According to the de Broglie’s theory, electron (with mass m and velocity v) has a wave nature with wavelength λ = h/mv (h: Planck constant), which is 0.0037 nm, if accelerated at 100 kV, and is quite small compared with λ of visible light (= 500 nm) or even X-ray (= 0.1 nm). This indicates that electron can be a source of diffraction experiments with smaller λ. Nowadays, reflection type of diffraction as illustrated in red ray-diagram of Fig. 123.1a is mostly used for surface analysis using low-energy beam (LEED), while the transmission electron diffraction (TED), as illustrated by green ray-diagram in Fig. 123.1a, is applied mainly for inner structure of thin crystals by using TEM (transmission electron microscope) with relatively higher energy. Open image in new window Fig. 123.1 a Ray diagrams for both “transmission” and “reflection” type of electron diffraction modes are illustrated with green and red arrows, respectively. b Principle of diffraction condition in transmission case is shown schematically