Fine Particles

The morphology of a wide range of fine particles has been studied by electron holography by making use of Eq. (33). The projected thickness of a polyhedral particle provides information about the locations and sizes of the particle's edges and faces without the need to tilt the sample to multiple zone axes, although complications may arise because of the unknown relative inclinations of the entrance and exit surfaces. A significant advantage of examining small particles (in particular those below 50 nm in diameter) is that dynamical contributions to the phase are more likely to be small. When examining magnetic particles in low-field conditions, the mean inner potential and magnetic contributions to the phase must still be separated.

Figure 9a shows a high-resolution lattice image of a 15 nm cuboctahedral ZrO2 crystal that has {111} and {001} faces [90, 120]. Schematic diagrams of the expected shape and [110] projection of the crystal are shown in Figure 9b. A reconstructed electron holographic phase image of the particle, and line profiles obtained across different sections of the phase image, are shown in Figure 9c-g and are consistent with the expected geometry. A void within the particle is apparent in one of the profiles (Fig. 9g). Similar surface structures and internal voids have been reported for 5-15 nm Pd particles supported on amorphous silica spheres [121-124].

Electron holography has been applied to the study of cubic single crystals of MgO, which have {100} faces and are produced by burning magnesium wire or ribbon and collecting the resulting smoke (e.g., [120, 125-127]). Further applications of holographic phase imaging have included the formation of 1.3 nm thickness contours on a 200 nm Be particle by making use of 32-times phase amplification [127] and the examination of carbon nanotubes [128, 129], voids in Si [130, 131], SiGe quantum dots [132], the tobacco mosaic virus [133], bacterial flagellae [134], strands of DNA [135], and both organic polymers and mesoporous silica [136].

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