University of California, Berkeley March 9, 2007
Use of NEXAFS in Materials Science
Tony van Buuren
Nanoscale Synthesis and Characterization Laboratory
Lawrence Livermore National Laboratory
and
UC Merced
School of Natural Sciences
Outline
•
Overview of the X-ray absorption process
•
How do you measure NEXAFS or XANES
–
–
–
–
•
Element specific
Measures partial density of empty states
Sensitive to local bonding
Polarization depended
Applications of NEXAFS:
– Surface Chemistry
• Catalysts
– Environmental Chemistry
• Oxy-state -> mobility
– Material science experiments using NEXAFS
•
•
•
•
Quantum dots
Self assembled monolayer
Chemical mapping (Imaging)
Magnetic structures
Density of states from x-ray absorption

occupied, VB
variable hn
emission
Intensity
unoccupied, CB
VB
CB
XES
core level
XANES
Photon energy
XANES=NEXAFS
XANES = x-ray absorption near-edge structure
NEXAFS = near-edge x-ray absorption fine structure
XAS = x-ray absorption
EELS = electron energy loss spectroscopy, provides
very similar information to XANES
XANES: Partial density of unoccupied states
unoccupied, CB

Wif ~ fTi2 (Ef-Ei-E)
occupied, VB
variable hn
Il(E) ~ l-1(E)Ml-1(E)2 + l+1(E)Ml+1(E)2
core level
Dipole selection rules apply (l1):
XANES edges:
1s – K edge
2s, 2p – L edges
3s, 3p, 3d – M edges
4s, 4p, 4d, 4f – N edges
sp
p  s and d
d  p and f
f  d and g
Quadrupole transitions (l 2 or 0) are typically much (102-103 times) weaker.
Element-specific, angular-momentum resolved density of unoccupied states
Zinc K-edge X-ray Absorption Spectroscopy Spectrum
9450
0.6
9650
9850 10050 10250
XANES
Signal (Arb. Units)
0.4
EXAFS
0.2
0
-0.2
PreEdge
-0.4
X-ray
Pre-Edge
Absorption
Region
Near Edge
Extended
X-ray
Absorption
Structure(EXAFS)
Fine Structure
•Prior to adsorption of the
(XANES)
•atom
Oscillations
dependant upon
of interest
•Absorption
internal
type, positioninduces
and number
of
•Composed
ofatoms
the
electronic
transitions
neighbouring
absorption ‘tails’ of elements
•Oxidation
state
is would
obtained
•A
monatomic
gas
not
with
lower binding
energies
from
thethe
position
of the ‘edge’
display
fine structure
•Edge
features
are1000eV
• Extend
for up to
characteristic
of the local
beyond the edge
environment of the atom of
• Only elastically scattered
interest
electrons contribute to the
EXAFS – local order
•Unlike the XANES,
interpretation of the EXAFS by
inspection is limited
-0.6
Photon Energy (eV)
Extended X-ray Absorption Fine Structure (EXAFS)
Irradiate the sample with X-ray photons stepwise over a range
encompassing the binding energy of a core electron
Adjacent atoms backscatter the ejected photoelectrons which then
interfere with outgoing wave
Constructive interference near the nucleus promotes X-ray photon
absorption, destructive interference reduces absorption
XAS is:
•Element specific
•Non-intrusive
Whilst probing:
•Short-range order
Structure Extraction From the EXAFS
Quantitative analysis of the EXAFS was first realised by Sayers et al.
working with polycrystalline and amorphous Ge samples
–Fourier transformation of c(k) into real space
–Peaks correspond to shells of atoms distributed around the central atom
and comprise an entire radial structure function
Data analysis
• Subtract pre- and post-edge backgrounds
• Create structural models and computer generate
EXAFS and FTs from them to compare with the real
data
• Least squares regression gives a fit-parameter, R
Model parameters varied include:
• Position of backscatterers and their identity
• Co-ordination numbers
• Thermal disorder (Debye-Waller factors)
• Atomic potentials
NEXAFS spectra can be recorded in different ways.
The most common methods are transmission and electron yield
measurements. Note that the absorption coefficient µ is obtained either as the
logarithm or the direct ratio of the detected intensities It and Ie and incident
intensity Io
www-ssrl.slac.stanford.edu/stohr/nexafs
NEXAFS measurements are element specific
X-ray absorption spectra of a wedge sample,
revealing the composition at various points along
the wedge.
www-ssrl.slac.stanford.edu/dichroism/xas
BN Thin Films: NEXAFS Determination of Bonding
•Cubic phase (sp3 bonded) Boron Nitride
films and coatings are desirable for their
hardness and electronic (wide band gap)
properties (GM - G. L. Doll)
B 1s Photoabsorption,
hBN and cBN vs BN film
p*
sp2
•Hexagonal (sp2, graphitic) BN is the
energetically favorable phase
•Metastable growth conditions (magnetron
sputtering, laser ablation) greatly affect the
film’s bonding and morphology
sp3
I. Jimenez, L. J. Terminello, et al. Appl. Phys. Lett. 68, 2816
(1996)
Polarization Dependent NEXAFS
Synchrotron radiation
sources:
BL8.2
SSRL
→ high flux
→ polarized light
Bond/functional group orientation:
NEXAFS resonance strength
E.
Surface Chemistry: Catalysts
SnO2 aerogels are attractive gas sensor materials
Sn 3d XANES
B4 C4
Intensity (arb. units)
B5
C5
D4
D5
A5
o
20 C
o
250 C
o
400 C
o
500 C
o
550 C
bulk
480
M5
485
490
495
Photon energy (eV)
S. O. Kucheyev, PRB 72 (3): Art. No. 035404 (2005).
M4
500
505
Surface Chemistry: Polymers
This clearly illustrates the power of
NEXAFS to distinguish chemical bonds and
local bonding. In many ways it is superior to
XPS, which doe s not provide local
structural information.
Often one can use a spectral "fingerprint"
technique to identify the local bonding
environment.
Carbon K-edge NEXAFS spectra of different
polymers, revealing the sensitivity to molecular
functional groups.
www-ssrl.slac.stanford.edu/stohr/nexafs
NEXAFS Imaging
Chemical mapping of polymer blend
XAS images at 283, 285.1, 288.4 and 290
eV, at the C 1s region of an annealed 28:72
(w/w) PS:PMMA blend thin film spun cast
on native oxide Si
(b) Spectra from the indicated spots.
(c and d) Component maps of PS and
PMMA derived by singular value
decomposition of the C 1s image
sequence.
(e) Color coded composite map (red: PS;
green: PMMA
C. Morin J. Electron Spectosc. 137-140 (2004) 785-794
Environmental Chemistry: Oxy state
Comparison of plutonium LII XANES
spectra for plutonium in oxidation
states III, IV, V, and VI with RFETS
soil and concrete samples
http://wwwssrl.slac.stanford.edu/research/highli
ghts_archive/rocky_flats.pdf
Polarization depended NEXAFS to study self
assembled monolayers
Self-Assembled Monolayers (SAMs):
molecules which adsorb on a surface,
spontaneously order via intramolecular forces.
Most common type of SAM: alkanethiols on gold.
Headgroup of molecule
changed for chemical
functionality
extremely easy to make
dip gold substrate in mM solution
rinse in clean solvent
relatively stable
under ambient conditions ~hrs.
Under N2 > 1 year
Van der Waals
Interactions between chains
cause alignment, ordering
Sulfur bound to gold
Au(111)
Mica or 5nm Ti on Si substrate
“Hot” Applications
switchable surfaces
switching interlocking molecules molecular electronics
SCIENCE VOL 299 17 JANUARY 2003
60nm
trapping of proteins,
viruses, etc.
60nm
www.sciencemag.orgSCIENCEVOL 289 18 AUGUST2000
Barry Cheung et. Al., LLNL, to be published
C(1s) NEXAFS of Organothiol SAMs
Collect and compare NEXAFS
spectra at multiple angles of
incidence
• Vary from grazing to normal angles of
incidence between X-rays and sample
• Polarization dependent resonances
denote well-defined orientation in the
orbital of interest
• Peak direction in difference spectra
provides a preliminary indication of
functional group orientation
C(1s) NEXAFS for MUA SAM on
Au(111)
Obtaining Molecular Orientation from C(1s)
NEXAFS
Linear regression analysis provides a more quantitative measure of
functional group orientation:
→
Peak fitting protocols resolve convoluted resonances and provide peak
intensities
→
Linear regression analysis yields bond orientation to within ± 5°
Orientation of MUA on
Au(111)?
Carboxyl Group:
Carboxyl group tilted ~ 45°
from the Au(111) surface
normal
Alkyl Chain:
Hydrocarbon backbone tilted
by ~ 42°
T.M. Willey et. al., Langmuir, 2004, 20,
2746
NEXAFS Characterization of MBA SAMs
on Au(111)
2-MBA
3-MBA
4-MBA
Quantum Confinement Effects in
Semiconductor Nanocrystals (NCs)
Semiconductor nanocrystals/‘Quantum dots’
→ Diverse range of potential technological applications
85 Å
60 Å
45 Å
37 Å
2.0
Absorbance/arb. units
Photoluminescence/arb. units
→ Unique, size-dependent, optical and electronic properties
3.0
Energy (eV)
Optoelectronic behavior explained in terms of quantum confinement effects:
Particle in a Box
Silicon nanocrystals are prepared and
deposited in situ out of the gas-phase.
BF
SAD
111
220
311
TEM
1.1 nm 1.5 nm
AFM
2.3 nm 2.8 nm
14 A
15 A
16 A
17 A
19 Å
Intensity [arb. units]
1.0
0.8
0.6
C. Bostedt, et al.,
J. Phys. Condens.
Matter 15, 1017
(2003).
0.4
0.2
0.0
0.5
1.0
1.5
2.0
2.5
Size [nm]
Crystalline particles with narrow size distribution.
3.0
3.5
4.0
First, the size-dependent properties are investigated
on sub-monolayer depositions of nanocrystals.
AFM:
Dilute systems need
element specific
measurements
Film-morphology:
Individual nanocrystals
Substrate:
Surface-passivated
germanium
X-ray absorption and Emission measurements
show shift in valence and conduction band of
isolated Si clusters
Conduction Band
Si 2p absorption from nanoparticles
Valence Band
Soft x-ray emission of Si nanoparticles
1.6 nm Clusters
Intensity (Arb. Units)
Intensity (Arb. Units)
1.6 nm Clusters
2 nm Clusters
2 nm Clusters
Bulk Silicon
Bulk Si
88
90
92
94
96
Emission Energy (eV)
98
100
99
99.5
100 100.5 101 101.5 102 102.5 103
Absorption Energy (eV)
Quantum Confinement in Nanoparticles
Measured and Compared to Theory
CB and VB band edge shift as a function of
particle size.
Band gap as a function of particle size
2.4
2.2
0.2
2.0
0.0
Band Gap (eV)
CB Shift (eV)
0.4
1.8
VB Shift (eV)
-0.2
1.6
-0.4
-0.6
1.4
-0.8
1.2
1
2
3
4
Particle Size (nm)
Ratio of CB to VB shift is 1:2
5
1.0
2.0
3.0
4.0
Particle Size (nm)
T. van Buuren, L. Dinh, L. L. Chase,
L. J. Terminello, Phys. Rev. Lett. 80, 3803 (1998)
Germanium exhibits much stronger
confinement effects than silicon.
3.5
Ge - extr. band gap
Ge - guide to the eye
Si - extr. band gap
Si - guide to the eye
Bandgap [eV]
3.0
T. van Buuren,
Phys. Rev. Lett. 80,
3803 (1998).
2.5
2.0
1.5
C. Bostedt, T. van
Buuren, APL (2004)
1.0
1.0
1.5
2.0
2.5
Particle Size [nm]
The band-gap of the Ge becomes larger than Si at
particles sizes below 2.0 nm
3.0
CdSe NCs: An Model System of
Technological Importance
Archetypal nanocrystalline binary semiconductor for technological applications
Model system for the study of quantum confinement
→ Readily synthesized with narrow size
distributions
→ Exhibit size-dependent photoluminescence
→ Extensively studied
Synthesis of CdSe NCs
TEM Image of CdSeTOPO
5nm
Murray, et. al., J. Am. Chem.
Soc., 115, 8706 (1993)
But…
Theories on electronic
structure conflict with one
another and experimental
results
Cd L3-edge XAS: We can probe the bottom of the
CB (vacant) DOS directly using this technique
Cd L3-edge XAS
L3-edge formally 2p → s, d empty
states
• Bottom of CB comprised of Cd
5s states
• Hybridized pd states located ~
4-5 eV above CB minimum
We find that only the ‘s’-states
move as a function of
particle size
J. Lee R. Meulenberg PRL 2007
Magnetic properties of materials can be studied by X-Ray
Magnetic Circular Dichroism (XMCD) spectroscopy
Electronic transitions in conventional
L-edge x-ray absorption (a), and xray magnetic circular x-ray dichroism
(b,c), illustrated in a one-electron
model.
The transitions occur from the spinorbit split 2p core shell to empty
conduction band states. In
conventional x-ray absorption the
total transition intensity of the two
peaks is proportional to the number
of d holes.
By use of circularly polarized x-rays
the spin moment (b) and orbital
moment (c) can be determined from
linear combinations of the dichroic
difference intensities A and B,
www-ssrl.slac.stanford.edu/dichroism/xas
according to other sum rules.
Circular dichroism at the Iron L-edge
If the photoelectron originates from the
p3/2 level (L3 edge), the angular
momentum of the photon can be
transferred in part to the spin through
the spin-orbit coupling.
Right circular photons (RCP) transfer
the opposite momentum to the electron
as left circular photons (LCP) photons,
and hence photoelectrons with opposite
spins are created in the two cases.
www-ssrl.slac.stanford.edu/dichroism/xas
Since the p3/2 (L3) and p1/2 (L2) levels
have opposite spin-orbit coupling, the
spin polarization will be opposite at the
two edges. In the absorption process,
"spin-up" and "spin-down" are defined
relative to the photon helicity or photon
spin.
Backup slides
NEXAFS Quantitative Orientation - Vector
(From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891)
Define polarization:
P
| E p |2
| E p |2  | E s |2
Intensity is electric field and TDM dot product squared
I S

P sin  cos  sin   1  P sin  sin   P cos  cos 

2
Due to 3-fold or higher substrate symmetry,
 d cos( )  0,  d sin(  )  0,  d cos
2
1
1
( )  ,  d sin 2 ( ) 
2
2
Squaring the dot product and averaging over azimuthal angle,
1  1

 1
I  S  P1  3 cos 2   1 3 cos 2   1   1  P sin 2  
 2
3  2




For raw intensities, use the ratio method by fitting
I  , 
I ( ,  fixed )
to experimental data.
Through this method we can determine quantitatively how molecules in ultrathin
organic layers are oriented on surfaces.
NEXAFS Quantitative Orientation - Plane
(From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891)
P
Define polarization:
| E p |2
| E p |2  | E s |2
Intensity: square of projection of E onto plane or sin(ε)

I  S E 2  ( Ep  sin  cos  sin   Es  sin   sin   Ep  cos   cos  ) 2

Due to 3-fold or higher substrate symmetry,
 d cos( )  0,  d sin(  )  0,  d cos
2
1
1
( )  ,  d sin 2 ( ) 
2
2
Squaring the dot product and averaging over azimuthal angle,
2  1


1 1

I  S   1   3 cos 2   1  3 cos 2   1   P     cos 2    1  P 

2 2

3  4




For raw intensities, use the ratio method by fitting
I  , 
I ( ,  fixed )
to experimental data.
Through this method we can determine quantitatively how molecules in ultrathin
organic layers are oriented on surfaces.
NEXAFS Quantitative Orientation - Difference
Use vector or plane intensity:
vector:
plane:
1  1

 1
I  S  P1  3 cos 2   1 3 cos 2   1   1  P sin 2  
 2
3  2




2  1


1 1

I  S  1  3 cos 2   1 3 cos 2   1  P    cos 2  1  P 

2 2

3  4




Take difference spectra between two incident angles
D( , a , b )  I ( , b )  I ( , a )
Determine parameter SP from a reference sample with known tilt
Solve for α or γ as all parameters are now known.
In either case,
D( ,  a ,  b )  cos 2  b  cos 2  a
Run linear regressions of D vs.
cos 2  b  cos 2  a
with multiple spectra
Through this method we can determine quantitatively how molecules in ultrathin
organic layers are oriented on surfaces.