14.Mag Prop of Gold Clusters

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R E S E A R C H P A P E R

Permanent magnetism in phosphine- and chlorine-capped

gold: from clusters to nanoparticlesMiguel A. Munoz-Marquez Estefana Guerrero

Asuncion Fernandez Patricia Crespo Antonio Hernando

Raquel Lucena Jose C. Conesa

Received: 20 September 2009 / Accepted: 20 January 2010 / Published online: 6 February 2010

Springer Science+Business Media B.V. 2010

Abstract Magnetometry results have shown that

gold NPs (*2 nm in size) protected with phosphine

and chlorine ligands exhibit permanent magnetism.

When the NPs size decreases down to the subnano-

metric size range, e.g. undecagold atom clusters, the

permanent magnetism disappears. The near edge

structure of the X-ray absorption spectroscopy data

points out that charge transfer between gold and the

capping system occurs in both cases. These results

strongly suggest that nearly metallic Au bonds are

also required for the induction of a magneticresponse. Electron paramagnetic resonance observa-

tions indicate that the contribution to magnetism from

eventual iron impurities can be disregarded.

Keywords Gold clusters Gold nanoparticles

EPR spectroscopy SQUID magnetometry

Ferromagnetic behaviour

Introduction

Currently, many nanoscale applications such as

electronic devices, systems with catalytic properties,magnetic and optical mechanisms, and biological

systems (e.g. Andres et al. 1996; Valden et al. 1998;

Sun et al. 2000; Boyen et al. 2002; Daniel and Astruc

2004; Turner et al. 2008) include transition metal

nanoparticles (NPs) as essential parts to perform their

function. Despite the widespread study and applica-

tion of these particular systems, there are still many

unknowns to be addressed regarding the solid-state

properties and structure of capped transition metal

NPs (Whetten and Price 2007), for instance, their

magnetic properties which can certainly help theunderstanding of essential questions in magnetism. It

is known that due to size and surface effects that

appear when the system size is reduced down to the

nanometre range, e.g. the electronic properties of NPs

are significantly different from bulk-like systems of

the very same materials (Alivisatos 1996); however,

sometimes not only the system size is the key factor in

the physical properties and even the capping systems

play an important role in this profound change of the

M. A. Munoz-Marquez (&) E. Guerrero A. Fernandez

Instituto de Ciencia de Materiales de Sevilla (CSIC-US),

Av. Americo Vespucio 49, 41092 Sevilla, Spain

e-mail: [email protected]

P. Crespo A. Hernando

Instituto de Magnetismo Aplicado (UCM-ADIF-CSIC),

P.O. Box 155, 28230 Las Rozas, Madrid, Spain

P. Crespo A. Hernando

Departamento de Fsica de Materiales, UCM,

Av. Complutense s/n, 28040 Madrid, Spain

R. Lucena J. C. Conesa

Instituto de Catalisis y Petroleoqumica (CSIC), Marie

Curie 2, Campus de Cantoblanco, 28049 Madrid, Spain

123

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DOI 10.1007/s11051-010-9862-0


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electronic structure (Zhang and Sham 2002). There-

fore, further investigations of the parameter space,

which define the physical and chemical properties of

nanoscale systems, are always welcomed.

Regarding the NPs magnetic properties, it is now

widely accepted that a ferromagnetic-like behaviour

is observed in one-dimensional nanoscale systemssuch as gold, silver, palladium and copper which are

diamagnetic in their bulk-like form. This claim leans

on a wealth of experimental results already published

in world-leading scientific journals (e.g. Shinohara

et al. 2003; Sampedro et al. 2003; Crespo et al. 2004;

Yamamoto et al. 2004; Negishi et al. 2006; Suber

et al. 2007; Dutta et al. 2007; Garitaonandia et al.

2008; de la Venta et al. 2009), performed by wholly

independent research groups throughout the world. In

addition, this experimental work is now supported by

recent theoretical total energy calculations (Gonzalezet al. 2006; Luo et al. 2007; Michael et al. 2007)

which yielded encouraging results on size-dependent

magnetization and spin symmetry-breaking that

would explain the experimental results found for

thiol-capped gold and silver nanoparticles. Probably,

the most striking advance at the light of the latest

magnetization studies based on element-specific

techniques (Yamamoto et al. 2004; Negishi et al.

2006; Garitaonandia et al. 2008; de la Venta et al.

2009), such as X-ray magnetic circular dichroism

(XMCD) and Mossbauer spectroscopy, is that it hasbeen unequivocally determined that the gold atoms

neighbouring the chemisorption site at the nanopar-

ticle surface are the carriers of the magnetic moment.

At this point, it has to be mentioned that all the

previous studies mainly concern thiol-capped transi-

tion metal nanoparticles and, as a result, the magnetic

behaviour has been partly attributed to the charge

transfer effect measured in the metal-S bond (Guer-

rero et al. 2007); including the fact that magnetism

was not observed in polymer-like gold compounds

such as Au2S. Of course, as it will be discussed later,this is not the only effect responsible of the

ferromagnetic-like behaviour. However, the well-

known Brust method used to synthesize highly

monodisperse thiol-capped gold NPs from gold salt

(HAuCl4) reduction (Brust et al. 1994) has lead to a

widespread use and thorough study of the AuS bond

properties in nanometre-size systems, being these

gold-thiolate species the building blocks of many

self-assembled systems. Despite the Brust method is

a highly reproducible and straightforward synthesis

procedure, it is not free of experimental difficulties.

In fact, it has been proved almost impossible to obtain

subnanometric gold clusters using this synthesis

method which would justify the importance of the

synthesis methods developed by Weare et al. (2000)

for small phosphine-stabilized Au NPs (diameter *2nm) and, the procedures established by Bartlett et al.

(1978) to obtain phosphine-capped undecagold clus-

ters (subnanometric particles with a diameter around

0.8 nm) that have played a very important role in the

research presented in this article. These synthesis

methods allowed the fabrication of smaller particles

than the ones obtained by the Brust method and, in

addition, they have the potential of exchanging the

phosphine shell with a protecting thiolated ligand

layer (Song et al. 2003; Woehrle and Hutchison

2005); so the final result is a set of very small thiol-capped gold NPs or clusters.

Moreover, the obtained NPs have been subject of

controversy regarding their magnetic properties

which some authors have ascribed to natural occur-

ring iron impurities that can be found in the reagent

grade chemicals used across the synthesis process or,

as a result of using stainless steel tools whilst

manipulating the synthesis products and reactants

(Abraham et al. 2005). However, it has been demon-

strated that superparamagnetic iron impurities cannot

be responsible of the ferromagnetic behaviour in goldNPs (Crespo et al. 2006). The electron paramagnetic

resonance (EPR) studies carried out in this investi-

gation are key to probe the magnetic state of eventual

impurities and, most important, to check their role on

the macroscopic magnetic behaviour.

The structural and electronic properties of phos-

phine-capped clusters and nanoparticles have been

deeply studied in the past (Weare et al. 2000; Bartlett

et al. 1978; Mingos 1976; Briant et al. 1981; Teo

et al. 1992; Mingos 1996; Schmid 2008). However,

nobody before has explored the magnetic propertiesof these systems in order to tackle the eventual

relationship between the magnetic behaviour and the

electronic/atomic structure. Moreover, there is a

long-standing issue regarding the physical and chem-

ical properties of these nanomaterials where the

driving factors of the thermodynamic stability are

still unknown (Walter et al. 2008). These were the

reasons that lead us to study the magnetic properties

of phosphine-capped gold nanoparticles and clusters.

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As previously said, the nanoparticle surface atom

bonding with the capping ligand is not the unique

concurring factor in the emergence of a ferromag-

netic-like behaviour in gold NPs. The size effect is

also a key point: the nanoparticle diameter is very

similar to the wavelength of the confined electrons in

the nanoparticle and this will determine many of thephysical properties of these particular systems at the

quantum level. In addition, the ferromagnetic-like

behaviour shall be strongly linked to the surface

effects originated by the dramatic increase of atoms

that lose the bulk 3D symmetry in the NP surface; the

atoms in the surface are only constrained in a 2D

fashion, hence, the NP physical properties undergo

significant changes associated to a modification in the

chemical environment and atomic structure. For

instance, considering the thiol-capped NPs, due to a

huge increase in the mobility of the thiol-Au species(Yu et al. 2006), the bonding of the sulphur atom in

the thiol head to a gold surface atom leads to a deep

change of the outermost surface atoms structure

which necessarily must involve a dramatic modifica-

tion of the electronic structure.

Experimental details

Sample preparation

Au11-TPP synthesis

This subnanometric gold cluster is synthesized by

reduction of the AuCl(PPh3) precursor which was

synthesized according to the procedure described by

Braunstein et al. (1990). Following the synthesis

method established by Bartlett et al. (1978), 0.18 g

of Au(PPh3)Cl are dissolved in 8 ml of absolute

ethanol and then reduced with a solution of NaBH4 in

ethanol (0.0137 g of solute in 2 mL of solvent) overa 15 min period whilst the reducing agent is slowly

added under nitrogen atmosphere. The mixture is

then stirred for 2-h. The formed solid is precipitated

with hexane and, filtered and washed with CH2Cl2/

hexane. The remaining solid is dissolved in CH2Cl2and filtered to eliminate a colourless and insoluble

powder. Finally, the product is precipitated with

hexane.

nAu-TPP synthesis

Phosphine-chlorine-capped Au NPs could be directly

obtained from HAuCl4 reduction in presence of

triphenylphosphine (PPh3 or TPP), using NaBH4 as

reducing agent. The synthesis method used was first

reported by Weare et al. (2000) which is a modifica-tion of the well-known Brust method (1994). Au(III) is

transferred from an aqueous solution of 0.2 g of

HAuCl4 in 12 mL of milli-Q water to degassed and

dried toluene; a solution of 0.28 g of tetraoctylammo-

nium bromide in 12 mL of toluene was used as the

phase-transfer agent. The mixture is strongly stirred

for 5 min whilst 0.46 g of PPh3 are added to the

solution which is further stirred for 10 more minutes. A

fresh solution of NaBH4 (0.4 g of solute in 2 mL of

milli-Q water) is quickly added whilst stirring then, the

mixture is continuously stirred for three hours. Theorganic phase is then separated from the aqueous

phase. Toluene is removed under reduced pressure by

means of a rotary evaporator. The solid was precip-

itated with hexane and, finally, filtered and washed

with hexane and MeOH/H2O.

Sample characterization

Particle characterization using elemental analysis,

ICP, TEM and SEM

Aliquots of the synthesized nanoparticles were

dropped onto the carbon film on a 200-mesh copper

grid and air dried. Nanoparticles were imaged

employing a Philips CM200 TEM microscope work-

ing at 200 kV at the Microscopy Service of the

Instituto de Ciencia de Materiales de Sevilla. The

approximate particle size distribution histograms were

measured using image analyser software that deter-

mines the cluster radii from a digital image of the

micrographs. The chemical composition by elementalanalysis in a LECO CNS-2000 and ICP-MS in a

Thermo Elemental X-7 was determined at the Centro

de Investigacion, Tecnologa e Innovacion d e l a

Universidad de Sevilla (CITIUS). In addition, the

chemical composition was also checked using EDX in

the TEM microscope and in a SEM Hitachi S-4800.

Samples for EDX-SEM analysis were placed in

carbon tape.

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Optical transitions

UVVis absorption spectra were recorded in transmis-

sion mode.In this experiment, the gold clusters andnano-

particles were dispersed in liquid solutions (1 mg/mL)

of ethanol and placed into a high transmission quartz

cuvette; so good transmission is achieved in the UVregion. The spectra were collected in the range

350850 nm with a Shimadzu UV-2102 PC spectrom-

eter at room temperature.

SQUID and EPR measurements

The hysteresis curves were obtained using a Quan-

tum Design S6000 magnetometer at the Instituto de

Magnetismo Aplicado in Madrid. Data were col-

lected at temperatures between 5 and 300 K using aliquid helium cooling system. The samples were

placed in adhesive kapton stuck to a quartz tube. The

diamagnetic contribution from the sample holder was

measured and subtracted from the total magnetiza-

tion. Meanwhile, the EPR measurements were

carried out at the Instituto de Catalisis y Petroleoqu-

mica in Madrid on a Bruker ER200D instrument

operating in the X-band and interfaced to a digital

data acquisition system. Aliquots of the studied

samples were placed into a special spectroscopically

pure quartz cell. All the spectra were recorded in aTE104-type double cavity. The frequency of the

microwave was calibrated for each experiment

using a standard of the stable-free radical Diph-

enylpicrylhydrazyl (DPPH) with g = 2.0036 located

in the second cavity.

X-ray absorption spectra measurements

XAS of the Au samples were recorded in transmis-

sion mode at the BM29 beamline of the European

Synchrotron Radiation Facility (ESRF) storage ring.The samples were measured as thin self-supported

pressed pellets diluted in boron nitride. Spectra were

recorded at the Au L2- and L3-edge, at 13,734 and

11,919 eV, respectively. Simultaneously, a gold foil

standard was measured to calibrate the X-ray energy.

To compare the X-ray absorption near-edge structure

region, a linear background was fitted in the pre-edge

region and subtracted before normalization to the

edge jump.

Results and discussion

Chemical composition: elemental analysis,

ICP and EDX

Two different types of gold nanomaterials were

chemically synthesized for this study: triphenylpho-sphine-capped undecagold clusters labelled as Au11-

TPP and gold nanoparticles which were named nAu-

TPP. As it will be discussed later, both particles have

a significant chlorine content coming from the

precursors AuCl(PPh3) and HAuCl4 used in the

fabrication. Across the entire synthesis process,

Teflon-coated stainless steel tools, as well as, brand

new laboratory glassware were used to avoid

unwanted effects on the magnetometry results com-

ing from eventual ferromagnetic contamination.

The undecagold compound was first synthesizedby Bartlett et al. (1978), although there were previous

theoretical studies on this system (Mingos 1976) that

focused on various Au11 and Au13 based phosphine-

passivated clusters. Since the synthesis method

followed here claims the production of undecagold

gold clusters, then, despite the obtained sample could

contain both phases (Au11 and Au13) we will refer to

it as undecagold cluster which seems to be the most

frequent phase. According to the literature, the result

of this synthesis should be a Au11(PPh3)7Cl3 cluster

which agrees pretty well with the chemical compo-sition results reported in this section (cf. Table 1).

Regarding the phosphine-stabilized gold NPs, We-

are et al. (2000) slightly modified the Brust method for

thiol-capped Au NPs synthesis so phosphine-capped

gold NPs could be obtained. This method was born as

an alternative to the more complicated synthesis

method originally formulated by Schmid et al. (1981)

which required from strictly anaerobic conditions and

diborane gas as a reducing agent, and resulted in 1.4 nm

Au55(PPh3)12Cl6 clusters. The NPs obtained following

Weare et al. (2000) method should have a mean size of1.5 nm which according to the authors corresponds to a

metal core of*101 gold atoms that leads to an

estimated formula of Au101(PPh3)21Cl5. However, as it

will be shown later and despite having reproduced the

synthesis method described by Weare et al. (2000), the

nanoparticles produced in our laboratory have a mean

diameter of*2 nm which, according to the composi-

tional analysis performed, would lead to an estimated

composition Au225(PPh3)80Cl16.

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Determining the exact composition of the syn-

thesized samples has been proved a very difficult

task. Compositional data were obtained from induc-

tively coupled plasma-atomic emission spectrometry

analysis (ICP) for the heaviest elements (Au and Fe)

whilst the quantity of lightest elements (P, C and H)was determined by elemental chemical analysis. The

results are summarized in Table 1. Unfortunately,

despite having performed a filtration procedure, still

some residual ligands remained free, not bonded to

any gold particle and, therefore, this led to an

overestimation of the gold-ligand ratio. One should

remember that ICP and elemental chemical analysis

provide a chemical composition quantification of the

sample as a whole. Instead, energy dispersive X-ray

analysis (EDX), measured in a transmission electron

microscope (TEM), is able to quantify the chemicalcomposition in a very small region mainly occupied

by nanoparticles which results in a more accurate

determination of the gold-ligand ratio: essentially,

Au:P ratio. TEM studies also allowed the size

determination of the clusters and nanoparticles.

However, some difficulties were found when mea-

suring the Cl content in the studied samples: in

addition to the low Cl content, the effect of a low

electron scattering cross section at 200 kV lead to a

non measurable emission line from the Cl K-shell. A

supplementary EDX spectrum was recorded in adifferent electron microscope; in this case, a 20 kV

scanning electron microscope (SEM) where the Cl

K-shell at 2822 eV could be measured since the

electron cross section is noticeably higher at 20 kV.

This was used to qualitatively determine the pres-

ence of chlorine in the synthesized samples. The

EDX results and the TEM size determination are

included in Table 1.

Cluster and nanoparticle topological structure

According to previous results, an incomplete icosa-

hedral structure with an approximately C3v symmetry

axis (Bartlett et al. 1978; Nunokawa et al. 2006;

Barnard et al. 2009) is assumed for the phosphine-capped undecagold clusters which have a core diam-

eter of 0.8 nm. As it has already been described by

Menard et al. (2006b), this value is slightly overes-

timated when measured by conventional bright field

(BF) TEM (cf. Fig. 1). For clusters below 1 nm, the

poor contrast between the smallest metal NPs and the

support films used for the electron microscopy studies

tends to bias microscopic measurements towards any

subpopulation of larger sized particles (Narayanasw-

amy and Marks 1993; Wilcoxon et al. 2000).

Meanwhile, the nAu-TPP sample with an averagediameter of*2 nmas shown inFig. 1, and considering

a slight experimental overestimation, would correspond

to a bulk-like fcc structure with*225 gold core atoms,

which is consistent with one of the magic number

structures already reported in previous studies (Brust

et al. 1994; Whetten et al. 1996): on the basis of X-ray

diffraction (XRD) experiments recorded by Whetten at

al. (Whetten et al. 1996), a 2% expansion of the bulk

gold lattice constant is considered. In Fig. 2 there is a

perspective view diagram of the assumed models for

both, undecagold clusters and 2 nm nanoparticles. Itshould be noted that the theoretical Au:P atomic

ratio shown in the nAu-TPP model nearly match the

experimental results obtained from the EDX analysis.

Despite the Au:P ratio might seem very low compared

to the same ratio as calculated in the well-

established Au55-TPP clusters, it is very similar to the

Au:P ratio obtained for the also well-characterized

[Au39(PPh3)14Cl6]Cl2 cluster (Teo et al. 1992). This

Table 1 Summary of the chemical composition determined by ICP, elemental analysis and EDX (TEM and SEM), along with some

structural parameters determined from the TEM micrographs, of the phosphine-capped particles studied in this article

Sample Au Fe P C H Cl

(% at)c

Au:P

(at. ratio)d

Dm

(nm)

SD r

(nm)(% wt)a

(% wt)b

Au11-TPP 50.1 \0.03 5.2 36.0 2.5 3.0 1.3 1.41 0.14

nAu-TPP 43.5 \0.03 5.9 41.1 2.9 3.6 3.3 2.1 0.2

aAs determined by the ICP

b As determined by elemental analysisc

Qualitative determination by SEM-EDXd

As determined by TEM-EDX

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illustrative figure provides a very good idea on how

well-packed the PPh3 molecules must be in the NP

surface, so*80 of them, along with 16 chlorine atoms,

will fit in one single particle. Incidentally, regarding

the molecular packing structure of phenyl rings on

Au(111) surfaces, it has been reported that phenyl rings

which usually lie flat on the gold surface when thecoverage is below half-monolayer, can also adsorb

along the ring edge in stand up position to improve the

surface coverage in saturated conditions (Mullegger

et al. 2006). The adsorption site of PPh3 molecules and

Cl ligands is in agreement with previous structural

studies of solvated nanoparticles (Periyasamy and

Remacle 2009) and conventional surface science

investigations (e.g. Steiner et al. 1992; Kastanas and

Koel 1993; Baker et al. 2008); the adsorption of

electronegative species are found to affect signifi-

cantly the gold surface structure, in particular, theseadsorbates can lift gold atoms from the surface. The

preferred adsorption sites are surface defects such as

adatoms, step edges, kinks and gold vacancies. Hence,

according to previous results by Periyasamy and

Remacle (2009), the PPh3 molecules have been coor-

dinated to the face edge atoms of the gold core outer

layer, i.e. to the corner atoms. Meanwhile, the chlorine

ligands are symmetrically coordinated to the face-

centred gold atoms in the sides of the nanoparticle core.

The formation of a self-assembled monolayer which, as

it has been proved before (Yu et al. 2006), would have

profound implications concerning the shell-core bond-

ing that might go beyond a plain chemisorption. A

strong bonding to the nanoparticle gold atoms will

deeply change the outermost surface core atoms

topological structure and, subsequently, the electronicstructure: this point may be crucial to explain the origins

of the magnetic properties.

Unfortunately, no additional information could be

extracted from these nanoparticles and clusters by

means of high-resolution transmission electron

microscopy (HRTEM). Although valuable informa-

tion has been gathered from HRTEM studies in larger

supported nanoparticles, the application of this method

to smaller clusters (D\3nm) is limited by image-

contrast, momentum-transfer, and beam-induced

mobility considerations (Cleveland et al. 1997). How-ever, the UVVis absorption spectra were extremely

valuable since the quality of the synthesized particles

could be checked using the absorption features. The

UV-Vis spectra are shown in Fig. 3. The dashed line

corresponds to the absorption spectrum of the Au11-

TPP cluster which shows an absorbance feature at

*420 nm typical of subnanometric phosphine-stabi-

lized undecagold (Bartlett et al. 1978). Meanwhile, the

solid line represents the absorption spectrum of the

Fig. 1 Transmission

electron micrographs of

phophine-capped Au

clusters and nanoparticles,

and their corresponding size

distribution histograms. The

solid line represents the

fitting curve assuming alog-normal function. The

calculated mean particle

diameter (Dm) and the

standard deviation (r) are

shown in the histograms

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nAu-TPP nanoparticles with a broad plasmon reso-

nance feature at 520 nm, characteristic of*2 nm

nanoparticles (Alvarez et al. 1997).

Magnetic behaviour: SQUID and EPR

measurements

The magnetic behaviour of the systems presented here

was studied by means of superconducting quantum

interference device (SQUID) magnetometry. The

recorded hysteresis cycles at the lowest and highest

temperature, presented in Fig. 4a and c, provided

sound information regarding the macroscopic magne-

tization of the studied systems and, clearly show the

ferromagnetic-like behaviour of the phosphine-stabi-

lized gold nanoparticles (nAu-TPP) whilst the phos-phine-capped clusters (Au11-TPP) do not present any

magnetic feature being diamagnetic at low and high

temperature. Moreover, the magnetization versus

temperature is shown in Fig. 4b and d for samples

nAu-TPP and Au11-TPP, respectively. The depen-

dence of the magnetization with temperature of the

nAu-TPP sample is consistent with the observed

behaviour in thiol-capped gold nanoparticles (Crespo

et al. 2004): the hysteresis phenomenon is observed

up to room temperature which means that this

particular system has a blocking temperature above300 K corresponding to an anisotropy constant of

3 9 107 J m-3. Only the coercive field decreases from

850 Oe down to 300 Oe when raising the temperature

from 5 K up to room temperature. Meanwhile, the

magnetic saturation of the nAu-TPP is 0.08 lB/NP.

This value is similar to the magnetization values

associated to the Au 5d orbitals as measured with

XMCD experiments by Garitaonandia et al. (2008)

and, more recently, by de la Venta et al. (2009). The

fractional magnetization values (\1 lB) confirm that

the electron is partially shared between the surfacegold atom and the organic ligand. Since, the blocking

has its origins in the strong spin-orbit field (Hernando

et al. 2006b) then, the blocking behaviour will exist

for magnetization values well below 1 lB.

At this point an important question arises: is the

magnetic behaviour due to the existence of ferro-

magnetic and/or paramagnetic iron impurities? At the

light of the compositional results which show an iron

content below 0.01% wt., the aforementioned possi-

bility sounds more than plausible. However, since the

measured Fe content is just below the detection limitof the available chemical analysis device, then a more

sensitive technique was needed. Here is where the

electron paramagnetic resonance spectroscopy comes

into play. This technique is able to detect very low

quantities of chemical species that have one or more

unpaired electrons. Therefore, the EPR results will

provide information on the presence of magnetic

impurities and also on their magnetic state. These

results are shown in Fig. 5. The spectra were

Fig. 2 Perspective view of the cluster and nanoparticle

structure. On the left hand side panel, the subnanometric

cluster Au11-TPP with an icosahedral structure, whereas the fcc

bulklike *2 nm nAu-TPP nanoparticle is represented in the

right hand side panel. The gold core atoms are yellow, the red

balls correspond to phosphorus and the green ones to chlorine.

For simplicitys sake, the aromatic rings of the triphenylphos-

phine molecule are not included

Fig. 3 UVVis absorption spectra of nAu-TPP (solid line) and

Au11-TPP (dashed line) in ethanol

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absorption near-edge structure (XANES) data of the

Au-L3 edge, which have a first resonance around

5 eV above the threshold energy (whiteline) associ-

ated with a 2p3=2 ! 5d5=2;3=2 dipole transition that

is actually probing the density of unoccupied d states

at the Fermi level (Zhang and Sham 2002). Despite

gold should have a nominally full 5d band, due to s-

p-d hybridization, a faint whiteline is still detected

for bulk gold (cf. Fig. 6). The area under the XANES

curve will provide an accurate estimation on the

charge redistribution, i.e. a charge transfer phenom-

enon taking place in the gold-ligand bond. As it has

been previously discussed, the charge transfer effect

that occurs in the AuS bond of the thiol-capped gold

NPs is an essential point to explain the origins of the

magnetic behaviour. However, considering that

according to the XANES results reported in this

article where subnanometric phosphine-protected

gold clusters present a noticeably large charge

transfer in the Au-ligand bond and, according to the

SQUID experiments on the very same clusters where

a diamagnetic behaviour is observed, then the charge

transfer effect can be ruled out as the only responsibleeffect of the ferromagnetic-like character. This claim

is reinforced if the charge transfer of the nAu-TPP is

considered. In this case, the measured resonance of

the XANES edge is significantly less intense than in

the Au11-TPP sample and, in spite of this, the

ferromagnetic-like character appears in the nanopar-

ticles whilst it is not observed in the subnanometric

clusters. Recently, Walter et al. (2008) have per-

formed density functional theory calculations of

structurally characterized ligand-protected gold clus-

ters and nanoparticles. As a result, their determina-tion of the electronic structure, i.e. d-hole generation,

agrees with the magnetic behaviour previously

observed in thiol-capped gold NPs (Crespo et al.

2006). Walter et al. (2008) also claim that, in the

Au11(PPh3)7Cl3 clusters, the phosphine ligands are

weak surfactants that cannot significantly modify the

electron shell structure of the gold cluster core and, it

would be the chlorine ligand the one that plays the

role of the thiol ligand in the aforementioned

magnetic nanoparticles (Crespo et al. 2006). In our

case, the strong charge transfer observed from theXANES spectrum of the undecagold clusters might

be solely attributed to the AuCl bond. Unfortu-

nately, the data presented in this research work are

not able to discern whether the major charge transfer

effect is due either to phosphine or chlorine ligands.

The presence of possible individual Au?-species has

not been detected by EPR. However, regardless of

what is the exact relative number of AuP and AuCl

bonds, it seems to be very clear that a balance

between Au-ligand charge transfer and AuAu bond

is indeed necessary in order to originate a netmagnetic moment. This balance as observed from

the XANES whiteline represents a slight charge

transfer large enough to originate a strong electric

dipole along the bond but, at the same time, weak

enough to keep the metallic character of the inner

gold atoms in the nanoparticle core. The optimum

balance has been found for the thiol-capped gold NPs

synthesized following the Brust method and also for

the nAu-TPP NPs.

Fig. 5 Electron paramagnetic resonance spectra from thestudied samples: nAu-TPP upper panel and Au11-TPP bottom

panel. Data were collected at T= 298 and 77 K. In both

samples, spiky features appear between 5,500 and 8,000 gaussmagnetic fields in the EPR spectra taken at 77 K; these

correspond to molecular oxygen present in the analysis

chamber due to vacuum conditions slightly higher than the

typical base pressure

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Beyond this argument, there is a point to be

considered yet: the electronic structure of the system

as a whole. As it has been previously investigated

(Bartlett et al. 1978; Yang and Chen. 2003; Menard

et al. 2006a), the phosphine-capped gold clustershave a molecule-like electronic structure which

presents almost discrete electron energy levels below

500 nm in the UVvisible absorption range. Instead,

the electronic structure of the phosphine-capped gold

NPs presents a broad plasmon feature around

520 nm. This particular behaviour is due to the

presence or not of metallic AuAu bonds in the

system; the surface plasmon resonance feature is

strongly marked for gold particles above 10 nm

which have an almost metallic character. The pres-

ence of adsorbates in the nanoparticle surface,namely PPh3 molecules, which reduce the surface

electrons mobility would significantly damp the

surface plasmon resonance as it has already been

proved (Guerrero et al. 2008). However, if the

adsorbate molecules, in this case phosphine and

chlorine ligands, form a well-ordered and close-

packed structure then, small domains of ordered PPh3molecules and Cl atoms could self-assemble in the

nanoparticle core facets. This would finally lead to an

arrangement of the charge dipoles formed at the Au-

ligand bond and, consequently, the nanoparticle

surface electrons would collectively move along the

domain boundaries. The coexistence of a collective

electron movement that leads to a weak surface

plasmon resonance and some degree of mobility is a

key point in the appearance of a permanent magneticmoment (Hernando et al. 2006a). The well-ordered

structure of the capping system, far from ideal, is a

more than possible situation. In fact, a close-packed

layer is the only way to explain such a high number

of ligands:*80 PPh3 molecules and 16 Cl atoms for

a given nanoparticle, as it has been determined by the

chemical composition study.

Conclusions

In summary, the existence of a permanent magnetic

behaviour in phosphine-chlorine-capped gold nano-

particles has been shown for the first time by means

of SQUID magnetometry. The EPR experiments

confirm that Fe3? species are not present in the

samples; therefore, the origins of the observed

ferromagnetic behaviour must be somewhere else.

These results agree with previous investigations on

thiol-capped gold nanoparticles carried out using

XMCD and Mossbauer spectroscopy. The peculiar

magnetic behaviour does not have its origins in onesingle factor; instead, there are several factors that

contribute to the appearance of a permanent magnetic

moment. One factor is a charge transfer effect

occurring at the Au-ligand bond which must be

linked to the presence of AuAu bonds that provide a

metallic character: this is the reason that explains the

diamagnetic behaviour of phosphine-chlorine-capped

clusters, where there is very probable that the Cl

anions are dominating the charge transfer pulling the

electrons towards the ligand and, therefore limiting

the mobility of the nearby electrons that finallyvanishes any chance of generating an orbital mag-

netic moment. Additionally, the EPR spectra also

show a faint peak at low g values in the nAu-TPP

sample which could be attributed to the ferromagne-

tism observed in these nanoparticles. Finally, the self-

assembling of an ordered capping layer is also a key

factor since the formation of charge dipole domains

will drive the movement of the surface electrons that

finally lead to the appearance of a magnetic moment.

Fig. 6 Au L3-edge XANES spectra of the the synthesized

samples: nAu-TPP and Au11-TPP, compared with bulk Au and

the polymeric precursor Au(PPh3)Cl used in the cluster

synthesis

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Acknowledgements The authors would like to acknowledge

the support of the European Synchrotron Radiation Facility and

BM29 beamline staff. This research has been supported by the

Spanish Ministry of Science Ministerio de Ciencia e Innovacion

(MICINN) (Strategic Action NAN2004-09125-C07) and the

Andalusian Government Junta de Andaluca (Excellence

Project P06-FQM-02254 and P09-FQM-4554, group TEP127).

M.A. Munoz-Marquez thanks the Spanish Research Council

Consejo Superior de Investigaciones Cientficas (CSIC) I3P

programme, E. Guerrero acknowledges the MICINN for

financial support and R. Lucena thanks CSIC for a PhD grant.

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14.Mag Prop of Gold Clusters