Synchrotron Radiation: a novel research tool in materials ...users.auth.gr/katsiki/crete.pdfPhase shift ANALYSIS OF THE EXAFS SPECTRA * Nearest neighbor distances * Coordination numbers

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)


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


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


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).


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)


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


F. Pinakidou et al, JAP 102, 113512 (2007)


-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


F. Pinakidou et al, JAP 102, 113512 (2007) & J. non Cryst. Solids 351, 2474 (2005)


-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)


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


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


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


Incoherent scattering factor

atomic form factor

x=sinϑ/λ

3

M. Katsikini et al, J. Nanoscience & Nanotechnology (in press)

polarization factor (=0.84)


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

Documents

Synchrotron Radiation: a novel research tool in materials ...users.auth.gr/katsiki/crete.pdfPhase shift ANALYSIS OF THE EXAFS SPECTRA * Nearest neighbor distances * Coordination numbers