The basic mechanism of the transmission electron microscope is similar to that of XRD, but using the electron beams instead of the X-rays. The successful application of the TEM is beneficial to the discovery of the de Broglie (matter) wave. In this case, the crystallince lattice serves as a grating to lead to the diffraction of the electrons. With the TEM, essentially all incoming electrons are transmitted through a specimen that is suitably thin. Rather than being absorbed, electrons may be scattered (i.e., deflected in their path) by the atoms of the specimen. The fast electrons used in a TEM are capable of penetrating many atomic planes and so are diffracted by crystalline regions of material just like X-rays. Their wavelength (~0.04 nm for E0~100 keV) is much less than a typical atomic-plane spacing (~ 0.3 nm) so that, according to the Bragg equation of n\ = 2d sin 6, Bragg angles 6 are small. In addition, the integer n is usually taken as unity, since n'th order diffraction from planes of spacing d can be regarded as first-order diffraction from planes of spacing d/n.

Diffraction represents elastic scattering of electrons (i.e., deflection by the Coulomb field of each atomic nucleus) in a crystal. The regularity of the spacing of these nuclei results in a redistribution of the angular distribution of scattered intensity. Instead of a continuous distribution over scattering angle, there are sharp peaks centered around certain scattering angles, each twice the corresponding Bragg angle 6. In the TEM, this angular distribution can be displayed by magnifying the diffraction pattern first formed at the back focal plane of the objective lens. Examination of the TEM image of a polycrystalline specimen shows that there is a variation of electron intensity within each crystallite. This diffraction contrast arises either from atomic defects (including point defects, dislocation, planar defects) within the crystal or from the crystalline nature of the material itself, combined with the wave nature of the transmitted electrons. In addition to diffraction (or amplitude) contrast, image contrast features can result from the phases of the electron waves after they have traversed the specimen. Although this phase cannot be detected directly, it can give rise to interference between electron waves that have passed through different regions of a specimen, and these electrons are brought together if the specimen image is defocused by changing the objective-lens current slightly. Phase contrast also occurs as a result of the regular spacing of atoms in a crystal, particularly if the specimen is oriented so that its atomic columns are parallel to the electron beam. The contrast seen in high magnification (>500,000) images is of this type and allows the lattice of atoms to be seen directly (in projection). The atoms may appear as either bright or dark spots, depending on the amount of objective-lens defocus. A more complete interpretation of the atomic-scale structure may require recording of a through-focus series of images.

The nanoparticle/nanocapsule samples for TEM observation can be prepared in two steps: (a) the nanocapsules are first dispersed in ethyl alcohol by an ultrasonic field and (b) a drop of the suspension is then allowed to evaporate onto a carbon coated TEM mesh grid. At present, most commerial TEMs are Philips, Hitashi, JEOL, etc. A TEM can be used to obtain the detailed structural information, such as space group, characteristic interplanar spacings, lattice parameters, atomic site coordinate, atomic occupation, etc., for the phases in the materials. Figure 11 shows TEM photographs of Fe and Ni nanoparticles prepared by arc discharge. The size distribution of the nanoparticles can be seen and/or analyzed from the TEM image.

The resolution of the TEM could be improved by special arrangements of the magnetic fields serving as focuses

Figure 11. TEM photographs of (a) Fe and (b) Ni nanoparticles. After [604], X. L. Dong et al., J. Mater. Res. 14, 398 (1999). © 1999, Materials Research Society.

Figure 11. TEM photographs of (a) Fe and (b) Ni nanoparticles. After [604], X. L. Dong et al., J. Mater. Res. 14, 398 (1999). © 1999, Materials Research Society.

of the electron beams, by enhancing the energy of the electrons, and so on. The main advantages of the HRTEM are the direct observations of the detailed nanostructures, such as the core/shell structure, the interfaces, and the surfaces, the atomic defects (including point defects, dislocation, planar defects), the twin structures, etc. of the nanocapsules. Because the electron beam can be focused in a narrow range, the detailed structural information of the core and also the shell of the nanocapsules can be illustrated clearly. The most advanced HRTEMs can show the images of the arrangement of the atoms. Figure 12 represents a HRTEM image showing the shell/core structure of Fe(B) nano-capsules with an amorphous boron oxide shell of about 4.0 nm thick and a crystalline Fe core with d = 2.0268 A [122]. Figure 13 shows a HRTEM image for the shell/core structure of Gd(C) nanocapsules with a graphite shell and crystalline Gd core. The shell is characterized by curved lattice fringes of interplanar spacing 0.34 nm corresponding to the (0002) lattice plane of graphite carbon. HRTEM observation can indicate whether different types of the core/shell structures form, whether the interface between the shell and the core is sharp, whether the intermediate phase exists between the shell and the core, and whether different phases coexist in the cores. Furthermore, energy dispersive X-ray (EDX) microanalysis and parallel electron energy-loss spec-troscopy (PEELS) data equipped in a HRTEM can indicate the composition and its distribution in nanoscale of the phases in both the core and the shell of the nanocapsules. However, it is difficult to derive the relative amount of different phases in the nanocapsules from the HRTEM observation because only limited numbers of the particles can be observed.

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