2D Monoelements. Группа авторов

Figure 1.6 Monolayer phosphorene under different values of in-plane compressive strain at 300 K in the two directions.

      1.2.3 Applications

      The remarkable properties and the strong anisotropy observed in phosphorene make it an ideal candidate for photodetectors, modulators, and sensors. Below, we will report some applications.

       1.2.3.1 Gas Sensors

      In 2D materials, the large ratio of surface area to volume renders them promising for gas sensors. In addition, the unique supplementary advantages of phosphorene, like its in-plane anisotropy, structural stability, and high chemical reactivity with molecules, make it highly desirable as a superior gas sensor [61]. Indeed, under gases exposition, phosphorene undergoes multiple modifications [25]. For example, the gas molecules adsorption on phosphorene leads to a reduction or an increase of the resistance that is very required for the markers in sensing applications. Furthermore, phosphorene depends mainly on certain toxic gases because of the high binding strength gas molecules. As a result, the selective behavior of the phosphorene in adsorbing gases influences significantly its transport properties along the two axis directions [61].

       1.2.3.2 Battery Applications

According to [16, 17], phosphorene exhibits a specific capacity of 2,596 mAh/g, which is larger than the ones of sulfur and graphene. In addition, the discharge potential in phosphorene, that ranges in the interval [0.4–1.2] V, is smaller than 2.1 V obtained for lithium/sulfur battery [18]. It follows that 2D BP is a potential candidate for new generation of battery [10]. Compared to 2D anode materials, such as graphene and MoS₂, phosphorene exhibits an ultrahigh diffusivity, that is 10²–10⁴ times faster. This is owing to its strong anisotropic diffusion barrier that is 0.68 eV, with respect to the small value of 0.08 eV found in the ZZ-axis. Furthermore, phosphorene-based Li battery shows a voltage of 2.9 V which is larger than the one of other 2D layered materials, making this kind of battery of potential use as a rechargeable battery for different electronic and energetic devices.

       1.2.3.3 FETs

      Another significant application of phosphorene is the fabrication of field-effect transistors (FETs) [38]. Phosphorene devices can offer many advantages over graphene transistors due to the good saturation of the current and their band gap [62]. Phosphorenes have attractive characteristics that are critical for advanced circuits and sophisticated amplifiers [10]. In particular, phosphorene exhibits drain current modulation of 10⁵, high flexibility, and high carrier mobilities of about 1,000 cm²V⁻¹s⁻¹ which is larger than the other flexible transistors based on 2D monolayers like WSe₂ and MoS₂. Furthermore, when the length of channel is 300 nm, the measurements show that phosphorene exhibits a cutoff frequency of 12 GHz for the short-circuit current while frequency oscillation reaches the value of 30 Hz.

      Beyond the multi-GHz frequency, phosphorene constitutes one of the best candidates for future generations of ultrathin layer transistors [10]. Moreover, phosphorene is not only used in field-effect transistor applications, but also in other electronic devices based on semiconductor materials due to its electronic properties and its charge mobility.

1.3 Phosphorene Oxides

      The non-bonding pairs of electrons present on the surface of phosphorene leads to degradation of this 2D monolayer under ambient conditions, namely, oxygen, water, and light. This impedes phosphorene from some of its potential applications. To overcome this obstacle, phosphorene oxides with different O-concentration were investigated. It follows that phosphorene is stable at low O-concentrations. More precisely, half-oxidation is the best concentration to construct a stable material.

1.3.1 Challenges: Degradation of Phosphorene

      In contrast to the unique properties and great potential of phosphorene, which distinguish it from other 2D materials, phosphorene remains unstable under atmospheric conditions, for example, in the presence of oxygen, water, and light, due to the non-bonding pairs of electrons present at its surface [27, 63]. The unprotected surface of phosphorene develops significant roughness, causing important changes and consequent degradation in the compositional and physical features of the material. In some cases, the degradation poses a serious performance problem of phosphorene-based devices [64].

       1.3.1.1 Light Exposure

      In phosphorene, the ambient degradation in the atmosphere is divided into three stages. In the first stage, the reaction induced by the ambient light O₂ leads to the formation of oxygen. In that case, the reaction expressing the transfer if charge is given by: where P corresponds to phosphorene and h⁺ is a hole with positive charge. In the second stage, the oxygen molecule is separated at the surface leading to the following: . Finally, in the last step that is a hydrogen-bond interaction, the P atom is removed from the surface and the bonded O is absorbed by water molecules. It follows that the top layer of phosphorene is broken and excitons can be produced under ambient light.

      To evaluate the evolution of BP degradation for various light’s wavelengths and at different time scales, six representative BP flakes were studied individually. Using atomic force microscopy (AFM), the exposures were evaluated in a dark room at six values of wavelengths ranging from 280 to 1,050 nm and imaged before and after identical exposure durations varying from 30 to 120 min with a step of 30 min [65]. The maximum degradation was observed for the UV light (280 nm), then for the blue one (455 nm). In contrast, phosphorene does not show any degradation for green, red, and infrared light, namely, 565, 660, 850, and 1,050 nm. Consequently, the UV light is the predominant contributor to the degradation of BP.

      In addition, the engineering of the phosphorene’s band gap renders this material a good candidate for a photodetector, with a large spectral response ranging from the UV towards IR region. For instance, phosphorene photon detector shows a very fast response of 1.82 A/W in the presence of visible light irradiation of 550 nm. With photon energy and a bias of 0.1 V, the photoresponsivity attains the value of 175 A/W in the NIR regime, and at a higher bias of 3 V, it reaches 9×10⁴ A/W offering phosphorene potential as a UV detector [66].