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Soft x-ray spectroscopies: Photoemission and x-ray absorption
Bryan Doyle
S@S 2009, Pretoria, 10th February 2009
Physics at the “surface”
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Plan of talk
o Soft x-rays
o X-ray absorption
o Case 1: carbon nanotubes
o Photoemission
o Case 2: organic molecules on surfaces
o Case 3: YbGaGe and zero thermal expansion
o ARPES – electronic structure in the valence band
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Soft x-rays
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Soft x-rays
• from the ultraviolet (UV) to hard x-rays 20 – 1600 eV
• correspond to many atomic energy levels – K shell of light elements (organics), L shells of transition metals, M shells of rare-earths
• high cross sections for absorption and electronic excitations
• limited penetration depth: 20 – 500 nm
• experiments carried out in high vacuum < 10-6 mbar
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occupied valence band
unoccupied valence band
core level
EF
h
X-ray absorption
if the photon energy is just higher than the binding energy of the core level
bound state
X-ray absorption (NEXAFS/XANES): a brief introduction
2p3/2-3d (LIII)
2p1/2-3d (LII)
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O
NH
NH
O
O
O
NH
O
NH2
NH2
O
O
OOH
OH
OH
OOH
O
O
OOH
OH
OH
OOH
HDA
hOK-edge(530 eV)
hCK-edge(280 eV)
NH2
hNK-edge(400 eV)
Case 1: Functionalization and doping of carbon nanotubes
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Nanotube functionalization: grafting of an amine
525 530 535 540 545 550
inte
nsity
(A. U
.)
Energy (eV)
Pristine
525 530 535 540 545 550
HDA
Oxidized
inte
nsity
(A. U
.)
Energy (eV)
Pristine
525 530 535 540 545 550
Oxidized
inte
nsity
(A. U
.)
Energy (eV)
Pristine
525 530 535 540 545 550
inte
nsity
(A. U
.)
Energy (eV)
Pristine
C--C
=OHD
AC-
-C=O
OH
Oxygen K-edge
525 530 535 540 545 550
Energy (eV)
CH3COOH
*C=O
*C-OH
CH3COCH3
(Acetic acid)
(Acetone)
chemical environment of the oxygen
M.-R. Babaa, J.-L. Bantignies, L. Alvarez, P. Parent, F. Le Normand, M. Gulas, J. Mane Mane, P. Poncharal and B.P. Doyle, J. Nanosci. Nanotechnol. 7 (2007) 3463-3467
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: carbon hybridized sp2
Multiwalled tubes30 % nitrogen
Single walled tubes1 % nitrogen
Substitutional doping of nanotubes: CNx
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h
macroscopic probe
high energy resolution
nanoprobe
spatial resolution
Electron Energy Loss Spectroscopy (EELS)
*he-
*
1s
N K-edge 1s *1s *
NEXAFS e-
Use complementary spectroscopies: EELS and NEXAFS
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High density of nitrogen within the tube cavity
(a)
(b)
(a)
(b)
(c) (d)Carbon
Nitrogen
(c) (d)Carbon
Nitrogen
(d)Carbon
Nitrogen
Energy Loss (eV)
Inte
nsity
(a.u)
Energy Loss (eV)
Inte
nsity
(a.u)
EELS on multiwalled nanotubes
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395 400 405 410 415 420
inte
nsity
(A. U
.)
Energy (eV)
1s*
Multiwalled tubes: NEXAFS spectroscopy
Nitrogen K-edge
?
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395 400 405 410 415 420
inte
nsity
(A. U
.)
Energy (eV)
*
molecular N2
400,0 400,4 400,8 401,2 401,6 402,01,30E-011
1,40E-011
1,50E-011
1,60E-011
1,70E-011
1,80E-011
1,90E-011
Inte
nsity
(A. U
.)
Energy (ev)
MWNT
S. Enouz, J. L. Bantignies, M. R. Babaa, L. Alvarez, P. Parent, F. Le Normand, O. Stéphan, P. Poncharal, A. Loiseau and B.P. Doyle, J. Nanosci. Nanotechnol. 7 (2007) 3524-3527
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Molecular N2 mostly adsorbed on the internal tube walls
N2
The state of molecular N2 in MWNTs
405 410 415 420 425
Abso
rptio
n in
tens
ity (A
. U.)
Energy (eV)
405 410 415 420 425
Abso
rptio
n in
tens
ity (A
. U.)
Energy (eV)
N 1s Rydberg series
Double excitation
Shape resonance
N2
MWNT CNx
405 410 415 420 425
Abso
rptio
n in
tens
ity (A
. U.)
Energy (eV)
405 410 415 420 425
Abso
rptio
n in
tens
ity (A
. U.)
Energy (eV)
N2
MWNT CNx
405 410 415 420 425
Abso
rptio
n in
tens
ity (A
. U.)
Energy (eV)
N2
MWNT CNx
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395 400 405 410 415 420
inte
nsity
(A. U
.)
Energy (eV)
chemical sites are the same but the relative concentrations are very different
Presence of substitutional N verified
Single walled tubes
Multiwalled tubes
Comparison of the N chemical environments in single walled and multiwalled tubes
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Photoemission: a brief introduction
• based on photoelectric effect: photon in – electron out
• other names are photoelectron spectroscopy (PES), XPS, UPS
ANALYSER
h
SAMPLE
MICRO-CHANNEL PLATES
PHOSPHOR SCREEN
CCD CAMERA
EXPERIMENTAL CHAMBER
ELECTROSTATIC LENSES
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occupied valence band
unoccupied valence band
core level
Photoemission: x-rays in electrons out
h
valence band photoemission
EF
core level photoemission
Information obtained is: The binding energy of the electron emitted: • electronic structure (VB)• chemical state (core levels)• different bonding configurations
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But electrons have a much smaller escape depth than photons!
100
10
1
0,1 10
01000
101h (eV)
Mea
n fr
ee p
ath
(nm
)Electron inelastic mean free path
Experiments usually carried out in ultra high vacuum < 10-9 mbar
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O
S
O
nn
1. OFET2. OLED3. Solar Cells4. Antistatic coatings5. Anticorrosion coatings6. “smart windows”7. Sensors8. ...
Interface with metal of great importance in any device
Case 2: 3,4-ethylenedioxythiophene (EDOT) on noble metal surfaces
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thiol or poly-Sthiophene
C(1s) h = 385 eV
S(2p) h = 260 eV
C3C2C1
10 mM aqueous solution
Pt polycrystalline surface20
15
10
5
0
Inte
nsity
(arb
.uni
ts)
294 292 290 288 286 284 282 280 278 276Binding energy (eV)
14
13
12
11
10Inte
nsity
(arb
.uni
ts)
540 536 532 528Binding Energy (eV)
O(1s) h = 630 eV
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6543210in
tens
ity (a
rb.u
nits
)
540 536 532 528binding energy (eV)
10
8
6
4
2
0
inte
nsity
(arb
.uni
ts)
175 170 165 160binding energy (eV)
40
30
20
10
0
inte
nsity
(arb
.uni
ts)
292 290 288 286 284 282 280binding energy (eV)
Au polycrystalline surfaceC 1s
h = 385 eV
S 2p h = 260 eV
thiophene
thiolor
poly-S
atomicS
SOx
10 mM aqueous solution
O1s h = 630 eV
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3.0
2.5
2.0
1.5
1.0
0.5
0.0
inte
nsity
(arb
.uni
ts)
290 288 286 284 282 280binding energy (eV)
4
3
2
1
0
inte
nsity
(arb
.uni
ts)
175 170 165 160binding energy (eV)
Au(111) single crystalC 1s
h = 385 eV
S 2p h = 260 eV
thiophene
thiolor
poly-S
atomicS
NO oxygen!
exposure to EDOT vapours
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X-ray photoemission – conclusions
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Most solids expand upon heating.
This is due to the anharmonic potential formed from the sum of the internuclear forces
However some systems exhibit negative (or zero) thermal expansion: Invar (Fe65Ni35) and some oxides (ZrW2O8, Y2W3O12, CuScO2).
Case 3: Rare earth valence transitions as a mechanism for zero or negative thermal expansion: the case of YbGaGe
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Zero thermal expansion (ZTE) reported in YbGaGe
J.R. Salvador et al., Nature 425 (2003) 702
= -3×10-7 K-1
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2 calculated Yb valences:Yb(1) = 2.6 ; Yb(2) = 2.0
Yb3+
Yb2+
Magnetic susceptibility
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YbGaGe
Yb2+ (r = 1.16 Å), significantly larger than
Yb3+ (r = 1.008 Å)
They proposed temperature dependent electron transfer: between Yb (4f) and Ga (4p)
Density of States
Yb2+ (4f14)
Yb 5d
Yb 4f
Yb2+ (4f13 5d1)
Yb2+ (4f14)
Yb 5d
Yb 4f
Ga 4p
Ener
gyEn
ergy
Yb3+ (4f13)
Density of States
Yb 5d
Yb 4f
Ga 4p
Yb 5d
Yb 4f
increasing temperature
Yb2+ ion
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S. Schmidt et al., PRB 71 (2005) 195110
J.L. Sarrao et al., PRB 54 (1996) 12207
YbInCu4: Resistivity
h = 1486.6 eV
S. Schmidt et al., PRB 71 (2005) 195110
4f electrons in the rare earths reflect the valence
h = 5.95 keV
H. Sato et al., PRL 93 (2004) 246404
Photoemission
L.I. Johansson et al., PRB 21 (1980) 1408
Yb3+
Yb2+
Shows a transition at T = 42 K
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YbGaGeProblem! Other groups cannot replicate the data
Direct spectroscopic probe of valence needed
Normal thermal expansion found:S. Bobev et al., Solid State Commun. 131 (2004) 431
No magnetic transition:Y. Janssen et al., J. Alloys Compd. 389 (2005) 10
Normal thermal expansion found:Y. Muro et al., J. Phys. Soc. Jpn. 73 (2004) 1450
from original ref.
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h = 182 eVon resonance
Yb 4f12 and 4f13 as a function of temperature
no clear change with temperature
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Need to determine parameters for fit
Yb2+: h = 72 eV to increase surface sensitivityYb3+: h = 182 eV to increase intensity
1
4
4
12
13
14131
f
fh I
In 2 hYb nv
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Use high energy photons to be more bulk-sensitive
h = 1421 eV
Electron mean free paths:
35 Å at 1421 eV10 Å at 182 eV
again no clear change with temperature
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YbGaGe conclusions
B.P. Doyle, E. Carleschi, E. Magnano, M. Malvestuto, A. Dee, A.S. Wills, Y. Janssen and P.C. Canfield, Phys. Rev. B 75 (2007) 235109
No valence change detected over temperature range of interest (T: 115 K 316 K: v = 2.29)
Other groups fail to replicate ZTE results (although doping does lower thermal expansion coefficient)
Photoemission is a sensitive tool to measure valence changes (our limit <0.03)
Now are looking at other candidates: Yb2.75C60,Yb8Ge3Sb5
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Angles resolved photoemission (ARPES)
Information obtained:
Which electrons are free to move and how?
€
Ar k ,E,σ( )
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Courtesy: A. Damascelli
ARPES: Widespread impact in complex materials
Diamond
Nature 2005
Nature 2003
Nanotubes
Nature 2000
Quasicrystals
CMR's
Nature 2005Science 2006
Graphene
C60
Science 2003
as well as high TC superconductors
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In conclusion …
• soft x-ray techniques can also be applied to ex-situ samples
• greater bulk sensitivity now available with (1) hard x-ray photoemission < 15 keV (2) very low energies < 15 eV (ARPES only)
• most techniques also available as microscopies
• There are many available soft x-ray beamlines in the world today
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Thanks to …
• L. Pasquali and F. Terzi, University of Modena, Italy • E. Carleschi and E. Magnano, TASC Laboratory, Italy
• M. Malvestuto, Elettra, Italy
• J.-L.Bantignies and L. Alvarez, Université Montpellier II, France
• A.S. Wills and A. Dee, University College London, UK
• Y. Janssen, Brookhaven National Laboratory, USA
• P.C. Canfield, Ames Laboratory and Iowa State University, USA
and to all of you for listening!