Electronic and Optical Properties of Vertex-Sharing Homo- and Hetero

Article pubs.acs.org/JPCC

Electronic and Optical Properties of Vertex-Sharing Homo- and Hetero-Biicosahedral Gold Clusters Takeshi Iwasa,†,‡ Katsuyuki Nobusada,§ and Atsushi Nakajima*,†,‡ †

JST-ERATO, Nakajima Designer Nanocluster Assembly Project, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan § Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan ‡

ABSTRACT: Electronic and optical properties of vertex sharing homo- and heterobiicosahedral gold clusters [M1M2Au23(PH3)10(SCH3)5Cl2]2+ (M1/M2 = Au/Au, Pt/Hg) are investigated by means of density functional computations, focusing on superatom assembly. In the homo clusters, the projected density of states onto the spherical harmonics reveals that each icosahedral unit behaves as a superatom with the Au(6s6p) electrons and electron shell closings of S and P orbitals followed by unoccupied D orbitals; each unit satisfies an 8-electron superatom condition. Because the Au(5d) band appears between the S and P orbitals, the electronic structure is characterized as S, Au(5d)-band, and P. The lowest electronic excitation in the homo cluster is the P to D transition. In the hetero biicosahedral cluster, the central Au atoms of each icosahedron are replaced by Pt and Hg. The HOMO and LUMO are well localized to the PtAu12 and HgAu12 units, respectively, forming a cluster-assembled heterojunction in a single stable structure. The absorption spectrum of the heterobiicosahedral cluster is characterized by charge transfer from PtAu12 to HgAu12 in the visible region, accompanied by a weak charge transfer in the opposite direction. Despite low numbers of charge carriers, the heterobiicosahedral cluster shows a large dipole moment.

1. INTRODUCTION Throughout the past decade, gold clusters protected by organic ligands have been extensively researched,1−6 leading to precise chemical synthesis,7,8 total structural determination,9−12 and enhanced theoretical understanding.3,12−17 Because of their optical12,18−21 and magnetic22−24 properties and catalytic activities,25,26 ligand-protected gold clusters are candidate materials for several applications. Among members of the ligand-protected gold cluster family, mixed phosphine- and thiolate-protected gold clusters are particularly interesting from a cluster assembly perspective, since they comprise clusters of cluster structures. Mixed bimetallic Au−Ag clusters, first reported by Teo et al.,27,28 were followed by reports of an all-gold cluster.10,15 In biicosahedral clusters, two 13-atom icosahedral clusters share a single atom to realize a 25-atom core, which is then protected by ligands. The stability of these clusters is attributable to superatomic shell closing of eight electrons for each Au13 unit, although one of the Au atoms is shared.15,28,29 The superatomic electronic shell is described by a spherical jellium model, in which the electronic structure is similar to that of atoms but with different order of electronic levels, for example, 1s, 1p, 1d, 2s, and so on.30,31 This model underlies the physics of magic-numbered atomic clusters.9,16,32,33 The electronic shell model explains the magic-numbered behavior of Na clusters. The model was used to explain the stability of a series of abundant-Na clusters in terms of the atom-like electronic shell closings.30,31,34 While this concept is generally applicable to bare metal or inorganic atomic clusters, © 2013 American Chemical Society

it does not extend to ligand-protected clusters such as vertexsharing biicosahedral clusters, which undergo charge transfer from the metallic core to the ligand. The stability of ligandprotected biicosahedral clusters is simply explained by the 8electron counting rule developed by Mingos, in which 16 electrons contribute to electronic shell closure, while 8 electrons are assigned to each 13-atom icosahedral cluster.29 Recently, the stabilities of various small ligand-protected gold clusters have been explained by the 8-electron rule, and the electronic structures of these gold clusters have been pictorially represented by using the projected density of states onto spherical harmonics.16 The chemical and physical properties of these electronically closed shell species may be precisely tuned by replacing their constituent atoms using state-of-the-art chemical synthetic techniques. In particular, if the central atom is replaced with a neighboring element in the periodic table, the new electronic structure resembles that of a halogen or an alkali metal, with a deficit or excess of electrons, respectively.35−37 In semiconductor physics, elements with electron deficits and excesses are interpreted as hole and electron carriers, respectively. Assembling functional clusters is fundamental to bridging the gap between cluster science and material science. In our previous study involving density functional computations, we demonstrated that heteroassemblies of doped silicon clusters Received: August 14, 2013 Revised: October 17, 2013 Published: November 7, 2013 24586

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possibly possess a strong dipole moment, can be used to develop cluster-based p−n or p−i−n junctions, and undergo photoinduced charge transfer.38 In vertex-sharing biicosahedral gold clusters, electron deficit (excess) is introduced by replacing Au with Pt (Hg) in each icosahedral unit. The resultant compound mimics p-type (n-type) semiconductor materials. Introduction of Pd atoms into gold clusters has been investigated experimentally.39,40 In this study, we use density functional computations to investigate vertex-sharing homo- and heterobiicosahedral gold clusters, focusing on their superatomic nature. In particular, heteroassemblies of doped gold clusters are characterized by a large dipole moment and emergent photoinduced charge transfer, desirable properties for cluster-based nanomaterials.

2. COMPUTATIONAL DETAILS Spin-restricted density functional computations were performed at the RI-B3LYP level41−43 using the def-SV(P) basis sets,44 with 60-electron relativistic effective core potentials45 for Au, Pt, and Hg, as implemented in TURBOMOLE 6.4 and 6.5.46,47 The electronic absorption spectra were simulated by time-dependent density functional theory,48−51 in which the line spectra are convoluted by a Lorentzian of width 0.01 eV. The absorption spectrum of the heterobiicosahedral cluster was characterized by charge transfer between the composite icosahedra using the Mulliken population of metal cores, as described in our previous study.38 Note that range-separated functionals are ideally suitable for analyzing heterobiicosahedral clusters because the clusters undergo charge transfer excitations to some extent. However, in our previous study on the M@Si16 heterodimer, the absorption spectrum of the dimer was less affected by long-range corrections than the trimer because of the dimer’s smaller size. To analyze the electronic structures in a superatom context, the Kohn−Sham (KS) orbitals ψi were projected onto a real basis of spherical harmonics Zlm, generated by SymPy.52 This projection process is well documented in Walter et al.16 In the present study, ψi(r, θ, φ) and Zlm(θ, φ) were calculated on pregenerated polar grids (r, θ, φ). To obtain the projection ⟨ψi| Zlm⟩, the product was integrated over θ and φ for each fixed r, and |⟨ψi|Zlm⟩|2 was then integrated over r. The projections were performed on doubly generated polar grids. To prevent grid overlap, the origin was placed at the central atom of the icosahedral unit in the half space. When generating the projected density of states, a suitable polar grid radius was determined as 10 au, with a radial grid spacing of 0.5 au and an angular interval of 10°. Each peak was convoluted by a Gaussian of width 0.001 au.

Figure 1. Optimized structures of (a) [M1M2Au23(PH3)10(SCH3)5Cl2]2+ and (b) M1M2Au23+ (M1/M2 = Au/Au, Pt/Hg). Gold, black, and silver spheres represent Au, M1, and M2, respectively, while red, yellow, gray, and white spheres represent P, S, C, and H, respectively.

imaginary frequencies, whereas bare Au25+ and PtHgAu23+ are optimized under D5h and C5v symmetries, respectively. The local minimum structures of the bare clusters are highly distorted with lower symmetries than those of the protected clusters. Therefore, symmetric structures are useful for comparing the protected and bare biicosahedral clusters. The representative bond lengths, HOMO−LUMO gaps, and dipole moments are summarized in Table 1. Table 1. Bond Lengths in Å, HOMO−LUMO Gaps (HLG) in eV, and Dipole Moments d in Debye of [M1M2Au23(PH3)10(SCH3)5Cl2]2+ and M1M2Au23+ (M1/M2 = Au/Au, Pt/Hg) [Au25(PH3)10 (SCH3)5 Cl2]2+ M1−Au M2−Au Au−Au Au−Cl Au−S Au−P HLG d

2.86−2.92 2.86−2.92 3.02−3.07 2.37 2.47 2.40 2.13 0.01

Au25+ 2.76−3.01 2.76−3.01 2.75−3.15

1.23 0.00

[PtHgAu23 (PH3)10 (SCH3)5 Cl2]2+ 2.83−2.92 2.89−2.94 3.00−3.14 2.37 2.45−2.50 2.40 1.68 7.64

PtHgAu23+ 2.73−3.04 2.75−3.02 2.76−3.22

1.09 2.07

3.1. Homo Biicosahedral [Au25(PH3)10(SCH3)5Cl2]2+. 3.1.1. Geometric Structures. Though the symmetry of the present structure of 1 is C5, higher than that obtained in our previous study,15 the geometric and electronic properties, such as bond lengths and HOMO−LUMO gaps, are almost identical between the two structures. According to Table 1, the Au−Au distances are more constrained in the protected structure and the HOMO−LUMO gap is widened, but these effects might arise from combined protection and charging. 3.1.2. Electronic Properties. Figure 2 shows the density of states obtained by decomposing the KS orbitals into the s, p, and d atomic orbitals of Au (DDOS) and projecting them onto the spherical harmonics (PDOS). Note that the ligand states for DDOS and higher angular momentum states for PDOS are

3. RESULTS AND DISCUSSION Figure 1 shows the optimized structures of [M1M2Au23(PH3)10(SCH3)5Cl2]2+ and M1M2Au23+ (M1/M2 = Au/Au, Pt/Hg), which are detailed in our previous study.15 Briefly, the 25-atom core of the ligand-protected structure in Figure 1a consists of two 13-atom icosahedral clusters sharing a central gold atom (Figure 1b). The cluster is protected by thiols, phosphines, and chlorines; in particular, the pair of icosahedral units is bridged by five thiols. Sections 3.1 and 3.2, respectively, discuss the homobiicosahedral cluster (M1, M2) = (Au, Au), hereafter denoted as 1, and the heterobiicosahedral cluster (M1, M2) = (Pt, Hg), denoted as 2. Both ligandprotected clusters 1 and 2 possess C5 symmetry with no 24587

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Figure 2. Density of states (DOS) of (a, b) ligand-protected and (c, d) bare Au25. (a) and (c) are obtained by decomposing the KS orbitals into atomic orbitals (DDOS), while (b) and (d) are obtained by projecting the Kohn−Sham orbitals onto spherical harmonics (PDOS, see text for details).

mainly comprise Au(5d6p), while those of bare Au25+ are Au(6s5d6p) hybridizations. The difference can be attributed to the Au−Cl bonds, because the Au(6s) contribution is given to the P state in the bare Au25+ by the terminal Au atoms. Upon the ligation, this P state includes Au−Cl σ-bond which is formed by Au(6p) orbital. By natural charge analysis, the total charge of Au25 in 1 was found to be about 3+. The difference from the expected charge of 9+ is explained as follows. Even though the Au−S and Au−Cl bonds localize parts of Au(6sp) electrons which cannot participate in superatom orbitals, these bonds have covalent nature rather than purely ionic bonds. The polarizations at Au−S and Au−Cl bonds are expressed in terms of natural charge as Au(+0.2)−S(−0.3)/Cl(−0.6), causing the deviation from the formal electron counting, where the total number of Au(6s) electrons is reduced by its cationic charge and the number of thiolates and chlorines, assuming that the electron is completely transferred from Au to S or Cl. Although interactions between the ligands and surface gold atoms may influence the electronic structure of Au25 core in the cluster 1, superatom states such as S and P are constructed irrespective of the ligand protection, implying that the jellium model is robust. It should be noted that, when Au259+ is geometrically optimized with P-shell closure, the cluster is broken into two separate Au12 units with release of the shared gold atom. Under D5h symmetry, geometry optimization of the D-shell-closed Au255− yields a cage structure, in which the central Au of the icosahedra escapes toward the Au25 termini while the Au initially located at both termini move toward the Cl positions in 1. Also, Au25+/3± are geometrically optimized into distorted structures after relaxations along the vibrational modes with the highest imaginary frequencies. Thus, to compare the bare and protected Au25 clusters, we must impose D5h symmetry on bare Au25. 3.1.3. Optical Properties. Figure 3 shows the simulated absorption spectrum of 1. Previously, we had assigned the first peak at 1.8 eV to the HOMO−LUMO transition.15 In a superatom context, this can be restated as the Pz(σ*) to Dz2(σ)

partially cut off; that is, the y scale of DOS is selected to visualize the features of interest. As shown in the DDOS (Figure 2a), the occupied states of 1 comprise the atomic orbitals of Au(6s), Au(5d)+ligands, and Au(6p), while the unoccupied states comprise primarily of Au(6s6p). According to the PDOS (Figure 2b), the symmetry of the KS orbitals around −15 eV (consisting of Au(6s)) and around −9 to −8 eV (consisting of Au(6p)) is S and P, respectively. The Au(5d) band between the S and P states is complicated, with much higher angular momentum. Here the S and P shells are closed for Au(6sp) valence electrons, supporting the previously mentioned 8-electron counting rule,10,15,29,53 which has been recently applied to other ligand-protected gold clusters.16 The PDOS of 1 is very similar to that of an Au40 gold−thiolate cluster enclosing two bare Au13,54 although a vertex Au atom is shared in 1. For comparison, the DDOS and PDOS of the bare Au25+ are shown in Figure 2c and d, respectively, to examine how the S and P states are modified by the ligand protection. Although the monocationic bare Au25+ has eight more valence electrons than the dicationic protected cluster, the superatom orbitals are also found in the same order to each other, only the difference herein is that D orbitals for Au25+ are occupied by the excess electrons. A practical reason to adopt the monocationic state is to avoid degenerate orbitals in the self-consistent field calculations. The electronic structure of the DDOS is similar to that of its protected counterpart: Au(6s), Au(5d) band, and Au(6s6p). Note that the HOMO of the bare Au25+ shows less Au(6p) character than the protected cluster, and the angular momentum is more clearly seen in the PDOS of bare Au25+ (Figure 2d). The two S orbitals around −15 eV correspond to bonding of the antibonding S orbitals, followed by the Au(5d) band starting from the D orbital. P and D orbitals (where both D orbitals are doubly degenerate) appear within the range −10 to −8 eV. The D orbitals become LUMO in the ligandprotected Au25+ core, indicating that Au25 is ionized to 9+ leaving 16 valence Au(6s6p) electrons. The P orbitals of 1 24588

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Figure 3. Simulated absorption spectrum of cluster 1. Inset shows HOMO and LUMO. The peak at 1.8 eV consists of the HOMO− LUMO transition, interpretetable as the P(σ*) to Dz2(σ) transition in a superatom context.

transition, as shown in Figure 3. The absorption peak at 2.54 eV is assigned to P to D transitions, while that at 2.58 eV is assigned to P to D transitions (50%) and Au(5d) to D transitions (25%). Finally the peak at 2.77 eV is assigned to P to D transitions (28%) and Au(5d) to D transitions (>25%). 3.2. Heterobiicosahedral [PtAu11−AuHgAu11(PH3)10(SCH3)5Cl2]2+. The optimized structure of the heterobiicosahedral cluster [PtAu11−AuHgAu11(PH3)10(SCH3)5Cl2]2+ also possesses C5 symmetry, as shown in Figure 1a. From the geometric parameters tabulated in Table 1, the Pt−Au and Hg−Au distances are comparable to the corresponding Au−Au distances, with the average length of Hg−Au being slightly longer. The bond lengths in the Auligands appear unrelated to the dopants in the icosahedral units. Like Au25+/3±, PtHgAu23+ is geometrically optimized to a distorted structure, and the C5v symmetry is again enforced for a comparison of electronic properties. The DDOS of 2 is shown in Figure 4a. The spectrum is similar to the DDOS of 1. Figure 4b shows the DDOS of the metal cores, PtAu12 and HgAu11, above and below the horizontal line, respectively. Note that the shared central Au atom is included in PtAu11 as PtAu12, because the opposite inclusion (in PtAu11) or inclusion in HgAu12 produces negligible effect on the DDOS (Figure 4a). Thus, PtAu12/11 and HgAu11/12 are hereafter denoted as Pt@Ih and Hg@Ih. As shown in Figure 4b, Hg(5d) states appear below the Hg(6s) state (around −17 eV). HOMO and HOMO-1, mainly comprising Pt(6p), localize to Pt@Ih, while LUMO to LUMO+2, mainly comprising Hg(6s), localize to Hg@Ih. The superorbitals of the HOMOs and LUMOs possess P and D symmetry, respectively (Figure 4c). To achieve P-shell closing, the D electron of Hg@Ih is transferred to the unoccupied P orbital of Pt@Ih. This electronic shell closure may induce the large HLG (1.68 eV). The shell closing can also be interpreted as depletion of charge carriers originally present within each icosahedral unit. The natural charges of the icosahedral units are [PtAu12]1.01 − [HgAu12]1.62, whereas that of the isoelectronic homobiicosahedral clusters (M1 = M2 = Au, Pt−, Hg+) are [Au13]1.35, [PtAu12]0.90, and [HgAu12]1.74. This shows the small amount of electron transfer of 0.1e from Pt@Ih to Hg@Ih when the heteroassembly is formed. The natural charge of the shared Au atom is evenly split between the units. Here, the deficit and excess charges of 2 relative to 1 are +0.34 and −0.27 in the Pt@ Ih and Hg@Ih units, respectively, where the inconsistent charge separation between the M@Ih units can be attributed to the different electronegativities of the dopants which cause different charge transfers from M@Ih to the ligands. It is worth mentioning that the natural charge of Pt and Hg for ligand-

Figure 4. Density of states for cluster 2, constructed similarly to Figure 2: (a) the overall DDOS; (b, c) the DDOS and PDOS, respectively, of PtAu12 (above horizontal line) and HgAu11 (below horizontal line).

protected homobiicosahedral clusters, Pt2Au23 and Hg2Au234+, are −1.02 and 0.04, respectively, and for 2 they are −1.04 and 0.08. Although the charge carriers are not large, 2 possesses a large dipole moment (7.64 D) due to the large size of the cluster framework compared to the diatomic molecule, suggesting applications for this heterobiicosahedral structure as a strong dipole material. The obtained dipole moment is comparable to that of the heterodimer of doped silicon clusters Sc@Si16−[email protected] However, the present cluster is more suitable for molecular-scale devices, such as molecular rectifiers,55,56 than thin-film heterojunctions, because the two icosahedral units are inseparably assembled into a single stable structure. In single-molecule devices, electrodes could be inserted by replacing the terminal Cl atoms with (for example) dithiol molecules. Furthermore, by varying the dopant atoms, the occupations, energy levels, and magnetic moment could be tailored for desired single-molecule junctions while retaining the geometric structure. Finally, the simulated absorption spectrum of 2 is shown in Figure 5. Charge transfers between the icosahedral units are also plotted in this figure. The ubiquitous charge transfer from Pt@Ih to Hg@Ih is accompanied by minor contributions from the opposite charge transfer. This charge transfer behavior is consistent with the PDOS of Figure 4c, in which the occupied and unoccupied frontier orbitals are biased toward Pt@Ih and Hg@Ih, respectively. The peak at 1.49 eV is assigned to the HOMO to LUMO+1 transition. While HOMO is localized to Pt@Ih, LUMO+1 is delocalized over the cluster. In addition, because LUMO+1 corresponds to the LUMO of 1, the signature of the HOMO to LUMO+1 transition in 2 is similar to that of the HOMO−LUMO transition in 1. The electronic 24589

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REFERENCES

(1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. MonolayerProtected Cluster Molecules. Acc. Chem. Res. 2000, 33, 27−36. (2) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications Toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (3) Häkkinen, H. Atomic and Electronic Structure Of Gold Clusters: Understanding Flakes, Cages and Superatoms From Simple Concepts. Chem. Soc. Rev. 2008, 37, 1847−1859. (4) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Gold Nanoparticles: Past, Present, and Future. Langmuir 2009, 25, 13840− 13851. (5) Parker, J. F.; Fields-Zinna, C. a; Murray, R. W. The Story of a Monodisperse Gold Nanoparticle: Au25L18. Acc. Chem. Res. 2010, 43, 1289−1296. (6) Jin, R. Quantum Sized, Thiolate-Protected Gold Nanoclusters. Nanoscale 2010, 2, 343−362. (7) Price, R. C.; Whetten, R. L. All-Aromatic, Nanometer-Scale, Gold-Cluster Thiolate Complexes. J. Am. Chem. Soc. 2005, 127, 13750−13751. (8) Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-Protected Gold Clusters Revisited: Bridging The Gap Between Gold(I)− Thiolate Complexes and Thiolate-Protected Gold Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261−5270. (9) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Structure Of A Thiol Monolayer-Protected Gold Nanoparticle At 1.1 A Resolution. Science 2007, 318, 430−433. (10) Shichibu, Y.; Negishi, Y.; Watanabe, T.; Chaki, N. K.; Kawaguchi, H.; Tsukuda, T. Biicosahedral Gold Clusters [Au25(PPh3)10(SCnH2n+1)5Cl2]2+ (n = 2−18): A Stepping Stone To Cluster-Assembled Materials. J. Phys. Chem. C 2007, 111, 7845−7847. (11) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. Crystal Structure Of The Gold Nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]. J. Am. Chem. Soc. 2008, 130, 3754−3755. (12) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating The Crystal Structure Of A Thiol-Protected Au25 Cluster And Optical Properties. J. Am. Chem. Soc. 2008, 130, 5883−5885. (13) Iwasa, T.; Nobusada, K. Theoretical Investigation of Optimized Structures Of Thiolated Gold Cluster [Au25(SCH3)18]+. J. Phys. Chem. C 2007, 25, 45−49. (14) Iwasa, T.; Nobusada, K. Gold-Thiolate Core-In-Cage Cluster Au25(SCH3)18 Shows Localized Spins In Charged States. Chem. Phys. Lett. 2007, 441, 268−272. (15) Nobusada, K.; Iwasa, T. Oligomeric Gold Clusters With VertexSharing Bi- And Triicosahedral Structures. J. Phys. Chem. C 2007, 111, 14279−14282. (16) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. A Unified View of Ligand-Protected Gold Clusters as Superatom Complexes. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157−9162. (17) Jiang, D.-E.; Overbury, S. H.; Dai, S. Structure Of Au15(SR)13 and Its Implication for the Origin of the Nucleus in Thiolated Gold Nanoclusters. J. Am. Chem. Soc. 2013, 135, 8786−8789. (18) Schaaff, T. G.; Whetten, R. L. Giant Gold−Glutathione Cluster Compounds: Intense Optical Activity In Metal-Based Transitions. J. Phys. Chem. B 2000, 104, 2630−2641. (19) Bigioni, T. P.; Whetten, R. L.; Dag, Ö . Near-Infrared Luminescence From Small Gold Nanocrystals. J. Phys. Chem. B 2000, 104, 6983−6986. (20) Huang, T.; Murray, R. W. Visible Luminescence Of WaterSoluble Monolayer-Protected Gold Clusters. J. Phys. Chem. B 2001, 105, 12498−12502. (21) Sfeir, M. Y.; Qian, H.; Nobusada, K.; Jin, R. Ultrafast Relaxation Dynamics Of Rod-Shaped 25-Atom Gold Nanoclusters. J. Phys. Chem. C 2011, 115, 6200−6207. (22) Yamamoto, Y.; Miura, T.; Suzuki, M.; Kawamura, N.; Miyagawa, H.; Nakamura, T.; Kobayashi, K.; Teranishi, T.; Hori, H. Direct

Figure 5. Simulated absorption spectrum of cluster 2, constructed similarly to Figure 3. In addition to the absorption curve (black line), charge transfer from PtAu12 to HgAu12 and its reverse are indicated in red and green, respectively. Inset shows the KS orbitals involved in the transition at 1.49 eV.

excitations in the 1−4 eV range occur among KS orbitals of energies −11.60 to −4.36 eV. Such efficient charge transfer suggests the applicability of this heterobiicosahedral cluster to photovoltaic cells. Since the degree of charge transfer greatly depends on the combination of the dopants, the dopants further influence the extent of the symmetry-breaking in the electronic structure of the cluster. Consequently, the proper selection of the dopants can enhance the present results.

4. CONCLUDING REMARKS We have investigated in detail the electronic and optical properties of homo- and heterobiicosahedral gold clusters. We focused on their superatomic nature and potential applicability to cluster-based devices. From the density of states projected onto the spherical harmonics, we identified 8-electron shell closures in each icosahedral unit of the homobiicosahedra. The electronic structure was characterized as S, Au(5d) band, P, D, and F, with the superatom orbitals mainly comprising Au(6s6p). In natural charge analysis, the Au25 core of the homobiicosahedral cluster was ionized to 3+, inconsistent with the 9+ charge expected in the 16-electron system. This difference might arise from compensation by a dense Au(5d) band. The electronic structures of the hetero- and homobiicosahedral clusters are similar. In the heterobiicosahedral cluster, the HOMO and LUMO are localized to separate icosahedral units, and a large dipole moment (7.64 D) was observed. Furthermore, photoinduced charge transfers from Pt@Ih to Hg@Ih were observed in the simulated absorption spectra. These results indicate that the heterobiicosahedral cluster is suitable for use in single-molecule junctions or photoactive materials, or may be aligned to create a strong dipole layer. To further understand the physics and chemistry of superatom assemblies, this analysis could be extended toward superatomic bondings.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-45-566-1697. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partly supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2009−2013. 24590

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4081405 | J. Phys. Chem. C 2013, 117, 24586−24591