Photoemission
Spectroscopy
Cheiron School 2014
September 29, 2014 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

In 1905 Albert Einstein proposed the concept oflight quanta (photons) to explain the photoelectriceffect, which was pivotal in establishing the quantum
theory in physics
In 1921 he was awarded the Nobel Prize in Physics“for his services to theoretical physics, andespecially for his discovery of the law of thephotoelectric effect”

Since the late 1940’s Kai Siegbahn has been working
on the Electron Spectroscopy for Chemical Analysis(ESCA) also termed the X-ray Photoelectron
Spectroscopy (XPS)In 1981 he was awarded the Nobel Prize in Physics“for his contribution to the development ofhigh-resolution electron spectroscopy”

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)

What are the samples and probed states?
Atoms
Molecules
Nanoparticles
Solidsatomic orbitals (states)
molecular orbitals
core level states (atomic like)valence bands/statescore level states (atomic like)valence bands
core level states (atomic like)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
Conservation of energy
Density ofStates(DOS)

Ek = hv – EB – φ
Ev : vacuum (energy) level
EF : Fermi (energy) level
φ = 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
Energy Distribution Curve (EDC)

Hufner, Damascelli
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/λ(E)λ(E): IMFP depending on KINETIC ENERGY inside solid or relative to EF
SR (VUV,SX,HX)
Universal 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

X-Ray Data Booklet
http://xdb.lbl.gov/

Core level photoemission: chemical analysis of elements
ESCA (XPS)
hv = Mg Kα = 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

Se 3d

Bulk CdSe

Dm = 42 A

Dm = 32 A

Dm = 18 A

410 409 408 407 406 405 404 403

59

58

Binding Energy (eV)57

Intensity (a. u.)

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

8

6

4

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(IMFP)Pi, SS 2001 NSRRC

Chemical Analysis of C1s core levels

http://surfaceanalysis.group.shef.ac.uk

Auger Electron Spectroscopy
Core electron ionized byphotons 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
energyhttp://xdb.lbl.gov/(synchrotron)
Handbook of XPS

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 (ARPES)
Conservation of linear momentum
parallel to the surface

=
k
z
θ

x

2m
Ek ⋅ sin θ2k||(inside solid) = k||(outside in vacuum)k┴ is not conserved, obtained bychanging photon energy

xz plane: crystal symmetry plane

Electron emission angle ө with respect to the crystalline
surface normal and symmetry planes is also measured⇒ Electronic band dispersion E(k||, k┴)inside (ordered) crystalline solids

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)

Γ

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 ΓLof 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Surface state of topological insulators
Insulating bulk but with conductive
Bi2Se3 single crystal surface
surface by strongly dispersive surface
kF
statesS
Electron occupation ina surface band can be
determined by kF:

SB

2*π*kF2/(2D B.Z. area)
2 : spin

Luo et al., Adv. Optical Mater. 1, 804 (2013)
Luo et al., Nano Lett. 13, 5797 (2013)

BOne-particle spectral function near EF measured by
ARPES with many-particle correction (quasi-particle)
A( k , ω ) = −Σ" ( k , ω )π [ω − ε k − Σ' (k , ω )]2 + [Σ" (k , ω )]2
1

εk : single particle energy without many-particle correction
ω= 0 : EF

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

Σ( k , ω ) = Σ' ( k , ω ) + iΣ" ( k , ω )Real part: shift observed peak energy from single particle energy
Imaginary part: peak FWHM = 2 Σ”

Peak position
Featurelesssingle particle
dispersion
curve

Kink ~25 meV due to electron-phonon scattering

Peak position – single particle curve

Width due toelectron-electron
scattering ~ ω2
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 of
1. Better absolute photon energy resolution for most BLs designed
as nearly const ∆E/E.
2. Larger photoionization cross section at low photon energies.
3. Better momentum resolution for a given angular resolution.
∆k||(1/Å)= 0.5123 √(Ek(eV)) cos(θ) ∆θ
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.

TLS U90 undulator beamline & ARPES end station
2 white light branches

ARPES
End station

2 spectroscopy branches

U90 plane undulator w/ 90 mm period and 4.5 m long
CGM spectroscopy BL: 6 ~ 120 eVBest ∆E ~ 0.15 meV @ 16 eV, R.P. ~ 100,000; for gas phase
Beam spot size at sample: ~200 (H) x 80 (V) um^2Current status of ARPES end station

Preparation chamber

Electron energy analyzer: Scienta R4000
with ± 15° collecting angle
Overall energy resolution:< 7 meV at 64 eV and 5 meV at 22 eV
6-axis motorized manipulator, < 10 K

Load-lock system

R4000

Au(111) Surface State data taken in 3 minutes

Hemispherical electron energy analyzer
R1 : radius of inner sphereR2 : radius of outer sphere
R0=(R1+R2)/2 : mean radius
and along electron path
V1: inner potential
V2: outer potential
Ep: pass energy = electron
kinetic energy along mean
radius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.
Buried interface is mostly undetectable by PE using VUV/SX photons
because IMFP is too small compared to thickness of outermost thin layer.

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.

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

2000

surfacesurfaceBy using Ce 3d  4f Res. PE near 880 eV surface 4f component becomes
greatly reduced compared to 4d  4f Res. PE near 120 eV, the resulting spectra
are closer to true bulk 4f DOS.

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(in collaboration with the
Max Planck Institute for
Chemical Physics of Solids(MPI-CPfS) in Dresden,
Germany)World wide efforts on SR based HAXPES* SPring-8 BL29XU (RIKEN, HAXPES end station can move in, pioneer in HAXPES)* SPring-8 BL15XU (National Institute Materials Science (NIMS) WEBRAM, fixedinstallation)* SPring-8 BL19LXU (RIKEN long undulator BL, HAXPES end station can move in)* SPring-8 BL46XU (JASRI Engineering Science Reseach, fixed installation)* SPring-8 BL47XU (JASRI HXPES, fixed installation)* SPring-8 BL12XU-SL (NSRRC, fixed installation) unique with dual analyzers* ESRF ID16 (mainly for IXS, used by VOLPE)* ESRF ID32 (fixed installation, shared with XRD)
* ESRF BM32 SpLine (fixed installation, PXD/XAS/SRD/HAXPES+SXRD)* BESSY II KMC-1 BM (HIKE and XUV diffraction, fixed installation)* NSLS X24A BM (fixed installation)* DESY BW2 Wiggler (fixed installation)* DLS I09 (Surface and Interface Analysis (SISA))* SOLEIL Galaxies (RIXS and HAXPES)* CLS SXRMB BM (wide range 1.7-10 keV)* APS (?)

Why Hard X-rays?
Electron IMFP (probing depth) and Cross section
HAXPESXPS
UPSSPring-8
Higher Ek for deeper probing depth or more bulk sensitivity, for strongly
correlated systems and interface properties
Photoemission signal (σ⋅λ) 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)

Ionization cross section (kb)

A serious issue on going to hard x-rays
XPS 1487 eV

1.E+05

Ni 3d8_y

1.E+04
1.E+03

Ni 4s2_y

1.E+02

O 2p6_y

1.E+01
1.E+00
1.E-01

10

100

1000

1.E-0210000

Ni 3d8Ni 4s2

1.E-03O 2p6

1.E-04

Photon energy (eV)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 keVLDA+U simulation

O-2p

Intensity (arb. units)

Intensity (arb. units)

XPS
1.5 keVLDA+U simulation

Cu-3d

HAXPES
7.7 keVLDA+U simulation

Zn-3d

HAXPES
7.7 keVLDA+U simulation
Zn-4s (LDA)(LDA) Cu-4s

18 16 14 12 10 8

O-2p

6

4

2

Binding Energy (eV)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 β-asymmetry parameter+ ... ]
β-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 β ≈ 2:
• intensity is enhanced for e- emission || E-vector• intensity vanishes for e- emission ⊥ E-vector

photoelectron emissionθpolarization E vector→ choose suitable experimental geometry !

HAXPES Commissioning: Horizontal vs Vertical geometriesdσ1∝ 1 + β ⋅ (3 cos2 θ − 1)dΩ2polarization

horizontal
θ =0o

θ: angle between electron emission andpolarization vector
β: electron emission asymmetry parametervertical
β~2 for s-orbitals, strong emission near θ=0o
θ =90o(horiz.) while suppressed near θ=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.

Analyzer
HRM

KB
Platform

Horizontal geometry

Vertical geometry

For selecting different orbital symmetries in valence band

BL12XU Sideline

MBS-A1HE
Analyzer(Vertical)

Diamond Mono.

High Resolution
Mono. (HRM)

MBS-A1HE
Analyzer(Horizontal)

KB focus mirrors

New end station installed in January, 2013

Example 1: Metal-Insulator Transition of
Ca2-xSrxRuO4 (vertical geometry)
There is a rich phase diagram of Ca2-xSrxRuO4
regarding their structure, transport and magnetic
properties as Sr doping and temperature are varied.
Ru is a 4d element. Despite its larger wave function
than 3d TM, structure distortion induced large
Ru-O bond distance and Ru-O-Ru bond angle canresult in strong correlation effect, particularly
Metal-Insulator Transition (MIT).

Phase diagram

x=0, TMIT ~ 360 K
1sr order structural transition

x=0.09, TMIT~170 K
1st order structural transition

tetrag.
orthorh.

x=2, no MIT
except Tsc=1.5 K

SC: superconductor
P-M: paramagnetic metal
P-I: paramagnetic insulator
CAF-I: canted antiferromagnetic
insulator
M-M: AF correlated metal

Sr2RuO4: tetragonal, elongatedperovskite
Complex structural distortion
with replacement of Sr by Ca
Ca2RuO4: orthorhombic
Nakatsuji & Maeno, PRB 62, 6458 (2000)
Friedt & Maeno et al. PRB 63, 174432 (2001)

Metal Insulator Transition (MIT) of Ca1.91Sr0.09RuO4
300 K  140 K  200 K (TMIT ~ 160 K)* Dramatic PE spectral change across MIT due to 1st order phase transition* Gap opening about 300 meV near EF* Reversible

Example 2: Interface of LAO/STO(horizontal geometry)
Interface of two large gap band insulators
LaAlO3 and SrTiO3 becomes metallic-like.
Evidence of charge transfer from LAO to
STO is observed but the amount is less than
prediction of a 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 depthθhv = 6.5 keV
5 UC (2nm) LAOSTOLaAlO3/SrTiO3 (001)0(AlO2)0.5−

30µgrown with PLD, 10-5 torr O2, 850 C
annealed at 100 mtorr of O2

5 u.c.
LaAlO3Ti3+?

conducting interfaces
substrate(TiO2 terminated)

SrTiO3

LaAlO3
Ti3+/Ti4+

d1
d2 (2DEG)(Ti4+)SrTiO3

Ti4+

Ti
2p1/2

Ti
2p3/2

Ti3+

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

LaAlO3
Ti3+/Ti4+

d1
d2 (2DEG)(Ti4+)SrTiO3

d2 = 48.3 ± 20.3 A~ 12 u.c.
α = 0.021(Ti3+/T4+ in d2)
Total carriers~ 0.24 e / 2D u.c.

Chu et al., Appl. Phys. Lett. 99, 262101 (2011)

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 Fresnel 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