Electron Microscopy Basics

Electron Optics (PHYS 312)

• Course title: Electron Optics
• Code no.: PHYS 312
• Number of hours: 2 lecture hours per week
• Course objectives: The course introduces principles and applications of electron optics to equip students in various applications.

Intended Learning Outcomes (ILOs)

• Knowledge and Understanding
  • Identify principles of electron optics
  • Recognize types and design of electron lenses
  • Describe electron motion in uniform fields
  • Demonstrate different types of electron microscopes and their applications
• Intellectual Skills
  • Choose optimal solutions for physical problems using analytical thinking
• Professional and Practical Skills
  • Use learning resources effectively for research tasks
• General and Transferable Skills
  • Acquire lifelong learning, considering community-linked problems

Chapter 1: Introduction

• Electron optics is a branch of physics applying optical principles to electron beams
• Electron beams are studied, formed, focused and manipulated using electric and magnetic fields
• Electron optical devices are more complex than light optical devices due to differences in electron and light characteristics
• Electron speeds vary greatly from being at the speed of light when emitted, whereas electrons require acceleration using fields to be effective for practical devices
• Light beams generally travel in straight lines; electron paths are curved by fields

1.1 Classical Optics

• Classical optics is divided into geometrical (ray) and physical (wave) optics
  • Geometrical optics: studies light propagation as 'rays'.
  • Physical optics: treats light as a wave. This explains phenomena like interference, diffraction, and polarization not easily elucidated by geometrical optics

1.2 Types of Light Lenses

• Converging (positive) lenses: bend light rays toward the axis with a positive focal length. They produce real, inverted images when the object is placed beyond the focal point and virtual, erect images when the object is placed between the focal point and the lens
• Diverging (negative) lenses: bend light rays away from the axis with a negative focal length. They always produce virtual, erect images, regardless of the object position

1.3 Lens Defects

• Chromatic aberration: Different wavelengths (colors) of light are focused at different points, causing blurring and color distortion.
  • Corrected by combining lenses of different types as "doublet" lenses
• Spherical aberration: Parallel light rays passing through the centre of the lens focus at different points, creating a blurred image
  • Corrected by appropriate lens design and aperture control

1.4 Limitations of the Human Eye

• The human eye has a limited resolving power (0.2 mm); it can only distinguish two points that are at least a certain distance apart.
• The eye is sensitive to visible light (300–700 nm)

1.5 Resolution of the Human Eye

• Given sufficient light, the unaided human eye can distinguish two points 0.2 mm apart
• Microscopes can magnify this distance allowing the eye to view smaller objects

1.6 Optical (Light) Microscope

• Uses visible light and a system of lenses to magnify images of small samples
• The oldest type of microscope
• Can be very simple or complex
• Images from an optical microscope can be captured by normal light-sensitive cameras (generating a micrograph). Images can also be captured digital using CCD cameras.
• Purely digital microscopes using CCD cameras show the image directly on a computer
• Alternatives to optical microscopy: electron microscopy
• Types of Optical microscopes: Simple and compound microscopes

1.7 Applications of Optical Microscope

• Used extensively in microelectronics, nanophysics, biotechnology, pharmaceutics, mineralogy, and microbiology
• For medical diagnosis and in smear tests

1.8 Types of Optical Microscopes

• Simple optical microscope
• Compound optical microscope

Chapter 2: Electron

• The electron is a subatomic particle with a negative electric charge.
• Electron properties are similar to those of light, including diffraction

2.1 Electron

• Electrons have wave properties
• Their De Broglie wavelength is greater than that of light, which is significant for experiments
• Electrons, in a magnetic field, behave like light
• They can be influenced by electric fields and magnetic fields

2.2 Electron Motion in Uniform Fields

• Electron motion in an electric field: Electrons are deflected towards the positively charged plate, following a parabolic path
• Electron motion in a magnetic field: Electrons move in a circular path if the field is uniform; helical path if electron's motion has a component perpendicular to the magnetic field
  • The force acting on an electron in a uniform magnetic field follows the Lorentz force law F = Bev.

2.3 Types and Design of Electron Lenses

• Electron lenses use electric or magnetic fields to control electron beams, converging or diverging them just as light lenses manipulate light
• Electrostatic lenses use electric fields between charged plates
• Magnetic lenses use magnetic fields produced by coils to control electron beams

2.4 Physical Similarity and Difference between Light and Electron Lenses

• Similarity: Both types of lenses can converge or diverge beams.
• Difference: Light lenses use glass; electron lenses use electrostatic or electromagnetic fields; light lenses used to manipulate light while electron lenses are used to manipulate electron beams

2.5 Types of Microscopy

• Optical (light) microscopy
• Electron microscopy (transmission and scanning)

2.6 Electron Microscopy

• Uses electrons to create images of samples
• Has higher magnification and resolution than light microscopes

Chapter 3: Types of Electron Microscopes

3.1 Types of Electron Microscopes

• Transmission electron microscope (TEM)
• Scanning electron microscope (SEM)

3.1.1 Transmission Electron Microscope (TEM)

• Uses an electron beam transmitted through a thin specimen
• The interaction with the specimen will produce an image from electron scattering
• A typical TEM consists of: vacuum system, the necessary electronics to for focusing and deflecting the electron beam, control software

3.1.2 Electron Source

• Tungsten
• Lanthanum hexaboride
• Field emission gun

3.1.3 What is the TEM?

• Is a technique for producing magnified images of objects using a beam of electrons passing through the object

3.1.4 How does TEM Work?

• Tungsten filament generating electron beam to focus on specimen
• Condensed optical lenses directing the beam
• Vacuum system to avoid air molecule deflection or collision
• Electron lenses to capture a magnified image on a screen or film
• Photographic film to capture permanent record

3.1.5 Sample Preparation

• Samples must be thinned.
  • Conducting samples: Thinned
  • Non-conducting samples: Thinned and coated with a conductive material

3.1.6 Electron Sample Interaction

• Backscattered, secondary, and auger electrons creation based upon electron beam hitting a sample or interaction, as well as a potential for X ray emissions

3.1.7 Advantages of TEM

• High resolution
• Crystal structure analysis
• Morphological information
• Defect analysis
• Compositional and nanoscale analysis

3.1.8 Disadvantages of TEM

• Requires thin specimens
• Sample preparation complexity

3.2 Scanning Electron Microscope (SEM)

• Uses a focused electron beam to scan the surface of a sample and analyse
• Gives information on the surface features and chemical composition of the sample

3.2.1 How SEM Works

• Electron gun to generate an electron beam
• Condenser lenses focus the beam to a fine point
• Scanning coils to move the beam across the surface of the sample
• Detectors to collect the emitted signals from the sample interaction

3.2.2 Advantages of SEM

• High speed
• 3D surface imaging
• High resolution images from angled view
• Depth of field
• Useful in EDS analysis (energy-dispersive X-ray spectroscopy)

3.2.3 Disadvantages of SEM

• Expensive instruments
• Requires special training
• Not suitable for chemical analyses (like EDX) in non-surface structures

3.3 SEM and TEM Images

• Shows examples of images obtained from nanoparticles, nanoparticles, core-shell nanoparticles, viruses and cells using SEM and TEM

3.4 Comparison Between TEM and SEM

• Electron beam: TEM uses broad, static beams while SEM focuses on a fine point that scans
• Voltage needed: TEM requires higher voltages (60-300,000 volts) compared to SEM (1-5 kV)
• Specimen interaction: TEM interacts with the whole specimen; whereas, SEM interacts based on surface.
• Imaging: TEM interacts to generate a real image, whereas SEM performs an imaging by scanning
• Preparation: TEM requires extremely thin specimens requiring multiple processing steps; SEM can use a wider range of samples, including thick samples or non-conducting materials

3.5 Sample Preparation in Electron Microscopes

• Step 1: Primary Fixation (using aldehydes, like formaldehyde or glutaraldehyde) : Fix and stabilise the ultrastructure of biological samples
• Step 2: Secondary Fixation (using osmium tetroxide): Adds detail and increases sample conductivity by fixing elements like the lipid membranes
• Step 3: Dehydration (using ethanol or acetone): Prevents artefacts from the process

3.6 Applications of Electron Microscopy

• Biology and Life Sciences
• Materials Research
• Industry

This quiz explores the fundamental concepts of electron microscopy, including its advantages over light microscopy and key differences in specimen preparation. Test your understanding of electron optics, magnification limits, and the complexity of electron optical devices.