Stan Lecture Notes Flashcards
Magnification Differences Between Optical Microscopes and Electron Microscopes,
Optical microscopes typically offer magnifications ranging from 4x to about 1000x.
SEMs can achieve magnifications from 8x up to more than 3,000,000x.
TEMs can reach magnifications exceeding 50,000,000x
Depth of Field Differences Between Optical Microscopes and Electron Microscopes,
Optical microscopes have a depth of field ranging from 0.19 microns to 15 microns.
SEMs offer a broader range, from 0.4 microns to 4 mm.
TEMs have a very shallow depth of field, typically in the nanometer range
Resolution Differences Between Optical Microscopes and Electron Microscopes
Optical microscopes can achieve a spatial resolution of approximately 0.2 microns.
SEMs can reach resolutions as fine as 0.4 nm with advanced models and lenses.
TEMs provide even higher resolutions, often below 0.1 nm
Applications of EM
Product Design Failure Analysis
Surface Texturing Characterization
Surface Defect Analysis and Quality Control
Contaminant Study
Morphological and Structural Analysis
Competitive Analysis:
Define EM
Electron microscopy encompasses several techniques, (SEM and TEM) which exploit the wave-like nature of electrons to achieve resolutions down to the atomic level. This allows for detailed investigations of surface morphology, crystallography, and elemental composition in a variety of materials.
Electron Microscopy uses a beam of electrons to visualise structures at very high magnifications.
Method of SEM
SEM,
developed in the 1940s,
scans a sample’s surface
with electrons, producing
high-resolution, 3D images
of surface topography,
ideal for complex textures
Define SEM
Scanning Electron
Microscopy
Method of TEM
Developed in the late 1930s
and 1940s, TEM passes
electrons through a thin
sample, revealing highly
detailed internal structures
at atomic levels
2D
Define TEM
Transmission Electron
Microscopy
Two types of electron sources in TEMs
hermionic sources, which generate electrons when
heated
field-emission sources, which produce electrons under a strong electric potential.
Histroy of electron mircroscopy
▪ 1931: First Electron Microscope developed by Ernst Ruska.
▪ Ongoing advancements in resolution and applications
How does electron microscopy work?
filament that generates a beam of electrons that impact the
sample.
These electrons interact with the sample that is being studied and return different signals that are interpreted by different detectors.
With this information we are able to obtain superficial information from:
*Shape and topography
*Texture
*Composition
The interaction of the electron beam with the surface of the sample takes place in a ‘pear’
shape
Electron beam
A focused stream of electrons interacts with the sample, penetrating its surface and interacting with atoms
As the high-energy electrons penetrate the material, they undergo different types of scattering events
Inelastic Scattering
During this interaction, high-energy electrons from
the beam collide with the atoms in the sample and transfer energy. This
process leads to the ejection of
-Secondary electrons
-characteristic x-rays
-auger electrons
Elastic Scattering
This occurs when electrons collide with atoms in the
sample without losing energy. This process generates:
1. Backscattered Electrons
Secondary Electrons (SE)
*Description: Low-energy electrons emitted from the sample’s surface due to inelastic scattering.
*Technique: Crucial in Scanning Electron Microscopy (SEM), these electrons enable the generation of detailed topographical images of the sample surface
Back-Scattered Electrons (BSE)
*Description: Electrons that are reflected back from the sample due to elastic scattering.
*Technique: Primarily utilized in Scanning Electron Microscopy (SEM) to provide compositional
contrast based on differences in atomic number
Why BSE is Atomic Number Dependent ?
Atomic Number and Scattering:
The likelihood of backscattering depends on the atomic
number (Z) of the sample’s elements. Heavier elements possess more protons, resulting in a stronger electric field that can effectively deflect incoming electrons, thus increasing the
number of back-scattered electrons.
Contrast Mechanism:
This atomic number dependence creates compositional contrast in BSE
imaging. Areas containing heavier elements appear brighter in SEM images, while regions with lighter elements appear darker, allowing researchers to differentiate materials based on elemental composition.
determining the
type of information obtained from scanning electron microscopy (SEM):
Electrons’ energy levels and resulting detection depth play a crucial role in determining the
type of information obtained from scanning electron microscopy
Backscattered electrons vs Secondary electrons
- High-energy backscattered electrons (BSEs) can escape from greater depths within the
sample, allowing for the examination of internal structures and compositional variations. - Low-energy secondary electrons (SEs) predominantly originate from the surface layers,
providing insights into the sample’s morphology and surface topography
BSE ideal for ?
analysing
internal features like phase distributions and material density,
SE ideal for?
excel in revealing fine
surface details and textures.
DDC-SEM
density-dependent colour SEM
used to combine SE and BSE signals to highlight variations in material density and composition
Auger Electrons
*Description: Electrons emitted through the Auger effect, where the vacancy of an inner-shell electron is filled by an outer-shell electron, releasing energy that ejects another electron.
*Technique: Used primarily in Auger Electron Spectroscopy (AES) for surface analysis
Characteristic X-rays:
*Description: When an electron beam interacts with a sample, it can knock out an inner-shell electron from an atom. To fill
the vacancy, an outer-shell electron transitions down to the inner shell, releasing energy in the form of an X-ray. The
energy of the emitted X-ray is unique to each element, allowing for the identification of the elemental composition of the sample.
*Technique: Employed in Energy-Dispersive X-ray Spectroscopy (EDX), typically coupled with SEM for elemental analysis
Transmitted Electrons (TE)
- Description: Electrons that pass through a thin specimen, generating an image based on the sample’s density and thickness. Denser
regions scatter more electrons (both elastic and inelastic scattering), allowing fewer to pass through, leading to darker areas in the image,
while less dense or thinner areas transmit more electrons and appear brighter. - Technique: Primarily utilized in Transmission Electron Microscopy (TEM) for high-resolution imaging of internal structures at the atomic
level, providing detailed contrast based on electron scattering and material composition
Elastically Scattered Electrons
*Description: These electrons are deflected by the atoms in the sample without losing energy. As they pass through the material, their
paths are bent by the atomic planes, producing interference patterns that correspond to the crystal structure.
*Technique: Elastically scattered electrons are employed in Selected Area Electron Diffraction (SAED) within Transmission Electron
Microscopy (TEM). The diffraction patterns generated reveal information about the crystallographic arrangement, phase identification, and
lattice spacing of the sample
define SAED
Selected Area Electron Diffraction
Principle of SAED
- SAED operates on the principle that when an electron beam passes through a crystalline material, the
crystallographic planes (if the material is crystalline) act as diffraction gratings. - The diffracted electrons create a pattern based on constructive interference, forming distinct spots that correspond
to specific crystallographic planes.
SAED Diffraction Process:
- Why Electrons Diffract: Electrons have wave-like properties. When they interact with the periodic atomic
arrangement in a crystal, they diffract, and the resulting pattern depends on the crystal’s symmetry and lattice
spacing. - Pattern Analysis: The resulting diffraction pattern can be used to determine the crystal structure by analysing the
distance and arrangement of the spots, which are directly related to the lattice parameters.
Application of SAED
- SAED is used for crystal structure identification, phase determination, and lattice parameter calculation in
nanomaterials. - Provides insight into grain orientation, defects, and phase transitions
HITACHI S-2600N
- SEM
-high sample thourghput
-precise imaging
-FEG
-Uperf
-ESEM
-BSE detector
FEG
feild image gun
Wide range of accelerating voltages
-30kV-8kV
Uperf
Ultramagnification-over-perfect-feild
-larger depth of feild than SEM
-easier detailed observations
ESEM
Envoirnemntal scanning electron microscope
-allows imaging in natural state- hydrated and low vaccume conditions
-doesnt need dehydration or coating
-perfect for biological or soft materials
Hitachi S-4700 SEM
0.5-30kV
-load lock for quick sample loading and unloading
-resolution less than 10nm
-CD
-EDX
EDX
energy dispersive X-ray
measures X-rays emitted by the sample when excited by the electron beam.
This data is used by Bruker software to graph the sample’s
elemental composition and create elemental maps of the image, highlighting where selected elements are located
CD
Critical dimension
Zeiss GeminiSEM 360 SEM
designed for materials and life sciences, as well as industrial applications
High- resolution, surface-sensitive imaging at
low voltages
It features Inlens
secondary and backscatter electron imaging, which allows simultaneous acquisition of surface and compositional
information,
Essential preperation steps
1)fixation
2) dehydration
3) coating
4) embedding
5) sectioning
Fixation
- Preserves sample structure by halting biological activity.
- Prevents autolysis (self-digestion or breakdown of cells and tissues by their own enzymes) and decay (breakdown of organic matter by bacteria, fungi, or other microorganisms).
- Common fixatives: Glutaraldehyde (for biological samples) or osmium tetroxide (for lipids and membranes)
Dehydration
- Gradual removal of water using an ethanol or acetone series.
- Maintains structural integrity, avoiding collapse.
- Critical for SEM preparation, particularly for soft or biological samples.
Coating
- Applied to non-conductive samples (e.g., biological or organic materials).
- Prevents charging under the electron beam.
- Common materials: Gold, platinum, or carbon coating
Embedding
- Crucial for TEM to stabilize samples, except for nanoparticles which can be observed directly.
- Involves embedding in resins like epoxy for ultra-thin sectioning.
- Ensures the sample remains intact during slicing.
Sectioning
- Precision cutting of embedded samples into ultra-thin slices (50–100 nm) for TEM.
- Enables electron transparency for detailed internal structure imaging.
Two SEM preparation techniques for inorganic nanoparticles
1) nanopartical suspension
2)nanoparticle dry powder
SEM preperation - nanopartical susupension
- Sample Collection: Obtain a small, representative amount of dry nanoparticles.
- Deposition: Place the dry sample directly onto a conductive substrate (carbon or aluminium stub) mounted
with conductive double-sided carbon tape. - Dusting: Gently tap or blow off any excess particles that are not adhered to the tape, ensuring only well-
adhered particles remain for imaging. - Coating: Apply a conductive layer (gold, carbon) if the sample is non-conductive.
- Imaging: Secure the stub onto the SEM sample holder for imaging.
method 1 is not suitable for resolving individual particles within the aggregate
Sem preperation - dry powder
- Sample Collection: Obtain a small, representative amount of dry nanoparticles.
- Suspension: Disperse the nanoparticles in a solvent (e.g., water or ethanol), using a small amount, such as the tip of a spatula in around 5 mL of solvent.
- Deposition: Place a drop of the nanoparticle suspension onto a conductive substrate (carbon or aluminium
stub) mounted with conductive double-sided carbon tape. - Drying: Allow the solvent to evaporate, leaving the nanoparticles adhered to the tape.
- Coating: Apply a conductive layer (gold, carbon) if the sample is non-conductive.
- Imaging: Secure the stub onto the SEM sample holder for imaging
What is charging?
Charging occurs when a non-conductive sample accumulates excess electrons during SEM
imaging, causing image distortions.
Charging is an undesirable effect in SEMs where the negative charge of the incident electron beam accumulates on the surface of a non-conductive specimen.
This occurs when the total number of electrons emitted from a sample, the sum of backscattered electrons, secondary electrons and absorbed electrons, is less than the incident electrons. The potential on the specimen surface becomes negative and causes various distortions in secondary electron
imaging.
Charging- impact on image quality
This can result in bright areas, scan shifts, or distorted features, making the image difficult to interpret
Stratigies to mitigate issue of charging in non-conductive samples
Mitigation Techniques:
* Lower Beam Energies (e.g., 1 keV): Reducing the energy of the electron beam limits the number of electrons penetrating deep into the sample, minimizing excess negative charge accumulation
and reducing image distortions.
- Variable-Pressure SEM (Low-Vacuum Mode): Operating in low vacuum allows positive ions from the gas in the chamber to neutralize the negative charge buildup on non-conductive samples.
- Thin Conductive Coatings (e.g., metals): Applying a conductive coating creates a pathway for
excess charge to dissipate, preventing buildup and ensuring clearer imaging.
coating selection stragie to deal with charging in non-conductive samples
- Coatings should be selected based on imaging requirements (low vs. high magnification).
- Ideal coatings are thin, minimize grain structure, and improve secondary electron yield
metal section for SEM coating
-gold
-gold/palladium
-platinum
-iridium
-chromium
-tungsten
metal coating gold
*High conductivity & stability; ideal for low magnification
(<5000x).
*Produces large grains that interfere at high magnifications.
*Does not oxidize; potential X-ray interference with elements
like S, Nb, Ge
metal coating Au/Pd
*Smaller grain size; optimal for general research applications.
*High SE yield; minimal X-ray interference compared to pure gold
metal coating pt
*Finer grain size; best for high-magnification imaging (>100,000x).
*Prone to stress cracking in the presence of oxygen,
requiring high-purity argon gas.
metal coating iridium
*Fine grain, excellent for high-resolution imaging; most
expensive.
*High SE yield; preferred when carbon analysis is needed
metal coating chromium
*Very fine grain, ideal for high-magnification BSE imaging.
*Oxidizes easily, requiring turbo-pumped coaters with pure argon flushing
-moderate yeild
metal coating tungstun
*Extremely fine grain size; high SE yield.
*Oxidizes rapidly, must be imaged immediately after
coating.
avoiding image distortion and charging
Conventional imaging parameters cause image distortions and charging (A) whereas lowering
the vacuum (B) and lowering the accelerating voltage (B) reduce sample charging but also lead to less contrast and signal-to-noise. Coating
the sample with a thin layer of gold allows the beam parameters to be optimized while avoiding charging artifacts (D)
highest to lowest magnification |(metals )
W-Cr-Ir-Pt-Au/Pd-Au
recommended metal coatings for general research purposes
Gold/palladium sputtered alloys (60/40 and
80/20)
-have smaller grain size
cause of beam damage?
- High-energy electron beams can cause physical and chemical damage to sensitive samples,
particularly organic and biological materials. - Damage includes breaking bonds, heating, and charge buildup leading to sample deformation
How to image with beam damage
begin imaging at low magnification to minimise exposure then zoom into areas of interest
mitigation stratagies for beam damage
- Reduce Beam Voltage: Lowering the electron beam energy (e.g., 1–5 kV) minimizes damage.
- Use Shorter Dwell Time: Decrease the time the beam scans a specific area to limit exposure.
- Apply conductive coatings or use low-Vacuum Mode:
This reduces charging by allowing ionized gas to
neutralize sample charges
Sample preperation for TEM imaging of inorganic nanoparticles
- Sample Collection: Obtain a small, representative sample of the nanoparticles.
- Suspension: Disperse the nanoparticles in a solvent (e.g., ethanol or water), using a minimal amount, such
as the tip of a spatula in approximately 5 mL of solvent. - Grid Deposition: Place a small drop of the nanoparticle suspension onto a TEM grid (usually copper or nickel
grids) covered with a carbon or formvar support film. - Drying: Air-dry the grid to remove the solvent. Optionally, use blotting paper to absorb excess solvent from
the grid edges. - Staining (Optional): For biological or hybrid materials, a contrasting agent like uranyl acetate may be used to
improve contrast under the electron beam. - Imaging: Place the dried TEM grid into the holder for analysis in the TEM
