Nanomagnetism
Chemically Induced Magnetism in Atomically Precise Gold Clusters Katla Sai Krishna, Pilarisetty Tarakeshwar, Vladimiro Mujica, and Challa S. S. R. Kumar* Recent investigations demonstrate unexpected and unusual magnetic behavior in a wide range of nanoscale materials – metal nanoparticles, metal oxide nanoparticles, nanocrystalline films- which are otherwise diamagnetic (i.e. nonmagnetic) in their bulk phase.[1] Most prominent among these are ligand stabilized ferromagnetic noble metal nanoparticles such as Au, Ag, Cu and Pt.[2,3] A number of gold nanoparticles stabilized by different ligands have been investigated for their magnetic behavior and it is now well documented that the magnetism is chemically-induced and strongly sizedependent.[4] These magnetic properties are intrinsically related to their electronic structure, which is influenced both by the size of the nanoparticle and the nature of the ligand decorating it.[5] While strongly binding dodecanethiol-capped Au nanoparticles of 1.4 nm in diameter exhibit ferromagnetism, a weakly binding tetraoctylammonium capped ∼1.5 nm Au nanoparticle is diamagnetic.[6] We have also recently demonstrated ferromagnetism in peptide-capped gold nanoshells (∼0.5 nm thick) and the possibility to modulate their magnetic behavior by step-wise functionalization of stabilizing ligands.[7] It is generally observed that ferromagnetism at room temperature in gold nanoparticles prevails
Dr. K. S. Krishna, Dr. C. S. S. R. Kumar Center for Advanced Microstructures and Devices (CAMD) Louisiana State University Baton Rouge, LA 70806, USA Center for Atomic-Level Catalyst Design, #324 Cain Department of Chemical Engineering Louisiana State University Baton Rouge, LA 70803, USA E-mail: [email protected] Dr. P. Tarakeshwar, Prof. V. Mujica Department of Chemistry and Biochemistry Arizona State University Tempe, AZ 85287–1604, USA Prof. V. Mujica Department of Chemistry Northwestern University Evanston, Illinois 60208, USA Center for Nanoscale Materials Argonne National Laboratory Argonne, IL, 60439, USA DOI: 10.1002/smll.201302393 small 2014, 10, No. 5, 907–911
in thiol-stabilized Au nanoparticles.[8] The crucial electronic event involved on the onset of magnetization in capped gold nanoparticles is a spin symmetry breaking associated with the 5d and 6s electrons of the Au atoms involved in the chemical bond with the ligands, which in turn modifies the relative spin densities at the Fermi energy thus creating a non-zero magnetic moment and a corresponding magnetization. Miyake and coworkers have recently reported diameter dependence (size effect) on ferromagnetism of dodecanethiol-capped gold nanoparticles.[9] Table S1 (Supporting Information) summarizes the previously reported literature on size-dependent magnetic properties observed in ligand-stabilized gold nanoparticles. A simple correlation between size and magnetic behavior in gold nanoparticles from the previously published results can be drawn. In several instances, thiol-capped gold nanoparticles of size around 2 to 3 nm have predominantly exhibited ferromagnetic behavior. When metal nanoparticles enter the quantum size regime (typically < 2 nm), their properties are extremely sensitive to the particle size. For instance, the characteristic surface Plasmon resonance (SPR) band which dominates the optical spectrum of larger gold nanoparticles is replaced by steplike multiband, due to the discretization of the energy spectrum.[10,11] Impressive progress has been made in the past few years in bringing atomic level control in the synthesis of nanoparticles starting from the original Brust's method[12] followed by size focusing methodology developed by Jin and coworkers[13] and expanded further by other researchers.[14] Despite all this significant progress in studying the size-controlled optical and magnetic behavior in ligand-stabilized gold nanoparticles, they are still more or less heterogeneous in terms of size with respect to atomic precision. Surprisingly, there are no published reports focusing on the experimental magnetic behavior of atomically precise gold clusters (APGCs) which are < 2 nm. Investigations in this direction are extremely important as ligand stabilized APGCs provide a means to chemically turn-on and tune-in their magnetism and thereby providing an opportunity to tailor-make atomically precise nanomagnets. The ability to probe magnetism with atomic precision is hitherto an unchartered area of investigation. As an example of the potential of our technique, we have fabricated atomically precise, ligand-stabilized Au25, Au38 and Au55 clusters and characterized them.[15–18] We investigated their magnetic properties experimentally using the superconducting quantum interference device (SQUID).
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Figure 1. Ligand-stabilized APGCs with their experimentally observed magnetic behavior; structures adapted from the literature.[15,19,20]
Figure 1 illustrates the different ligand-stabilized APGCs along with their experimentally observed magnetic behavior. In addition, we provide reasonable theoretical justification of our experimentally observed magnetic behavior by examining their electronic structure and the density of states (DOS). Figure S1 shows the UV-Vis absorption spectra of the as-prepared clusters, which are in agreement with the reported literature. The mixed-ligand-stabilized [Au25(PPh3)10(SC12H25)5Cl2]2+ clusters show discrete peaks typical of molecule-like electronic levels. The spectrum is similar to that of the earlier reported spectrum by Tsukuda et al. for the biicosahedral Au25 clusters.[15] The spectrum for the thiol-stabilized Au25[(SC2H4Ph)18]− clusters show broad absorption bands at 400, 450 and 670 nm, which are similar to characteristic bands reported previously for the clusters.[16] The anionic Au25[(SC2H4Ph)18]− clusters were however oxidized to neutral Au25(SC2H4Ph)18 clusters through aerial oxidation[21] before the magnetic measurements were carried out on them. The spectrum for Au38(SC12H25)24 clusters show step-wise absorption bands at 400 nm and 625 nm respectively, which represent the characteristic bands for the clusters.[17] The spectrum for Au55(PPh3)12Cl6 clusters exhibit no peaks but an approximately exponential decay with increasing wavelength, that matches the spectra reported by Schmid et al.[22] Figure S2 shows the matrix-assisted laser desorption ionization mass spectrum (MALDI-MS, using trans-2-[3(4-tert-butylphenyl)-2-methyl-2-propenylidene] malanonitrile (DCTB) as the matrix) for the as-synthesized thiol and mixed-ligand stabilized Au25 clusters and Au38 clusters. The mass spectrum for thiol-stabilized Au25 clusters shows the molecular ion peak at 7388 m/z (theoretical: 7390 m/z) with its predominant fragment at ∼6054 m/z due to the loss of Au4(SC2H4Ph)4 unit. The electrospray ionization (ESI) spectra for the mixed ligand-stabilized Au25 clusters shows the major peak at 4312 m/z which is equivalent to the mass/ charge ratio of [Au25(PPh3)10(SC12H25)5Cl2]2+ clusters.[15] The Au38 clusters show the molecular ion peak at 12316.7 m/z
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(theoretical: 12317.9 m/z) with its fragment peak at 10556.1 due to the loss of Au4(SC12H25)4 unit. The mass spectral characterization for Au55 clusters was difficult to obtain due to the dissociation of the stabilizing phosphines during the analysis. However, the transmission electron microscope (TEM) image (Figure S3) shows nanoparticles of ∼1.4 nm, which are previously established for 55-atom clusters by the highlycited literature report.[23] The thermo gravimetric analysis (TGA) of the clusters show 75.6% gold and 24.4% ligand mass (Figure S4), which is also in good agreement for Au55 clusters with the ratio of the ligand shell to the total cluster mass M(ligands)/M(Au55(PPh3)12Cl6) = 23.7%. Unlike gold nanoparticles, the APGCs are made up of a specific number of atoms and ligands, whose electronic configuration can be theoretically calculated, which in turn could be used to predict their magnetic behavior.[24,25] The electronic structure calculations on bulk gold show that the d band lies below the Fermi level where the density of states is too low to stimulate noticeable Pauli paramagnetism. As a result, the weak 5d band paramagnetism is overcome by a combination of Landau and core diamagnetism giving rise to diamagnetism in bulk gold.[6] However, the presence of ligands (specifically thiols) has been shown to increase the number of holes in the 5d band of gold nanoparticles.[26] The strong affinity between the Au surface atoms and the ligand atoms induces a charge transfer from the Au surface atoms to the ligand atoms where the participation of 5d electrons can also be implied leading to the generation of unoccupied d states on Au atoms resulting in magnetism induced by the spin symmetry breaking alluded to in the Introduction. Figure 2a shows M vs H curves obtained for each of the four clusters at 5 K. The curve for mixed ligand-stabilized Au25 clusters (black) shows a typical diamagnetic behavior for these clusters. A similar result was observed by Conesa and coworkers[27] for 1.4 nm triphenylphosphine-capped particles. The valence electron count for this cluster calculated using Hakkinen’s method[28] gives 16 electrons, which is an even number (the PPh3 ligands here are considered weak and
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Figure 3. Calculated spin density of the thiol-capped Au25.
Figure 2. M vs H curves for ligand-capped Au25, Au38 and Au55 clusters at different temperatures (a) 5 K and (b) 300 K.
do not contribute to the electron count). It has been shown earlier, that following Mingos's electron counting rule, this cluster can be regarded as a dimer of a closed-shell, 8-electron system.[29,30] Based on this interpretation, the cluster can be expected to be diamagnetic. The curve for thiol-stabilized Au25 clusters (red) shows a paramagnetic behavior with an experimentally observed saturation magnetic moment of μB = 0.0516/cluster or μB = 0.0020/Au atom. The Hakkinen's valence electron count for this cluster is 17 electrons, which is an odd number. This result is also consistent with the DFT calculations of Schatz et al.[31] that show that for this species with an odd number of electrons the ground state is a magnetic doublet. An interesting aspect of this system is that we have analyzed in detail the spin density for this cluster and found that it is larger in the surface than in the core (Figure 3), which indicates that magnetism is a surface effect, a result that might have considerable importance in understanding its influence on catalysis[32] and that also differs from earlier observations attributing the magnetic behavior to the core.[31] The dominance of surface magnetization is strongly sizedependent as eventually, as the number of atoms increases, small 2014, 10, No. 5, 907–911
the behavior of the core will dominate the magnetic behavior of the nanoparticle. The curve for thiol-stabilized Au38 clusters (green) shows a diamagnetic behavior. This result is rather surprising as the majority of dodecanethiol stabilized gold nanoparticles which are less than 2 nm exhibit room temperature ferromagnetism. The unexpected experimental result in Au38 is possibly due to the formation of localized spin-singlet states reducing the magnetization in dodecanethiol capped particles.[9] The Hakkinen's valence electron count for this cluster is 14 electrons, which is an even number. Calculations on the bare Au38 cluster, which has an even number of electrons, indicate that there are several magnetic states that are energetically quasidegenerate with the ground singlet state.[10] In particular, the calculations reveal that the density of states (DOS) at the Fermi level is very similar for both the singlet and polarized singlet cases. However, the polarized singlet of Au38 is characterized by an increased participation of the localized d electrons, compared with the spherically symmetric s electrons. This translates into a higher effective electronic repulsion and hence provokes the transition from diamagnetic to ferromagnetic behavior. In the ligand-decorated cluster, the localized d electrons of Au are involved in the bonding with the ligands. This leads to a lower effective electronic repulsion and hence the ligand decorated Au38 cluster is diamagnetic. These changes in the nature of bonding, can be noted in the highest occupied molecular orbitals (HOMOs) of bare and ligand-decorated Au38 clusters, shown in Figure 4. Our results are in close agreement with earlier calculations carried out on highly symmetric forms of ligand-decorated Au38.[33] The lesson of the calculations and the associated experiments is that the energy balance that eventually leads to spin symmetry breaking and the onset of magnetism is a delicate one and that it is system-dependent because of the large degeneracy close to the ground state. Under some
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Three of the four clusters, mixed ligand-stabilized and thiolstabilized Au25 and Au38 clusters show diamagnetic behavior whereas Au55 clusters show a ferromagnetic behavior with saturation magnetic moment of μB = 0.0015/cluster or μB = 2.77 × 10−5/Au atom and a negligible coercive field. Figure S5 shows the ZFC/FC curves for the clusters. In summary, atomically precise ligand-stabilized Au25, Au38 and Au55 clusters were prepared and a comparative chemically induced magnetism was investigated using SQUID magnetometer and theoretical calculations. The mixed ligand-stabilized Au25 and Au38 clusters were found to be diamagnetic, thiol-stabilized Au25 clusters were paramagnetic and Au55 clusters were ferromagnetic. Even though, Au55(PPh3)12Cl6 has been investigated for the last two decades, this is the first report of its room temperature permanent magnetism. Detection of room temperature ferromagnetism in APGCs is anticipated to lead to hitherto undiscovered applications involving a combination of magnetism and quantum size effects. These findings in combination with recent efforts to correlate fundamental electronic structure of atomically precise gold clusters with their catalytic properties[32], [34] are just the first steps in meeting one of the grand science challenges to understand how remarkable properties emerge from complex correlations of their atomic or electronic constituents.[35]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Figure 4. Plots of the HOMOs of (a) bare and (b) ligand-decorated Au38 clusters.
conditions the process of measuring magnetization might break the degeneracy and favor the polarized singlet which is a magnetic state, but this aspect of the problem requires further investigation.[10] The curve for the phosphine-stabilized Au55 clusters (blue) shows a ferromagnetic behavior with a saturation magnetic moment of μB = 0.0584/cluster or μB = 0.0010/Au atom, a coercive field of Hc = 1125 Oe and reminiscent magnetization of 0.0067 emu. The Hakkinen's valence electron count for this cluster is 49 electrons, which is an odd number. The result is in agreement with permanent magnetism observed for triphenylphosphine capped nAu-TPP particles reported previously and with our DFT calculations that indicates the ground state to be a magnetic doublet.[10,25,27] The results clearly show two different magnetic behaviors for clusters with same core size (i.e. 25 atoms). This complex behavior is partly a consequence of the ligand effect (and hence through chemical induction) as the clusters are stabilized by different ligands, but, perhaps more importantly, a result of a delicate electronic and energetic balance at the Fermi level that controls the spin symmetry breaking ultimately responsible for the magnetic behavior. Figure 2b shows M vs H curves for the respective clusters at 300 K.
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Acknowledgements This research is supported as part of the Center for Atomic Level Catalyst Design (CALC-D), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0001058. We also thank the Louisiana Board of Regents for an equipment grant (LEQSF (2008–10)-ENH-TR-07) to purchase the SQUID magnetometer.
[1] S. Trudel, Gold Bull. 2011, 44, 3. [2] J. S. Garitaonandia, M. Insausti, E. Goikolea, M. Suzuki, J. D. Cashion, N. Kawamura, H. Ohsawa, I. G. Muro, K. Suzuki, F. Plazaola, T. Rojo, Nano Lett. 2008, 8, 661. [3] L. Suber, D. Fiorani, G. Scavia, P. Imperatori, W. R. Plunkett, Chem. Mater. 2007, 19, 1509. [4] G. L. Nealon, B. Donnio, R. Greget, J. P. Kappler, E. Terazzi, J. L. Gallani, Nanoscale 2012, 4, 5244. [5] P. Crespo, E. Guerrero, M. A. Munoz-Marquez, A. Hernando, A. Fernandez, IEEE Trans. Mag. 2008, 44, 2768. [6] P. Crespo, R. Litran, T. C. Rojas, M. Multigner, J. M. de la Fuente, J. C. Sanchez-Lopez, M. A. Garcia, A. Hernando, S. Penades, A. Fernandez, Phys. Rev. Lett. 2004, 93, 087204. [7] C. S. S. R. Kumar, F. Mohammad, J. Phys. Chem. Lett. 2010, 1, 3141.
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[8] P. Dutta, S. Pal, M. S. Seehra, M. Anand, C. B. Roberts, Appl. Phys. Lett. 2007, 90, 213102. [9] H. Hori, Y. Yamamoto, T. Iwamoto, T. Miura, T. Teranishi, M. Miyake, Phys. Rev. B 2004, 69, 174411. [10] A. Roldan, F. Illas, P. Tarakeshwar, V. Mujica, J. Phys. Chem. Lett. 2011, 2, 2996. [11] R. Jin, H. Qian, Y. Zhu, A. Das, J. Nanosci. Lett. 2011, 1, 72. [12] M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. J. Whyman, Chem. Soc., Chem. Commun. 1994, 801. [13] R. Jin, H. Qian, Z. Wu, Y. Zhu, M. Zhu, A. Mohanty, N. Garg, J. Phys. Chem. Lett. 2010, 1, 2903. [14] D. Jose, J. E. Matthiesen, C. Parsons, C. M. Sorensen, K. J. Klabunde, J. Phys. Chem. Lett. 2012, 3, 885. [15] Y. Shichibu, Y. Negishi, T. Watanabe, N. K. Chaki, H. Kawaguchi, T. Tsukuda, J. Phys. Chem. C 2007, 111, 7845–7847. [16] M. Zhu, E. Lanni, N. Garg, M. E. Bier, R. Jin, J. Am. Chem. Soc. 2008, 130, 1138. [17] H. Qian, M. Zhu, U. N. Andersen, R. Jin, J. Phys. Chem. A 2009, 113, 4281. [18] J. E. Hutchison, E. W. Foster, M. G. Warner, S. M. Reed, W. W. Weare, Inorganic Syntheses, (Wiley, Ed. J. R. Shapley), 2004, 34, 228. [19] R. Jin, Y. Zhu, H. Qian, Chem. Eur. J. 2011, 17, 6584. [20] Y. Pei, N. Shao, Y. Gao, X. C. Zeng, ACS Nano 2010, 4, 2009–2020. [21] M. Zhu, W. T. Eckenhoff, T. Pintauer, R. Jin, J. Phys. Chem. C 2008, 112, 14221. [22] R. E. Benfield, J. A. Creighton, D. G. Eadon, G. Schmid, Z. Phys. D Atoms, Molecules and Clusters 1989, 12, 533. [23] M. Turner, V. B. Golovko, O. P. H. Vaughan, P. Abdulkin, A. Berenguer-Murcia, M. S. Tikhov, B. F. G. Johnson, R. M. Lambert, Nature 2008, 454, 981–983. [24] S. S. Pundlik, K. Kalyanaraman, U. V. Waghmare, J. Phys. Chem. C 2011, 115, 3809.
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[25] A. Ayuela, P. Crespo, M. A. Garcia, A. Hernando, P. M. Echenique, New J. Phys. 2012, 14, 013064. [26] P. Zhang, T. K. Sham, Appl. Phys. Lett. 2002, 81, 736. [27] M. A. Munoz-Marquez, E. Guerrero, A. Fernandez, P. Crespo, A. Hernando, R. Lucena, J. C. Conesa, J. Nanopart. Res. 2010, 12, 1307. [28] M. Walter, J. Akola, O. Lopez-Acevedo, P. D. Jadzinsky, G. Calero, C. J. Ackerson, R. L. Whetten, H. Gronbeck, H. Hakkinen, Proc. Natl. Acad. Sci. 2008, 105, 9157–9162. [29] K. Nobusada, T. Iwasa, J. Phys. Chem. C 2007, 111, 14279–14282. [30] Z. Lin, R. P. F. Kanters, D. M. P. Mingos, Inorg. Chem. 1991, 30, 91–95. [31] M. Zhu , C. M. Aikens , M. P. Hendrich , R. Gupta , H. Qian , G. C. Schatz , R. Jin , J. Am. Chem. Soc. 2009 , 131 , 2490 – 2492 . [32] J. Liu, K. S. Krishna, Y. B. Losovyj, S. Chattopadhyaya, N. Lozova, J. T. Miller, J. J. Spivey, C. S. S. R. Kumar, Chem. Euro. J. 2013, 19, 10201. [33] O. Lopez-Acevedo, H. Tsunoyama, T. Tsukuda, H. Häkkinen, C. M. Aikens, J. Am. Chem. Soc. 2010, 132, 8210–8218. [34] a) H. Li, L. Li, Y. Li Nanotechnol. Rev. 2013, 2, 515; b) D. Stellwagen, A. Weber, G. L. Bovenkamp, R. Jin, J. H. Bitter, C. S. S. R. Kumar, RSC Adv. 2012, 2, 2276; c) L. M. Rossi, N. J. S. Costa, F. P. Silva, R. V. Goncalves, Nanotechnol. Rev. 2013, 2, 597. [35] Directing Matter and Energy: Five Challenges for Science and the Imagination, A report from the basic energy sciences advisory committee, DOE, USA 2007.
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Chemically Induced Magnetism in Atomically Precise Gold Clusters Katla Sai Krishna, Pilarisetty Tarakeshwar, Vladimiro Mujica, and Challa S. S. R. Kumar* Recent investigations demonstrate unexpected and unusual magnetic behavior in a wide range of nanoscale materials – metal nanoparticles, metal oxide nanoparticles, nanocrystalline films- which are otherwise diamagnetic (i.e. nonmagnetic) in their bulk phase.[1] Most prominent among these are ligand stabilized ferromagnetic noble metal nanoparticles such as Au, Ag, Cu and Pt.[2,3] A number of gold nanoparticles stabilized by different ligands have been investigated for their magnetic behavior and it is now well documented that the magnetism is chemically-induced and strongly sizedependent.[4] These magnetic properties are intrinsically related to their electronic structure, which is influenced both by the size of the nanoparticle and the nature of the ligand decorating it.[5] While strongly binding dodecanethiol-capped Au nanoparticles of 1.4 nm in diameter exhibit ferromagnetism, a weakly binding tetraoctylammonium capped ∼1.5 nm Au nanoparticle is diamagnetic.[6] We have also recently demonstrated ferromagnetism in peptide-capped gold nanoshells (∼0.5 nm thick) and the possibility to modulate their magnetic behavior by step-wise functionalization of stabilizing ligands.[7] It is generally observed that ferromagnetism at room temperature in gold nanoparticles prevails
Dr. K. S. Krishna, Dr. C. S. S. R. Kumar Center for Advanced Microstructures and Devices (CAMD) Louisiana State University Baton Rouge, LA 70806, USA Center for Atomic-Level Catalyst Design, #324 Cain Department of Chemical Engineering Louisiana State University Baton Rouge, LA 70803, USA E-mail: [email protected] Dr. P. Tarakeshwar, Prof. V. Mujica Department of Chemistry and Biochemistry Arizona State University Tempe, AZ 85287–1604, USA Prof. V. Mujica Department of Chemistry Northwestern University Evanston, Illinois 60208, USA Center for Nanoscale Materials Argonne National Laboratory Argonne, IL, 60439, USA DOI: 10.1002/smll.201302393 small 2014, 10, No. 5, 907–911
in thiol-stabilized Au nanoparticles.[8] The crucial electronic event involved on the onset of magnetization in capped gold nanoparticles is a spin symmetry breaking associated with the 5d and 6s electrons of the Au atoms involved in the chemical bond with the ligands, which in turn modifies the relative spin densities at the Fermi energy thus creating a non-zero magnetic moment and a corresponding magnetization. Miyake and coworkers have recently reported diameter dependence (size effect) on ferromagnetism of dodecanethiol-capped gold nanoparticles.[9] Table S1 (Supporting Information) summarizes the previously reported literature on size-dependent magnetic properties observed in ligand-stabilized gold nanoparticles. A simple correlation between size and magnetic behavior in gold nanoparticles from the previously published results can be drawn. In several instances, thiol-capped gold nanoparticles of size around 2 to 3 nm have predominantly exhibited ferromagnetic behavior. When metal nanoparticles enter the quantum size regime (typically < 2 nm), their properties are extremely sensitive to the particle size. For instance, the characteristic surface Plasmon resonance (SPR) band which dominates the optical spectrum of larger gold nanoparticles is replaced by steplike multiband, due to the discretization of the energy spectrum.[10,11] Impressive progress has been made in the past few years in bringing atomic level control in the synthesis of nanoparticles starting from the original Brust's method[12] followed by size focusing methodology developed by Jin and coworkers[13] and expanded further by other researchers.[14] Despite all this significant progress in studying the size-controlled optical and magnetic behavior in ligand-stabilized gold nanoparticles, they are still more or less heterogeneous in terms of size with respect to atomic precision. Surprisingly, there are no published reports focusing on the experimental magnetic behavior of atomically precise gold clusters (APGCs) which are < 2 nm. Investigations in this direction are extremely important as ligand stabilized APGCs provide a means to chemically turn-on and tune-in their magnetism and thereby providing an opportunity to tailor-make atomically precise nanomagnets. The ability to probe magnetism with atomic precision is hitherto an unchartered area of investigation. As an example of the potential of our technique, we have fabricated atomically precise, ligand-stabilized Au25, Au38 and Au55 clusters and characterized them.[15–18] We investigated their magnetic properties experimentally using the superconducting quantum interference device (SQUID).
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Figure 1. Ligand-stabilized APGCs with their experimentally observed magnetic behavior; structures adapted from the literature.[15,19,20]
Figure 1 illustrates the different ligand-stabilized APGCs along with their experimentally observed magnetic behavior. In addition, we provide reasonable theoretical justification of our experimentally observed magnetic behavior by examining their electronic structure and the density of states (DOS). Figure S1 shows the UV-Vis absorption spectra of the as-prepared clusters, which are in agreement with the reported literature. The mixed-ligand-stabilized [Au25(PPh3)10(SC12H25)5Cl2]2+ clusters show discrete peaks typical of molecule-like electronic levels. The spectrum is similar to that of the earlier reported spectrum by Tsukuda et al. for the biicosahedral Au25 clusters.[15] The spectrum for the thiol-stabilized Au25[(SC2H4Ph)18]− clusters show broad absorption bands at 400, 450 and 670 nm, which are similar to characteristic bands reported previously for the clusters.[16] The anionic Au25[(SC2H4Ph)18]− clusters were however oxidized to neutral Au25(SC2H4Ph)18 clusters through aerial oxidation[21] before the magnetic measurements were carried out on them. The spectrum for Au38(SC12H25)24 clusters show step-wise absorption bands at 400 nm and 625 nm respectively, which represent the characteristic bands for the clusters.[17] The spectrum for Au55(PPh3)12Cl6 clusters exhibit no peaks but an approximately exponential decay with increasing wavelength, that matches the spectra reported by Schmid et al.[22] Figure S2 shows the matrix-assisted laser desorption ionization mass spectrum (MALDI-MS, using trans-2-[3(4-tert-butylphenyl)-2-methyl-2-propenylidene] malanonitrile (DCTB) as the matrix) for the as-synthesized thiol and mixed-ligand stabilized Au25 clusters and Au38 clusters. The mass spectrum for thiol-stabilized Au25 clusters shows the molecular ion peak at 7388 m/z (theoretical: 7390 m/z) with its predominant fragment at ∼6054 m/z due to the loss of Au4(SC2H4Ph)4 unit. The electrospray ionization (ESI) spectra for the mixed ligand-stabilized Au25 clusters shows the major peak at 4312 m/z which is equivalent to the mass/ charge ratio of [Au25(PPh3)10(SC12H25)5Cl2]2+ clusters.[15] The Au38 clusters show the molecular ion peak at 12316.7 m/z
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(theoretical: 12317.9 m/z) with its fragment peak at 10556.1 due to the loss of Au4(SC12H25)4 unit. The mass spectral characterization for Au55 clusters was difficult to obtain due to the dissociation of the stabilizing phosphines during the analysis. However, the transmission electron microscope (TEM) image (Figure S3) shows nanoparticles of ∼1.4 nm, which are previously established for 55-atom clusters by the highlycited literature report.[23] The thermo gravimetric analysis (TGA) of the clusters show 75.6% gold and 24.4% ligand mass (Figure S4), which is also in good agreement for Au55 clusters with the ratio of the ligand shell to the total cluster mass M(ligands)/M(Au55(PPh3)12Cl6) = 23.7%. Unlike gold nanoparticles, the APGCs are made up of a specific number of atoms and ligands, whose electronic configuration can be theoretically calculated, which in turn could be used to predict their magnetic behavior.[24,25] The electronic structure calculations on bulk gold show that the d band lies below the Fermi level where the density of states is too low to stimulate noticeable Pauli paramagnetism. As a result, the weak 5d band paramagnetism is overcome by a combination of Landau and core diamagnetism giving rise to diamagnetism in bulk gold.[6] However, the presence of ligands (specifically thiols) has been shown to increase the number of holes in the 5d band of gold nanoparticles.[26] The strong affinity between the Au surface atoms and the ligand atoms induces a charge transfer from the Au surface atoms to the ligand atoms where the participation of 5d electrons can also be implied leading to the generation of unoccupied d states on Au atoms resulting in magnetism induced by the spin symmetry breaking alluded to in the Introduction. Figure 2a shows M vs H curves obtained for each of the four clusters at 5 K. The curve for mixed ligand-stabilized Au25 clusters (black) shows a typical diamagnetic behavior for these clusters. A similar result was observed by Conesa and coworkers[27] for 1.4 nm triphenylphosphine-capped particles. The valence electron count for this cluster calculated using Hakkinen’s method[28] gives 16 electrons, which is an even number (the PPh3 ligands here are considered weak and
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Figure 3. Calculated spin density of the thiol-capped Au25.
Figure 2. M vs H curves for ligand-capped Au25, Au38 and Au55 clusters at different temperatures (a) 5 K and (b) 300 K.
do not contribute to the electron count). It has been shown earlier, that following Mingos's electron counting rule, this cluster can be regarded as a dimer of a closed-shell, 8-electron system.[29,30] Based on this interpretation, the cluster can be expected to be diamagnetic. The curve for thiol-stabilized Au25 clusters (red) shows a paramagnetic behavior with an experimentally observed saturation magnetic moment of μB = 0.0516/cluster or μB = 0.0020/Au atom. The Hakkinen's valence electron count for this cluster is 17 electrons, which is an odd number. This result is also consistent with the DFT calculations of Schatz et al.[31] that show that for this species with an odd number of electrons the ground state is a magnetic doublet. An interesting aspect of this system is that we have analyzed in detail the spin density for this cluster and found that it is larger in the surface than in the core (Figure 3), which indicates that magnetism is a surface effect, a result that might have considerable importance in understanding its influence on catalysis[32] and that also differs from earlier observations attributing the magnetic behavior to the core.[31] The dominance of surface magnetization is strongly sizedependent as eventually, as the number of atoms increases, small 2014, 10, No. 5, 907–911
the behavior of the core will dominate the magnetic behavior of the nanoparticle. The curve for thiol-stabilized Au38 clusters (green) shows a diamagnetic behavior. This result is rather surprising as the majority of dodecanethiol stabilized gold nanoparticles which are less than 2 nm exhibit room temperature ferromagnetism. The unexpected experimental result in Au38 is possibly due to the formation of localized spin-singlet states reducing the magnetization in dodecanethiol capped particles.[9] The Hakkinen's valence electron count for this cluster is 14 electrons, which is an even number. Calculations on the bare Au38 cluster, which has an even number of electrons, indicate that there are several magnetic states that are energetically quasidegenerate with the ground singlet state.[10] In particular, the calculations reveal that the density of states (DOS) at the Fermi level is very similar for both the singlet and polarized singlet cases. However, the polarized singlet of Au38 is characterized by an increased participation of the localized d electrons, compared with the spherically symmetric s electrons. This translates into a higher effective electronic repulsion and hence provokes the transition from diamagnetic to ferromagnetic behavior. In the ligand-decorated cluster, the localized d electrons of Au are involved in the bonding with the ligands. This leads to a lower effective electronic repulsion and hence the ligand decorated Au38 cluster is diamagnetic. These changes in the nature of bonding, can be noted in the highest occupied molecular orbitals (HOMOs) of bare and ligand-decorated Au38 clusters, shown in Figure 4. Our results are in close agreement with earlier calculations carried out on highly symmetric forms of ligand-decorated Au38.[33] The lesson of the calculations and the associated experiments is that the energy balance that eventually leads to spin symmetry breaking and the onset of magnetism is a delicate one and that it is system-dependent because of the large degeneracy close to the ground state. Under some
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Three of the four clusters, mixed ligand-stabilized and thiolstabilized Au25 and Au38 clusters show diamagnetic behavior whereas Au55 clusters show a ferromagnetic behavior with saturation magnetic moment of μB = 0.0015/cluster or μB = 2.77 × 10−5/Au atom and a negligible coercive field. Figure S5 shows the ZFC/FC curves for the clusters. In summary, atomically precise ligand-stabilized Au25, Au38 and Au55 clusters were prepared and a comparative chemically induced magnetism was investigated using SQUID magnetometer and theoretical calculations. The mixed ligand-stabilized Au25 and Au38 clusters were found to be diamagnetic, thiol-stabilized Au25 clusters were paramagnetic and Au55 clusters were ferromagnetic. Even though, Au55(PPh3)12Cl6 has been investigated for the last two decades, this is the first report of its room temperature permanent magnetism. Detection of room temperature ferromagnetism in APGCs is anticipated to lead to hitherto undiscovered applications involving a combination of magnetism and quantum size effects. These findings in combination with recent efforts to correlate fundamental electronic structure of atomically precise gold clusters with their catalytic properties[32], [34] are just the first steps in meeting one of the grand science challenges to understand how remarkable properties emerge from complex correlations of their atomic or electronic constituents.[35]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Figure 4. Plots of the HOMOs of (a) bare and (b) ligand-decorated Au38 clusters.
conditions the process of measuring magnetization might break the degeneracy and favor the polarized singlet which is a magnetic state, but this aspect of the problem requires further investigation.[10] The curve for the phosphine-stabilized Au55 clusters (blue) shows a ferromagnetic behavior with a saturation magnetic moment of μB = 0.0584/cluster or μB = 0.0010/Au atom, a coercive field of Hc = 1125 Oe and reminiscent magnetization of 0.0067 emu. The Hakkinen's valence electron count for this cluster is 49 electrons, which is an odd number. The result is in agreement with permanent magnetism observed for triphenylphosphine capped nAu-TPP particles reported previously and with our DFT calculations that indicates the ground state to be a magnetic doublet.[10,25,27] The results clearly show two different magnetic behaviors for clusters with same core size (i.e. 25 atoms). This complex behavior is partly a consequence of the ligand effect (and hence through chemical induction) as the clusters are stabilized by different ligands, but, perhaps more importantly, a result of a delicate electronic and energetic balance at the Fermi level that controls the spin symmetry breaking ultimately responsible for the magnetic behavior. Figure 2b shows M vs H curves for the respective clusters at 300 K.
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Acknowledgements This research is supported as part of the Center for Atomic Level Catalyst Design (CALC-D), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0001058. We also thank the Louisiana Board of Regents for an equipment grant (LEQSF (2008–10)-ENH-TR-07) to purchase the SQUID magnetometer.
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