Stan Lecture Notes

Question 1

Magnification Differences Between Optical Microscopes and Electron Microscopes,

Answer 1

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

Question 2

Depth of Field Differences Between Optical Microscopes and Electron Microscopes,

Answer 2

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

Question 3

Resolution Differences Between Optical Microscopes and Electron Microscopes

Answer 3

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

Question 4

Applications of EM

Answer 4

Product Design Failure Analysis

Surface Texturing Characterization

Surface Defect Analysis and Quality Control

Contaminant Study

Morphological and Structural Analysis

Competitive Analysis:

Question 5

Define EM

Answer 5

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.

Question 6

Method of SEM

Answer 6

SEM,
developed in the 1940s,
scans a sample’s surface
with electrons, producing
high-resolution, 3D images
of surface topography,
ideal for complex textures

Question 7

Define SEM

Answer 7

Scanning Electron
Microscopy

Question 8

Method of TEM

Answer 8

Developed in the late 1930s
and 1940s, TEM passes
electrons through a thin
sample, revealing highly
detailed internal structures
at atomic levels
2D

Question 9

Define TEM

Answer 9

Transmission Electron
Microscopy

Question 10

Two types of electron sources in TEMs

Answer 10

hermionic sources, which generate electrons when
heated

field-emission sources, which produce electrons under a strong electric potential.

Question 11

Histroy of electron mircroscopy

Answer 11

▪ 1931: First Electron Microscope developed by Ernst Ruska.
▪ Ongoing advancements in resolution and applications

Question 12

How does electron microscopy work?

Answer 12

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

Question 13

Electron beam

Answer 13

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

Question 14

Inelastic Scattering

Answer 14

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

Question 15

Elastic Scattering

Answer 15

This occurs when electrons collide with atoms in the
sample without losing energy. This process generates:
1. Backscattered Electrons

Question 16

Secondary Electrons (SE)

Answer 16

*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

Question 17

Back-Scattered Electrons (BSE)

Answer 17

*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

Question 18

Why BSE is Atomic Number Dependent ?

Answer 18

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.

Question 19

determining the
type of information obtained from scanning electron microscopy (SEM):

Answer 19

Electrons’ energy levels and resulting detection depth play a crucial role in determining the
type of information obtained from scanning electron microscopy

Question 20

Backscattered electrons vs Secondary electrons

Answer 20

• 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

Question 21

BSE ideal for ?

Answer 21

analysing
internal features like phase distributions and material density,

Question 22

SE ideal for?

Answer 22

excel in revealing fine
surface details and textures.

Question 23

DDC-SEM

Answer 23

density-dependent colour SEM

used to combine SE and BSE signals to highlight variations in material density and composition

Question 24

Auger Electrons

Answer 24

*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

Question 25

Characteristic X-rays:

Answer 25

*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

Question 26

Transmitted Electrons (TE)

Answer 26

* 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

Question 27

Elastically Scattered Electrons

Answer 27

*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

Question 28

define SAED

Answer 28

Selected Area Electron Diffraction

Question 29

Principle of SAED

Answer 29

* 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.

Question 30

SAED Diffraction Process:

Answer 30

* 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.

Question 31

Application of SAED

Answer 31

* SAED is used for crystal structure identification, phase determination, and lattice parameter calculation in nanomaterials. * Provides insight into grain orientation, defects, and phase transitions

Question 32

HITACHI S-2600N

Answer 32

- SEM -high sample thourghput -precise imaging -FEG -Uperf -ESEM -BSE detector

Question 33

FEG

Answer 33

feild image gun Wide range of accelerating voltages -30kV-8kV

Question 34

Uperf

Answer 34

Ultramagnification-over-perfect-feild -larger depth of feild than SEM -easier detailed observations

Question 35

ESEM

Answer 35

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

Question 36

Hitachi S-4700 SEM

Answer 36

0.5-30kV -load lock for quick sample loading and unloading -resolution less than 10nm -CD -EDX

Question 37

EDX

Answer 37

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

Question 38

CD

Answer 38

Critical dimension

Question 39

Zeiss GeminiSEM 360 SEM

Answer 39

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,

Question 40

Essential preperation steps

Answer 40

1)fixation 2) dehydration 3) coating 4) embedding 5) sectioning

Question 41

Fixation

Answer 41

- 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)

Question 42

Dehydration

Answer 42

- 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.

Question 43

Coating

Answer 43

- Applied to non-conductive samples (e.g., biological or organic materials). - Prevents charging under the electron beam. - Common materials: Gold, platinum, or carbon coating

Question 44

Embedding

Answer 44

- 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.

Question 45

Sectioning

Answer 45

- Precision cutting of embedded samples into ultra-thin slices (50–100 nm) for TEM. - Enables electron transparency for detailed internal structure imaging.

Question 46

Two SEM preparation techniques for inorganic nanoparticles

Answer 46

1) nanopartical suspension 2)nanoparticle dry powder

Question 47

SEM preperation - nanopartical susupension

Answer 47

1. Sample Collection: Obtain a small, representative amount of dry nanoparticles. 2. Deposition: Place the dry sample directly onto a conductive substrate (carbon or aluminium stub) mounted with conductive double-sided carbon tape. 3. Dusting: Gently tap or blow off any excess particles that are not adhered to the tape, ensuring only well- adhered particles remain for imaging. 4. Coating: Apply a conductive layer (gold, carbon) if the sample is non-conductive. 5. Imaging: Secure the stub onto the SEM sample holder for imaging. method 1 is not suitable for resolving individual particles within the aggregate

Question 48

Sem preperation - dry powder

Answer 48

1. Sample Collection: Obtain a small, representative amount of dry nanoparticles. 2. 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. 3. Deposition: Place a drop of the nanoparticle suspension onto a conductive substrate (carbon or aluminium stub) mounted with conductive double-sided carbon tape. 4. Drying: Allow the solvent to evaporate, leaving the nanoparticles adhered to the tape. 5. Coating: Apply a conductive layer (gold, carbon) if the sample is non-conductive. 6. Imaging: Secure the stub onto the SEM sample holder for imaging

Question 49

What is charging?

Answer 49

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.

Question 50

Charging- impact on image quality

Answer 50

This can result in bright areas, scan shifts, or distorted features, making the image difficult to interpret

Question 51

Stratigies to mitigate issue of charging in non-conductive samples

Answer 51

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.

Question 52

coating selection stragie to deal with charging in non-conductive samples

Answer 52

* Coatings should be selected based on imaging requirements (low vs. high magnification). * Ideal coatings are thin, minimize grain structure, and improve secondary electron yield

Question 53

metal section for SEM coating

Answer 53

-gold -gold/palladium -platinum -iridium -chromium -tungsten

Question 54

metal coating gold

Answer 54

*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

Question 55

metal coating Au/Pd

Answer 55

*Smaller grain size; optimal for general research applications. *High SE yield; minimal X-ray interference compared to pure gold

Question 56

metal coating pt

Answer 56

*Finer grain size; best for high-magnification imaging (>100,000x). *Prone to stress cracking in the presence of oxygen, requiring high-purity argon gas.

Question 57

metal coating iridium

Answer 57

*Fine grain, excellent for high-resolution imaging; most expensive. *High SE yield; preferred when carbon analysis is needed

Question 58

metal coating chromium

Answer 58

*Very fine grain, ideal for high-magnification BSE imaging. *Oxidizes easily, requiring turbo-pumped coaters with pure argon flushing -moderate yeild

Question 59

metal coating tungstun

Answer 59

*Extremely fine grain size; high SE yield. *Oxidizes rapidly, must be imaged immediately after coating.

Question 60

avoiding image distortion and charging

Answer 60

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)

Question 61

highest to lowest magnification |(metals )

Answer 61

W-Cr-Ir-Pt-Au/Pd-Au

Question 62

recommended metal coatings for general research purposes

Answer 62

Gold/palladium sputtered alloys (60/40 and 80/20) -have smaller grain size

Question 63

cause of beam damage?

Answer 63

* 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

Question 64

How to image with beam damage

Answer 64

begin imaging at low magnification to minimise exposure then zoom into areas of interest

Question 65

mitigation stratagies for beam damage

Answer 65

* 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

Question 66

Sample preperation for TEM imaging of inorganic nanoparticles

Answer 66

1. Sample Collection: Obtain a small, representative sample of the nanoparticles. 2. 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. 3. 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. 4. Drying: Air-dry the grid to remove the solvent. Optionally, use blotting paper to absorb excess solvent from the grid edges. 5. Staining (Optional): For biological or hybrid materials, a contrasting agent like uranyl acetate may be used to improve contrast under the electron beam. 6. Imaging: Place the dried TEM grid into the holder for analysis in the TEM