Transmission Electron Microscopy

The distinguishing feature of transmission electron microscopy (TEM) is its ability to form images of atomic arrangements at localized regions within materials. It provides a view of the microstructure, that is, the variations in structure from one region to another, and the interfaces between them. TEM plays a critical role whenever macroscopic properties are controlled or influenced by defects or interfaces, for example, in the development of advanced structural materials with their complex microstructure of second phases or electronic materials which rely on the exquisite control of interfaces and multilayers. X-ray or neutron diffraction provide quite complementary information. These techniques can determine the average structure of complex materials very precisely, but not the structure of a local region or individual nanostructure. As condensed matter physics moves toward the study of ever more complex materials, and at the same time interest in nanoscale physics and devices is increasing, TEM or its scanning counterpart STEM, is finding a rapidly increasing role in basic condensed matter physics research. The unique role of the TEM arises because electrons are charged particles, and therefore, unlike X-rays or neutrons, are able to be accelerated and precisely focused by electromagnetic fields. The scattered beams can be collected by a lens, and refocused to form a true real space image in the manner of an optical microscope, where each point in the image corresponds to a specific point in the object. Electrons also interact much more strongly with matter and electron diffraction can be performed on materials of nanometer dimensions. For accelerating voltages of 100–1000kV, the electron wavelength ranges from 0.004 to 0.001 nm, orders of magnitude lower than atomic spacings in materials. It would therefore appear relatively trivial to form atomic resolution images of materials. However, it is only very recently that atomic resolution imaging could be said to be at all trivial. The major limitation was realized early in the history of the microscope to be the high inherent aberrations of a round magnetic lens. Spherical aberration is the dominant aberration, and leads to a ray deviation of Csa , where a is the semiangle of the objective lens. With electron lenses, Cs is of the same order as the focal length. This means that only very small apertures could be used, and since microscope resolution is given by 0.61l/ sin a, where l is wavelength, the best resolution would be limited by diffraction to B50l. It took B50 years from the development of the first transmission electron microscopes in the 1930s to achieve sufficient electrical and mechanical stability to allow imaging of crystal lattices at 0.2–0.3 nm resolution in the 1980s. Over the next 20 years, resolution improved incrementally to 0.1–0.2 nm. Now electron microscopy is in the midst of a revolution. Thanks to the development of solid-state devices, specifically the computer and charge-coupled-device (CCD) detectors, a series of nonround magnetic lenses can be used to correct the aberrations of the (round) objective lens. The gain in resolution over the last few years is comparable to that seen in the last few decades, an extraordinary advance that has pushed TEM into the sub-Angstrom regime for the first time in history. Aberration-correction brings more than just resolution; better resolution brings increased sensitivity, and recent results have demonstrated the imaging of single atoms within materials and on their surfaces, together with their spectroscopic identification by electron energy loss spectroscopy (EELS). Furthermore, entirely new modes of microscopy now appear feasible. Aberration-correction is allowing the objective aperture to be opened up, and, just as in an optical microscope, the depth of focus reduces. We are seeing the beginnings of three-dimensional (3D) TEM.