Tuning Magnetic Moments by 3d Transition-Metal-Doped Au6 Clusters

J. Phys. Chem. C 2009, 113, 6491–6496

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Tuning Magnetic Moments by 3d Transition-Metal-Doped Au6 Clusters Meng Zhang, Li-Ming He, Li-Xia Zhao, Xiao-Juan Feng, and You-Hua Luo* Department of Physics, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed: December 16, 2008; ReVised Manuscript ReceiVed: March 5, 2009

The geometries, electronic, and magnetic properties of the 3d transition-metal-doped gold cluster: M@Au6 clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. It is found that all the ground states of the M@Au6 clusters doped with an open d shells transition-metal atom are planar structures, in which the transition metal atom is located in the center of an Au6 ring. All doped clusters show larger relative binding energies compared with pure Au7 cluster, indicating doping by 3d transition-metal atoms could stabilize the Au6 ring and promote the formation of a new binary alloy cluster. The magnetism calculations demonstrate that the magnetic moments of M@Au6 clusters vary from 0 to 4 µB by doping different transition-metal atoms into Au6 ring, suggesting that M@Au6 could have potential utility in new nanomaterials with tunable magnetic properties. I. Introduction Gold is an element whose unique properties are strongly influenced by relativistic effects. The strong relativistic effect leads to reduced 5d-6s energy gap and strong s-d hybridization. During the past two decades, small gold clusters have been intensively studied by both experiment1-13 and various theoretical methods14-47 due to their unique catalytic, electronic, and optical properties. On the other hand, clusters doped with the transition metal atoms with a partially filled d shell are important as they can be tuned for the tailored physicochemical properties, for example, the hybridization of the impurity d states with the host metal plays a crucial role in determining the local magnetic moments of the clusters.48-52 In the past few years, a considerable amount of experimental and theoretical work have been carried out on gold clusters doped with a transition metal (TM) atom in order to tailor the desired structural, magnetic, and chemical properties for potential applications.53-69 Neukermans and co-workers70,71 have investigated the stability of cationic gold clusters doped with a 3d TM atom, AunTM+, with TM from Sc to Zn, and extended their investigations to multiply TM atoms doped gold clusters AuNXM clusters (X ) Sc, Ti, Cr, Fe; N ) 1-40, M ) 0-3) by means of photofragmentation experiments.72 Torres et al.73 investigated the atomic and electronic structure of AunM+ clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Au; n e 9) using first-principles density functional calculations in which the magnetic moment showed pronounced odd-even effects as a function of the cluster size and resulted in values very sensitive to the geometrical environment. Recently, Li et al.74 studied the electronic structures and magnetic properties both experimentally and theoretically through density functional theory (DFT) calculations in a series of anionic transition-metal-doped Au clusters, M@Au6- (M ) Ti, V, Cr). Although a number of experimental and theoretical studies reported the geometric structures and electronic properties of transition-metal atom doped small gold clusters, most of these works were focused on anionic or cationic clusters. To the best of our knowledge, systematic investigation of the neutral Au6 * To whom correspondence should be addressed.

TABLE 1: Calculated Bond Distance d (Å), Vibrational Frequency ωe (cm-1), Binding Energies Per Atom Eb (eV), Vertical Electron Detachment Energies VDE (eV), Adiabatic Electron Detachment Energies ADE (eV), and Ionization Potential IP (eV) of the Au2, Au2-, Au7, Au7-, and Ti@Au6Clusters system Au2

Au2Au7 Au7Ti@Au6-

property

this work

experiment1-7,13,40,74

d ωe Eb IP d VDE ωe1 ωe2 ωe3 VDE ADE VDE ADE

2.49 178 1.18 9.43 2.587 2.08 168.2 190.0 205.5 3.45 3.28 3.36 3.35

2.47 191 1.15, 1.18 9.50, 9.22 2.582 2.01 165 185 203 3.46 3.40 3.32 3.32

cluster doped by 3d transition-metal atoms, (M@Au6, with M from Sc to Ni) has not been reported. Thus, motivated by the available results of the experiment of the neutral Au7 cluster in the gas phase13 and the experiment of the transition-metal-doped M@Au6- clusters (M ) Ti, V, Cr),74 we perform a firstprinciples study of the transition-metal-doped gold clusters: M@Au6 (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) to explore their structural and magnetic properties. It is found that all transition metal atoms with open d shells can stabilize the Au6 ring structure, and the magnetic moments of the M@Au6 clusters vary from 0 to 4 µB by doping different transition-metal atom. We describe our computational details in Section II and present our results and discussion in Section III. Finally, the conclusion is given in Section IV. II. Computational Methods The calculations in this work are carried out by using DFT75,76 implemented in the DMol3 package.77 The electron density functional is treated by the generalized gradient approximation (GGA) with the PW91 exchange-correlation functional param-

10.1021/jp811103u CCC: $40.75  2009 American Chemical Society Published on Web 03/27/2009

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Figure 1. Equilibrium geometries of the lowest energy structure and low-lying isomers for pure Au7 clusters and transition-metal-doped M@Au6 clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) using density functional theory with the PW91 functional, DND basis, and VPSR pseudopotential (see text for details). The M atom is represented by green spheres. See Table 2 for the corresponding energetic and structural information.

TABLE 2: Various Structures with the Symmetry Type, the SM, the Binding Energy (BE) Per Atom, DE per atom, HOMO-LUMO energy gap Egap, and energy separation of the isomeric structures from the ground-state structures (∆E) of M@Au6 (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) clustersa ground state

low-lying isomer

cluster

structure

SM

symmetry

BE (eV)

DE (eV)

Egap (eV)

structure

∆E (eV)

structure

∆E (eV)

structure

∆E (eV)

Au7 Sc@Au6 Ti@Au6 V@Au6 Cr@Au6 Mn@Au6 Fe@Au6 Co@Au6 Ni@Au6 Cu@Au6 Zn@Au6

a c c c c c c c c d d

2 2 3 4 5 4 3 2 1 2 1

Cs D6h C2h C2h D2h D6h C2h C2V C2V Cs Cs

2.22 2.75 2.70 2.52 2.28 2.35 2.38 2.29 2.30 2.17 2.01

2.26 5.96 5.62 4.36 2.68 3.20 3.35 2.78 2.86 1.96 0.83

1.08 0.22 0.18 0.12 0.76 0.72 0.50 0.77 0.33 1.07 1.14

b d d d d d d d d e e

0.15 0.52 0.31 0.65 0.05 1.83 0.10 0.35 0.26 0.14 0.18

e e e e e e e e c f

0.58 0.42 0.68 1.65 2.07 0.33 0.65 0.31 1.04 0.38

f f f f f f f f f c

2.20 1.76 0.93 -

a

The hyphen (-) means the structure is not stable with an imaginary vibrational mode.

etrized by Perdew and Wang.78 All-electron spin-unrestricted calculations with scalar relativity (via VPSR tag) and doublenumerical basis set that included d polarization functions (DND)79 are employed in this work. The direct inversion in iterative subspace (DIIS)80,81 approach is used to speed up SCF convergence. Mulliken population analysis is made to obtain the net spin populations on each atom. The Global orbital cutoff quality is set as “fine” to describe the electronic wave functions. The thermal smearing applied to the orbital occupation is not chosen. Self-consistent field calculations were done with a convergence criterion of 2 × 10- 5 hartree on total energy in our calculation. We confirm the stability of the lowest-energy structures as minima of the potential energy surface by considering vibra-

tional frequency. There is no imaginary frequency for structures reported here. In addition, for geometry optimization of each isomer, the spin multiplicity (SM) was considered at least 1, 3, and 5 for even-electron clusters (Ti, Cr, Fe, Ni) and 2, 4, and 6 for odd-electron clusters (Sc, V, Mn, Co). If the total energy decreases with increasing of SM, we will consider higher spin state until the energy minimum with respect to SM is reached. In order to check the validity of the computational method used for the M@Au6 clusters, we perform the calculation on Au2, Au2-, Au7, Au7-, and Ti@Au6 clusters, respectively, and the results are summarized in Table 1 All of the properties of these clusters computed using PW91 functional, DND basis, and VPSR pseudopotential in our work are in excellent agreement with available experiment data. For example, the

Tuning Magnetic Moments by M@Au6 Clusters

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TABLE 3: Magnetic Moment (µB) of 3d, 4s, and 4p States for Guest M Atom, Mulliken Charge (au), Total Magnetic Moment (µB) of the M Atom, and Total Magnetic Moment of the Ground-State M@Au6 (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) Clusters moment (µB)

computed vertical and adiabatic detachment energies of Ti@Au6- are 3.36 and 3.35 eV, which are quite consistent with the experimental results,74 3.32 ( 0.02, 3.32 ( 0.02 eV, respectively. This indicates that our methods are reliable and accurate enough to describe the structures and properties of M@Au6 clusters.

M atom system

charge (au)

3d

4s

4p

local

total

Sc@Au6 Ti@Au6 V@Au6 Cr@Au6 Mn@Au6 Fe@Au6 Co@Au6 Ni@Au6

0.951 0.786 0.686 0.593 0.624 0.616 0.445 0.443

0.102 1.185 2.494 3.737 4.086 2.977 1.885 -0.008

0.003 0.025 0.102 0.113 0.098 0.082 0.056 0

0.003 0.038 0.059 0.107 0.095 0.052 0.031 0

0.108 1.249 2.507 3.954 4.277 3.110 1.968 -0.009

1 2 3 4 3 2 1 0

III. Results and Discussion A. Geometric Structure. We have explored a number of low-lying isomers and determined the lowest-energy structures for M@Au6 clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) using the computation scheme described in Section II. Here, we considered extensive two-dimensional (2D) structures and threedimensional (3D) structures with six Au atoms surrounding a

Figure 2. The TDOS of M@Au6 (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) clusters. Spin up (positive) and spin-down (negative) densities are given in each case. The dashed lines indicate the location of the HOMO level.

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Figure 3. The PDOS for (a) Sc atom in Sc@Au6 cluster; (b) Au6 ring in Sc@Au6 cluster; (c) Co atom in Co@Au6 cluster; (d) Au6 ring in Co@Au6 cluster. Spin up (positive) and spin-down (negative) densities are given in each case. The dashed lines indicate the location of the HOMO level.

Figure 4. The PDOS of s, p, and d orbital for (a) Cr atom in Cr@Au6 cluster; (b) Au6 ring in Cr@Au6 cluster; (c) Ni atom in Ni@Au6 cluster; (d) Au6 ring in Ni@Au6 cluster. Spin up (positive) and spin-down (negative) densities are given in each case. The dashed lines indicate the location of the HOMO level.

central impurity 3d atom. The obtained ground-state structures and some low-lying metastable isomers are shown in Figure 1, where a green sphere refers to the impurity atom M. The lowestenergy structures for pure Au7 clusters are also plotted in Figure 1 for the purpose of comparison. We found a planar edge-capped triangle with Cs symmetry (Figure 1a) to be lowest in energy for pure Au7 cluster. This structure has been previously unambiguously assigned by comparing the experimental spectrum (far-IR multiple-photon dissociation (FIR-MPD) spectroscopy in the gas phase) with the calculated vibrational spectra for multiple isomers.13 A hexagonal planar structure with D6h symmetry (Figure 1b) is the metastable structure of Au7; it is higher in energy by 0.154 eV than the edge-capped triangle one. A 2D structure with the transition metal atom sitting in the center of an Au6 ring (see Figure 1c) is found to be the most stable structure for all M@Au6 clusters (M ) Sc, Ti, V, Cr,

Mn, Fe, Co, Ni). Other lower-symmetry 2D structures (see Figure 1d,f) and a distorted isomer of 3D octahedral arrangement (see Figure 1e) are all much higher in energy for all these clusters. Our calculations show that the ground-state 2D structures of Sc@Au6 and Mn@Au6 clusters form perfectly symmetrical D6h structures, although the Ti@Au6, V@Au6, Cr@Au6, Fe@Au6, Co@Au6, and Ni@Au6 have a slight inplane distortion due to the Jahn-Teller effect. It is interesting to know if the structure and stability of the clusters will be changed when an atom with full 3d shell is doped into Au6 cluster compared with the case of by doping open 3d transitionmetal atoms. Here, we also calculated the Cu@Au6 and Zn@Au6 clusters. It should be noted that the ground-state of Cu@Au6 and Zn@Au6 is different and turned to be another 2D structure, a planar edge-capped triangle (see Figure 1d). Both Cu and Zn atoms have completely filled d shells. However, Sc, Ti, V, Cr, Mn, Fe, Co, and Ni, all have open d shells. So we can conclude

Tuning Magnetic Moments by M@Au6 Clusters that a partially filled d shell of the transition-metal atoms plays a crucial role in determining the structures and properties of M@Au6 clusters. Table 2 gives various structural and energetic characteristics of M@Au6 clusters. Generally, the binding energy (BE) of a given cluster is a measure of its thermodynamic stability, which is defined as the difference between the energy sum of all the free atoms constituting the cluster and the total energy of the cluster. It can be seen from the table that the binding energies per atom for the ground state of M@Au6 (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) clusters are higher than that for pure Au7, indicating that all doped transition metal atoms with open d shells can stabilize the Au6 ring structure. To further study the stabilities of M@Au6 clusters, we will discuss the doping energy (DE) of the ground-state structure, which is defined as the energy sum of the Au6 and the M atom minus the total energy of the M@Au6. From the Table 2 we can see that the Sc@Au6 cluster has the highest DE by 5.96 eV than those of its neighbors. So, Sc@Au6 should be the most stable one among these clusters. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap is another useful quantity for examining the kinetic stability of clusters. A large energy gap corresponds to a high strength required to perturb the electronic structure. We compare the Egap’s of all M@Au6 and find the Egap of the Zn@Au6 with 1.14 eV is the largest one, and larger than the Egap of the pure Au7 cluster. This suggests that Zn@Au6 is relatively more chemically stable than the neighboring clusters. B. Magnetic Moment. In what follows, we will discuss the magnetic moments of open 3d transition-metal-doped Au clusters M@Au6. The spin populations of M atom and total magnetic moments in the ground-state M@Au6 are listed in Table 3 It is interesting to note certain trends of the magnetic moments of M@Au6 in this work as follows. (1) Table 3 shows that all the clusters are magnetic except the Ni@Au6. A Ni atom inside an Au6 ring is found to exhibit no magnetic moment, which means the magnetic moment of the impurity Ni atom is completely quenched. (2) The magnetic moments of the M@Au6 clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) are 1, 2, 3, 4, 3, 2, 1, and 0 µB, respectively. Since the magnetic moments of the M@Au6 clusters vary from 0 to 4 µB, we can come to the conclusion that doping the different 3d transition metal into the Au6 ring has potential utility in new nanomaterials with tunable magnetic properties. In the previous work74 of Li et al., they reported that all the neutral M@Au6 clusters preferred higher spin multiplicities (SM ) 3, 4, and 5 for M ) Ti, V, and Cr, respectively) and carried large magnetic moments (2, 3, and 4 µB for M@Au6, M ) Ti, V, and Cr, respectively). This is quite consistent with the result obtained in our work. Torres et al.73 investigated the atomic and electronic structure of AunM+ clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Au; n e 9) using firstprinciples density functional calculations in which the local magnetic moment of the doped atom showed a pronounced odd-even oscillation with the number of Au atoms. By comparing the data of different charge states between the neutral M@Au6 clusters in this work and the cation AunM+ clusters in Torres,73 it may provide more insight into the electronic and magnetic properties of these clusters. As an effort to explain this phenomenon of the magnetic moment in M@Au6, here we plot the total density of states (TDOS) of the clusters M@Au6 as shown in Figure 2 The DOS is obtained by Gaussian extension applied to the eigenvalues and the broadening width parameter is chosen to be 0.1 eV. The Fermi level of cluster is presented as a dashed vertical line

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6495 and shifted to zero. Spin-up (positive) and spin-down (negative) densities are given in each case. To explore the hybridization between 3d transition-metal atom and Au atoms, in Figure 3 we plot the partial densities of states (PDOS) of Sc and Au6 in Sc@Au6 cluster as well as the PDOS of Co and Au6 in Co@Au6 as two examples. For Sc@Au6 and Co@Au6, the valence electron configurations of the free atoms Sc and Co are 3d14s2 and 3d74s2, respectively. In accordance with the Hund’s rule, the magnetic moments of Sc and Co atoms are 1 and 3 µB, respectively. As the Sc is entrapped into the Au6 ring, because there is some hybridization between the atomic orbitals of the guest atom Sc and host atom Au, the magnetic moment of Sc atom is reduced to only 0.108 µB, although the total magnetic moment of Sc@Au6 is still 1 µB. The PDOS plots in Figure 3 give a good indication of this hybridization. As we can see there are several sharp peak superpositions of PDOS both for Sc and Au6, and for Co and Au6. Similar behavior is observed for all the other M@Au6 (M ) Ti, V, Cr, Mn, Fe, Ni) clusters. Figure 4 gives the PDOS of s, p, and d orbitals of dopant atoms Cr, Ni, and Au6 ring in Cr@Au6 and Ni@Au6, respectively. Cr@Au6 has the largest total magnetic moment with 4 µB in the series of M@Au6; on the other hand, Ni@Au6 has completely no magnetic moment. We find that the shapes of the R and β DOS of the d states not only in Cr atom but also in Au6 ring are always quite different (see Figure 4a,b). This situation is the same to s and p states of Cr@Au6 cluster, while the shapes of R and β DOS of the s, p, and d states for Ni@Au6, respectively, become perfectly matching (see Figure 4c,d). It also can be clearly seen from the Figure 4 that the electronic states below Fermi level are mainly come from d state and the contributions from s and p states are very little. The same situation happens to the other M@Au6 cluster. IV. Conclusion The lowest-energy geometries and magnetic properties of 3d transition-metal impurity doped Au6 clusters have been systematically studied by using relativistic all-electron density functional theory with generalized gradient approximation. The results are summarized as follows. (1) The most stable structure for all M@Au6 clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) is a 2D structure with the transition metal atom sitting in the center of an Au6 ring. (2) All transition-metal atoms with open d shells can stabilize the Au6 ring structure. This indicates that the Au6 ring could serve as a perfect host to stably store a single 3d transition metal atom in the M@Au6 system. (3) The magnetic moments of these clusters vary from 0 to 4 µB by doping different transition metal atoms with open d shells into Au6 ring. This indicates that M@Au6 clusters (M ) Sc, Ti, V, Cr, Mn, Fe, Co, Ni) could be used as a new nanomaterials with tunable magnetic properties. Acknowledgment. This work is financially supported partly by the National Natural Science Foundation of China (Grant 10747130) and the Foundation for the research starting of East China University of Science and Technology (Grant YK0157103). References and Notes (1) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (2) Simard, B.; Hackett, P. A. J. Mol. Spectrosc. 1990, 142, 310. (3) Ho, J.; Ervin, K.; Lineberger, W. J. Chem. Phys. 1990, 93, 6987. (4) Taylor, K.; Pettitte-Hall, C.; Cheshnovsky, O.; Smalley, R. J. Chem. Phys. 1992, 96, 3319.

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