Outline

Photoemission (I)Spectroscopy

1. What is photoemission spectroscopy?
2. Fundamental aspects of photoemission.
3. Examples.
4. Increase bulk sensitivity: HAXPES.
5. Challenging future directions.

Cheiron School 2012
October 2, 2012 SPring-8, Japan

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

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 spectroscopy?
(photoelectron spectroscopy) (PES)Electron energy analyzer

hv

Monochromatized
photonse-h+sampleSince 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”

Initial 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?

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

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)Na metal
E

3p

EF

3s

Fermi levelE (nearly free electron like)
Valence (sp)
Band
(DOS)

2p3/2
2p1/2
Core levels

2s

1s
N(E)
(DOS)

Energetics in PES
Conservation of energy
Density ofStates(DOS)

Ek = hv – EB – I
Ev : vacuum (energy) level
EF : Fermi (energy) level
I = 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)

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

Hufner, Damascelli
HufnerInelastic Electron Mean Free Path (IMFP)
I(d) = Io e-d/O(E)Core level binding energies arecharacteristic of each orbital ofeach element

O(E): IMFP depending on KINETIC ENERGY inside solid or relative to EF
SR (VUV,SX,HX)
Universal curve

Finger prints
XPS

Core level BE independent ofphoton energy used

UPS

Hufner

X-Ray Data Booklet

Minimum due to electron-electron scattering, mainly plasmons

http://xdb.lbl.gov/

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

surface component

Core level photoemission: chemical analysis of elements
ESCA (XPS)
hv = Mg KD = 1253.6 eV

Synchrotron hv = 160 eV

Bulk CdSe

Dm = 42 A

Dm = 32 A

Dm = 18 A

Bulk CdSe

Dm = 42 A

Dm = 32 A

59

58

Binding Energy (eV)

PJW, NSRRC

Se 3d

Dm = 18 A

410 409 408 407 406 405 404 403

57

Bulk CdSe
Dm = 42 A
Dm = 32 A
Dm = 18 A

10

8

6

4

56

55

54

Binding Energy (eV)Intensity (a. u.)Different photon energy Ædifferent relative cross section
for various core levels Æ
Relative intensity changes
with photon energy

bulk component(b)

Cd 3d5/2

Photoemission Intensity (a. u.)

Photoemission Intensity (a. u.)(a)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

Chemical Analysis of C1s core levels

higher oxidation state=> higher BEBEhigher emission angle
Æ more surface sensitive
Pi, SS 2001 NSRRC(IMFP)

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

Auger Electron Spectroscopy

Photoemission Process

Core electron ionized byphotons or high energy
electrons
Non-radiative core hole decay
Æ Auger electron emission
Radiative decayÆ 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

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

Electron kinetic energy inside and outside of solids(valence band initial states)
Inner potential: EV – E0
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

Angle-resolved photoemission (ARPES)

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

Band Mapping (3D) E(k, k||=0)

Conservation of linear momentum
parallel to the surface

k
z
T

x

2m
Ek  sin T!2

k||(inside solid) = k ||(outside in vacuum)k is not conserved, obtained bychanging photon energy

xz plane: crystal symmetry plane

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

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 surfaces

Quantum well states: manifestation of particle in a box
in real materials

Cu

Ag(111) thin films expitaxiallygrown on Au(111) substrate

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

Au E Ag vacuum
EFBand gap
below EFLAg QWS canexist within
Au gapGap below EF(=0) at L-point(nearly free electron like) sp-bandz
Small dispersion d-band
more localized state

Quantized discretely along z-direction
Energy levels depend on film thickness L

Large dispersion sp-band
extended state

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

Nearly free electron like in xy-plane
L
(111)

*

X
(001)

HufnerÅ Ag S.S.
2D
Int.
plots

Ag QWS

Anneal to 180 K
QWS appear
minimal flat
dispersion
Small localized
domains within
xy-planeAnneal to 189 K
Coexistence of
two kinds ofdispersionk|| = 0EDCs

Luh et al.
PRL 2008
NSRRC
BL21B1

Clean Au(111)
surface state

Deposit 22 ML Ag
at 37 K
disordered form

Bulk projected bands along *Lof Au and Ag, respectively

Anneal to 258 K
Atomically flat
22 ML thin film

Anneal to 258 K
Well developed
dispersion
Large, good
crystalline
domainsin xy-plane

Proposed growth model
Annealing Temp

Same QWS energies ÆSame 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)
A( k , Z )

6" ( k , Z )S [Z  H k  6' (k , Z )]2  [6" (k , Z )]2
1

Hk : single particle energy without many-particle correction

Z= 0 : EF

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

6( k , Z )6' ( k , Z )  i6" ( k , Z )Real part: shift observed peak energy from single particle energy
Imaginary part: peak FWHM = 2 6”

Peak position
Featurelesssingle particle
dispersion
curve

Kink ~25 meV due to electron-phonon scattering

Peak position – single particle curve

Width due toelectron-electron
scattering ~ Z2
Width due toelectron-phonon
scattering

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

Const bkg width
due to impurities

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. Better photoionization cross section at low photon energy.
3. Better momentum resolution for a given angular resolution.
  Ek(eV)) cos(T) 'T
'k||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

Comments on photoelectron IMFP
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
radiusValence 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

Resonance photoemission(near-edge absorption followed by Auger likeelectron emission)e.g. Ce3+ (4f1)

HArd X-ray PhotoEmission Spectroscopy(HAXPES)4f mixed with other DOS

e-

e4f

intermediate
state
EF

4f

Intensity enhanced by absorption
Predominantly 4f DOS

4f

4f

4f

4f

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.

Absorption +

3d/4d

3d/4d
Direct PE

3d/4d
Resonance PE

EF
Auger likeemission

2000

bulk

surfacesurface

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

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

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

NSRRC HAXPES projectat 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)

World wide efforts on SR based HAXPES

Why Hard X-rays?
Electron IMFP (probing depth) and Cross section* 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 (under construction, RIXS and HAXPES)* CLS SXRMB BM (wide range 1.7-10 keV)* APS (?)

HAXPESXPS
UPSSPring-8
Higher Ek for deeper probing depth or more bulk sensitivity, for strongly
correlated systems and interface properties
Photoemission signal (VO) 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)

Unexpected lineshapes in HAXPES compared to XPS
Valence band of simple oxides – e.g. Cu2O and ZnO

XPS 1487 eVĲįņĬıĶġ

ŏŪġĴťĹŠźġ

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band-insulators, no electron-correlation effects – LDA should do (+Udd+Upp)ŏŪġĵŴĳŠźġ

ZnO – valence band

Cu2O – valence bandĲįņĬıĳġ

ŐġĳűķŠźġ

ĲįņĬııġ
ĲįņĮıĲġ

Ĳıġ

Ĳııġ

Ĳıııġ

ĲįņĮıĳġ

Ĳııııġ

ŏŪġĴťĹġ
ŏŪġĵŴĳġ

ĲįņĮıĴġ

Őġĳűķġ

ĲįņĮıĵġ

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?

XPS
1.5 keVLDA+U simulation

XPS
1.5 keVLDA+U simulation

O-2p

Intensity (arb. units)ĲįņĬıĲġ

Intensity (arb. units)

Ionization cross section (kb)

A serious issue on going to hard x-rays

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 HAXPES

HAXPES Commissioning: Horizontal vs Vertical geometries
1dV
v 1  E  (3 cos2 T  1)
2
d:

How to suppress the 4s spectral weight?
• photo-ionization cross-section depends on
e- emission direction and light polarization• make use of E-asymmetry parameterpolarization+ ... ]

horizontalT=0oE-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

photoelectron emission

In general: s orbitals have E|:• intensity is enhanced for e- emission || E-vector• intensity vanishes for e- emission A E-vectorTpolarization E vectoro choose suitable experimental geometry !

Optical design concept

T: angle between electron emission andpolarization vector
E: electron emission asymmetry parameterverticalE~2 for s-orbitals, strong emission near T=0o (horiz.)
T =90owhile suppressed near T=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

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

DM: horizontal dispersion

Layout of the side beamline of BL12XU
HRM: vertical dispersion

BL12XU Sideline

Diamond Mono.

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

Analyzer
HRM

KB
Platform

Horizontal geometry

Vertical geometry

For selecting different orbital symmetries in valence bandNiO above Neel temperature at 523 K

NiO: a prototypical strongly correlated electron system

How important is long range AF ordering?
NiO impurity in MgO

single site

725K
675K
625K
575K
550K
525K
500K
475K
450K
425K
400K
375K

HAXPES hQ=6500 eV
Ni 2p

HAXPEShQ=6.5keV
300K
525K
300K after 525K

Intensity (arb. units)

NSRRC Dragon BL

Intensity (arb. units)

A
B

14

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 2pV (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.

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)polar catastrophe
LaAlO3

HAXPES Example 2:
Interface of LAO/STO
SrTiO3

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

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)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 depthSing 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 dependentT

hv = 6.5 keV
5 UC (2nm) LAOSTOLaAlO3/SrTiO3 (001)

LaAlO3
Ti3+/Ti4+

d1
d2 (2DEG)(Ti4+)SrTiO3

0(AlO2)

30P

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

5 u.c.
LaAlO3

Ti4+

Ti3+?

conducting interfaces
substrate(TiO2 terminated)

SrTiO3

Ti3+
Ti
2p1/2

Ti
2p3/2

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

d1

LaAlO3
Ti3+/Ti4+(Ti4+)SrTiO

d2 (2DEG)3d2 = 48.3 Ʋ 20.3 A~ 12 u.c.
D = 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 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