sp magnetism in clusters of gold-thiolates
A. Ayuela, P. Crespo, M.A. García, A. Hernando, P. M. Echenique
aa r X i v : . [ c ond - m a t . m t r l - s c i ] J a n sp magnetism in clusters of gold-thiolates A. Ayuela ∗ Centro de F´ısica de Materiales CFM-MPC CSIC-UPV/EHU,Donostia International Physics Center (DIPC),Departamento de F´ısica de Materiales, Fac. de Qu´ımicas,Univ. del Pais Vasco UPV-EHU, 20018 San Sebasti´an, Spain
P. Crespo † and A. Hernando ‡ Instituto de Magnetismo Aplicado, UCM-CSIC-ADIF. Las Rozas. P. O. Box 155,Madrid 28230 and Dpto. F´ısica de Materiales, Universidad Complutense
M.A. Garc´ıa § Instituto de Cer´amica y Vidrio. CSIC c/Kelsen,5 Madrid 28049
P. M. Echenique ¶ Centro de F´ısica de Materiales CFM-MPC CSIC-UPV/EHU,Donostia International Physics Center (DIPC),Departamento de F´ısica de Materiales, Fac. de Qu´ımicas,Univ. del Pais Vasco UPV-EHU, 20018 San Sebasti´an, Spain bstract Using calculations from first principles, we herein consider the bond made between thiolate witha range of different Au clusters, with a particular focus on the spin moments involved in eachcase. For odd number of gold atoms, the clusters show a spin moment of 1. µ B . The variation ofspin moment with particle size is particularly dramatic, with the spin moment being zero for evennumbers of gold atoms. This variation may be linked with changes in the odd-even oscillationsthat occur with the number of gold atoms, and is associated with the formation of a S-Au bond.This bond leads to the presence of an extra electron that is mainly sp in character in the goldpart. Our results suggest that any thiolate-induced magnetism that occurs in gold nanoparticlesmay be localized in a shell below the surface, and can be controlled by modifying the coverage ofthe thiolates. ∗ [email protected] † [email protected] ‡ [email protected] § [email protected] ¶ [email protected] − -10 − Bohr magnetons per atom,and is weakly dependent on temperature in the range from 4K up to room temperature.The low magnitude of the magnetic moment is not caused by the few magnetic impuritiespresent, because any increase in the number of impurities causes the magnetic signal todisappears [3]. The results of both X-ray magnetic circular dichroism (XMCD) and AuM¨ossbauer spectroscopy have shown that the Au atom possses a magnetic moment [4, 5],which was previously thought to be due to the spins of Au d-holes introduced by the ligands[6]. However, the observed saturation at room temperature and the shape of the magnetiza-tion curve still lack convincing explanation. A more detailed understanding of the nature ofthe gold-alkenethiol bond is therefore required. In particular, understanding of the interac-tion between alkenethiols and Au NPs is currently one of the main challenges in the studyof their magnetic structure.The contribution of the sp levels in Au NPs is particularly significant, and it is the filling ofthese levels that explains the effects of the electronic shells in noble metal clusters in generaland in gold clusters in particular [7]. Using a theoretical approach, it has been shown thatthe coverage of the gold clusters by thiols that have well-defined compositions is ratherdisordered, and that the charge transfer depends strongly on the coordination of sulphuratoms [8]. A range of different coordination numbers must therefore be taken into accountin any investigation. Alkenethiols with longer chains on Au surfaces have been the focusof detailed theoretical studies [9] and the presence of several different metastable states inenergy have been shown. The sulphur atoms in the gold layers becomes arranged in a bridgeconfiguration for the ground state, and this must then be primarily considered in assesingtheir magnetic signal. Other NPs such as ZnO capped with a range of different organicmolecules, e.g. amine, thiol and topo, also exhibit ferromagnetism at room temperature [10].The results of XMCD show that the magnetic signal of ZnO NPs is due to the conductionband, which is mainly formed from empty 4sp Zn surface states [11]. The d-electrons ofZn are too far away in terms of energy and clearly make no contribution to the magneticsignal. It has been shown theoretically that a carbon oxide CO molecule on ZnO surface3onates a single electron to the 4sp surface orbitals, which are initially empty [12]. Althoughthere have been a number of studies of the changes in the geometrical structure induced byalkenethiols in gold surfaces and clusters, to our knowledge no studies have yet focused onthe magnetism of the NPs covered by such molecules. The results of such a study would beof particular interest when considered toghether the effect of magnetism on the electronicproperties of gold nanoparticles covered by thiols.An all-electron local-orbitals scheme, namely the ADF (Amsterdam Density Functional)method [13], was used herein for calculations involving density-functional-theory (DFT). Thevalence basis set was composed of the d gold, sulphur, carbon and hydrogen states, whichwere expanded in triple zeta with two polarization functions at each atom. The relativisticeffects of gold were modeled using the Zeroth Order Regular Approximation (ZORA) andits scalar relativistic version was used for the structural optimizations. The data presentedherein were obtained using generalized gradient density approximations (GGA) with theparametrized exchange-correlation functional of Perdew-Burke-Ernzerhof according to Ref.[14]. The optimization of the geometry was carried out until the forces were all smaller than0.02 eV/˚A. The magnetic anisotropy (MAE) was obtained by means of self-consistent ZORArelativistic calculations, including spin-orbit. The resulting GGA data were regarded as alower estimate for the MAE. In order to obtain an upper estimate, an orbital polarizationcorrection (OPC) [15] should be considered, but this work is beyond the scope of the presentstudy.The singly or doubly occupied levels of the Au clusters are responsible for the odd-evenoscillations. These oscillations do not vary strongly much with the number of atoms in thecluster. We have herein therefore chosen to investigate the interaction between alkenethi-ols − SCH and minimal gold clusters. The interactions of gold clusters with thiols areconsidered to be a suitable model of adsorption of thiols onto larger nanoparticles [8].We begin our analysis with a cluster of Au , which allows us to investigate a numberof different coordination numbers for the sulphur atom. Au SCH has an even number ofelectrons, and thus has a spin moment equivalent to an even number of Bohr magnetons.In order to determine the lowest-energy geometry and the spin magnetic state, four possiblehigh-symmetry structures were optimized for each of the possible values of low spin µ S =0,1, 2 etc. We only compared the data for µ S =0 and 1 in Fig. 1, because the other spinstates produce an even higher total energy. The point group symmetries were fixed in our4alculations to reproduce those of the geometries in the surfaces of the nanoparticles. Thegeometry of the gold atoms was not fixed because the adsorbates of thiols on gold can alterthe atomic distances of Au-sulphur and Au-Au binding.Calculations using DFT yield energies that depend on the exchange-correlation approachused. In order to confirm the qualitative validity of the calculated energies, structuralsequence, and spin states of Fig. 1, we performed calculations with many other functionals,as implemented in ADF, using the fixed geometry obtained with the previous GGA approach.We obtain that the energetic difference between the different spin states was sometimeshigher [though never lower] than that obtained from the previous GGA values using the PBEapproach. The other functionals produced energetic differences of about 1.4 eV between thenon- and spin-polarized structure. These supporting calculations therefore confirm the mainfindings obtained using PBE, in that the bonds of an SCH molecule with a three-atomcluster form the structure depicted in the bottom of Fig. 1 which has a total spin of zero.Our findings show a gain in total energy of 0.12 eV for the − SCH adsorbate with thegold cluster in a bridge position, this being the ground-state structure with respect to themolecule in the Au plane. We are not aware of any previous investigation of this structuralform of Au SCH . Although other molecules of Au-thiols have been studied previously [16],no report has been made of a model of the surface of gold nanoparticles. Evidence for such’tilt’ arrangement for the molecule was previously provided by Ref. [9] for thiols on goldsurfaces. Furthermore, thiols have theoretically been predicted [9] to form a mat on a goldsurface with perpendicularly oriented molecules. This is given by the bridge position withall the atoms bonded to S in the same plane, as shown in the third geometry of Fig. 1.However, this form has a larger energy than the tilt case, because it is a metastable positionby 0.56 eV. Althought this form has a similar coordination number to that of the studiedcase of C H − S − Au [17], is not considered in the results that follow, despite its spinpolarization for alkanethiols.The structural forms of other gold cluster structures are given in Fig. 2. The panel on thefar right hand side shows the structures of S=0, as previously discussed for the ground stateof Au -thiol, while the left-hand panel refers to clusters with spin polarization (S=1). Thenumbers shown below the structures give the spin polarization energy in each case, whichare almost independent of the size of the cluster. The related energy differences ∆ E arearound 0.5 eV and -0.2 eV for the odd and even cases, respectively. These values should be5onsidered to be lower estimates for the polarization, because they were evaluated withoutthe so-called orbital polarization. The most important characteristic is the shift betweenodd-even oscillations, by a single gold atom, with respect to bare gold clusters. The structureis spin compensated for an even number of gold atoms in the thiol-cluster, while it is spinpolarized otherwise.It is perhaps somewhat surprisingly that the bonding of thiols on gold clusters does notlead to the elimination of the spin magnetic moment, but rather to a new paradigm, i.e. thespin moments induced by the thiols are unchanged, as in free particles. It is noteworthy thatour spin polarized state can be more stable in energy, by > Au n SCH clusters that contain an even number of gold atoms, as alsoseen in Fig. 2, show that, while the local magnetic moments of the thiols nearest the goldatoms are mostly negligible ( < µ B per atom), those of the lower gold atoms ( ∼ µ B per atom) are even higher than the experimental values. These very high values of magneticmoment per atom may also be accompanied by large ground-state orbital moments, of about1 µ B per molecule. The spin moments are largely suppressed when the thiols are locatedparallel to the gold surface. Depending on the orientation of the molecule, the spin momentvaries due to different numbers of thiol-gold bonds. We have herein neglected variations inthe bond distances.Although some of the gold-thiol molecules described herein were investigated both ex-perimentally [1], by quantum mechanical calculations [16], and using spin-polarized GGA[8], their behaviour in terms of their magnetic moment merits further study. Figure 3 showsthe bonding mechanism of thiolates on metal clusters. A sulphur atom is shown here joinedto two Au atoms with sp hybridization. The spin population is smaller in these sulphurand bonded gold atoms than in that of the next nearest neighbors below the sulphur-goldbonds. There are five sulphur electrons in total: the two bonds with gold atoms share twoelectrons, an electron is shared in the bond with the carbon atom, and two more form thelone pair in the sp hybridization. Because each sulphur atom contributes with a total of sixelectrons, a single electron is left over and is passed to the gold cluster. It is this sp electronthat contributes to the spin polarization.A related discussion that supports the back-donation of a sp electron from the thiols6o gold arises from a comparison between the eletronegativities of thiols and those of theirgold counterparts. We calculated the ionization potentials (IPs) and eletroaffinities (EAs),thereby obtaining the Mulliken electronegativities M = ( IP + EA ) /
2. This M value is thethe negative of the electrochemical potential. The electronegativity of the
SCH unit isslightly higher (5.43 eV) [18] than the work function of around 5.22 eV for the Au surface[19] and is comparable with the M value for gold nanostructures ( / SCH unit behaves rather differently from the sulphur atom (M=6.21 eV). In consecuence, thetransfer of charge from gold to sulfur cannot be assumed a priori. In fact, we stress thatindependently of the partition scheme used, the calculated transfer of charge between thethiols and the gold clusters is less than 0.1 e . Such a charge neutrality is also seen inother systems, even in those assumed to be highly ionic [21], and supports the previouslycommented mechanism of back-donation.We now compare our findings with those obtained from experiments on thiols and goldnanoparticles. To this end, it must be stressed that the sulphur atom is not negativelycharged, even though XANES experiments have shown that the 5d orbitals of gold areslightly open. The sp contribution to the magnetic signal is larger than that of the dorbitals. The back-donated electron from sulphur to gold also sits in the sp states of the Aubond, and cannot be differentiated by the previous results using XANES method. Thesesp electrons are almost free in the second gold layer and could orbit, and it is these thatare responsible for the magnetism. These findings are consistent with experimental resultsin which a number of gold layers are needed in order to engender thiol induced magnetism.This general picture is in agreement with the donor mechanism of carbon monoxide to Znatoms on a ZnO surface [12], in that there is donation of a single electron from the moleculeto the 4sp surface orbitals of Zn, which are initially empty. More importantly, it is also inagreement with the 4sp magnetism observed by XMCD in thiol capped ZnO nanoparticles[11].The method of application of thiols is via their deposition in layers on the surface ofgold nanoparticles. The magnetism that results should be that of the electrons donated bythe capping molecules, which are confined to a thin layer below the surface, and form atwo-dimensional electron gas. We have herein determined the means by which this surfacelayer is unfilled, thereby giving rise to a permanent magnetic moment [22]. Orbital and spinangular moments are formed at the generally unfilled Fermi level of the system, which is7onfined to a spherical shell at the surface. The unfilled Fermi level of the surface band ischaracterized by a collective magnetic moment that is rather similar to an atomic orbital,but with a much larger quantum number. The order of magnitude (0.1 or 0.01 µ B per atom)is in agreement with that measured experimentally [1–5].Among all the possible combinations of thiol and noble metals, we have herein describedour investigation of Au clusters, but it should be mentioned that copper shows similarlygood bonding behavior with thiols. The related magnetic ground states have the same spinstate (1 µ B ) and lie at similar energies with respect to the state with zero moment. Thisfinding is similar to recent experimental work carried out for these nanoparticles [5] anddeserves further investigation.By including spin-orbit coupling, we find that thiols attached ot Au and Au show amagnetic anisotropies MAE of 1.43 and 0.98 meV per SCH molecule, respectively, whilethe change of MAE in the plane parallel to the surface is less than an order of magnitudesmaller. These figures are comparable with the giant magnetic anisotropy of single cobaltatoms on a Pt surface (9 meV per Co atom) [23]. Such large values of MAE demonstratethe stability of the magnetism discussed herein at room temperature, and are a factor often times larger than the equivalent values for tetragonal Ni (0.2 meV). The results of arecent experiment [1] showed that the magnetism of gold nanoparticles covered by thiolscan indeed survive almost unchanged at room temperature, and it could therefore be usedin hyperthermic applications.In summary, we predict that the bonding of alkanethiols on gold clusters (i.e. SCH Au n )results in a bridge arrangement with a deviated axis in a polarised ground state for evenn values of n. This structure is characterized by a division of functions, in that whilethe sulphur atom and the close pair of atoms are responsible for the chemical bonding,it is the outer gold atoms that host the larger part of the magnetic moment. The largemagnetic moment is preserved in the structure of the nanoparticles because they have lowersymmetries. Thus, it should be possible for the particles of other noble metals to showsimilar magnetic behavior. Further avenues of investigation could include the fabricationof separated thiols on the surface of nanoparticles, and the development of novel magneticnanoparticles for medical applications. Acknowledgement.
We gratefully acknowledge the support of the Basque Departamentode Educaci´on and the UPV/EHU (Grant No. IT-366-07), the Spanish Ministerio de In-8ovaci´on, Ciencia y Tecnolog´ıa (Grant No. MAT2009-14741-C02-01, CSD2007-00010, andFIS2007-66711-C02-02), and the ETORTEK research program funded by the Basque De-partamento de Industria and the Diputaci´on Foral de Guip´uzcoa. [1] P. Crespo, R. Litr´an, T. C. Rojas, M. Multigner, J. M. de la Fuente, J. C. S´anchez-L´opez,M. A. Garc´ıa, A. Hernando, S. Penad´es, and A. Fern´andez, Phys. Rev. Lett. , 087204(2004).[2] K. Rumpf, P. Granitzer, and H. Krenn, J. Phys. Condens. Matter , 45421 (2008).[3] P. Crespo, M. A. Garc´ıa, E. Fern´andez-Pinel, M. Multigner, D. Alc´antara, J. M. de la Fuente,S. Penad´es, and A. Hernando, Phys. Rev. Lett. , 177203 (2006).[4] J. de la Venta, V. Bouzas, A. Pucci, M. A. Laguna-Marco, D. Haskel, S. G. E. te Velthuis,A. Hoffmann, J. Lal, M. Bleuel, G. Ruggeri, C. de Juli´an Fern´andez, and M. A. Garc´ıa, J.Nanosci.Nanotecnol. , 6434 (2009).[5] J. S. Garitaonanind´ıa, M. Insausti, E. Goikolea, M. Suzuki, J. D. Cashion, N. Kawamura,H. Ohsawa, I. G. de Muro, K. Suzuki, F. Plazaola, and T. Rojo, Nanoletters , 661 (2008).[6] A. Hernando, P. Crespo, and M. A. Garc´ıa, Phys. Rev. Lett. , 057206 (2006).[7] J. A. Alonso, Chem. Rev. , 637 (2000).[8] H. H¨akkinen, R. N. Barnett, and U. Landman, Phys. Rev. Lett. , 3264 (1999).[9] Y. Yourdshahyan and A. M. Rappe, J. Chem. Phys. , 825 (2002).[10] M. A. Garc´ıa, J. M. Merino, E. F. Pinel, A. Quesada, J. de la Venta, M. L. Ru´ız-Gonz´alez,G. R. Castro, P. Crespo, J. Llopis, J. M. Gonz´alez-Calbet, and A. Hernando, NanoLetters ,1489 (2007).[11] J. Chaboy, R. Boada, C. Piquer, M. A. Laguna-Marco, M. Garc´ıa-Hern´andez, N. Carmona,J. Llopis, M. L. R. z Gonz´alez, J. Gonz´alez-Calbet, J. F. Fern´andez, and M. A. Garc´ıa, Phys.Rev. B , 064411 (2010).[12] A. B. Anderson and J. A. Nichols, J. Am. Chem. Soc. , 1385 (1986).[13] G. te Velde, F. Bickelhaupt, S. J. A. van Gisbergen, C. F. Guerra, E. J. Baerends, J. G.Snijders, and T. Ziegler, J. Comput. Chem. , 931 (2001).[14] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. , 3865 (1991).[15] O. Eriksson, M. S. S. Brooks, and B. Johansson, Phys. Rev. B , 7311 (1990).
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FIG. 1. DFT energies for various states of spin magnetic moments and thiol- Au configurations.The energies refer to the spin and geometry of the ground state. In order of size, the atoms aregold, sulphur, carbon, and hydrogen. Note the different types of Au-sulfur bonds. B µ B µ B µ B µ B µ B µ B µ B −0.13 eVS = 10.63 eVS = 00.58 eV −0.11 eV FIG. 2. Relaxed structures for sulphur-Au n clusters. The numbers below the structures refer tothe energetic difference between spin polarized and spin compensated structure. The numbers onthe atoms refer to the local spin densities on the atoms for the ground states with spin polarization.Note that the odd-even oscillations in spin are shifted by 1 compared to the bare gold clusters. u Aue − sAu AuC FIG. 3. Bonding mechanism of thiols on gold metal. The orbitals around the sulphur denote sp like configurations and the arrows indicate electrons from a sulphur atom. In order to form bondswith two gold Au atoms on a surface, a single sp electron must pass from sulphur to the metalcluster.like configurations and the arrows indicate electrons from a sulphur atom. In order to form bondswith two gold Au atoms on a surface, a single sp electron must pass from sulphur to the metalcluster.