**3. Defects**

As we know, BN is a good choice for low-dimensional semiconductors (especially 2D) with remarkable thermal, mechanical, and dielectric properties. Theoretically, BN should exhibit perfect lattice structures free from defects, but as we know, the actual material is marginally different from theoretical models. Similarly, BN has some inevitable structural defects that arise due to the imperfection of the growth/preparation processes. These defects are unintentionally induced because of the dramatic influence over the material's physical properties, even in a low-dimensional state.

#### **3.1 Point defect**

The point defect, known as Stone-Wales (SW), is prevalent in semiconductor materials. It involves the connectivity change of π-bonded carbon atoms leading to their rotation by 90° [45]. This defect forms two separate vertically bonded rings instead of two rings sharing a common edge. This specific structural defect, such as graphene, is common in sp2 -bonded carbon allotropes. Similarly, SW or point defects are observed in BN material with low dimensions [46]. The formation of point defects in BN nanoribbons is due to the structural geometry. The SW defects in BN nanoribbons (zigzag and armchair) are shown in **Figure 5**. These defects decrease the bandgap regardless of BN nanoribbon orientation but maintain ultrawide bandgap behavior (insulating). At the same time, the defect site of this particular nature is far more reactive when compared to the defect-free site in BN nanoribbon [47–49].

Atomic vacancy is also a point defect observed by high-resolution transmission electron microscopy (HRTEM). The HRTEM analysis of BN monolayers reveals triangle-shaped vacancies that have been observed. It is also revealed that the monovacancy of boron (VB) and the monovacancy of nitrogen (VN) coexist in nature. The boron atom has low knock-on energy compared to that of nitrogen. Hence, it favors the formation of boron vacancies rather than nitrogen vacancies [49]. Therefore, VN is not observed during HRTEM observation.

On the other hand, the coexisting vacancies like VB, V3B + N, and V6B + 3N, etc., are evident. However, Alem et al. suggested that besides knock-on damage, there might be other mechanisms of forming coexisting vacancies, such as replacing ejected atoms with nearby atoms [50]. Furthermore, the interlayer distance with a missing boron atom was enlarged, which indicates that the dangling bonds for each N atom might be repulsive to each other. No stable divacancy (VBN) was observed as VBN would immediately transform into V3B + N due to further removal of boron atoms [48].
