SurfaceAnalysis by means of Electron and Vibrational ... · Photoelectron emission by means of X-rays Ultraviolet Photoelectron Spectroscopy UPS UV photon E kin = h -E V-X-ray photon

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


Contents

• Electron Spectroscopy of Surfaces

• Theory

• Applications

• Optical Spectroscopy

• FT-IR spectroscopy - Theory

• Applications


Electron Beam - Sample Interaction

5-50 nm

Continuum X-rays

(Bremsstrahlung – “breaking radiation”)

X-ray

Fluorescence


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


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)


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


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


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


Typical XP Spectrum


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


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


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)


PE structure: Binding energy of core electrons


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)


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+


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


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.


Examples of XP-spectra

Si2p

After removal

of background

and Si2p1/2

Oxide covered Si:

N-containing adsorbates on Si:

N1s


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



• 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



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)


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 %


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


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


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!


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]


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


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


Information depth: Native oxide on top Si

native oxide

Si-substrate

2-3 nm-

-

Si2p

Si2s

Si-substrate

Si-substrate

native oxide

native oxide


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


Contents

• Electron Spectroscopy of Surfaces

• Theory

• Applications

• Optical Spectroscopy

• FT-IR spectroscopy - Theory

• Applications


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:


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


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


A Simple Spectrometer Layout

Fourier Transform IR-Spectrometers: All frequencies are examined simultaneously

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


BRUKER Vertex 70


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


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


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


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


Al2O3

Water immersion cycle 1-4 = A-D


[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]



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


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)

Documents

SurfaceAnalysis by means of Electron and Vibrational ... · Photoelectron emission by means of X-rays Ultraviolet Photoelectron Spectroscopy UPS UV photon E kin = h -E V-X-ray photon