university of california, berkeley march 9, 2007 tony van buuren nanoscale synthesis and...
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University of California, Berkeley March 9, 2007
Tony van Buuren
Nanoscale Synthesis and Characterization LaboratoryLawrence Livermore National Laboratory
and
UC MercedSchool of Natural Sciences
Use of NEXAFS in Materials Science
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
unoccupied, CB
occupied, VB
variable h
core level
emission
VB CB
XES XANES
Intensity
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
occupied, VB
variable h
core level
Wif ~ fTi2 (Ef-Ei-E)
Il(E) ~ l-1(E)Ml-1(E)2 + l+1(E)Ml+1(E)2
Dipole selection rules apply (l1):s pp s and dd p and ff d and gQuadrupole transitions (l 2 or 0) are typically much (102-103 times) weaker.
Element-specific, angular-momentum resolved density of unoccupied states
XANES edges: 1s – K edge2s, 2p – L edges3s, 3p, 3d – M edges4s, 4p, 4d, 4f – N edges
Zinc K-edge X-ray Absorption Spectroscopy Spectrum
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Pre-Edge
XANES
EXAFS
Pre-Edge Region
•Prior to adsorption of the atom of interest
•Composed of the absorption ‘tails’ of elements with lower binding energies
X-ray Absorption Near Edge Structure(XANES)
•Absorption induces internal electronic transitions•Oxidation state is obtained from the position of the ‘edge’•Edge features are characteristic of the local environment of the atom of interest
Extended X-ray Absorption Fine Structure (EXAFS)
• Oscillations dependant upon type, position and number of neighbouring atoms
•A monatomic gas would not display the fine structure
• Extend for up to 1000eV beyond the edge
• Only elastically scattered electrons contribute to the EXAFS – local order
•Unlike the XANES, interpretation of the EXAFS by inspection is limited
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, RModel 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
www-ssrl.slac.stanford.edu/dichroism/xas
X-ray absorption spectra of a wedge sample, revealing the composition at various points along the wedge.
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)
•Hexagonal (sp2, graphitic) BN is the energetically favorable phase
•Metastable growth conditions (magnetron sputtering, laser ablation) greatly affect the film’s bonding and morphology
B 1s Photoabsorption,hBN and cBN vs BN film
I. Jimenez, L. J. Terminello, et al. Appl. Phys. Lett. 68, 2816 (1996)
sp2
sp3
p*
Polarization Dependent NEXAFS
Synchrotron radiation sources:→ high flux
→ polarized light
BL8.2BL8.2SSRLSSRL
Bond/functional group orientation:NEXAFS resonance strength E .
Surface Chemistry: Catalysts SnO2 aerogels are attractive gas sensor materials
480 485 490 495 500 505
D4
20 oC
250 oC
400 oC
500 oC
550 oC
bulk
C4B
4
D5
C5B
5
A5
M4
M5
Intensity (arb. units)
Photon energy (eV)
Sn 3d XANES
S. O. Kucheyev, PRB 72 (3): Art. No. 035404 (2005).
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 ImagingChemical mapping of polymer blend
C. Morin J. Electron Spectosc. 137-140 (2004) 785-794
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
Environmental Chemistry: Oxy state
http://wwwssrl.slac.stanford.edu/research/highlights_archive/rocky_flats.pdf
Comparison of plutonium LII XANES spectra for plutonium in oxidation states III, IV, V, and VI with RFETSsoil and concrete samples
Self-Assembled Monolayers (SAMs):molecules which adsorb on a surface,spontaneously order via intramolecular forces.Most common type of SAM: alkanethiols on gold.
Polarization depended NEXAFS to study self assembled monolayers
extremely easy to makedip gold substrate in mM solutionrinse in clean solvent
relatively stableunder ambient conditions ~hrs.Under N2 > 1 year
Mica or 5nm Ti on Si substrateAu(111)
Sulfur bound to gold
Van der WaalsInteractions between chainscause alignment, ordering
Headgroup of molecule changed for chemical functionality
“Hot” Applications
www.sciencemag.orgSCIENCE VOL289 18AUGUST2000
SCIENCE VOL 299 17 JANUARY 2003
60nm
60nm
switchable surfaces switching interlocking molecules -molecular electronics
trapping of proteins, viruses, etc.
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)
Ph
oto
lum
ines
cen
ce/a
rb. u
nit
s
3.02.0Energy (eV)
Ab
sorb
ance/arb
. un
its
37 Å
45 Å
60 Å
85 Å
Semiconductor nanocrystals/‘Quantum dots’
→ Unique, size-dependent, optical and electronic properties
→ Diverse range of potential technological applications
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.
19 Å
111220
311
SAD
BF
AFM
C. Bostedt, et al., J. Phys. Condens. Matter 15, 1017 (2003).
TEM
1.0
0.8
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0.0
Intensity [arb. units]
4.03.53.02.52.01.51.00.5
Size [nm]
1.1 nm 1.5 nm 2.3 nm 2.8 nm 14 A 15 A 16 A 17 A
Crystalline particles with narrow size distribution.Crystalline particles with narrow size distribution.
First, the size-dependent properties are investigated on sub-monolayer depositions of nanocrystals.
Dilute systems need element specific measurements
Film-morphology:Individual nanocrystals
Substrate:Surface-passivated
germanium
AFM:
X-ray absorption and Emission measurements show shift in valence and conduction band of isolated Si clusters
99 99.5 100 100.5 101 101.5 102 102.5 103
Intensity (Arb. Units)
Absorption Energy (eV)
Bulk Silicon
2 nm Clusters
1.6 nm Clusters
88 90 92 94 96 98 100
Intensity (Arb. Units)
Emission Energy (eV)
Bulk Si
2 nm Clusters
1.6 nm Clusters
Valence BandSoft x-ray emission of Si nanoparticles
Conduction Band Si 2p absorption from nanoparticles
Quantum Confinement in Nanoparticles Measured and Compared to Theory
T. van Buuren, L. Dinh, L. L. Chase, L. J. Terminello, Phys. Rev. Lett. 80, 3803 (1998)
2.4
2.2
2.0
1.8
1.6
1.4
1.2
4.03.02.01.0
Band Gap (eV)
Particle Size (nm)
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CB Shift (eV)
54321
VB Shift (eV)
Particle Size (nm)
CB and VB band edge shift as a function of particle size.
Band gap as a function of particle size
Ratio of CB to VB shift is 1:2
Germanium exhibits much stronger confinement effects than silicon.
3.5
3.0
2.5
2.0
1.5
1.0
Ban
dgap
[eV
]
3.02.52.01.51.0Particle Size [nm]
Ge - extr. band gap Ge - guide to the eye Si - extr. band gap Si - guide to the eye
C. Bostedt, T. van Buuren, APL (2004)
T. van Buuren, Phys. Rev. Lett. 80, 3803 (1998).
The band-gap of the Ge becomes larger than Si at particles sizes below 2.0 nmThe band-gap of the Ge becomes larger than Si at particles sizes below 2.0 nm
CdSe NCs: An Model System of Technological Importance
→ Readily synthesized with narrow size distributions
→ Exhibit size-dependent photoluminescence
→ Extensively studied
Archetypal nanocrystalline binary semiconductor for technological applications
Model system for the study of quantum confinement
Murray, et. al., J. Am. Chem. Soc., 115, 8706 (1993)
5nm
TEM Image of CdSe-TOPO
Synthesis of CdSe NCs
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 x-ray magnetic circular x-ray dichroism (b,c), illustrated in a one-electron model.
The transitions occur from the spin-orbit 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,
according to other sum rules. www-ssrl.slac.stanford.edu/dichroism/xas
Circular dichroism at the Iron L-edge
www-ssrl.slac.stanford.edu/dichroism/xas
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.
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
Define polarization: 22
2
||||
||
sEpE
pEP
+=
Intensity is electric field and TDM dot product squared
[ ]2coscossinsin1sincossin αθαφαφθ PPPSI +−+=
Due to 3-fold or higher substrate symmetry,
∫ ∫ ∫ ∫ →→→→2
1)(sin,
2
1)(cos,0)sin(,0)cos( 22 φφφφφφφφ dddd
Squaring the dot product and averaging over azimuthal angle,
( )( ) ( ) ⎥⎦
⎤⎢⎣
⎡−+⎟
⎠
⎞⎜⎝
⎛ −−+= θ 222 sin12
11cos31cos3
2
11
3
1PPSI
For raw intensities, use the ratio method by fitting ( )),(
,
fixedI
I
=θα
θα to experimental data.
(From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891)
Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.
NEXAFS Quantitative Orientation - Plane
Define polarization: 22
2
||||
||
sEpE
pEP
+=
Intensity: square of projection of E onto plane or sin(ε)
[ ]22 )coscossinsinsincossin( γθγφγφθ ⋅⋅+⋅⋅+⋅−= EpEsEpESI
Due to 3-fold or higher substrate symmetry,
∫ ∫ ∫ ∫ →→→→2
1)(sin,
2
1)(cos,0)sin(,0)cos( 22 φφφφφφφφ dddd
Squaring the dot product and averaging over azimuthal angle,
( ) ( ) ( )⎥⎦
⎤⎢⎣
⎡−⋅⎟
⎠
⎞⎜⎝
⎛ ⋅++⋅⎟⎠
⎞⎜⎝
⎛ −⋅−⋅−⋅= PPSI 1cos2
1
2
11cos31cos3
4
11
3
2 222 γγθ
For raw intensities, use the ratio method by fitting ( )),(
,
fixedI
I
=θα
θα to experimental data.
(From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891)
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:
Take difference spectra between two incident angles
( )( ) ( )⎥⎦
⎤⎢⎣
⎡−⎟
⎠
⎞⎜⎝
⎛ ++⎟⎠
⎞⎜⎝
⎛ −−−= PPSI 1cos2
1
2
11cos31cos3
4
11
3
2 222 γγθ
Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.
vector:
plane:
( )( ) ( ) ⎥⎦
⎤⎢⎣
⎡−+⎟
⎠
⎞⎜⎝
⎛ −−+= θ 222 sin12
11cos31cos3
2
11
3
1PPSI
),(),(),,( abba IID θθθθ −=
Determine parameter SP from a reference sample with known tilt
Solve for α or γ as all parameters are now known.
In either case, abbaD θθθθ 22 coscos),,( −∝
Run linear regressions of D vs. ab θθ 22 coscos − with multiple spectra