Chemical analysis of surfaces and organic thin films
by means of XPS
X-ray photoelectron spectroscopy – The photoelectric effect
– Wilhelm Hallwachs (1886), Albert Einstein (1905) und Ernest Rutherford (1914) –
K
after 500 000 s
Wilhelm Hallwachs and Philipp Lenard, around 1900:
D
= 10-5 Wm-2
DAK = 10-5 Wm-28.610-20 m2
= 8.610-25 W
energy at 1 s irradiation:
Eprim = 8.610-25 Ws
energy to remove one electron: Ei
= 2.24 eV = 3.610-19 Ws
flux density:
flux density per K atom:
During irradiation only the 1/2,000,000 part of energy is
supplied which is necessary to remove one electron.
X-ray photoelectron spectroscopy – The photoelectric effect
– Wilhelm Hallwachs (1886), Albert Einstein (1905) und Ernest Rutherford (1914) –
h. n
Ekin
core level
valence band
Einstein’s Photochemical Equivalence Law (1905)
h n = Ebind + Ekin
ii[eV]
to determine experimentally
value specific for each element and level
primary energy
X-ray photoelectron spectroscopy – The photoelectric effect
– Wilhelm Hallwachs (1886), Albert Einstein (1905) und Ernest Rutherford (1914) –
K
after 500 000 s
Wilhelm Hallwachs and Philipp Lenard, around 1900:
D
= 10-5 Wm-2
DAK = 10-5 Wm-28.610-20 m2
= 8.610-25 W
energy at 1 s irradiation:
Eprim = 8.610-25 Ws
energy to remove one electron: Ei
= 2.24 eV = 3.610-19 Ws
flux density:
flux density per K atom:
During irradiation only the 1/2,000,000 part of energy is
supplied which is necessary to remove one electron.
X-ray photoelectron spectrometer
– Kai Manne Börje Siegbahn (1954/1955) – the first spectrum was recorded from common salt –
Born in Lund (Sweden), April 20, 1918
Son of K.M.G. Siegbahn
Studies at the Univ. of Uppsala
1944: PhD (Univ. of Stockholm)
1951:Professor in Stockholm at the
Royal Institute of Technology
since 1954: University of Uppsala
Nobel Prize of Physics 1981
hn = Ebind + Ekin
Siegbahn, K.: Beta-ray-spectrometer theory and design.
High resolution spectroscopy. In: Beta- and gamma-ray
spectroscopy. (ed. by K. Siegbahn). Nort-Holland Publ.
Co., Amsterdam (1955), S. 52
Modern X-ray photoelectron spectrometer
– Axis Ultra (Kratos Analytical, Manchester, United Kingdom) –
Excellent vacuum: ca. 10-12 mbar, at 20°C, one collision each 30 years (at 1000 mbar: ca. 109 collisions/s)
Modern X-ray photoelectron spectrometer
– X-ray source –
hn = Ebind + Ekin
mirror
analyzer
Sh. ni
i
filament
trajectories of
image mode
outer
hemisphere
anode
(Mg, Al or Ag)
block of
copper
+ 20 kV
cooling water
slit plate
slit plate
detector system
electrostatic
lenses
inner
hemisphere
photoelectrons
I
DE  0.2 eV
K a3...6 K a1,2 characteristic
radiation
Kb
L
Bremsstrahlung
lmin
Emax = e. U
monochromator
CCD camera
deflection plates
charge compensation
unit
X-ray
magnetic lense
sample on
sample holder
analysis chamber
l
E
screen
vacuum pumps
Measuring the kinetic energy of the photoelectrons (Ekin)
– e.g. 180° hemispheric analyzer –
E
mirror
analyzer
outer hemisphere
E
E
trajectories of
image mode
detection
system
E
outer
hemisphere
slit plate
slit plate
detector system
electrostatic
lenses
inner
hemisphere
photoelectrons
monochromator
screen
E - dE
CCD camera
deflection plates
charge compensation
unit
X-ray
magnetic lense
sample on
sample holder
analysis chamber
vacuum pumps
E + 3dE
E + 2dE
E + dE
inner hemisphere
E
XPS – A surface sensitive technique
– How depth we do analyze? –


dI = A N exp - xldx
High surface sensitivity vs. X-rays


Sh. ni
i
+ 20 kV
cooling water
filament
Middle free path of photoelectrons
In the sample material
l (C 1s, Al a) = 1.2 nm
anode
Mg, Al or Ag
block of Cu

 d 
Id = I  1 -exp - 
 l 

Photoelectron contributes to the spectrum
inelastically scattered
photoelectrons only contribute
X-rays
to the spectrum background
1.0 probability W
0.8
60 Å at W = 0.01
0.6
10 nm at W = 0.0001
0.4
nl
d
0.2
0
0
20
40
information depth [nm]
Interpretation of XPS spectra
– Wide-scan spectrum of poly(3-hydroxybutyrate) –
I [kcounts/s]
O 1s
350
300
250
O KLL series

150

50
 qualitative surface anlysis
 all elements were detected (without
hydrogen and helium)
 elements with higher numbers in the
periodic table show a couple of peaks.
Si 2s
Si 2p
O 2s
200
100
C 1s

0
1000
O
CH3
CH
800
CH2
600
O
C
n
400
200
0
binding energy [eV]
 background
 element peaks
 relaxation phenomena
 (Auger peaks)
Interpretation of XPS spectra
– Designation of the peaks –
s=±½
l
Coupling of orbit quantum number and electron spin: |j| = l + s
Cu 2p3/2
total orbit quantum number j
orbit quantum number l
main quantum number s
chemical symbol
E
d (l = 2)
M
3
s (l = 0)
DBE = 19.9 eV
Cu 2p 3/2
p (l = 1)
L
2
p (l = 1)
s (l = 0)
Cu 2p 1/2
K
970
960
950
940
930
binding energy [eV]
1
s (l = 0)
n
l
5/2
3/2
3/2
1/2
3d5/2
1/2
3d3/2
3p3/2
3p1/2
3s
3/2
2p3/2
1/2
2p1/2
1/2
2s
1/2
1s
| j|
BE [eV]
Interpretation of XPS spectra
– Quantification –
I [kcounts/s]
O 1s
350
300
250
C 1sC 1s
 qualitative surface analysis
(showing all elements),
 quantitative surface analysis
O KLL series
Si 2s
Si 2p
O 2s
200
150
100
50
0
1000
O
CH3
CH
800
CH2
600
O
C
n
400
200
0
Binding energy [eV]
[C]
[O]
[Si]
= 68.39 At-%
= 31.22 At-%
= 0.39 At-%
[O]:[C] = 0.456
[Si]:[C] = 0.005
[O]:[C] should be = 0.5
Interpretation of XPS spectra
– Quantification –
I(X) = F [ n(X), photon, (X,i, h  n), l (E kin ),  , A, T(E kin ) ]
Usually only relative values are the matter of interest:
transmission
e.g.SiO2 means
an elemental or atomic ratio of [Si]:[O] = 1:2
area of acceptance

take-off angle
Hence,
results are given in atom percents [At-%]
middle free path of
electrons
cross section of the photon-electron interaction
I(X1 ) n(X1 ) (X1flux
) ldensity
(E kin [Xof
])theTphotons
(E kin [X1 ])
c(X1 ) T(E kin [X1 ]) SF(X1 )
1 
=


=


I(X2 ) n (X2number
) (X2 )ofl(atoms
E kin [X2 ]) T(E kin [X2 ])
c(X2 ) T(E kin [X2 ]) SF(X2 )
measured intensity
Relative sensitivity factors are
experimentally determined
Peak
C 1s
F 1s
SF
0.278
1.000
O 1s
0.549
O 2s
0.033
Ag 3d5/2
3.592
Different elements have a different sensitivity and will be
analyzed with different minimum concentration limits.
However, all elements having a minimum concentration
of ca. 0.1 at-% will be trustworthy analyzed.
Analyzing the chemical structure and binding states
– Chemical shifts in the C 1s and O 1s spectra of poly(3-hydroxybutyate) –
1 : 11 : 1
O O CH3CH3
O CH
C OC CH
1 : 1:1:1
O O CH3CH3
O CH O
C OC CH
CH3
CH
O
C O
535 535530 530
CH2
O
C
 chemistry of the element’s neighbourhood controls the electron
density  chemical shift
(types of functional groups,
degree of oxidation),
CH3
290
 qualitative surface analysis
(showing all elements),
 quantitative surface analysis
280
Bindungsenergie [eV]
O
CH3
H 2C C O CH


O
CH3
H 2C C O C H


O
CH3
CH
CH2
O
C
n
Chemical shifts and binding states
– Qualitative and quantitative detection of functional groups –
285
286
287
288
289
binding energy [eV]
290
291
292
primary shift
–C–C–; –C–H
–C=C–C=C
C–O–C (ether)
C–OH
C–O– C=O (ester)
C =O (keto group)
Oxirane
C–O–C =O (ester)
HO–C =O (carbonic acid)
anhydride
carbonate
CF3
C–Cl
CF2
C–NR2 ; C–NR3+
0
1
2
3
4
5
6
7
D BE [eV]
CH2
CH2

CH2 C
OH
b-shift by strongly electronegative groups
O




CH2 C OH
DBE
HO–C–C– 0.05...0.2 eV
O–C(O)–C– 0.40...0.6 eV
F–C–C–
0.5...0.7 eV
Cl–C–C–
0.4 eV
C 1s spectra to analyze chemical structures
– Polymethacrylates with different alcohol rests –
R
C
CH2
A CH3
ACH B C
2
EC O
F
F F
F GF G
F
F GGG
F
F
F
F F
O
C1 CH
2
G CF
2
H CF
3
n
R
C
CH2
B1CH
ACH3
ACH BC
2
EC O
O
C1CH
2
HCF
3
2
G1CF
2
GCF
2 4
HCF3
n
A
CH3
A
B
CH2 C
E
C O
ACH3
ACH BC
2
EC O
O
CCH
3
300 295 290 285 280
Binding energy [eV]
n
n
300 295 290 285 280
Binding energy [eV]
O
C1
CH2
G
CF2
GCF
2
H
CF3
n
n
C 1s spectra to analyze surface reactions
– Hydrophobic fluorocarbon films should get hydrophilic properties –

PEO-dimethacrylate
UV-grafted onto an oxygen
plasma treated CF film



CF film with PEO-PPO-PEO
cross-linked in an argon
plasma

unmodified fluoro
carbon film (CF film)
PEO-monoacrylate
melt-grafted onto an
oxygen plasma treated
CF film
PEO-monoacrylate
UV-grafted onto an oxygen
plasma treated CF film
PEO-monoacrylate
grafted onto an oxygen
plasma treated CF film
Derivatization to analyze of functional surface groups
– Double bonds and carbonic acids –
Double bonds
Carbonic acids
+
Br2
Br
Br
+
OsO4
O O
Os
O O
O
O
+
OH
CH3
H3C Si N
CH3
O
N
O
+ R3N
OH
O
+
OH
NaOH
R3N H
O
in MeOH
O
Na
O
O
+
OH
CH3
O Si CH3 + HN
CH3
AgNO3
in H2O
O
O
+ HNO3
Ag
N
Derivatization to analyze of functional surface groups
– Alcohol, oxiran and amino groups –
Alcohol groups
Oxiran groups
O
O
OH + F3C
O
OH
CF3
O + HCl
O
HO
F3C
O
Cl
CF3
O
O
O
O +
F
O
F
O
Cl
O CH2
F
F
F
F
R
N
NH2 +
CS2
O
O CH2
F
F
H
N
SH
S
F
F
O
CF3
O
CF3
O
F3C
Amino groups
NR2 +
F3C
O
Chemical shift in the Si 2p spectrum
– Detection of silanol groups and the condensation of silanes on glass surfaces –
H2N CH2 CH2 CH2
No significant chemical shift
at BE ca. 102 eV.
OC2H5
Si OC2H5
OC2H5
H2N CH2 CH2 CH2
CH3
Si OCH3
CH3
Separation of component peaks
of O–Si(–O)3, Si–OH und Si–C
seemed to be impossible.
Si 0
F
H2N CH2 CH2 HN
F
CH2
Si 2p 1/2
Si 2p 3/2
CH2 CH2
F
OCH3
Si OCH3
OCH3
F
F
CH3
Si Cl
CH3
Si _ O
110
106 102
98
binding energy [eV]
106
102
98
binding energy [eV]
106
102
98
binding energy [eV]
Chemical shift in the Si 2p spectrum
– Detection of silanol groups and the condensation of silanes on glass surfaces –
Ag L
Al K a
Si 1s
Si 2p
O 1s
Si 2s
C 1s
N 1s
0
500
Si KLL
1000
1500
O KLL
C KVV
2000
2500
Bindungsenergie [eV]
Pleul, D.; Frenzel, R.; Eschner, M.; Simon, F.:
X-ray photoelectron spectroscopy for the detection of different Si–O bonding states of silicon.
Analytical and Bioanalytical Chemistry, 375 (2003) 1276-1281.
Chemical shift in the Si 2p spectrum
– Detection of silanol groups and the condensation of silanes on glass surfaces –
O
O Si O
O
C
O
O Si O Si C
O
C
1840
1844
1848
O
O Si O
O
O Si OH
O
O
1836
1840
1844
1848
Angle-resolved X-ray photoelectron spectroscopy (ARXPS)
– Depth profiles in the range of a few nanometers –
CF3 CF2
Spectrometer
CF3
ED = ID =
5l
75°
 = 0
(CF2 )5
CF2
O
H N
hydrophobic
end-group
60°
sample
O
O
0°
ID = 5 l. cos ()
300

ED = 5 l
290
280
binding energy [eV]
O
O
[X]:[C]
O
0.6
0.4
0.2
0
[F]:[C]
[O]:[C]
O
n
information depth
O
OCH3
XPS employed to study biological surfaces
– XPS spectra recorded at low temperature (Cryo-XPS) –
O 1s
Na KLL
gel
Na 1s
Zn 2p
O KLL
Ca 2p
N 1s
C 1s
Cl 2p
S 2p
P 2p
skin
S 2p
gel
10 µm
Baum, C.; Simon, F.; Meyer, W.;
Fleischer, L.G.; Siebers, D.:
Surface properties of the skin of
Delphinids. Biofouling 19 (2003)
181-186
170
166
162
Si 2p
K 2p
skin
1000
800
600
300
400
200
0
binding energy [eV]
290
280
Imaging XPS
colloidal silver
Ag 3d
Ag 4d 3/2
Y
Y
Y
polyelectrolyte layer
N 1s
S
reactive anchor
HS–C10H20 –COOH S 2p
laterally structured
gold dots Au 4f
semi-fluorinated
silane F 1s
S
S
metal oxide Si 2p
800
400
0
binding energy [eV]
silicone wafer
ø 3.05 mm
silicon wafer
laterally structured
gold dots
gilded surface
Imaging XPS
– Laterally structured samples –
a)
a)
b)
b)
100 µm
Eschner, M.; Simon, F.; Pleul, D.; Spange,
S.: In: "Dresdener Beiträge zur Sensorik",
Band 16, w.e.b. Universitätsverlag, Dresden (2002) 149-153, ISBN 3-935712-71-5
100 µm
Si 2p
c)
N 1s
before silanisation
S 2s
d)
e)
N 1s
after silanisation
N 1s + Ag 3d
after silver deposition
Small spot XPS
real XPS-imaging
1250
50 µm
60 µm
27 µm spot size
hollow fibre of an artificial kidney cut along its
length axis
0
C 1s
27
55 µm
110 µm
O 1s
S 2s
N 1s
S 2p
O KLL
1000
800
600
400
200
0
binding energy [eV]
Summary
XPS
XPS is a method oriented to detect elements in the surface region of samples

qualitative surface analysis: identification of all elements (H and He are excluded),

quantitative surface analysis  relative amounts of the elements, e.g. [X]:[Y],

peak deconvolution allows to analyze (qualitatively and quantitatively) functional
surface groups or the oxidation states, labelling can be helpful to identify the species,

depth profile analysis on molecular scale without destroying the sample, sputtering
is also able,

special features: cryo-XPS, thermo- XPS, imaging XPS, small spot XPS
Auger electron spectroscopy (AES)
– Relaxation: Competition between Auger process and X-ray fluorescence –
E
EAuger = hn’ – Ekin
eh . n'
Small atomic number:
Auger process is preferred
High atomic number:
X-ray fluorescence takes place
e-
I [kcounts/s]
O 1s
350
300
250
h.n
C 1s
photo ionisation
O KLL Serie
Si 2s
Si 2p
O 2s
200
150
100
50
0
1000
800
600
h . n'
400
200
0
Bindungsenergie [eV]
relaxation
Auger emission
Auger electron spectroscopy (AES)
– Auger electron spectrometer –
X-ray source
electron gun
 qualitative surface analysis
 quantitative surface analysis
 chemistry of the element’s neigh bourhood controls the electron
 density  chemical shift
 (types of functional groups,
 degree of oxidation)
 depth profiling combined with
 sputtering
 imaging AES in nanometer resolution
detection system
sample
cylindrical analyzer
trajectories
SEM
Al
10 µm
Si
Auger electron spectroscopy (AES)
– Spectral information –
O KL23L23
O KL1L23
O KL1L1
F KL23L23
F KL1L23
F KL1L1
2p
2s
1s
2s0 2p6
2s1 2p5
2s1 2p5
KL1 L 1 ( 1S) KL1 L 23 ( 1P) KL1 L 23 ( 3P)
1000
900
800
binding energy [eV]
Bindungsenergie
1st derivation
2p
2s
0
1s
2s2 2p4
2s2 2p4
2s2 2p4
KL23L23( 1S) KL23L23 ( 3P) KL23L23( 1D)
500
600
700
kineticEnergie
energy [eV]
kinetische
Summary
AES
AES is also a method oriented to detect elements in the surface region of samples

qualitative surface analysis: identification of all elements (H and He are excluded),

quantitative surface analysis  relative amounts of the elements, e.g. [X]:[Y],

peak deconvolution allows to analyze (qualitatively and quantitatively) functional
surface groups or the oxidation states, labelling can be helpful to identify the species,

depth profile analysis by sputtering techniques

imaging of the lateral distribution of elements in nanometer resolution