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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
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
ˆI (x,y,z,ε )
ˆ ˆI (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 )hνˆI (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
1253.6 - Auger Energy (eV)
O KLL
1253.6 - Auger Energy (eV)
65 70 75 80 85Kinetic Energy (eV)
65 70 75 80 85Kinetic Energy (eV)
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
• 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
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
• Photon sources• X-ray tubes
• Synchrotrons
30
InstrumentationInstrumentation
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
• Photon sources• X-ray tubes
• Synchrotrons
• Free-electron lasers
42
InstrumentationInstrumentation
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
• Photon sources• X-ray tubes
• Synchrotrons
• Free-electron lasers
• Electron analyzers• Hemispherical analyzers
46
InstrumentationInstrumentation
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
• Photon sources• X-ray tubes
• Synchrotrons
• Free-electron lasers
• Electron analyzers• Hemispherical analyzers
• Momentum microscope
50
InstrumentationInstrumentation
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
• Photon sources• X-ray tubes
• Synchrotrons
• Free-electron lasers
• Electron analyzers• Hemispherical analyzers
• Momentum microscope
• Time-of-Flight analyzer
52
InstrumentationInstrumentation
CoESCA endstation at BESSY-II, BerlinArTOF with 2D delay line detection
53
Time-of-flight analyzers
Need pulsed sources (FEL, lasers)
• Photon sources• X-ray tubes
• Synchrotrons
• Free-electron lasers
• Electron analyzers• Hemispherical analyzers
• Momentum microscope
• Time-of-Flight analyzer
• 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
• Photon sources• X-ray tubes
• Synchrotrons
• Free-electron lasers
• Electron analyzers• Hemispherical analyzers
• Momentum microscope
• Time-of-Flight analyzer
• 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)
• 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 (SW-XPS)
• Practical aspects of XPS analysis• Strategies of data collection• Introduction to online and offline tools• Hands-on course in data treatment
59
Outline