Photoemission spectroscopy: fundamentals and applications

Photoemission spectroscopy: fundamentals and applications

Lecture 1

Slavomír Nemšák

Advanced Light Source, Lawrence Berkeley National Laboratory

• X-ray Photoelectron Spectroscopy (XPS)• Brief history - XPS or ESCA?• Resources (online and offline)• Basics of the XPS (binding energy shifts, information depth, Auger e-, inelastic losses)• Instrumentation (photon sources, analyzers & detectors)

• Advanced techniques in photoemission• Ambient Pressure Photoelectron Spectroscopy (APXPS)• Resonant Photoelectron Spectroscopy (ResPES)• Photoelectron Electron Microscopy (PEEM)• Standing-Wave Photoelectron Spectroscopy

• Practical aspects of XPS analysis• Strategies of data collection• Introduction to online and offline tools• Hands-on course in data treatment

2

Outline

Photoelectric effect

Photoemission or

Photoelectron spectroscopy

(PS, PES)

X-ray photoelectron

spectroscopy

(XPS) or

Electron spectroscopy

for chemical analysis

(ESCA)

Studies of chemical

processes on solid

surfaces – surface

science

3

Nobelists that shaped XPS as we know it

XPS – X-ray Photoelectron SpectroscopyESCA – Electron Spectroscopy for Chemical Analysis

Resources (textbooks):

4

Resources (textbooks):

Resources (online databases):

https://vuo.elettra.eu/services/elements/WebElements.html https://cross-sections.lbl.gov/

http://henke.lbl.gov/optical_constants/ https://srdata.nist.gov/xps/main_search_menu.aspx

5

Resources (software and offline databases):

http://cxro.lbl.gov/x-ray-data-booklet http://www.quases.com/products/quases-imfp-tpp2m/

https://www.nist.gov/srd/nist-standard-reference-database-100 https://www.kolibrik.net/kolxpd/ 6

http://cxro.lbl.gov/x-ray-data-booklet

http://www.quases.com/products/quases-imfp-tpp2m/

https://www.nist.gov/srd/nist-standard-reference-database-100

https://www.kolibrik.net/kolxpd/

X-ray Photoelectron spectroscopy (XPS)

Reinert, Hufner,New J. Phys. (2005)

inc

En

erg

y

K,s y

x z

No. e-

Kin

etic

En

erg

y

7

Photoelectric Effect (Einstein):

Kinetic Energy = Photon Energy – Binding Energy

Apparent Kinetic Energy vs. Binding Energy

Core level

E’kin = hν - EB - φS

Ekin = E’kin + φS - φSP

Ekin = hν - EB - φSP

E’kin

Detected Ekin depends on the spectrometer work function (ΦSP)

8

Core levels - chemical shifts

F. J. Himpsel, et al., Phys. Rev. B 38, 6084 (1988)

• Naturally occurring silicon oxide on Si substrates

• In general, when atom loses valence charge (Si0->Si1+), binding energy increases due to the change in the electrostatic potential

Si

SiO2

9

Core level shifts – more examples

Polyethylene terephthalate (PET)

R. R. Deshmukh, et al., Mat. Res. Innovat. 7, 283 (2003)G. Beamson, et al., Polymer 37, 379 (1996)

C 1s

E. Casero, et al., Microchim Acta 181, 79 (2014)

Graphene oxide

Be careful with your interpretation!

10

Atomic (core) photoelectron intensities: the three-step model

Q

Qn

hν

hν

j

e ki

i

Q

n

n0

k

=

ˆdσ (hν ,ε ) zC exp dxdydz

dΩ Λ (E )sinθ

Î (x,y,z,ε )

ˆ Î (x,y,z,ε ) = x-ray flux, ε = pol

I(Qn

ariz

Ω(E , x,y )

atio

ρ (x,y,z)

ρ (x,y,z)

j)

= density of

n

a

Qn j

e ki tn sca

ˆdσ (hν ,ε )differential photoelectric cross section foenergy-dependent

energy-

r subshell Qn jdΩ

Λ (E ) inelastic attenuation lengt

toms Q quantitative a

elastic scatdep terh i + ng

nal

: fende (θn

y s

t

i

)

s

kin

kin kin

energy-depende

Ω(E , x,y ) spectrometer acceptance s

Effective Attenuation Length (EAD) Mean Emission Dept

olid angle

Ω(E , x,y )dxdy = T(E ) = trans

h

m

(M

ission funct

ED

=

i

)

nt

0V inner potential

on

Atom Q

Level nj

h

z

x

y

e-

kinΩ(E , x,y )hνÎ (x,y,z,ε )

ε̂

I(Qn j)

V0

Analyzer manufacturer

11

Information Depth (Inelastic Attenuation Length)

100

101

102

103

101

102

103

104

41 Elemental SolidsIn

ela

stic M

ea

n F

ree P

ath

(Å

)

Electron Energy (eV)

KCs

NaLi E0.78

Typical XPS

Typical valence ARPES

HAXPES, HXPS

Tanuma, Powell, Penn, Surf. and Interf. Anal. 43, 689 (2011)

“Bulklike”+

Buried layers &

interfaces

: Optical theory,

TPP-2M

400 6000

Universal curve

• Electron inelastic mean free path (λ) calculated from TPP-2M equation (Quases TPP-2M)

• Typical Soft X-ray PES: 100-1500 eV

• Higher photon energies → more bulk sensitive

12

Depth profiling in XPS

• Exponential attenuation of the signal with depth (Beer-Lambert law)

• λ is inelastic mean free path (dependent on material and function of kinetic energy)

• At close to normal electron exit angle (θe ≈ 90°), 95% of the signal comes from atoms within 3λ of the surface

• Depth profiling:

• θe: change the emission angle

• λ: vary photon energy (consequently KE)

Signal originating from the depth z

𝐼(𝑧) = 𝐼0 . 𝑒−𝑧/ λsin θ𝑒

z

θe

e-

13

C. S. Fadley, Prog. In Surf. Sci. 16, 275 (1984)

Signal originating from the depth z

𝐼(𝑧) = 𝐼0 . 𝑒−𝑧/ λsin θ𝑒

z

θe

e-

Si

SiOx

14

Angle-resolved XPS

15

Depth profiling in XPS

𝐼𝐴∞ = 𝐾. 𝑛𝐴. 𝜙𝑥(ℎ𝜈). 𝜎𝐴 ℎ𝜈 . 𝐿𝐴(𝛾, ℎ𝜈). 𝑇 𝐸𝑘 . 𝜆 𝐸𝑘 . sin 𝜃

𝐼𝐴𝑑𝐴 = 𝐼𝐴

∞. 1 − 𝑒−𝑑𝐴/𝜆𝐴 sin 𝜃

XPS core-level intensity from a semi-infinite flat sample

A flat layer of thickness dA

A flat layer of thickness dA buried under dB of material B

𝐼𝐴𝑑𝐴 = 𝐼𝐴

∞. 1 − 𝑒−𝑑𝐴/𝜆𝐴 sin 𝜃 . 𝑒−𝑑𝑩/𝜆𝐵 sin 𝜃

Photon flux

Photoionizationcross-section

Assymetry parameter

Analyzer transmission

Attenuation length

Speciesconcentration

Emission angle

1000 800 600 400 200 0

La4dSr3d

La4p

Sr3p &La4s

Sr3s

O1s

Mn 2p &Mn+La Auger

O KLL

Auger

Mn2s

La3d

Survey scanhν = 1253.6 eV

Co

un

ts (

A.U

.)

Binding Energy (eV)

La0.6Sr0.4MnO3

80 60 40 20 0

Sr4p

Sr4s

O2s

La5p

La5s

VB

Mn3pMn3s

h = 950 eV

Co

un

ts (

A.U

.)

Binding Energy (eV)

Multiplets

Mn 2p1/2

Mn 2p3/2

16

Identifying peaks in survey spectra

Can be a complicated and tedious job!

X-Ray Data Booklet: Electron Binding EnergiesThe energies are given in eV relative to the vacuum level for the rare gases and for

H2, N2, O2, F2, and Cl2; relative to the Fermi level for the metals; and relative to the

top of the valence bands for semiconductors (and insulators).

Missing

valence

B.E.s

Electronic

configuration

45 17 17

9 9

13 13Interpolated,

extrapolated

Valence levels

Valence levels

17

Distinguishing core-level and Auger peaks

Photoemission process

1s

2s

2p

Photon

Photoelectron

Auger electron emission

1s

2s

2p

Auger electron

Constant kinetic energyConstant binding energy

18

Ekin = E1s – E2p – E’2p

Ag MNN

1253.6 - Auger Energy (eV)

Ni LMM


O KLL


65 70 75 80 85Kinetic Energy (eV)



Au N6,7O4,5O4,5

X-Ray Data Booklet Fig. 1.4

19

Ekin = E1s – E2p – E’2p

Au spectrum with various excitation energies

Auger: constant kinetic energy Core level: constant binding energy

Auger Auger

Core leveldoublets

Core levelsingle-peaks

20

CLEAN ALUMINUM

Bandgap of

Al2O3

Plasmons:

Eplasmon = p = (nvalencee2/me0)

1/2

AlAl2O3

O 1sAl 2s, 2p

21

Typical photoemission spectra of oxidized aluminum


• Advanced techniques in photoemission• Ambient Pressure Photoelectron Spectroscopy (APXPS)• Resonant Photoelectron Spectroscopy (ResPES)• Photoelectron Electron Microscopy (PEEM)• Standing-Wave Photoelectron Spectroscopy


22

Outline

inc

En

erg

y

K,s y

x z

No. e-K

ine

ticE

nerg

y

Lab X-ray sources, synchrotrons, free-electron lasers

Hemispherical analyzers, Momentum microscopes, Time-of-Flight analyzers

Multi-channel detectors, multi-channel plates, delay-line detectors

23

Instrumentation

Instrumentation

• Photon sources• X-ray tubes

24

InstrumentationInstrumentation

Bremsstrahlung

Al or Mg or...

10-20 keV

+Ze

e-

1s

2p

3pe-

See Section 1.2 in

“X-Ray Data

Booklet”

Siegbahn notation

25

Producing x-rays: the good old-fashioned way

K

K

λmin≈20.7 pm (60 keV)

26

X-ray spectrum from a Rhodium target at 60 keV electron energy

Popular laboratory sources

for photoelectron spectroscopy

27

X-Ray energies from the “X-Ray Data Booklet”

28

Popular laboratory sources

for photoelectron spectroscopy

X-Ray energies from the “X-Ray Data Booklet”

XR 50 by SPECS

Non-monochromated Al Kα

• Expanding selection of anode materials

• Non-monochromated sources:• Poor energy resolution (~ 1

eV)• Large beam size (~ 1 cm)

• Monochromated sources:• Beam size < 1 mm• Energy resolution < 0.5 eV

29

Lab X-Ray tubes

Instrumentation


• Synchrotrons

30


Synchrotron radiation sources of the world- about 41 and growing

Nature Photonics 9, 281 (2015)

31

Advanced

Light Source

San Francisco

Group offices

& lab.

UC Berkeley

Marin County

32

ALS and view of the bay area

u

u / as

seen by

electrons

0=c/u

+Doppler

=2c/ u

Electron

speed near c:

0.99999994 c,

= 3719

Radiofrequency

Cavity

See page 137 33

Inside a synchrotron radiation source

2

2

1γ

v1

c

max

c = Critical energy [keV] = 0.665 E2[GeV]B[T]

c

B0

B0

B0

B0

B0

34

Synchrotron radiation sources

Intensity 1 n n2

1 n n

Intensity 1 n n2

1 n n

e-vc

30 psec long

RCP

LCP

Horiz. Lin.

LCP

Intensity 1 n n2

1 n n

Intensity 1 n n2

1 n n

Below plane

Above plane

LCP

RCP

Horiz. Lin.

35

Variable polarization with a bend magnet: above and below plane

Sasaki-Carr Elliptically-Polarized Undulator: Variable light polarization

RCP

LCP

Horiz. Lin.

Vert. Lin.

Translating magnet arrays

36

Range of wavelengths produced by undulators

Typical materials

science expts.

Vacuum Ultraviolet

(VUV)-8-200 eV

Soft x-rays200-2000

eV

“Tender” x-rays

2000-10000 eV

x(Å) = 12,398/[h(eV)]

h =

37

100m

spot

1

2 1’

2’

x

38

Typical soft X-ray beamline layout

HAXPES endstation

Solid-State, Gas-

Phase, Liquid Jet

30x80 µm² beam spotRIXS endstation

Source: High-flux

undulator

(2.3 to 12 keV)

Double-crystal

monochromator

(Si 111 and 333)High res. mono.

(∆E ~ 100 meV)

Quarter-wave plate

Variable polarization

hν

J.-P. Rueff, J. Rault

10x10 µm² beam spot

HAXPES

RIXS

x

x

Bragg:

nx = 2dhksin

n = 1, 2,…39

Typical hard X-ray beamline layout

40

ALS Beamlines

Scienta

soft x-ray

spectrometer

Sample prep.

chamber: LEED,

Knudsen cells,

electromagnet,...

ALS

BL 9.3.1

h = 2-5 keVChamber

rotation

5-axis

sample

manipulator

Permits using all relevant soft and hard x-ray spectroscopies on a single sample:

PS, PD, PH; XAS (e- or photon detection), XES/RIXS, with MCD, MLD

Scienta

electron

spectrometer

(hidden)

41

Multi-technique spectrometer difractometer

Instrumentation


• Synchrotrons

• Free-electron lasers

42


43

Ultra-fast X-ray science

• High coherent flux• Ultra-short pulses (~fs)• Soft and hard (LCLS-II) X-ray sources

The Next Generation: The Free-Electron Laser

44

The next generation: Free-electron laser

“X-Ray Data

Booklet”

See Fig. 2.10

PRESENT

&

Average brightness

45

Instrumentation


• Synchrotrons


• Electron analyzers• Hemispherical analyzers

46


inc

En

erg

y

K,s y

x z

No. e-

Kin

etic

En

erg

y

47

Hemispherical analyzers

Retardinglens

Deflecting and focusing lens Slits

Vout

Vin

R0

Rout

Rin

EpassE>Epass

E<Epass

e-

Lens imaging: (position)/(angle) vs KE at the detector

Distance along lens (mm)Dis

tan

ce

fro

m s

am

ple

ce

nte

r (m

m)

z = vertical angle

Em

iss

ion

an

gle

Kinetic energy

E,k band mapping Imaging Au stripes

A.X. Gray et al., Nat. Mat. 10, 759 (2011) J. Cai et al., Nucl. Sci. Tech. 30, 81 (2019)48

SPECS Phoibos 150

Hemispherical Analyzer

• High energy resolution (down to ~1 meV)

• 2D detection allows:• 1D microscopy• Momentum mapping of valence

electrons

• Working with KE up to ~10 keV

• Widely commercially available

49

Hemispherical analyzers

Instrumentation


• Synchrotrons



• Momentum microscope

50


Momentum microscope

Courtesy of C. Tusche, FZJ Meyerheim et al., Phys. Status Solid

RRL 1800078 (2018)

full B.Z. with μm and meV resolution Spin-resolved ARPES – 2D spin filters

51

Instrumentation


• Synchrotrons




• Time-of-Flight analyzer

52


CoESCA endstation at BESSY-II, BerlinArTOF with 2D delay line detection

53

Time-of-flight analyzers

Need pulsed sources (FEL, lasers)


• Synchrotrons





• Detectors• Channeltrons and multi-channel plates

54

Instrumentation

55

Channeltrons and channel-plates

H.Cai et al., Materials 2019, 12(7), 1183

56

Electron signal amplification: channel-plates


• Synchrotrons





• Detectors• Channeltrons and multi-channel plates

• Delay-line detector57

Instrumentation

• Coupled to a MCP

• Each event is recorded as (x,y,t)

• Time resolution down to ~10 ns

• Limited max. count-rate due to relatively high dead time

• Can be connected to all types of analyzers

58

2D delay line detector

K. Muller-Kaspary et al., Appl. Phys. Lett. 107, 072110 (2015)


• Advanced techniques in photoemission• Ambient Pressure Photoelectron Spectroscopy (APXPS)• Resonant Photoelectron Spectroscopy (ResPES)• Photoelectron Electron Microscopy (PEEM)• Standing-Wave Photoelectron Spectroscopy (SW-XPS)


59

Outline

Photoemission spectroscopy: fundamentals and applications

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Transcript of Photoemission spectroscopy: fundamentals and applications