PRINCIPLE OF SEM
-Study topography of solid samples, resolution around 0.3 µm to
0.15 nm.
-A source of high energy electrons, condenser system and probe
lens to focus the electron beam into fine probe that impinges on
specimen.
-Image is obtained by scanning the electron probe over surface
and collect image signal – display after suitable amplification and
processing.
-Sample : electrically conductive, nonconductive materials require
thin conductive coating to prevent electrical charging of the
specimen
-An energetic electron penetrating solid sample undergoes both
elastic & inelastic scattering. Inelastic predominates, reducing
energy in beam to kinetic energy kT.
-Release of secondary electron emission from surface generate
electron current in sample due to impact of high energy incident
beam.
-a fine beam of electrons is scanned across surface of sample,
-a detector counts the number of low energy secondary electrons,
or other radiation, emitted from each point of the surface.
-The brightness of each image pixel is modulated according to the
output current of the detector for each point of surface and an
image is build up in this way as the beam scans.
-Because secondary electrons come from near surface region, the
brightness of signal depends on surface area that is exposed to
the primary beam.
-Inelastic collision (electron – electron) secondary electrons, X-ray,
phonon (heat).
-Elastic collision (electron – nucleus) produce backscattered
electrons (BSE).
BASIC SIGNAL OF SEM
=The electrons interact with the atoms at or close to sample
surface produces signals that contain information about the
sample's surface topography, composition, and other properties
such as electrical conductivity. The types of signals produced by
an SEM include secondary electrons, back-scattered electrons
(BSE), characteristic X-rays, light (cathodoluminescence),
absorption specimen current
=Secondary electrons: they are electrons generated as ionization
products. They are called 'secondary' because they are generated
by other radiation (the primary radiation). This radiation can be in
the form of ions, electrons, or photons with sufficiently high
energy, i.e. exceeding the ionization potential. Secondary electron
detectors are common in all SEMs.
=Back-scattered electrons (BSE): they are beam electrons that are
reflected from the sample by elastic scattering. BSE are often
used in analytical SEM along with the spectra made from the
characteristic X-rays. Because the intensity of the BSE signal is
strongly related to the atomic number (Z) of the specimen, BSE
images can provide information about the distribution of different
elements in the sample. If mean atomic wt (Z) of specimen is low,
e.g plastic, probability of backscattering event is lower than if Z is
higher. So high-Z metal release greater number of BSE than low Z
specimens.
Characteristic X-rays: they are emitted when the electron beam
removes an inner shell electron from the sample, causing a higher
energy electron to fill the shell and release energy. These
characteristic X-rays are used to identify the composition and
measure the abundance of elements in the sample.
3.
PRINCIPLE OF TEM
2 TYPES:
Scanning Tunneling Microscopy (STM)
=In transmission electron microscopy (TEM), a beam of highly
focused electrons are directed toward a thinned sample (<200
nm).
=Normally no scanning required --- helps the high resolution,
compared to SEM.
=These highly energetic incident electrons interact with the atoms
in the sample producing characteristic radiation and particles
providing information for materials characterization.
=Image is focused by objective lens while imaging lenses enlarge
the final image.
=Information is obtained from both deflected and non-deflected
transmitted electrons, backscattered and secondary electrons,
and emitted photons.
=The specimen must be sufficiently thin is to facilitate
transmission of a beam of electrons without a great loss of
intensit
PRINCIPLE OF SPM
-examine materials with a solid probe scanning the surfaces;It
examines surfaces features whose dimensions range form atomic
spacing to a tenth of a millimeter.
-main characteristic of the SPM is a sharp probe tip that scans a
sample surface;
-tip must remain in very close proximity to the surface because
the SPM uses near-field interactionsbetween the tip and a sample
surface for examination;
-enable to obtain a true image of surface atoms, it can accurately
measure the surface atom profiles in the vertical and lateral
directions;
-SPM must operate in a vibration-free environment because the
probe is only an atomic distance away from the examined surface;
4.
5.
6.
Scanner- a piezoelectric scanner which moves the
sample under the tip in a rastes pattern
Feedback system-control the vertical position of the tip
Probe motion sensor-sensing the vertical position of
the tip
Computer-Computer system that drives the scanner,
measures data& content form image.
-Based on quantum mechanical “tunneling” effect – when 2
electrodes (2 metals) are brought sufficiently close together to
allow overlapping of electronic wave functions associated with
each electrode.
-Apply small bias voltage – tunneling current is formed in
direction given by the sign of applied voltage.
-Use very sharp tip (one atom’s) possible to detect current
variation as tip is scanned across surface.
Samples must be conductive but medium can be air, liquid or
vacuum.
 Resolution up to nanoscale range.
 An electronic feedback system is used to keep the
current (and hence the gap) constant as we move the
tip sideways across the surface
 Recording the tip's vertical position at points on a grid
we can make a 3D map of the surface
The tunneling current it has very important
characteristic –it exhibits an exponentially decay with
an increase of the gap d. Very small changes in tipsample separation induce large changes in tunneling
current!!
1.
2.
Probe-as a tip
Vibration isolation system-provide vibration
environment
free
Atomic Force Microscopy (AFM)
 Operates by measuring forces between sample and probe
tip.
 Force depends on:
C- constant of probe,
S- spring constant of probe,
P- probe geo.,
D- distance between probe & sample,
N- nature of sample
 An atomically sharp tip is scanned over surface with
feedback mechanisms that enable the piezo-electric
scanners to maintain the tip at a constant force (to obtain
height info) or height (to obtain force info) above the sample
surface.
 Tip is brought close enough to surface to detect repulsive
force between atoms in tip and sample.
 Probe tip is mounted on cantilever. Interatomic forces will
induce bending and can be detected by laser beam. Tips
typically made of Si3N4 or Si .
 Surface topography of sample is tracked by monitoring
deflection of the cantilever.
 Optical detection system – diode laser is focused onto the
back of a reflective cantilever. As tip scans surface, moving
up & down with the contour, laser beam is deflected off into
a dual element photodiode.
 Measures difference in light intensities between upper &
lower photodetectors & converts to voltage
.
view of surface features (no
coating necessary).
secondary, backscattered
electron and absorption
specimen current
Comparison between TEM and AFM
AFM
TEM
-3D image of AFM obtained
-2D image with a
without expensive sample
expensive and critical
preparation
sample preparation
-complete info than 2D profiles
-Less information due
from cross-sectioned sample.
to 2D topography only
-Magnification-100,000times
deflected transmitted
electrons backscattered
and secondary
electrons, and emitted
photons.
-Magnification-1 million
time
COMPASIRON BETWEEN OM, SEM AND SPM
Comparison between OM and SEM
OM
SEM
Source-light wave
Source-high energy
electron
-Magnification 10-1500x
--Magnification
800,000x
-Resolution<= 0.2um
-Resolution>= 5nm
-wavelength, 2000 Å
-Imaging modes – transmitted
& reflected light, polarized
light, bright-field, dark field,
differential interference
contrast, and phase contrast.
< 0.5 Å
-Imaging mode –
Backscattered electron,
secondary , and
absorbed specimen
current
-Limited depth of field
-Low frequency
- larger depth of field
-higher frequency
XRD & XRF
Comparison between STM and AFM
AFM
STM
Both-conducting or nonOnly conducting material
conducting
Less resolution
Better resolution
Similarities:
-AFM and STM are both using the technique to examine materials
with a solid probe scanning the surfaces;
-It examines surfaces features whose dimensions range form
atomic spacing to a tenth of a millimeter.
Comparison between SEM and AFM
AFM
STM
-extraordinary, topographic
Need coating
contrast, direct height
measurements & unobscured
Comparison between TEM and SEM
SEM
TEM
-Image is developed by point-image is focused by
to-point by collecting signal
objective lens, imaging
generated by electron
lens enlarge the final
interaction as it scan over the
image
surface
-3D topography
-2D image
- thickness is not critical as
-required thin sample
TEM
250-500nm()
-Image shown on TV monitor
-Electron beam scan over the
surface and the signal obtained
by reflected electron.
-Information is obtained from
deflected electron on the
sample surface such as
-Image shown on
fluorescence screen
-Electron beam pass
through thin sample,
-Information is
obtained from both
deflected and non-
PRINCIPLE:
XRD
XRF
-X-ray diffraction is a method of -When an element is placed in
X-ray crystallography, in which a a beam of x-rays, the x-rays
beam of X-rays strikes a sample are absorbed. The absorbing
(crystalline solid), land on a piece atoms become ionized (e.g.
of film or other detector to due to the x-ray beam ejects
produce scattered beams.
the electron in the inner
-These beams make a diffraction shell).
pattern of spots; the strengths -An electron from higher
and angles of these beams are energy shell (e.g., the L shell)
recorded as the sample is then fall into the position
gradually rotated
vacated by dislodged inner
electron and emit x-rays or
characteristic wavelength
DIFERENTIATE:
Sources:
-Crystalline materials in
forms of bulk, powder,
sheet or thin films can be
Powders:
-Grinding (<400 mesh if
possible) can minimise
analyzed.
-It is important that the
specimen
chosen
is
representative
of
the
materials.
-For powder specimen – a
thin layer of crystalline
powder is spread on to a
planar substrate, which is
often a nondiffracting
material such as a glass
microscope
slide
and
exposed to x-ray beam.
scatter affects due to
particle size. Additionally,
grinding insures that the
measurement is more
representative of the entire
sample, vs. the surface of
the sample.
-Pressing (hydraulically or
manually) compacts more
of the sample into the
analysis area, and ensures
uniform density and better
reproducibility.
Solids:
-Orient surface patterns in
same manner so as
minimise scatter affects.
-Polishing surfaces will also
minimise scatter affects.
-Flat samples are optimal
for quantitative results.
Liquids:
-Samples should be fresh
when
analysed
and
analysed
with
short
analysis time - if sample is
evaporative.
-Sample should not stratify
during analysis.
-Sample should not contain
precipitants/solids, analysis
could show settling trends
with time.
Sources:
beam of X-rays strikes a sample
(crystalline solid), land on a
piece of film or other detector
to produce scattered beams
When an element is placed in a
beam of x-rays, the x-rays are
absorbed.
The absorbing
atoms become ionized
XPS:
X-Rays
-Irradiate the sample surface, hitting the core electrons (e-) of the
atoms.
-The X-Rays penetrate the sample to a depth on the order of a
micrometer.
WHY CORE ELECTRON:
-An electron near the Fermi level is far from the nucleus, moving
in different directions all over the place, and will not carry
information about any single atom.
-Fermi level is the highest energy level occupied by an electron in
a neutral solid at absolute 0 temperature.
-Electron binding energy (BE) is calculated with respect to the
Fermi level.
-The core e-s are local close to the nucleus and have binding
energies characteristic of their particular element.
-The core e-s have a higher probability of matching the energies of
AlK and MgK.
Binding Energy (BE)
- determined by the attraction of the electrons to the nucleus
- If an electron with energy x is pulled away from the nucleus, the
attraction between the electron and the nucleus decreases and
the BE decreases
PRINCIPLE:
- A monoenergetic x-ray beam emits photoelectrons from the
from the surface of the sample.
-The X-Rays either of two energies:
Al Ka (1486.6eV)
Mg Ka (1253.6 eV)
-The x-ray photons The penetration about a micrometer of the
sample
-The XPS spectrum contains information only about the top 10 100 Ǻ of the sample.
-Ultrahigh vacuum environment to eliminate excessive surface
contamination.
-Cylindrical Mirror Analyzer (CMA) measures the KE of emitted e-s.
-The spectrum plotted by the computer from the analyzer signal.
-The binding energies can be determined from the peak positions
and the elements present in the sample identified.
UHV for Surface Analysis:
-Remove adsorbed gases from the sample.
-Eliminate adsorption of contaminants on the sample.
-Prevent arcing and high voltage breakdown.
-Increase the mean free path for electrons, ions and photons.
X-Rays on the Surface
-The X-Rays will penetrate to the core e- of the atoms in the
sample.
-Some e-s are going to be released without any problem giving the
Kinetic Energies (KE) characteristic of their elements.
-Other e-s will come from inner layers and collide with other e-s of
upper layers
-These e- will be lower in lower energy.
-They will contribute to the noise signal of the spectrum.
e-s can the Cylindrical Mirror Analyzer Detect
-The CMA not only can detect electrons from the irradiation of XRays, it can also detect electrons from irradiation by the e- gun.
-The e- gun it is located inside the CMA while the X-Ray source is
located on top of the instrument.
-The only electrons normally used in a spectrum from irradiation
by the e- gun are known as Auger e-s. Auger electrons are also
produced by X-ray irradiation.
Two Ways to Produce Auger Electrons
1.
The X-Ray source can irradiate and remove the e- from
the core level causing the e- to leave the atom
2.
A higher level e- will occupy the vacancy.
3.
The energy released is given to a third higher level e-.
4.
This is the Auger electron that leaves the atom.
The axial e- gun can irradiate and remove the core e- by collision.
Once the core vacancy is created, the Auger electron process
occurs the same way.
Cylindrical Mirror Analyzer (CMA)
-The electrons ejected will pass through a device called a CMA.
-The CMA has two concentric metal cylinders at different voltages.
-One of the metal cylinders will have a positive voltage and the
other will have a 0 voltage. This will create an electric field
between the two cylinders.
-The voltages on the CMA for XPS and Auger e-s are different.
-When the e-s pass through the metal cylinders, they will collide
with one of the cylinders or they will just pass through.
-If the e-’s velocity is too high it will collide with the
outer cylinder
-If is going too slow then will collide with the inner
cylinder.
-Only the e- with the right velocity will go through the
cylinders to reach the detector.
-With a change in cylinder voltage the acceptable kinetic energy
will change and then you can count how many e-s have that KE to
reach the detector.
XPS Spectrum:
-The X-Ray will hit the e-s in the bulk (inner e- layers) of the sample
-e- will collide with other e- from top layers, decreasing its energy
to contribute to the noise, at lower kinetic energy than the peak .
-The background noise increases with BE because the SUM of all
noise is taken from the beginning of the analysis.
Weight loss – vaporization (physical), desorption (physical),
oxidation (physical), decomposition (chemical), dehydration &
desolvation (chemical).
-The XPS peaks are sharp.
-In a XPS graph it is possible to see Auger electron peaks.
-The Auger peaks are usually wider peaks in a XPS spectrum
Identification of XPS Peaks
-The plot has characteristic peaks for each element found in the
surface of the sample.
-There are tables with the KE and BE already assigned to each
element.
-After the spectrum is plotted you can look for the designated
value of the peak energy from the graph and find the element
present on the surface.
XRAY
Hit
all
sample
area
simultaneously permitting data
acquisition that will give an
idea
of
the
average
composition of the whole
surface.
e-BEAM
It can be focused on a
particular area of the sample
to determine the composition
of selected areas of the sample
surface.
XPS Technology
-Consider as non-destructive
-because it produces soft x-rays to induce photoelectron emission
from the sample surface
-Provide information about surface layers or thin film structures
Applications in the industry:
- Polymer surface, Catalyst, Corrosion, Adhesion, Semiconductors,
Dielectric materials, Electronics packaging, Magnetic media, thin
film coatings.
Thermogravimetric Analysis (TGA)
-Used to measure changes in weight (mass), m, of sample as a
function of T and/or time.
-Commonly used to
*Determine polymer degradation temperature,
*Residual solvent level,
*Absorbed moisture content, and amount of inorganic
(noncombustible) filler in polymer or composite material
compositions.
*Decomposition temperature of materials-impurities in ceramic
etc
Response ;
Weight gain – adsorption (physical), oxidation (chemical).
Applications of TGA
-Determines temperature and weight change of decomposition
reactions, which often allows quantitative composition analysis.
May be used to determine water content.
-Allows analysis of reactions with air, oxygen, or other reactive
gases (see illustration below).
-Can be used to measure evaporation rates, such as to measure
the volatile emissions of liquid mixtures.
-Allows determination of Curie temperatures of magnetic
transitions by measuring the temperature at which the force
exerted by a nearby magnet disappears on heating or reappears
on cooling.
-Helps to identify plastics and organic materials by measuring the
temperature of bond scissions in inert atmospheres or of
oxidation in air or oxygen.
-Used to measure the weight of fiberglass and inorganic fill
materials in plastics, laminates, paints, primers, and composite
materials by burning off the polymer resin. The fill material can
then be identified by XPS and/or microscopy. The fill material may
be carbon black, TiO2, CaCO3, MgCO3, Al2O3, Al(OH)3, Mg(OH)2,
talc, Kaolin clay, or silica, for instance.
PRINCIPLE
-Sample is placed into a tared TGA sample pan which is attached
to a sensitive microbalance assembly.
-Sample holder is then placed into high temperature furnace.
-Balance assembly weigh the initial sample at room T & then
continuously monitors changes in sample weight (losses or gains)
as heat is applied to sample.
Heat applied at certain rate, in various environment
*ambient air, vacuum, inert gas, oxidizing/reducing gases,
corrosive gases, carburizing gases, vapors of liquids or "selfgenerating atmosphere".
*The pressure can range from high vacuum or controlled vacuum,
through ambient, to elevated and high pressure; the latter is
hardly practical due to strong disturbances.
*Typical weight loss profiles are analyzed for the amount or % of
weight loss at any given temperature, amount or % of
noncombusted residue at final temperature, & temperature of
various sample degradation processes.
-Factors affecting TG curve
*heating rate, sample size ,particle size of sample ,the way it is
packed ,crucible shape ,gas flow rate
-DTG – derivative of TG curve, often useful in revealing extra
detail.
-TG also often used with DTA (differential thermal analysis).
-DTA – record difference in T (∆T) between sample and reference
material. Each DTA curve should be marked with either
endothermic or exothermic direction.
Curve – peak represent exothermic or endothermic reaction
Differential Thermal Analysis (DTA)
-Record temperature difference between sample & reference
material.
-If endo event (e.g melting) temperature sample will lower than
reference material.
-If exo event (e.g oxidation) response will be in opposite direction.
-Reference material:
*thermally stable at a certain temperature range
*Not react with sample holder or thermocouple
*both thermal conductivity
*heat capacity should be similar to those of sample
-Both solid sample & reference material usually powdered form.
Differential Scanning Calorimetry (DSC)
-A thermal analysis technique in which the amount of energy
absorbed (endothermic) or released (exothermic) by a material is
measured.
-Both events are the result of physical and/or chemical changes in
a material.
-Normally the weight of sample is 5 – 10 mg,
-Sample can be in solid or liquid form.
-Many of the physical (e.g evaporation) or chemical (e.g
decomposition) transformation are associated with heat
absorption (endothermic) or heat liberation (exothermic).
DSC – Applications :
-Identify melting point, glass transition, Curie temperature,
energy required to melt material. Evaluation of phase
transformation.
-Decomposition, polymerization, gelation, curing.
-Evaluation of processing, thermal & mechanical histories.
-Process modeling, material’s min process temperature
(processing condition).
-Determine crystallization temperature upon cooling.
-Perform oxidative stability testing (OIT).
-Compare additive effects on material
Differential Scanning Calorimetry (DSC)
-2 pans sit on a pair of identically positioned platforms connected
to a furnace by common controlled heat.
-Record any energy difference – endothermic or exothermic
depending on whether more or less energy has to be supplied to
the sample relative to the reference material.
-Endothermic response usually represented positive, opposite of
usual DTA convention (endothermic as negative side)
-Correlate endothermic or exothermic peaks with thermal events
in sample.
-One way – test if readily reversible on cooling & reheating.
Exothermic process usually not, unlike melting & many solid-solid
transitions.
DSC Responses
Physical changes :

Exothermic – adsorption, crystallization.

Chemical changes :


Endothermic
vaporization.
–
desorption,
melting,
Exothermic – oxidation, decomposition,
curing.
Endothermic – reduction, decomposition,
dehydration.

Differential Scanning Calorimetry (DSC)
 DSC have many applications in field of polymer science
& engineering.
 Tg, Tc & Tm transitions are characteristic of each
polymer  identification.
 Curing conditions for thermoset – heat for curing
which allows calculation of degree of curing.
 But, DSC technology is not sensitive to detect Tg in
cross-linked or highly crystalline resins. Also for
polymer with high filler content.
 Handling liquid also difficult. Interpretation of phase
transition requires further info – XRD, etc.