Modern characterization

MODERN
TECHNIQUES
OF MATERIALS
CHARACTERIZATION
Dr. N. SELVAKUMAR, M.E., Ph.D., FIE.,
Senior Professor,
Department of Mechanical Engineering,
Mepco Schlenk Engineering College, Sivakasi.
E mail: nselva@mepcoeng.ac.in
ATAL Sponsored FDP on
“Research Aspects on Thermal Barrier Coatings”
06/12/2021 to 10/12/2021

EVOLUTION OF NANO-TECHNOLOGY
Mol. Biology

Basic concept
• Source – What kind of “probe“ is used?
• How does the probe reach the sample?
• Interaction between probe and sample.
• How does the signal of interest reach the analyzer?
• Characteristics of the analyzer.
Source
Sample
Analyzer
Interaction

What kind of probes are available?
• Each and every analysis is based on the interaction between a
probe and a sample. The following probes are generally available:
• Electrons - Hot cathode, field emission
• Ions - Plasma, liquid metal tips
• Neutrons - Nuclear reactions
(e.g. Spallations-sources)
• Photons - Laser
X-ray
Synchrotron radiation
• Heat - …
• A field - electric, magnetic fields

Analysis Techniques (principle)
Electrons Ions Neutrons Photons Heat A field
Electrons
Ions
Neutrons
Photons
Heat
A field
Signal
Probe

Energy of a particle → Wavelength

Analysis of the structure
• Usually one starts with the direct physical imaging of a sample
surface
– Optical microscope
– SEM / Auger (scanning electron microscopy)
– TEM (transmission electron microscopy)
– STM / AFM (scanning tunneling microscopy / atomic force microscopy)
– LEERM (low energy electron reflection microscopy)

Indirect analysis of the structure
• Diffraction of electrons, atoms or ions is used to gain insight to the
atomic structure of the sample surface
– XRD (X-ray diffraction) – surface analysis by crazing incidence X-ray
diffraction
– LEED (low electron energy diffraction) - MEED
– ABS (atomic beam scattering)
– LEIS (low energy ion scattering) – MEIS, HEIS
– RBS (Rutherford back scattering)
– RHEED (reflection high energy electron diffraction)
– SEXAFS (surface enhanced X-ray absorption fine structure)
– XANES (X-ray absorption near edge structure)
– SEELFS (surface extended energy loss fine structure)

Chemical analysis of the surface
• Basic determination of elements present at the surface
• Determination of chemical bonding and atomic or molecular states in
the surface region
• Lateral and depth profiling of elemental distribution
– XPS (X-ray photoelectron spectroscopy)
– UPS (ultraviolet photoelectron spectroscopy)
– AES (Auger electron spectroscopy)
– SIMS (secondary ion mass spectrometry)
– FTIR (Fourier transform infrared spectroscopy)
– ATR (attenuated total reflectance spectroscopy)
– Raman spectroscopy

IMAGING OF THE OBJECTS
Under the best of conditions
• Human eye - 200 µm
• Optical microscope - 0.40 µm
• SEM - 5-10 nm
• SPM - 0.1-2 nm
TEM - 0.1 nm

11
The Light Microscope
– Bright-field microscope
– Dark-field microscope
– Phase-contrast microscope
– Fluorescence microscopes
• The best optical microscopes can see structures in the 200 – 400
nm range, at their limits of resolution.
• Optical microscopes are “diffraction limited”— the wavelength range
of visible light (400-750 nm) sets the size of the smallest thing that
can be imaged: ~ 400 nm.

12
The Bright-Field Microscope
• Produces a dark image against a brighter background
• Has several objective lenses
– Parfocal microscopes remain in focus when objectives are changed
• Total magnification
– Product of the magnifications of the ocular lens and the objective lens

Electron microscope techniques
-
Scanning Electron Microscope (SEM)
Transmission Electron Microscope (TEM)

Characteristic Information: SEM
Topography
• The surface features of an object or "how it looks", its texture;
direct relation between these features and materials properties
Morphology
• The shape and size of the particles making up the object; direct
relation between these structures and materials properties
Composition
• The elements and compounds that the object is composed of and
the relative amounts of them; direct relationship between
composition and materials properties
Crystallographic Information
• How the atoms are arranged in the object; direct relation between
these arrangements and material properties

BASIC PRINCIPLES OF SEM
• Similar to metallographic microscope, where the specimen is
illuminated and viewed from the same side.
• SEM makes use of finely focused electron beam (dia 10nm)
• Strikes the specific point of specimen.
• Interaction of primary electron with the specimen surface
provides back scattered and induced emission of electron
with different properties (depends on physical
characteristics).
• Secondary electron generated carry a variety of physical
chemical and electrical information.

SEM
Electron/Specimen Interactions
When the electron beam strikes a sample,
both photon and electron signals are emitted.
Incident Beam
Specimen
X-rays
- composition info
Auger electrons
- Surface sensitive
compositional
Backscattered
electrons
- Atomic number
and topographical
Cathodoluminescence
- Electrical
Secondary electrons
- Topographical
Specimen Current
Electrical

What kind of species are generated?
Probe-sample interaction results in the “generation“ of
• Secondary electrons
• Backscattered electrons
• X-rays
• Auger electrons
• Plasmons

SEM
Gun supply
Lens supply
Signal amplifier
Display unit
Record unit
Magnification
unit
Aperture
Stigmator
Scanning
coils
Specimen
Vacuum
system
Signal collector
ANODE
SHIELD
FILAMENT
ELECTRON
GUN
First Condenser
lens
Second
Condenser lens
Final Condenser
lens
Scan generator

ELECTRON GUN
F-filament
s-shield
a-anode
Types of electron gun:
TUNGSTEN FILAMENT CATHODE
LANTHANUM HEXA BORIDE CATHODE
FIELD EMISSION GUN

LANTHANUM HEXA BORIDE CATHODE
Highest possible current density.
High melting point

FIELD EMISSION GUN
Ultra high vacuum
Instability of the cathode tips

Electron sources
Electron guns:
• Various examples of
gun design
– Thermionic
– Schottky
– Field emission
• Cathode material
– Tungsten
– Lanthanum
hexaboride (LaB6)
– Others…
• Cathode material
determines emission
current density Energy scheme of various gun types

The Objective Lens - Aperture
Since the electrons coming from the electron gun have spread in kinetic
energies and directions of movement, they may not be focused to the same
plane to form a sharp spot.
By inserting an aperture, the stray electrons are blocked and the remaining
narrow beam will come to a narrow “Disc of Least Confusion”

• Secondary electron collector comprises a cylindrical metal box with a
metal gauze window.
• Inside is an Al-coated plastic scintillator that is attached by a short
connection tube to a light pipe whose other end rests against a
photomultiplier window.
• Hitting the scintillator-each electron produces many photons–guided by
the light pipe to the photo multiplier.
• SE (exit energies < 50 eV) are generated if the energy gain of these
species is large enough to overcome the work function.
Secondary
Electron
Detector

Amplification and Detection Sequence
 Secondary electrons are accelerated to the front of the detector by a bias
voltage of 100 - 500 eV.
 They are then accelerated to the scintillator by a bias of 6- 12 keV, (10 KeV
is normal).
 Scintillator is doped plastic or glass covered with a fluorescent material (e.g.
Europium). A thin (700Å) layer of Al covers it to prevent light from causing
fluorescence. The 10keV potential allows the SE to get through the Al and
fluorescence.
 The light photons travel down the tube (guide) to a photocathode which
converts them into electrons.
 The electrons move through the detector, producing more electrons as they
strike dinodes. An output electron pulse is then detected.

Backscattered electrons (BSE)
• BSE are present in the whole energy range from 50 eV (definition) to the
maximum acceleration energy of the primary electrons (PE).
• Their spectrum shows a broad peak overlapped by SE and Auger peaks as
well as plasmon loss.
• BSE and SE are the most important signals for imaging. Knowledge about
the dependence of the backscattering coefficient and the SE yield on surface
tilt, material and electron energy is essential for any interpretation.

X-ray
• Acceleration of a charged particle (electron) in the screened Coulomb
potential of the nucleus leads – with a low probability – to an emission of a
X-ray quantum (usually elastic scattering is observed)
• Electron is decelerated by h (energy of the X-ray quantum) → continuous
X-ray spectrum
• This continuous spectrum is superposed on the characteristic X-ray
spectrum generated by filling of inner shell vacancies

Morphology
The size, shape and arrangement of the particle atomic diameters.
Crystallographic Information
The arrangement of atoms and their degree of order,
detection of atomic-scale defects in areas a few
nanometers in diameter
Compositional Information (if so equipped)
Specimens must be very thin and able to withstand the
high vacuum present inside the instrument
Preparation techniques to obtain an electron transparent
region include ion beam milling and wedge polishing. The
focused ion beam (FIB) is a relatively new technique to
prepare thin samples for TEM
Crystal structure can also be investigated by HRTEM
TEM Study

TEM - transmission electron microscopy
Typical accel. volt. = 100-400 kV
(some instruments - 1-3 MV)
Spread broad probe across specimen
- form image from transmitted
electrons
Diffraction data can be obtained from
image area
Many image types possible (BF, DF,
HR, ...) - use aperture to select signal
sources
Main limitation on resolution -
aberrations in main imaging lens
Basis for magnification - strength of
post- specimen lenses

Instrument components
Electron gun (described previously)
Condenser system (lenses &
apertures for controlling illumination
on specimen)
Specimen chamber assembly
Objective lens system (image-forming
lens - limits resolution; aperture -
controls imaging conditions)
Projector lens system (magnifies
image or diffraction pattern onto final
screen)

Examples
Matrix - '-Ni2AlTi
Precipitates - twinned L12 type '-Ni3Al

Examples
dislocations
in superalloy
SiO2 precipitate
particle in Si

Specimen preparation
Foils
3 mm diameter disk
very thin (<0.1 - 1 µm - depends on material, voltage)
Types
replicas
films
slices
powders, fragments
foils
as is, if thin enough
ultramicrotomy
crush and/or disperse on carbon film

Specimen preparation
Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)
mechanical thinning (grind)
chemical thinning (etch)
ion milling (sputter)
examine region
around perforation

TEM
Limitations
Many materials require extensive sample preparation to produce a sample thin
enough to be electron transparent, which makes TEM analysis a relatively time
consuming process
The structure of the sample may also be changed during the preparation
process
The sample may be damaged by the electron beam, particularly in the case of
biological materials

Scanning Probe Microscopy
-
A plethora of possibilities

The scanning part of SPMs
• Based on the piezoelectric
effect:
– Piezo Tri-Pods
– Piezo-Tube-Scanner
• Problems of these
scanners are:
– Hysteresis, creep
– Aging
– Cross-correlations
between the individual
axis
• These are addressed by
extensive calibration-
functions or closed-loop-
systems utilizing laser-
interferrometry
Piezo-tube scanner and
sketch of a piezo tripod

AFM - interaction
• Lennard-Jones
potential is often cited
• Consisting of a van-
der-Waals and a
Pauli-part
• Distance-dependence
of interaction is
changed in case of
nanoscale objects
• Basic behavior,
however, is
comparable

Non-contact mode
• Sense the sample
without touching it →
essential in the
context of most
polymer and
biological samples
• Cantilever is operated
close to its resonance
frequency via a piezo
actuator

Magnetic Force Microscopy
MFM mode measures the magnetic variations over a sample surface
by detecting the interaction between a magnetized cantilever and the
sample surface.
The cantilever measures surface topography on the first scan, then lifts
and follows either the stored surface topography (lift mode) maintaining
a constant distance from the sample surface or a variable distance
above the sample surface at a fixed height.

TiC coated by PLD
at IGCAR
Source: Mepco

Diffraction based techniques
-
X-ray, neutron, and electron based methods

Basic definition of diffraction
• Diffraction is the bending, spreading and
interference of waves when they pass by an
obstruction or through a gap. It occurs with any
type of wave, including sound waves, water
waves, electromagnetic waves such as light and
radio waves, and matter displaying wave-like
properties according to the wave–particle duality.
Two-slit diffraction presented to the Royal Society in 1803
Thomas Young (1773-1829),
ophthalmologist and physicist

X-ray sources
Energy regime of Gamma- and X-ray radiation overlap – naming criteria is the
heritage: X-ray is created by electron processes whereas Gamma radiation is
a nuclear reaction product
Typically X-ray radiation is generated by deceleration of electrons

Bragg relation
• The diffraction equation
postulated by Bragg and
his son in 1914 (Nobel
laureate in 1915)
Waves that satisfy this condition interfere constructively and result
in a reflected wave of significant intensity

X-ray diffraction – phase analysis
Rietveld method (Hugo Rietveld (1932) allows a quantitative
phase analysis in the context of X-ray and neutron
diffractogramm
• Analysis of the whole diffractogramm
• Refinement of structure- as well as real-structure-
parameters
– Quantitative phase analysis
– Lattice parameters and temperature effects
– Grain size and micro strain
• Its not a structure analysis!
– Basic lattice parameters,
– phase composition, and
– Space group
Hugo Rietveld

Raman spectroscopy
• The phenomenon behind this technique was first
reported by Sir Chandrasekhara Venkata Raman in
1928 was awarded the Nobel Prize in physics for his
findings
• A small percentage of light scattered at a molecule is
in-elastically scattered (1 in 107 photons)
Sir C.V. Raman

Raman spectroscopy - basics
• At room temperature majority of molecules in initial (ground) state  anti-
Stokes signal will be less pronounced: Ratio of anti-Stokes to Stokes can be
used for temperature measurement
• The energy of a vibrational mode depends on molecular structure and
environment. Atomic mass, bond order, molecular substituents, molecular
geometry and hydrogen bonding all effect the vibrational force constant
which, in turn dictates the vibrational energy
• Vibrational Raman spectroscopy is not limited to intramolecular vibrations.
Crystal lattice vibrations and other motions of extended solids are Raman-
active
• Raman scattering occurs when it features a change in polarizability during
the vibration
• This rule is analogous to the rule for an infrared-active vibration (that there
must be a net change in permanent dipole moment during the vibration) -
from group theory it is possible to show that if a molecule has a center of
symmetry, vibrations which are Raman-active will be silent in the infrared,
and vice versa

Raman spectroscopy vs. IR
IR = Change in dipole of molecule
Extended Equilibrium Compressed
Raman = Polarizability of Molecules

Raman spectroscopy - examples
• The frequency of the
RBS mode is inversely
proportional to the
diameter of the
nanotube.
• RBS mode and double
peaked high energy
modes are prove of the
existence of single-wall
nanotubes in a sample.
• In metallic carbon
nanotubes the lower
high-energy mode is
strongly broadened and
shifted to smaller
energies (1540 cm-1).

Modern characterization

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