ABSTRACT
Small objects about 100 nm in size can be closely observed with an electron microscope, but are impossible to observe with a light microscope because of the long wavelength of the detecting light. The transmission electron microscope (TEM) has been widely used in such fields as material and biological sciences, as electrons going through specimens contain significant information pertaining to crystal structure and bonding state. Figure 6.1 shows the schematics of (a) a light microscope and (b) a TEM. The light microscope is mainly made up of a visible light source and a system of optical glasses such as condenser, objective, and eyepiece lenses. The specimen is mounted between the condenser and objective lenses, and the light transmitted through the specimen produces its magnified view. The resulting image is directly detected by eye, whereas the resolving power of an ordinary light microscope is about 0.2 µm because of the diffraction limit of visible light having a wavelength range of about 400-700 nm. Specimens with smaller sizes than the resolution of the light microscope can not be observed, for example, viruses, voids and inclusions in crystal, and crystalline lattices. The TEM operates under a similar basic principle as the light microscope, but uses electrons instead of visible light. Max Knoll, a German electrical engineer, and Ernst Ruska, a German physicist, developed an electron microscope in 1931, which had about 13 magnifications. The first practical TEM was built by Albert Prebus and James Hillier at the University of Toronto in 1938. As can be seen in Figure 6.1b, the electrons generated by an electron gun are applied to the specimen. A high resolution of about 0.3 nm is theoretically obtained in the TEM, because the electron beam accelerated at several hundred kilovolts indicates an extremely shorter wavelength compared with visible light. The relationship between the wavelength (λ) and the acceleration voltage (V) of electrons is expressed as follows:
λ ∼ 1 23. ( ). V
nm
(6.1)
As the TEM apparatus (FEI, Tecnai F30) in Figure 6.2 can be operated at V = 300 kV, the accelerated electrons have a λ of ~0.02 Å. In fact, even
the atomic arrangement of crystals has been demonstrated to use the extremely short wavelength of electrons. During the generation of electrons via the electron gun, thermionic emission results from the heating of tungsten (W) or lanthanum hexaboride (LaB6) filaments with low work function and high melting point, whereas with a field emission gun this is achieved by applying a cathode (of several kilovolts to watts) with about 10-nm tip radius at room temperature. The field emission gun has superior brightness and lifetime compared to the thermionic emission gun, and a smaller beam size obtained by the field emission gun also represents a remarkable improvement in resolution for the TEM. In addition, since electrons are not able to move in the atmosphere, a column chamber of the TEM maintains a high vacuum above 10−5 Pa to enhance the mean free path of electrons. The traveling direction of electrons with negative charge can be altered with electromagnetic lenses that correspond to condenser, objective, and projective lenses in Figure 6.1b. The electron beam that becomes parallel to the specimen by using the condenser lens strikes the specimen to ascertain its crystal information. In TEM analysis, as electrons must travel the electron gun to the phosphor screen, there is an ultrathin specimen of about 100-nm thickness on the TEM grid consisting of metal frame and carbon (C)-based support film. Figure 6.3 shows electron, x-ray, and light signals generated by the interaction between electron beam and specimen. The component elements of the specimen can be analyzed by measuring the characteristic x-rays with an energy-dispersive x-ray spectroscope (EDX), which must be located near the specimen to detect the signals. Although backscattered, Auger, secondary electrons, and so on, occur when applying the electron beam to the specimen, a part of the electron beam goes through the specimen for its thin thickness. The objective and projective lenses in Figure 6.1b allow handlers to enlarge the TEM image on the phosphor screen. Moreover, if there is an electron energy loss spectroscope (EELS) below the phosphor screen, the bonding information of the specimen can be evaluated by detecting the direct beam.
