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Insights into the structural, electronic and magnetic properties of V-doped copper clusters: comparison with pure copper clusters

Scientific Reports volume 6, Article number: 31978 (2016) | Download Citation

Abstract

The structural, electronic and magnetic properties of Cun+1 and CunV (n = 1–12) clusters have been investigated by using density functional theory. The growth behaviors reveal that V atom in low-energy CunV isomer favors the most highly coordinated position and changes the geometry of the three-dimensional host clusters. The vibrational spectra are predicted and can be used to identify the ground state. The relative stability and chemical activity of the ground states are analyzed through the binding energy per atom, energy second-order difference and energy gap. It is found that that the stability of CunV (n ≥ 8) is higher than that of Cun+1. The substitution of a V atom for a Cu atom in copper clusters alters the odd-even oscillations of stability and activity of the host clusters. The vertical ionization potential, electron affinity and photoelectron spectrum are calculated and simulated for all of the most stable clusters. Compare with the experimental data, we determine the ground states of pure copper clusters. The magnetism analyses show that the magnetic moments of CunV clusters are mainly localized on the V atom and decease with the increase of cluster size. The magnetic change is closely related to the charge transfer between V and Cu atoms.

Introduction

During the last few decades, copper clusters have been demonstrated to have similar catalytic activities with those of gold clusters for the low temperature CO oxidation and partial oxidation of hydrocarbons1,2,3,4,5,6. At the same time, theoretical and experimental work has also shown that the nature of small clusters can be considerably modified by the addition of impurity atom(s)7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39. Copper clusters doped with different transition-metal atoms have been expected to tailor the desired catalytic, electronic, magnetic and optical properties for potential applications in solid state chemistry, microelectronics, nanotechnology and materials science40,41,42,43,44,45,46,47,48,49,50. For instance, Yang et al. reported that the adsorption property of copper cluster to CO2 can be modified by doping it with Ni atoms and icosahedral Cu42Ni13 cluster, which is used as catalysts for methanol synthesis via CO2 hydrogenation, exhibits the strongest CO2 adsorption ability compared to Cu55 and Cu54Ni clusters40. Wang et al. found that the melting behavior of Cu-Co bimetallic clusters, which is different from that of pure copper clusters, is closely related to the component materials, stoichiometries and local structure. The Kondo temperature of a Co atom embedded in Cu clusters on Cu(111) exhibits a nonmonotonic variation with the cluster size41. Han et al. noted that though most of the CunNi clusters possess similar geometrics to those of pure copper clusters, Ni-doping introduces a dramatic modulation of the electronic structures, such as the density of states and d-band centers42. Recently, the Cu-V alloys have investigated due to their unique physical properties. It was shown that the addition of V in Cu alloys can improve mechanical properties and heat resistance of Cu alloys. This effect can be reinforced by increasing the solubility of V in Cu during the synthesis of the alloys43. The fcc crystalline structure of Cu-V alloys can be preserved in the solid solution model until the concentration of V reaches a critical value of 23 at.%. When the V concentration in a model is over 23 at.%, a crystal-to-crystal transition will take place44. To the best of our knowledge, however, there is a lack of work on small V-doped copper clusters. It has been proved that the gold cluster doped with V can bind a high number of oxygen molecules over pure gold cluster and is an improved novel catalyst for CO oxidation51. As it is known, Cu and Au have a similar electronic configurations nd10(n + 1)s1. Presumably, the copper clusters doped with V should also be a potential catalyst for the oxidation of CO. On the other hand, some experiments which are used to shed light on the structures of clusters must rely on theoretical calculations of geometries of possible low lying isomers. Therefore, in this paper, the geometric, electronic, and magnetic moments of the small Cun+1 and CunV (n = 1–12) clusters will be studied systematically on the basis of density functional theory (DFT). It is wished that this work would be helpful to understand the influence of material structure on its properties and could provide practical guidelines for coming experimental research.

Computational Methods

Geometry optimizations and vibrational frequency analyses of Cun+1 and CunV clusters have carried out in the framework of a DFT-based method using the GAUSSIAN09 package52. The exchange-correlation functional B3LYP and an effective core potential basis set LanL2DZ were used for all of the computations53,54,55,56. The convergence thresholds are set to 4.5 × 10−4 a.u. for maximum force, 3.0 × 10−4 a.u. for root mean square (RMS) force, 1.8 × 10−3 a.u. for maximum displacement and 1.2 × 10−3 a.u. for RMS displacement. The accuracy of the theoretical level has been checked by calculations on copper dimer and vanadium dimmer. The results have summarized in Table 1. To search the lowest energy structures of Cun+1 and CunV clusters, lots of initial isomers, which include one-, two- and three-dimensional (3D) configurations, had been taken into account in our geometry optimizations. Owing to the spin polarization, every initial configuration was optimized at possible spin multiplicities. If an imaginary vibraional mode is found, a relaxation of the structure is performed until the true local minimum is actually obtained.

Table 1: The geometries and electronic properties of Cu2 and V2 dimers.

Results and Discussion

Geometrical structures and vibrational spectra

The optimized results for Cu2 and CuV dimmers show the former in singlet spin state is 1.86 eV lower than in triplet spin state and the latter in single, triplet and septet spin states is less stable than in quintet spin state by 2.94, 0.47 and 1.38 eV, respectively. Accordingly, the singlet Cu2 and quintet CuV are the ground states. Their bond lengths are 2.26 Å for Cu2 and 2.49 Å for CuV. The bond length of the Cu2 is shorter than that of the CuV. This may be attributed to the fact that the radius of Cu atom (1.28 Å) is smaller than that of V atom (1.34 Å). For each Cun+1 and CunV (n = 2–12) clusters, Figs 1 and 2 display the ground state structure and low-lying isomers. According to the energy order from low to high, these isomers are denoted by nA, nB, nC, nD, nI, nII, nIII, and nIV, where n represents the number of Cu atoms in pure copper and CunV clusters. Meantime, their symmetry, spin multiplicity, and energy difference compared to each of the ground state structures are also indicated in the two figures. The geometric features and mean static polarizabilities () of the ground state Cun+1 and CunV (n = 1–12) clusters are listed in Table 2.

Figure 1: The ground-state structures of Cun (n = 3–13) clusters, and three low-lying isomers for the Cun (n = 6–13) clusters.
Figure 1

The point group, spin multiplicity, and energy difference compared to each of the ground state structures are given below them.

Figure 2: The ground-state structures and three low-lying isomers for CunV (n = 2–12) clusters.
Figure 2

The point group, spin multiplicity, and energy difference compared to each of the ground state structures are given below them. The grey and black balls represent Cu and V atoms, respectively.

Table 2: The maximum and minimum bond lengths (Rmax, Rmin) and chemical bond per atom (C) for the most stable Cun+1 and CunV clusters.

The most stable structures of Cun+1 and CunV (n = 2–5) clusters evidently prefer planar configurations. All isomers of copper clusters, which do not include 5C, are found to be in the lowest spin state. The ground state structures of Cu3, Cu4, Cu5 and Cu6 clusters are angular, rhombic, trapezoidal and triangular structures, respectively, and no low-lying 3D isomer is obtained for Cu4 cluster. When a Cu atom in copper clusters is replaced by one V atom, the number of optimized CunV structures apparently increases. But only four isomers of each CunV cluster are depicted in Fig. 2. The lowest energy structures of Cu2V, Cu3V, Cu4V and Cu5V clusters is similar to those of Cu3, Cu4, Cu5 and Cu6 clusters. The energies of similar structures decrease as the coordination number of V atom increase. The 3IV isomer is the first 3D structure of CunV clusters. The tetragonal bipyramid and pentagonal pyramid is not unstable for Cu5V cluster. Other isomers, which not displayed in Figs 1 and 2, are higher in energy than the nD or nIV isomer.

Starting from n = 6, many stochastic configurations were optimized for Cun+1 and CunV clusters. The optimized structures show that almost all lower energy isomers possess 3D configurations and V atom in lower energy CunV cluster tend to occupy the position with the more ligands. As a result, a series of 3D structures for the Cun+1 and CunV clusters (n = 6–12) were considered and optimized again. Moreover, various 3D CunV isomers with V atom occupying the most highly coordinated site were optimized further to ensure that the lowest energy structures obtained are the true minimum. To avoid missing the ground state structures, we had also used the strategies of substituting a Cu by one V atom from the pure copper cluster or adding Cu atom(s) to former Cun or CunV clusters in geometry optimizations.

The ground state structure of Cu7 cluster is a pentagonal bipyramid (7A), lying just below the 7B. The 8A isomer with Td symmetry, which can be treated as a face-capped 7B, is found to be the lowest energy structure of Cu8 cluster. The 9A and 9B are nearly degenerate and ref. 57 suggests 9B as the most stable structure. Nevertheless, in view of vertical ionization potential (VIP) which will be discussed later, we deduce that the 9A is the ground state structure of Cu9 cluster. Simultaneously, the most stable structures of small Cun (n = 2–9) clusters had studied by means of optical absorption spectra58. Our results are consistent with the previous conclusion. From Cu10 to Cu13 clusters, the flat cage-like configurations are more stable than other structures, e.g. close-packing and globe-shaped structures. The 10A, 11A, 12A and 13A are the lowest energy structures of Cu10, Cu11, Cu12 and Cu13 clusters, respectively. Several isomers reported in ref. 56 have also been optimized at B3LYP/LanL2DZ level and are higher in energy than our lowest energy structures. This is in agreement with Ramirez et al.’s studies57.

With regard to CunV (n = 6–12) clusters, the ground state structures (6I, 7I, 8I, 9I, 10I, 11I and 12I) are entirely different from the most stable structure of the corresponding Cun+1 clusters. The 6I, 7I and 8I structures are similar to the low-lying isomers (7D, 8D and 9C) of pure copper clusters. The 9I, 10I, 11I and 12I configurations are unstable or do not exist for Cu clusters in the lowest spin state. The 11I is obtained by distorting the geometry starting from C5v to Cs symmetry. The 12I has a small deviation from Ih symmetry. The CunV isomers, which resemble the lowest energy structures and low-lying isomers of Cun+1 clusters, lay above each of the ground state structures (nI). The most stable structures of CunV (n = 7–12) clusters all contain a pentagonal bipyramid. In addition, due to the Jahn-Teller effect, the 6IV and 7II isomers with Cs symmetry have a slight deviation from C3v symmetry. The 10I and 12I strucutres are more stable in doublet spin state than in quartet spin state. The V atom in CunV clusters tends to occupy the site with the maximum coordination number. This may be ascribed to the principle of maximum overlap in molecular orbital theory. Because the orbital overlap between Cu and V atoms increases, the energy of CunV cluster will decrease.

The combination of theoretical and experimental vibrational spectra is a good method for the structural determination of small isolated clusters and the method has been successfully applied in practice59. Consequently, the vibrational spectra of the lowest energy Cun+1 and CunV (n = 1–12) clusters are shown in Fig. 3. The Cu2 dimer merely has a stretching vibration without change of dipole moment, so there is no absorption peak. The absorbed peaks of planar or highly symmetrical clusters are less than those of other configuration clusters. The intense peaks of 3D CunV (n = 6–12) clusters are more than those of corresponding pure Cun+1 clusters. The vibrational fundamentals of all Cun+1 and CunV clusters are found to be in the range of 5 to 330 cm−1. The most intense peak of vibrational spectum of each CunV clusters is related to the V-Cu stretching vibrations. The characteristic frequencies of the ground state structures and several low-lying isomers are given as Supplementary Material.

Figure 3: Vibrational spectra of the ground state Cun+1 of CunV (n = 1–12) clusters.
Figure 3

Relative stabilities and electronic properties

In this part, the relative stabilities and electronic properties of the ground state Cun+1 and CunV (n = 1–12) clusters are discussed by means of the atomic averaged binding energies, second-order energy differences, energy gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the VIP, the vertical electron affinity (VEA) and photoelectron spectroscopy (PES).

The atomic averaged binding energies (EB) of the Cun+1 and CunV clusters can be calculated as follows

where E(Cun+1), E(Cu), E(CunV) and E(V) are the energy of Cun+1 cluster, Cu atom, CunV cluster and V atom, respectively. The calculated binding energies per atom for the lowest energy Cun+1 and CunV clusters are shown in Fig. 4. As seen from this figure, the size dependence of EB for Cun+1 clusters have an apparent peak at n = 7. That is to say, the Cu8 cluster possesses relatively higher thermic stability. The EB of CunV clusters, which is larger than that of Cun+1 clusters for n≥8, is a monotonically increasing function of the number of atoms in clusters. This implies that the doped clusters can continue to gain energy during growth process. The substitution of a V atom for a Cu atom in Cun+1 (n≥8) clusters can evidently enhance the stability of the host clusters. The phenomenon may be caused by structural changes. The configuration of CunV (n≥8) clusters is entirely different from that of Cun+1 clusters.

Figure 4: Size dependence of the averaged binding energies for the ground state Cun+1 and CunV (n = 1–12) clusters.
Figure 4

In cluster physics, the second-order energy differences (Δ2E), which can be compared with the relative abundances determined in mass spectroscopy experiment, is a particularly sensitive quantity that reflects the relative stability of clusters. For the ground state Cun+1 and CunV clusters, it can be calculated as

where E is the energy of the ground-state clusters. The calculated second-order energy differences as a function of the cluster size are illustrated in Fig. 5. It is obvious from Fig. 5 that the even-numbered copper clusters are more stable than the odd-numbered ones. However, the introduction of a V atom in copper cluster alters the stable pattern of the host clusters significantly. For the CunV clusters, three maxima are observed at n  = 4, 7 and 9. Accordingly, it can be inferred that the Cu4V, Cu7V and Cu9V clusters are magic clusters and have an enhanced abundance in mass spectra.

Figure 5: Size dependence of the second-order energy differences for the lowest energy Cun+1 and CunV (n = 1–12) clusters.
Figure 5

The HOMO-LUMO energy gap (Eg), which relies on the eigenvalues of the HOMO and LUMO energy levels, is viewed as an important parameter that characterizes chemical stability of small clusters. A big energy gap usually relates to a high chemical inertness. For the ground state Cun+1 and CunV clusters, the energy gaps are plotted in Fig. 6. The pure copper clusters show an odd-even alternation in their energy gaps. This phenomenon can be interpreted by the electron pairing effect that the electron in a doubly occupied HOMO has stronger effective core potentials because the electron screening is weaker for electrons in the same orbital than for inner shell electrons. When a Cu atom ([Ar]3d104s1) in Cun+1 cluster is replaced by a V ([Ar]3d34s2) atom, the closed electronic shell will become an opened electronic shell. So, the Eg of CunV cluster for n = odd is smaller than that of Cun+1 cluster. For n = 2, 4, 6 and 8, the unpaired electrons of CunV cluster is more than those of the corresponding Cun+1 cluster. The energy of the LUMO of CunV cluster will rise because of the electrostatic interaction of unpaired electrons. The Eg of CunV are larger than that of the Cun+1 cluster. For n = 10 and 12, the CunV cluster is equal to Cun+1 cluster in unpaired electrons. However, the formers have a highly symmetrical geometry. Hereby, the Eg of Cu10V and Cu12V clusters is also larger than that of the Cu11 and Cu13 clusters, respectively.

Figure 6: Size dependence of the HOMO-LUMO energy gaps of the most stable Cun+1 and CunV (n = 1–12) clusters.
Figure 6

The VIP and VEA are two basic quantities to get an insight into the electronic property and can be estimated as follows

where E(cluster cation) and E(cluster anion) are the single-point energies of the cationic and anionic clusters in the neutral geometry. For the lowest energy Cun+1 and CunV clusters, Table 3 give the calculated VIP and VEA along with the available experimental data. The calculated VIPs of pure copper clusters are in good agreement with previous measurements obtained at discrete 2.5 nm intervals. The agreement confirmed reliability of the present theoretical method again. Meanwhile, we can distinguish the ground state structure of Cu9 cluster by the aid of VIPs. The present and preceding calculations have shown that the 9A and 9B isomers are the candidate for the lowest energy structure of Cu9 cluster. Our calculated VIPs are 5.27 eV for 9A and 5.99 eV for 9B. The measured value is 5.36 ± 0.05 eV60. Thus, we deduced that the 9A structure is the most stable structure of Cu9 cluster. To offer reference material for PES experiment in future, the theoretical PES spectra of the global minimum structures of Cun+1 and CunV (n = 1–12) clusters were simulated by adding the occupied orbital energy relative to the HOMO to the VIP and fitting them with a broadening factor of 0.1 eV, as plotted in Fig. 7. The distribution of energy level for all clusters is in the range of 6 to 11 eV. The doped V atom made a change for the PES spectra of copper cluster. This change is relatively pronounced for Cun+1 and CunV (n = 1–5) clusters. The pronounced change might be related with the planar configuration.

Table 3: VIP and EA of the most stable Cun+1 and CunV clusters.
Figure 7: Simulated photoelectron spectra of the ground state Cun+1 (in red) and CunV (in black) (n = 1–12) clusters.
Figure 7

Magnetic properties

The magnetic properties of the clusters are not only widely used in the preparation of nano electronic devices and high density magnetic storage materials, but also have a very important theoretical significance in the basic research of physics. The total magnetic moments of cluster mainly include the orbital and spin magnetic moments of electrons. The orbital magnetic moment of an electron is far less than the spin magnetic moment and, consequently, the magnetic moment of cluster is dominated by the spin magnetic moment. For the ground-state Cun+1 and CunV (n = 1–12) clusters, the total magnetic moments are calculated and displayed in Fig. 8. The lowest energy copper clusters show an odd–even alternations with the increase of Cu atom in the total magnetic moment. The magnetic moment of Cun+1 clusters with odd n is completely quenched. For the doped clusters, the magnetic moment of CunV (n = 1–8) cluster is far larger than that of Cun+1 clusters. The substitution of a V atom for a Cu atom can enhance the magnetism of the small host cluster. The Cu2V, Cu4V, Cu6V and Cu8V clusters have a magnetic moment of 3 μB, which is also the magnetic moment of a V atom. The magnetic moment (4 μB) of each CunV (n = 1, 3, 5 and 7) clusters is just equal to the sum of the magnetic moments of the Cun cluster (1 μB) and an isolated V atom (3 μB). These imply that the interaction of Cu and V atoms may have similarities among CunV (n = 1–8) clusters. In case of big CunV (n = 9–12) cluster, the Cu10V and Cu12V clusters have the same magnetic moments as Cu11 and Cu13 clusters. The magnetic moment (2 μB) of Cu9V and Cu11V clusters be greater than that (1 μB) of Cu9 and Cu11 clusters and less than that (3 μB) of V atom. The foregoing relation indicates that the big CunV (n = 9–12) clusters have a different interaction between Cu and V atoms relative to CunV (n = 1–8) clusters. As an effort to explain the magnetism, Fig. 9 gives the spin density of states (SDOS) for the global minimum structures of Cun+1 and CunV clusters. All the ground states have an intense band between −5 and −2 eV, which consists principally of the valence s and d orbitals of the constituent atoms. It is clear from the density difference that the magnetic moment of CunV clusters mostly comes from the electrons near the HOMO (EEH = −2~0 eV). The CunV clusters have some very small magnetic domains, which vary with the size of cluster.

Figure 8: Total magnetic moment of the ground state Cun+1 and CunV (n = 1–12) clusters and local magnetic moment on the dopant atom V.
Figure 8
Figure 9: SDOS of the lowest energy CunV clusters.
Figure 9

A broadening factor δ = 0.1 eV is used. Spin up (positive) and spin down (negative) densities are given in each case. The black part is the density difference (spin up minus spin down). The dashed line indicates the location of the HOMO level.

To gain insight into the magnetic properties further, we have performed the natural bond orbital analysis61 for the lowest energy CunV clusters. The local magnetic moments on V atom are 4.16 μB for CuV, 4.13 μB for Cu2V, 3.90 μB for Cu3V, 3.29 μB for Cu4V, 3.82 μB for Cu5V, 3.58 μB for Cu6V, 3.73 μB for Cu7V, 3.34 μB for Cu8V, 2.88 μB for Cu9V, 2.33 μB for Cu10V, 2.72 μB for Cu11V and 1.88 μB for Cu12V, as shown in Fig. 8. Overall, with the increase of cluster size, the magnetic moments of V atoms gradually decrease. The magnetic moments of V atom in CunV clusters is larger for n = 1–8 and smaller for n = 9–12 than that of free V atom. Compared to the free V atom, the change of magnetic moments of V atoms in CunV clusters (see Fig. 10) should reflect the strength of the interaction between V and Cu atoms. The magnetic moment provided by Cu atoms is very small. Furthermore, Cu atoms in CunV (n = 1, 9, 11 and even) clusters exhibit an antiferromagetic alignment with respect to the V atom’s magnetic moment. That is to say, the magnetic moments of these CunV clusters primarily are from a paramagnetic V atom. The charge and magnetic moment on 4s, 3d, 4p and 5p orbitals of V atom in CunV clusters are listed in Table 4. It can be seen from the table that the partially filled 3d orbital play a substantial role in determining the magnetism of V atom. The magnetic moment of 3d orbital is 1.81~3.98 μB. The 4s and 4p orbitals, which are non-magnetic for a free V atom, contribute a few of magnetic moment, apart from 4p orbital of V in CuV dimer. This may be ascribed to the internal charge transfer from 4s to 3d, 4p and 5p orbitals. Simultaneously, there are an interatomic charge transfers in CunV clusters. Namely, 0.13–0.36 electrons transfer from V atom to Cu atoms for n = 1, 3–5 and 0.28–3.46 electrons from Cu atoms to V atom for n = 2, 6–12. As we know, the d orbital can contain up to 10 electrons. If N represents the sum of valence electron on V atom in CunV clusters, we found that 10-N and the magnetic moment of V atom have the same change trend, as shown in Fig. 11. The charge transfer hints that the V atom in CunV clusters has a hybridization among s, p and d orbitals. The energy of d orbital of V atom is gradually decreased with the increase of clusters size and more and more electrons are transferred to the d orbital. Hence, the larger the cluster, the smaller the magnetic moment of V atom. The orbital hybridization and charge transfer should be responsible for the magnetic moment alteration of the dopant atom.

Figure 10: Free V atom as the reference point, the change of magnetic moments of V atom in CunV clusters.
Figure 10
Table 4: The charge (Q) and local magnetic moment (M) of 3d, 4s, 4p, and 5p states for the V atom in the ground state CunV clusters.
Figure 11: The relation between the charge and magnetic moment of V atom in CunV clusters.
Figure 11

Conclusions

Density functional calculations have been performed for the structural, electronic, and magnetic properties of Cun+1 and CunV (n = 1–12) clusters. The results show that V atom in low energy CunV clusters tend to occupy the position with the maximum coordination number and changes the geometry of the 3D host clusters. The vibrational and photoelectron spectroscopy spectra are given to identify the most stable structures in times to come. The substitution of a Cu atom in copper clusters by a V atom enhances the binding energy of big clusters and alters the odd-even oscillations of relative stability and chemical activity of the host clusters. The ground states of copper clusters are confirmed by comparing the theoretical vertical ionization potential with experimental findings. At the same time, we predict the vertical ionization potential and electron affinity of CunV clusters and electron affinity of Cun+1 cluster. The magnetism calculation indicates that V atom in CunV clusters carries most of the total magnetic moment. The local magnetic moment of the doped atom decreases with the increase of cluster size because of the orbital hybridization and charge transfer.

Additional Information

How to cite this article: Die, D. et al. Insights into the structural, electronic and magnetic properties of V-doped copper clusters: comparison with pure copper clusters. Sci. Rep. 6, 31978; doi: 10.1038/srep31978 (2016).

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Acknowledgements

This project was supported by the education department of sichuan province (grant No. 15233422) and by the key scientific research fund of Xihua University (grant No. Z0820401).

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Author notes

    • Dong Die
    •  & Ben-Xia Zheng

    These authors contributed equally to this work.

Affiliations

  1. School of Science, Xihua University, Chengdu 610039, China

    • Dong Die
    • , Ben-Xia Zheng
    • , Lan-Qiong Zhao
    • , Qi-Wen Zhu
    •  & Zheng-Quan Zhao
  2. key Laboratory of Advanced Scientific Computation, Xihua University, Chengdu 610039, China

    • Dong Die

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Contributions

D.D. and B.-X.Z. conceived the idea. B.-X.Z., L.-Q.Z., Q.-W.Z. and Z.-Q.Z. performed the calculations. D.D. and B.-X.Z. wrote the manuscript and all authors contributed to revisions.

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The authors declare no competing financial interests.

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Correspondence to Dong Die.

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https://doi.org/10.1038/srep31978

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