2D Monoelements. Группа авторов

1.3.1.2 Phosphorene vs Air

      When phosphorene is exposed directly to air, its reaction causes rapid degradation of phosphorene-based devices. Moreover, the exothermic process reveals that H₂O will react with oxidized phosphorene. Both theoretical calculations and experiments have shown that at room temperature, phosphorene undergoes a spontaneous oxidation when it is exposed to O₂. The oxidation pathway leads to the formation of phosphoric acid and defective phosphorene [29]. Besides, humidity (the presence of H₂O) is very important to determine the stability of phosphorene in air. To avoid surface degradation of phosphorene and overcoming the oxidation barrier, some passivation techniques must be introduced. For example, graphene, h-BN, AlOx, Al₂O₃, PxOy, and polymeric materials are used to protect it from mechanical and chemical degradation [28]. Under low oxidation, phosphorene is stable and tend to be less stable when increasing the oxygen concentrations [25]. Consequently, 50% oxidation of phosphorene is the best amount to stabilize phosphorene after a two-day exposure to the atmosphere [67].

       1.3.1.3 Functionalized Phosphorene

Non-metallic adatoms can also be strongly bound to phosphorene due to the lone electrons pair. Functionalization of phosphorene by adsorption of non-metallic atoms with a [He] core electronic, namely, B, C, N, F, and Al showed different site preferences as schematically illustrated in Figure 1.7. Indeed, while the adatoms B, C, and Al prefer to adsorb to the hollow (H) site, F adatom is adsorbed at the top site and N adatom prefers the bridge one [68]. Furthermore, the chemical functionalization of these non-metallic adatoms exists in three classes [69]. In the first one, the C and B adatoms get located at the interstitial site after breaking the P-P bonds. However in the second group, the N and F atoms remain on the surface of the P atoms and preserve the lattice structure of phosphorene. The last group is formed by the Al impurity, located at the top of the centre of the hexagon. The interatomic distances show that the smaller the adatom, the closer it is to the P monolayer, which implies a higher binding energy compared to the larger ones. This result is confirmed by the calculations of the binding energy (Eb).

Figure 1.7 The three possible adsorption sites.

Figure 1.8 DOS of spin up and down of adatoms.

For B, C, N, F, and Al adatoms on phosphorene, Eb is −5.08, −5.16, −2.98, −2.30, and −3.18 eV, respectively [68, 70]. The adsorption process is more stable in phosphorene since the values of Eb are much greater than the case of adsorbed graphene [71–73]. The higher values of Eb are mainly deserved to the buckled sp³ configuration of the reactive material as reported in [68]. Mid-gap states are observed in the spin-polarized density of states plotted in Figure 1.8 with 1 μB for B, N, and F systems. However, the curves for the C and Al impurities reveal the same number of electrons having up-spin and down-spin, indicating the absence of magnetic order in these configurations [70].

      Moving to 3D transition metal (TM), such as Cu, Ti, V, Ni, Cr, and Fe adsorbed at the H site in phosphorene. According to [69], TM adatoms induce a magnetic moment ranging from 1.00 to 4.93 μB. In particular, the Ti adatom states contribute in the midgap and the conduction band (CB), which reduces the band gap to 0.41 eV in the presence of a magnetic order of 1.87 μB [70]. A magnetic of 2.00 μB is observed for Fe adatom systems. In the case of Cr and V adatoms, the spin-down is observed in CB. However, the spin-up of V states dominates the Fermi level and splits into two peaks for Cr adatom. The situation is different for the Ni and Cu adatoms which exhibit no spin-polarization.

      1.3.2 Half-Oxided Phosphorene

      Similar to graphene oxide, oxygen adsorption on phosphorene can be used efficiently to tune the optoelectronic properties as well as the protective layer of phosphorene. The absorption of a single oxygen atom on phosphorene can occupy numerous positions like interstitial, horizontal, diagonal ones [74]. As mentioned previously, phosphorene is stable at low oxygen concentrations.

1.3.2.1 Electronic Structure

An oxidation with a degree of 50% generates nine possible configurations among which only six are stable. As illustrated in Figure 1.9, the unit cell comprises four P-atoms assigned as P₁, P₂, P₃, and P₄ and two O-atoms, namely, OA and OB. Oxygen atoms bind to two P-atoms on the same side (more precisely, they are attached either to the up-side or to the down-side of the surface and or they are in opposite sides, namely . The index U and D referred to the up and the down side of the P-atoms. For example, to get , one should place the OA-atom up on P₁ and the OB down on P₂ (OB is in the opposite side of OA) forming a fashion on either side of the plane. The resulting new derivatives can be divided onto three main groups. The first group contains the structures and that have only dangling bonds P = O. In three conformers constituting the second group, dangling oxygen motif and bridging bond alternate, respectively. The third class concerns the configurations exhibiting only bridging bonds which are energetically less favorable (see [56] for more details). All the half-oxidized conformers exhibit a high buckling parameter confirming the anisotropic behavior of their properties. It was found, using different methods [24, 56], that half-oxidation of phosphorene (P₄O₂) allows to build a stable material.

Half O-functionalization influences significantly the electronic structure of phosphorene mono-layer as depicted in Figure 1.10. The oxidation induces a band gap modulation with the highest value observed in the bridge structures [56]. In all structures the band gap ranges from 0.54 (1.19) to 1.57 (2.88), calculated by GGA (GW) approximation. Moreover, a half O-functionalization tunes the band gap from direct to indirect in all the conformers, except P₂O_D which presents a direct band gap.

Figure 1.9 Configurations of 50% oxidized phosphorene: (a) dangling structures and (b) bridge structures.

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