Scanning Electron Microscopy

Digital imaging over a range of magnifications depending on sample quality  (~ x20 to x100 000) in the following modes:

• Secondary electron emission (SEI): when the specimen is irradiated with a fine electron beam, secondary electrons are emitted from the specimen surface. Topography of the surface can be observed by two-dimensional scanning of the electron probe over the surface and acquisition of an image from the detected secondary electrons.
• Backscattered electrons (BSE): backscattered electrons are those scattered backward and emitted out of the specimen when the incident electrons are scattered in the specimen. They have higher energy than secondary electrons and therefore provide information on a deeper region of the sample. They are sensitive to the composition of the specimen (i.e. the higher the atomic number of the element, the more backscattered electrons emitted). This means that an area where heavy atoms are located in a sample looks bright in a backscattered electron image (atomic number contrast).

When the sample is irradiated with an electron beam, electrons in the inner shells are emitted from the constituent atoms in the specimen and the vacant orbits are filled with outer-shell electrons. This leads to the sample emitting X-rays whose energies correspond to the energy difference between the outer-shell electrons and the inner-shell electrons. These X-rays are called ‘characteristic X-rays’ because their energies are characteristic of individual elements. EDX detectors allow the ID and semi-quantification of elements from carbon (C) upwards. Users should note that quantification of light elements (e.g. C, N and O) cannot be performed with accuracy.

This imaging mode is used for observation of non-conductive specimens without coating. When operating in low vacuum mode, samples are exposed to a lower energy electron beam in the chamber than used during standard SEM examination, so there is less risk of damage to fragile or sensitive samples (e.g. bio-engineering materials).

EMScope TB500 using carbon filaments to evaporate and generate a thin layer of carbon on the specimen surface. Carbon coating is used on low conductivity samples and it is compatible with EDX work.

BioRad SC500 using a gold target to sputter and generate a thin layer of gold on the specimen surface. Gold coating is best for high quality imaging but it is not suitable for EDX work.

LabRam HR-800 Jobin Yvon. Raman spectroscopy identifies molecules from their interaction with monochromatic radiation. The molecules scatter radiation and can rotate or vibrate depending on the energy of the incident radiation and the molecular structure. Characteristic spectra of intensity versus wavenumber are generated. 

• Non-conductive materials can be sputtered coated with carbon if imaging and EDX is required, or with gold if high resolution imaging is needed
• Advice on sample requirements and preparation is available on request

• MSDS and COSSH forms must be supplied with the samples where appropriate
• Samples should be clearly labelled and to limit contamination they should be stored in clean glass or plastic containers. Clients should avoid touching the samples as much as possible
• Clients should either collect their samples at the end of the work, or a charge will be applied for their disposal

Contamination can be a big problem in research/manufacturing when particles can appear buried in products and coatings affecting their final look and/or performance.

Analysing the composition of the contaminating particles can generally help in identifying the source for that particle and once that source is known, it will become easier to put measures in place to control it.

Scanning electron microscopes (SEMs) can be fitted with a range of detectors to analyse different aspects of a sample. One of these detectors is the EDX (Energy Dispersive X-rays) detector. This detector provides chemical composition of a sample, including what elements are present and their concentration.

Read further on contamination‌ to understand the benefits of using SEM: Identification of Contamination using SEM and EDX

Scanning electron microscopes (SEMs) are a powerful tool in determining the causes of failure on the micro and nano scales on a wide range of materials.

The large depth of field characteristic of SEMs allows high spatial resolution imaging of sample surfaces at higher magnification than conventional light microscopes. Different modes of failure show different characteristic features on the surface of the sample, therefore looking at a SEM image of a fracture surface can help us identify the cause of failure in the material.

Read more on failure analysis: Study of Failure Analysis of Materials using Scanning Electron Microscopy