Low-Energy Isomer Identification, Structural Evolution, and Magnetic

Article pubs.acs.org/JPCA

Low-Energy Isomer Identification, Structural Evolution, and Magnetic Properties in Manganese-Doped Gold Clusters MnAun (n = 1−16) Meng Zhang,† Hongyu Zhang,† Lina Zhao,‡ Yan Li,*,§ and Youhua Luo*,† †

Department of Physics, East China University of Science and Technology, Shanghai 200237, China CAS Key Lab for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Beijing 100049, China § Department of Physics, East China Normal University, Shanghai 200062, China ‡

S Supporting Information *

ABSTRACT: The size-dependent electronic, structural, and magnetic properties of Mn-doped gold clusters have been systematically investigated by using relativistic all-electron density functional theory with generalized gradient approximation. A number of new isomers are obtained for neutral MnAun (n = 1−16) clusters to probe the structural evolution. The two-dimensional (2D) to three-dimensional (3D) transition occurs in the size range n = 7−10 with manifest structure competitions. From size n = 13 to n = 16, the MnAun prefers a gold cage structure with Mn atom locating at the center. The relative stabilities of the ground-state MnAun clusters show a pronounced odd−even oscillation with the number of Au atoms. The magnetic moments of MnAun clusters vary from 3 μB to 6 μB with the different cluster size, suggesting that nonmagnetic Aun clusters can serve as a flexible host to tailor the dopant’s magnetism, which has potential applications in new nanomaterials with tunable magnetic properties.

1. INTRODUCTION Gold clusters have received intense research interests in nanoscience because of their unique catalytic, electronic, and optical properties.1−4 The strong relativistic effects of gold cause the Aun clusters to exhibit unique geometrical structures that isolate them from other metallic clusters. During the past two decades, pure gold clusters Aun in the small-to-medium size range have been assigned through experimental5−18 and theoretical studies.19−45 The planar gold clusters have been understood by the relativistic effects of gold and the structure of Au7 was identified in earlier theoretical studies.46,47 In 2008, Gruene et al. determined the structures of neutral Au7, Au19, and Au20 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.18 Huang et al. probed the two-dimensional (2D) to three-dimensional (3D) structural transition in gold cluster anions using argon tagging.48 Assadollahzadeh et al. performed a systematic search for minimum structures of small gold clusters Aun (n = 2−20) using density functional theory together with a relativistic pseudopotential.49 Recently, De et al. investigated the finite temperature behavior of gas phase neutral Aun (n = 3−10) clusters using relativistic density functional theory based molecular dynamical simulations.50 Previous experimental and theoretical studies showed that small gold clusters exhibit a variety of structures from 2D to 3D, including planar, pyramidal,18,29 shell-like flat-cage,38 tube-like structure,51 and fullerene-like cage.52,53 © 2012 American Chemical Society

However, a considerable amount of experimental and theoretical work has 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.54−73 Tian and co-workers studied the detailed geometric and electronic structures of PtAu bimetallic clusters within density functional theory (DFT) and demonstrated that the bonding pattern of metal cluster with impurity associates with the bonding nature of the metal of majority and the doping metal.74,75 Later, Neukermans and co-workers have investigated the stability of cationic gold clusters doped with a 3d TM atom, TMAun+, with TM from Sc to Zn,76,77 and extended their investigations to multiple TM atom doped gold clusters XnAum clusters (X = Sc, Ti, Cr, and Fe; n = 0−3; m = 1−40) by means of photofragmentation experiments.78 Li et al. studied the electronic structures and magnetic properties both experimentally and theoretically through DFT calculations in a series of anionic transition metal doped Au clusters, MAu6− (M = Ti, V, and Cr).79 Mn is considered as a prospective dopant of semiconductor materials for their conversion into a ferromagnetic phase.80 Interesting cases are the Mn−Au bimetallic systems, which have shown a variety of intriguing magnetic properties with potential applications in new nanomaterials. The icosahedral Au12 cage Received: September 30, 2011 Revised: January 6, 2012 Published: January 6, 2012 1493

dx.doi.org/10.1021/jp2094406 | J. Phys. Chem. A 2012, 116, 1493−1502

The Journal of Physical Chemistry A

Article

true minima. There is no imaginary frequency for structures presented here. In addition, for geometry optimization of each isomer, the spin multiplicity (SM) is considered at least 1, 3, 5, 7, or 9 for even-electron clusters and 2, 4, 6, 8, or 10 for oddelectron clusters. 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, we perform the calculation on Au2, Au2−, Au7, Au7−, and Au20 clusters, and the results are summarized in Table 1. All of the

filled with a single Mn atom, which shows a large binding energy compared with the icosahedral Au13 cluster, has a large local spin magnetic moment.81 Torres et al. investigated the atomic and electronic structure of MnAun+ clusters (n ≤ 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.82 In 2007, Wang et al. reported a novel gold-coated icosahedral cluster Mn13Au20− with an ultrahigh magnetic moment.83 In our previous work, we have shown that the magnetic moment of MAu6 clusters (M = Sc−Ni) can be tuned by the incorporation of different dopant atoms, which renders these clusters perfectly applicable in the design of nanomagnets.84 Later, Hö ltzl et al. used the phenomenological shell model of metal clusters to provide a reasonable rationalization for the magnetic properties of MnAu6 cluster obtained in our results.85 Recently, Yang et al. investigated the energetic and magnetic properties of the tubular cluster Au24 doped by a 3d TM atom from V to Ni.86 They found that the atom-like magnetism is retained for MnAu24 cluster. However, these previous studies focused on the ionic or special-size clusters. To the best of our knowledge, systematic investigation of the neutral Aun (n = 1−16) cluster doped with one Mn atom has not been reported. However, small metal clusters usually exhibit extraordinary size-dependent properties, and clusters of mixed metal−gold systems are known to have structures very different from pure gold clusters. In the current study, we perform a first-principles study of the manganese-doped gold clusters MnAun (n = 1−16) to systematically explore their structural evolution and properties. We focus on the size-dependent growth behavior related to the stability of the bimetallic MnAun clusters, and then, we investigate the electronic and magnetic properties of MnAun in comparison to pure gold clusters. We hope that our work would be useful to understand the influence of material structure on its properties and stimulate the experimentalist to validate our theoretical prediction.

Table 1. Calculated Bond Distance d (Å), Vibrational Frequency ωe (cm−1), Average Binding Energies Per Atom Eb (eV), Vertical Electron Detachment Energies VDE (eV), Adiabatic Electron Detachment Energies ADE (eV), Ionization Potential IP (eV), and Adiabatic Electron Affinity Energies AEA (eV) of the Au2, Au2−, Au7, Au7−, and Au20 Clusters system

property

this work

expt5−12,18,29

Au2

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

2.49 184.2 1.18 9.44 2.588 2.06 171.4 192.7 207.9 3.45 3.29 133.7 2.742

2.47 191 1.15, 1.18 9.50, 9.22 2.582 2.01 165 185 203 3.46 3.40 146 2.745

Au2− Au7

Au7− Au20

properties of these clusters computed using PBE functional, DNP basis, and VPSR pseudopotential in our work are in excellent agreement with available experimental data. For example, the computed vertical and adiabatic detachment energies of Au7− are 3.45 and 3.29 eV, which are well consistent with the experimental results,11 3.46 and 3.40 eV, respectively. Additionally, the computed adiabatic electron affinity energy of Au20 is 2.742 eV, in good agreement with the experiment, 2.745 eV.29 This indicates that our methods are reliable and accurate enough to describe the structures and properties of MnAun clusters.

2. COMPUTATIONAL METHODS We have performed density functional theory calculations of MnAun clusters using the DMOL3 program.87 Relativistic calculations are performed with scalar relativistic corrections to valence orbitals relevant to atomic bonding properties via a local pseudopotential (VPSR). All-electron spin-unrestricted calculations with a double-numerical basis set that included d polarization functions (DNP) are employed in this work. Generalized gradient approximation in the Perdue−Burke− Ernzerhof (PBE) functional form is chosen.88 The quality of self-consistent field (SCF) convergence tolerance is set as fine with a convergence criterion of 1 × 10−5 Hartree on total energy and electron density, 2 × 10−3Hartree/Å on the gradient, and 5 × 10−3 Å on the displacement in our calculation. In the present work, we choose the initial structures of MnAun clusters by two ways as follows: first, the low-lying structures are obtained by placing the Mn atom on each possible site of the Aun host cluster as well as by substituting one Au by a Mn atom in the Aun+1 cluster or substituting another impurity atom by a Mn atom from the doped clusters MAun with the configurations available from the previous studies.26,38,49−51,72−82 Second, we use the basin-hopping global optimization method to produce a large number of isomers for further DFT optimization.89,90 Harmonic vibrational frequencies are computed to confirm that the low energy isomers are

3. RESULTS AND DISCUSSIONS 3.1. Geometric Structure. In the following, we will discuss the geometric structures and properties of manganese-doped gold clusters MnAun by using the computation scheme described in section 2. We have considered extensive 2D and 3D structures to determine the lowest-energy geometry for each MnAun cluster. The obtained ground-state structures and some low-lying isomers are shown in Figure 1. As successive Au atoms are added to a Mn atom, the progression of the geometry can be seen from Figure 1. The differences of the total binding energies between an isomer and the lowest-energy structure are given. The lowest-energy structures of the corresponding bare Aun+1 clusters are also given in Figure 1 for the purpose of comparison. The lowest-energy geometries of pure Aun+1 clusters from our optimized results are in agreement with the previously reported results.36,42,45−50 The calculated vibrational frequency ωe of pure Au7 cluster with a planar edge-capped 1494



Article

Figure 1. continued

1495



Article

Figure 1. Lowest-energy structures and low-lying isomers with relative energies (in eV) of MnAun (n = 1−16) clusters. The ground-state geometries of the corresponding bare Aun+1 clusters are also given on the left. See Table 2 for the corresponding energetic and structural information. The Mn atom is represented by the purple sphere.

triangle in our work is in good agreement with the result of the experiment (see Table 1).18 In general, the 2D → 3D transition of neutral Aun is predicted to occur somewhere between n = 13 and 15. From the Figure 1, we can see that the 2D → 3D transition is found to occur at n = 14 in our calculations, in accord with the previous calculations of Assadollahzadeh et al.49

Moreover, the structural transition from flat-cage to hollowcage structure is at n = 17, which is in agreement with the results of Bulusu et al.91 For small Mn-doped Aun clusters from MnAu1 to MnAu5, the isomers of these clusters are not numerous, so we only show the lowest-energy structures. The ground-state of diatomic 1496



Article

Table 2. Various Structures with the Symmetry Type, the Spin Multiplicity (SM), the Average Binding Energy Per Atom (Eb), and HOMO−LUMO Energy Gap Egap of MnAun Clusters and the Corresponding Bare Aun+1 Clusters (n = 1−16) for the Lowest-Energy Structures pure Aun

MnAun

cluster

symmetry

Eb (eV)

Egap (eV)

cluster

symmetry

SM

Eb (eV)

Egap (eV)

Au2 Au3 Au4 Au5 Au6 Au7 Au8 Au9 Au10 Au11 Au12 Au13 Au14 Au15 Au16 Au17

D∞h C2v D2h Cs C3v Cs D4h C2v D2h Cs C2 C2v C2 Cs C2v C2v

1.23 1.34 1.74 1.90 2.16 2.11 2.25 2.25 2.35 2.36 2.44 2.44 2.54 2.48 2.56 2.61

1.758 0.369 1.017 0.355 1.764 0.253 1.001 0.253 1.211 0.197 1.012 0.153 1.499 0.176 0.762 0.114

MnAu MnAu2 MnAu3 MnAu4 MnAu5 MnAu6 MnAu7 MnAu8 MnAu9 MnAu10 MnAu11 MnAu12 MnAu13 MnAu14 MnAu15 MnAu16

C∞v D∞h C2v C2v C2v D6h C2v C2v C2v Cs C1 Cs C1 C1 C2v Td

7 6 7 6 5 4 5 6 5 6 5 6 5 4 5 6

1.29 1.89 2.02 2.16 2.22 2.32 2.30 2.42 2.37 2.53 2.51 2.57 2.57 2.63 2.68 2.74

0.779 2.278 0.402 1.254 0.707 0.717 0.526 0.793 0.267 0.841 0.365 1.44 0.412 0.094 0.116 1.284

MnAu exhibits a C∞v symmetry. For the triatomic MnAu2 cluster, the linear configuration in which the Mn atom occupies the central position with the D∞h symmetry is found to be the most stable structure. The angular isomer with the C2v symmetry, which relates to the lowest-energy structure of the Au3 cluster, is 1.06 eV higher in energy than the linear isomer. Similar to the configuration of the ground-state Au4 and Au5 clusters, a C2v rhombic form and a C2v trapezoidal isomer are the most stable structures for MnAu3 and MnAu4, respectively. The lowest-energy structure of the MnAu5 is the C2v planar structure in which the Mn atom is surrounded by five Au atoms. Small MnAun clusters containing up to six Au atoms have planar structures, and MnAu6 is the first cluster to have an interior Mn atom. A 2D structure with the transition metal Mn atom sitting in the center of an Au6 ring, which forms perfect D6h symmetry, is found to be the most stable structure for MnAu6 clusters. Two lower-symmetry 2D structures and a distorted isomer of 3D octahedral arrangement are all much higher in energy. The number of isomers for MnAun is increasing as successive Au atoms are added to a Mn atom. The top 5 lowest-energy structures located for MnAu7 to MnAu16 are depicted in Figure 1. The most stable structure for MnAu7 is the 3D configuration with C2v symmetry. The second and third lowest-energy isomers are also 3D geometries and have a large structural distortion relative to the most stable cluster with 0.22 eV and 0.59 eV higher in energy, respectively. The 7d isomer, which is the most stable 2D configuration, is 1.42 eV less stable than the 7a isomer. The planar 8a and 9a isomers are the lowest-energy structures for MnAu8 and MnAu9, which are relative to the most stable pure Au9 and Au10 clusters, respectively. All the 3D configurations of these two clusters are less stable than the lowest-energy planar isomers. Nevertheless, the energy of metastable 3d isomers of MnAu8 and MnAu9 is very close to that of the most stable 2d isomers, respectively. Figure 1 provides clear evidence for a transformation from planar to 3D structures in the size range n = 7−10 of MnAun clusters. The 2D to 3D structural evolution from MnAu7 to MnAu10 reflects that there are strong competition between the tendency to form

3D structures around the Mn atom and the tendency to form planar structures at size n = 7−10. From MnAu10 to MnAu16, all the most stable structures are 3D configurations. The geometry structures of these isomers from n = 10 to n = 16 are very different from the corresponding pure Aun+1 clusters, respectively. This means when one 3d transition Mn atom is doped into Aun clusters, it can evidently influence the structures of the gold clusters. The compact geometries 10a and 11a with low symmetry are found to be the most stable structures for MnAu10 and MnAu11 clusters, respectively. The planar isomers of these two clusters are higher in energy than the present ground-state structures. A two-double 3D structure with Cs symmetry (12a in Figure 1) is found to be the ground-state structure of MnAu12. This isomer can be considered as adding two gold atoms on the distorted ground-state structure of MnAu10. It should be mentioned that previous studies show that both of WAu12 and MoAu12 clusters with the perfect Ih symmetry in which the TM atom is located in the center of the Au12 cage are particularly stable in accord with the 18-electron rule.55,68 Here, the icosahedral structure with a Mn atom in the center is found to be 0.24 eV less stable than the lowest-energy structure 12a. Moreover, this icosahedral isomer has two imaginary frequencies in our calculations, so it is not presented here. The octahedral isomer with Oh symmetry has 0.54 eV higher in energy than the 12a isomer. From n = 13 to n = 16, the gold atoms adopt a cage-like structure, and a Mn atom occupies at the center site in the ground-state structures. The isomers with the Mn atom at a surface site are all found to be higher in energy. As successive Au atoms are added, the structures of MnAun evolve from a compact geometry toward a high Td symmetry cage with a central Mn atom, as shown in Figure 1 (16a). 3.2. Relative Stabilities. Table 2 gives various structural and energetic characteristics for the lowest-energy structures of MnAun clusters and the corresponding bare Aun+1 clusters (n = 1−16). As mentioned above, the optimization included both the geometry and the spin multiplicity. The spin multiplicities in the ground-states are also given in Table 2. The average binding energy (Eb) per atom of a given cluster is a measure of 1497



Article

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 using the following formula: Eb(MnAu n) = [nE(Au) + E(Mn) − E(MnAu n)] /(n + 1)

(1)

Eb(Au n + 1) = [(n + 1)E(Au) − E(Au n + 1)]/(n + 1) (2)

It can be seen from the Table 2 that all the average binding energies per atom for the ground-state of MnAun (n = 1−16) clusters are significantly higher than those of the corresponding pure Aun+1. This indicates that doping with the Mn atom that has the partially filled d shells can enhance the stability of the Aun clusters. Figure 2 gives the average binding energy per atom of the MnAun and bare Aun+1 clusters for the lowestenergy structures. As seen from the Figure 2, the binding energy of MnAun clusters increases monotonically with cluster size 1 ≤ n ≤ 5 and 13 ≤ n ≤ 16, but for middle size 6 ≤ n ≤ 12 there is a stair−step pattern phenomenon, where the clusters with even numbers of Au atoms have higher Eb than the preceding clusters with odd numbers of Au atoms. To further study the stabilities of MnAun clusters, we also discuss the fragmentation energy (Ef) of the ground-state structures, which is defined as the energy that is released when an Au atom is separated from these clusters. Where E(MnAun) and E(MnAun−1) represent the energy of the lowest energy structure of MnAun and MnAun−1 clusters, respectively. E f (MnAu n) = [E(MnAu n − 1) + E(Au) − E(MnAu n)] (3)

E f (Au n + 1) = [E(Au n) + E(Au) − E(Au n + 1)]

(4)

The second-order difference of the cluster’s binding energy is a sensitive quality to reflect the relative stability of a cluster compared to its neighbors, which can be defined as follows: Δ2En = [Eb(n + 1) + Eb(n − 1) − 2Eb(n)]

(5)

The size dependence of the fragmentation energy (Ef) and the second-order difference of binding energy (Δ2E) of the MnAun and bare Aun+1 clusters for the lowest-energy structures as a function of the number of Au atoms are also depicted in Figure 2 for the purpose of comparison. It is interesting to see that both Ef and Δ2E of MnAun and Aun+1 clusters exhibit strong odd−even oscillations. However, we can clearly find that odd− even oscillations of MnAun and Aun+1 clusters are in reverse order with cluster size, indicating that the presence of the Mn atom changes the stable pattern of the host clusters. The evennumbered gold atom MnAun clusters are relatively more stable than the neighboring odd-numbered gold atom clusters, in agreement with the above analysis based on average binding energy. Two conspicuous maxima for MnAun clusters are found at n = 8 and n = 10 in Figure 2. The analyses of the secondorder difference of energies and fragmentation energies indicate that MnAu8 and MnAu10 clusters possess relatively higher stability than their respective neighbors. We perform Mulliken population analysis for the groundstate structures, and the atomic charge on Mn atom of MnAun clusters is given in Table 3 and plotted in Figure 3. The charge on the impurity Mn atom, ranging from 0.357 au to 0.843 au, clearly shows that there are obvious charge transfers from Mn

Figure 2. Comparison of the average binding energy per atom (Eb), fragmentation energies (Ef), and the second-order difference of binding energy (Δ2E) of the MnAun and bare Aun+1 clusters (n = 1− 16) for the lowest-energy structures.

to Au atoms. This indicates that strong interactions between Mn and Au atoms occur in these clusters due to ionic-like bonding through the charge transfers, which can rationalize the enhancement of the interaction between Mn and Au. The charge transfers from Mn to Au atoms in MnAu8 and MnAu10 clusters are 0.649 au and 0.638 au, respectively, larger than their neighbors, which is responsible for the higher stability of these two clusters than their respective neighbors. To further have a clear view of how the Mn atom have interaction with the Au atom, the deformation electron densities of the bare Au5 and MnAu4 clusters for the lowest-energy structures as examples are plotted in Figure 4, which are defined as the total charge 1498



Article

electron deformation density distributes not only around the Mn and Au atoms but also in the intervals of MnAu4 cluster, which shows some covalent character in the Mn−Au bonds. In comparison to pure Au5 cluster, when the Mn atom substitutes one Au atom, the electron accumulation between Mn and Au atoms shows a marked increase, verifying that the strong interaction between the Mn and Au exists in MnAu4. The energy gap (Egap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is another useful quantity for examining the kinetic stability of small clusters. A large energy gap corresponds to a high energy required for electron excitation. We plot the energy gap of the most stable MnAun and Aun+1 clusters in Figure 5. There are clear odd−even

Table 3. Mulliken Charge (a.u.) on Mn Atom, Local Magnetic Moment (μB) of the Guest Mn and Au Atoms, and Total Magnetic Moment of MnAun (n = 1−16) Clusters for the Lowest-Energy Structures moment (μB) Mn system

charge (a.u.)

Aun

3d

4s

4p

local

total

MnAu MnAu2 MnAu3 MnAu4 MnAu5 MnAu6 MnAu7 MnAu8 MnAu9 MnAu10 MnAu11 MnAu12 MnAu13 MnAu14 MnAu15 MnAu16

0.357 0.561 0.471 0.583 0.600 0.618 0.549 0.649 0.530 0.638 0.616 0.609 0.738 0.756 0.783 0.843

0.412 0.095 0.778 0.238 −0.486 −1.283 −0.457 0.392 −0.640 0.422 −0.402 0.348 −0.226 −1.139 −0.243 0.482

4.751 4.603 4.603 4.467 4.276 4.089 4.256 4.401 4.443 4.371 4.224 4.446 4.090 4.022 4.096 4.386

0.702 0.236 0.468 0.184 0.123 0.098 0.108 0.118 0.151 0.094 0.082 0.103 0.041 0.032 0.057 0.041

0.137 0.070 0.155 0.112 0.098 0.096 0.093 0.089 0.052 0.110 0.095 0.105 0.093 0.083 0.088 0.090

5.588 4.905 5.222 4.762 4.486 4.283 4.457 4.608 4.640 4.578 4.402 4.652 4.226 4.139 4.243 4.518

6 5 6 5 4 3 4 5 4 5 4 5 4 3 4 5

Figure 5. HOMO−LUMO energy gap (Egap) of the MnAun and bare Aun+1 clusters (n = 1−16) for the lowest-energy structures.

oscillations in their energy gap spectrum. The Egap of MnAun clusters with even gold atoms is larger than that of the neighbors with odd gold atoms, except for MnAu14 cluster, which has the smallest Egap (0.094 eV) among these clusters. The energy gap of MnAu2 is found to be 2.278 eV, larger than any other cluster size. For the pure Aun+1 clusters, Au6 has a particularly large Egap with 1.764 eV. The energy gap of MnAu2 is 0.514 eV larger than that of Au6, indicating that MnAu2 is relatively more stable in electronic structure, and can be used as a fundamental building block for constructing the clusterassembled materials. 3.3. Magnetic Properties. In what follows, we will discuss the magnetic moments of 3d TM manganese doped Aun clusters (n = 1−16). The local magnetic moment of the Mn atom and Au atoms, and the total magnetic moment of the ground-state MnAun clusters are listed in Table 3 and plotted in Figure 6. The total magnetic moment of bare Aun+1 clusters shows a pronounced odd−even alternation (0 μB and 1 μB, respectively) with the number of Au atoms. When a Mn atom with the half-filled 3d orbital is doped into Aun clusters, the formed MnAun clusters all generate a large magnetic moment. It is found that the MnAu and MnAu3 clusters have the largest magnetic moment of 6 μB. A Mn atom has a 4s23d5 electron configuration, which results in 5 μB depicted of the free Mn atom. This indicates that the magnetic moment of these two doped clusters is enhanced upon the Mn atom. The magnetic moment of the clusters at n = 2, 4, 8, 10, 12, and 16 is 5 μB, retaining the atom-like magnetism in these bimetallic clusters. These Aun clusters act as a host protecting the spins of the

Figure 3. Aomic charge (a.u.) on Mn atom of the MnAun clusters (n = 1−16) for the lowest-energy structures.

Figure 4. Deformation electron density of the bare Au5 and MnAu4 clusters for the lowest-energy structures. The surface isovalue for molecular orbital plotting is 0.023 e/Ǻ 3.

density of a cluster with the density of the isolated atoms subtracted. The blue area indicates electron accumulation when atoms form a cluster. From the Figure 4, we can see that the 1499



Article

generalized gradient approximation. For each cluster, an extensive search of the most stable structure is conducted by considering a number of structural isomers and spin multiplicity. The results are summarized as follows: (1) A Mn atom does substantially change the atomic and electronic structures of the pure Aun clusters. The structures of MnAun clusters are quite different from the bare Aun clusters. The Mn atom prefers to stay at the center of these clusters. In the lowest-energy structures of MnAun clusters, the geometry transition from planar to 3D structure occurs in the size range n = 7−10 with strong competition between 2D and 3D structures. A structural transition from the compact structure to a hollow cage takes place at n = 13, and the structure evolves toward a high symmetry cage at n = 16. (2) A transition metal Mn atom with open d shells can stabilize the Aun clusters. The calculated fragmentation energy and the second-order difference of binding energy of the MnAun clusters show the same odd−even alternation tendency with cluster size. The clusters with an even number of Au atoms are relatively more stable than the neighbors with an odd number of Au atoms. (3) Mn-doped Aun clusters can result in variable magnetic properties. The total magnetic moments of MnAun clusters vary from 3 μB to 6 μB by changing the number of the host Au atoms. Thus, Aun cluster can serve as a flexible host to tailor the spins of the dopant magnetic atom, which are very interesting in designing novel types of nanomaterials with desired magnetic properties. We are now extending these studies to explore whether similar properties may occur by doping other 3d transition metal atoms into the Aun clusters. We hope that our work would offer relevant information to stimulate further experimental work.

Figure 6. Local magnetic moment on Mn atom and total magnetic moment of the MnAun and bare Aun+1 clusters (n = 1−16) for the lowest-energy structures.

dopant Mn atom, while the Mn atom helps stabilize the Aun structures. The clusters with odd Au atoms at n = 5, 7, 9, 11, 13, and 15 have 4 μB magnetic moments, suggesting that the magnetic moment of the Mn atom is partly reduced as doped into these Aun clusters. MnAu6 and MnAu14 have the smallest magnetic moments of 3 μB among those clusters. Höltzl et al. demonstrated that the stability of the quartet state MnAu6 with the D6h point group can be explained using an extension of the phenomenological shell model of metal clusters.85 It is noteworthy that previous studies showed that the magnetic moment of 3d dopants in the doped Ag clusters and Cu clusters was fully quenched because these clusters attain 18-electron shell closing.61,92 In contrast, the total magnetic moment of MnAun clusters is not quenched. Consequently, Aun cluster can serve as a flexible host to enhance, protect, or reduce the spins of the dopant magnetic atom. The local magnetic moment on 3d, 4s, and 4p states of the Mn atom for the most stable MnAun clusters obtained with the Mulliken populations are listed in Table 3. We can see that, the total magnetic moment of the clusters is mainly localized in the Mn atom. A small amount of magnetic moments are found in host Au atoms. The local magnetic moments contributed by the 3d state electrons of Mn are about 4.022−4.751 μB. The 4s and 4p subshells, which are nonmagnetic for a free Mn atom, provide a little contribution to the magnetic moments. This may be ascribed to the internal charge transfer from the 4s and 4p orbital to the 3d orbital and the hybridization between the 3d state and the 4s and 4p states. To perform a detailed analysis of molecular orbitals, we depict the HOMO and LUMO orbitals of the MnAun (n = 1−16) clusters for the lowest-energy structures (Figure 7 in Supporting Information). From Figure 7 (Supporting Information), we can easily see that there is an obvious overlap of the sd orbitals of the host Au atoms and the d orbitals of the impurity Mn atom in MnAun clusters. Such hybridization can lead to an effective change of d occupancy on the Mn atom, which plays a crucial role in determining the magnetic moments.

■

ASSOCIATED CONTENT

S Supporting Information *

HOMO and LUMO orbitals of the MnAun (n = 1−16) clusters for the lowest-energy structures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y. Li); [email protected] (Y. Luo).

■

ACKNOWLEDGMENTS This work is financially supported partly by the National Natural Science Foundation of China (Grant no. 11104075) and by the Fundamental Research Funds for the Central Universities of China (Grant no. WM0911005).

■

REFERENCES

(1) Haruta, M. Catal. Today 1997, 36, 153. (2) Schwerdtfeger, P. Angew. Chem., Int. Ed. 2003, 42, 1892. (3) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (4) Pyykkö, P. Nat. Nanotechnol. 2007, 2, 273. (5) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (6) Simard, B.; Hackett, P. A. J. Mol. Spectrosc. 1990, 142, 310. (7) Ho, J.; Ervin, K.; Lineberger, W. J. Chem. Phys. 1990, 93, 6987. (8) Taylor, K.; Pettitte-Hall, C.; Cheshnovsky, O.; Smalley, R. J. Chem. Phys. 1992, 96, 3319.

4. CONCLUSIONS The geometrical structures, growth-pattern behaviors, relative stabilities, and electronic and magnetic properties of manganese impurity doped Aun clusters have been systematically studied by using relativistic all-electron density functional theory with 1500



Article

(9) Jackschath, C.; Rabin, I.; Schulze, W. Ber. Bunsenges. Phys. Chem. 1992, 96, 1200. (10) Barnett, R. N.; Cleveland, C. I.; Häkkinen, H.; Luedtke, W. D.; Yamouleas, C.; Landsman, U. Eur. Phys. J. D 1999, 9, 95. (11) Häkkinen, H.; Yoon, B.; Landman, U.; Li, X.; Zhai, H.-J.; Wang, L.-S. J. Phys. Chem. A 2003, 107, 6168. (12) Balasubramanian, K.; Liao, D. W. J. Chem. Phys. 1991, 94, 5233. (13) Gingerich, K. A.; Blue, G. D. J. Chem. Phys. 1973, 59, 185. (14) Schaaff, T. G.; Cullen, W. G.; First, P. N.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutieérez-Wing, C.; Ascensio, J.; Jose-Yacamaá, M. J. J. Phys. Chem. B 1997, 101, 7885. (15) Koga, K.; Takeo, H.; Ikeda, T.; Ohshima, K. I. Phys. Rev. B 1998, 57, 4053. (16) Cleveland, C. L.; Landman, U.; Schaaff, T. G.; Shafigullin, M. N.; Stephens, P. W.; Whetten, R. L. Phys. Rev. Lett. 1997, 79, 1873. (17) Palpant, B.; Prevel, B.; Lerme, J.; Cottancin, E.; Pellarin, M.; Treilleux, M.; Perez, A.; Vialle, J. L.; Broyer, M. Phys. Rev. B 1998, 57, 1963. (18) Gruene, P.; Rayner, D. M.; Redlich, B.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Science 2008, 321, 674. (19) Liao, D. W.; Balasubramanian, K. J. Chem. Phys. 1992, 97, 2548. (20) Häberlen, O. D.; Schmidbauer, H.; Rösch, N. J. Am. Chem. Soc. 1994, 116, 8241. (21) Michaelian, K.; Rendon, N.; Garzón, I. L. Phys. Rev. B 1999, 60, 2000. (22) Fernández, E. M.; Soler, J. M.; Balbás, L. C. Phys. Rev. B 2006, 73, 235433. (23) Hess, B. A.; Kaldor, U. J. Chem. Phys. 2000, 112, 1809. (24) Grönbeck, H.; Andreoni, W. Chem. Phys. 2000, 262, 1. (25) Omary, M. A.; Rawashdeh-Omary, M. A.; Chusuei, C. C.; Fackler, J. P.; Bagus, P. S. J. Chem. Phys. 2001, 114, 10695. (26) Furche, F.; Ahlrichs, R.; Weiss, P.; Jacob, C.; Gib, S.; Bierweiler, T.; Kappes, M. J. Chem. Phys. 2002, 117, 6982. (27) Zhang, Z.; Berg, A.; Levanon, H.; Fessenden, R.; Meisel, W. D. J. Am. Chem. Soc. 2003, 125, 7959. (28) Zheng, J.; Petty, J. T.; Dickson, R. M. J. Am. Chem. Soc. 2003, 125, 7780. (29) Li, J.; Li, X.; Zhai, H. J.; Wang, L. S. Science 2003, 299, 864. (30) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (31) Wang, J.; Wan, G.; Zhao, J. Phys. Rev. B 2002, 66, 035418. (32) Zhao, J.; Yang, J.; Hou, J. G. Phys. Rev. B 2003, 67, 085404. (33) Niemietz, M.; Gerhardt, P.; Ganteför, G.; Kim, Y. D. Chem. Phys. Lett. 2003, 380, 99. (34) Sugawara, K.; Sobott, F.; Vakhtin, A. B. J. Chem. Phys. 2003, 118, 7808. (35) Kim, Y. D.; Fischer, M.; Ganteför, G. Chem. Phys. Lett. 2003, 377, 170. (36) Olson, R. M.; Varganov, S.; Gordon, M. S.; Metiu, H.; Chretien, S.; Piecuch, P.; Kowalski, K.; Kucharski, S. A.; Musial, M. J. Am. Chem. Soc. 2005, 127, 1049. (37) Fernández, E. M.; Soler, J. M.; Garzón, I. L.; Balbás, L. C. Phys. Rev. B 2004, 70, 165403. (38) Bulusu, S.; Li, X.; Wang, L. S.; Zeng, X. C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 8326. (39) Lechtken, A.; Schooss, D.; Stairs, J. R.; Blom, M. N.; Furche, F.; Morgner, N.; Kostko, O.; Von Issendorf, B.; Kappes, M. M. Angew. Chem., Int. Ed. 2007, 46, 2944. (40) Häkkinen, H.; Landman, U. Phys. Rev. B 2000, 62, R2287. (41) Gilb, S.; Weis, P.; Furche, F.; Ahlrichs, R.; Kappes, M. J. Chem. Phys. 2002, 116, 4094. (42) Idrobo, J. C.; Walkosz, W.; Yip, S. F.; Ö ğüt, S.; Wang, J. L.; Jellinek, J. Phys. Rev. B 2007, 76, 205422. (43) Bonačić-Koutecký, V.; Burda, J.; Mitric, R.; Ge, M. J. Chem. Phys. 2002, 117, 3120. (44) Ferrighi, L.; Hammer, B.; Madsen, G. K. H. J. Am. Chem. Soc. 2009, 131, 10605. (45) Li, X. B.; Wang, H. Y.; Yang, X. D.; Zhu, Z. H.; Tang, Y. J. J. Chem. Phys. 2007, 126, 084505.

(46) Häkkinen, H.; Moseler, M.; Landman, U. Phys. Rev. Lett. 2002, 89, 033401. (47) Tian, W. Q.; Ge, M.; Sahu, B. R.; Wang, D.; Yamada, T.; Mashiko, S. J. Phys. Chem. A 2004, 108, 3806. (48) Huang, W.; Wang, L. S. Phys. Rev. Lett. 2009, 102, 153401. (49) Assadollahzadeh, B.; Schwerdtfeger, P. J. Chem. Phys. 2009, 131, 064306. (50) De, H. S.; Krishnamurty, S.; Mishra, D.; Pal, S. J. Phys. Chem. C 2011, 115, 17278. (51) Yuan, D. W.; Wang, Y.; Zeng, Z. J. Chem. Phys. 2005, 122, 114310. (52) Johansson, M. P.; Sundholm, D.; Vaara, J. Angew. Chem., Int. Ed. 2004, 43, 2678. (53) Ji, M.; Gu, X.; Li, X.; Gong, X. G.; Li, J.; Wang, L. S. Angew. Chem., Int. Ed. 2005, 44, 7119. (54) Chen, M. X.; Yan, X. H. J. Chem. Phys. 2008, 128, 174305. (55) Pyykkö, P.; Runeberg, N. Angew. Chem., Int. Ed. 2002, 41, 2174. (56) Fa, W.; Dong, J. M. J. Chem. Phys. 2008, 128, 144307. (57) Wang, L. M.; Bulusu, S.; Huang, W.; Pal, R.; Wang, L. S.; Zeng, X. C. J. Am. Chem. Soc. 2007, 129, 15136. (58) Tanaka, H.; Neukermans, S.; Janssens, E.; Silverans, R. E.; Lievens, P. J. Am. Chem. Soc. 2003, 125, 2862. (59) Janssens, E.; Tanaka, H.; Neukermans, S.; Silverans, R. E.; Lievens, P. New J. Phys. 2003, 5, 46. (60) Yamada, Y.; Castleman, A. W. Jr. J. Chem. Phys. 1992, 97, 4543. (61) Sun, Q.; Gong, X. G.; Zheng, Q. Q.; Sun, D. Y.; Wang, G. H. Phys. Rev. B 1996, 54, 10896. (62) Sun, Q.; Wang, Q.; Yu, J. Z.; Li, Z. Q.; Wang, J. T.; Kawazoe, Y. J. Phys. I 1997, 7, 1233. (63) Van De Walle, J.; Tarento, R. J.; Joyes, P. Surf. Rev. Lett. 1999, 6, 307. (64) Bouwen, W.; Vanhoutte, F.; Despa, F.; Bouckaert, S.; Neukermans, S.; Kuhn, L. T.; Weidele, H.; Lievens, P.; Silverans, R. E. Chem. Phys. Lett. 1999, 314, 227. (65) Heinebrodt, M.; Malinowski, N.; Tast, F.; Branz, W.; Billas, I. M. L.; Martin, T. P. J. Chem. Phys. 1999, 110, 9915. (66) Koyasu, K.; Mitsui, M.; Nakajima, A.; Kaya, K. Chem. Phys. Lett. 2002, 358, 224. (67) Li, X.; Kiran, B.; Li, J.; Zhai, H. J.; Wang, L. S. Angew. Chem. 2002, 114, 4980. (68) Li, X.; Kiran, B.; Li, J.; Zhai, H. J.; Wang, L. S. Angew. Chem., Int. Ed. 2002, 41, 4786. (69) Häkkinen, H.; Abbet, S.; Sanchez, A.; Heiz, U.; Landman, U. Angew. Chem., Int. Ed. 2003, 42, 1297. (70) Wang, L. M.; Pal, R.; Huang, W.; Li, X.; Zeng, X. C.; Wang, L. S. J. Chem. Phys. 2010, 132, 114306. (71) David, J.; Guerra, D.; Hadad, C. Z.; Restrepo, A. J. Phys. Chem. A 2010, 114, 10726. (72) Ghanty, T. K.; Banerjee, A.; Chakrabarti, A. J. Phys. Chem. C 2010, 114, 20. (73) Nhat, P. V.; Nguyen, M. T. Phys. Chem. Chem. Phys. 2011, 13, 16254. (74) Tian, W. Q.; Ge, M.; Gu, F.; Yamada, T.; Aoki, Y. J. Phys. Chem. A 2006, 110, 6285. (75) Tian, W. Q.; Ge, M.; Gu, F.; Aoki, Y. J. Phys. Chem. A 2005, 109, 9860. (76) Neukermans, S.; Janssens, E.; Tanaka, H.; Silverans, R. E.; Lievens, P. Phys. Rev. Lett. 2003, 90, 033401. (77) Tanaka, H.; Neukermans, S.; Janssens, E.; Silverans, R. E.; Lievens, P. J. Chem. Phys. 2003, 119, 7115. (78) Janssens, E.; Tanaka, H.; Neukermans, S.; Silverans, R. E.; Lievens, P. Phys. Rev. B 2004, 69, 085402. (79) Li, X.; Kiran, B.; Cui, L. F.; Wang, L. S. Phys. Rev. Lett. 2005, 95, 253401. (80) Ohno, H. Science 1998, 281, 951. (81) Wang, S. Y.; Yu, J. Z.; Mizuseki, H.; Sun, Q.; Wang, C. Y.; Kawazoe, Y. Phys. Rev. B 2004, 70, 165413. (82) Torres, M. B.; Fernández, E. M.; Balbás, L. C. Phys. Rev. B 2005, 71, 155412. 1501



Article

(83) Wang, J. L.; Bai, J.; Jellinek, J.; Zeng, X. C. J. Am. Chem. Soc. 2009, 129, 4110. (84) Zhang, M.; He, L. M.; Zhao, L. X.; Feng, X. J.; Luo, Y. H. J. Phys. Chem. C 2009, 113, 6491. (85) Höltzl, T.; Lievens, P.; Veszprémi, T.; Nguyen, M. T. J. Phys. Chem. C 2009, 113, 21016. (86) Yang, A.; Fa, W.; Dong, J. M. J. Phys. Chem. A 2010, 114, 4301. (87) DMOL3 is a density functional theory program distributed by Accelrys, Inc. See also Delley, B. J. Chem. Phys. 1990, 92, 508. (88) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (89) Doye, J. P. K.; Wales, D. J. J. Phys. Chem. A 1997, 101, 5111. (90) Wales, D. J.; Scheraga, H. A. Science 1999, 285, 1368. (91) Bulusu, S.; Zeng, X. C. J. Chem. Phys. 2006, 125, 154303. (92) Janssens, E.; Neukermans, S.; Nguyen, H. M. T.; Nguyen, M. T.; Lievens, P. Phys. Rev. Lett. 2005, 94, 113401.

1502


Low-Energy Isomer Identification, Structural Evolution, and Magnetic

Recommend Documents