A brief background to TEM
TEM or transmission electron microscopy is a common analytical technique used to study very thin specimens in a 2D plane, rather than producing 3D images like in SEM (scanning electron microscopy).
TEM uses similar principles to conventional light microscopy, but electrons are transmitted through a sample to produce an image, rather than light. This technique is a very reliable way to produce high-resolution images, which rely on the interaction of electrons with the atoms in the sample.
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Why is a higher resolution better?
High-resolution TEM uses a slightly different methodology to other types of TEM, resulting in a high resolution that can structurally characterize samples at an atomic level. It is, therefore, a very useful technique in the study of all nanoscale structures.
Structures at this scale normally are very difficult to visualize and characterize, so using this method opens up a whole new way to research into areas such as nanotechnology, nanomedicine, or material science.
How HR-TEM works – electron scattering
To appreciate how HR-TEM works, it is important to understand the concept of electron scattering.
Transmitted electrons interact with atoms within the sample via elastic and inelastic scattering. Electrons that undergo inelastic scattering have a change in energy after they are transmitted through the sample, whereas elastically scattered electrons maintain their initial transmitted energy and therefore can be more useful for the final data interpretation.
The electrons that undergo elastic scattering leave the sample and move through the lenses of the microscope to form the high-resolution image. Inelastically scattered electrons are not normally used in this technique but they can produce data from the sample using a technique called EELS, electron energy loss spectroscopy.
The scattering of electrons and photons (i.e. light) can be used to produce data in several other analytical techniques including Raman spectroscopy and medical ultrasound. The scattering of particles generates important information, as it changes depending on the sample’s molecular or atomic structure.
Image formation in HR-TEM
HR-TEM relies on a principle called ‘phase contrast’ for image formation which can often come with some problems when used at such a small scale. When the image is formed through the objective lens of the microscope, there is some significant oscillation present due to the small scaling used, which can often reduce the quality of the resolution of the image.
A function called the phase contrast transfer function (TF) describes how these limitations affect image formation. The TF is universal for all lenses and for all specimens used so it can be easily measured in all scenarios and used to predict the resolution produced in certain procedures.
What is HR-TEM used for?
There are many different applications for HR-TEM currently used, and there are even more options after potential development. It is often used for the study of crystals, but also materials such as metals, semiconductors, and nanoparticles. Special focus is usually aimed at defects in these materials at the atomic level.
These defects can include point faults, linear faults, or planar faults. These depend on if the structural defect affects just one exact point, one dimension along a line, or two dimensions, respectively.
Point defects can often be corrected but can be harder to detect. Planar faults such as crystallographic shear planes, stacking faults, or interfaces can be a bigger problem in the quality control of material production, and normally depend on specific atomic interactions within a material.