High Resolution Soft X-Ray Emission and Resonant Inelastic ... · • Synchrotron-radiation excited...
Transcript of High Resolution Soft X-Ray Emission and Resonant Inelastic ... · • Synchrotron-radiation excited...
High Resolution Soft X-Ray Emission
and Resonant Inelastic X-Ray Scattering:
New Probes of Electronic Structure in
Complex Materials.
Kevin E. Smith
Department of Physics
Department of Chemistry
Boston University
NSLS-2 Workshop, BNL; March 15, 2004
Novel Materials LaboratoryDepartment of Physics
BOSTONUNIVERSITY
Acknowledgements
• Cormac McGuinness, Per-Anders Glans, James Downes, TimothyLearmonth and Yufeng Zhang< Department of Physics, Boston University;
• Steven D. Hulbert< National Synchrotron Light Source, Brookhaven National Laboratory;
• Martha Greenblatt and William McCarroll,< Department of Chemistry, Rutgers University
• J.M. Honig< Department of Chemistry, Purdue University
• Russel Egdell, < Inorganic Chemistry Laboratory, Oxford University.
• Ted Moustakas,< Electrical and Computer Engineering, Boston University
• Joseph Nordgren, Laurent Duda, and Jinghua Guo< Department of Physics, Uppsala University
• Research supported by DOE, NSF, ARO, and PRF
• Experiments performed at the NSLS, the ALS, MAX-Lab and HASYLAB
Outline
• Limitations of Photoemission Spectroscopy
• Soft X-Ray Emission Spectroscopy< Example: Thin Film Nitride Alloys
• Resonant Inelastic X-Ray Scattering< Example: Cr-doped V2O3
• Resonant Soft X-Ray Emission< Example: CuPc thin films
• Future Directions
SXE/RIXS can measure1) the local, chemically and site resolved, partial density of states, 2) shallow core level hybridization, and 3) electronic excitations across a gap or E
F,
for materials that are:1) non-crystalline2) insulating3) in applied electric or magnetic fields4) in non-vacuum environments5) have ill-defined surfaces.
The Bottom Line:
Photoemission Spectroscopy
h<
Valence
Band
Conduction
Band
e-
h<
e-
Core
Levels
‚ Measure the kinetic energy of emittedelectrons - get binding energy
‚ Angle resolved photoemission (ARP)measures momentum of emitted electrons -get band dispersion
‚ Angle integrated photoemission (AIP)integrates momentum of emitted electrons-get joint density of states
‚ X-ray photoemission spectroscopy (XPS)-get core level binding energies
‚ Highly surface sensitive (- 5 6 50 Å) -need single crystals, and atomically cleansurfaces
‚ UHV required
‚ Inapplicable to insulating samples
‚ Inapplicable in applied electric ormagnetic fields
Soft X-Ray Emission Spectroscopy
‚ h< = 50 6 1000 eV
‚ Bulk sensitive (- 1000 Å) Y no need for large crystals, clean ordered surfaces
‚ Atomic and chemical specific
‚ Dipole selection rules Y measure occupied PDOS; hybridization
‚ Operational in external fields and (windowed) liquid environments
Valence
Band
Conduction
Band
Core
Levels
h<N
e-
Photon In Photon Out
h<
Photon Energy Photon Energy
Soft X-Ray Emission Spectrometer
! Rowland circle instrument
! Detector = 5 stacked microchannel plates + resistiveanode encoder
! Gratings cover energy range 50 eV - 1000 eV
! high resolution (100's of meV @ hν = 500 eV)
! Compact and “portable”
! UHV compatible
Rowland circles
Diffraction gratings
Detector
Aperture
Slit Sample
SynchrotronRadiation
The Boston University
High Resolution Photoemission and
Soft X-Ray Emission Spectrometer SystemFunded by the U.S. ArmyResearch Office, the Fritz
Haber Institute, andBoston University.
Stationed at the NationalSynchrotron Light Source,
Brookhaven NationalLaboratory.
Sample preparation chamber: pumped witha 360 l/s turbo pump, titanium sublimationpump, and cryoshield. Features a LowEnergy Electron Diffraction optics, CMAAuger spectrometer, multiple metalevaporators and gas dosing system.
Sample manipulator, with liquid heliumcooling, electron beam heating, 5 degrees offreedom for sample motion, sample transferand load lock.
Spectrometer Level: double µ metal linedchamber, housing 100 mm Scientaelectron analyzer, and soft x-ray emissionspectrometer
Pumping level for Spectrometer Chamber:400 l/s ion pump, titanium sublimationpump, cryoshield
3 m
Wurtzite GaN: Crystal Stuctureand Bulk Brillouin Zone
< Wurtzite GaN has a hexagonal crystal structure< Lattice parameters: a = 3.18 Å, c = 5.185 Å< Band gaps< GaN: 3.4 eV; < AlN: 6.2 eV< InN: 1.8 eV,
< Bulk and surface Brillouin zones are hexagonal
HH
A
L
Γ
A
K
K
M
K
L
M
H
c
<a
N 2p PDOS in GaN
Ga PDOS in GaN
N 2p PDOS for AlxGa
1-xN
-6 -4 -2 0 2 4-8
Binding Energy (eV) relative to AlN VBM
X
0
0.1
0.25
0.5
0.7
1.0
Elementally resolved motion of the valence band maximum in Al
xGa
1-xN
0.80 0.4 0.6 1.00.2
0.4
0.6
0.8
1.0
1.2
1.4
0.2
0
Al concentration (x) in AlxGa1-xN
0.0 0.1 0.2 0.3
2.0
2.5
3.0
3.5
InxGa
1-xN
Ban
d G
ap (
eV)
Indium content (x)
0.0 0.2 0.4 0.6 0.8 1.01.0
2.0
3.0
4.0
Elementally resolved band gapevolution in InxGa1-xN
N 2p - Ga 3d hybridization in AlxGa
1-xN
Binding Energy (eV)
Resonant Inelastic X-Ray Scattering
Photon Energy Photon Energy
ValenceBand
ConductionBand
CoreLevel
hνN
h<
Eex
= hν - hνN
Schematic Electronic Structure for V2O
3
O 2p
V 2p1/2
O 1s
V 3d
V 2p3/2
522.8
EF
530.9
515.9
6 eV
3 eV
O 2p hybrid states
V 3d hybrid states
500 510 520 530 540
510
515
520
525
530
535
540
545
SXE/RIXS from
1.5% Cr-doped V2O
3
at room temperature
Intensity
Exc
itat
ion
ener
gy (eV
)
Emission energy (eV)
ElasticScattering
dd
V 3d PDOSmanifold
CT
O 2pPDOS
490 495 500 505 510 515 520
Photon Energy (eV)490 495 500 505 510 515 520
Intensity
Photon Energy (eV)
RIXS from 1.5% Cr-doped V2O
3
Paramagnetic insulator
Metal
Antiferromagnetic insulator
hν = 514 eV hν = 511.5 eV
Elastic
d-d
CT
Valence
Band
Conduction
Band
Core
Levels
hνN
h<
Valence
Band
Conduction
Band
Core
Levels
hνN
e-
h<
RSXESXE
Resonant and Non-Resonant Soft X-Ray Emission
• TDATA - hole transport layer inOLED devices
• QAD - dopant/emitter in OLEDs
• CuPc - electron transport inOLEDs
ORGANIC SEMICONDUCTORS
260 270 280 290 300
Inte
nsity
Photon Energy (eV)
C Kα XES
hν = 294.0 eV
Translated
Stationary
SXE from CuPc - Translated vs.Stationary samples
260 270 280 290
285 290 295
hgfe
dc
b
a
Intensity
Photon Energy (eV)
294.0 eV
290.3 eV
288.1 eV
286.9 eV
286.3 eV
285.9 eV
285.3 eV
284.6 eV
C Kα RXESNEXAFS
viii
vii
vi
v
iv
iii
ii
i
Photon Energy (eV)
In
ten
sity
! Presence of two C 1s levels is masked in NEXAFS, but visible in RXES
Copper phthalocyanine RXES
ii
iii iv
v viviii
i
vii
286
285
284
283
282
6
5
4
3
2
1
(Ef)
pyrole C 1s
ring C 1s
CuPc
b2u
a2u
b2g
1eg
a1u
b1g
2eg
b1u
Bin
din
g E
nerg
y (
eV
)280 285 290 295
hνexcite
284.6 eV
285.3 eV
285.9 eV
286.3 eV
286.9 eV
288.1 eV
290.3 eV
294.0 eV
Intensity
Photon Energy (eV)
284 286 288
CuPc - RXES detail
β
γ
ε
α
δ
ii
iii
iv
v
vi
vii
viii
i
a1u
b1g
370 380 390 400 410
3 9 5 4 0 0 4 0 5 4 1 0 4 1 5
edcb
a
Inte
nsity
P h o to n E n e rg y (e V )
413.6 eV
403.8 eV
401.8 eV
400.5 eV
399.6 eV
NEXAFS N Kα RXES
e
d
c
b
a
Intensity
Photon Energy (eV)
Copper phthalocyanine RXES
-25 -20 -15 -10 -5 0 5
0 7 14
N
C
Intensity
Energy Relative to HOMO (eV)
Energy Relative to HOMO state(eV)
N RXES
C RXES
UPS
-5 0
Energy Relative to HOMO state(eV)
CuPc RXES / UPS comparison
! RXES represents 2p character of C and N states of the valence band
! UPS represents joint or total density of states of the valence band
UPS from Chasse et al, J. Appl.Phys. 1999, 85(9), p6589
Conclusions
• Synchrotron-radiation excited soft x-ray emission(SXE) is a new, versatile probe of electronicstructure in complex materials wherephotoemission is inapplicable.< Organic solids< Insulating or poorly conducting materials< Powdered or nanostructured samples< Materials in liquid suspension < Samples in applied electrical or magnetic fields.
• Resonant inelastic x-ray scattering (RIXS) allowsdirect measurement of low energy excitationsunder the same circumstances as SXE
• Signal strength for RIXS experiments is asignificant limitation, indicating a need for newspectrometer designs.......
Next Generation RIXS Spectrometer
{
ParabolicMirror
Sample
PlaneGrating
Detector
ParabolicMirror
1.5 m
• High transmission
• Fixed detector
• Design resolution <10 meV @ 100 eV photon energy