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Applications of XAFS to materials and nano sciencematerials and nano– science
Federico BoscheriniD f Ph i Department of Physics
University of Bologna, Italyfederico boscherini@unibo [email protected]
www.df.unibo.it/fismat/rad-sinc
1/67SILS School on SR, Duino (Trieste)Italy, September 2009
Plan
• Introduction: why use XAFS in materials Introduction: why use XAFS in materials science & nanostructure research?
• Examples of applications, using bothresults which have “stood the test of time”– results which have stood the test of time
– recent results
2SILS School on SR, Duino (Trieste)Italy, September 2009
X-Ray Absorption Fine StructureX y p FEn
ergy
“ E ” b b d l / d 1s2s2p ωh
ωh“XANES”: Absorber symmetry and valence/oxidation state
Electronic structure of unoccupied statesMedium range structurem g
“EXAFS”: Coordination numbersInteratomic distancesDisorder of distancesDisorder of distances
3SILS School on SR, Duino (Trieste)Italy, September 2009
EXAFSEXAFS• Extended X-ray Absorption Fine Structure• Fit fine structure oscillations with• Fit fine structure oscillations with
From ab initio calculations or from reference compounds
2222 ]22[)()( jkkSikANSk σδ −∑
From ab-initio calculations or from reference compounds
120 ]22[)()( j
jjjshellsj
j ekrSinkANSk δϕχ=
++= ∑
M s :
( )h
h BEmk
−=
ω2
Debye Waller factor-thermal vibration
d d
Coordinationnumber
Interatomic distance
Measure:
-static disorder4SILS School on SR, Duino (Trieste)
Italy, September 2009
XANESXANES• X-ray Absorption Near Edge Structure
(also NEXAFS)
)(ˆ422
fEfri ρεωαπσ rh •=
)(
0,1
d
m =±=light) pol. (lin.
ll ∆∆
• “Molecular orbital” approach: 1 electron approximation, i l b i d
)(, sdpps →→
constant matrix element: probe site and symmetryprojected density of states of final states
• “Multiple scattering” approach: structural interpretation Multiple scattering approach: structural interpretation through simulation
5SILS School on SR, Duino (Trieste)Italy, September 2009
Characteristics of XAFS• Atomic selectivity• High resolution for first few
di ti h ll
• Today’s topics– Dopants, defects
coordination shells• Sensitivity to high dilutions &
surfaces/interfaces
– Alloys• local distortions • orderingsurfaces/ nterfaces
• Equally applicable to ordered or disordered matter
ordering– Phase transitions– Thin films, interfaces
• XANES:– valence/oxidation state– 3D structural sensitivity
– Nanostructures• Semiconductor dots• Metallic clusters3D structural sensitivity
• µm spot size now available• Metallic clusters
6SILS School on SR, Duino (Trieste)Italy, September 2009
Role of XAFS in Materials ScienceGrowth Physical Properties
Structure
Nakamura et al., Jpn. J. Appl. Phys. 35, L217, 1996MBE@TASC
XAS
7SILS School on SR, Duino (Trieste)Italy, September 2009
XAFS and dopants/dilute elementsXAFS and dopants/d lute elements• Only the structure around the photo-
excited atom is probed• Fluorescence detection greatly g y
enhances sensitivity• Present sensitivity limitPresent sensitivity limit
– dopants in the bulk ∼ 1018 at/cm3, – thin films (single layer) ∼ 1014 at/cm2thin films (single layer) ∼ 10 at/cm
8SILS School on SR, Duino (Trieste)Italy, September 2009
Arsenic in a-Si:H• As in a-Si expected to be
3-fold coordinated
ber
(CN
)
3.0 • But: doping is observed• Doping in c-Si due to 4-fold As
Knights et al observe increase ion
Num
b
• Knights et. al. observe increasein CN at 1% As concentration– evidence of active As oo
rdin
ati
• Knights, Hayes, and MikkelsenJr.Phys Rev Lett 39 712 (1977)
As
Co
1 % 10 %2.5
Phys. Rev. Lett. 39, 712 (1977)
9SILS School on SR, Duino (Trieste)Italy, September 2009
Si/GaAs δ-doped layers and superlatticesB h i i F tti B i O i
• δ doping: physical separation
Boscherini, Ferretti, Bonanni, Orani, Rubini, Piccin, and Franciosi APL 81, 1639 (2002)
• δ-doping: physical separationof dopants and chargecarriers– Si-GaAs a technologically
relevant caseOp n issu : Numb f ch• Open issue: Number of chargecarriers not proportional tonumber of dopants above a number of dopants above a critical concentration. Structural origin?
10SILS School on SR, Duino (Trieste)Italy, September 2009
Si-GaAs: samplesp• Growth conditions optimized for Si/GaAs
superlattice formation: p– MBE, 540 °C, 4 nm/h, @TASC
• Si thickness: 0.02 ML – 4 ML; repetition optimized for XAS
• Fraction of electrically active Si atoms:4 22×10–4 – 6× 10-2
11SILS School on SR, Duino (Trieste)Italy, September 2009
Si-GaAs: dataSi GaAs data• Si K edge XAS @
LURELURE• Fluorescence
mode bulk
Si - As theory
-1)mode, bulk
sensitivity• Si – Si clustering
0.2 ML Si
2 ML Sik χ(
k) (Å
-
• Si – Si clustering apparent in raw data 1
2 ML Si
Bulk Siliconk
data2 4 6 8 10 12
k (Å-1)
0.5 Å-1 u S co
12SILS School on SR, Duino (Trieste)Italy, September 2009
Si-GaAs: results
• Si – Si CN > 2 4 10-1
always• Presence of Si
l t3.5
10-2
N (Si) R*
Num
ber Fraction
clusters• This is the origin
of low 3
10
ordi
natio
n
n of activeof low concentration of charge carriers
2.510-3
Si -
Si C
oo
e Si atoms
2 10-4
10-2 10-1 100 101
S
Thi k (ML)Thickness (ML)
13SILS School on SR, Duino (Trieste)Italy, September 2009
Fe:GaNM. Rovezzi, F. D’Acapito, A. Navarro-Quezada, B. Faina, T. Li, A. Bonanni, F. Filippone, A. Amore Bonapasta, and T. Dietl, Phys. Rev. B 79, 195209 (2009)
A Bonanni A Navarro-Quezada Tian Li M Wegscheider Z Matěj V Holý A. Bonanni, A. Navarro Quezada, Tian Li, M. Wegscheider, Z. Matěj, V. Holý, R. T. Lechner, G. Bauer, M. Rovezzi, F. D’Acapito, M. Kiecana, M. Sawicki, and T. Dietl Phys. Rev. Lett. 101, 135502 (2008)
– Aims:– Define the site of Fe in GaN for – Define the site of Fe in GaN for
different metal concentrations and growth rategrowth rate
– Determine the effect of Si co-dopant
14SILS School on SR, Duino (Trieste)Italy, September 2009
Fe:GaN samplesFe:GaN samples
• Grown by MOVPE
SQUID
• Grown by MOVPE• Fe concentrations 4 × 1019 cm-3 - 3 × 1020 cm-3
SQUID
Ferromagnetic (FM) signal detected for all samples
15SILS School on SR, Duino (Trieste)Italy, September 2009
Fe:GaN dataFe:GaN dataLow Fe content:• only two Fe N (R ) • only two Fe-N (R1)
and Fe-Ga (R3) bonds. • Fe substitutional;
b d l h i e bond length in agreement with DFT for Fe3+ea
sing
Fe
High Fe content• Appearance of Fe-
Incr
e
FTs of the EXAFS spectra at
• Appearance of Fe-Fe (R2) coming from a precipitated Fe3N hFTs of the EXAFS spectra at
various Fe content phase16SILS School on SR, Duino (Trieste)
Italy, September 2009
Si Fe:GaN dataSi,Fe:GaN data
•For the same Fe ng F
e
content Si co-doping prevents the formation of Fe3Nnc
reas
i
formation of Fe3N•No evidence of the Fe-Fe bond at R2
InFTs of the EXAFS spectra at various Fe content and
codoped with Sicodoped with Si17SILS School on SR, Duino (Trieste)
Italy, September 2009
ff h h f FSi affects the charge state of Fe
P l d f F 3 F 2 ddPartial reduction of Fe3+ ions to Fe2+ upon Si addition
18SILS School on SR, Duino (Trieste)Italy, September 2009
Si F G N l iSi,Fe:GaN conclusions
• For high [Fe] FM due to Fe3N precipitates• For low [Fe] FM due to substitutional FeFor low [Fe] FM due to substitutional Fe• Si codoping prevents the formation of
Fe N aggregates and permits a higher Fe3N aggregates and permits a higher incorporation of Fe.Si l d t ti l d ti t F 2+• Si leads to a partial reduction to Fe2+
19SILS School on SR, Duino (Trieste)Italy, September 2009
Metal precipitates in Si solar cellsMetal precipitates in Si solar cellsBuonassisi et al, Nature Mat. 4, 676 (2005)
• Supply of high purity Si << demand• Use of lower purity materialUse of lower purity material• Problem: impurities decrease efficiency
µ XRF and µ XAFS to characterize • µ-XRF and µ-XAFS to characterize metal precipitates and suggest processing to improve efficiencyprocessing to improve efficiency
20SILS School on SR, Duino (Trieste)Italy, September 2009
µ-XRF & µ-XAFS: two defects~ 20 nm silicide
~ 10 µm oxideµm
2121SILS School on SR, Duino (Trieste)
Italy, September 2009
Processing & characterization
Best process: slow cool (5 °C s-1) from 1200 °Cµm sized, low density precipitatesgreater carrier diffusion lengths
22
greater carrier diffusion lengths22SILS School on SR, Duino (Trieste)
Italy, September 2009
Low Z dopants and XAS• C, N & O often used as dopants• Experimentally difficult: low fluorescence
i ld ft X UHVyield, soft X-rays, UHV100
10-2
10-1
nceY
ield
K edge yield
10-3
10
Fluo
resc
en
K d L d
LIII
edge yield
ALOISA beamline @ ELETTRA10-4
5 10 15 20 25 30Z
K edges LIII
edges
Z
23SILS School on SR, Duino (Trieste)Italy, September 2009
Dilute nitrides: GaAs1-yNy, InxGa1-xAs1-yNy
Anomalous non-linear opticaland electronic proprieties of
d
3.25 eV
Eg (eV) GaN
III-V nitrides• Red shift of the band gap by adding
few % of nitrogen (≈ 0 05-0 1 eV
GaAs
1.42 eV
few % of nitrogen (≈ 0.05 0.1 eVper N atomic percent in InGaAsN )
• Huge and composition dependentoptical bowing
0.0 0.5 1.0y
optical bowing
24SILS School on SR, Duino (Trieste)Italy, September 2009
Hydrogen – nitrogen mpl x s in dil t nit id scomplexes in dilute nitrides
• Hydrogenation leads to reversible changesy g g– compressive strain– opening of Eg
X-ray diffraction photoluminescence(004)opening of Eg (004)
GaAsN
EK~100 eV
GaAs
GaAsN
25SILS School on SR, Duino (Trieste)Italy, September 2009
Hydrogen – nitrogen complexes in dilute nitrides
• Which is the hydrogen –nitrogen y g gcomplex responsible for these changes?g
Some candidate Some candidate low energy structures
2626SILS School on SR, Duino (Trieste)
Italy, September 2009
Hydrogen – nitrogen complexes i dil t it idin dilute nitrides
ion
• DFT calculations to determine lowest energy geometries os
s-se
ct As grown
lowest energy geometries• Full multiple scattering XANES
simulationsA C lik l tio
n cr
o
N-HBC
N-H3
C• Answer: C2v – like complexes are mostly present
• 3-D sensitivity of XANES!! Abs
orp C
2vHydrogenated
y390 400 410 420 430
AEnergy (eV)
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Annealing studiesAnnealing studies
28SILS School on SR, Duino (Trieste)Italy, September 2009
XAFS and alloysy• High resolution in probing the local
di ti i fi t f di ti coordination in first few coordination shells
• Study, as a function of composition– Deviation of local structure from
average structure – Atomic ordering
29SILS School on SR, Duino (Trieste)Italy, September 2009
Bond lengths in InxGa1-xAs• Does the local structure follow
the average structure suggested by Vegard’s law?y g
• NO! Bond legths stay close to sum of covalent radii
• Mikkelsen Jr and Boyce • Mikkelsen Jr. and Boyce, Phys. Rev. Lett. 49, 1412 (1982)
30SILS School on SR, Duino (Trieste)Italy, September 2009
Disorder in Sr2FeMoO6 double perovskitesC. Meneghini, S. Ray, F. Liscio, F. Bardelli, S. Mobilio
λ
g , y, , ,& D.D. Sarma, Phys. Rev. Lett. 103, 046403 (2009)
SL
Fe
0.90.80.5
Mo
OSrS
0.40.2 A = Fe sublattice
B = Mo sublattice
long range order parameter Sdetermined from powder XRD
P(FeA) = N(FeA)/N(Fe) = xdetermined from powder XRD
S = 2x - 131SILS School on SR, Duino (Trieste)
Italy, September 2009
Double perovskites: disorder from a local perspective
R d A i i d fRandom Antisite defects Defect clusters
S = 0.51S 0.51
Antiphase regions
S takes into account only for the number of defects (FeB or MoA), not their arrangement
32
not their arrangement
SILS School on SR, Duino (Trieste)Italy, September 2009
Double perovskites: XAFS very similar spectra for all Sspectra for all S
ξ = P(Fe-Mo)
ξ EXP
0.90.80.50.40.2
0.90.80.50.40.2
AS
33SILS School on SR, Duino (Trieste)Italy, September 2009
ξ=0.63=S2 S=0.51 Double perovskites:a model for local disorder
Randomly distributed defect clusters
a model for local disorder
having average diameter DD is refined in order to obtain S and x close to their experimental values
ξ=0.85 S=0.51
1 5 nm close to the r exper mental values1.5 nm
ξ=0.99 S=0.51 Conclusion:ξ Conclusion:• Fe-Mo antisite defects appear as small (~1.5 nm)domains with antiphase Fe/Mo order.
34SILS School on SR, Duino (Trieste)Italy, September 2009
XAFS and phase transitionsXAFS and phase trans t ons
• Measure local structure through the gphase transition
• XAFS has highlighted the difference XAFS has highlighted the difference between the real local structure and the average structurethe average structure
35SILS School on SR, Duino (Trieste)Italy, September 2009
Ferroelectric Phase transitions in PbTiO3
low temp
Sicron, Ravel, Yacoby, Stern, Dogan and Stern, Phys. Rev. B 50, 13168 (1994)
low temp
• At Tc = 763 K PbTiO3 undergoes tetragonal to cubic At c 763 K Pb O3 undergoes tetragonal to cub c phase transition
• T < Tc it is ferroelectric (permanent dipole moment)• Phase transition believed to be purely displacive (no
local distortion for T > Tc)
36SILS School on SR, Duino (Trieste)Italy, September 2009
Ferroelectric Phase transitions in PbTiO3
• Ti and Pb XAFS data • Local lattice parameters cp
and local distortionsdo not change at TcThe ferroelectric
a• The ferroelectric
transition has a large order – disorder character: the orientation of local distortions changes at Tchanges at Tc
37SILS School on SR, Duino (Trieste)Italy, September 2009
La1-xCaxMnO3 Manganites
• For 0.2 < x < 0.5For 0.2 x 0.5– Metallic ferromagnet T < Tc
– Insulating paramagnet T > Tcg p g c
• Colossal Magneto Resistance• Double exchange model: g
hopping conductivity between adjacent Mn ions is enhanced f l dif Mn ions are spin aligned Mn3+
Mn4+
38SILS School on SR, Duino (Trieste)Italy, September 2009
La1-xCaxMnO3 Manganites• Mn(3+)O6 octahedron in
LaMnO3 is Jahn-Teller distorteddistorted– Mn4+ is not
• Conductivity in the ypresence of polarons: an electron + a lattice distortiondistortion
• Range of polaronic distortion?
39SILS School on SR, Duino (Trieste)Italy, September 2009
La1-xCaxMnO3 Manganites• Presence of
localized localized structural distortions at distortions at MI transition
Booth, Bridges, Kwei, Lawrence, Cornelius and NeumeierPhys. Rev. Lett. 80, 853 (1998)
40SILS School on SR, Duino (Trieste)Italy, September 2009
XAFS and thin films / interfacesXAFS and thin films / interfaces• With specific detection schemes p
sensitivity to very thin films achievable– Grazing incidence– Electron / fluo detection
• Exploit linear polarization of SR to obtain directional informationobtain directional information
41SILS School on SR, Duino (Trieste)Italy, September 2009
Using SR linear polarization to study thin films
Electron orbit
ε̂
θθσ 2)( Cos∝θ
ε
42SILS School on SR, Duino (Trieste)Italy, September 2009
Cubic and hexagonal GaN• N K-edge XAS to study N K edge XAS to study
relative amounts of cubic and hexagonal GaN
• Exploit– linear polarization of SR
polarization dependence of – polarization dependence of cross-section
• XAS signal must exhibit (at g (least) point group symmetry of the crystal
T : isotropic si nal– Td: isotropic signal– C6V: σ tot(E,θ)=σ iso(E)+(3Cos2θ −1)σ1(E)
43SILS School on SR, Duino (Trieste)Italy, September 2009
GaNCUBIC HEXAGONAL
• Katsikini et. al., APL 69, 4206 (1996); JAP 83, 1440 (1998)
44SILS School on SR, Duino (Trieste)Italy, September 2009
The Fe/NiO(001) interfaceThe Fe/NiO(001) interfaceP. Luches, V. Bellini, S. Colonna, L. Di Giustino, F. Manghi, S. Valeri, and F. Boscherini, Phys. Rev. Lett. 96, 106106 (2006).
heating up to TN
FM +cooling in H
AFM
h biexchange bias
45SILS School on SR, Duino (Trieste)Italy, September 2009
The Fe/NiO(100) interface: structurebcc
FeFM TC=1040 K
rock-salt
aFe= 2.866 ÅdFe = 4.053 Å
m=-2.8%
NiOAF TN=520 K
aNiO= 4.176 Å46SILS School on SR, Duino (Trieste)
Italy, September 2009
Fe/NiO XANES
• deviation from deviation from metallic character
• shift of the hit lin white line
towards FeO
47SILS School on SR, Duino (Trieste)Italy, September 2009
Fe/NiO XANES: polarization dependence
kE
θ
• planar FeO phase
48SILS School on SR, Duino (Trieste)Italy, September 2009
Fe/NiO EXAFS-quantitative analysis
N BCC Fe, R (Å)
BCT Fe, R (Å)
fit results, R (Å)
1st shell 8 2.48 2.50 2.50
2nd shell 2 2.77 2.81the epitaxial strain is partially 2.87
3rd shell 4 2.95 2.87
4th shell 84.05
4.04 3.92
5th shell 4 4.17 4.07
6th h ll 8 4 64 4 74
released at 10 ML
SILS School on SR, Duino (Trieste)Italy, September 2009
49
6th shell 84.75
4.64 4.74
7th shell 16 4.87 4.84
8th shell 8 4.96 5.01 5.09
Fe/NiO 2 ML thickness
50SILS School on SR, Duino (Trieste)Italy, September 2009
Fe/NiO: atomic structure of interface
BCT Fe
buckled pseudomorphicp pFeO layer
• expanded FeO-NiO and FeO-Fe distance
51SILS School on SR, Duino (Trieste)Italy, September 2009
XAFS and nanostructuresF l l h ff• XAFS is a local, short range, effect
– Origin: core hole lifetime (τhole = 10-16 – 10-15 s) and electron mean free path (5 – 10 Å)electron mean free path (5 10 Å).
• Same formalism applies to molecule, cluster or crystalline solid– insenstive to variations of morphology– sensitive to low thicknesses, high dilutions
Ex ll nt p b f i ti ns in l l n i nm nt • Excellent probe of variations in local environment due to– Size effectsSize effects– Change 3D / 2D / 1D
52SILS School on SR, Duino (Trieste)Italy, September 2009
S i d dSemiconductor quantum dots
• Self organized hetero-epitaxial growth can lead to quantum dots with g qnarrow size distribution
• Due to small size the quantum dots Due to small size the quantum dots exhibit new physical properties
53SILS School on SR, Duino (Trieste)Italy, September 2009
Ge Quantum DotsBoscherini, Capellini, DiGaspare, Rosei, Motta, and Mobilio, Appl. Phys. Lett. 76, 682 (2000)
• Need for understanding of
p p pp y
Need for understanding of local bonding
• Preparation:p– Ge/Si(001) by CVD @ 600 °C,
Univ. Roma TreE it AFM d t h t i • Ex-situ AFM used to characterize degree of relaxation
– Ge/Si(111) by MBE @450 - 550 y°C, Univ. Roma II
• In-situ STM/AFM
54SILS School on SR, Duino (Trieste)Italy, September 2009
Energetics of island formationEnergetics of island formation• Competing energies:
– strain• Contributions from:
- wetting layer– strain– surface– dislocations
wetting layer- islands
d s ocat ons
Wetting layer WL+Strained island WL+Relaxed islandWL+2D plateletWetting layer WL+Strained island WL+Relaxed island
"Coverage"
WL+2D platelet
55SILS School on SR, Duino (Trieste)Italy, September 2009
Q. Dots: AFMQ• Analysis of aspect ratio
provides measurement 80
100
%)
Ge/Si(001)provides measurement of relative amount of relaxed islands
60
80
volu
me
( Ge/Si(001)
• Ge/Si(001): Full range of relaxation examined 20
40
Rel
axed
v
00 5 10 15 20 25 30 35 40
R
equivalent thickness (nm) (1 ML = 0 135 nm WL = 3 ML)(1 ML 0.135 nm, WL 3 ML)
56SILS School on SR, Duino (Trieste)Italy, September 2009
Q. Dots: Ge edge dataGe in Si
ansf
4
Ge:Si
dots
Fou
Tra
2
3
(Å-1
) (111)1 nm
(001)Ge bulk
Ge
F
1
2
k χ(
k) ( (001)
7.8 nm
(001)38 nm 0 2 4 6
interatomic distance (Å)0
38 nm
Ge
2 4 6 8 10 12 14 16k (Å-1)
•Assuming random alloy average composition is Ge0 70Si0 30is Ge0.70Si0.30
57SILS School on SR, Duino (Trieste)Italy, September 2009
Conventional SK growthConventional SK growth
58SILS School on SR, Duino (Trieste)Italy, September 2009
SK growth with interdiffusion SK growth with interdiffusion
59SILS School on SR, Duino (Trieste)Italy, September 2009
Metallic nanostructuresMetallic nanostructures
• One of the first applications of XAFS• Exploits high resolution in first Exploits high resolution in first
coordination shells
60SILS School on SR, Duino (Trieste)Italy, September 2009
Clusters: bond length contraction
Apai et. al.Phys. Rev. Lett. 43, 165 (1979) Montano et. al.
Phys. Rev. Lett. 56, 2076 (1986)• A bond length contraction has been
found for weakly supported metallic l st s (Ni C A ) f d 100 Å
y , ( )
clusters (Ni, Cu, Au……) for d < 100 Å61SILS School on SR, Duino (Trieste)
Italy, September 2009
Bond length contractiong• A macroscopic surface tension interpretation
(“liquid drop”) can explain the bond length ( liquid drop ) can explain the bond length contraction– f surface tension fR κ2∆f surface tension, κ bulk modulusr radius of spherical particleM l
rf
RR κ
32
−=∆
– Montano et. al.Phys. Rev. B 30, 672 (1984)Balerna et. al.,h ( )Phys. Rev. B 31 5058 (1985)
62SILS School on SR, Duino (Trieste)Italy, September 2009
Dynamic properties of Au clustersy p p• In the harmonic approximation,
the Mean-Square-Relative-qDisplacement, σ2, damps the EXAFS signal with a term
222 σke−
2( ) 2
0020
ˆjjj Ruu •−=
rrσ
Bulk Au63SILS School on SR, Duino (Trieste)
Italy, September 2009
Dynamic properties of Au clusters h l d • As the cluster dimensions
decrease an enhacement of σ2 is evidentσ is evident
• Surface atoms have less motion constraints
• High surface-to-volume ratio for nanoclusters
• Values reproduced by numerical model for free sphere phonon DOS which sphere phonon DOS which includes surface modes
• Balerna and Mobilio, Phys Rev B 34 2293 (1986)Phys. Rev. B 34, 2293 (1986)
64SILS School on SR, Duino (Trieste)Italy, September 2009
Cross-over from thermal expansion to contraction
Comaschi Balerna and Mobilio Phys Rev B 77 075432 (2008)
Extremely good
Comaschi, Balerna and Mobilio, Phys. Rev. B 77, 075432 (2008)
spectra of Au nanoparticles (error bar on bond length = 0.2 pm)
6518 Å-1
Cross-over in expasion / pcontraction behaviour
R d d di i • Reduced dimensions increase splitting of l t i t telectronic states
• Change of total energy dependence on bond length (R)
40 Å diameter NP • At high T states with low R are accessed
66SILS School on SR, Duino (Trieste)Italy, September 2009
ConclusionsXAFS h b d dd i• XAFS has been used to address importantstructural issues in materials/nano science
• It has specific advantages especially• It has specific advantages, especially
• Atomic selectivity• High resolution for first few coordination shells• Sensitivity to high dilutions & surfaces/interfaces• Equally applicable to ordered or disordered matterEqually applicable to ordered or disordered matter• XANES:
– valence/oxidation state3D structural sensitivity– 3D structural sensitivity
• µm spot size now available
67/67SILS School on SR, Duino (Trieste)Italy, September 2009