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TEM and STEM are related techniques which can be considered as the most powerful tools to characterise nanomaterials and indispensable for nanotechnology. In both cases, high energy electrons, incident on ultra-thin samples, allow for image resolutions that below the Ansgtrom order. The electron beam travels through the specimen and, depending on the density, crystallinity, orientation, etc. of the material present, electrons are scattered differently, giving rise to an image of the specimen with different contrast features according to specimen properties. In the STEM mode, electrons pass through the specimen, but, the electron optics focus the beam into a narrow spot which is scanned over the sample in a raster. The rastering of the beam across the sample makes these microscopes suitable for analysis techniques such as mapping by energy dispersive X-ray (EDX) spectroscopy, electron energy loss spectroscopy (EELS) and annular dark field imaging (ADF). These signals can be obtained simultaneously, allowing direct correlation of image and quantitative data. By using a STEM and a high-angle detector, it is possible to form atomic resolution images where the contrast is directly related to the atomic number.
Traditionally, TEM/STEM have been mainly applied for imaging, diffraction, and chemical analysis of solid materials. For biological samples, cell structure and morphology are commonly determined whilst the localization of antigens or other specific components within cells is readily undertaken using specialised preparative techniques and, when required specific TEM cooling holder. Nowadays, the availability of specimen holders designed to perform in situ experiments including heating, cooling, biasing, liquid or gas atmospheres, etc. have turned the TEM into a real nanolaboratory suitable for an increasing range of specimens both in Materials and Llife Science.
A TEM can also be used to do Electron Tomography, which allows obtaining detailed three dimensional (3D) structural and chemical characterisation of 3D objects. This is accomplished by multiple views of the same specimen obtained by rotating the angle of the sample along an axis perpendicular to the beam. By taking multiple images and/or chemical maps of a single TEM sample at differing angles, its 3D structure and composition can be retrieved.
In the last few years, there has been a considerable revolution in electron microscopy with the arrival of aberration correctors for the condenser and objective lenses with the consequent improvement in the attainable resolution limits. The obtainable resolution limit now lies at around 0.05 nm in both TEM and STEM, and the improved images from these aberration-corrected microscopes are opening up new avenues in the characterisation of materials. The use of aberration correctors allows using lower accelerating voltages while keeping atomic resolution and, in combination with the last generation of direct detection cameras and dose modulators, beam sensitive materials can now be observed in the TEM at very low energy and radiation doses.
Sample preparation is the most crucial part in TEM experiments. High quality TEM specimens have a thickness that is comparable to the mean free path of the electrons that travel through the samples, which may be only a few tens of nanometres. Preparation of TEM specimens is specific to the material under analysis and the desired information to obtain from the specimen. Sample preparation laboratories are equipped with the basic tools (diamond saw, polisher, dimpler, electropolisher, ultrasonic cutter, precision ion polishing system, gentle mill, plasma cleaner) commonly used in conventional chemical and mechanical thinning procedures.