## 3 Results and Discussion
### Electronic Properties  
To comprehend the electronic properties of the 2D ZnO/graphene heterostructures, CDD analysis and band structure calculations were performed. As shown in Figure 2a, the charge accumulation (yellow-colored region) occurs mainly in the middle of the pristine 2D ZnO/graphene structure. This is also evidenced by an appearance of a strong CDD peak in the corresponding region of the \(xy\)-plane averaged charge redistribution curve presented \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Structure**} & \multirow{2}{*}{\(\mathbf{D}\), Å} & \multicolumn{2}{c}{**Mean Zn-O**} & \multicolumn{2}{c}{\(\mathbf{E_{\text{adhr}}}\)} & \multirow{2}{*}{\(\mathbf{E_{\text{D}}}\), eV} & \multirow{2}{*}{\(\mathbf{\varphi_{n}}\), eV} & \multirow{2}{*}{\(\mathbf{\varphi_{p}}\), eV} \\  & & **Bond Length**, Å & & & & & \\ \hline \multirow{2}{*}{2D ZnO/G} & \multirow{2}{*}{2.931} & \multirow{2}{*}{2.001} & \multirow{2}{*}{\(-0.076\)} & 0 & 1.578 & 0.520 \\  & & & & [0] & [1.571] & [0.516] \\ \hline \multirow{2}{*}{1V\({}_{\text{O}}\)-2D ZnO/G} & \multirow{2}{*}{2.925} & \multirow{2}{*}{2.022} & \multirow{2}{*}{\(-0.077\)} & \(-0.231\) & 1.354 & 0.636 \\  & & & & [\(-0.231\)] & [1.354] & [0.635] \\ \hline \multirow{2}{*}{2V\({}_{\text{O}}\)-2D ZnO/G} & \multirow{2}{*}{2.913} & \multirow{2}{*}{2.027} & \multirow{2}{*}{\(-0.080\)} & \(-0.524\) & 0.915 & \(-0.016\) \\  & & & & [\(-0.524\)] & [0.915] & [\(-0.018\)] \\ \hline \multirow{2}{*}{1 Fe-2D ZnO/G} & \multirow{2}{*}{2.924} & \multirow{2}{*}{2.019} & \multirow{2}{*}{\(-0.091\)} & \(-0.619\) & 0.931 & 1.508 \\  & & & & [\(-0.643\)] & [0.897] & [0] \\ \hline \multirow{2}{*}{2 Fe-2D ZnO/G} & \multirow{2}{*}{2.912} & \multirow{2}{*}{2.032} & \multirow{2}{*}{\(-0.096\)} & \(-0.663\) & 0.816 & 1.495 \\  & & & & [\(-0.691\)] & [0.832] & [0] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of selected parameters for pristine and defective 2D ZnO/graphene structures obtained by spin-polarized GGA-PBE+U-D2 calculations. The main parameters describing the electronic properties of the heterostructure are given for the spin-up channel. The parameters for spin-down channels are included in square brackets. Figure 1: Optimized structures of (**a**) pristine and (**b**–**d**) defect-engineered 2D ZnO/graphene heterostructures, including: (**b**) 1 oxygen vacancy, (**c**) 2 oxygen vacancies, (**d**) 1 Fe substitutional atoms, and (**e**) 2 Fe substitutional atoms, respectively. Grey-, red-, mustard-, and brown-colored balls represent zinc, oxygen, iron, and cardon atoms, respectively. in Figure 2b. In turn, the other two charge accumulation regions (as well as the maxima of the CDD curve) are located directly in 2D ZnO and graphene layers, while the charge depletion regions are manifested as deep minima of CDD curves were observed below (above) the 2D ZnO (graphene) layer. The nature of the charge redistribution in the system leads to the coincidence of the Dirac point energy with the Fermi level (Figure 2c). This was probably because the positive and negative charge carriers canceled each other out, making the graphene charge neutral. It is noteworthy that the band structure and density of states of the pristine 2D ZnO/graphene system were the same for the spin-up and spin-down channels. The conduction band mainly consisted of C 2\(p\) states, while the contribution from unoccupied Zn 4s states became evident above 2 eV. The valence band was mainly dominated by the occupied O 2\(p\) states; however, the contribution of occupied C 2\(p\) and Zn-related states was also significant. A careful analysis of the band structure also enabled determining a contact type of the 2 DZnO/graphene heterostructure (Figure 2c). According to the Schottky-Mott model [39; 40], the \(n\)-type Schottky barrier \(\Phi_{\mathrm{n}}\) refers to the difference in energy between the conduction band minimum (CBM) at the high-symmetry point \(\Gamma\)-point and the Fermi level \(E_{\mathrm{F}}\left(\Phi_{\mathrm{n}}=E_{\mathrm{CBM}}-E_{\mathrm{F}}\right)\).