Research Centre for Surface and Materials Science


Scanning Electron Microscopy (SEM)

A microscope in which a finely focused beam of electrons is scanned across a specimen, and the electron intensity variations are used to construct an image of the specimen. This type of microscope can effectively achieve magnifications from 200 to 35,000 times.

Predominantly an imaging technique, in Scanning Electron Microscopy an electron beam strikes the surface of a specimen with several results:

  • Some electrons "bounce" back, called back-scattered electrons, or BSE, the likelihood of this event is determined by atomic number of the specimen. Heavier elements lead to more back scattered electrons. Back scattered electrons have relatively high energies and can escape from penetration depths of anything up to a couple of ┬Ám. A BSE image relates strongly to atomic number variations within the sample, less strongly to topography.
Back scattered electron image of the polished cross section of a thermally sprayed coating
Back scattered electron image of the polished cross section of a thermally sprayed coating (contrast is due to variations in atomic number)

SEM is a highly flexible technique. Recent research applications include:

  • Fracture analysis of light metal alloys.
  • Identification of geological morphology.
  • Processing/property relationship of coating materials.
Secondary electron image of dendrites in Al casting
Secondary electron image of dendrites in Al casting.
  • Electrons interact with atoms within the sample causing other electrons to be knocked out of their orbitals. These are called secondary electrons, SE, and are heavily influenced by the angle of the beam to the surface, less so by atomic number. As such an SE image shows the topography of the specimen.
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Our equipment


RCSMS has the following two SEM instruments.

 

FEI (Philips) XL30 S-FEG

The XL30 S-FEG with Gatan Alto cryo-trans system. State of the art field emission instrument capable of 1.5nm resolution. Cryo-trans system extends the range of samples that can be examined to include fully hydrated specimens in a frozen state.

Samples must be vacuum compatible as the specimen chamber is under vacuum, they must also be conductive, but can be coated with a fine layer of Pt, Au or carbon.

 

FEI Quanta 200 field emission Environmental SEM

The ESEM is a versatile high performance, low vacuum, field emission scanning electron microscope. Based on FEI’s patented ESEM technology, it can operate at a range of pressures from 10-6 Torr to 25 Torr, allowing examination of fully hydrated specimens, without freezing or immobilising the sample.

The low vacuum detectors are not sensitive to light generated during heating of samples, so in-situ dynamic heating experiments can be imaged and recorded live at temperatures up to 1500° C.

The ESEM also has Scanning Transmission EM (STEM) detectors for thin section analysis of both wet and dry samples.
 

Melting sucrose crystals in the ESEM
Melting sucrose crystals in the ESEM.
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