Electron Microscopy

Electron probe

Electrons can be accelerated by electrical potential decreasing their wavelength

Wavelength of electron

Resolution

Scanning Electron microscopy

Produces images of a sample by scanning it with a focused beam of electrons

Microscope Applications:

▪ Topography (texture/surface of a sample)

▪ Morphology (size/shape of particles)

▪ Composition (elemental composition of sample

▪ Crystallographic information (arrangement present within sample)

Instrumentation

Electron Beam Generation

• Electron gun – generates a beam of electrons and accelerates them to energies in the range

• Thermoionic gun

• Field emission gun (FEG)

Electromagnetic Lenses

The purpose of a lens is to change the path of the rays in a desired direction.

The deflection experienced by a charged particle in a magnetic field is given by the Lorentz force law.

• Condenser lens: adjust diameter of electron beam

• Objective lens: Focuses the beam onto the sample

Electromagnetic Lenses: Focal length

The focal plane of the lens is the plane in which the parallel rays are brought to a focus.

Vacuum

It minimises scattering of the electron beam before reaching the specimen

Scattering or attenuation of the electron beam will increase the probe size and reduce resolution

Electron–Matter Interactions

Interactions: Auger

Relaxation of electron ionized atoms lea to emission of Auger electrons

Interactions: Secondary Electrons

Caused by an incident electron whcih imparts some of it energy to a lower energy electron as kinetic energy ~ 5eV.

Can produce several secondary electrons

Topogical

• Bumps: bright image

• Indents: dark image

Interactions: Back scattered electrons

Caused when an incident electron collides with an atom normal to incident path, and is scattered backwards 180^o.

• High atomic number: bright

• Low atomic number: dark

Interactions: X-rays

As the electron falls it emits energy, usally as x-rays to balance the total energy.

Charge Coating

For non conducting materials: coating layer grounds excess negative charge build-up.

SEM Resolution: Spherical abberations

lens more powerful at edge then at the centre

Disc of minimum confusion (d_min) results instead of point focus

SEM Resolution: Chromic abberations

SEM Resolution: Astigmatism

Occurs when the electrons sense a non-uniform magnetic field as they spiral round the optic axis.

Transmission electron Microscopy

A beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it.

2D image

Interactions: Transmitted unscattered electrons

No interactions between electrons and sample (no deflection from the original path) as electrons transverse the specimen.

• Thick specimin: dark (less transmitted)

• Thin specimin: bright

Interactions: Transmitted elastically scattered electrons

Incident electrons scattered by the atoms in the specimen in an elastic fashion (no loss of energy).

Crystalline materials coherent.

At higher angles elastic scattering becomes more incoherent.

Interactions: Inelastic Scattered Electrons

Electrons interact with the specimen atom in an inelastic fashion.

Large angles - incoherent

Amount of backscatter scales with thickness

• Kakuchi Bands: alternating dark/light lines relating to atmonic spacing

Instrumentation

Bright Field Mode

When the aperture is positioned to pass only the transmitted (un-diffracted) electrons.

Image is bright where diffraction in specimin is weak.

• Thick regions: dark

• No sample: bright

Mass Thickness Contrast

High-Z/Thicker specimins scatter more

Number of unscattered electrons decreases exponentially with thickness

Amorphous no diffraction

Density and thickness determine contrast

Dark Field Mode

When the aperture is positioned to pass only some diffracted electrons (exclude primary beam).

No scattering (primary beam): dark

study crystal defects

Diffraction Pattern

Fourier transform of the periodic crystal lattice, giving us information on the periodicities in the lattice and hence the atomic positions.

Information on crystallinity

Braggs Law

Point resolution limit

Aberration Corrected TEM

Spherical aberration in compensated by combining the converging lens with a suitable diverging lens - enabling a smaller electron beam