Photoemission (I)Spectroscopy
Cheiron School 2011
October 2, 2011 SPring-8, Japan

Ku-Ding Tsuei
tsuei@nsrrc.org.tw
National Synchrotron Radiation Research Center

Outline
1. What is photoemission spectroscopy?
2. Fundamental aspects of photoemission.
3. Examples.
4. Increase bulk sensitivity: HAXPES.
5. Challenging future directions.
Reference books:
1. "Photoelectron Spectroscopy" 3rd Ed. by S. Hufner,
Springer-Verlag 2003
2. "Angle-Resolved Photoemission: Theory and Current
Applications", S. D. Kevan, ed., Amsterdam; Elsevier 1992
3. "Very High Resolution Photoelectron Spectroscopy“
Ed. by S. Hufner, Springer 2007

What is photoemission?

Photon in –> electron out (emission)What are the samples and probed states?
Atoms
Molecules
Nanoprticles
Solidsatomic orbitals (states)
molecular orbitals
core level states (atomic like)valence bands/statescore level states (atomic like)valence bands
core level states (atomic like)What is photoemission spectroscopy?
(photoelectron spectroscopy) (PES)Electron energy analyzer

hv

Monochromatized
photonse-h+sampleInitial state: ground (neutral) state

Final state: hole (excited) state

Conservation of energy

N(Ek)Ek = hv + Ei – Ef (most general expression)
Ek : photoelectron kinetic energy
Ei (N) : total initial state system energy
Ef (N-1): total final state system energy

Ek
Energy Distribution Curve (EDC)(Spectrum)

Single particle description of energy levels(Density of States) (most convenient in PE)
Na atom

Na metal
E

3p
3s

EF
Fermi level√E (nearly free electron like)
Valence (sp)
Band
(DOS)

2p3/2
2p1/2

2s

Core levels

1s
N(E)
(DOS)

Energetics in PES
Ek = hv – EB - 
Conservation of energy
Ev : vacuum (energy) level
EF : Fermi (energy) level
 = Ev – EF : work function
E0 : bottom of valence band
V0 = Ev – E0 : inner potential
Ekmax marks EF in spectra
EB measured relative to EF = 0
Hufner, Damascelli

Usually fixed photon energy
scanning not neededAn XPS Energy Distribution Curve (EDC) Most spectra expressed
this way
HufnerLight sources and terminology
Ultraviolet Photoemission Spectroscopy (UPS)UV He lamp (21.2 eV, 40.8 eV)
valence band PE, direct electronic state info

X-ray Photoemission Spectroscopy (XPS)(Electron Spectroscopy for Chemical Analysis) (ESCA)
x-ray gun (Al: 1486.6 eV, Mg: 1253.6 eV)core level PE, indirect electronic state info
chemical analysis

Synchrotron radiation:
continuous tunable wavelength
valence band: <100 eV, maybe up to several keV
core level: 80-1000 eV, maybe up to several keV
depending on core level binding energies

Inelastic Electron Mean Free Path (IMFP)
I(d) = Io e-d/(E)(E): IMFP depending on kinetic energy relative to EF
SR (VUV,SX,HX)
Univeral curve
XPS
UPS

Hufner

Minimum due to electron-electron scattering, mainly plasmons

PE is a surface sensitive technique! (requires UHV)High energy photoemission: several keV to increase bulk sensitivity

Core level binding energies arecharacteristic of each orbital ofeach element
Finger prints
Core level BE independent ofphoton energy usedCore level photoemission: chemical analysis of elements
ESCA (XPS)
hv = Mg K = 1253.6 eV

Synchrotron hv = 160 eV

Different photon energy different relative cross section
for various core levels 
Relative intensity changes
with photon energy
PJW, NSRRC

surface component(b)

Cd 3d5/2

Photoemission Intensity (a. u.)

Photoemission Intensity (a. u.)(a)Bulk CdSe

Dm = 42 A

Dm = 32 A

Dm = 18 A
410

409

408

Se 3d

Bulk CdSe

Dm = 42 A

Dm = 32 A

Dm = 18 A
407

406

405

404

403

59

58

Binding Energy (eV)Intensity (a. u.)

Dm = 42 A
Dm = 32 A
Dm = 18 A

8

6

4

57

56

55

54

Binding Energy (eV)

Bulk CdSe

10

bulk component

2

Binding Energy (eV)

0

-2

53

52

Surface core level shift(chemical and/or
environmental)
A case study of IMFP
applied to PE of CdSenano particles withtunable SR
How to choose photon
energies for valenceand different core levels
with the max surface
sensitivity?

Actual choices:
Cd 3d5/2 :Se 3d:
Valence band:

480 eV
120 eV
50 eV

Ek ~ 45-74 eV, most surface sensitive
Wu, PRB 2007 NSRRC

Core level photoemission:
chemical shift
higher oxidation state=> higher BEBEhigher emission angle
 more surface sensitive
Pi, SS 2001 NSRRC(IMFP)

Auger Electron Spectroscopy
Core electron ionized by photons
or high energy electrons
Non-radiative core hole decay
 Auger electron emission
Radiative decay Fluorescent x-ray emission

Comparison between PES
and AESPES: constant BE, Ek shift
with changing photon energy
AES: constant Ek, apparent
BE shift with changing
photon energy(synchrotron)

Photoemission Process

Explicitly responsible for IMFP

Implicitly responsible for IMFP

Hufner, Damascelli

Conceptually intuitive,
Simple calculation works

Rigorous,
requires sophisticated calculation

Schematic wave functions of initial and final states(valence band initial states)

Bulk band gap

Bulk band gap(a) Surface resonance(b) Surface state(c) Bulk Bloch state(d) Surface resonance(e) in-gap evanescent state(f) Bulk Bloch final stateElectron kinetic energy inside and outside of solids
Inner potential: EV – E0

Concept of inner potential is used to deduce 3D band structure from PE dataassuming free electron like final state inside solids

Angle Resolved Photoemission Spectroscopy (ARPES)zxxz plane: crystal symmetry plane

Electron emission angle: өPhoton incident angle: ψ, s- and p-polarization

Conservation of linear momentum parallel to the surface

2mk // 
Ek  sin 2k // ( Å -1 )  0.5123 Ek ( eV )  sin k||(inside) = k ||(outside)
Conservation of liner momentum
Important for 3D and 2D band mappingk┴ (inside) ≠ k ┴ (outside) because of inner potential
Ultimately to deduce band dispersion E(k┴) or E(k┴,k||)Normal emission:  = 0, or k|| = 0, most used detection geometryBand Mapping (3D) E(k┴, k||=0)k┴Pilo, Damarscelli

1st B.Z.

Vertical transition (using visible, uv and soft x-rays) at normal emission
For hard x-ray photon momentum cannot be neglected
Using different hv at normal emission to map out E(k┴)

Bulk band structure andFermi surfacesCu

Fermi surfaces:
Electron pockets and hole pockets
Related toHall coefficientElectric conductivity
Magnetic susceptibility

Gap below EF(=0) at L-point(nearly free electron like) sp-band
Small dispersion d-band
more localized state
Large dispersion sp-band
extended state

Dispersion of a band can tell how localized
or extended a state is in a solid

L
(111)



X
(001)

HufnerQuantum well states: manifestation of particle in a box
in real materials
Ag(111) thin films expitaxiallygrown on Au(111) substrate
Au E Ag vacuum
EFBand gap
below EFLAg QWS canexist within
Au gapz
Quantized discretely along z-direction
Energy levels depend on film thickness L
Nearly free electron like in xy-plane

Bulk projected bands along Lof Au and Ag, respectively

 Ag S.S.
2D
Int.
plots

Ag QWSk|| = 0EDCs

Luh et al.
PRL 2008
NSRRC
BL21B1

Clean Au(111)
surface state

Deposit 22 ML Ag
at 37 K
disordered form

Anneal to 258 K
Atomically flat
22 ML thin film

Anneal to 180 K
QWS appear
minimal flat
dispersion
Small localized
domains within
xy-planeAnneal to 189 K
Coexistence of
two kinds ofdispersion
Anneal to 258 K
Well developed
dispersion
Large, good
crystalline
domainsin xy-plane

Proposed growth model
Annealing Temp

Same QWS energies Same crystalline film
thickness along z
even though lateralcrystalline domains
grow from small tolargeOne-particle spectral function near EF measured by
ARPES with many-particle correction (quasi-particle)
" (k ,  )A( k ,  )   [   k  ' (k ,  )]2  [" ( k ,  )]2
1

k : single particle energy without many-particle correction
= 0 : EF

Self energy correction due to interaction with phonons, plasmonsand electrons, etc.

( k ,  )  ' ( k ,  )  i" ( k ,  )Real part: shift observed peak energy from single particle energy
Imaginary part: peak FWHM = 2 ”Peak position
Featurelesssingle particle
dispersion
curve

Kink ~25 meV due to electron-phonon scattering

Peak position – single particle curve

Width due toelectron-electron
scattering ~ 2
Width due toelectron-phonon
scattering
Const bkg width
due to impurities

Total W=We-e+We-ph+Wim

ARPES for valence band PE uses primarily VUV light because
1. Better absolute photon energy resolution for most BLs designed
as const E/E.
2. Better photoionization cross section at low photon energy.
3. Better momentum resolution for a given angular resolution.
k|| = 0.5123 √(Ek) cos() 
SX ARPES has been tried for increasing bulk sensitivity, more
free electron like final states and reduced matrix element effects.
The increasing bulk sensitivity will be discussed.

NSRRC U9 BL21B1 BL andhigh resolution photoemission end station

U9
undulator

CGM

Scienta
SES 200
analyzer

Hemispherical electron energy analyzer
R1 : radius of inner sphereR2 : radius of outer sphere
Ro=(R1+R2)/2 : mean radius
and along electron path
V1: inner potential
V2: outer potential
Ep: pass energy = electron
kinetic energy along mean
radiusResonance photoemission(near-edge absorption followed by Auger likeelectron emission)e.g. Ce3+ (4f1)
4f mixed with other DOS

e-

e4f

intermediate
state
EF

4f

Intensity enhanced by absorption
Predominantly 4f DOS

4f

4f

4f

4f
Absorption +

3d/4d

3d/4d
Direct PE

3d/4d
Resonance PE

EF
Auger likeemission

Comments on photoelectron IMFP
Valence band PE using VUV and SX has IMFP near minimum,very surface sensitive. It is great to probe surface electronic structuresuch as surface states and surface resonances.
Many strongly correlated systems have electronic structure sensitive tocoordination, thus surface contains different electronic structure fromthat of deeper bulk. Great surface sensitivity posts a serious problem toprobe true bulk properties.
Need larger IMFP by using higher energy photons to enhance bulk sensitivity.

Drive to go to even higher photon energies into
hard x-ray regime

HArd X-ray PhotoEmission Spectroscopy(HAXPES)HAXPES not only reach even closer to true bulk properties ofstrongly correlated systems, but also becomes capable of probinginterface electronic structure, Very difficult using conventional VUV/SX.

Nature 2000

Heavy Fermion
Kondo systems
Mixed valence Ce f0 & f1
E ~ 100 meVLess surf. sensitive

More surf. sensitive

Drastically diff. electronic structure btn. surface and bulk: f0 vs. f1
No Kondo peak in an itinerant 4f system of CeRu2

bulk

5950 eV, 75 A
800 eV, 15 A
43 eV, 5 A

surface

Bulk sensitive HAXPES can determine sharp first order valence band transition

HAXPES example: Hard x-ray photoemission on
Si-high k insulator buried interface
Kobayashi, APL 2003 SPring-8

Annealed sample
HfSix formation
hv = 6 keV, E ~ 0.24 eV
Take-off angle dependence => non-destructive depth profileCan probe buried interface at 35 nm ! (achievable only by hard x-ray PE)

NSRRC HAXPES projectat SPring-8

Why Hard X-rays?
Electron IMFP (probing depth) and Cross section
2

Inelastic Mean Free Path (A)

Au
SiO2

HAXPES

100
7
6
5
4

XPS

3
2

10

UPS

7
6
5
5 6 7

2

10

2

3

4 5 6 73

10
Kinetic Energy (eV)23

4 5 6 74

10

SPring-8

Higher Ek for deeper probing depth or more bulk sensitivity, for strongly
correlated systems and interface properties
Photoemission signal () decreases rapidly > 1 keV
Need photon source of higher flux/brightness (modern SR),efficient BL design and good electron analyzers

HAXPES is a low count rate, photon hungry experiment!
(except at a grazing incident angle)

A serious issue on going to hard x-rays

Yeh,1992
Trzhaskovskaya et al., ADNDT 2001,2006

Cross sections of 3d TM s-orbitals go down more slowly than d-orbitals
which are the needed information on 3d TM strongly correlated electron
systems. Hard x-ray PE spectra could be dominated by contribution fromless desired s-orbitals
How to cope with this problem?

Unexpected lineshapes in HAXPES compared to XPS
Valence band of simple oxides – e.g. Cu2O and ZnO
band-insulators, no electron-correlation effects – LDA should do (+Udd+Upp)
ZnO – valence band

Cu2O – valence bandXPS
1.5 keVXPS
1.5 keVLDA+U simulation

O-2p

Intensity (arb. units)

Intensity (arb. units)

LDA+U simulation

Cu-3d

HAXPES
7.7 keV

O-2p

Zn-3d

HAXPES
7.7 keVLDA+U simulation

LDA+U simulation

Zn-4s (LDA)(LDA) Cu-4s

18 16 14 12 10

8

6

4

Binding Energy (eV)

2

0

-2

18 16 14 12 10

8

6

4

2

Binding Energy (eV)

HAXPES: TM-4s overwhelms TM-3d and O-2p

0-2

Tjeng et al.

Polarization dependent cross sections in HAXPESHow to suppress the 4s spectral weight?
• photo-ionization cross-section depends on
e- emission direction and light polarization• make use of -asymmetry parameter+ ... ]
-parameters @ hv= 5-10 keV
Cu 3d 0.48 - 0.32
Cu 4s 1.985
Zn 3d 0.50 - 0.33
Zn 4s 1.987 - 1.986
In general: s orbitals have  :• intensity is enhanced for e- emission || E-vector• intensity vanishes for e- emission  E-vector

photoelectron emissionpolarization E vector

 choose suitable experimental geometry !

HAXPES Commissioning: Horizontal vs Vertical geometries1d
 1    (3 cos2   1)2dpolarization

horizontal
=0o

: angle between electron emission andpolarization vector
: electron emission asymmetry parameterverticalo
~2 for s-orbitals, strong emission near =0 (horiz.) =90owhile suppressed near =90o (vertical);Tjeng
can be used to distinguishet al.
s-orbital, important in chemical bonding, andd-orbital, important in strongly correlated systems
Zn 4s has relatively larger cross section than 3d at 7.6 keVcompared to 1.486 keV, enhanced in horizontal geometry at
7.6 keV, while suppressed in vertical geometry

Optical design concept

Diamond (111) reflection
6-12 keV

DM: horizontal dispersion

HRM: vertical dispersion

6-12 keV using
diamond (111) reflection
Designed for HAXPES

Layout of the side beamline of BL12XU

BL12XU Sideline

Diamond Mono.

MBS Analyzer
HRM

KB
Platform

Horizontal geometry

Vertical geometry

HAXPES Example 1:
NiO at RT and high temp.
Interpretation of XPS valence spectra of NiOagainst argument of surface effectFirst ionization (photohole final) valence state
identified as Ni2+ low spin state
Indication of non-local screening in valence bandof bulk NiO compared to impurity NiOPeaks splitting due to non-local screening invalence and Ni 2p core level diminishes astemp. approaches TN=523 K

NiO: a prototypical strongly correlated electron systemA
B

NSRRC Dragon BL
NiO impurity in MgO

single site

Peak B is a true bulk feature

Peak B is absent in single NiO6 cluster

Implications:1. First ionization state is 2E (compensated spin, (photo)hole in the mixed state made ofeg (d7) and O 2p (d8L) (ZR‐doublet), instead of 4T1 (atomic‐like Hund’s rule high spin, d7,
quasi‐core) as previously suggested.
2. Peak B due to non‐local (neighboring sites) effect.

NiO above Neel temperature at 523 K
725K
675K
625K
575K
550K
525K
500K
475K
450K
425K
400K
375K

Intensity (arb. units)

HAXPES h=6500 eV
Ni 2p

Intensity (arb. units)

How important is long range AF ordering?
HAXPESh=6.5keV
300K
525K
300Kafter 525K

14

900

890

880 870 860 850
Binding Energy (eV)

840

12

10

8 6 4 2
Binding Energy (eV)0-2

• Ni 2p3/2 splitting due to non-local screening mechanism(Veenendaal and Sawartzky PRL1993)• Splitting goes smaller with increasing temp.
• Valence band doublet structure also changes w/ temp.
(Why need bulk sensitive HAXPES? Because O decomposes leaving surf. at high T)

HAXPES Example 2:
Interface of LAO/STO
Interface of two band insulators LaAlO3 andSrTiO3 becomes metallic-like. Evidenceof charge transfer from LAO to STO isobserved but the amount is less than
prediction of simplest modelpolar catastrophe
LaAlO3

SrTiO3

Polarity discontinuity between
LAO and STO plays a crucial role.
Electronic reconstruction
1/2
1Charge transfer balances thepolar discontinuity and leads to
conducting behavior of theinterface.

½ e transferred Nakagawa et al., Nature Mater. 5, 204 (2006)

Hard X‐ray Photoemission Spectroscopy (HAXPES)Sing et al.,PRL 102,176805 (2009)hv = 3 keV
 Conducting interface due to electronic reconstruction
 2DEG confined to only ~ 1 or at most a few u.c. thicktotal Ti3+ density < 0.28 e /2D u.c. for 5 u.c. LAO (<0.5 e); sample dependentOur approach:* grazing incidence near total external reflection
to enhance photon field near the surface and interface
region for better detection of Ti3+ near the interface* higher photon energy (6.5 keV) to increase probing depthhv = 6.5 keV
5 UC (2nm) LAOSTOLaAlO3/SrTiO3 (001)(AlO2)

30

0grown with PLD, 10-5 torr O2, 850 C
annealed at 100 mtorr of O2
1200

Ti3+?

conducting interfaces
Heating up
Cooling down

1000
Resistance (Ohm)

5 u.c.
LaAlO3

substrate(TiO2 terminated)

800
600

SrTiO3

400
200
0
50

100

150
Temp (K)

200

250

300

d1

LaAlO3
Ti3+/Ti4+

d2 (2DEG)(Ti4+)SrTiO3
1.6

60
150
200

0.4

100

468

220 14
200

Ti
2p1/2

466

462

460

458

110

3

0.885

TH = 4.5 degree

100

80

0.6

458

0.4

457

456

455

BE (eV)

0.2

Ti
2p3/2

464

1.090 Pure STO

12
10

Ti3+

100 20

0.2

16

180 8
459

40

120

18

240

6

0.6

260 20x10

3200 80Pure STO
TH = 4.5 degree

x103
x103
x10

300

5uc LAO/STO
TH = 0.32 degree
TH = 0.62 degree95
TH = 1.12 degree

1.2
3

100

100

130

22

x10

0.8

280x10

3

250

1.4

3

Ti4+

x10

5uc LAO/STO
TH = 0.32 degree
TH = 0.62 degree
TH = 1.12 degree

400
120

Intensity (arb. units)

1.0x10300 140x10

Intensity (arb. units)6456

BE (eV)

Measure intensity ratio Ti3+/Ti4+ as a function of incident angle

LaAlO3
Ti3+/Ti4+

d1
d2 (2DEG)(Ti4+)SrTiO3
2.2

d2 = 48.3 ± 20.3 A~ 12 u.c.

-6

14x10

12

2.0

 = 0.021(Ti3+/T4+ in d2)

8
6
4

1.8

2
0

20

40

3+

Ti /Ti

60

80

100

e

4+

ratio (%)





10

Total carriers~ 0.24 e / 2D u.c.

1.6

3+

4+

Ti /Ti
fit

1.4

ratio

2
e= 42.6± 15.8 A, d= 48.3 ± 20.3 A, p=0.021± 0.001,  = 2.51e-06

1.2
0.0

0.5

1.0

1.5

Incidence angle (Deg.)

2.0

Consistent withelectronic
reconstruction but
only half the amountO vacancies?

Challenging future directions of
Photoemission Spectroscopy
1. ARPES at submicron to tens of nanometer scale,using Schwatzchild optics or zone plates.
Need brighter light sources.
2. Time-resolved PES.
Pump-probe: dynamics.
Need efficient detection and brighter sources.
lasers or laser+SR.

Thanks for your attention