2D Monoelements. Группа авторов

2D Monoelements

GW band structures of half-oxidized structures."/>

Figure 1.10 GGA and GW band structures of half-oxidized structures.

POs can provide oxygen in its solid-phase to valve regulated Li-O₂ batteries. In addition, when the Li atom is absorbed at the surface of the POs, it binds strongly to the O atoms indicating a strong ionic characteristic of the bond between oxygen and lithium [75]. The absolute values of binding energies of the Li atom adsorbed on the PO surface are greater than those of the Li atom on pure phosphorene, MoS₂ and graphene [76–78]. POs promise high diffusivity owing to the anisotropy of POs cathode barrier that is reduced by half with respect with the armchair axis for Li diffusion on POs. Besides, Li-PO structures with a number of Li atoms lower than O atoms show stable discharge products for PO cathodes [75].

1.3.2.2 Optical Response

The optical absorption spectra of half oxidized phosphorene in Figure 1.11 show maximum values for dangling structures observed at 1.46, 1.71, and 2.62 eV in images , and images , respectively.

This absorption behavior is required for photodetector with high efficiency. In contrast, pics of spectra describing the bridge structures coincide with the ultraviolet part and the visible light, since the they are located at 1.81, 2.03, and 3.18 eV for images , and images , respectively. One deduces that 50% oxidation is an effective manner to enlarge the absorption range of phosphorene along the light spectrum [20].

Schematic illustration of absorption coefficient of dangling structures (on the left) and bridge structures (on the right) of half-oxidized phosphorene sheets obtained using the GW-BSE methods.

Figure 1.11 Absorption coefficient of dangling structures (on the left) and bridge structures (on the right) of half-oxidized phosphorene sheets obtained using the GW-BSE methods.

Half oxidation is also used to modify the reflectivity of phosphorene as illustrated in Figure 1.11. Indeed, its maximum value in the UV region is around 38%, 50%, and 34% located at 8.21, 8.04, and 7.06 eV in images , images , and images , respectively. In the visible part, these structures show a reflectivity lower than 15% indicating their potential use for transparent electronics. The situation is different for the dangling configurations P₂OU, P₃OD, and P₄OD structures that reflect the visible light with a maximum value of 42%, 38%, and 39% found for the energy 1.48, 2.58, and 1.68 eV, respectively [20].

In contrast to pure phosphorene, the first peak of the optical absorption in all P₄O₂ structures is characterized by a dark exciton with long-lifetime. This result makes these new systems promising candidates as molecular sensors or applications in on-chip communication. Studying the excitonic effects of half-oxidized phosphorene conformers reveals that the wave function in the dangling phosphorene extends along the armchair direction, which is similar to pure phosphorene (see Figure 1.12). These results indicate that six half-oxidized phosphorene conformers are potential candidates for electronic devices and photovoltaic applications [20].

1.3.2.3 Strain Effect

Besides the high flexibility and strong anisotropic elastic properties of phosphorene, oxidation is so important to tune its elastic properties and extending the corresponding applications. Phosphorene half-oxides can be stretched becoming ideal for devices requiring flexibility [79]. Moreover, the degree of oxidation influences significantly the elastic parameters [30–80]. The polar plot of Young modulus and Poisson ratios reveals that the maximal values are attempted for armchair-strain resulting super flexible structures. However, it is hard to implement zigzag deformation direction that shows minimal values of elastic parameters (see Figures 1.13a, b). Importantly, the Poisson ratios of images and images conformers take negative values for some ranges of angles, which lead to an auxetic behavior (see [79]). Moreover, in all the conformers, the Poisson ratio is lower than 0.5, which correspond to incompressible materials. Stress-strain responses, under the armchair and zigzag tensile strain, show that half oxidation leads to much higher critical points, compared to pure phosphorene. It is found that both the dangling and bridge structures resist to a large axial deformation up to 27%–39% along the AC-axis with respect to the ZZ one. More precisely, the maximum values coincide with the dangling structures P₄O_U, P₂O_D, and P₃O_U which can resist a tensile strain up to 30%, 33%, and 39%, respectively, showing a high flexibility in the armchair direction. This result is owing to the high buckled honeycomb structures that exhibit these configurations in this direction. The mechanical flexibility of half-oxidized phosphorene make these structures an ideal candidate for wearable optoelectronic devices.

Schematic illustration of excitons wave functions.

Figure 1.12 Excitons wave functions. Black balls represent the holes.

Schematic illustration of the part of polar plots of (a) Young modulus, (b) Poisson ratios.

Figure 1.13 Part of polar plots of (a) Young modulus, (b) Poisson ratios.

Under half oxidation, the Debye temperature of phosphorene increases, with a maximum value reached in the ZZ-axis relative to AC. The high Debye temperature values indicate an important thermal conductivity in these new derivatives lattice lattice [79]. Furthermore, the curves describing the normal electrical polarization of the PO configurations in terms of applied strain are linear. With respect to pure phosphorene, the piezoelectric stress parameters increase under 50% oxidation while the piezoelectric strain coefficients d11 are three times lower than 2D BP [80].

When axial deformation is implemented, the electronic features of the POs become modulated. The band gap of dangling and bridge structures increases with low tensile strain, then it reduces to achieve a metallic state for large deformations. Besides, both groups of POs can maintain the semi-conductor behavior along the armchair direction for a strain ranging from 20% to 40% [81]. One can deduce that the adjustment of the phosphorene oxides features makes this class of materials potential candidates for advanced devices.

1.3.3 Surface Oxidation on Phosphorene

In contrast to the functionals H, F, and −OH which work like scissors by breaking down phosphorene into nanoribbons [82], a complete oxidation maintains the initial

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