Information that XAS provides (case study) Oxidation states: Si and SiO2 Si K-edge 6 eV L. Liu., et...
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![Page 1: Information that XAS provides (case study) Oxidation states: Si and SiO2 Si K-edge 6 eV L. Liu., et al Small, (2012)15, 2371 (E): absorption coefficient.](https://reader035.fdocuments.us/reader035/viewer/2022062322/5697bf981a28abf838c912e8/html5/thumbnails/1.jpg)
Information that XAS provides (case study)
Oxidation states: Si and SiO2
Si K-edge
6 eV
L. Liu., et al Small, (2012)15, 2371
(E): absorption coefficient
What is the origin of the shift? Oxidation state, ionicity!
VB occupied
CB unoccupied
DO
S
crystalstructure
bandstructure
band gap
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Information that XAS provides
Hybridization: sp3 and sp2 hybridized BN
NBO
π*
B K-edge
X.-T. Zhou, Anal. Chem. (2006) 78, 6314
graphite diamond
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Local symmetry: anatase and rutile TiO2
Ti L3,2-edgeTiO
Information that XAS provides
Liu et al. J. Phys. Chem. C (2010) 114, 21353
1s K-edge
2p3/2,1/2 L3,2-edge 1sK-edge
OTi
t2g eg
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0.1-1 nm
nm-10 nm
10-103 nm
surface
near surface
bulk-like
hard x-ray
soft x-rayInterface
(photon)photon e- ion
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Graphite 300 eV (K-edge) t1:160 nm Au 11919 eV (L3 -edge) t1: 2.8 m
Soft X-ray vs. Hard X-rays
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Soft X-ray energy and core level threshold (binding energy) of element
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Soft X-ray (< ~5000eV)
Soft X-ray is associated with deep core levels of low z elements
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Soft vs Hard X-rays and core level threshold (binding energy) of element
Soft X-ray is associated with shallow core levels of intermediate z elements
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Soft X-ray is associated with shallow core levels of high z elements
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What edge can soft X-ray probe ?
• Li K-edge (54.7 eV) to ~ Ti K-edge (~5 keV)• Soft X-ray can access many shallow core
level of many important elements, K-edge of C, N, O; L, K-edges of Al, Si, S, P, Cl, L-edge of 3d metals, L, M-edges of 4d metals, L-edge of Ga, Ge, As, Se and Cd, In, Sn, Sb, Te and
N4,5 edge (giant resonance) of 4f rare-earths
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Soft X-ray vs. hard X-ray
Soft X-ray: shallow penetration depth Measurements in transmission is difficult or undesirable Yield is normally used !
One absorption length (t1 = 1 or t1 = 1/ )Attenuation length
Element density(g/cm3) hv (eV) mass abs (cm2/g) t1(m)
Si 2.33 1840 (K-edge) 3.32 103 1.3 100 (L-edge) 8.60 104 0.05
Graphite 1.58 300 (K-edge) 4.02 104 0.16
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X-ray absorption coefficient and the one-absorption length
• The x-ray absorption coefficient (cm-1) = ; (g/cm3), (cm2/g);
Beer- Lambert law: Transmission measurements
)/ln( tot
ot IItoreII
t
Io It
V V
AAA
Ionization chamber:Can be used to measure absolute photon flux
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Io I Iref
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Experimental considerations(transmission is not always practical)
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(i) Suitable sample thickness: SiO2
t = 0.2m
t = 0.5m
t = 5m
t = 5m
5%
50%
90%
0% 10%
0%
0.1%
40%
0.% 0.1%
Si L Si K
O K edge
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How does an ion chamber work in X-ray measurements
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102 -103 V
io ion pair/sec Io
t (gas)W (gas)
sample
io
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W and G values of gases
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W value: energy required to produce one ion pair.
For nitrogen, W = 35.8 eV/ ion pair
G values: number of ion pairs produced by 100eV of energy absorbed;e.g. 100 eV may come from one 100eV photon of 5 20 eV photon or 0.01 10,000 eV photon.
These values are reasonably constant at hard X-ray and gamma ray energies but varies for soft X-ray photons.
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Experimental considerations(transmission is not always practical)
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(ii) Specimen geometry
Thin over layer/multilayer film
hv (X-ray fluorescence)bulk sensitive
e-hv (optical photons)can be surface and bulk sensitive depending on the origin of the luminescence
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Why soft x-rays ( ~ 40 - 5000 eV) ?
Shallow penetration depth: t1( 1/e attenuation) is ~10 – 103 nm. E.g. t1 ~ 60 nm at Si L-edge
High resolution monochromators (SGM, PGM): E/E ~10,000 obtainable. e.g. at 300 eV, E= 0.03 eV
Narrow inherent linewidths (core hole lifetime). Decay often involves the shallow core and valence electrons.
Chemical and site specificity; depth profile
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Soft x-ray spectroscopy: unique features
• Yield spectroscopyElectron yield (total, partial- surface sensitive)X-ray fluorescence yield (total, selected wavelength-bulk sensitive)Photoluminescence yield (visible, UV- site specific)
- XEOL (X-ray excited optical luminescence)- TR-XEOL (Time-resolved)
• Excitation of shallow core levelsThe near-edge region (XANES/ NEXAFS) is the focus of interest in low z elements and shallow cores
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• High energy and spatial resolution
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Soft X-ray attenuation
Si3N4 TiO2
Si L3,2-edge
N K-edge
Si K-edge
Ti M-edge
Ti L3,2-edge
O K-edge
Ti K-edge
Source: CXRO
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Surface sensitivity & electron attenuation length
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Secondary electronsMain contribution to TEY
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The Interplay of TEY and FLY @ B K-edge
c-BN
h-BN
Si(100)
a-BN
80 nm
Diamond-like
Graphite-like
Amorphous
Cubic BN film grown on Si(100) wafer ? Texture of the h-BN underlayer (film)?
hv TEY FLY
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B K-edge (188 eV)
B fluorescence(<183 eV)
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190 200 210 220
4
3
2
1
b
600
200
400
900
Inte
nsi
ty (
a.
u.)
Photon Energy (eV)
190 200 210 220
4
32
1a
600400200
900
Inth
esi
ty (
a.
u.)
Photon Energy (eV)
normal (90o)
glancing (20o)
TEY
FLY
c-BN
h-BN
Si(100)
a-BN
hv TEY FLY
X.T. Zhou et.al. JMR 2, 147 (2006)
Boron K-edge
*
I (1s - *) minimum
I (1s - *) maximum
*
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sp3 and sp2 hybridized BN
NB
π*
B K-edge
2013 summer school
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3d, 4d and 5d
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Probing d band metals
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d – band 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)
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Transition metal systematic
3d 4d 5d
Filling of the d band across the period
d1
d4
d9
d10
Atom:ns2(n-1)dxn
Ni:4s23d7
Metal:ns1(n-1)dx+1
Ni:4s13d8
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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 d5/2 and d3/2 hole populations
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3d metals
Hsieh et al Phys. Rev. B 57, 15204 (1998)
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3d compounds: TiO2 Nanotube
F.M.F. de Groot, et al., PRB 41, 928 (1990)
J. Zhou et al. J. Mater. Chem, 19, 6804-6809 (2009)
H. Fang et al. Nanotechnology (2009)
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d- band filling across 4d and 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
3p3/2-5d2p3/2-5d
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d - charge redistribution in 4d metals
Ag Pd 4d charge transfer upon alloying
> charge transfer to Pd d bandIn PdAg3
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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
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Au L3
2p3/2 - 5d 5/2,3/2
Au L2
2p1/2 - 5d3/2
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*
* Phys. Rev. B22, 1663 (1980)
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De-excitation spectroscopy I (Auger)Core hole decays via two competing channels• Auger
normal (hv > threshold)Coster- Kronig (hole and e from same shell)Resonant Auger (hv~ threshold)
• Fluorescence X-raynormal (hv > threshold)XES (x-ray emission, valence e) RIXS (resonant inelastic x-ray scattering,
hv ~ threshold), RXES (resonant XES)• Other secondary processes, fragmentation/defect creation, luminescence, etc.
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Auger nomenclature
1s
2s2p1/2
2p3/2
1s (K)
2s (L1)2p1/2
2p3/2L3
1s
2s2p1/2
2p3/2
KL1L3 Auger KL2L3 Auger
In general, Auger involving n = 2 electrons to fill the 1s hole are called KLL Auger electrons ; selection rule: coulombicIn low z elements, Auger is the dominant decay channel; it determines the life time of the corehole; i.e. that the shorter the lifetime, the broader the peak (Uncertainty Principle).
1s (K shell) core hole, normal Auger
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Auger nomenclature
1s
2s2p1/2
2p3/2
1s (K)
2s (L1)2p1/2 L2
2p3/2L3
1s
2s2p1/2
2p3/2
L1L2L3 Auger(Coster-Kronig)
L1L3L3 Auger
Auger process involving 2 electrons from the shell with the same quantum number n are called Coster-Kronig transitions; CK transitions are fast; it leads to short life time of the corehole.
2s (L1) core hole, filled by other L subshell → Coster Kronig
KE (Auger) = BE (1s) - BE (L) - BE(L’) - V (2 hole state)
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Auger in molecular systems
*
*Normal Auger
Resonant Auger
* e: spectator
* e: participates
hv >> I.P.
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Excited electron participates in the decay
Excited electron acts as a spectatorin the decay
Note: corresponding normal Auger has a two hole final state without the excited electron acting as the spectator
Resonant photoemission
Normal Auger
Resonant Auger
Kinetic energy
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How to identify Auger and photoelectrons ?hv is significantly above the thresholdi) Normal Auger electrons (2 hole final state) associated with a core hole have constant KE, ii) Photoelectrons have constant BE regardless of photon energy. In gas phase, BE = hv – KE
hv = resonance energy, Auger Peaks appear at higher kinetic energy (2 hole + spectator electron)
Auger electrons excited at resonance
*
CO
Excitation energy significantly below C 1s threshold, no C -KLL Auger
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Other examples
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Resonant Photoelectron and Auger spectroscopy
**
XPS of VB at * resonance, resonant Auger turns on
XPS of VB below * resonance, no C K-edge Auger
No C K Auger
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Resonance in NXAFS/XANES (bound to bound transitions)
Peak position (E): atomic one-electron energy of the core level modified by chemical environment, to LUMO, LUMO+ transitione.g. oxidation state, electronegativity etc.Peak width ( ): convolution of core hole lifetime (uncertainty principle), and instrumentation resolution, EI
i r f2
x p t E 2
Lifetime of core hole, ; lifetime broadening
E E I2 2
Note: in classical theory, the peak intensity is also referred to as oscillator’s strength
:LWHMLine width at half maximum
Peaks intensity (I): (area under the curve)transition matrix elementand the occupancy densities of states of the final states (E)
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42J Stöhr, NEXAFS Spectroscopy
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Resonance in NXAFS/XANES (bound to quasi bound transitions)
Peak position (E): atomic one-electron energy of the core level modified by chemical environment to MS statee.g. qualitatively to semi-quantitatively, inter-atomic distancePeak width ( ): convolution of core hole lifetime (uncertainty principle), band width, instrumentation resolution, E Peaks intensity (I): (area under the curve)transition matrix elementand the occupancy densities of states of the final states (E)
i r f2
:LWHMLine width at half maximum
, Ef and EI
222If EEE
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Core hole life time and widths (radiative, X-ray and Auger)
fluorescence
Auger
Totalhigh z
low z
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Photo-fragmentation of molecules
• One of the more interesting consequence of core hole decay is the fragmentation of molecules (radiation chemistry, radiation damage)
• Photo-excitation at selected edges and resonances can lead to site specific photo-fragmentation of molecules (photon scalpel)
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Ion and Auger yield of CO on and off resonance
XANES
Auger
Time of flight MS
CO+
C+
O+
CO+
O+
C+
no core hole
C 1s core hole, e in
* orbital
C 1s core hole, e in
Rydberg orbital
C 1s core hole, e in
contiuum
C 1s *
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Photo-fragmentation of CO and (CH3)2CO at C K-edge
Coulomb explosion: Moddeman et
al.
J. Chem. Phys. 55, 2371 (1971)
Eberhardt et al.
Phys. Rev. Lett.
50, 1038 (1983)
Doubly charge fragments are produced in small molecules