Density Functional Study on the Structural, Electronic, and Magnetic

Article pubs.acs.org/JPCA

Density Functional Study on the Structural, Electronic, and Magnetic Properties of 3d Transition-Metal-Doped Au5 Clusters Zheng Ben-Xia,† Die Dong,*,†,§ Wang Ling,† and Yang Ji-Xian† †

School of Physics and Chemistry, and §Key Laboratory of Advanced Scientific Computation, Xihua University, Chengdu 610039, China ABSTRACT: Density functional calculations have been performed for the structural, electronic, and magnetic properties of Au5M (M = Sc−Zn) clusters. Geometry optimizations indicate that the M atoms in low-energy Au5M isomers prefer to occupy the most highly coordinated position. The ground-state clusters except Au5Sc possess a planar structure. The vibrational spectra of the doped clusters are completely different from that of a pure gold cluster. The relative stability and chemical activity are investigated through the averaged binding energy and energy gap for the most stable Au5M clusters. It is found that the impurity atoms (not including the Zn atom) can enhance the thermal stability of the host cluster. The chemical activity of Au5M clusters is higher than that of the Au6 cluster. The calculated energy gaps are in accord with available approximate experimental data. The vertical ionization potential, the electron affinity, and photoelectron spectrum are computed and simulated theoretically for all of the ground-state clusters. The magnetism analyses show that the magnetic moment of these Au5M clusters varies from 0 to 5 μB by substituting a Au atom in a Au6 cluster with various M atoms and is mainly localized on the M atom for M = Ti−Ni. the cluster increase.35 For Au5X+ (X = Sc, Ti, Cr, and Fe) systems, the number of delocalized electrons not only depends on the kind and amount of constituting atoms but also on the shape of the cluster.36 At the same time, most of these investigations focused on the neutral 3p-atoms- and cationic transition-metal-doped Au5 clusters. As far as we know, there are relatively few systematic works regarding the doped Au5 clusters. On the other hand, it is well-known that the bimetallic clusters often have intriguing properties, which should be very different from those of the atoms or bulk materials, in virtue of the so-called surface and size effects. Consequently, in this paper, the geometrical, electronic, and magnetic properties of small Au5M (M = Sc−Zn) clusters will be studied by means of density functional theory (DFT). It is hoped that this work could provide detailed information to understand the influence of dopant atom and would be of help to chemists, especially those designing new materials.

1. INTRODUCTION During the last few decades, the binary clusters have attracted considerable attention of a wide range of researchers.1−18 Theoretical and experimental studies have demonstrated that the nature of the cluster can be altered significantly with the addition of a single impurity atom.11,19−29 Doping of gold clusters with an impurity atom has been actively sought to tailor the desired structural, electronic, catalytic, and magnetic properties for potential applications in nanotechnology, microelectronics, solid-state chemistry, and materials science.30−50 For example, Zhang et al. proved that doping with different 3p impurity atoms (Al, Si, P, S, and Cl) can strongly influence the structure, stability, electronic property, and growth pattern of gold clusters, which is very distinct from the case of 3d transition-metal-doped Aun clusters.30 Kumar et al. carried out ab initio calculations on Gd-doped gold clusters and found a magic cage cluster Gd@Au15 that has the attractive features of a large highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap and a great magnetic moment that could make it useful for both phototherapies of cancer cells as well as bioimaging.31 Yang et al. reported that the infrared spectrum of tubular cluster Au24 can be changed by the dopant atoms (V, Cr, Mn, Fe, Co, and Ni) and the atom-like magnetism is retained for all of the doped gold clusters.32 Recently, the Au5 cluster doped with foreign atoms has been investigated owing to their unique physical and chemical properties. It was shown that Au5Na, Au5Mg, Au5S, and Au5S− clusters prefer planar triangular structures, while Au5Al, Au5Si, and Au5P clusters have threedimensional (3D) geometries.33,34 The positive charge of cationic Au5M+ (M = Sc, Ti, V, Cr, Mn, and Fe) clusters is mostly localized in the M atom and decreases when the size of © XXXX American Chemical Society

2. COMPUTATIONAL METHODS The exchange−correlation functional proposed by Perdew (PW91), as implemented in the GAUSSIAN09 program package,51 has been selected in geometry optimizations of Au5M (M = Sc−Zn) clusters. The def2-TZVPP and LANL2DZ basis sets52−54 were used for Au and M atoms, respectively. To search the global minimum, more than 20 principal initial configurations for each Au5M cluster had been taken into account in their geometry optimizations. Due to the spin Received: April 7, 2014 Revised: May 13, 2014

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dx.doi.org/10.1021/jp503391p | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Table 1. Bond Length (r), Dissociation Energy (De), Vertical Ionization Potential (VIP) or Vertical Detachment Energy (VDE), and EA of Au2, AuNi, Au6Ti−, Au6V−, and Au6Cr−− Clusters r(Å) dimer Au2

AuNi Au6Ti− Au6V− Au6Cr− a

De(eV)

VIP/VDE(eV)

EA(eV)

functional/basis set

calc.

expt.

calc.

expt.

calc.

expt.

calc.

expt.

PW91/TZVPP PBE/TZVPP BLYP/TZVPP B3LYP/TZVPP PW91/PAWb PW91/Ni/LanL2DZ PW91/Ti/LanL2DZ PW91/V/LanL2DZ PW91/Cr/LanL2DZ

2.51 2.52 2.56 2.55 2.52b 2.37

2.47a

2.30 2.28 2.10 1.95 2.34b 2.55

2.29a

9.51 9.44 9.37 9.36

9.50a

1.88 1.82 1.78 1.79

1.94a

3.55 3.29 3.28

3.32d 3.25d 3.25d

2.35c

2.55c

References 55 and56. bReference 58. cReferences 59 and60. dReference 37.

Figure 1. Ground-state structures of Au6 and Au5M (M = Sc−Zn) clusters. Bond lengths are in Å. The point group and spin multiplicity are given below them. The yellow and black balls represent Au and the doped atoms, respectively.

Figure 2. Low-lying configurations of Au5M (M = Sc−Zn) clusters.

3. RESULTS AND DISCUSSION A. Geometrical Structures and Vibrational Spectra. An extensive structural search has been carried out for the pure and doped gold clusters. The most stable structures of Au6 and Au5M (M = Sc−Zn) clusters are plotted in Figure 1. Three low-lying isomers (LLIs) of each Au5M cluster, which closely approach the most stable structure in energy, are depicted in Figure 2. According to the energy difference from low to high, the three isomers are denoted by LLI-1, LLI-2, and LLI-3 and listed in Table 2 for each doped cluster. Geometry optimizations for Au6 manifest that the closepacked equilateral triangle is energetically lower than other

polarization, every initial configuration was optimized at various possible spin states. Harmonic vibrational frequencies were computed at the same level of theory to confirm that the optimized structure corresponds to a local minimum of the potential energy surface. In all computations, the convergence thresholds were set to 1.5 × 10−5 Hartree/Bohr for the forces, 6.0 × 10−5 Å for the displacement, and 10−6 Hartree for a total energy. The accuracy of the current scheme has been tested by calculations on Au2, AuNi, Au6Ti−, Au6V−, and Au6Cr− clusters. The calculated results summarized in Table 1 are in good agreement with available experimental findings. B



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Table 2. LLI, Spin Multiplicity (SM), and Energy Difference (ΔE) Compared to the Ground-State Structure and Characteristic Frequency (fc) of Au5M (M = Sc−Zn) Clusters clusters

LLI-1

SM

ΔE(eV)

fc(cm−1)

LLI-2

SM

ΔE(eV)

fc(cm−1)

LLI-3

SM

ΔE(eV)

fc(cm−1)

Au5Sc Au5Ti Au5V Au5Cr Au5Mn Au5Fe Au5Co Au5Ni Au5Cu Au5Zn

C7 C8 C1 C6 C1 C2 C2 C3 C6 C4

1 2 5 6 7 4 3 2 1 2

0.02 0.002 0.10 0.44 0.54 0.10 0.30 0.55 0.46 0.05

273 304 234 162 10 257 259 57 208 179

C10 C9 C9 C3 C11 C1 C6 C11 C3 C5

1 2 3 6 5 4 3 2 1 2

0.11 0.008 0.17 0.74 0.67 0.24 0.38 0.96 0.50 0.11

331 300 274 31 249 236 224 245 178 133

C8 C10 C8 C12 C3 C3 C3 C4 C5 C3

1 2 3 6 5 4 3 2 3 2

0.23 0.06 0.35 0.98 0.83 0.63 0.62 1.00 1.45 0.31

290 332 278 201 43 39 181 184 227 173

Figure 3. Vibrational spectra of the most stable Au6 and Au5M (M = Sc−Zn) clusters.

occupy the most highly coordinated site. This phenomenon is in accord with the principle of maximum overlap in molecular orbital theory. The energy of isomers with similar configuration increases as the coordinated number of the dopant atom in these isomers decreases. The most stable structures of Au5M (M = Sc−Zn) clusters possess 3D structure for M = Sc and a planar structure for M = Ti−Zn. The ground-state 3D structure of Au5Sc may be attributed to the fact that the radius of the Sc atom is much bigger than that of the Au atom. The combination of calculated and recorded vibrational spectra is a good method for the structure determination of isolated clusters, and this method has been successfully applied in practice.49 Therefore, the vibrational spectra of the most stable Au6 and Au5M (M = Sc−Zn) clusters were computed and are displayed in Figure 3. The characteristic frequency in vibrational spectra of M-doped Au5 clusters is 279, 275, 270, 219, 259, 166, 243, 238, 234, 212, and 181 cm−1 for M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Au atoms, respectively. The 3D structure (Au5Sc) is larger in characteristic frequency than the planar structures (Au5M, M = Ti−Zn). Thereinto, the characteristic frequencies of a planar butterfly-like configuration are greater than those of the planar triangle structure. This may be related to the coordination number of the dopant, and the coordination number is five for the butterfly-like one and four for the triangle structure. In addition, the Au5Ti, Au5V, and Au5Mn clusters, whose structures are very much alike, have a

configurations and the ground-state structure. This is in accord with previously reported results using different theoretical methods.56−58 The most stable structure of the Au5Sc cluster was obtained by optimizing a face-capped square-pyramid and possesses a 3D configuration. The Sc and three Au atoms form a dihedral angle of 161.2° for ∠1234 (see Figure 1). The nexthigher-energy isomer is a C7 structure in Figure 2. Due to the Jahn−Teller effect, the C7 isomer of Au5Sc has a slight distortion from C2v to C2 symmetry. A planar structure with the dopant atom occuping the five-fold coordination site was found to be the lowest-energy structure of Au5Ti, Au5V, and Au5Mn clusters. The triangle structure, which resembles the groundstate Au6 cluster, is a LLI for Au5Ti, Au5V, and Au5Mn clusters. Meantime, this structure for the Au5Ti cluster lies 0.62 eV above the LLI-3 isomer. With regard to Au5M (M = Cr, Fe, Co, Ni, Cu, Zn) clusters, the most stable structures, where these M atoms are four-fold-coordinated, exhibit a planar triangle structure similar to the lowest-energy Au6 cluster. Another planar triangular isomer with the M atom occupying the apex position has a big energy difference relative to the former. In addition, the LLIs of Au5Fe and Au5Zn clusters evidently favor planar structures. As for their 3D configurations, the pentagonal pyramid, tetragonal bipyramid, and so forth, were found to be unstable or substantially higher in energy than the third LLI. From our optimized equilibrium isomers, it is clear that the 3d transition-metal atoms in low-energy Au5M clusters tend to C



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similar vibrational spectrum. Nevertheless, the vibrational spectra of Au5M (M = Cr, Fe−Zn) clusters that have a similar structure are entirely different. For the LLIs, a number of bands are found in the region between 10 and 332 cm −1, and their characteristic frequencies are given in Table 2. The most intense peak is related to the M−Au stretching vibration. B. Electronic Properties. In this part, the electronic properties of the most stable Au6 and Au5M (M = Sc−Zn) clusters are analyzed on the basis of the atomic averaged binding energies, the HOMO−LUMO energy gaps, the vertical ionization potential (VIP), the vertical electron affinity (VEA), and photoelectron spectroscopy (PES). The atomic averaged binding energies (Eb) of the Au6 and Au5M clusters can be calculated as follows E b(Au6) =

[6E(Au) − E(Au6)] 6

E b(Au5M) =

[5E(Au) + E(M) − E(Au5M)] 6

Figure 5. HOMO−LUMO energy gaps of the most stable Au5M (M = Sc−Zn) and Au6 clusters.

(M = Sc−Zn) atom can enhance the chemical activity of the host cluster. For all neutral clusters, the HOMO−LUMO energy gaps can be estimated experimentally by PES spectra of the corresponding anionic clusters.61 The experimental PES spectra of Au6−, Au5Ni−, Au5Cu−, and Au5Zn− were reported by Koyasu et al.62 The energy difference between the first and second peaks in PES spectra of anionic clusters are an approximate measure of the HOMO−LUMO gap of the corresponding neutral clusters. From the experimental findings,62 the energy gaps of Au6, Au5Ni, Au5Cu, and Au5Zn clusters have been measured and are 2.20, 0.30, 2.00, and 0.48 eV, respectively. The measured values are in agreement with our calculated results (2.15, 0.28, 1.93, and 0.30 eV for Au6, Au5Ni, Au5Cu, and Au5Zn). The large energy gap of the Au5Cu cluster suggests that this cluster should be chemically inert and useful as a building block for constructing the cluster-assembled materials. The VIP and VEA are two basic quantities to get an insight into the electronic property and can be estimated as follows

(1)

(2)

where E(Au6), E(Au), E(M), and E(Au5M) denote the energy of the Au6 cluster, Au atom, dopant atom, and Au5M cluster. The calculated binding energies per atom for the most stable Au6 and Au5M clusters are shown in Figure 4. The Eb of Au5M

Figure 4. Atomic-averaged binding energy of the most stable Au5M (M = Sc−Zn) and Au6 clusters.

VIP = E(cluster +) − E(cluster)

(3)

VEA = E(cluster) − E(cluster −)

(4)

+

−

where E(cluster ) and E(cluster ) are the single-point energies of the cationic and anionic clusters in the neutral geometry. For the most stable Au6 and Au5M (M = Sc−Zn) clusters, the calculated VIP, VEA, the available experimental findings, and previous theoretical data are presented in Table 3. The calculated values are in agreement with previous results. The reliability of the present project was again validated. The VIPs of doped clusters Au5M are smaller for M = Sc−Co and Zn and greater for M = Ni and Cu than that of the Au6 cluster. From Figure 1 and Table 3, one can see that the EAs correlate highly with the structure and are 2.00−2.09 eV for the triangular structure, 2.34 eV for the 3D one, and 2.50−2.77 eV for the butterfly-like ones. In other words, similar configurations have a close EA. To facilitate comparison with further experimental results, the simulated PES spectra of the most stable Au6 and Au5M clusters were drawn 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 shown in Figure 6. Referring to the PES spectra, it is obvious that the distribution of energy levels in the range of 7−14 eV is smaller for the pure Au6 cluster than that for doped clusters Au5M (M ≠ Cu). The PES spectrum of Au5Cu is slightly different from that of Au6, and this can be explained by similar electronic configurations nd10(n +1)S1 of Cu and Au atoms. Several sharp peaks occur in the PES spectra of doped clusters due to the substitution of a M atom for a Au atom, which reduces the symmetry of the host

clusters is bigger for M = Sc−Cu and smaller for M = Zn than that of the Au6 cluster. That is to say, the Zn atom apart, the substitute of a Au atom by one 3d transition-metal atom increases the thermal stability of the host cluster. This change should be caused by the different electronic shell of the dopant atoms. As we know, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu atoms have opened the d or s shell, but the d and s shells of the Zn atom are completely closed. Therefore, the binding energy between M (M = Sc−Cu) and Au atoms is greater than that between Zn and Au atoms. The HOMO−LUMO energy gap (Eg), which depends on the eigenvalues of the HOMO and LUMO energy levels, is a very important parameter that reflects chemical activity of small metal clusters. In general, a cluster with high symmetry has a large energy gap that corresponds to a high chemical stability. For the most stable Au6 and Au5M (M = Sc−Zn) clusters, the HOMO−LUMO energy gaps are calculated and displayed in Figure 5. The Au6 cluster with D3h symmetry has an energy gap of 2.15 eV, which approaches the calculated value (2.06 eV) of Xiao et al.58 The addition of a 3d transition-metal atom disrupts the highly symmetrical geometry of the Au6 cluster. Therefore, the energy gaps of all Au5M clusters are smaller than that of the pure Au6 cluster. This means that the presence of a single M D



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Table 3. VIP and EA of the Ground-State Au5M (M = Au and Sc−Zn) Clusters and the Charge (Q) and Local Magnetic Moment (M) of 3d, 4s, 4p, and 5p States for the Doped Atom in the Most Stable Au5M Clusters M-3d

M-4s

M-4p

M-5p

doped clusters

VIP (eV)

VEA (eV)

Q(e)

M (μB)

Q(e)

M (μB)

Q(e)

M (μB)

Q(e)

M (μB)

Au6 Au5Sc Au5Ti Au5V Au5Cr Au5Mn Au5Fe Au5Co Au5Ni Au5Cu Au5Zn

8.41(8.80)a 7.69 7.70 (7.20)b 7.31 7.29 7.24 6.92 7.42 10.16 8.55(8.53)b 6.14(6.30)b

2.09(2.06)a 2.34 2.50(2.46)b 2.77 2.00 2.55 2.02 2.09 2.03 2.03 2.08

1.55 2.77 3.78 4.93 5.53 6.78 7.85 8.96 9.88 9.98

0 1.09 2.32 4.61 4.33 3.10 2.01 0.90 0 0

0.51 0.48 0.47 0.38 0.51 0.53 0.54 0.46 0.55 0.92

0 0.04 0.05 0.12 0.07 0.05 0.02 0.04 0 0.10

0.04 0.04 0.04 0.08 0.39 0.13 0.12 0.10 0.11 0.50

0 0 0 0 0.03 0.01 0 0 0 0.02

0.92 0.74 0.75 0.20 0.43 0.28 0.26 0.26 0.25 0

0 0 0.01 0.02 0.01 0 0 0 0 0

a Experimental values of Au6 cluster in parentheses; see refs 57 and63. bPrevious theoretical results of Au5Ti, Au5Cu, and Au5Zn clusters in parentheses; see refs 64−66.

Figure 6. Simulated photoelectron spectra of the most stable Au5M (M = Sc−Zn, in black) and Au6 (in red) clusters.

cluster. The distinct PES spectra can be used by experimenters to identify the cluster structures. C. Magnetic Properties. Atomic clusters are a remarkable medium to explore magnetism because their size, local structure, and atomic compositions can be readily controlled. For the most stable Au5M (M = Sc−Zn) clusters, the total magnetic moments are calculated and displayed in Figure 7. It can be seen from this figure that the magnetic moment of Au5M (M = Sc−Cu) is 1 μB smaller than that of the corresponding free M atom. This implies that an unpaired electron of a gold cluster paired up with the unpaired electron of a M atom. The Au5Zn cluster has an odd number of valence electrons and a total magnetic moment of 1 μB. The total magnetic moments of Au5M increase gradually from Au5Sc to Au5V and decrease linearly from Au5Cr to Au5Cu. The variable magnetic moments may have potential utility in new nanomaterials with tunable magnetic moment. Compared to the

Figure 7. Total magnetic moment of the most stable Au5M (M = Sc− Zn) and local magnetic moment on the dopant atom.

E



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Figure 8. SDOS of the most stable Au5M (M = Sc−Zn, in black) and Au5 (in red) clusters. A broadening factor of δ = 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.

and 5p shells. Simultaneously, there are an interatomic charge transfers in the Au5M clusters. Namely, 0.02−0.04 electrons transfer from Au atoms to the M (M = Sc−V) atom, and 0.14− 0.60 electrons transfer from the M (M = Cr−Zn) atom to Au atoms. The orbital hybridization and charge transfer should be responsible for the magnetic moment change of the magnetic dopant atoms.

nonmagnetic Au6 cluster, the substitution of a Au atom by an individual M (M = Ti−Ni, Zn) atom can enhance the magnetism of the host cluster. As an effort to interpret the magnetism, Figure 8 shows the spin density of states (SDOS) for the most stable Au6 and Au5M clusters. All clusters have an intense band between −6 and 0 eV, which consists principally of the valence s and d orbitals of the constituent atoms. It is evident from Figure 8 that the magnetic moment of Au5Ti, Au5V, Au5Cr, and Au5Zn clusters principally comes from the electrons near the HOMO (E − EH = −1−0eV). The Au5Fe, Au5Co, and Au5Ni clusters have a similar SDOS, and the majority of their magnetic moment originated from the electrons in the range of −2.0 to −0.5 eV. The magnetic moment of the Au5Mn cluster is randomly distributed between 0 and −5.5 eV. Other nonmagnetic clusters (Au5Sc and Au5Cu) have a symmetric SDOS. To understand the magnetic properties further, we have performed the natural bond orbital analysis67 for the most stable Au5M (M = Sc−Zn) clusters. The local magnetic moments on M atom are completely quenched for M = Sc and Cu atoms and are 1.13, 2.38, 4.75, 4.44, 3.16, 2.03, 0.94, and 0.12 μB for M = Ti, V, Cr, Mn, Fe, Co, Ni, and Zn atoms, as shown in Figure 7. The combination of a magnetic M atom and Au atoms reduces the magnetic moments of the former on account of the orbital hybridization. However, the magnetic moment of Ti, V, Cr, Mn, Fe, Co, and Ni atoms still predominates in the total magnetic moment of the corresponding doped cluster. A small amount of magnetic moments are found on the Au atoms. Moreover, Au atoms in Au5M (M = Ti, V, Mn, Fe, and Co) clusters exhibit an antiferromagetic alignment with respect to the M atom’s magnetic moment. The charge and magnetic moment on 3d, 4s, 4p, and 5p shells of all dopant atoms are listed in Table 3. It is found that the magnetic moment of Ti, V, Cr, Mn, Fe, Co, and Ni atoms originates chiefly from the open 3d shell, and the magnetic moment of the open 3d shell has been reduced by 0.39−1.10 μB. Most of the electrons in the 4s shell of each M atom have transferred to its 3d, 4p, and 5p shells for M = Sc−V and Mn−Ni. The Cr, Cu, and Zn atoms have an internal charge transfer from 4s to 4p

4. CONCLUSIONS The geometrical, electronic, and magnetic properties of small Au5M (M = Sc−Zn) clusters have been studied by firstprinciples density functional calculations. The structural searches show that the M atoms in the most stable Au5M clusters, which adopt a planar structure except for the Au5Sc cluster, favor the most highly coordinated position. The analysis of the binding energy per atom indicates that the 3d transitionmetal M atom in the Au5M cluster can improve the thermal stability of the gold cluster apart from the Zn atom. The calculated HOMO−LUMO gaps are in line with the approximate experimental findings. The chemical activity of Au5M clusters is higher than that of Au6 clusters. The simulated vibrational and PES spectra should be helpful for the identification of the most stable Au5M structures in the coming experiments. The magnetism calculations reveal that the magnetic moment of all Au5M clusters varies from 0 to 5 μB, and the M (M = Ti−Ni) atom in the corresponding doped clusters carries most of the total magnetic moment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Key Scientific Research Fund of Xihua University (Grant No: Z1213320) and the Open Research Fund of the Key Laboratory of Advanced F



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Scientific Computation, Xihua University (Grant No: szjj2012035).

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