# Characterization of Monovacancy Defects in Vanadium Diselenide Monolayer: A DFT Study
## 3 Results
In the case of TMDs, however, defect engineering plays an important role in the control of their electronic, optical, and magnetic properties, as well as surface chemical activity. For instance,  
Figure 5: The atomic structure of the 5 \(\times\) 5 supercell structure (**a**,**d**), the local density of states (**b**,**e**) and band structure (**c**,**f**) of the T-VSe\({}_{2}\) monolayer containing V\({}_{\rm Se}\) and V\({}_{\rm V}\) defects, respectively. Figure 6: Atomic structure of the 5 \(\times\) 5 supercell structure (**a**,**d**), the local density of states (**b**,**e**) and band structure (**c**,**f**) of the H–VSe\({}_{2}\) monolayer containing V\({}_{\rm Se}\) (marked by the black circle) and V\({}_{\rm V}\) defects, respectively. optoelectronic devices may be limited by the presence of chologene monovacancies, as these vacancy sites function as non-radiative recombination centers for photoexcited carriers [91]. It is well-known that sulfur vacancies are the most energetically favorable among point defects in exfoliated TMD monolayers [47]. The results of this work are well in line with this theory, as the formation of the Se vacancy has been found to be the most energetically favorable in both the T-VSe\({}_{2}\) and H-VSe\({}_{2}\) monolayers. It has been shown earlier that chologene monovacancies in 2D TMDs change their electronic structure by the formation of vacancy-induced localized electronic states [92]. These localized electronic states can increase the electrical conductivity of 2D TMDs through the hopping mechanism [93]. In the case of the T-VSe\({}_{2}\) and H-VSe\({}_{2}\) monolayers, the observed localized electronic states created by the VSe defect may also enhance electron transport and lead to a change of magnetic moment in them [72; 94]. The STM images of perfect monovacancy-containing T-VSe\({}_{2}\) and H-VSe\({}_{2}\) monolayers are obtained to facilitate experimental observation of these point defects. STM images of the pristine and monovacancy-containing T-VSe\({}_{2}\) and H-VSe\({}_{2}\) monolayers are shown in Figure 7. The constant height tip position at 4 A from the topmost Se atom is recorded in the image. This method is faster, despite being mainly useful for relatively smooth surfaces [42]. The STM image for the pristine T-VSe\({}_{2}\) monolayer shows protrusions from the Se atoms and looks similar to the STM image of the VSe\({}_{2}\) monolayer obtained previously in experiments and DFT simulations [72]. Specifically, the STM image of the T-VSe\({}_{2}\) monolayer (Figure 7a) is identified as six Se atoms arranged in a hexagon and surrounded by one V atom, with these atoms Se atoms depicted as bright spots due to their small atomic number and the V atom as a dark spot due to its large atomic number [95]. The H-VSe\({}_{2}\) monolayer (Figure 7b) is represented by three bright spots forming a triangle and one dark spot in the middle of that triangle [96]. The VSe defects (Figure 7c) in the T-VSe\({}_{2}\) monolayer show STM images with three bright spots (Se atoms) forming a triangle with a dark area in the middle of that triangle. The VSe defects (Figure 7d) in the H-VSe\({}_{2}\) monolayer are depicted as three blurred bright spots. The V\({}_{\rm V}\) defects in the T-VSe\({}_{2}\) monolayer (Figure 7e) are represented by five bright spots forming a pentagon with a dark area in the middle of that pentagon, while the V\({}_{\rm V}\) defects in the H-VSe\({}_{2}\) monolayer (Figure 7f) are represented by three bright blurred spots forming a triangle and located inside of a pentagon forming by another five bright spots. All the defects considered correlate with their corresponding atomic structures and are easy to recognize, as can be seen from the comparison of STM images in Figure 7 with the corresponding structures in Figures 2, 5 and 6.