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Phosphorene: A 2D New Derivative of Black Phosphorous

Lalla Btissam Drissi1,2,3*, Siham Sadki1 and El Hassan Saidi1,2,3

1Faculté des Sciences, Université Mohammed V de Rabat, Rabat, Morocco

2Academie Hassan II des Sciences et Techniques, Rabat, Morocco

3CPM, Centre of Physics and Mathematics, Faculty of Science, Mohammed V University, Rabat, Morocco

Abstract

Phosphorene is a stable 2D elemental material obtained via the exfoliation of 3D layered phosphorus. Phosphorene exhibits several interesting features, including its unique highly buckled structural characteristics which lead to strong anisotropies in the transport, electronic, optical, mechanical, and thermal properties of this material along its two directions: zigzag and armchair. These excellent properties render phosphorene an ideal platform for various optoelectronic devices. However, under atmospheric conditions, for example, in the presence of oxygen, water, and light, phosphorene is very reactive due to the free non-bonding electrons existing on its surface. Consequently, the O-concentrations effect on the optoelectronic response, the elastic parameters, and thermal conductivity of phosphorene is significant and indicates interesting results.

Keywords: Phosphorene, synthesis, chemical functionalization, optoelectronic properties, mechanical response, thermal conductivity

1.1 Introduction

The 3D phosphorus is a very abundant element existing in several polymorphous forms. Among the allotropes of phosphorus, namely, white, red, violet, black, and blue; the black one (BP) constitutes the most thermodynamically stable phase under ambient conditions. This layered allotrope was discovered for the first time more than a century ago through the high-pressure [1]. Recently, 3D BP was synthesized from red phosphorus using the new sonochemical method [2]. In bulk BP, the layers are weakly stacked together via VDW interactions [3]. In each layer, the P atoms are connected to their three nearest neighbors by covalent bonds that form a rippled honeycomb structure [4]. BP is a semi-conductor with a direct-gap, a strong in-plane anisotropy and a density greater than 2.5 g/cm3 [5, 6].

Like its counterpart graphene, stable 2D phosphorene can be mechanically extracted from 3D BP. In 2014, phosphorene was synthesized, for the first time, using a scotch tape based microcleavage method [7-9]. The phosphorene's unit cell is composed of four P atoms and appears highly buckled in the armchair (AC) axis [10]. Because of its geometric characteristics, phosphorene exhibits highly anisotropic physical properties along its AC with respect to its zigzag one [11, 12]. Phosphorene is a p-type semiconductor [13-15] that shows a high flexibility, an important specific capacity and discharge potential that are very required for advanced battery applications [16-18]. In addition, it exhibits a strong excitonic effect [19], an optical gap located at 1.2 eV and its absorbs infrared to near ultraviolet radiation [20]. This new hexagonal material has great potential applications in optoelectronics and photovoltaic devices [21].

Furthermore, the puckered structure of phosphorene attributes its interesting elastic properties such as great structural flexibility and a resistance to 27% and 30% deformations along the zigzag and armchair directions, respectively [22, 23], which makes this material very suitable for wearable optoelectronic devices. Furthermore, the Young's modulus and Poisson ratio exhibit their maximum values along ZZ-axis indicating how it is difficult to strain it. Consequently, phosphorene is super flexible along the armchair axis [23]. It is also well to mention that phosphorene is an auxetic material [24, 25] and that its non-centrosymmetric point group leads to a large piezoelectric response [23] showing that phosphorene can convert mechanical energy into electrical one [26].

Despite all the exceptional properties of phosphorene, it is very reactive with oxygen due to the non-bonding pairs present at its surface [27]. This fact limits its applications in optoelectronics, sensors, energy conversion, photocatalytic, and so on. To overcome this obstacle, many different techniques have been used to fabricate air-stable phosphorene. The passivated phosphorene by graphene, h-BN, Al2O3, and the polymeric material is a promising technique to avoid chemical debasement and to modulate its features [28]. The measurements shown smaller degradation when phosphorene only exposes to O2 or H2O [29]. Phosphorene with different oxygen concentrations confers excellent new properties in these 2D materials [30, 31]. At high concentration, oxidation leads to a new family, namely, 2D planar and 1D tubular forms, with a transition in the band gap from semiconductors to insulators [32].

In this chapter, we first present pure phosphorene starting from its crystalline structures, its fabrication methods, its physical properties, and ending with certain applications. Secondly, we will investigate how the oxidation's arrangement and concentrations influence the electronic, elastic, and optical characteristics of phosphorene oxides.

1.2 Pristine 2D BP

Owing to its great buckle height, phosphorene has fascinating properties such as anisotropic optoelectronic and mechanical features which make it very attractive for nanodevices.

1.2.1 Synthesis and Characterization

Similar to graphene, 2D BP can be exfoliated from buckled material trough the top down method. The bottom-up method is still not promising for phosphorene CVD growth since most of the phosphorus precursors used in thermal depositions show a high amount of toxicity and cannot be adapted for CVD manufactures [33, 34]. It follows that the large-scale bottom-up method requires more effort from experimental scientists.

1.2.1.1 Top-Down Approaches

The mechanical exfoliation is an effective widely used method for cleaving 3D materials from mutilayers to some layers and then to isolate a single layer [34]. Graphene monolayer, for example, has been isolated from graphite simply by using adhesive tape [35, 36].

Monolayer, bi- and tri-BP sheets were successfully exfoliated using micromechanical cleavage of 3D BP with PDMS in 2014. This method was carried out using an adhesive tape in three steps.

First, the exfoliated phosphorene layers were transferred to PMMA/PVA (polymethyl methacrylate/Polyvinyl Alcohol) composites, and then, the resulting layers with the composites were moved to a SiN substrate with a thickness of 200 nm. Several chemicals are used to separate the obtained specimens from the PMMA/PVA composites and to ensure that no more scotch tapes was left [37]. Despite the success of the mechanical exfoliation process, it was found that it was not scalable and hence limited to be used in academic laboratories for fundamental studies. Thus, to obtain a phosphorene sheet, a more efficient manufacturing process has been introduced. In particular, an Ar+ plasma was used to produce monolayer phosphorene through thermal ablation. This process provides an improved means of controlling the phosphorene thickness, unlike it is still challenging for mass production [38, 39].

The interesting technique to fabricate large quantities of exfoliated phosphorene is the liquid phase preparation. The solution-based phosphorene synthesis is placed into the BP interlayers which enlarge the distance and allows the exfoliation. This approach is widely used to manufacture several 2D and 3D materials that have shown good performance in dispositive [40].

1.2.1.2 Bottom-Up Methods

Advanced chemical techniques were used intensively to fabricate large quantities of innovative devices based on new 2D sheets like graphene, germanene, borophene, silicene, and stanene [38]. For other synthesized 2D materials, this new processing route based on the deposition via thermal evaporation of their elemental forms is done on available suitable substrates/surfaces like Ag(111), Au(111), Pt(111), and Al(111) [34]. In parallel, other means, such as the successful epitaxial growth of graphene and TMDCs on insulating substrates made of sapphire or 300 nm of SiO2 on Si (SiO2/Si) [41] open up also the way to a possible phosphorene. These bottom-up methods are very used for materials stable under moisturizing conditions and at high temperature. In contrast, large-scale phosphorene CVD and epitaxial growth are still incubating and breakthroughs due to various reasons, such as lack of suitable substrate, high toxicity of phosphorus, as well as instability of phosphorene in the presence of moisture under high pressure [38, 42].

1.2.1.3 Geometric Structure and Raman Spectroscopy

Crystallographic data and elemental details of phosphorene were gained both theoretically and also experimentally using different techniques such as X-ray cristallography, high performance spectrometers, SEM microscope, and EDX analysis. Phosphorene has been shown to be a nonplanar lattice along and seems to be a bilayer material in the zigzag direction as displayed in Figure 1.1a.

Figure 1.1 Optimized crystallographic structure of (a) 3D BP and (b) 2D BP.

Measurements made by means of preliminary X-ray investigations indicate lattice constants of 3.31 Å and 4.38 Å in ZZ- and AC-axes, respectively, with four atoms forming the unit cell of phosphorene [43, 44]. The experimental result concorde with the theoretical values...