soft x ray spppyectroscopy for physics (and...
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SESAME-JSPS School
Soft XSoft X‐‐ray Spectroscopyray Spectroscopyy p pyy p pyfor Physics (and Chemistry)for Physics (and Chemistry)
Kenta Kenta AmemiyaAmemiya (KEK(KEK--PFPF))yy (( ))
Studies using Soft XStudies using Soft X--rayraySoft X-ray (& VUV) Beamlines
~14/50 at Photon Factory (2.5 & 6.5 GeV)y ( )~5/50 at Spring-8 (8 GeV)
E i t l T h iExperimental TechniquesX-ray Absorption Spectroscopy (XAS)y y ( )Photoemission SpectroscopyX ray ScatteringX-ray Scattering
Applications:Organic Molecules & PolymersMagnetic Materials (Fe Co Ni )Magnetic Materials (Fe, Co, Ni, …)Surface & Thin Film
Absorption Edges in the Soft XAbsorption Edges in the Soft X--ray Regionray Region
M edgeum
ber
L edge
tom
ic N
u L edge
At
K edge
Li.
Absorption-edge Energy (eV)
Soft XSoft X‐‐ray Absorption Spectroscopyray Absorption Spectroscopy
1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
3. Soft X-ray Beamlines
4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS
Soft XSoft X--ray Absorption Spectroscopy ray Absorption Spectroscopy (~100(~100--4000 4000 eVeV))
1. Element selectivity Core-hole excitation (1s, 2p…)(C: 290 eV O: 530 eV Fe: 710 eV Ni: 850 eV )
10
12
h
arb.
uni
ts)
C K edge
(C: 290 eV, O: 530 eV, Fe: 710 eV, Ni: 850 eV…)
2. Information on chemical species Characteristic spectral features (π*,σ*…)
4
6
8 = 15 o
= 55 o
= 90 o
on Y
ield
(a
C-S
Characteristic spectral features (π ,σ …)
3. Structural information (bond length, etc.)EXAFS (Extended X-ray Absorption Fine Structure)
0
2
4
rtial
Ele
ctro C S
C-C
S ( te ded ay bso pt o e St uctu e)
4. Information on anisotropy Linear polarization
280 290 300 310
Photon Energy (eV)
Par (molecular orientation, lattice anisotropy)
5. Magnetic information Circular polarization Circular polarizationXMCD (X-ray Magnetic Circular Dichroism)
6. High sensitivity
XAS XAS Measurement in the Soft XMeasurement in the Soft X--ray Regionray Region
0 025
3 ML Fe / Cu(100) Fe L-edge XAS How can we measureX-ray absorption spectrum ?
0 02
0.025
units
)
X-rays
2p3/2→3d(LIII)
~mm ~nm
A0 015
0.02
y (a
rb. u
X rays
Auger electrons
Core hole
0.01
0.015
Inte
nsity
Detector
(core hole relaxation)2p1/2→3d
(LII)
0.005sorp
tion
etecto(Retarding voltage)
Electron yield XAS
0700 710 720 730 740 750
Abs
ect o y e d STotal electron yield (TEY)
Partial electron yield (PEY)700 710 720 730 740 750
Photon Energy (eV)Partial electron yield (PEY)
cf. Fluorescence yield (FY)
Advantages and Disadvantages of SXASAdvantages and Disadvantages of SXASSh t P t ti L thShort Penetration Length
Transmission mode can be available only for a very thin sample on avery thin or without substrate.
Electron yield mode is usually adopted because of high efficiency.Special care is necessary for insulators (powders might be OK).
Fluorescence yield efficiency is very small for light elements.y y y g<1 % for C, N, O Be careful for the self absorption (saturation) effect.
Samples should be usually kept in vacuum (NOT ultra-high vacuum).
Surface Sensitive
Samples should be usually kept in vacuum (NOT ultra high vacuum).Some attempts have been made to realize ambient-pressure or liquid-state measurements.
Surface SensitiveSub-monolayer samples can be investigated.
= several nm for electron yield.)B lk information is hardl obtained especiall in the electron ield mode
Sensitive to Electronic and Magnetic States of light elements
Bulk information is hardly obtained, especially in the electron yield mode.( ~ 0.1 μm for fluorescence yield.)
g gValence electrons can be directly investigated by 1s2p excitation ofC, N, O,… and 2p3d excitation of 3d transition metals (Fe, Co, NI,…).
1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
3. Soft X-ray Beamlines
4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS
NearNear--edge Spectroscopyedge SpectroscopyNear edge X ray Absorption Fine Structure (NEXAFS)Near-edge X-ray Absorption Fine Structure (NEXAFS)X-ray Absorption Near-edge Structure (XANES)
Chemical speciesStructural information (orientation)
Determination of Chemical Species
Thiophene (C4H4S) molecule on Au(111)NearNear--edge Spectroscopyedge SpectroscopyDetermination of Chemical Species
Initial oxidation process of SiDipped in thiophene
Molecular O2
Dipped in thiophenesolution
OxidationC6H13SH
Exposed to thiopheneposed to t op e egas in vacuum
Existence of molecular oxygen Different chemical species depending ygin the initial stage of Si oxidation
p p gon preparation processes
Matsui et al., Phys. Rev. Lett. 85, (2000) 630. Sako et al., Chem. Phys. Lett. 413, (2005) 267.
Determination of Atomic StructureDetermination of Atomic StructureE t d d X Ab ti Fi St t (EXAFS)Extended X-ray Absorption Fine Structure (EXAFS)
• Coordination number (N)• Inter-atomic distance (r)• DW factor (σ2)
(k) S 2 NiFi(ki) e2ki2 i
2
e2ri /i (k ) sin(2k r (k ))
EXAFS oscillation
(k) S0 kiri2 e i i e i i
i
sin(2kiri i(ki))
Fe K-edge XAFS spectrum μ(E) of FeOFe K edge XAFS spectrum μ(E) of FeO
Amemiya et al Phys Rev B 59 (1999) 2307Determination of Atomic StructureDetermination of Atomic Structure
Application to surface molecule (CH3O)
CH O/Cu(111)
Amemiya et al., Phys. Rev. B 59, (1999) 2307.
CH3O/Cu(111)
CH3O/Cu(111)
EXAFSNEXAFS
CH3O/Ni(111)
CH3OH (solid)Oscillation period -> O-Cu bond lengthAngle (θ) dependence -> bond angle
adsorption site
Peak B (1s→σ*CO) -> C-O bond length
adsorption site
Peak B (1s→σ CO) C O bond lengthAngle (θ) dependence -> molecular orientation
3 ML F / C (100)
Magnetic structures studied by XMCDMagnetic structures studied by XMCD
0.03s) H li it S i
3 ML Fe / Cu(100)Fe L-edge XMCD X-ray Magnetic Circular Dichroism (XMCD)
0.025
b. u
nits
μ↑↑ (μ+)μ (μ )
Helicity Spin2p3/2→3d
(LIII)Difference in absorption intensities between right- and left-hand circular polarizations
0.015
0.021. Element selectivity
resonant absorptionity (a
rb μ↑↓ (μ-)
2p →3d
0 005
0.01
p2p -> 3d excitation for 3d transition metal3d -> 4f excitation for rare-earth elements
Inte
ns
2p1/2→3d(LII)
0
0.0052. Determination of spin and orbital
magnetic moments Sum rulesor
ptio
n
-0 01
-0.005 Sum rules
3. High sensitivityAbs
o μ↑↓ -μ↑↓
0.01700 710 720 730 740 750
Photon Energy (eV)
XMCD Sum RulesXMCD Sum Rules
Orbital moment
Spin moment
B.T. Thole et al., PRL 68, 1943 (1992).P. Carra et al., PRL 70, 694 (1993).
Utilization of Element Selectivity of XMCDUtilization of Element Selectivity of XMCD
3
4
5
Fe L-edge
b. un
its)
Fe
1
2
3Co L-edge
Inte
nsity
(ar
Co
Fe
680 700 720 740 760 780 800 820 840
0
Photon Energy (eV)
Fe(6 ML)/Co(2 ML)/Cu(001) Fe(5 ML)/Co(2 ML)/Cu(001)
Photon Energy (eV)
Magnetic-field dependence of XMCD at Fe and Co L edges
0.5
1.0
(arb
. uni
ts)
0 0
0.5
1.0
(arb
. uni
ts)
1 0
-0.5
0.0
Inte
nsity
(
Fe Co -1.0
-0.5
0.0
Inte
nsity
(
Fe Co
-30 -20 -10 0 10 20 30
-1.0
H (Oe)-30 -20 -10 0 10 20 30
-1.5
H (Oe)
Angle Dependence of XMCDAngle Dependence of XMCD
0.0
0.5
ity [a
.u.]
0.0
0.5
ity [a
.u.]
0.0
0.5
ity [a
.u.]
-0.5
XM
CD
inte
ns
Grazing Normal
-0.5
XM
CD
inte
ns Grazing Normal
-0.5
XM
CD
inte
ns
Grazing Normal
840 860 880
-1.0
Photon Energy [eV]840 860 880
-1.0
Photon Energy [eV]840 860 880
-1.0
Photon Energy [eV]
Fe (4 ML)Fe (0 7 ML)
Cu(100)Ni (7.5 ML)Fe (4 ML)
Cu(100)Ni (7.5 ML)Fe (0.7 ML)
Cu(100)Ni (7.5 ML)
in plane XMCD reflects magnetic component which is parallel to X-ray beam.
d t i ti f i f ti tiperpendicular
determination of easy axis of magnetization
Information on orbital momentti ti f ti i tin plane estimation of magnetic anisotropy
Abe et al., J. Magn. Magn. Mater. 206 (2006) 86.
1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
3. Soft X-ray Beamlines
4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS
3.1. principle - wavelength dispersion -Diff ti ti P i di b t tDiffraction grating: Periodic grooves on a substrate
1 m
10 nm
Principle: Interference between the rays reflected at different grooves
Enhanced when light path difference = msin + sin = nm n: groove density
m: diffraction order
* ≠
depends on ⇒ Wavelength dispersion(conversion of wavelength to angle)
* ≠ (if = , any satisfies the above condition at m = 0)
⇒ zero-th order light
*
3.2. energy resolution - dispersion and focus -
How can we monochromatize by using a diffraction grating?Most basic mode: collimated-light illuminationg
sin + sin = nm
P bl 1 SR i lli d li h
Collimated whitelight
dispersed
Problem 1: SR is not a collimated light!Problem 2: Superposition of diffracted lights
⇒ difficult to be resolved
light
⇒ difficult to be resolvedSolution 1: Collimation of diverging light with a parabolic mirrorSolution 2: Focusing of diffracted lights with another parabolic mirrorg g p
Focused diffracted lights are well resolved in wavelength
Parabolic mirror(collimation)
Parabolic mirror(focusing) Exit slit well resolved in wavelength
at the exit slit !Plane grating
(wavelength selection)
S Dispersion and Focus (dispersion)Source
3.2. energy resolution - dispersion and focus -
The simplest monochromator
Point source
Exit slit(wavelength selection)
Both the “dispersion” and Point source )
“focus” are achieved by
a diffraction grating only. Concave grating
(dispersion & focusing)
Is that really possible?
It is impossible to obtain a perfect focus at all wavelengthp p g
But, ”perfect focus” is not necessary !
Small number of optical elements
3.3. Monochromator operation
Parabolic mirror(collimation)
Parabolic mirror(focusing) Exit slit
How can we select wavelength?
Plane grating
(wavelength selection)
SourceBy moving exit slit?
(dispersion)Source-> Sample should be moved!
i i d itSimple rotation of grating!
sin + sin = nm n: groove densitym: diffraction order
- = const. (included angle)*
( g )“Constant included angle monochromator”
Variable included angle system Wide energy range
Included angle
Variable included angle system Wide energy range
3.4. Beamline components
Overview of a typical soft X-ray beamline
Pre-focusing optics: focuses X rays onto the entrance slit
Pre-focusing optics Monochromator Post-focusing optics
Pre focusing optics: focuses X rays onto the entrance slitMonochromator: from the entrance slit to the exit slitPost-focusing optics: focuses monochromatized X rays onto sample position
22
Higher-order suppression:utilizes energy dependence of reflectivity (or transmittance)
1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)
2. SXAS studies on Surface and Thin films
3. Soft X-ray Beamlines
4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS
Conventional Technique for Magnetic Depth ProfilingConventional Technique for Magnetic Depth ProfilingSQUID, VSM, MOKE, XMCD…
Gives averaged information over the whole sample.⇒ also averaged in depth
SQUID, VSM, MOKE, XMCD…
3ent
2
netic
Mom
e
0
1
0 1 2 3 4 5 6 7Tota
l Mag
n
Based on an assumptionthat magnetic structure of surface and interface
Film Thickness
Tthat magnetic structure of surface and interface dose not change upon layer growth
Direct technique for depth profiling
XAS XAS Measurement in the Soft XMeasurement in the Soft X--ray Regionray Region
0 025
3 ML Fe / Cu(100) Fe L-edge XAS How can we measure
X-ray absorption spectrum ?
0 02
0.025
units
)
X-rays
y p p2p3/2→3d
(LIII)~mm ~nm
A0 015
0.02
y (a
rb. u
X rays
Auger electrons
Core hole
0.01
0.015
Inte
nsity
Detector
(core hole relaxation)2p1/2→3d
(LII)
0.005sorp
tion
etecto(Retarding voltage)
Electron yield XAS
0700 710 720 730 740 750
Abs
ect o y e d STotal electron yield (TEY)
Partial electron yield (PEY)700 710 720 730 740 750
Photon Energy (eV)Partial electron yield (PEY)
cf. Fluorescence yield (FY)
Principle of DepthPrinciple of Depth--resolved resolved XASXAS
X-raysSurface
X-rays
e-Microchannel plate
Sample
d
e-
p d
e-Auger electrons
XAS data CCD camera
Phosphor screen
e-
XAS data CCD camera
Surface sensitive
Electron yield XAS measurements at different detection angles
A f XAS d i h diff bi d hA set of XAS data with different probing depths
Ni (5.5 ML)OSurface of O/Ni/Cu(100) Surface of O/Ni/Cu(100) ––raw dataraw data--
6
O/Ni(5.5 ML)/Cu(001) Ni L-edge XAS
Cu(100)
(X-ray Absorption Spectrum)
5
6
10
12 (b) e0.6 nm e0.8 nm e1.0 nm
(a)
Surface sensitiveIncrease of L3 peak
4 1.58
rb. u
nits
)
e = 0.6 nm
rb. u
nits
)
2
3
868 870 872
1.0
4
6
ensi
ty (a
r
tens
ity (a
r
Growth of shoulder
1
868 870 872
2In
te
Int
850 860 870 880 890
0
850 860 870 880 890
0 e = 1.0 nm
Ph E ( V)
Decrease of Plateau
Photon Energy (eV)Photon Energy (eV)
K. Amemiya and M. Sakamaki, Appl. Phys. Lett. 98 (2011) 012501.
Extracted XASExtracted XAS
O
Ni (5.5 ML)10
O/Ni(5.5 ML)/Cu(001)Extracted Ni L-edge XAS
) ( )
Cu(100)6
8
1
2
Surface
arb.
uni
ts)
2
4868 870 872
0
nten
sity
(a XAS spectra at surface
shows NiO-like features
4
0
Inner Layers
nits
)In
2
3
ty (a
rb. u
n
NiO thin film
850 860 870 880 890
0
1
Inte
nsit
850 860 870 880 890Photon Energy (eV)
Haverkort et al., Phys. Rev. B 69, 020408(R).
K. Amemiya and M. Sakamaki, Appl Phys Lett 98 (2011) 012501
Extracted XMCD spectraExtracted XMCD spectra
8 Surface
nits
) Ni L-edge XMCD
Appl. Phys. Lett. 98 (2011) 012501.
4
6
ty (a
rb. u
n
O
Ni (5.5 ML)
0
2
Inte
nsit
Cu(100)
Surface layer shows small
negative magnetization.3
4
0
Inner Layers
units
)
g g
1
2
nsity
(arb
.
-1
0Inte
n
840 850 860 870 880 890
Photon Energy (eV)
Depth profile of atomic structureDepth profile of atomic structure
6Co(3 ML)/Ru(0001) Normal Incidence
Layer-resolved EXAFS data
6
1 2
1.3
ts)
4
1.0
1.1
1.2
y (a
rb. u
ni
2 900 1000 1100 1200
Inte
nsity
800 900 1000 1100 1200
0
1st layer (surface) 2nd 3rd
Relaxation of strain
800 900 1000 1100 1200Photon Energy (eV)
Normal incidence: dominated by in-plane distance
Surface shows longer oscillation period: shorter bond length
DepthDepth--resolved EXAFS at grazing incidenceresolved EXAFS at grazing incidence
Grazing Incidence
Longer out-of-plane bond length at surface?
3Grazing Incidence
1st (surface)2nds)
2
2nd 3rd (interface)
rb. u
nits
2
nsity
(ar
Ru
1
Inte
n
800 900 1000 1100Ph t E ( V)Photon Energy (eV)
Preliminary Preliminary analysesanalysesIn plane
0.251 nm
Out of plane
0.260 nm0.251 nm
Ru
0.255 nm 0.252 nm
0.255 nm 0.252 nm
Let’s try soft XLet’s try soft X--ray absorption!!ray absorption!!
You don’t have to be afraid of soft X ray.You don’t have to be afraid of soft X ray.yy