. 3 reveals the size dependence of the second-order energy differences for the ground state CunFe clusters. Four local peaks of Δ2E are found at n ¼ 1, 3, 7 and 9, implying that the CuFe, Cu3Fe, Cu7Fe and Cu9Fe clusters possess relatively higher stability. In addition, the local maximum at n ¼11 is not considered because Δ2 E of Cu12Fe cluster is unknown and its calculation needs the energy of Cu13Fe cluster. Secondly, the structural transition occurs at n¼ 5–6 from planar to 3D. The second-order energy differences show an

W. Ling et al. / Journal of Physics and Chemistry of Solids 76 (2015) 10–16

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Table 2 The averaged bond length between Fe and Au atoms (Rv), minimum bond length (Rmin), maximum bond length (Rmax), coordination number of Fe atom (N) and chemical bond per atom (C) for the most stable CunFe clusters. Clusters

Rv (Å)

Rmin (Å)

Rmax (Å)

N

C

CuFe Cu2Fe Cu3Fe Cu4Fe Cu5Fe Cu6Fe Cu7Fe Cu8Fe Cu9Fe Cu10Fe Cu11Fe Cu12Fe

2.35 2.50 2.46 2.52 2.50 2.54 2.55 2.52 2.57 2.60 2.57 2.50

2.35 2.50 2.37 2.40 2.45 2.48 2.49 2.47 2.49 2.49 2.50 2.50

2.35 2.50 2.50 2.57 2.64 2.58 2.58 2.58 2.66 2.67 2.62 2.63

1 2 3 4 5 5 6 8 7 7 8 12

0.50 0.67 1.25 1.40 1.50 2.14 2.25 2.56 2.60 2.64 2.67 2.77

Fig. 4. Size dependence of the HOMO–LUMO energy gap of the ground-state CunFe clusters.

Table 3 VIP and EA of the ground state CunFe clusters, and the charge (Q) and local magnetic moment (M) of 4s, 3d and 4p shells for Fe atom in the ground state CunFe clusters. Clusters

Fig. 2. Size dependence of the averaged binding energies for the ground state CunFe clusters.

CuFe Cu2Fe Cu3Fe Cu4Fe Cu5Fe Cu6Fe Cu7Fe Cu8Fe Cu9Fe Cu10Fe Cu11Fe Cu12Fe

VIP (eV)

6.98 6.57 6.40 6.28 6.55 6.45 6.13 5.67 5.87 5.82 6.08 5.49

EA (eV)

0.46 1.29 1.34 1.10 0.90 0.96 0.92 1.37 1.45 2.05 1.71 1.82

Fe-4s

Fe-3d

Fe-4p

Q (e)

M (μB)

Q (e)

M (μB)

Q (e)

M (μB)

0.99 1.20 0.59 0.86 0.76 0.74 0.61 0.60 0.53 0.63 0.57 0.53

0.11 0.10 0.07 0.20 0.06 0.30 0.05 0.06 0.05 0.09 0.05 0.01

6.92 6.39 6.92 6.66 6.81 6.76 6.92 7.02 6.89 6.71 6.85 7.21

3.04 3.59 3.02 3.30 3.09 3.16 2.98 2.82 2.99 3.23 3.03 2.69

0.02 0.30 0.33 0.64 0.95 0.78 0.75 1.45 0.91 1.03 1.36 2.50

0 0.06 0.30 0.12 0.05 0.10 0.03 0.05 0.03 0.07 0.02 0.02

we notice that the energy gaps of copper-caged iron clusters, such as 10b, 10c, 11d, 12a, 12b and 12d, are smaller than that of other configurations. The Cu12Fe cluster with 18 valence electrons does not have a relatively big energy gap. The phenomenon is also found in other doped-clusters [28] and means that the 18-electron rule perhaps has a restrictive applicability. The VIP and EA are two fundamental quantities to gain an insight into the electronic structures and are defined as

Fig. 3. Size dependence of the second-order energy differences for the groundstate CunFe clusters.

odd–even oscillation for n r5 or n Z6. Accordingly, it can be deduced that for the planar or 3D configurations, the ground state CunFe clusters with odd n are more stable than that with even n. The HOMO–LUMO energy gap (Eg) is usually considered as an important quantity that reflects chemical activity of small metal cluster. A big energy gap corresponds to a high chemical stability. For the ground state CunFe clusters, the energy gaps are displayed in Fig. 4. It is clear from this figure that the Cu5Fe and Cu7Fe clusters have a very large energy gap relative to other clusters. In other words, the Cu5Fe and Cu7Fe clusters presumably are less reactive and should be useful as a building block for constructing the cluster-assembled materials. In particular, the highly stable Cu7Fe cluster with large energy gap (2.61 eV) may be perceived as a superatom that tends to retain its structure integrity and chemical identity in cluster-assembled solids [54]. Simultaneously,

VIP ¼ EðCun Fe þ Þ  EðCun FeÞ

ð3Þ

EA ¼ EðCun Fe  Þ  EðCun FeÞ

ð4Þ

where EðCun Fe þ Þ and EðCun Fe  Þ are the single point energy of the corresponding cationic and anionic clusters in the neutral geometry. For the ground state CunFe clusters, no experimental datum is available and the calculated VIP and EA are listed in Table 3. The VIP tends to decrease in the mass with the growth of the clusters, while the EA shows an inverse relationship. In chemical reactivity, a small VIP and a great EA for CunFe clusters mean that these clusters are easy to lose and accept an electron, respectively. Thereby, the CuFe dimer with a small EA (0.46 eV) will be very unstable after it acquires an electron. To compare with measured PES spectra, simulated PES spectra [23] of the ground state CunFe clusters are obtained by adding the occupied orbital energy relative to the HOMO to the first VIP and fitting them with a Lorentz expansion scheme and broadening factor of 0.1 eV, as plotted in Fig. 5. The PES spectra of Cu2Fe, Cu3Fe and Cu4Fe clusters have two intense peaks at  8.4 and 12.0 eV. One intense peak at 8–10 eV is apparent in the PES spectra of CunFe (n ¼5–12) clusters.

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W. Ling et al. / Journal of Physics and Chemistry of Solids 76 (2015) 10–16

Fig. 5. Simulated PES spectra for the ground state CunFe clusters.

The distinct PES spectra of the ground state CunFe clusters can be used to identify the ground state structures in future experiments. 3.3. Magnetic properties Atomic clusters provide an extraordinary medium to probe magnetism because its size, local structure and atomic compositions can be readily controlled. The total magnetic moment of the ground-state CunFe (n¼ 1–12) clusters has been calculated and their spin density of states (SDOS) is shown in Fig. 6. The groundstate CunFe clusters with odd n have the total magnetic moment of 3 μB, which can be derived from the odd number of valence electrons. For the ground-state CunFe clusters with even n, the number of valence electrons is even and total magnetic moments are 4 μB for n ¼2, 4, 6 and 10 and 2 μB for n ¼8 and 12. The magnetic moment of Cu8Fe and Cu12Fe clusters is smaller than that of other clusters. This may be ascribed to the fact that the Cu8Fe and Cu12Fe clusters have a highly symmetric structure and Fe atom is partially or completely encapsulated with Cu atoms. Compared to the magnetic moment of pure copper cluster [33], the doping of Fe atom enhances the magnetism of the host clusters. Meanwhile, it is also evident from Fig. 6 that an intense band below the HOMO gradually moves toward lower energy as the size of cluster increases. All clusters have an intense band between  5 and 1.5 eV, which consists principally of the valence s and d orbitals of the constituent atoms. The total magnetic moment of small CunFe (n ¼1–4) clusters mainly comes from the energy level on the left of sharp peak. In order to realize the magnetic properties further, we have performed the natural bond orbital analysis [55] for the ground state CunFe (n ¼1–12) clusters. The local magnetic moments on Fe

atom is 3.15 μB for CuFe, 3.75 μB for Cu2Fe, 3.39 μB for Cu3Fe, 3.62 μB for Cu4Fe, 3.20 μB for Cu5Fe, 3.56 μB for Cu6Fe, 3.06 μB for Cu7Fe, 2.93 μB for Cu8Fe, 3.07 μB for Cu9Fe, 3.39 μB for Cu10Fe, 3.10 μB for Cu11Fe, 2.72 μB for Cu12Fe. The magnetic moment provided by Cu atoms is very small. Furthermore, the Cu atoms in the CunFe clusters with n ¼ 8, 12 and odd numbers exhibit the antiferromagnetic alignment with respect to the Fe atom's magnetic moment. That is to say, the magnetic moment of these CunFe clusters primarily comes from the paramagnetic Fe atom, as shown in Fig. 7. The magnetic moment and charge on 3d, 4s and 4p shells of the Fe atom are presented in Table 3. It can be seen from Table 3 that the magnetic moment of Fe atom mainly originates from the partially filled 3d shell. A few of magnetic moments are found on the 4s and 4p shells. In contrast to the free Fe atom, the 3d and 4p shells gain 0.69–3.71 electrons and the 4s shell loses 0.80–1.47 electrons. At the same time, there is an interatomic charge transfer in the CunFe clusters from Fe atom to Cu atoms for n ¼1–3 and from Cu atoms to Fe atom for n ¼4–12. The charge transfer and orbital hybridization should be the major reasons for the magnetic moment changes of the dopant Fe atom.

4. Conclusions The structural, electronic, and magnetic properties of small CunFe (n ¼1–12) clusters have been studied by first-principle density functional calculations. Geometry optimizations indicate that the dopant Fe atoms in low-energy CunFe isomers prefer to occupy the most highly coordinated position. The most stable CunFe clusters adopt planar structure for n ¼2–5 and 3D structure for n ¼6–12. The analyses of electronic properties manifest that

W. Ling et al. / Journal of Physics and Chemistry of Solids 76 (2015) 10–16

15

Fig. 6. The SDOS of the ground-state CunFe clusters. A broadening factor of 0.1 eV is used. Spin-up (positive) and spin-down (negative) densities are given in each case. The dashed line indicates the location of the HOMO level.

Fig. 7. The magnetic moment of the ground-state CunFe clusters and dopant Fe atom.

the CunFe (n ¼ 1, 3, 7 and 9) clusters are more stable than their neighboring clusters. The Cu7Fe cluster with a large energy gap can be perceived as a superatom. The VIP and EA have an inverse variation with the size of cluster increasing. The simulated PES should be helpful for the identification of the ground state CunFe clusters in the coming experiments. The magnetism calculations show that the total magnetic moment of CunFe clusters is mainly localized on Fe atom and the introduction of single Fe atom improves the magnetic moment of the host cluster.

Acknowledgments This project was supported by the Key Scientific Research Fund of Xihua University (Grant no. Z1213320), the Innovation Fund of Postgraduate of Xihua University (No. ycjj2014131) and the Open Research Fund of Key Laboratory of Advanced Scientific Computation, Xihua University (Grant no. szjj2012-035). References [1] J. Yano, J. Kern, K. Sauer, M.J. Latimer, Y. Pushkar, J. Biesiadka, B. Loll, W. Saenger, J. Messinger, A. Zouni, V.K. Yachandra, Where water is oxidized to dioxygen: structure of the photosynthetic Mn4Ca cluster, Science 314 (2006) 821–825.

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