1

Part II

• XAFS: Principles

• XANES/NEXAFS

• Applications

2

X-ray Absorption Spectroscopy (XAS)

X-ray Absorption spectroscopy is often referred to as

- NEXAFS for low Z elements (C, N, O, F, etc. K-edge, Si, P, S, L-edges) or

- XAFS (XANES and EXAFS)

for intermediate Z and high Z elements.

3

NEXAFS, XANES and EXAFS

• NEXAFS (Near Edge X-ray Absorption Fine Structures) describes the absorption features in the vicinity of an absorption edge up to ~ 50 eV above the edge (for low Z elements for historical reasons).

• It is exactly the same as XANES ( X-ray Absorption Near Edge Structures), which is often used together with EXAFS (Extended X-ray Absorption Fine Structures) to describe the modulation of the absorption coefficient of an element in a chemical environment from below the edge to ~ 50 eV above (XANES), then to as much as 1000 eV above the threshold (EXAFS)

• NEXAFS and XANES are often used interchangeably• XAFS and XAS are also often used interchangeably

4

XAFS of free atom

• In rare gases, the pre-edge region exhibits a series of sharp peaks arising from bound to bound transitions (dipole: 1sto npetc.) called Rydberg transitions

5

XAFS of small molecules

• Small molecules exhibit transitions to LUMO, LUMO + and virtual orbital, MO in the continuum trapped by a potential barrier (centrifugal potential barrier set up by high angular momentum states and the presence of neighboring atoms), or sometimes known as multiple scattering states

6

XAFS: the physical process

Dipole transition between quantum states• core to bound states (Rydberg, MO below vacuum level,-ve energy, the excited electron remains in the vicinity of the atom)- long life time-sharp peaks• core to quasi-bound state (+ve energy, virtual MO, multiple scattering states, shape resonance, etc.); these are the states trapped in a potential barrier, and the electron willeventually tunnel out of the barrier into the continuum-short lifetime, broad peaks• core to continuum (electron with sufficient kinetic energy to escape into the continuum) - photoelectric effect.XPS &EXAFS

7

XAFS: the physical process cont’

Scattering of photoelectron by the molecular potential– how the electron is scattered depends on its kinetic energy

• Low kinetic energy electrons - Multiple scattering (typically up to ~ 50 eV above the threshold, the region where bound to quasi-bound transitions take place); e is scattered primarily by valence and shallow inner shell electrons of the neighboring atoms -XANES region•High kinetic energy electrons (50 -1000 eV) are scattered primarily by the core electrons of the neighboring atoms, single scattering pathway dominates - EXAFS region

8

Electron scattering

• Free electron (plane wave) scattered by an atom (spherical potential) travels away as a spherical wave

• Electrons with kinetic energy > 0 in a molecular environment is scattered be the surrounding atoms

• Low KE e- is scattered by valence electrons, undergoes multiple scattering in a molecular environment

• High KE e- is scattered by core electrons, favors single scattering

Multiple Scattering

Single Back-scattering

9

10

11

Asymptotic wing of the coulomb potential supports the Rydberg states

States trapped in the potential barrier are virtual MO’s or multiple scattering states (quasi bound states)

bound states

free e with KE >0

Free atom

diatomic with unsaturated bondingN2, NO, CO etc.

NEXAFS of free atom & unsaturated diatomic molecules

π*

12

NEXAFS of Free Atom: Ar L3,2-edge

244 245 246 247 248 249 250 251

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

244.00 244.25 244.50 244.75

Rel

ativ

e In

tens

ity

Photon Energy (eV)

123meV

2p3/2

- 4s

2p1/2

- 4s

3d

4d

5d

6d

3d

5d6d

4d

Photon Energy

Ar (gas)[Ne]3s23p6

2p6 →2p54s1

2p1/2

2p3/2(lower binding)

j = l ±s

2p14s1Core hole

Hund’s rule

hv

13

Rydberg transitions in HBr remain strong and a broad transition to molecular orbital emerges

HBrKr

NEXAFS of atom and small molecules

M5,4 (3d5/2,3/2)M5,4 (3d5/2,3/2)

MO

14

Inte

nsity

(ar

b. u

nits

)

402.0401.5401.0400.5400.0Photon Energy (eV)

Vibronic structures in the 1 s to π* transition

N2

1s -π*

IP

π*

σ*

NEXAFS of Nitrogen

15

C 1s C 1s

π*

π*σ*

Molecular orbital illustrationC2H2CON2 MO axis

16

17

∆E = (Eσ*-Eo) ∝ 1/r

Why would this correlation exist?i) Multiple scattering theoryii) Simple picture- particle in a box-

the shorter the bond the farther the energy separation

iii) Scattering amplitude ∝ Z

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NEXAFS of molecules oriented on a surface(angular dependence of resonance intensity)

When molecules adsorb on a surface, their molecular axis is defined by the axis of the substrate. Therefore, angle dependent experiments can be made by rotating the substrate with respect to the polarization of the photon beam. Using selection rules, the orientation of the molecule on a surface can be determined.

H HC=C

H H

C-C

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Carbon Allotropes

Diamond

Graphite

Graphene

Nanotube

Hexagonal Diamond(Lonsdaleite)

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core

LUMO

LUMO + 1

XANES of Benzene

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ππππ*1 C-H* ππππ*2

XANES systematic

Tom Regier unpublished

Incr

easi

ng n

o. o

f ben

zene

rin

gs

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Chemical systematic

CNT

C60

24

25

d - electron in transition metals

The dipole selection rule allows for the probing of the unoccupieddensities of states (DOS) of dcharacter from the 2p and 3p levels –M3,2 and L3,2-edge XANES analysis(relevance: catalysis)

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Transition metal systematic

3d 4d 5dFilling of the d band across the period

Dec

rea

sin

g w

hite

line

inte

nsi

ty a

cros

s th

e p

erio

d a

s th

e d

ba

nd g

ets

fille

d

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L3,2/M3,2 Whiteline and unoccupied densities of d states

• 3d metal: the L-edge WL for early 3d metals is most complex due to the proximity of the L3,L2 edges as well as crystal field effect; the 3d spin-orbit is negligible

• 4d metals: the L3,L2 edges are further apart, WL intensity is a good measure of 4d hole population

• 5 d metals: The L3 and L2 are well separated but the spin orbit splitting of the 5d orbital becomes important, j is a better quantum number than l, WL intensity is a good measure of d5/3 and d3/2 hole populations

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3d metals

Hsieh et al Phys. Rev. B 57, 15204 (1998)

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d- band filling across 5d row

5d metal d band

Ta

W

Pt

Au

Intensity (area under the curve) of the sharp peak at threshold (white line) probes the DOS

EFermi

T.K. Sham et al. J. Appl. Phys. 79, 7134(1996)

3p3/2-5d2p3/2-5d

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d - charge redistribution in 4d metals

Ag →Pd 4d charge transfer upon alloying

I. Coulthard and T.K. Sham, Phys. Rev. Lett., 77, 4824(1996)

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Analysis of d hole in Au and Pt metal In 5 d metals with a nearly filled/full d bandsuch as Pt and Au, respectively, spin orbit coupling is large so d5/2 and d3/2 holes population is not the same

Selection rule: dipole, ∆l = ± 1, ∆j = ± 1, 0

L3, 2p3/2 → 5d5/2.3/2

L2, 2p1/2 → 5d3/2

Pt5d3/2

5d5/2

DOS

EF

T.K. Sham et al. J. Appl. Phys. 79, 7134(1996)

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Au L32p3/2 - 5d 5/2,3/2

Au L22p1/2 - 5d3/2

M. Kuhn and T.K. Sham Phys. Rev. 49, 1647 (1994)

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*

* Phys. Rev. B22, 1663 (1980)

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Probing depth profile with Probing depth profile with light: applications

Morphology Structure

Electronicproperties

0.1-1 nm

nm-10 nm

10-103 nm

surface

near surface

bulk-like

hard x-ray

soft x-ray

Interface

X-ray probes:

(photon)photon

Light-matter Interaction:Absorption & Scattering

e ion

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Soft X-ray vs. hard X-ray

Soft x-ray usually cannot penetrate the entire sample → measurements cannot be made in the transmission mode.Yield spectroscopy is normally used !

One absorption length (µt1 = 1 or t1 = 1/ µ) is a goodmeasure of the penetration depth of the photon

Example of one absorption lengths

Element density(g/cm3) hv(eV) mass abs (cm2/g) t1(µm)Si 2.33 1840 3.32 x103 1.3

100 8.6 x104 0.05Graphite 1.58 300 4.02x104 0.16

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Soft x-ray spectroscopy: unique features

• XAFS measurement in the soft x-ray (short absorptionlength) region is often made using yield spectroscopyElectron yield (total, partial, Auger)X-ray fluorescence yield (total, selected wavelength)Photoluminescence yield (visible, light emitting materials)XEOL (X-ray excited optical luminescence) and TRXEOL (Time-resolved X-ray Excited Optical Luminescence)

• Since soft X - ray associates with the excitation of shallow core levels, the near-edge region (XANES/ NEXAFS) is the focus of interest for low z elements and shallow cores

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E.G.: the Interplay of TEY and FLY

c-BN

h-BN

Si(100)

a-BN

80 nm

Diamond-like

Graphite-like

Amorphous

Cubic BN film grown on Si(100) wafer? Preferred orientation of the h-BN film?

hv TEY FLY

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190 200 210 220

4

3

2

1

b

600

200

400

900

Inte

nsity

(a.

u.)

Photon Energy (eV)

190 200 210 220

4

32

1a

600400200

900

Inth

esity

(a.

u.)

Photon Energy (eV)

normal

glancing

TEY

FLY

c-BN

h-BN

Si(100)

a-BN

hv TEYFLY

X.T. Zhou et.al. JMR 2, 147 (2006)

Boron K-edge

π* ε

I (1s -π*) minimum

I (1s -π*) maximum

π* ε

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hvop

hvex

PLY

Mono

XEOL

XA

FS

Abs.

E

200 850

Wavelength (nm)

CB

VB

EF

XAFS and XEOL (Optical XAFS)

Edge

40

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• X-ray photons in, optical photons outIllustrations

BN nanowiresSi nanowiresZnS nanoribbons

XEOL - Energy Domain

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W.-Q. Han et al. Nanoletter. 8, 491-494 (2008)

Boron nitride nanotube (BNNT)

Liu et. al. (UWO) unpublished

BNNT

h-BN

B-O bond

Oxygen

4242

W a v e l e n g t h ( n m )

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0

Inte

nsit

y (a

rb. u

nits

)

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

7 0 0 0

8 0 0 0

9 0 0 0

1 0 0 0 0

1 1 0 0 0

1 2 0 0 0

4 6 0 n m 6 3 0 n m

5 3 0 n m

h v e x ( e V )1 8 9 0

1 8 6 7

1 8 5 11 8 4 7 .5

1 8 4 5

1 8 4 21 8 4 0

1 8 3 8 .5

1 8 3 0

W a v e le n g t h ( n m )3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

Inte

nsi

ty

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0 1 8 4 7 .5 e V

1 8 4 2 e V

d i f f e r e n c e c u r v e

P h o t o n E n e r g y1 8 4 0 1 8 5 0 1 8 6 0 1 8 7 0

TE

Y

0

1

2

3S i K - e d g e

S i O 2

S i

( a ) ( b )

Si K-edge XEOL of silicon nanowires

T.K. Sham et al. Phys. Rev. B 70, 045313 (2004)

4343

Si nanowire

Photon Energy (eV)

1830 1840 1850 1860 1870 1880

Inte

nsit

y (a

rb. u

nits

)

0

10

20

30

40

PLY zero order

TEY

FLY

630 nm

460 nm

530 nm

SiSiO2

Photoluminescence: Si K-edge

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1050104010301020

Energy (eV)

332 nm

OL

W XAS

1050104010301020

Energy (eV)

520 nm

OL

ZB XAS

ZnS hetero-crystalline nano-ribbon

ZnS nw (wurtzite)

ZnS nw (zinc blend)

700600500400300

Wavelength (nm)

280 K

10 K Total

700600500400300

Wavelength (nm)

Total

0-14 ns

520 nm

332 nm

X.-T. Zhou et al., J. Appl. Phys. 98, 024312(2005)

R.A. Rosenberg et al., Appl. Phys. Lett. 87, 253105(2005)

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45

• Timing - resolved XEOL (TRXEOL)• Illustrations

ZnO: Nanodeedle vs NanowireCdSe-Si: Hetero nanostructures

XEOL - Time Domain

46

46

TRXEOL at APS

T.K. Sham & R.A. Rosenberg, ChemPhysChem 8, 2557-2567 (2007)

47

47

Time-gated spectroscopy

• Select time window(s)

• Obtain spectra /yields of photons arriving within that window

15010050

Time (ns)

153 ns

Fast time window Slow time window

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48

CdSe -Si hetero nanoribbons hv = 1100 eV

total optical yield

0-20 ns

20-150 ns

Si

CdSe

R.A. Rosenberg et. al. Appl. Phys. Lett., 89, 243102(2006)

X.H. Sun et al.J. Phys. Chem., C, 111, 8475(2007)

4949

CdSe-Si Heterostructure: Se L3,2 - edge

1400 1420 1440 1460 1480 1500 1520

0.1

0.2

20 - 150 ns

0 - 20 ns

Un-gated

TEY

Inte

nsity

(ar

b. u

nits

)

Photon Energy (eV)

Se L3,2

-edge

50

50

STXM: Scanning Transmission X-ray Microscopy

STXM @ the SM beamline @CLS

J.G. Zhou, J. Wang, H.T. Fang, C.X. Wu, J.N. Cutler, T.K. Sham, Chem. Comm. 46, 2778 (2010)

Fresnel Zone Plates

30nm

Courtesy of A.P. Hitchcock, McMaster

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S1S2 S9

S3

S7

S8

(S2)

Mico/nano spectroscopy of N-CNT

J. Zhou, J. Wang et al.J. Phys. Chem. Lett. (2010) DOI: 10.1021/jz100376v

TEM STXM

______500 nm