Investigating individual point defects, e.g. monovacancies, using TEM-related techniques was believed to be difficult because this requires both atomic sensitivity and atomic resolution and the specimens need to be very thin such that one can detect the individual point defects from the image contrast.
Since a monovacancy was first observed by TEM in low-dimensional carbon structures , the studies of point defects in monolayered materials using TEM have been attracting scientists' interest. For instance, the vacancies and topological defects in grapheme, edge structures and point defects in single layer hexagonal boron nitride (h-BN) [2, 7], monovacancies in WS2 nanoribbons  have been successfully identified at atomic level [4–6]. Unfortunately, this type of analyses was only achieved in “natively” grown nanomaterials but no applications have been performed successfully in samples prepared from bulk materials, e.g. by FIB (focused ion beam), mainly because of preparation-induced damages.
Figure 2227 shows a boron monovacancy [8, 9] (indicated by the darkest contrast in (a)), in single-layered h-BN (hexagonal boron-nitride), induced by the knock-on effect.
Figure 2227. (a) A boron monovacancy shown by the darkest contrast in the HAADF-STEM image of single-layered h-BN and (b) A corresponding atomic model (red: nitrogen, blue: boron). Adapted from 
Table 2227. Other Detections of Vacancies.
|Electron diffraction and simulation
Diffraction pattern of a planview TEM sample of ErSi2-x on Si(001) annealed at 700 °C. The extra spots are due to ordered vacancy structure are marked by "O". 
Schematic plots showing: (a) the ordered vacancy structure, (b) a simulated electron diffraction pattern viewed along the [1-100] silicide direction and (c) three-dimensional vacancy ordering structure. The vacancy positions are indicated as empty squares. 
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