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At high concentration, oxidation leads to new derivatives of oxided phosphorene. As shown in Figure 1.14, up to fully oxidation, the interatomic P-P lengths increase to 2.32 and 2.37 Å while the direct gap located at point Γ reduces with respect to pure phosphorene. VBM is characterized by py orbitals of P- and O-atoms, while the P-s and O-p_z orbitals dominate the conduction band minimum (CBM) [30]. Both interstitial and dangling oxygen form no states in the middle of the gap, while the horizontal and diagonal oxygen introduce levels in the gap, which deals with a deep acceptor state at near the conduction band. Furthermore, the planar phosphorene oxides exhibit a monotonic increase to reach a maximum value with a deoxidation degree of 0.25, then start to decrease to attempt the value of 0.6 eV in a fully oxidized PO structure. For the tubular structure, the band gaps take the values from 0.4 (1.62) to 5.56 (7.78) eV at PBE (HSE level) [32]. Interestingly, the GW corrected band gap shows that the increasing oxygen coverage leads to an increase in the band energy from 4 eV to 10 eV, indicating that the VBM and CBM part become more localized [83].

Figure 1.14 (a) Top and (b) side views of phosphorene oxides PO.

The application of electric field reduces the gap energy of PO to a minimum of about 0.4 eV for a field E = 1.5 V/Å. The band gap fluctuates also from direct found for 100% to indirect for O-concentrations of 12.5%, 25%, and 50 %. Also, the work function in phosphorene increases linearly with the increased of the oxidation degree. The calculated values for PO_0.125, PO_0.25, and PO_0.5, are 4.9, 5.2, and 5.8, respectively, compared to PO that has 7.2 eV [30].

Under ambient conditions, phosphorene oxide is a stable material that did not exhibit any negative frequencies in its phonon dispersion curve [30]. Moreover, the simulation indicates that oxided structure is still robust and intact at low temperature, confirming its stability, while the material cut for large temperature values [84]. Unlike pure phosphorene, the phonon dispersion of PO exhibits three main regions as displayed in Figure 1.15b, namely, (i) the acoustic region, (ii) the middle region, and (iii) the high frequency range. Moreover, in contrast to the electron effective mass, the effective masses of holes are anisotrope [30].

Besides the band structure modification, the oxidation tunes also the optical features of phosphorene. In PO systems, the absorption spectrum reveals that the 1st absorption peak is located at 2.7 eV, in P₄O₂, it is also found that both phosphorus and oxygen atoms contribute in the transition and extend the wavefunction along the AC-axis (see Figure 1.16a). Further, in the P₄O₁₀ system, the absorption peak moves to high energy with a peak located at 7.0 eV. The wavefunction only localized on oxygen atoms adsorbed at the surface (see Figure 1.16b). This changes the binding energy Eb from −1.4 eV to reach −3.0 eV for P₄O₂ and P₄O₁₀. The electronic and optical band gap as well as the binding energy of P₄O₂ and P₄O₁₀ are very close to those of benzene [83].

Figure 1.15 Phosphorene oxide (a) band structure and density of states, (b) phonon dispersion curves and density of states.

Figure 1.16 Absorption spectrum and exciton wavefunction for the first transition peak for (a) P₄O₂ and (b) P₄O₁₀ structures.