1PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Title
Surface Analysis by means of Electron
and Vibrational Spectroscopy
Prof. Dr. Guido Grundmeier
University of Paderborn
2PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Contents
• Electron Spectroscopy of Surfaces
• Theory
• Applications
• Optical Spectroscopy
• FT-IR spectroscopy - Theory
• Applications
3PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Electron Beam - Sample Interaction
5-50 nm
Continuum X-rays
(Bremsstrahlung – “breaking radiation”)
X-ray
Fluorescence
4PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Energy distrib. of backscattered/emitted electrons
Regions of interest:
I. Elastically scattered primary electrons (structure, vibrational information)
II. Electronic excitations (phonon and plasmon losses)
III. Auger electrons (inelastic background)
IV. Secondary electrons (“True” emission electrons)
e-
e-
G.A. Somorjai, “Introduction to Surface Chemistry and Catalysis” (Wiley, New York, 1994), p.384
5PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Mean free path of electrons in solid matter
G.A. Somorjai, “Introduction to Surface Chemistry and Catalysis” (Wiley, New York, 1994), p.383
Mean free path of electrons in the order of 1 nm surface sensitivity
The surface sensitivity depends on the probability of the electron to reach
the surface without a loss of energy (e.g. inelastic collisions). The
penetration depth of the excited particles/radiation can be orders of
magnitude higher!
UPS
XPS and AES
(1 ML ~ 2.5 Å)
6PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Photoelectron emission by means of X-rays
Ultraviolet Photoelectron
Spectroscopy
UPS
UV photon
Ekin = h -EV-
X-ray photon
Ekin = h -EK-
Electron or X-ray
photon
Ekin = EK-EL1-EL23-
Evac
V
EL2,3
EL1
EK
X-ray Photoelectron
Spectroscopy
XPS
Auger Electron
Spectroscopy
AES
Evac
1s1/2
2s1/2
2p1/2, 2p3/2
7PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Photoelectron spectroscopy
Principles:
• Ejection of electrons from atoms, molecules, amorphous or crystalline solids
following a bombardment by monochromatic photons (compare: photoelectric effect)
• Photoelectrons are emitted above a treshold frequency of the incoming photons
2
evm2
1Ih
eMhM
:UPS/XPSFor
8PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Photoelectron emission by means of X-rays
Now excitation with X-rays: h >>
Determination of core levels
Reflect inner electronic structure
Energy balance:
Ekinetic = h – Ebinding (solid)
Since each element has unique set of core levels
Ekin can be used to fingerprint element
Regarding the photoemission process:
• Needed: monochromatic (X-ray) incident beam
• Absorption very fast: t ~ 10-16 s
• No photoemission for hν <
• No photoemission from levels with EB + > hν
• Ekin of photoelectron increases as EB decreases
Evac = 0
0
Ekin
EB
VB
core level
DOS
I
EB
EF
h
h
h
h
core level
9PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Typical XP Spectrum
10PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Binding Energy of Electrons
Calculation of Binding Energies of Electrons:
Determination of the binding energy (BE):
KE = h – BE ↔ BE = h – KE
The Binding Energy of electrons is normally is referred to the Fermi Level. (BE = EBF).
Attention: For the measurement, the work function of the spectrometer is relevant!
Calibration via a reference of known binding energy!
Usually the negative value of the BE is used leading to positive values in the spectra.
Contributions to the binding energy:
1. Atomic base-contribution : E0bind
2. Chemical shift: Echem
3. Relaxation term: Erelax
BE = E0bind + Echem + Erelax
11PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Contribution of the atomic species E0bind:
The binding energy represents strength of the electromagnetic interaction between the
electron (n,l,m,s) and the charge of the nucleus
• In gases: BE ≡ Ionization potential (n, l, m, s)
• BE follows energy of levels: BE(1s) > BE(2s) > BE(2p) > BE(3s) …
• BE of a selected orbital increases with Z: BE(Na 1s) < BE(Mg 1s) < BE(Al 1s)…
• BE of a selected orbital is not affected by isotope effects: BE(7Li 1s) = BE(6Li 1s)
Primary PE structure: Contrib. of the atomic species
12PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
PE structure: Peak labeling
Binding Energy / eV
Spin-orbit coupling: |j| = l + sCu 2p3/2
Total angular momentum quantum number j
Azimuthal quantum number l
Principal quantum number n
Chemical symbol
E
M
L
K
3
2
1
n l |j| BE / eV
3d5/2
2s
3s
1s
2p3/2
2p1/2
3d3/23p3/2
3p1/2
1/2
1/2
1/2
3/2
1/2
1/2
3/2
3/2
5/2
s (l=0)
s (l=0)
p (l=1)
s (l=0)
p (l=1)
d (l=2)
13PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
PE structure: Binding energy of core electrons
14PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Primary PE structure: Chemical shifts
The chemical shift Echem:
The energy of the core shell electron level is
influenced by the atom´s electron density of the
outer shells.
Shielding of the core shell electrons!
Influenced by the electro-negativity of the next
neighbor atoms!
The BE depends on the chemical state
Chemical shift as a fingerprint
Electron density of H2O
(Calculated with density functional
DFT)
15PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Primary PE structure: Chemical shifts
Core level chemical shifts:
• related to the overall charge on the atom
reduced charge increased B.E.
• number of substituents
• electronegativity of the substituent
• formal oxidation state
Chemical Shift is important for identifying:
• functional groups
• chemical environments
• oxidation states
A simplified explanation:
Remove a d shell electron, e.g. by bonding to oxygen
Levels shift down (higher BE) simply by
electrostatic reasons
M M+
16PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
PE structure: Chemical shift as a fingerprint
Experimental BE / eVJ.H. Scofield, J. Elect. Spec. 8 (1976) 129.
Cal
cula
ted
BE
/ e
V
2
BAAB
B
ABA,p
ii
i
i
ENEN25.0exp1
:Ionicity
eq
q
:eargchs'PaulingEffective
17PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Classical example: Chemical shifts for the C1s peak
Ethylene-trifluoroacetate:
C2H5-O-CO-CF3
All four carbon atoms have a different
neighboring atoms species that have different
electro-negativities.
All four carbon atoms have a different
chemical environment.
XPS spectrum shows 4 C1s peaks with
three different chemical shifts.
18PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Examples of XP-spectra
Si2p
After removal
of background
and Si2p1/2
Oxide covered Si:
N-containing adsorbates on Si:
N1s
19PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
XPS investigations on TiO2 powder samples
• chemical composition of particles?
• oxidation state of Ti?
• immobilization of nanoparticles (d = 110 nm) on an Indium foil
detector
In
monochromatoranalyzer
TiO2 particles
Indium foil
X-ray source
20PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
XPS investigations on TiO2 powder samples
• survey spectrum: XPS- and Auger-Peaks
O K
LL
Ti
LM
MT
iL
MM
1
Ti2
sO
1s
Ti2
p
C1s
Ti3
sT
i3p
O2s
21PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
XPS investigations on TiO2 powder samples
C-C
C-O
x 102
2
4
6
8
10
12
CP
S
291 288 285 282Binding Energy (eV)
O
OH
x 102
20
30
40
50
60
70
80
CP
S
534 531 528 525Binding Energy (eV)
Ti2p3/2
Ti2p1/2
x 102
10
20
30
40
50
60
CP
S
468 464 460 456 452Binding Energy (eV)
Ti2p O1s
13.3 at% 37.6 at%
C1s
peak area of 1:2
is fixed by spin-
orbit coupling!
1.1 at%
30,1 at%17.9 at%
quantification of sample composition via detail spectra
oxidation state of Ti is Ti4+ as followed by the Ti2p doublet
significant amount of carbon: residues of particle synthesis and/or contaminations
oxides and hydroxides, in total excess in oxygen compared to the 1:2 stochiometry of TiO2
no information about TiO2 modification (Anatase vs. Rutile vs. amorphous) in XPS
Binding Energy (eV)Binding Energy (eV)Binding Energy (eV)
22PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Chemical shifts of Ti compounds
Binding Energy / eV
454456458460
Ti vale
ncy
0
1
2
3
4
5
Ti (metallic)
TiO
Ti2O
3
TiO2
Ti 2p3/2
470 465 460 4550
2
4
6
8
10
22.0 %
CP
S /
10
3
Binding energy / eV
Ti2p exp.
Ti2p3 458.8 eV
44.8 %
23PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Secondary Structure: Surface charging
Surface charging:
Electrical insulators cannot dissipate charge generated by
the photoemission process!
Surface picks up excess positive charge!
All peaks shift to higher binding energies
Especially: Organic compounds, oxides and ceramics
Uncritical: Metals, semiconductors, very thin films
Can be reduced by exposing the surface to a neutralizing
flux of low energy electrons: “flood gun” or “neutralizer”
But the charge can be overcompensated by the neutralizer!
Good reference peak important!
Often used C1s: C-C 284.5 eV- -
- --
Insulating
sample
Insulating
sample
24PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Quantitative analysis: Photoelectron intensity
)cos(MAKINAKINAA NETLEDhI
x
z
y
hν
δ
γ
Φ
θ ē
IA – Photoelectrons current from A-Element
A – Element in Matrix M
LA(γ) – Angular asymmetry of the intensity
of the intensity of the photoemission from each atom
T(xyγΦEA) – analyzer transmission
NA – atom density of the A atoms at (xyz)
(hν) – cross-section for emission of a
photoelectron from the relvant inner shell
per atom of A by photon of enrergy hν
z ∞
AKINAAA NETLSI )(
SA( ) – Sensitivity factor for element A
– Take-off angle of photo electrons
25PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Quantification: sensitivity factors
Quantification:
If is difficult to apply calculated cross sections directly to the measured data sets
(e.g. other instrumental data sets need to be included, as well as loss processes lowering the
intensity at the peak position)
Most analyses use empirical calibration factors
(called atomic sensitivity factors) derived from
standards:
Imeasured = SA x Na
Note: Sensitivity for each element in a
complex mixture can vary!
A typical accuracy of less that 15% can be reached
by using sensitivity factors!
26PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Quantification: Background subtraction
How to determine Imeasured?
• Remember: “Stepped” structure of the
background signal
• Suitable background determination needed:
Shirley background usually used!
By using a suitable background:
• Accuracy better than 15 % using ASF's
• Use of standards measured on same
instrument or full expression above
accuracy better than 5 %
• In both cases, reproducibility (precision)
better than 2 %
Shirley background
linear background
step background
[D.A. Shirley, Phys. Rev. B5, 4709, 1972]
27PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Quantification example: TiO2 on top of stainless steel
0200400600800100012000.0
0.2
0.4
Ti
LM
M
CP
S /
10
5
Ti
LM
M1
Ti2
s
O2
sTi3
pT
i3s
O K
LL
O1
s
C1
s
Binding energy / eV
Ti2
p
Atomic %
Ti2p3 22.9
O1s 50.8
C1s 26.3
sputtered TiO2 film
stainless steel
23 nm TiO2 film covers the whole surface
Ti oxidation state: Ti4+
Ti:O ratio ≈ 1:2
N
SampleX
SampleX
Sample
Sample
ASF
Area
ASF
Area
atc
1
1
1
%)(
Peak ASF
Ti2p3 1.385
O1s 0.733
C1s 0.314
28PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Quantification: Information depth
The Intensity of the photoelectrons is attenuated by the strong electron-electron interaction
Universal mean free path curve!
High surface sensitivity
The attenuation follows an exponential decay (Lambert-Beer):
The Intensity of the emitted PE can be calculated by integration:
SiO2
suboxide
Si-substrate
-
-
-
xI exp~
b
ax
dxx
II exp0
Mea
n f
ree
path
/
Å
29PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Information depth: Native oxide on top Si
native oxide
Si-substrate
2-3 nm-
-
Si2p
Si2s
Si-substrate
Si-substrate
native oxide
native oxide
30PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Angle resolved XPS: AR-XPS
-
10 nm
Detector
= 90°
sampling depth: 10 nm
-
Detector
= 35°
sampling depth: 5.7 nm
-
Detector
= 10°
sampling depth: 1.7 nm
sinexp
dI sin3sin)ln(Id
31PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Contents
• Electron Spectroscopy of Surfaces
• Theory
• Applications
• Optical Spectroscopy
• FT-IR spectroscopy - Theory
• Applications
32PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Optical spectroscopy - Introduction
Material
Layer
Environment
External reflection:
Material has to reflect
the corresponding
radiation
Internal reflection:
Material needs to be transparent in the
corresponding wavelength region
Information (can be gained most often in-situ):
Layer thickness, optical constants, chemical composition, diffusion constants,
reaction kinetics, orientation, adsorption, corrosion, …
Diffraction:
33PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Beer–Lambert–Bouguer law
xeIxIxIdx
xdI0;
Assumptions:
• Light has to be monochromatic and parallel
• Molecules have to be molecularly dispersed
• There is no diffraction or reflection
Beer:
dI is proportional to the concentration c!
dxIcdI
xceIxI 0
Path length x
I0 I
molar absorptivity of the absorbing species
34PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
IR Fundamentals
41 10min
1cmin~
Section of the electromagnetic spectrum: 12500 – 10 cm-1 (0.8 – 1000 m)
Transmittance, T:
Ratio of radiant power transmitted by the sample (I) to the radiant power
incident on the sample (I0).
Absorbance, A:
Logarithm to the base 10 of the reciprocal of the transmittance (T)
010 IIlgTlg
T1logA
Measurement:
1. Measure Reference Single Beam Spectrum (I0)
2. Measure Sample Single Beam Spectrum (I)
3. Divide I/I0
35PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
A Simple Spectrometer Layout
Fourier Transform IR-Spectrometers: All frequencies are examined simultaneously
Source: http://mmrc.caltech.edu/FTIR/FTIRintro.pdf
36PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
BRUKER Vertex 70
37PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Attenuated Total Reflection (ATR) Spectroscopy
• The electric field of a wave reflected from an interface probes slightly beyond the
interface.
• This penetrating wave is called an evanescent wave.
• It sends energy back and forth across the interface so that absorptions on one side
are transmitted back to the other
• The intensity decays exponentially away from the interface so the signal is
weighted in favour of species closer to the interface.
ZnSe(n=2.4) or Ge (n=4.00)
n2< n1dp
D
n1
38PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Reflection FTIR-spectroscopy
Attenuated Total Reflection (ATR) and Multiple Internal Reflection (MIR) Spectroscopy
2
1
22
1 sin2n
nn
dp
crystal
sample
Multiple reflection unitsingle reflection unit
n1
n2< n1
D
dp
sample
• enables to measure surfaces as received without any
further sample preparation.
• permits characterisation of solid surfaces and thin
layers
• allows the in-situ-measurement of swelling of
polymeric films
• Penetration depth of the beam depends on diffraction
index of the crystal and the angle of incidence
Characteristics of
ATR-IR spectroscopy
39PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
ATR correction
Depth of penetration correction for ATR
spectra of PET: (a) spectrum as measured; (b)
correction function by which the original
spectrum is multiplied. The correction
function is linear in wavenumber (the
wavenumber scale is reproduced on the right-
hand axis); and (c) the corrected spectrum.
This spectrum has been scaled after
multiplication.
Fourier Transform Infrared Spectrometry, PETER
R. GRIFFITHS, JAMES A. de HASETH, A JOHN
WILEY & SONS, INC., PUBLICATION
40PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
IR-Spectroscopy of fine NP powders and SAMs
[1] http://www.nuance.northwestern.edu/keckii/ftir7.asp
[1]
Diffuse Reflection Infra-Red FT-Spectroscopy (DRIFTS)
DRIFTS collects and analyzes scattered IR radiation
It is used for measurement of fine particles and powders, as well as rough surfaces (e.g., the interaction of a surfactant with the inner particle, the adsorption of molecules on the particle surface)
Sampling is fast and easy because little or no sample preparation is required
Control of:
• Humidity
• Temperature
• IR-Radiation
41PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Al2O3
Water immersion cycle 1-4 = A-D
IR-Spectroscopy of fine NP powders and SAMs
[2] Thissen, Valtiner, Grundmeier; Langmuir 2009, 26(1), 156-164
Diffuse Reflection Infra-Red FT-Spectroscopy (DRIFTS) Example 1: Organic phosphonate Monolayer (ODPA-SAM) on Al2O3(0001) single
crystal
Problem: Al2O3 single crystals are non reflective IRRAS is not possible
Desorption of ODPA from Al2O3(0001) could be observed
[2]
42PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
IR-Spectroscopy of fine NP powders and SAMs
Diffuse Reflection Infra-Red FT-Spectroscopy (DRIFTS) Example 2: TiO2 NP powder changes surface OH-group density during
UV-light exposure
observation of particle ensembles instead of immobilized particles (IRRAS)
reactor allows the simultaneous surface modification by UV-light and water
adsorption and the control of the temperature
UV-source ON UV-source OFF
TiO2 - OH
TiO2 - CxHy
TiO2 - OH
TiO2 - CxHy
51PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier
Conclusions
Optical and electron spectroscopy allow the analysis of:
• Surface and thin film composition (XPSE, AES)
• Chemical states of elements (XPS, AES)
• Chemical groups (FTIR, Raman)
• Adsorbate formation (all)
• Processes at interfaces in-situ (FTIR, Raman)