PSI - Issue 65

Svetlana V. Ustiuzhanina et al. / Procedia Structural Integrity 65 (2024) 295–301 Ustiuzhanina S.V. et al. / Structural Integrity Procedia 00 (2024) 000–000

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Structural defects (Grishakov et al. (2016), Katin et al. (2018), Cai et al. (2019) Ustiuzhanina and Kistanov (2023)) are always present in low-dimensional materials. In most cases defects occur in synthetic materials, since various types of defects arise in these materials during the synthesis process (Hossen et al. (2024)). On the other hand, defects’ density can be controlled via defect engineering (Chen et al. (2022)). Nevertheless, surface defects can tune materials properties, therefore, it is important to calculate the formation energy of typical defects that can exist in the Zn 2 (V,Nb, Ta)N 3 monolayers. Common defects such as single vacancy defects: a single vacancies of a N atom (SV N ), of a Zn atom (SV Zn ) and of a V/Nb/Ta atom (SV V /SV Nb /SV Ta ) are considered. The bonding strength is calculated using the COHP analysis (Dronskowski and Blochl (1993), Chumakova et al (2022)), where the binding energy in a system is calculated as the area under a partial COHP (pCOHP) curve. Figure 3 presents the formation energies of above listed defects in the Zn 2 (V,Nb, Ta)N 3 monolayers, calculated based on the COHP analysis. The insets of Figure 3 show the defect-containing structures, and the vacant atom is shown as a yellow ball. It is estimated that the formation energy of the SV N defect is 6.36 eV for the Zn 2 VN 3 monolayer, -5.75 eV for the Zn 2 NbN 3 monolayer, and -5.78 eV for the Zn 2 TaN 3 monolayer. The formation of SV Zn requires an energy of ~ 3.02 eV in the case of the Zn 2 VN 3 monolayer, -2.03 eV in the case of the Zn 2 NbN 3 monolayer, and -3.70 eV in the case of the Zn 2 TaN 3 monolayer. High formation energy of ~18.80 eV for the SV V defect, ~16.75 eV for the SV Nb defect, and ~17.19 eV for the SV Ta defect are found in the Zn 2 VN 3 monolayer, Zn 2 NbN 3 monolayer, and the Zn 2 TaN 3 monolayer, respectively.

Fig. 3. Formation energies of SV N , SV Zn , and SV V /SV Nb /SV Ta defects in the Zn 2 (V,Nb, Ta)N 3 monolayers, calculated based on the COHP analysis. In the insets, the vacant atom is shown as a yellow ball.

Defects can also change the electronic properties of low-dimensional materials, such as charge carrier mobility (Cai et al. (2016), Katin et al. (2023), Kosarev and Kistanov (2024), Ilgamov et al. (2024)). The mechanism of carrier transport in pure and defect-containing Zn 2 (V,Nb,Ta)N 3 monolayers is further evaluated. The spatial structure of wave functions at the k points corresponding to valence band minimum (VBM) and conduction bands maximum (CBM) for the Zn 2 NbN 3 monolayers, as an example, is shown in Figure 4a. It can be seen, that the CBM and VBM in the Zn 2 VN 3 monolayer are localized in the x and y directions, which is similar in the cases of the Zn 2 VN 3 monolayer and the Zn 2 TaN 3 monolayer. Notably, the CBM in Zn 2 (V,Nb,Ta)N 3 monolayers is strongly delocalized in the out-of-plane direction, which suggests an increased electron mobility compared to hole mobility in these monolayers (Anikeeva et al. (2019)). For the case of the SV Zn defects possessing the lowest formation energy, a topological analysis of the electron localization function (ELF) is conducted to evaluate the impact of this defect to the electron transport in Zn 2 (V,Nb,Ta)N 3 monolayers. According to Figure 4b, which shows the the ELF of pure Zn 2 NbN 3 monolayer, electron localization basins are spherical and migrate to the respective cores of the N atoms, which is similar in the cases of the Zn 2 VN 3 monolayer and the Zn 2 TaN 3 monolayer. In the presence of the SV Zn defect (Figure 4c), electrons migrate to the neighboring to the vacancy site N1, N2 and N3 atoms, and strong electron localization occurs at the isolated vacancy site. The formation of such localized basins can be attributed to the presence of a trapped electron pair (Edwards (1988)), a high concentration of which can lead to a superconductivity (Gomes and Illas (2004)).