Bright-field (BF) Imaging in TEM
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Different from dark-field (BF) TEM imaging condition, in TEM bright-field (BF) mode as shown in Figure 3358a, an objective aperture is placed in the back focal plane of the objective lens which allows only the transmitted (also called direct) beam to pass. In this case, the specimen is imaged using a parallel incident electron beam. Therefore, only a small amount of the electrons that have passed through the specimen are used for imaging.
The electrons are diffracted by sets of crystallographic planes, e.g. (hkl) planes in Figure 3358a, and the angle between the transmitted beam and the diffracted beams depends on various crystallographic planes, e.g. 2θB (θB is a Bragg angle). Both the transmitted and diffracted beams are deflected by the objective lens. However, the transmitted beam is delivered to the origin (O*) of the back focal plane, and the diffracted beam to a spot denoted by ghkl*. The inverted bright-field (BF) and dark-field (DF) images are formed by the transmitted and the diffracted beams in the first image plane, respectively. However, in the bright-field case, mass-thickness and diffraction contrast contribute to image formation, that is thick areas, areas with heavy atoms, and crystalline areas in most crystalline orientations appear in dark contrast. The mixed contrasts make the interpretation difficult (see Table 3358).
Figure 3358a. TEM bright field (BF) mode. The white disk indicates the objective aperture.
The local contrast on the TEM image plane is proportional to the number of electrons striking the viewing screen or detector. In Figure 3358b, a0, b0, c0, and d0 represent the rays in the incident electron beam, a1, a2, b1, c1, and d1 the rays in diffracted beams, and a3, b2, c2, and d2 the rays in the directly transmitted beam. The direction deviation of the transmitted rays from that of the incident electron beam is due to a magnification caused by the lenses between the specimen and the imaging plane. According to the theory of diffraction contrast formation, in bright field TEM mode, only the a3, b2, c2, and d2 rays are selected by a proper objective aperture for imaging, i.e. without selecting any rays in diffracted beams.
Figure 3358b. Schematic illustration of diffraction contrast formation in TEM imaging mode.
Any optical defects, e.g. chromatic and spherical aberrations, affect the performance of the electron microscopes such as the contract of the bright field images and the quality of the dark field images in TEMs.
In TEM imaging, bright-field pictures are usually obtained at smaller angles and consist of a mixture of elastic, thermal diffuse, and inelastic scattering. The cause of the limited range of scattering angles is that these electrons do not travel very close to the centre of the atoms. In this case, the small scattering angle induced reflects a small phase change to the wave front of the incident probe. When sample thickness exceeds few inelastic mean free paths (e.g. few hundreds of nanometers), the contribution of inelastic scattering dominates. The spatial resolution degrades when a small objective aperture is used to reduce the inelastic scattering contribution to bright field images, or to exclude closely spaced spots to form dark field images.
In practical bright-field TEM imaging, the objective aperture is used to select the transmitted beam as well as a small amount of reflections by cutting out all the other electrons. Especially, high-resolution transmission microscopy (HRTEM) uses both the transmitted electron beam (wave) and several diffracted electron beams (waves) to form the image. In this way, the resolution is improved with the multiple beams comparing to bright field imaging because higher spatial frequencies are included.
Based on the optical reciprocity theorem , BF-STEM images can be related to the HRTEM images of an atom as a phase object. Like HRTEM images, BF-STEM images are very sensitive to the focal conditions, and contrast reversal is often observed because of multiple scattering and phase shift due to defocus condition of the lens. A full interpretation of BF-STEM images can be done with phase contrast simulation in the same way as the HRTEM images.