Electron Microscopy - Physics and Theories

Introduction to Electron Microscopy

Electron Microscopes

  • Hitachi SEM (High-End):

    • Equipped with multiple detectors for various applications.

    • Add-ons include EDS for elemental analysis, cryo transfers, and sero face imaging.

    • Versatile for diverse applications.

  • TEM (200 Kilowatts):

    • Features a lab 6 filament.

    • Capable of TEM and STEM.

  • Example Images:

    • Extracellular vesicles.

    • Biological structures.

    • Grains of metal alloy.

    • EDS elemental analysis results (colorful images).

What is Electron Microscopy?

  • Electron microscopes use a beam of electrons to inspect samples.

  • Enables high magnification images and provides information about the structure and chemistry of the sample.

  • Example: Close-up view of a bee's side using a benchtop SEM.

History of Electron Microscopy

  • Late 1800s: Discovery of the electron as a particle with known mass and charge.

  • 1924: Hypothesis of wave-particle duality.

  • 1927: JP Thompson's discovery that electrons behave as waves.

  • The concept of electron microscopy was born from the limitation of optical light microscopes due to the wavelengths of visible light.

  • The first electron microscope consisted of an electron source, lenses, a sample chamber, and a screen or camera.

Resolution in Microscopy

  • Microscopy involves imaging samples smaller than what can be resolved with the naked eye.

  • Common units used: micrometre, nanometer, and picometer.

  • Non-SI unit: Angstrom (1/10 of a nanometer), often used due to typical atomic bond lengths being 2-3 Angstroms.

  • Electron microscopy allows the study and imaging of viruses, individual molecules, and atomic resolution samples.

Resolution Limits

  • Maximum resolution for green light in fluorescence microscopy is about 227 nanometers (approximately half the wavelength of light).

  • The resolution limit at half the wavelength applies to all forms of imaging with waves.

Electrons as Waves

  • Electrons, like light, exhibit both particle and wave behavior.

  • The wavelength of an electron is related to its speed, which is related to its energy (accelerating voltage).

  • Equation: λ=h2m0eV\lambda = \frac{h}{\sqrt{2m_0eV}} , where:

    • λ\lambda is the wavelength of the electron.

    • hh is Planck's constant.

    • m0m_0 is the rest mass of an electron.

    • ee is the charge of an electron.

    • VV is the accelerating voltage.

  • Constants:

    • h=6.626×10−34 J sh = 6.626 \times 10^{-34} \text{ J s}

    • m0=9.109×10−31 kgm_0 = 9.109 \times 10^{-31} \text{ kg}

    • e=1.602×10−19 Ce = 1.602 \times 10^{-19} \text{ C}

  • Example:

    • At 20 kV, the calculated wavelength is 0.0086 nanometers, suggesting resolutions 10,000 times higher than with green light are possible.

  • Relativistic effects occur at these accelerating voltages, further reducing the real wavelength.

Image Formation

  • Image formation in electron microscopes relies on the interactions between electrons and the sample, broadly described as scattering.

  • Electrons strongly interact with matter due to their charge.

  • Interactions can be elastic or inelastic.

    • Elastic Scattering: Change in electron direction only, with no energy transfer.

    • Inelastic Scattering: Change in electron direction and transfer of energy to the electron or sample.

  • Different interactions provide useful information about the sample and electrons.

Modes of Scattering

  • Incoming high voltage electron beam interacts with the sample, resulting in:

    • Backscatter electrons

    • Secondary electrons

    • Unscattered electrons (if the sample is thin enough)

    • Forward scattered electrons

Elastic Scattering

  • Interaction that changes the electron's direction without altering its energy.

  • An electron is repelled by the electron cloud and attracted to the nucleus of an atom.

  • In crystalline materials, regularly spaced nuclei cause constructive and deconstructive interference.

  • Diffraction patterns result from electron interference when electron beams pass through a crystalline material (e.g., double-slit experiment).

Inelastic Scattering

  • Involves energy transfer between the beam and the sample.

  • Can damage the sample through localized heating (leading to diffusion or structural changes) and ionization (breaking covalent bonds in polymers and biological materials).

  • Provides valuable information about the sample's chemistry.

Elemental Analysis

  • Incoming electrons interact with the sample, transferring energy to electrons in the sample.

  • Excited electrons return to lower energy levels, releasing characteristic X-rays used for elemental analysis.

  • Another signal introduced by inelastic scattering is called Bras strong X-rays (breaking radiation).

    • Generated by primary electrons in interactions that don't eject core electrons.

    • Can have any energy up to the original energy of the electron and don't carry specific information.

Secondary Electrons

  • Electrons emitted from the sample due to direct interaction with the electron beam.

  • Low energy and can only escape from the surface of the sample.

  • Provide high-resolution images of the sample surface; very surface-sensitive.

Auger Electrons

  • Generated similarly to characteristic X-rays, but instead of photon emission, energy is transferred to another electron in a nearby band.

  • This electron is then emitted from the sample.

Collective Interactions

  • Electron-hole pairs and cathode luminescence (CL):

    • In direct gap semiconductors, electron-hole pairs are generated.

    • Recombination of electrons and holes gives off light (cathode luminescence).

  • Plasmons and phonons:

    • Plasmons: Collective oscillations of free electrons when a beam electron interacts with the free electron gas.

    • Phonons: Collective oscillations of atoms in a solid when the atom lattice is struck by a beam electron.

Beam Damage

  • Can cause alterations in structure, ionization, heating, phase changes, or diffusion.

  • Correct interpretation of data becomes more difficult after these events.

  • Polymers and biological samples are particularly prone to damage.

Electron Microscope Design

  • Electron microscopes are generally cylindrical.

  • Comparison of optical microscopes, TEM, and SEM:

    • Optical Microscope: Light bulb, condenser lenses, objective lens, projector lenses, eyepieces/camera.

    • TEM: Electron source, condenser lenses, objective lens, intermediate and projective lenses, camera/phosphor screen.

    • SEM: Similar to the top half of a TEM, with the objective lens focusing the beam into a small spot and raster scanning across the sample to generate pixel data based on signal intensity.

  • Typically requires more lenses (2-4 condenser lenses, 3-4 intermediate/project lenses) along with smaller alignment lenses.

  • Electronic lenses cannot be diverging.

Electromagnetic Lenses

  • Electrons cannot be focused with normal optics due to strong scattering.

  • Electromagnetic fields are used to focus the beam by taking several wraps of copper like a typical electromagnet, then they are wrapped in the soft iron case.

  • Electrons rotate around the central axis as they pass through the lens, causing image rotation.

Electron Gun

  • Materials contain electrons; the energy required to remove them is the work function.

  • Methods to extract electrons:

    • Heating (Thermionic Emission):

      • Tungsten: High melting point (3660K) allows it to be heated to extract electrons.

      • Lanthanum Hexaboride (LaB6): Lower work function, requiring heating to about 1700 degrees to extract electrons.

    • Field Emission:

      • The strength of an electric field on a spherical surface is proportional to the applied voltage and the radius of the sphere.

      • If the radius is less than 100 nanometers, the electric field gets so strong that electrons can tunnel out of the material

      • Requires a very small tip (radius < 100 nm) to create a high electric field, allowing electrons to tunnel out of the material.

      • Field emission guns are more expensive and require high vacuum for maintenance.

Electron Beam Formation

  • Components of the electron gun:

    • Filament (electron source)

    • Wehnelt: Negatively charged, focusing electrons inward.

    • Anode: Positively charged, attracting electrons downward with a small hole for beam formation.

Vacuum Requirements

  • Electron microscopes must operate under high vacuum to prevent electrons from scattering off gas molecules.

  • Higher resolution microscopes require higher performance vacuum systems.

  • Vacuum pumps: rough and a high vacuum pump. Also, Field emission guns and other high performance instruments usually add ion pumps or iron-getter pumps to achieve the highest possible vacuum.

Comparison with Other Radiation Sources

  • X-rays and neutrons can achieve similarly small wavelengths, dependent on energy.

  • Demonstrated resolution limits:

    • Electrons outperform X-rays and neutrons in terms of resolution due to ease of focusing.

    • Neutrons are difficult to focus due to their lack of charge, requiring crude focusing methods.

Advantages of Electron Microscopy

  • High resolution to observe nanoscale objects and individual atoms.

  • Strong scattering allows collection of useful information from small sample areas.

Summary of Part 1

  • Electrons exhibit wave-particle duality and have wavelengths 10,000 times smaller than visible light.

  • High-resolution imaging is possible due to the low diffraction limit.

  • Strong interaction with matter necessitates high vacuum to prevent scattering.

  • Inelastic scattering causes significant sample damage due to energy transfer.

  • Electron microscopy provides high spatial resolution and useful information from small areas and volumes.