Correlative Microscopy Network
Correlative microscopy combines the strengths of different characterisation techniques to provide more information than a single technique alone.
CoMic is an open network for those characterising a range of materials and processes across numerous scientific disciplines.
We focus on imaging, spectroscopy, scattering and diffraction techniques, as well as the software and hardware used for data acquisition and data post-processing (image analysis).
To learn more about correlative workflows — or if you'd like to share your knowledge and experience — there are a variety of ways to interact with CoMic.
- Join the CoMic mailing list | Get regular invites to our online research meetings and the latest news from CoMic
- Talk to us on Slack | Our Slack channel is a space for in-depth conversations about correlative workflows
- Follow us on twitter | Connect with us @UoP_CoMic
- View our research presentations | Explore past presentations from our speakers
The correlative workflow will depend upon the type of sample and the research question being asked. As such, workflows may look very different between projects. You can read a little more about some of the individual techniques below.
X-rays are useful because they can pass through matter, allowing internal structures to be visualised that would otherwise be hidden. This can be a good place to start a correlative workflow, using what’s called a ‘global’ scan to characterise an entire sample and create a 3D digital copy. Regions of interest within a sample can then be identified, and subsequent higher-resolution X-ray scans may follow which focus on smaller subsets within a sample. This is something regularly performed within the Future Technology Centre at the University of Portsmouth.
X-ray microscopy isn’t a purely non-destructive technique, as sometimes regions within a sample may need to be removed for experimental simplicity. Furthermore, the X-rays themselves may also degrade a sample. However, since large samples (mm to cm) can be processed at spatial resolutions ranging from hundreds of microns down to hundreds of nanometers, it’s an extremely versatile technique.
Imaging isn’t the only thing you can do with X-rays, of course, and there are a variety of spectroscopic techniques, such as X-ray Absorption Spectroscopy (XAS) and X-ray Photoelectron Spectroscopy (XPS), and scattering-based techniques, such as X-ray Diffraction (XRD) and ptychography. To read more about some of these techniques, and others, visit the Diamond Light Source website.
Sometimes higher spatial resolutions are needed, and this is where electron microscopy (EM) finds its place in the correlative workflow. Surface features measuring a few tens of nanometers can be imaged using scanning electron microscopes (SEMs), whilst transmission electron microscopes (TEMs) can show the internal structures of thin slices with spatial resolutions of a few nanometers. Most high-end SEMs and TEMs also have analytical subsystems, with the capability to perform Energy Dispersive Spectroscopy (EDS) for chemical and elemental analysis, and Electron Backscatter Diffraction (EBSD) for microstructural characterization.
Electron microscopy is not just a 2D technique either, with 3D electron microscopy (VolumeEM) achievable through techniques such as Focused Ion Beam SEM (FIB-SEM), Serial Block Face SEM (SBF-SEM), Array Tomography, Serial-Section SEM, and Serial-Section TEM. Though the sample volumes that can be processed in EM are typically smaller than for X-ray microscopy (microns to mm), the ability to perform concurrent analysis with EDS and EBSD in a laboratory setting makes EM a powerful tool for correlative studies. Take a look at the University of Portsmouth’s own EM website.
When an even higher resolution is required Atomic Force Microscopy (AFM) can be useful. Utilising an atomically sharp tip mounted onto a cantilever, which is scanned across a surface, atomic forces between the tip and sample cause it to move up and down. These movements are used to build a map of the surface topography. Unlike light microscopes that are diffraction limited, AFM is only limited by the radius of the tip, meaning nanometre resolution is possible in three dimensions.
Samples for AFM are typically smaller than those used in X-ray and electron microscopy, and they must also be flat to allow the tip to maintain good contact during imaging. The field of view is often much smaller, with maximum scan sizes often being around 100 µm, however the technique offers higher resolution and true 3 dimensional data sets.
AFM allows users mechanical properties of a surface to be measured by collecting a force curve, where the probe is pushed into the surface in a controlled manner, and bending of the cantilever is monitored. With careful calibration, this data can be fitted to measure properties such as Young's modulus and adhesion. In addition, the tip material can be changed to produce maps of other surface properties. This is the basis of techniques such as magnetic force microscopy (MFM), conductive AFM (c-AFM) and Kelvin Probe Microscopy (KPFM), and many others.
There are many other techniques to consider that could be included here, but for now a quick summary of some should hopefully suffice. More will be added later as CoMic develops.
Neutron microscopy is also useful for looking at entire samples, particularly for lighter atomic elements, such as hydrogen and lithium, that do not absorb X-rays very much. Combining neutron and X-ray microscopy can be a really useful correlative route for gaining unique insights into certain samples. These kinds of investigations are performed at national facilities, like the UK’s neutron and muon source, ISIS. See also previous CoMic presentations on neutrons for archaeology, palaeontology, materials science, food science, and medical devices.
Nuclear Magnetic Resonance (NMR), whether for imaging or spectroscopy, is not limited by penetration depth, unlike X-ray photons, and thus NMR is very useful for certain applications. However, the sensitivity of the technique is broadly dependent upon the magnetic field strength and the presence of nuclides within a sample environment susceptible to magnetic fields. Contrast agents can be used to overcome some of these limitations. Perhaps most commonly associated with the medical field for characterising soft tissues, NMR can also be used for investigating flow in porous media, assessing saturation and wetting properties, and for monitoring structural changes due to a chemical environment.
Ultrasound (acoustic) microscopy is another example of a technique that is perhaps better known for medical applications, but is equally applicable to other scientific disciplines. An advantage of this technique is that the experimental set-up is relatively less complex, with transmitters and receivers accommodated into a single scanning device, which can be handheld. Ultrasound microscopy is particularly useful for hard-soft interfaces, though penetration may be limited by acoustic impedance of the sample environment. While it’s a non-destructive technique, there must be direct physical contact between the handheld probe and the sample, with acoustically conductive gels assisting further.
Using all of these separate techniques is great, but without the hardware to enable the acquisition of data, correlative workflows wouldn’t exist. A sometimes forgotten aspect of correlative microscopy, hardware plays a vital role. Perhaps as diverse as correlative microscopy is broad, there are many hardware aspects to consider, including precision and repeatability, but perhaps a first step to consider is the sample holders used in each technique, and how these are then transferred between different pieces of equipment.
Another important part of correlative microscopy to understand is the difference between data and information. Correlative workflows generate a lot of data, but isolating the necessary information is difficult. Software solutions that can help with things like image registration, segmentation, and quantitative analysis are paramount. There are several existing packages aimed at doing just this, including Oxford Instruments' Relate, and Zeiss’ ZEN Connect.
To find out more about our correlative work and the equipment we use, for collaboration opportunities, and for commercial services, please contact the Correlative Microscopy (CoMic) Network leader, Dr Charles Wood.