Guido Grundmeier

Title 

Surface Analysis by means of Electron 

and Vibrational Spectroscopy 

Prof. Dr. Guido Grundmeier 

University of Paderborn 

PIKO Symposium, Bremen, 2011, Prof. Dr. G. Grundmeier 

1

Contents 

• Electron Spectroscopy of Surfaces 

• Theory 

• Applications 

• Optical Spectroscopy 

• FT-IR spectroscopy - Theory 



2

Electron Beam - Sample Interaction 

5-50 nm 

X-ray 

Fluorescence 

Continuum X-rays 

(Bremsstrahlung – “breaking radiation”) 


3

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 


4

Mean free path of electrons in solid matter 

(1 ML ~ 2.5 Å) 

UPS 

XPS and AES 

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! 

G.A. Somorjai, “Introduction to Surface Chemistry and Catalysis” (Wiley, New York, 1994), p.383 


5

Photoelectron emission by means of X-rays 

Electron or X-ray 

photon 

Auger Electron 

Spectroscopy 

AES 

X-ray Photoelectron 

Spectroscopy 

XPS 

X-ray photon 

Ultraviolet Photoelectron 

Spectroscopy 

UPS 

UV photon 

E vac 

E vac 

V 

E L2,3 

2p 1/2 , 2p 3/2 

E L1 

E K 

2s 1/2 

1s 1/2 

E kin = E K -E L1 -E L23 - 

E kin = h -E K - 

E kin = h -E V - 


6

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 

h 

I 

1 

2 

2 

m e 

v 

For XPS/ 

M h 

UPS: 

M 

e 


7

Photoelectron emission by means of X-rays 

E kin 

0 

Now excitation with X-rays: h >> 

Determination of core levels 

Reflect inner electronic structure 

Energy balance: 

h 

E vac = 0 

E F 

VB 

E B 

h 

h 

core level 

core level 

h 

I 

DOS 

E B 

E kinetic = h – E binding (solid) 

Since each element has unique set of core levels 

E kin 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 E B + > hν 

• E kin of photoelectron increases as E B decreases 


8

Typical XP Spectrum 


9

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 = E BF ). 

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. 

2. 

Atomic base-contribution : E 0 bind 

Chemical shift: E chem 

3. Relaxation term: E relax 

BE = E 0 bind + E chem + E relax 


10

Primary PE structure: Contrib. of the atomic species 

Contribution of the atomic species E 0 bind: 

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) 


11

PE structure: Peak labeling 

Cu 2p 3/2 

Spin-orbit coupling: |j| = l + s 

Total angular momentum quantum number j 

Azimuthal quantum number l 

Principal quantum number n 

Chemical symbol 

E 

M 

L 

3 

2 

d (l=2) 

p (l=1) 

s (l=0) 

p (l=1) 

s (l=0) 

5/2 

3/2 

3/2 

1/2 

1/2 

3/2 

1/2 

1/2 

3d 5/2 

3d 3/2 

3p 3/2 

3p 1/2 

3s 

2p 3/2 

2p 1/2 

2s 

Binding Energy / eV 

K 

1 

s (l=0) 

1/2 

1s 

n l |j| BE / eV 


12

PE structure: Binding energy of core electrons 


13

Primary PE structure: Chemical shifts 

The chemical shift E chem : 

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 H 2 O 

(Calculated with density functional 

DFT) 


14

Primary PE structure: Chemical shifts 

A simplified explanation: 

Remove a d shell electron, e.g. by bonding to oxygen 

Levels shift down (higher BE) simply by 

electrostatic reasons 

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 

M M + 


15

Calculated BE / eV 

PE structure: Chemical shift as a fingerprint 

EffectivePauling's charge : 

q 

p,A 

AB 

i 

q 

e 

Ionicity: 

B 

i 

AB 

1 exp 0.25 

i 

EN 

A 

EN 

B 

i 

2 

Experimental BE / eV 

J.H. Scofield, J. Elect. Spec. 8 (1976) 129. 


16

Classical example: Chemical shifts for the C1s peak 

Ethylene-trifluoroacetate: 

C 2 H 5 -O-CO-CF 3 

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. 


17

Examples of XP-spectra 

Oxide covered Si: 

Si2p 

After removal 

of background 

and Si2p 1/2 

N-containing adsorbates on Si: 

N1s 


18

XPS investigations on TiO 2 powder samples 

• chemical composition of particles? 

• oxidation state of Ti? 

• immobilization of nanoparticles (d = 110 nm) on an Indium foil 

detector 

X-ray source 

analyzer 

monochromator 

TiO 2 particles 

Indium foil 

In 


19

Ti LMM 

Ti LMM1 


• survey spectrum: XPS- and Auger-Peaks 

Ti3s Ti3p 

O KLL 

Ti2s 

O1s 

Ti2p 

C1s 

O2s 


20

CPS 

CPS 

CPS 


‣ quantification of sample composition via detail spectra 

60 

50 

40 

30 

20 

10 

x 10 2 

Ti2p 

peak area of 1:2 

is fixed by spinorbit 

coupling! 

Ti2p1/2 

Ti2p3/2 

17.9 at% 

80 

70 

60 

50 

40 

30 

20 

x 10 2 

O1s 

OH 

13.3 at% 37.6 at% 

O 

12 

10 

8 

6 

4 

2 

x 10 2 

C1s 

1.1 at% 

C-O 

C-C 

30,1 at% 

468 464 460 456 452 

Binding Energy (eV) 


534 531 528 525 



291 288 285 282 



‣ oxidation state of Ti is Ti 4+ 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 TiO 2 

‣ no information about TiO 2 modification (Anatase vs. Rutile vs. amorphous) in XPS 


21

Chemical shifts of Ti compounds 

5 

Ti 2p 3/2 

4 

TiO 2 

Ti valency 

3 

2 

10 

8 

Ti2p exp. 

Ti2p3 458.8 eV 

Ti 2 

O 3 

TiO 

6 

1 

CPS / 10 3 

4 

22.0 % 

2 

0 

0 

470 465 460 455 

Binding energy / eV 

44.8 % 

Ti (metallic) 

460 

458 

456 

454 

Binding Energy / eV 


22

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 

23

Quantitative analysis: Photoelectron intensity 

I 

A 

A 

h 

D E 

KIN 

L 

A 

T 

E 

KIN 

N 

A 

M 

cos( 

) 

I S ( ) L 

A 

A 

A 

T 

E 

KIN 

N 

A 

z 

∞ 

hν 

γ 

δ 

I A – Photoelectrons current from A-Element 

A – Element in Matrix M 

L A (γ) – Angular asymmetry of the intensity 

of the intensity of the photoemission from each atom 

T(xyγΦE A ) – analyzer transmission 

N A – atom density of the A atoms at (xyz) 

x 

θ 

Φ 

ē 

(hν) – cross-section for emission of a 

photoelectron from the relvant inner shell 

per atom of A by photon of enrergy hν 

z 

y 

S A ( ) – Sensitivity factor for element A 

– Take-off angle of photo electrons 


24

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: 

I measured = S A x N a 

‣ 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! 


25

Quantification: Background subtraction 

How to determine I measured ? 

• Remember: “Stepped” structure of the 

background signal 

step background 

• Suitable background determination needed: 

Shirley background usually used! 

By using a suitable background: 

linear background 

• Accuracy better than 15 % using ASF's 

• Use of standards measured on same 

instrument or full expression above 

accuracy better than 5 % 

Shirley background 

• In both cases, reproducibility (precision) 

better than 2 % 


[D.A. Shirley, Phys. Rev. B5, 4709, 1972] 

26

Ti LMM1 

Ti LMM 

O KLL 

Ti2s 

Quantification example: TiO 2 on top of stainless steel 

0.4 

0.2 

CPS / 10 5 

O1s 

Ti2p 

C1s 

O2s Ti3p Ti3s 

Atomic % 

Ti2p3 22.9 

O1s 50.8 

C1s 26.3 

c( 

at%) 

N 

1 

Area 

ASF 

Area 

ASF 

1 

Sample 

1 

Sample 

X 

Sample 

X 

Sample 

Peak ASF 

Ti2p3 1.385 

O1s 0.733 

C1s 0.314 

0.0 

1200 

1000 

800 600 400 

Binding energy / eV 

200 

0 

TiO 2 film covers the whole surface 

Ti oxidation state: Ti 4+ 

Ti:O ratio ≈ 1:2 

sputtered TiO 2 film 

stainless steel 

23 nm 


27

Mean free path / Å 

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: 

I 

x 

I ~ exp 

I 

b 

0 

x a 

exp 

x 

dx 

SiO 2 

- 

- 

suboxide 

Si-substrate 

- 


28

Information depth: Native oxide on top Si 

Si2p 

Si-substrate 

- 

native oxide 

Si-substrate 

- 

2-3 nm 

native oxide 

Si2s 

native oxide 

Si-substrate 


29

Angle resolved XPS: AR-XPS 

Detector 

Detector 

Detector 

- 

- 

- 

10 nm 

= 90° 

sampling depth: 10 nm 

= 35° 

sampling depth: 5.7 nm 

= 10° 

sampling depth: 1.7 nm 

I 

exp 

d 

sin 

d 

ln(I) 

sin 

3 

sin 


30

Contents 

• Electron Spectroscopy of Surfaces 

• Theory 


• Optical Spectroscopy 

• FT-IR spectroscopy - Theory 



31

Optical spectroscopy - Introduction 

Diffraction: 

Environment 

Layer 

External reflection: 

Material has to reflect 

the corresponding 

radiation 

Material 

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, … 


32

Beer–Lambert–Bouguer law 

Assumptions: 

• Light has to be monochromatic and parallel 

• Molecules have to be molecularly dispersed 

• There is no diffraction or reflection 

dI x 

dx 

I 

x 

; 

I 

x 

I 

0 

e 

x 

Beer: 

dI is proportional to the concentration c! 

dI 

I x I0 

e 

c I 

dx 

c x 

I 0 

I 

molar absorptivity of the absorbing species 

Path length x 


33

IR Fundamentals 

Section of the electromagnetic spectrum: 12500 – 10 cm -1 (0.8 – 1000 m) 

~ 

in 

cm 

1 

Transmittance, T: 

Ratio of radiant power transmitted by the sample (I) to the radiant power 

incident on the sample (I 0 ). 

1 

in 

Absorbance, A: 

Logarithm to the base 10 of the reciprocal of the transmittance (T) 

m 

10 

4 

A 

log 

1 

T 

lg T 

lg 

10 I 0 

Measurement: 

1. Measure Reference Single Beam Spectrum (I 0 ) 

2. Measure Sample Single Beam Spectrum (I) 

3. Divide I/I 0 

I 


34

A Simple Spectrometer Layout 

Fourier Transform IR-Spectrometers: All frequencies are examined simultaneously 

Source: http://mmrc.caltech.edu/FTIR/FTIRintro.pdf 


35

BRUKER Vertex 70 


36

Attenuated Total Reflection (ATR) Spectroscopy 

D 

d p 

n 2 < n 1 

n 1 

ZnSe(n=2.4) or Ge (n=4.00) 

• 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. 


37

Reflection FTIR-spectroscopy 

Attenuated Total Reflection (ATR) and Multiple Internal Reflection (MIR) Spectroscopy 

Characteristics of 

ATR-IR spectroscopy 

d p 

2 

n1 sin 

D 

2 

n 

n 

2 

1 

2 

• 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 

d p 

n 1 

n 2 < n 1 

single reflection unit 

sample 

crystal 

sample 

Multiple reflection unit 


38

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 righthand 

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 


39

IR-Spectroscopy of fine NP powders and SAMs 

Diffuse Reflection Infra-Red FT-Spectroscopy (DRIFTS) 

Control of: 

• Humidity 

• Temperature 

• IR-Radiation 

• 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 

[1] 


[1] http://www.nuance.northwestern.edu/keckii/ftir7.asp 

40



• Example 1: Organic phosphonate Monolayer (ODPA-SAM) on Al 2 O 3 (0001) single 

crystal 

Problem: Al 2 O 3 single crystals are non reflective IRRAS is not possible 

Water immersion cycle 1-4 = A-D 

Al 2 O 3 

[2] 

Desorption of ODPA from Al 2 O 3 (0001) could be observed 


[2] Thissen, Valtiner, Grundmeier; Langmuir 2009, 26(1), 156-164 

41



• Example 2: TiO 2 NP powder changes surface OH-group density during 

UV-light exposure 

UV-source ON 

UV-source OFF 

TiO 2 - OH 

TiO 2 - C x H y 

TiO 2 - OH 

TiO 2 - C x H y 

• 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 


42

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) 


51

Guido Grundmeier

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