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