Transcript X-ray Absorption Spectroscopy (XAS)

Part II
• XAFS: Principles
• XANES/NEXAFS
• Applications
1
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.
2
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 3
XAFS of free atom
 In rare gases, the pre-edge region exhibits a series
of sharp peaks arising from bound to bound
transitions (dipole: 1s to np etc.) called Rydberg
transitions
4
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
5
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 will
eventually 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
6
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
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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 Backscattering
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9
10
NEXAFS of free atom & unsaturated diatomic molecules
States trapped in the potential barrier are virtual MO’s
or multiple scattering states (quasi bound states)
Free atom
free e with KE >0
diatomic with
unsaturated
bonding
N2, NO,
CO etc.
bound
states
*
Asymptotic wing of the coulomb
potential supports the Rydberg states
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NEXAFS of Free Atom: Ar L3,2-edge
hv
3d
0.40
2p3/2 - 4s
123meV
0.35
Relative Intensity
0.30
4d
0.25
244.00
244.25
244.50
244.75
Photon Energy
Ar (gas)
2p6 2p54s1
[Ne]3s23p6
14s1
2p
Core hole
j=l±s
3d
0.20
2p1/2 - 4s
5d
4d
0.15
6d
2p1/2
2p3/2
5d
6d
0.10
0.05
Hund’s
rule
(lower binding)
244
245
246
247
248
Photon Energy (eV)
249
250
251
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NEXAFS of atom and small molecules
HBr
Kr
M5,4 (3d5/2,3/2)
M5,4 (3d5/2,3/2)
MO
Rydberg transitions in HBr remain strong and a
broad transition to molecular orbital emerges
13
NEXAFS of Nitrogen
π*
IP
σ*
Intensity (arb. units)
N2
400.0
400.5
401.0
401.5
Photon Energy (eV)
402.0
1s - *
Vibronic structures in
the 1 s to * transition
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Molecular orbital illustration
CO
N2
*
MO axis
C2H2
*
*
C 1s
C 1s
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16
E = (E*-Eo)  1/r
Why would this correlation exist?
i) Multiple scattering theory
ii) Simple picture- particle in a boxthe 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
H
C=C
H
H
C-C
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Carbon Allotropes
Hexagonal
Diamond
(Lonsdaleite)
Diamond
Graphene
Graphite
Nanotube
20
XANES of Benzene
LUMO + 1
LUMO
core
21
*1 C-H*
Increasing no. of benzene rings
XANES systematic
*2
Tom Regier unpublished
22
Chemical systematic
C60
CN
T
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d - electron in transition metals
The dipole selection
rule allows for the
probing of the
unoccupied densities
of states (DOS) of d
character from the 2p
and 3p levels –
M3,2 and L3,2-edge
XANES analysis
(relevance: catalysis)
25
Filling of the d band across the period
3d
4d
5d
26
Decreasing whiteline intensity across the period as the d band gets filled
Transition metal systematic
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
EFermi
Ta
W
Pt
Au
5d metal d band
Intensity (area under
the curve) of the
sharp peak at
threshold (white line)
probes the DOS
T.K. Sham et al. J. Appl. Phys.
79, 7134(1996)
3p3/2-5d
2p3/2-5d
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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 band
such as Pt and Au, respectively, spin orbit
coupling is large so d5/2 and d3/2 holes
E
population is not the same
F
DOS
5d5/2
Selection rule: dipole,
Pt
l = ± 1, j = ± 1, 0
L3, 2p3/2  5d5/2.3/2
L2, 2p1/2  5d3/2
T.K. Sham et al. J. Appl. Phys. 79, 7134(1996)
5d3/2
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Au L3
2p3/2 - 5d 5/2,3/2
Au L2
2p1/2 - 5d3/2
M. Kuhn and T.K. Sham Phys. Rev. 49, 1647 (1994)
32
*
* Phys. Rev. B22, 1663 (1980)
33
Probing depth profile with light: applications
photon
e
ion
(photon)
surface
0.1-1 nm
near surface
nm-10 nm
Interface
bulk-like
X-ray probes:
Light-matter
Interaction:
Absorption
& Scattering
Morphology Structure
Electronic
properties
soft x-ray
10-103 nm
hard x-ray
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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 (t1 = 1 or t1 = 1/ ) is a good
measure of the penetration depth of the photon
Example of one absorption lengths
Element density(g/cm3) hv(eV) mass abs (cm2/g)
Si
2.33
1840
3.32 x103
100
8.6 x104
Graphite 1.58
300
4.02x104
t1(m)
1.3
0.05
0.16
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Soft x-ray spectroscopy: unique features
• XAFS measurement in the soft x-ray (short absorption
length) region is often made using yield spectroscopy
Electron 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
hv
Diamond-like
Graphite-like
Amorphous
TEY
c-BN
h-BN
a-BN
FLY
80 nm
Si(100)
Cubic BN film grown on Si(100) wafer
? Preferred orientation of the h-BN film?
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Boron K-edge
b
3
TEY
FLY
c-BN
h-BN
a-BN
TEY
2
Intensity (a. u.)
hv
normal
*
4

1
20
0
40
0
60
0
90
I (1s - *) minimum
0
190
200
210
220
Photon Energy (eV)
1
* 
a
Si(100)
FLY
Inthesity (a. u.)
3
glancing
2
4
I (1s - *) maximum
0
20
0
40 0
60 0
90
190
X.T. Zhou et.al. JMR 2, 147 (2006)
200
210
220
Photon Energy (eV)
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XAFS and XEOL (Optical XAFS)
E
XEOL
XAFS
PLY
Mono
CB
hvop
Edge
Abs.
hvex
EF
200
850
Wavelength (nm)
VB
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XEOL - Energy Domain
• X-ray photons in, optical photons out
Illustrations
BN nanowires
Si nanowires
ZnS nanoribbons
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40
Boron nitride nanotube (BNNT)
B-O bond
Oxygen
BNNT
W.-Q. Han et al. Nanoletter. 8, 491-494 (2008)
h-BN
Liu et. al. (UWO) unpublished
41
Si K-edge XEOL of silicon
nanowires
4000
12000
3
Si K-edge
Si
10000
Intensity (arb. units)
9000
SiO2
1840
1850
1860
Photon Energy
TEY
2
(a)
11000
Intensity
3000
1847.5 eV
1842 eV
(b)
2000
1
1000
0
0
300
1870
difference
curve
400
500
600
700
Wavelength (nm)
hvex(eV)
1890
8000
7000
1867
1851
1847.5
6000
5000
1845
4000
1842
1840
3000
1838.5
1830
2000
530 nm
460 nm
1000
200
300
400
500
630 nm
600
700
800
Wavelength (nm)
T.K. Sham et al. Phys. Rev. B 70, 045313 (2004)
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Photoluminescence: Si K-edge
Si nanowire
TEY
40
Intensity (arb. units)
FLY
PLY
zero order
30
460 nm
20
530 nm
630 nm
10
Si
SiO2
0
1830
1840
1850
1860
1870
1880
Photon Energy (eV)
43
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ZnS hetero-crystalline nano-ribbon
520 nm
X.-T. Zhou et al., J. Appl. Phys. 98,
024312(2005)
520 nm
280 K
10 K
Total
332 nm
ZnSOLnw
XAS
(zincZB
blend)
1020
300
400
500
600
1030
1040
1050
Energy (eV)
700
Wavelength (nm)
332 nm
Total
0-14 ns
OLnw
ZnS
W XAS
(wurtzite)
300
400
500
600
Wavelength (nm)
R.A. Rosenberg et al., Appl. Phys. Lett. 87,
253105(2005)
700
800
1020
1030
1040
1050
Energy (eV)
44
XEOL - Time Domain
• Timing - resolved XEOL (TRXEOL)
• Illustrations
ZnO: Nanodeedle vs Nanowire
CdSe-Si: Hetero nanostructures
45
45
TRXEOL at APS
T.K. Sham & R.A. Rosenberg, ChemPhysChem 8, 2557-2567 (2007)
46
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Time-gated spectroscopy
• Select time
window(s)
• Obtain spectra
/yields of
photons arriving
within that
window
153 ns
50
Fast time window
100
Time (ns)
150
Slow time window
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47
CdSe -Si hetero nanoribbons hv = 1100 eV
CdSe
0-20 ns
Si
X.H. Sun et al. J. Phys. Chem., C, 111, 8475(2007)
total optical yield
20-150 ns
48
48
R.A. Rosenberg et. al. Appl. Phys. Lett., 89, 243102(2006)
CdSe-Si Heterostructure: Se L3,2 - edge
Intensity (arb. units)
0.2
TEY
Se L3,2-edge
Un-gated
0 - 20 ns
0.1
20 - 150 ns
1400
1420
1440
1460
1480
1500
1520
Photon Energy (eV)
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STXM: Scanning Transmission X-ray Microscopy
STXM @ the SM beamline @CLS
Courtesy of A.P. Hitchcock, McMaster
Fresnel Zone Plates
30nm
J.G. Zhou, J. Wang, H.T. Fang, C.X. Wu, J.N.
Cutler, T.K. Sham, Chem. Comm. 46, 2778 (2010)
50
50
Mico/nano spectroscopy of N-CNT
TEM
STXM
S7
S3
S1
S2
S8
S9
______
500 nm
(S2)
J. Zhou, J. Wang et al. J. Phys. Chem. Lett. (2010) DOI: 10.1021/jz100376v
51