e-, hv, ione-, hv, ion tip

Characterization Techniques

Primary Secondary

Characterization Techniques

The inelastic mean free path (IMFP) of electrons is less than 1 nm for electron

energies with 10~1000 eV.

Interaction of high energy (~kV) electrons with (solid) materials

1 Unscattered

2 Low angle elastically scattered

3 High angle elastically scattered

4 Back scattered

5 Outer shell inelastically scattered

6 Inner shell inelastically scattered

Secondary electron

< 50 eV

Backscatter electron

>80% of primary electron

energy

X-ray 0.5 – 20 KeV

Inelastic nuclear

scattering.This results in

radiation which is known as

“Bremsstrahlung”, so the

stopping power is the

“RadiativeStopping Power”.

Elastic scattering on

atoms. Incident electron is

scattered without any

change in energy

Keep in mind…

•A solid sample bombarded with low energy X-rays produces a number of

different types of electrons, all of which need to be understood.

•1,500 eV X-rays will penetrate 1-3 μm into a sample, producing

photoelectrons along the entire depth. Many of these electrons make it out of

the sample and into the analyzer. Most will have lost energy while traveling

through the sample, but some will not. We are primarily interested in the

photoelectrons (or Auger electrons) that have NOT lost energy to some

inelastic loss process before entering the spectrometer.

•Similar processes occur in many analytical techniques.

X-Ray Photoelectron Spectroscopy (XPS, ESCA)

XPS spectral lines are

identified by the shell

from which the electron

was ejected (1s, 2s, 2p,

etc.).

The ejected

photoelectron has

kinetic energy:

KE=hv-BE-

Following this process,

the atom will release

energy by the emission

of an Auger Electron.

Conduction Band

Valence Band

L2,L3

L1

K

Fermi

Level

Free

Electron

Level

Incident X-rayEjected Photoelectron

1s

2s

2p

Strengths of XPS

1.Can detect Li-U

2.Surface sensitive (10-100Å sampling depth)

3.Sensitive to differences in chemical environment

4.Quantitative without the use of standards

5. Manageable charging problems with insulators

Weaknesses of XPS

1.Relatively poor lateral resolution (>5μm) when compared with

charged particle surface techniques like Auger, SIMS.

2.Surface sensitive

3.Detection limits 0.01-1.0%

4.Cannot detect hydrogen

No other analytical technique offers these five features in one technique. This

makes XPS the workhorse surface analytical tool. It should be one of the first

things you consider when you have surface chemical issues.

Why surface sensitive ?

An electron with kinetic energy < 1500 eV

will NOT travel very far through a solid

Main Spectral Features in XPS

Chemical Shift

Electron Spectroscopy for Chemical Analysis (ESCA)

Peak intensity proportional to amount of

element/species present

• not affected by chemical environment

Peak intensity

Carbon 1s spectrum of ethyltrifluoroacetate

• 4 different carbon environments leads to

4 different carbon peaks

Siegbahn K. et al, ESCA: Atomic, Molecular and Soild

State Structure Studied by Means of Electron

Spectroscopy, Nova Acta Regiae (Uppsala), 1967.

Advantage: sensitive to local bonding environment

Large difference in relative amounts of

SiO2 and Si° with only small thickness

differences

Self assembled monolayers on gold

Methoxy terminated thiol:

C-C and C-O carbon

Methyl ester terminated thiol: C-

C and O=C-O carbon

Surface functionalization of carbon nanotubes

050100150200250

102010401060

Na1s

Zn2p1/2

Zn2p3/2

Na2s

Cl2pS2s

C1s

ZnLMM

S2p

Zn3s

Binding Energy (eV)

Zn3d

Zn3p

+ Au4f

Electrochemical Formation of ZnS

Tuba Oznuluer, Ibrahim Erdogan, and Ümit Demir, Electrochemically Induced Atom-by-Atom Growth of ZnS Thin Films: A New Approach for ZnS

Co-deposition, Langmuir 2006, 22, 4415-4419 4415

C1: C-C, C=C, C-H (285.0 eV)

C2: C-O, C-O-C (286.5-6 eV)

C3: O-C-O, C=O (287.9 eV)

C4: O-C=O, C (=O)OH (289.3 eV)

Grafen Oksit ve İndirgenmiş Grafen Oksit

Stoichiometry,

Morphology and Size-

Controlled

Electrochemical

Fabrication of CuxO

(x=1,2) by

Underpotential

Deposition Cemile Kartal, Yeşim Hanedar, Tuba

Öznülüer and Ümit Demir*

Prepared for submission

Electrochemical Synthesis and Photoelectrochemical Properties of Flower-Like and

Grass-Like Nanostructured α-Fe2O3 Photoanodes for Use in Solar Water Oxidation

Yeşim Hanedar, Tuba Öznülüer and Ümit Demir, Submitted for publication

General methods in assisting peak identification

(1) Check peak positions and relative peak intensities of 2 or more peaks

(photoemission lines and Auger lines) of an element

(2) Check spin orbital splitting and area ratios for p, d, f peaks

A marine sediment sample from Victoria Harbor

The following

elements are found:

O, C, Cl, Si, F, N, S,

Al, Na, Fe, K, Cu,

Mn, Ca, Cr, Ni, Sn,

Zn, Ti, Pb, V

Al 2p

Al 2s

Si 2pSi 2s

For p, d and f peaks, two peaks are observed.

The separation between the two peaks are named

spin orbital splitting. The values of spin orbital

splitting of a core level of an element in different

compounds are nearly the same.

The peak area ratios of a core level of an

element in different compounds are also nearly

the same.

Electronic Effects Spin-Orbit Coupling

Peak NotationsPrincipal quantum number

(n) describes the electron shell, Orbital quantum

number, , ℓ

describes the

subshell

The total angular momentum quantum

number: j = |ℓ ± s|Electron spin quantum number (ms or s)

%100% =

i i

i

A

A

S

I

S

I

Atomic

Quantitative Analysis

Peak Area of element A

Sensitivity factor of

element A

Peak Areas / Sensitivity

factors of all other elements

Peak Area measurement

Need background subtraction

Au 4f

Factors Affecting Photoelectron Intensities

ADTFNfI ciici = cos,,

For a homogenous sample, the measured photoelectron intensity is given by

d

Detector

Ii,c: Photoelectron intensity for core level c of element i

f: X-ray flux in photons per unit area per unit time

Ni: Number of atoms of element i per unit volume

i,c: Photoelectric cross-section for core level c of element i

: Inelastic mean free path of the photoelectron in the sample

matrix

: Angle between the direction of photoelectron electron and the

sample normal

F: Analyzer solid angle of acceptance

T: Analyzer transmission function

D: Detector efficiency

A: Area of sample from which photoelectrons are detected

An error of 15% is generally quoted

An error of 1-2% can be achieved if samples with

known concentrations are used as standards

If 20% of Cu is calculated, the Cu concentration should be

in the range of 17-23%.

e. g., determination of the concentrations of Si and N in

SiNx films with a Si3N4 standard.

Quantification Uncertainties

Auger electron

Incident

electron

KL1L2

KE = EK-EL1-E*L2- j,

3keV incident

electron beam

KLL LMM MNN

Auger Electron Spectroscopy (AES)

Auger process:

First remove an inner shell electron to form a

vacancy. This electron removal is generally

done with an electron beam, although x-rays

and ion beams also may be used. The vacancy

is filled by an electron from a higher shell. The

energy released by this process is transferred

to a 3rd electron, the Auger electron. Auger

electrons can be emitted with energies from

10s of eV, to thousands of eV. The alternative to

Auger emission is x-ray emission

The 3 letters specify the energy levels implied in the process of emission of the Auger electrons

Strengths of AES

1.Can detect Li-U

2.Surface sensitive (10-100Å sampling depth)

3.Semi-quantitative without the use of standards

4.Excellent lateral resolution (sub-micron is typical)

5.Sensitive to differences in chemical environment for selected

elements (C,Si,Al,Mg,Cu,Zn,Ti,Ga,Cd,Ag).

6.Rapid depth profiling

Weaknesses of AES

1.Bulk insulators generally cannot be analyzed

2.Quantification not as straightforward as XPS

3.Limited chemical state information

4.Detection limits are 0.01-1.0%

5.Sample damage due to focused electron beam

General fields of study for AES

1.Defect analysis

2.Thin film depth profiling

3.Grain boundary segregation

4.Inter- vs. intra-granular fracture

5.Interdiffusion studies

6.Reverse engineering

7.Surface contamination

Auger is recommended when… the analysis area is too small to be done by

XPS

Typical state-of-the-art XPS system can analyze with ~3-10 μm spatial

resolution.

Specialized XPS systems can obtain sub-micron (to 100nm) spatial resolution

State-of-the-art Auger systems can achieve ~10-15 nm spatial resolution

under ideal conditions

First derivative used to remove large background and to highlight small Auger peaks

AES spectrum from TiO2 thin film

AES spectum; S-Au (A) and CdS-Au (B)

Anthony Gichuhi, B. Edward Boone, Umit Demir, and Curtis Shannon J. Phys. Chem. B 1998, 102, 6499

A

B

Electrochemical formation of CdS monolayer

XPS profile of 200Å SiO2 layer

Ion Beam Depth Profiling in XPS (and AES)

Ions of sufficient energy (0.5-5 kV)

impinge a solid surface; momentum

from bombarding ions is transferred to

the sample, and a portion of that

momentum is re-directed towards the

surface, resulting in some atoms being

removed from the solid. The remaining

surface is analyzed with XPS or AES.

The sputter/analyze cycles are

repeated until a sufficient amount of

material has been etched away.

DEPTH PROFILE USING Ar ION ETCHING

Qualitative analysis

1. First, the main Auger peak positions are

identified.

2. These values are correlated with the

listed values in the Auger spectra book

or standard tables. The main chemical

elements are thus identified.

3. The identified element and transition

are labelled in the spectrum (close to

the negative jump in the differential

spectrum).

4. The procedure is repeated for so-far

unidentified peaks. The Auger spectrum of a sample under investigation

E0 = 3keV

Elemental identification procedure

Example:

From the differential AES spectrum

Ni, Fe and Cr have been identified.

Ni

Fe

Cr

Qualitative analysis

Information concerning chemical composition

• Peak shape and the energy values,

corresponding to maxima contain

information on the nature of the

environment, due to addition relaxation

effects during the Auger process

• A full theoretical model is difficult to

construct.

• In practice, Auger spectra of standard

samples are used and the results are

drawn from spectra comparison.

1. Measuring the peak-to-peak height

dEN(E)/dE vs. E

N(E) vs. E

2. Measuring the peak area

(after background subtraction)

Quntitative analysis

RDTFrNII iiiPi += cos)1(

For a homogeneous sample, the Auger peak intensity is given by:

Ii: Intensity of the detected current, due to the ABC Auger transition of the i element,

IP: Incident beam current,

Ni: Concentration of the element i in the surafce,

i: Ionization cross-section on the A-level of the element i by the electrons from the incident beam,

i: Probability of the Auger ABC transition of the element i,

r: ionization cross-section on the A-level of the element i by the electrons scattered in previous

processes,

: mean free path for inelastic collisions,

: incidence angle of the primary beam,

F: Correction factor, dependent on the entrance solid angle in the analyzer,

T: Transfer function of the analyzer,

D: Detection efficiency,

R: Roughness factor of the surface.

Remarks

1. Deriving Ni from the previous equation is rather difficult, due to large number of implied parameters,

2. In applications, empirical methods are used, which leave from: (a) utilization of standard

specimens; (b) utilization of sensitivity factors.

Factors affecting the peak intensity

Quantitative analysis using standard specimens

Scanning Auger Microscopy (SAM)

AES Auger Electron Spectroscopy

SAM Scanning Auger Microscopy: Same instrument can provide SEM

imaging, Auger spectra and chemical Auger mapping.

Specimen

Focussing &

scanning system of

the incident e-beam

Applications

• 1keV incident electron beam →

penetration depth of about 15 Å.

• Verification of surface

contamination freshly prepared in

UHV.

• Investigation of the thin film growth

process + elemental analysis.

• Depth profiling of concentration of

chemical elements.

SAM image:

red =Al; blue = F; magenta = Al + F

Red = Al; green = O red =Al; blue = F; green = O

SEM surface image

Al+F+O

SAM

SEM and Auger images of an aluminium oxide surface, in absence and presence of

fluorine contramination.

Energy-dispersive X-ray spectroscopy (EDS / EDX)

hv

Bremsstrahlung

Characteristic X-ray

Goldstein representation of EDS lines

Ka

Kb

L-series

Fe

Cr

Kb

Ka

Ni

KbKa

– Characteristic x-rays

• elemental identification

• quantitative analysis

Continuum x-rays

• background radiation

• must be subtracted for quantitative analysis

X-ray spectrum from stainless steel

Characteristic emission + bremsstrahlung = Complete spectrum

Qualitative Analysis

M-series

L-series

K-series

6.4

Fe(26)

Ka7.0

Fe(26)

Kb

4.5

4.9

26

Quantitative analysis

• Net intensity = complete intensity for the peak ÷ background intensity

• Net intensity is not completely proportional to the amount of atoms, but corrections should be made for

– Z: atomic number(Z) influence on the number of excitations

– A: Absorption of x-ray emission

– F: Fluorescence – generation of characteristic x-ray emission from higher energy x-ray

Absorption

X-ray

detector

d

e

1

-

• Absorption coefficient

depends on the angle

between detector and

the normal to the

surface

• A long travel = large

absorption

• The normal of the

surface should point

towards the detector

Fluorescense

The electron in the inner

shell can be loosened if the

x-ray has a higher energy

than the escape energy of

the electron.

During decay characteristic

x-ray is emitted and the

intensity of x-ray with the

high energy is diminished

and the intensity of x-ray

with the low energy

increases.

Quantitative analysis

hvP/B-ZAF – correction

Requires good background determination

Insensitive towards sample roughness –

because the background emission and the

characteristic x-ray emission come from

the same area in the sample and the

background goes into the calculations.

Inhomogeneous samples – no possibility of correction!

ZAF-correction requires that the interaction volume is homogeneous

No use just to sample from a larger volume

hv

hv

Al Si

hv

hv

Al Si

+÷

Quantitative analysis - Summary

Uncertainty on the quantitative analysis

• Determination of bremsstrahlung

• Overlapping peaks

• Absorbtion of the characteristic x-ray emission

• Fluorescence – excitation of atoms with lower atomic number

Limitations

• Quantitative analysis on elements with low atomic number (Z<10) is

difficult

• Lower detection limit ~0,1-1%

• Resolution ~1µm

Possibilities

• Elemental mapping

• Using backscattered electrons for

mapping gives much higher

atomic number contrast

• Using the signal from the EDS-

detector enables to make

mappings of a single element

Elemental mapping

EDS Applications

• Used to determine the elemental composition of a sample.

• Can perform both qualitative (What is it?) and quantitative (How much?) analysis.

• Depending on the window low atomic number elements may not be visible.

• Super ultra thin windows detect down to berilium.

• Older detectors may only detect fluorine and higher.

• Window less detectors are available.

Secondary Ion spectroscopy: SIMS

material

Ions in Ions out

Is very sensitive

for some elements

on the ppm level

Modes: static and dynamic, secondary electron

Probe: Ions (Ar+, Ga+, Cs+, C60+, etc.), keV

Signal: Secondary Ions (from sample)

Information: Elemental and Molecular Composition

Sample: Any that can withstand ultra-high vacuum

Principle: Bombardment ions liberate secondary particles from the surface, secondary ions can be detected for mass

Depth: 10 Å (more in dynamic mode)

Spatial Resolution: less than 1 mm2

Sensitivity: “very high”

Relative Cost: Expensive

Other: Semi-quantitative to quantitative

Can resolve isotopes

Imaging modes

ToF detectors lead to exact mass detection

This is a (by design) destructive technique…

Bombardment of a sample surface with a primary ion beam (Ip) followed by mass

spectrometry of the emitted secondary ions (Is) constitutes secondary ion mass

spectrometry. SIMS is a surface analysis technique used to characterize the surface

and sub-surface region of materials and based on m/e ratio measurement of ejected

particles under ion bombardment.

Primary ion beam:

Cs+, O2+, Ar+ and Ga+

at energies ~ a few keV.

Neutral & charged

(+/-) species

Advantages of SIMS

• Very sensitive-can reach parts per billion range

• Ability to do depth profiling

• Ability to identify all elements

Disadvantages of SIMS

• Only ionized particles are measured

• Sputtering not necessarily even

• Intrinsically destructive

• Dynamic SIMS, 1nA/cm2, 1mm/hr

• Static SIMS, 1mA/cm2, 1A/hr

• Expensive $$$$

Dynamic SIMS vs Static SIMS

Static mode involves the

bombardment of a sample

with an energetic (1-10

keV) beam of particles.

In static SIMS, the use of

low dose of incident

particles is critical to

maintain the chemical

integrity of the sample

surface during analysis

a focused beam of ions that

are sufficiently energetic to

cause ejection (sputtering) of

atom and small clusters of

atoms from the bombardment

region is used

Dynamic SIMS – Depth Profiling

Factors affecting

depth resolution

Coal Powder

• Identification of foreign materials

– Particulates

– Fibers

– Residues

• Identification of bulk material compounds

• Identification of constituents in multilayered

materials

• Quantitation of silicone, esters, etc., as

contamination on various materials

Fig. Comparison of Sample to FTIR Spectral Library

FT-IR Spectroscopy, Attenuated Total Reflectance (ATR)

FTIR passes IR radiation

through a sample, and

measures the wavelengths at

which energy is absorbed.

Because molecules can

vibrate by stretching, bending

and twisting and absorb

varying amounts of energy at

each frequency, FTIR

provides structural and

chemical information.

FTIR of (a) raw MWCNT, (b) oxidized MWCNT, (c)

chitosan and (d) the chitosan-MWCNT

nanocomposite

SPR

• A surface-plasmon-resonance is excited at a metal-dielectric interface by a monochromatic, p-polarized light beam, such as He-Ne laser beam

• The surface plasmon is sensitive to changes in the environment near the interface and therefore has potential as a sensing probe.

• Sensitive detection method that monitors variations in thickness and refractive index in ultra-thin films

A plasmon can be thought of as a ray

of light bound onto a surface -

propagating among the surface and

presenting itself as an

electromagnetic field.

Gold film

Incident light Reflected light

Glass slide

Bound antibodies

% R

efl

ec

tivit

y

Reflection angle

Prism

% R

efl

ec

tivit

y

Reflection angle

Bound antibodies

Flow cell system

Gold film

Incident light

Reflected light

Change in angle of reflection

Free antigen

Glass slide

Antigen-Antibody

complex

Prism

SIGNAL DETECTION

• The light source for SPR is a high efficiency near-infrared light emitting diode which has a fixed range of incident angles. The SPR response is monitored by a fixed array of light sensitive diodes covering the whole wedge of reflected light. The angle at which minimum reflection occurs is detected and converted to the resonance units. The SPR angle depends on several factors, most notably the refractive index into which the evanescent wave propagates on the non-illuminated side of the surface. In addition the other parameters are the wavelength of the incident light and the properties of the metal film.

Key questions in nanoanalysis

• Identification (What?)

• Localisation (Where?)

• Quantification (How much?)