Scanning Electron Microscopy and Elemental Microanalysis (EDX)

Scanning Electron Microscopy (SEM) is an electron imaging technique used to characterise the morphology and microstructure of bulk sample materials. A finely focused electron beam is scanned across the sample and the electron signals generated (including secondary electrons and back-scattered electrons) are then amplified and detected to produce an image of the surface or near surface features of the sample. Another important signal generated when the incident electron beam interacts with the sample is the characteristic X-ray signal, which is used to determine the elemental composition of small or larger areas of the sample. To do this an energy dispersive X-ray microanalysis (EDX) system is used and if the sample is suitably prepared it is possible to perform quantitative elemental analysis. An alternative chemical microanalysis approach (for improved light element, trace element analysis and better spectral resolution) is to utilise the wave length of the X-ray signal using a wave length dispersive (WDX) system. 

The nmRC is home to nine SEMs (all of which have EDX capability with WDX spectrometers on two of the SEMs).

A summary of the more specialised nmRC capabilities include:

• Focussed Ion Beam (FIB) SEM uses a focused ion beam for materials processing and sample preparation (deposition, ablation, sectioning etc.) or at low beam currents imaging in its own right.
• Environmental SEM (ESEM) allows examination of fully hydrated 'wet' samples and of poorly conductive uncoated materials, all of which cannot be imaged in the high vacuum conditions of a conventional SEM. 
• Cryo-SEM allows examination of rapidly frozen samples and subsequent fracturing to reveal the internal structure. This enables preservation of the true morphology and ultrastructure (and chemistry) of fully hydrated or liquid specimens. 
• Field Emission Gun (FEG) SEM uses a high brightness electron beam with a high spatial resolution (compared to thermionic tungsten filaments) and therefore more suitable for imaging features on the nanoscale. Another advantage of a FEG source over thermionic emitters is the much improved performance at lower accelerating voltages (<5kV).
• Wave-dispersive X-ray Analysis (WDX) - the lower peak to background inherent in the wavelength dispersive spectrum allows lower levels of detection and therefore much better analysis of trace elements compared to EDX. WDX is also better for light element analysis and the much improved spectral resolution means fewer spectral overlaps and more accurate analysis. The combination of the lower peak to background and the improved spectral resolution also produces 'cleaner' looking X-ray maps compared to EDX maps. 
• Mineral Liberation Analysis (MLA) with EDX software allows automated large area analysis of polished specimens to identify and quantify mineral composition and distribution. 
• Electron Back-Scatter Diffraction (EBSD) is used to study crystal structure at the surface of a sample in the SEM, with a spatial resolution approaching that of the SEM image, by identifying crystalline phases and measuring crystal orientations. This can be combined with EDX to correlate elemental composition and crystal structure. By mapping an area it is possible to generate images showing the distribution of phases and orientation, to identify grain and sub-grain boundaries, to calculate grain statistics, and to analyse preferred orientation (crystallographic texture) using pole figures, inverse pole figures and the orientation distribution function. 

Get in touch with us to discuss your SEM requirements.