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Instituto de Ciencia de Materiales de Sevilla

MICROSTRUCTURE AND MAGNETIC MICROSTRUCTURE AND MAGNETIC

BEHAVIOUR OF ALKANETHIOLBEHAVIOUR OF ALKANETHIOL--CAPPED CAPPED

GOLD NANOPARTICLESGOLD NANOPARTICLES

• Nanostructured Materials - A. Fernández- T.C. Rojas- E. Guerrero- M. A. Muñoz-Márquez

• Instituto de Magnetismo aplicado(RENFE-UCM-CSIC)

- A. Hernando- P. Crespo- M. Multigner- M.A.García

Gold and Pd-Nanoparticles GoldGold andand PdPd--Nanoparticles Nanoparticles

electronicoptical

magneticMetallic NPs NOVEL

The Magnetism in Pd and Au cluster is not completely understood.Further experiments are needed. Applications in Nanobiotechnology, Nanomaterials design, Nanoelectronic are foreseen

MICROSTRUCTURE AND MAGNETIC BEHAVIOUR OF ALKANETHIOL-CAPED GOLD NANOPARTICLES

Ferromagnetic hysteresis at room temperature in 1.7nm gold NPs cappedby alkanethiol molecules (PRL 93, 087204 (2004))

Ferromagnetic order in alkylammonium and alkanethiol protected Pd NPs(PRL 91, 237203 (2003); PRB 73, 054404 (2006); Nanotechn. 17, 1449 (2006)

Theory: Orbital ferromagnetism and giant anisotropy (PRL 96. 057206 (2006)

Synthesis Gold-NPSSynthesisSynthesis GoldGold--NPSNPS

AuCl4-(toluene) + RSH → (-AuISR-)n(polymer)

Method of Brust et al. Two-step process:

1-


In the first step Au(III) isreduced to Au(I) by oxidizingthe thiols to disulfides. In non polar solvents the AuI(SR) species aggregate forming a polymeric phase.We have isolated thisintermediate polymeric phase



Polym-aged Polymer e-irradiated

The polymeric phase evolves by aging at room temperature (even in dark) or by e-beam irradaition in the TEM microscope

This sample can also be isolated

Au/Thiol1:2 1:1


(-AuISR-)n + BH4- → Aux(SR)y

S

S

S

S

S

S

S S

S

S

S S

S

S

S

S

1 nm.

S

S

S

S

S

S

S S

S

S

S S

S

S

S

SSS S S SS

SSS

SSS SS S

S

Second step: reduction of the polymeric phase by borohydride. Formationof typical gold NPs functionalised by alkanethiol molecules.


2-

HREMHREM

CharacterizationCharacterizationCharacterization


2.4 Å

Sample AuWt%

FeWt%

Swt%

CWt%

HWt%

Au/S(at. Ratio)

Dm(nm)

TotalNr

% Nr

Au-SR1 70.5 0.018 3.7 23.6 4.1 3.0 2.2 249 162

Au-SR2 50.3 0.007 4.1 24.3 4.4 1.7 2.0 177 122

Au(I)-SR2-Polym

49.5 0.012 7.8 36.4 6.2 1.0 ¿ ?



SizeSize andand chemicalchemical compositioncomposition

The polymeric phase is minimising the Au:S atomic ratio

CharacterizationCharacterizationCharacterizationXANESXANES


White line

Unoccupieddensities of d

states

-10 0 10 20 30 40

-10 0 10 20 30 40

Inte

nsity

(a.u

.)

E - E0

Au Foil Au(I)-SR2- Polym Au(I) Sulfide

Au Foil Au-SR1 Au-SR2

E - E0

Au LII- edge

Charge transferfrom Au to S

CharacterizationCharacterizationCharacterizationEXAFS EXAFS Modulus of the Fourier Transform of EXAFS oscillations

at the Au L3-edge

1 2 3 4 5

IT.F

I

R(Å)

Au-SR1 Au-SR2 Au Foil


1 2 3 4 5

IT.F

I

R(Å)

Au(I)-SR2- Polym Au(I) Sulfide Au Foil

Au-S

Au-Au

Au-Au

Au-S

CharacterizationCharacterizationCharacterizationEXAFSEXAFS Best fitting of the Au L3– edge EXAFS oscillations

0 1 2 3 4

IF.T

I

R (Å)

Au(I)-SR2-Polym

0 1 2 3 4

Au-SR1

0 1 2 3 4

Au-SR2Au-SR1 Au-SR2

Au(I)-SR2-polym

2 4 6 8 10 12 14

Au-SR2

Au-SR1

X2 xK

K (Å-1)

Au(I)-SR2-PolymAu(I)-SR2-polym

Au-SR1

Au-SR2


SampleCoordination number

R: bondlength.

σ: Debye-Waller factors.

Δσ2x10-3

(Å2)ΔE0(eV)

Au Foil (Au-Au)m 12 2.850 0.073

Au-SR1 (Au-S)(Au-Au)m

0.458.8

2.3002.802

0.090.094

3.5 4.3

Au-SR2 (Au-S)(Au-Au)m

0.716.8

2.2892.772

0.0910.099

4.5 6.5

Au(I)-SR2-

Polym

(Au-S) 1.3 2.280 0.036

Au(I) Sulfide

(Au-S) 1.7 2.307 0.063



EXAFSEXAFS Best fitting parameters of the Au L3– edge EXAFS oscillations


XPSXPS


80 82 84 86 88 90 920,0

0,2

0,4

0,6

0,8

1,0 Energy shift (eV)Au 4f

norm

alis

ed in

tens

ity (a

rb. u

nits

)

binding energy (eV)

Au-SR1 Au-SR2 Au2S Polymer

0.00- 0.03+0.51+0.57

80 82 84 86 88 90 920,0

0,2

0,4

0,6

0,8

1,0

norm

alis

ed in

tens

ity (a

rb. u

nits

)shifted binding energy (eV)

Au Foil Au-SR1 Au-SR2 Au2S Polymer

Au 4f Peak width (eV)1.141.261.361.461.84

The Au(I)-SR polymeric sample is the one maximazing the chargetransfer from Au to S and the number of gold atoms bonded to sulfur

400 500 600 700 800

Abs

orba

nce

Wavelength (nm)

Au-SR1 Au-SR2 Au(I)-SR2-Polym Au(I)-SR2-Polym-aged


UVUV--VISVIS


Three samples have been obtained withdifferent microstructures at the nanoscale:

Gold nanoparticles (NPs) of 2.2 nm average diameter capped through Au-S bonds by alkanethiol molecules

Gold nanoparticles (NPs) of 2.0 nm average diameter capped through Au-S bonds by alkanethiol molecules

A polymeric Au(I)-SR phase containingAu atoms fully dispersed in an alkane-thiol matrix with 100% of the gold atomsbonded to sulfur

5 K 300 KSample Ms (emu/gr Au) Hc(Oe) Ms (emu/gr Au) Hc (Oe) Au-SR1 0.0045 (3050 Oe) 115 0,0034 (2023 Oe) 55 Au-SR2 0,094 (10000 Oe) 180 0,049 (10000 Oe) 20

Magnetic behaviourMagneticMagnetic behaviourbehaviour


-10000 -5000 0 5000 10000-0,010

-0,005

0,000

0,005

0,010

-10000 -5000 0 5000 10000

-0,10

-0,05

0,00

0,05

0,10a)Au-SR1 T = 5K T = 300K

M (e

mu/

g Au

)

H (Oe)

b)Au-SR2 T = 5 K T = 300K

M (e

mu/

g Au

)

H (Oe)

-200 -100 0 100 200-0,010

-0,005

0,000

0,005

0,010

M (e

mu/

g A

u)

H (Oe)

Au-SR2Au-SR1

Factors improving the ferromagnetic order:Smaller particle size, smaller Au:S atomic ratios, charge transfer from Au to S

Magnetic behaviourMagneticMagnetic behaviourbehaviour

-10000 -5000 0 5000 10000-0,010

-0,005

0,000

0,005

0,010

-10000 -5000 0 5000 10000-0,010

-0,005

0,000

0,005

0,010 T = 5K T = 300K

M (e

mu/

g Au

)

H (Oe)

T = 5K T = 300K

M (e

mu/

g Au

)

H (Oe)

Au(I)-SR2-Poym Au(I)-SR2-Poym-Aged


The polymeric phase is the one maximizing charge transfer from gold to sulfur andmaximizing the number of Au atoms bonded to S.However the simultaneous presence of Au-S and Au-Au bonds is necessary to detectthe ferromagnetic like behaviour.Orbital ferromagnetism: Orbital states associated to a certain cluster of gold atoms

ConclusionsConclusionsConclusionsThree samples have been studied containing gold atoms functionalised with alkane-thiol molecules (-SR), having three different microstructures at the nanoscale:

Gold NPs of 2.2 nm average particle size. Capped through Au-S bonds by alkanethiol molecules. Au:S at. ratio=3.0

Gold NPs of 2.0 nm average particle size. Capped through Au-S bonds by alkane-thiol molecules. Au:S at. ratio=1.7

Polymeric Au(I)-SR phase containing 100% of Au atoms bonded to S in the alkanethiol molecules. Total dispersion of gold. Au:S at.ratio=1.0

It has been experimentally demonstrated that the presence of a ferromagnetic likebehaviour is associated with the simultaneous presence of Au-Au and Au-S bonds.

The presence of metallic or condunting clusters or surfaces are necessary to produce orbital moments induced in atomic like localized states.

These crucial experiments point out the orbital contribution to the observedmagnetic behaviour.