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Maria KatsikiniLecturerDepartment of Solid State Physics
Synchrotron Radiation: a novel research tool in materials science
xafslab.physics.auth.gr
Outline
About Synchrotron Radiation
Principles of absorption & fluorescence spectroscopies
Applications • III-V nitrides • Solidified wastes• Solid biological samples
Summary
Synchrotron radiation
1947: Experimental verification 1994: 3rd generation sources
General Electric
Research Laboratory
New York
Electromagnetic radiation produced by accelerated particles circulating in close orbits with very high speed
SR sources
100 101 102 103 104 10510-2101104107
101010131016
BESSY-II: E=1GeV, I=400mA, B=1.3T
Bri
gh
tne
ss(p
ho
ton
s /
se
c
. m
rad
2
. 0
.1%
BW
)
E(eV)
εC
Electron orbit
br
il
li
an
ce
Bending magnet Undulator
Properties of SR
Very high brightnessSmall beam size / low divergence study of small samples & spatial resolved studies
Linearly / circularly polarized study of magnetic samples,
surfaces, catalysis
Pulsed character in-situreal-time chemical reactions
Continuous spectrum IR hard X-raysEnergy tunability spectroscopies
Techniques & beamlines
X-ray diffraction (XRD) & scattering (SAXS, WAXS)
X-ray absorption spectroscopy (XAFS)
X-ray photoelectronspectroscopy (XPS)
X-ray fluorescencespectroscopy (XRF)
Imaging techniques (microscopy, topography, tomography, XRF mapping)
Time resolved studies
Physics Materials Science Medicine & biology Chemistry Geology Environmental sciences
X-ray absorption & fluorescence
core shells
valence shell
KL
MN
impinging photon with E>Eb
emitted photoelectron
emitted fluorescence photonwith characteristic energy (EK-EL)
Absorption & fluorescence spectroscopies
energy dependence of the X-ray absorption coefficient above the absorption edge
XAFS X-Ray Absorption Fine Structure
7800 8000 8200 8400 86000.0
0.5
1.0
X-ra
y ab
sorp
tion
coef
ficie
ntEnergy (eV)
Co - K - edge
2 4 6 8 100.0
0.5
1.0
CuFe
Inte
nsity
(arb
. uni
ts)
Energy (keV)
SArCa
Zn
energy distribution of the emitted fluorescence photons
after excitation with X-rays
XRF X-Ray Fluorescence
of emitted fluorescence photons of absorbed photons
Number of absorbed photons or emitted fluorescence photons or emitted electrons (photoelectrons & secondary electrons)
X-r
ay
ab
sorp
tio
nc
oe
ffic
ien
t
Energy (eV)
X-ray absorption fine structure (XAFS)
hν
e
NEXAFS/XANES
NEXAFS The outgoing photoelectron interacts with the molecular orbitals
EXAFS
EXAFS
The outgoing photoelectron is scattered from the neighboring atoms
Modulation of X-ray
absorption coefficient
hv
Backscattered wave
Outgoing photoelectron
waveInterference
Ek=hν-Eb
Extended X-ray absorption fine structure (EXAFS)
Mean free pathof the φe-
Backscattering amplitude
Coordination number Debye-Waller
factor
distance
Phaseshift
ANALYSIS OF THE EXAFS SPECTRA
* Nearest neighbor distances* Coordination numbers* Thermal / static disorder
Least square fitting using various single or multiple scattering paths.Some structural parameters e.g. R, N are iterated.
Simulation using a proper structural model
4 6 8 10 12 14 16 18
-0.1
0.0
0.1
χ(k)
k(A-1)
0 1 2 3 4 5 6
Ga
N
R(Å)
|FT
|
Fourier transform
(radial distribution function)
Near edge X-ray absorption fine structure (NEXAFS)
NEXAFS -XANES• Density of empty states• Symmetry • Defect states
A d v a n t a g e s• non-destructive
• Applicability on solids (amorphous & crystalline) • Atom selective
EF
core states
OccupiedDOS
UnoccupiedDOS
0valence band
conduction band Dipole approximation
polarization unit vector density of
states
hv=Ef-Ei
XAFS characterization of III-V nitrides
Effect of symmetry & composition
Implantation & defect formation
Bonding configuration of impurities
Effect of alloying on the microstructure
Applications of III-V nitrides
Results
1
2
3
4
Samples:T. D. Moustakas (Boston Univ.)A. Georgakilas (Univ. of Crete)C.T. Foxon (Univ. of Nottingham)I. Akasaki (Nagoya Univ.)
• Optoelectronics: blue LED/laser• Micro-electronics: high power devices• Alloying (AlN, GaN, InN) band gap engineering
XAFS characterization of III-V nitrides Effect of symmetry
1
Quantitative determination of cubic and hexagonal fractions in mixed-phase samples .
M. Katsikini et al, APL 69, 4206 (1996) & JAP 83, 1437 (1998)
NEXAFS is strongly affected by the symmetry of the polytype
mixed phase
25% cub+75%hex
400 410 420 4300
1
2
3
Inte
nsi
ty (
arb
.un
its)
Energy (eV)
hexagonal
cubic
GaN
N-K edge
Linear Combination of Spectra
XAFS characterization of III-V nitrides Effect of composition
1
400 410 420 430 4400
1
2
3
4
Inte
nsi
ty (
arb
. un
its)
energy (eV)
AlN
InN
GaN
Absorption edge width : Wabs= 0.8-2me*
M. Katsikini et al, J. Synchr. Rad. 6, 558 (1999)
Absorption edge position : red shifted with ΖC
me*= 0.3m0(AlN) me*= 0.2m0(GaN) me*= 0.11m0(InN)
408 409 410 411 4120
1
2
Inte
nsi
ty (
arb
. un
its)
energy (eV)
AlN
InN
GaN
XAFS characterization of III-V nitrides Implantation & defect formation
2
Ion implantation• precise doping profile• formation of new phases, e.g. InGaN/GaN
disadvantage• lattice damage • annealing dopant activation & lattice recovery
400 410 420 430
0
2
4
6
8
10
1000
500
200
100
70
50
30
20
15
10
5
Inte
nsity
(arb
. uni
ts)
Energy (eV)
as grown
fluence (cm-2)
x1013
RL1
RL2
Effect of fluence • damping of the NEXAFS peaks • emergence of RL1 and RL2
XAFS characterization of III-V nitrides Implantation & defect formation
2
400 410 420 430
0
2
4
6
8
10
1000
500
200
100
70
50
30
20
15
10
5
Inte
nsity
(arb
. uni
ts)
Energy (eV)
as grown
fluence (cm-2)
x1013
RL1
RL2
400 401 402
Energy (eV)
RL2
236meV
vibronicstates of N2
RL1: ~ 1.7eV bellow the absorption edge
The N=N bonds give πpantibonding mid-gap states [J. Neugebauer et al , PRB 50, 8067 (1994)]
N-split interstitial
M. Katsikini et al, J. Phys.: Conf. Series, 190, 12065 (2009)
0 1 2 3 4 5 6 7 80
1
2
3
4
5
6
7 Ga
as grown
ann. (900oC)
ann. (800oC)
|FT
{k2χ(
k)}|
R(Å)
as implanted
N
Sample RGa-N(Å) ± 0.01
RGa-Ga (Å) ± 0.005
NGa-N NGa-Ga
as grown 1.94 3.178 4 12
as implanted
1.95 3.187 3.5±0.4 4.8±0.6
annealed (800oC)
1.94 3.192 3.8±0.6 11.4±2.1
annealed (900oC)
1.94 3.193 3.1±0.3 10.6±0.8
Fluence: 5x1015cm-2
• Implantation : reduction Ni
XAFS characterization of III-V nitrides Implantation & bonding of Ga
2
Effect of annealingGa - K - edge
M. Katsikini et al, Mat. Sci. Eng. B, 152, 132 (2008)
• Loss of nitrogen at 900oC
• Annealing at 800oC increase of the Ga-Ga distance and partial recovery of Ni
ϕ=55ολf~60nm
XAFS characterization of III-V nitrides Bonding configuration of impurities
3
Implantation of GaN with 70keV O ions
Why O?• Diluted: n-type dopant• at high concentrations formation of GaOxNy
400 410 420 430 4400
1
2
3
4
fluence (cm-2)
as grown
1x1016
1x1015
1x1017
Energy (eV)
Inte
nsi
ty (
arb
. un
its)
• implantation affects strongly the spectra variation of the O bonding
N K edge O K edge
M. Katsikini et al, APL 82, 1556 (2003) & NIMB (in press)
Self Consistent Fieldfor the electron density &
scattering potentials
Full Multiple Scattering
RFMS
RSCF
RSCF=5.6 - 6ÅRFMS=8Å (195-230 atoms)
N-K edge (hex GaN)
M. Katsikini et al, JAP 101, 83510 (2007)
XAFS characterization of III-V nitrides Bonding configuration of impurities
3
FEFF8: ab-initio MS calculations • scattering of the photoelectron wave from the muffin-tin potentials of the neighboring atoms
XAFS characterization of III-V nitrides Bonding configuration of impurities 3
M. Katsikini et al, Nucl. Instrum. Meth. B (in press)
O-Ga=1.84Å
fluence=1×1015cm-2
530 540 550 560
0
2
4
6
simul.
no
rma
lize
d in
ten
sity
Energy (eV)
exp.
channel interstitial (O in the Ga plane)
column interstitial
50% + 50%
O is interstitial
O-Ga=1.84Å
fluence=1×1016cm-2
O substitutes for N
530 540 550 5600
2
4
6
simul.
exp.
no
rma
lize
d in
ten
sity
Energy (eV)
0 2 4 6 8 10 120
10
20
30
40
disordered
nu
mb
er
of b
on
ds
radial distance (A)
crystalline
530 540 550 5600
2
4
6
8
10
12
Inte
nsi
ty (
arb
. uni
ts)
Energy (eV)
simul.
exp.
α-Ga2O3
50% O + 50% N
XAFS characterization of III-V nitrides Bonding configuration of impurities
3
formation of oxynitrides
fluence=1×1017cm-2
CVD poly-GaN O contamination varies with deposition temperature
N. H. Tran et al, J. Phys. Chem. B, 109, 18348 (2005).
XAFS characterization of III-V nitrides Effect of alloying on the microstructure
4
Alloying of InN and GaN band gap engineeringProblem: ~10% mismatch
0 1 2 3 4 5 6 7 80
2
4
x=1
0.9
0.8
0.7
|FT
{k2χ(
k)}|
R (A)
0.07
4 5 6 7 8 9 10 11 12
0.0
0.2
0.4
x=1
0.9
0.8
0.7χ(k
)
k (A-1)
0.07
In – K – EXAFS
InxGa1-xN
M. Katsikini et al, pss (a) 205, 2593 (2008).
Ferhat & Bechsted, PRB65, 075213 (THEORY)
Cation – cation distances
RGa-Ga
X-ray fluorescence spectroscopy
2 4 6 8 100.0
0.5
1.0
CuFe
Inte
nsity
(arb
. uni
ts)
Energy (keV)
SArCa
Zn
Quantitative analysis• using standards• using the fundamental parameter method
Uses:
CA: element concentration (mA/msample)ωA: fluorescence yieldgl: transition probability(rA-1)/rA: jump ratioμ/ρ : mass absorption coefficientEl: fluorescence energyA: analyte, M: matrixϕ,ψ: incidence, detection angles
ϕ
ψ
X-ray fluorescence spectroscopy
substance identification
Uses:
Chinese porcelains imported in Europe were re-decorated.Identification of metals (Fe, Cu, Mn) used to color the enamelLarge objects difficult to be studied by SEM
K. Janssens et al, Spectrochim. Acta, B51, 1161 (1996).
X-ray fluorescence spectroscopy
2 4 6 8 100.0
0.5
1.0
CuFe
Inte
nsity
(arb
. uni
ts)
Energy (keV)
SArCa
Zn
XRF mapping (spatial distribution of elements)
Uses:
• proper energy windows• micrometric sample movement• reduction of the beam size
Original ink, rich in Zn(Bøtinge i Asbo)
• Non- destructive
• Detection of high - Z elements (Fe, Cu, Sn, Sb, Ag) and mapping
• Different composition of inks although their visual appearance is the same
Sweden, 1499
Falsifying ink, rich in Ca(Gäsmestad i Böre)
X-ray fluorescence spectroscopy
XRF mapping (spatial distribution of elements)
Authenticity of valuable documents.
K. Janssens et al, X-ray spectrometry, 29, 73 (2000).
XAFS characterization of solidified wastes
Structural role of Fe in glasses
Annealing induced devitrification
Determination of crystallization ratio
Motivation• Problem: Management of Pb and Fe-containing toxic wastes from storage tanks from oil industry
• Stabilization & immobilization via vitrification (co-melting with SiO2 + Na2O)
Results
Glass or vitroceramic
Samples: Prof. T. Karakostas
Fani Pinakidou (PhD)
1
2
3
XAFS characterization of solidified wastes
7100 7120 7140 7160 7180 72000
2
FeS
In
ten
sity
(a
rb.u
nit
s)
Energy (eV)
Fe3O
4
Pre edge peak
Fe K edge
K
L
M
1s2
2s2, 2p6
N
3s2, 3p6, 3d6
4s2
The field of the O ligandsmodifies the Fe 3d energy levels
empty d-states
7110 7115 7120energy (eV)
non centrosymmetricDipole allowed -mixing of 3d Fe + 2p O
High intensity Small width
1 function to simulate the pre-peak
7110 7115 7120energy (eV)
centrosymmetric& distortionQuadruple allowed
Low intensity Large width
2 functions to simulate the pre-peak
XAFS characterization of solidified wastes1
10 20 30 40 50 60
3.5
4.0
4.5
5.0
5.5
Co
ord
ina
tio
n N
um
be
r
Fly ash (wt%)
Glasses with 10% < ash
XAFS characterization of solidified wastes2
F. Pinakidou et al, JAP 102, 113512 (2007)
Annealing induced devitrification
-150 -100 -50 0 50 100 150-150
-100
-50
0
50
100
150
μmμm
0.2
0.4
0.5
0.7
0.8
1.0
-400 -200 0 200 400
-400
-200
0
200
400
μm
μm
0.2
0.4
0.5
0.7
0.8
1.0
60% ash – 25% SiO2 – 15% Na2O
annealing (440oC) annealing (600oC)
Fe Fe
Annealing promotes the inhomogeneity
0 1 2 3 4 5 60
4
8
12
16
20
FT
(a
rb. u
nit
s)
R (Å)
Fe2O
3
XAFS characterization of solidified wastes2
F. Pinakidou et al, JAP 102, 113512 (2007) & J. non Cryst. Solids 351, 2474 (2005)
Annealing induced devitrification
-150 -100 -50 0 50 100 150-150
-100
-50
0
50
100
150
μm
μm
mid – range order(re-crystallization / devitrification )
7100 7120 7140 71600
1
2
Fe2O
3
Inte
nsi
ty (
arb
. uni
ts)
Energy (eV)
NEXAFSFT of EXAFS
octahedral / tetrahedral coordination
0 1 2 3 4 5 60
4
8
12
16
20
24
FT
(arb
. un
its)
R (Å)
XAFS characterization of solidified wastes3
F. Pinakidou et al, JAP 102, 113512 (2007)
Crystallization ratio
~80% glassy + 20% PbO ∙ 6 (Fe2O3)
μ-EXAFSFe –rich region -150 -100 -50 0 50 100 150-150
-100
-50
0
50
100
150
μm
μm
PbO ∙ 6 (Fe2O3)
μ-EXAFSLow-Fe region
conventional EXAFS (fitting using a mixed model)
XAFS & XRF characterization of human nails
Motivation Use of the human nails as bioindicators
Results
Structural role of metals in keratins which are also found in tumors
Spatial distribution of Fe
Effective atomic number
Bonding environment of Zn samples
Hospital of Dermatological & Venereal Diseases
Pulmonary Clinic of AUTH
1
2
3
Nail :modified type of epidermis keratinized matrix + heavier inorganic elements
major elements (e.g. Ca, Mg, K, Na)
trace elements (e.g. Fe, Cu)
Lack or excess is related to: • disorders or diseases • environmental factors• nutritional factors
hard insoluble protein, rich in cysteine
keratin
healthy
lung cancer
Distribution of Fe in nails1
0
20
40
60
80
100
density of clusters=1656 mm-2
nu
mb
er
of
clu
ste
rs
0 20 40 60 80 280 3000
20
40
60
80
100
H
size (μm)
C1
density of clusters=2594 mm-2
Spatial density of Fe-rich regions ~ x 1.5 higher in the sample from the cancer patient
M. Katsikini et al, Nucl. Instrum. Meth. B (in press)
Zn bonding in nails2
M. Katsikini et al, J. Phys.: Conf. Series, 190, 12204 (2009)
0 1 2 3 4 5 6 7 80
1
2
3
4
S/N = 0.43
S/N = 0.42
S/N = 0.38
S/N = 0.48
S/N = 0.53
clubbed nail
clubbed nail
FT
Inte
nsi
ty (
arb
. un
its)
R (Å)
healthy
healthy
obstructing lung disease
tuberculosis
fibrosis
N S
S/N = 0.81
Zn – N distance = 2.00ÅZn – S distance = 2.28Åcoordination number ~4
More S atoms are bonded with Zn
the lung tissue thickens, becomes stiff and inhibits O2 from entering the blood stream.
Predominant bonding of ZnZn (his) (cys)3
Healthy
Fibrosis
Zn S O N C
2 4 6 8 10-0.2
0.0
0.2
0.0
0.5
1.0
1.5
0 1 2 3 4 5 6 7 80.0
0.5
1.0
1.5
2 4 6 8 10-0.2
0.0
0.2
χ(k)
k (Å-1)
|FT
{k2χ(
k)}|
R(Å)
χ(k)
k (Å-1)
Zn bonding in nails2
Predominant bonding of ZnZn (his)2 (cys)2
M. Katsikini et al, J. Phys.: Conf. Series, 190, 12204 (2009)
9.0 9.5 10.0 10.5 11.0
scattered
RayleighCompton
Energy (keV)
Zn Kβ
X-ray scattering from human nailsIntense• in materials of small Zeff• at high energies
SplittedElastic (Rayleigh) at ER=E0Inelastic (Compton) at Ec
Effective atomic number of human nails
Rayleigh to Compton scattering ratio as a function of the fat content of liver.
3
Effective atomic number of human nails
Incoherent scattering factor
atomic form factor
x=sinϑ/λ
3
M. Katsikini et al, J. Nanoscience & Nanotechnology (in press)
polarization factor (=0.84)
Effective atomic number of human nails
x=sinϑ/λ
3
Phe 5.76Leu 5.92Ile 5.96Lys 6.00Val 6.01Trp 6.05Pro 6.11Gln 6.18Arg 6.18Tyr 6.20His 6.28Ala 6.29Ser 6.29Thr 6.42Asn 6.54Gly 6.54Glu 6.58Asp 6.74Met 8.45Cys 9.31
affect stronglythe Zeff
M. Katsikini et al, J. Nanoscience & Nanotechnology (in press)
Mean Zeff : 7.5±0.416 samples
Summary
Synchrotron Radiation is a valuable tool for the study of materials.
The NEXAFS spectrum is fingerprint of the symmetry and the composition. It is strongly affected by lattice damage & presence of defects.It is sensitive to the bonding geometry of impurities .
EXAFS provides information on the bonding environment of the absorbing atom. It provides information on local distortions. It can be applied in crystalline and amorphous materials.
The use of “focused” beams (1-5μm) permits the detection of inhomogeneities (XRF mapping) and the determination of the spatially resolved bonding configurations of the elements.
XAFS & XRF are useful and non destructive techniques for the atom specific structural and chemical analysis of materials.
xafslab.physics.auth.gr
• Prof. Eleni C. Paloura • Maria Katsikini, Lecturer• Dr. Fani Pinakidou, Pdoc• Katerina Mavromati, PhD candidate• 3 MSc students
Measurements
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