187 lines
121 KiB
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187 lines
121 KiB
JSON
[
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"id": 1,
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"chunk": "# Recent Developments in Stability and Passivation Techniques of Phosphorene toward Next-Generation Device Applications \n\nDavid K. Sang, Huide Wang, Zhinan Guo,\\* Ni Xie, and Han Zhang\\* \n\nPhosphorene as a rising star is a monolayer or few-layer form of black phosphorus (BP), which is used as a 2D material, in addition to graphene. This monoelemental 2D material has gained considerable attention in the fields of electronics, optoelectronics, and biomedicine due to its extraordinary physical properties. However, as both theoretical and experimental works show, the intrinsic instability of phosphorene under ambient conditions is a major challenge in practical applications. Various theoretical and experimental researches regarding the mechanism of the degradation and passivation strategies are proposed and reported to overcome the problem of the ambient instability of phosphorene. These strategies have enabled researchers to conduct fundamental studies on phosphorene’s extraordinary properties. Here, not only an extensive summary of these passivation strategies but also an overview of the fabrication methods, challenges, and suitable applications of phosphorene are provided.",
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"category": " Introduction"
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"id": 2,
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"chunk": "# 1. Introduction \n\nIn recent years, 2D layered materials (2DLMs) have made enormous progress through research after the discovery of graphene.[1] Graphene, as the first 2D material, was a major advance in science and technology because of its novel intrinsic physical properties, for example, thermal transport, electronic transport, and mechanical properties.[2] Since the exploration of graphene, researchers have discovered a series of 2D layered crystals, one of which is phosphorene. As a novel 2D material, phosphorene has been the focus of intense studies in recent years because of its excellent carrier mobility and high on/off ratio, tunable direct bandgap, and anisotropy in plane. Based on the bandgap value, phosphorene can be placed in between graphene and the 2D transitional metal dichalcogenides (2D-TMDs) in the hierarchy of 2D materials. Graphene \n\nis gapless,[3] and the 2D TMDs only have direct bandgap at the monolayer level,[4] while phosphorene has emerged as the star because it shows direct bandgap in bulk, and mono- and few-layer forms,[5] with layer-dependent bandgap.[6] Phosphorus is abundant in the earth’s crust making up $0.1\\%^{\\left[7\\right]}$ as monoelement P, and it also exists in various allotropes,[8] i.e., black phosphorus (BP), red phosphorus (RP), white phosphorus A7 phases, and violet phosphorus. BP is energetically stable among the phosphorus allotropes and possesses an orthorhombic structure with high density.[9] The breakthrough in phosphorene exfoliation[10] elicited intense research[11] due to phosphorene’s fascinating properties, such as layer-dependent bandgap,[6a,12] high carrier mobility with pronounced high hole mobility, and anisotropy between elec \ntrons and holes in the armchair and zigzag directions,[5a,13] and \nphonon and optical responses anisotropy.[14] The electronic \nband structures[15] of mono- and few-layer phosphorenes calcu \nlated via density function theory (DFT) are shown in Figure 1b. \nThe structures are for 1L, 2L, and 3L. The band structures[16] \nshow that they can be manipulated by varying the layer thick \nness where the gaps are ${\\approx}0.3\\ \\mathrm{eV}$ (bulk) and $2.0\\mathrm{eV}$ (monolayer), \nas shown in Figure 1c. Bandgap is a significant characteristic \nof the material, as it plays a crucial role in influencing the \nelectrical, optical, and thermoelectric properties. The interplay \nin the bandgaps is due to the strong out-of-plane quantum \nconfinement and interlayer interactions. The bandgap range \nin phosphorene covers the necessary technological spectral \nrange, i.e., from the visible to mid-infrared. The tunability of \nthe bandgap can offer unique interactions with photons and \npolarized light,[17] where the direct optical transition occurs \nand the strong in-plane anisotropy in phosphorene is seen. In \naddition, the strong layer thickness-dependent electronic prop \nerties of phosphorene offer a competitive edge over other 2D \nmaterials.[18] Moreover, these extraordinary properties make \nphosphorene admirable and attractive as a potential 2D-layered \nmaterial for applications in various fields, such as in energy \nstorage,[19] optoelectronic devices,[7,20] and field effect transis \ntors (FETs).[10] Over the last few years, the number of studies on phosphorene \n\nhas risen exponentially; however, the degradation of phosphorene under ambient conditions has largely limited its practical use and the actualization of its striking physical properties.[21] Therefore, innovative techniques for protecting phosphorene from degradation are much needed. The main cause of phosphorene degradation is its high reactivity with oxygen upon exposure to ambient conditions to form phosphate.[10b] Recently, substantial work has been conducted in search of effective and efficient protective modification methods of the exfoliated phosphorene with the sole aim of stabilizing exfoliated phosphorene, to make use of its intrinsic properties without interference from impurities. \n\nAmong the 2D layered materials, phosphorene suffers the most severe degradation; its nanoflakes degrade in a few hours after exfoliation, making it very difficult to handle in open air,[22] and the multilayer form degrades after a few days.[10b] Stabilization mechanisms involved in the passivation techniques are very good factors to consider in establishing the chemistry behind the process of stabilization. In addition, gaining insight into and solving the problem of the chemistry of degradation in phosphorene is a fundamental question that needs both theoretical and experimental analyses, and we thank the various research groups who have tried to unveil this kind of information. Therefore, in this review, we will illustrate the current state of phosphorene stabilization techniques, as shown in Figure 2. \n\nIn this review, we discuss the current state of the recent developments in phosphorene’s passivation techniques. In addition to the physical means of stabilizing BP, chemical methods and the most recently emerged chemical interaction methods are also reviewed in detail. Fabrication techniques are very crucial because they influence what the phosphorene flakes will look like at the end of the production process, thus dictating their susceptibility to oxidation; therefore, fabrication techniques will be discussed briefly. Stable phosphorene has been found to be suitable for application in various fields, such as electronics, optoelectronics, energy storage and conversion, and biomedicine, and therefore will be highlighted in the last section of this review. Finally, we will give conclusions on the status and perspectives of the stability of phosphorene, with an emphasis on passivation techniques.",
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"category": " Introduction"
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"id": 3,
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"chunk": "# 2. Fundamental Basic Structure, Properties, and Importance of BP",
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"category": " Introduction"
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"id": 4,
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"chunk": "# 2.1. Basic Structure \n\nThe preparation of BP involves the subjection of white phosphorus allotrope which is a precursor to high temperature and pressure.[9a,23] To understand the origin of BP structure is important to get the insight of white phosphorus which described by the molecular formula ${\\mathrm{P}}_{4}$ . The atoms in the white phosphorus form a tetrahedron with six single bonds of which the P atom consists of three covalent bonding with surrounding atoms. From the valence shell electron pair repulsion theory, shows that each P atom has a single lone pair of electrons and the three bonds which leads to the formation of $\\displaystyle\\mathrm{sp}^{3}$ hybridization of 3s and 3p atomic orbitals. In the $\\displaystyle\\mathbf{sp}^{3}$ hybridization, the bonds and lone pair of $\\mathrm{\\DeltaP}$ atom form an angle of $109.5^{\\circ}$ and because of the molecular structure of $\\mathrm{P_{4}}$ where angles between the bonds are $60^{\\circ}$ , which are small, hence induce some structural strain and this result in the instability seen in white phosphorus.[24] Turning to BP, obtained from white phosphorus, the \n\n \n\nDavid K. Sang received his masters degree in Chemical Engineering and Technology in 2016, from Beijing University of Chemical Technology, China. In 2019, he obtained his Ph.D. degree from Shenzhen University under the mentorship of Prof. Han Zhang in the Shenzhen Engineering Laboratory of Phosphorene \n\nand Optoelectronics, International Collaborative Laboratory of 2D Materials (ICL-2D) for Optoelectronics Science and Technology. His current research interests focus on 2D semiconductor materials’ design and simulations for optoelectronic and thermoelectric applications. \n\n \n\nZhinan Guo obtained his Ph.D. degree from Jilin University in 2014. Now he is an Associate Professor in the Institute of Microscale Optoelectronics (IMO) and a Deputy Director of the Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics in Shenzhen University. His current research interest is light–2D materials interactions and 2D material–based optoelectronics devices. \n\nHan Zhang is currently a Director of the Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics, Shenzhen University. His current research focus is the photo nics of low-dimensional materials and devices. \n\n \n\nthree bonds out of six bonds in $\\mathrm{\\DeltaP_{4}}$ have broken leaving three bonds with flattened ${\\mathrm{P}}_{4}$ with large angles; hence, no appreciable high-energy strained bonds make the BP most stable allotrope of the phosphorus element.[25] The flattened ${\\mathrm{P}}_{4}$ forms the basic building block of phosphorene and it forms a layer via linkage to two atoms from other blocks. The formed layer is not flat but is a puckered honeycomb lattice structure (orthorhombic) as shown in Figure 1a. The basic unit of phosphorene retained the $\\mathsf{s p}^{3}$ hybridization character of BP, and this shows that the orbitals contain s and p states. \n\n \nFigure 1. a) Side view of 3 layer phosphorene with armchair and zigzag orientations. b) Electronic band structures of few-layer phosphorene systems extracted from HSE06 hybrid functional calculations. c) Bandgap as a function of layer number extracted from different functions. Note that the direct bandgap character is maintained for all thickness up to 5L. Reproduced with permission.[15] Copyright 2015, American Chemical Society. \n\nIn 2014, phosphorene was successfully exfoliated through sticky-tape techniques[5b] similar to how the graphene was obtained. Its fundamental structure belong to the Cmca space group number 64.[9c] The exfoliated phosphorene exhibited layered structure like other layered 2D materials with an interlayer distances of ${\\approx}3.11$ Å.[26] The layered phosphorene consists of a single element P. The $\\mathrm{\\DeltaP}$ atoms form 2D orthorhombic closed-packed layers and thus give the phosphorene crystal orthorhombic structure with two subatomic layers composed of strong in-plane covalent bonding and weak out plane bound by van der Waals (vdW) interactions. The lattice geometry constants of single layer phosphorene are $a=4.58\\mathrm{~\\AA~}$ and $b=3.32\\mathring\\mathrm{A}$ . In general, these unique fundamental structural characteristics of phosphorene play a critical role in the origin of its instability.",
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"category": " Materials and methods"
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},
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"id": 5,
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"chunk": "# 2.2. Chemical Properties \n\nPhosphorene reacts with ambient species, i.e., water and oxygen are very significant because they influence fundamental properties, for examples, electronic structure in relation to air stability, transport charges, and wettability. Phosphorene is very reactive in air because of lone pair of electrons in $\\mathrm{\\DeltaP}$ atoms. The phosphorene surface’ reaction with air is enhanced by the presence of light forming $\\mathrm{P}_{x}\\mathrm{O}_{\\gamma}$ oxide. The oxide further reacts with moisture present in the ambient environment to form phosphoric acid. The visible light and oxygen $(\\mathrm{O}_{2})$ are the main contributing factors to the phosphorene degradation and, therefore, the reaction can be hampered by limiting either light or oxygen. In recent findings, phosphorene nanoparticles were established to be stable in the de-aerated aqueous dispersions for weeks.[27] In addition, phosphorene is prone to degradation because the effective surface is more exposed than in bulk BP, where the oxidized surface can be prevented by the oxide from further degradation,[28] and this makes the bulk BP more stable in ambient environment. \n\n \nFigure 2. Chart of various passivation techniques for black phosphorus.",
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"category": " Results and discussion"
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"id": 6,
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"chunk": "# 2.3. Physical Properties \n\nPhosphorene exhibits semiconducting behavior with considerable intrinsic physical anisotropy properties emanating from its unique structure as compared to graphene.[24,29] Phosphorene has aroused great research interests due to its layer-dependent bandgap and high mobility, which offer it distinct advantages over other 2D materials in electronic and optoelectronic appli cations.[5a,7,15,30] What especially exciting is the phosphorene’s nanosecond spin lifetime at room temperature (RT), which was theoretically predicted in $2016^{31}$ and experimentally confirmed in 2017.[32] Such long spin lifetimes afford BP new opportunities in spin-based electronics (such as spin diodes, spin transistors, and spintronics devices)[33] and which would significantly widen the application scope. Moreover, due to light, phosphorus (P) atoms in phosphorene can lead to a low spin–orbital coupling where the magnetoresistances in the phosphorene device can manifest and tune by gate voltage to optimal range for injection and detection of spin-polarized holes, thus making phosphorene suitable for spin transport devices.[33c] \n\nPhosphorene is a direct allowed transition which falls between $0~\\mathrm{eV}$ for graphene and $2.0\\ \\mathrm{eV}$ for TMDs.[18,34] Varying the thickness of phosphorene leads to different energy values, which tend to change electronics and associated properties which are depended on the layer thickness. Optical absorption range of phosphorene covers near-infrared and mid-infrared, and this is a key feature of phosphorene as a potential material for photovoltaic cells, photocatalysis, thermoelectric, and thermal imaging[35] In addition, optical responses such as nonlinear saturable absorption, Kerr nonlinear, and ultrafast carrier dynamics can be manipulated in phosphorene via varying lateral size, which are essential in the design of phosphorenebased electronics and optoelectronics devices.[36] \n\nMoreover, phosphorene crystal structure presents noticeable anisotropic properties unlike TMDs where physical properties are almost isotropic. Critical analysis of mechanical properties depicts a strong anisotropy along zigzag and armchair directions. Through first principles calculation, Jiang and Park studied the Young’s modulus and examined the variations in values from zigzag and armchair directions.[37] The determined Young’s moduli of monolayer in the zigzag and armchair directions are 56.3 and $21.9\\ \\mathrm{N}\\ \\mathrm{m}^{-1}$ , respectively. The magnitude of Young’s modulus in the zigzag direction was approximately twice the value along the armchair direction. Qin et al. calculated the thermal conductivities in both directions (zigzag and armchair) and found out that in both zigzag and armchair directions, thermal conductivities were 30.15 and 13.65 W $\\mathrm{m}^{-1}\\ \\mathrm{K}^{-1}$ , respectively.[38] Zhang et al. performed molecular dynamic simulation on a large single layer of phosphorene and demonstrated thermal conductivities along zigzag and armchair directions to be 42.553 and $9.891~\\mathrm{{W}~m^{-1}~K^{-1}}$ , respectively.[39]",
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"category": " Results and discussion"
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"id": 7,
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"chunk": "# 2.4. Importance of Phosphorene \n\nAmong the families of 2D and layered materials, phosphorene is the most celebrated and regarded as a rising star beyond graphene, due to the iconic intrinsic tunable direct bandgap at monolayer, and few-layer forms, with high charge transport in the order of $\\approx10^{5}\\ \\mathrm{cm}^{2}\\ \\mathrm{V}^{-1}\\ \\mathrm{s}^{-1}$ , on/off current ratio of ${\\approx}10^{5}$ and unique in-plane anisotropic structure.[10a,40] Also, phosphorene exhibits p-type character with weak layer interaction, making it possible to be fabricated to a single layer number.[5b] Phosphorene shows an outstanding electronic structure, which influences its ability to absorb the light with energy greater than the gap energy without intrinsic resistance as compared to other vdW solids with indirect allowed transitions, where extra phonon must be absorbed to compensate for the difference momentum; hence, photon absorption process is inefficient. Due to the tunable optical absorption via doping with vacancy or appropriate elements, application of external electric field, functionalization via chemical or surface co-ordinate and stress–strain engineering, all these have shown to modulate the optical responses within the UV–IR regimes. In addition, phosphorene has been demonstrated to be a potential material with applications in 2D-LEDs, solar cells, lasers, optical switches, transparent displays, sensors, and photodetectors.[41]",
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"category": " Introduction"
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"id": 8,
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"chunk": "# 3. Challenges Facing Phosphorene",
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"category": " Introduction"
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"id": 9,
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"chunk": "# 3.1. Air Instability \n\nPhosphorene has attracted great deal of interest because of its excellent electronic transport and high-performance tunable electronic band structure, which are suitable for design of electrical and optoelectronic devices;[42] however, due to instability, a oxidative process has made it difficult for practical actualization in the field of electronics and optoelectronics.[42] It has been established that few-layer phosphorene undergoes photo degradation within $^{2\\mathrm{h}}$ , hampering its accurate characterization procedure due to defects caused by the surface and edge degradation during the preparation process. Therefore, achieving stability in phosphorene demands a well-researched passivation techniques.",
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"category": " Results and discussion"
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"id": 10,
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"chunk": "# 3.2. Mass Production \n\nLarge production of phosphorene is more advantage for industrial purposes and commerce. The fundamental factor in meeting the demand phosphorene material is the production rate. Therefore, upscaling the production of monolayer or few-layer phosphorene still faces major challenges.[15] The mechanical exfoliation method is known to produce the bestquality samples, but the production scale is low and this will not meet the industrial demand. Even though, pulsed laser deposi$\\mathrm{tion}^{[43]}$ and liquid-phase exfoliation[44] can produce appreciable large samples, the quality is low and needs further modification to meet the standard for applications. Phosphorene fabrication poses a huge challenge to the researchers because the growth of bulk BP demands high pressure and temperature which are not attainable in the ordinary crystal growth approach[45] and phosphorene made via the bottom-up approach where the controllable synthesis of single- and multilayers are hard to attained. The method that can meet both the large samples’ production and good quality standard is considered in fabrication of phosphorene. Desirable techniques for phosphorene production should be simple, scalable, and cost effective, for feasible industrial purposes, maximum production, and good returns. Consequently, mass production of phosphorene with environmentally stability is quite difficult, though it is extremely important for industrial applications, and this challenge needs a synergy approach to ensure large production of phosphorene with air stable.",
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"category": " Results and discussion"
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"id": 11,
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"chunk": "# 4. Fabrication Techniques for Phosphorene \n\nResearch on bulk BP dates back to 1914, where the first successful synthesis of black phosphorus was done via roasting white phosphorus in the furnace under high temperature and pressure[9a] and was found to be the most thermodynamically stable allotrope of phosphorus. Few studies then highlighted the use of BP because of difficulties in the fabrication process and the conditions needed, i.e., high pressure and temperature, and it received very low attraction. About 100 years later, BP was rediscovered, where isolation of monolayer (phosphorene) was successfully done via sticky tape by two different groups[5b,10a] Production of phosphorene is very critical and it demands reliable methods which yield high-quality phosphorene. Up to date, there are two approaches applied in phosphorene synthesis, which are bottom-up approaches (growth method) and top-down approaches (dissection method).[46] The bottom-up approach is based on wet chemistry like wet-chemical method and chemical vapor-deposition (CVD) epitaxial process, while top-down approaches are purely exfoliation of either monolayers through breakage of interlayer interaction through mechanical or chemical processes. In recent years, electrochemical exfoliation of BP is shown to be a new and effective method to obtain few-layer phosphorene.[47] Based on the current publications, it is shown that top-down methods are the most studied approaches48 and have raised prospects for the synthesis of 2D materials which can be used in various applications.",
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"category": " Introduction"
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"id": 12,
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"chunk": "# 4.1. Liquid-Phase Exfoliation \n\nLiquid phase exfoliation (LPE) is the preparation of nanoparticles from bulk-layered materials through electrostatic separation in aqueous solvent and matching surface energy in the organic solvent system.[49] LPE has been demonstrated as a potential method with a possibility of upscaling to obtained phosphorene nanosheets’ dispersal in solution.[50] This method has several strategies such as ion intercalation/exchange and sonication-assisted exfoliation[51] as shown in Figure 3. Shapter et al.[52] recently developed an efficient production of phosphorene nanosheets through shear stress for a short processing time, and production of atomically thin phosphorene was achieved. Xu et al.[53] developed a scalable shear exfoliation of high-quality phosphorene nanoflakes by using simple means like a high-speed shear mixer or even a household kitchen blender, making this method relatively cheap and scalable to industrial applications. Guo et al.[54] carried out a simple and cost-effective LPE experiment to produce phosphorene with excellent water stability, controllable size, and layer number as well as high yield by using basic $N\\mathrm{.}$ -methly-2-pyrrolidone (NMP) as the solvent. This approach proved that LPE is relatively very cheap and can produce phosphorene with desirable thicknesses. \n\nMoreover, phosphorene with high quality has been produced through sonication of the liquid exfoliation method,[55] and the procedure was as follows: bulk black phosphorus crystal is sonicated in NMP solvent $(5~\\mathrm{mg}~\\mathrm{mL^{-1}}$ ) at $820~\\mathrm{\\textperthousand}$ and the frequency was set to $37\\mathrm{kHz}$ and powered for $24\\mathrm{h}$ under $30\\%$ power, and bath temperature was maintained below $30~^{\\circ}\\mathrm{C}$ using a watercooling coil, throughout the sonication process. Then, centrifugation of the final turbid was done to extract large flakes leaving the pale yellow/brown color dispersion of phospherene unchanged. \n\nBased on the demand of ultrathin phosphorene for various applications in electronic and optoelectronic, quality is considered and is determined by the sonication of solvents. Yasaei and his group exfoliate few-layer phosphorene from bulk BP using various organic solvents with the range of $(2.98\\mathrm{-}9.3\\ \\mathrm{MPa}^{1/2})$ polarity and (21.7–42.78 dyne $\\mathrm{cm^{-1}}$ ) surface tension. Finally, aprotic and solvent with polarity were found to be the best solvents to exfoliate phosphorene from bulk BP after sonication for a period of $^{6\\mathrm{~h~}}$ at $130\\mathrm{~W~}$ for $0.2~\\mathrm{mg}/10~\\mathrm{mL}$ solvent.[44] Therefore, The advantages of LPE technology are low cost, high efficiency, and simple operation. In addition, the sample prepared by this method has good stability in NMP and some other solvents. However, the disadvantage of this method is that the quality and size of the sample are not good enough.",
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"category": " Results and discussion"
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"id": 13,
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"chunk": "# 4.2. Plasma-Assisted Fabrication \n\nThe production of monolayer and few-layer of 2D materials via this method has been demonstrated to be faster and effective, and sizes are controllable.[10b,56] Lu et al. demonstrated this strategy approach using few-layer phosphorene by placing on $\\mathrm{SiO}_{2}/\\mathrm{Si}$ substrate and pre-treatment with optimized conditioned plasma to produced monolayer phosphorene through etching[56b] as shown in Figure 4a. The advantage of the plasma-assisted method is that the thickness of the sample can be artificially controlled. Furthermore, the method can be used to remove the oxide layer on the surface of the sample. Its disadvantage is that the speed and precision of etching process are difficult to be controlled accurately.",
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"category": " Results and discussion"
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"id": 14,
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"chunk": "# 4.3. Pulse Laser Deposition \n\nPulse laser deposition (PLD) is one of the physical bottomup synthesis methods, which involves physical deposition of vapor on nonmetallic substrate.[57] It is commonly used to grow the complex oxide thin films and has been demonstrated as the best alternative method for the CVD method, unlike the extreme conditions of pressure and temperature in the preparation of black phosphorus allotrope.[7] PLD exploits the gas segment attained after raising the temperature, and the formation of thin film sheets on cooling or via chemical reaction.[58] A defective BP crystal with a size ranging from several nanometers to tens of nanometers was successful developed on substrates through PLD by utilizing bulk BP as the precursor.[59] Recently, $\\mathtt{X u}$ et al. synthesized amorphous ultrathin phosphorene with a highly disordered structure arrangement film of $4\\ \\mathrm{nm}$ via deposing red phosphorus on a flexible polyester substrate, and the red phosphorus transformed into BP nanosheet in multi-anvil cell under high pressure[45] where they fabricated an amorphous ultrathin BP on graphene/Cu or $\\mathrm{SiO}_{2}/\\mathrm{Si}$ substrates via the PLD method as shown in Figure 5. The setup framework was as follows: a distance of $4\\ \\mathrm{cm}$ from the BP and substrate was set, and the chamber was evacuated to around $1.5\\times10^{-7}$ Torr before the PLD deposition. The microenvironment around the substrate was maintained at $150~^{\\circ}\\mathrm{C}$ , and the BP deposit (target) was evaporated through KrF pulse laser at $248~\\mathrm{nm}$ wavelength with a $5~\\mathrm{Hz}$ cycle. Throughout the PLD process, BP and the substrate were rotated simultaneously so as to achieve uniform film growth. Finally, the as-prepared amorphous ultrathin disordered BP film was cooled down to room temperature in a high vacuum chamber followed by characterization. Largescale film and favorable low-processing temperature are of great importance in device applications. It is anticipated that PLD-grown phosphorene will attract great attention from scientists and technologists. In this method, layers can be controlled by regulating the laser ablation exposure time, hence produce ultrathin 2D materials at low temperature and pressure, while the demerit of this method is the need of strict preparation conditions. \n\n \nFigure 3. Schematic descriptions of the main liquid exfoliation mechanisms. a) Ion intercalation: ions (yellow spheres) are intercalated between the layers in the liquid environment, swelling the crystal and weakening the interlayer attractions. Then, the agitation (like shear, ultrasonication, or thermal) can completely delaminate the layers, resulting in the exfoliated dispersion. b) Ion exchange: some layered compounds contain ions in between the layers in order to balance charge on the layers. Th (red spheres) can be chan ged with other ions with larger size (yellow spheres) in a liquid environment. As shown above, the agitation results inan exfoliated dis sisted exfoliation: the layered crystal is sonicated in a solvent, resulting into exfoliation and nanosheet format wh solvents (those with appropriate urface energy) the exfoliated nanosheets are stabilized against reaggregation. While in “bad” solvents, reaggregation and sedimentation will occur. Note that solvents molecules are not shown in this figure.",
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"category": " Materials and methods"
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"id": 15,
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"chunk": "# 4.4. Chemical Vapor Deposition \n\nThis method is one of the bottom-up synthesis approaches of nanomaterials and has been known to produce high-quality 2D materials, for example, graphene and TMDs.[60] More effort has been put up in the production of phosphorene in large scale in order to compete fairly with other 2D materials such as graphene and TMDs in the emerging fields.[61] Also, growing a single crystal of phosphorene for the purpose of studying its anisotropic properties in large scale is a priority in the design of production technique.[62] Smith and his group grew phosphorene film of $>3~\\upmu\\mathrm{m}^{2}$ and lateral size of approximately four layers[63] using a set as shown in Figure 6. In addition, the preparation of wafer scale on metal substrate and epitaxial growth on isolating substrate[64] have enabled the fabrication of device-based graphene in large scale. However, this method has not yet been fully explored in the production of highquality phosphorene as in the aforementioned 2D materials. \n\n \nFigure 4. a) Diagram showing the plasma etching process on the surface of BP nanosheet. b–d) Flake optical images at different storage durations. $\\mathsf{e{-}g})$ BP thickness at different regions, BP thickness as a function of duration exposed to plasma treatment, and Raman spectra as a function of thickness, respectively. Reproduced with permission.[56b] Copyright 2015, American Chemical Society. \n\nThe advantage of this technology is that it is expected to realize the industrial production of phosphorene with large size. However, the method needs further refinement by optimizing the processing conditions. Furthermore, this method is not yet fully explored in phosphorene production. \n\n \nFigure 5. a) Schematic setup of pulse laser deposition (PLD) for production of ultrathin BP. (b) Energy dispersion X-ray (EDX) spectrum of a-BP film grown on $\\mathsf{S i O}_{2}/\\mathsf{S i}$ . c) Raman spectra of a-BP films deposited under different temperatures on the PLD. Reproduced with permission.[45] Copyright 2014, IOP Publishing. \n\n \nFigure 6. a) Schematic production of BP film via the chemical vapor deposition method. b) Scanning electron microscopy (SEM) image of thin substrate black phosphorus (SBP) sample on the substrate. c) Thin film of SBP with an area of $0.35\\upmu\\mathrm{m}^{2}$ , inset: height profile of SBP thin film showing a thickness of ${\\approx}4$ layers $(3.4~\\mathsf{n m})$ . Reproduced with permission.[63] Copyright 2015, IOP Publishing.",
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"category": " Results and discussion"
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"id": 16,
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"chunk": "# 4.5. Wet Chemical Method \n\nThis method is one of the bottom-up approaches for preparing 2D materials,[65] and it comprises solvothermal, hydrothermal, template synthesis, and self-assembly of particles. Previous studies showed that this method has been applied in the production of graphene and TMDs.[66] Fan et al. carried out hydrothermal synthesis of functionalized phosphate carbon, which consists of composite carbon with porous support material via a model of hydrothermal approach, preceded by heat treatment,[67] as demonstrated in Figure 7. It was reported recently that synthesis of phosphorene via this method was achieved.[68] The advantages of this method are low production cost and high quality of samples while its limitation is that it is difficult to control thickness, large size, and large-scale production of the sample.",
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"category": " Materials and methods"
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"id": 17,
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"chunk": "# 4.6. Mechanical Cleavage \n\nThe Nobel Prize award for physics in 2010 was as a result of breakthrough in micromechanical cleavage of high-ordered pyrolytic graphite (HOPG) in 2004.[1,69] From this noble success, the idea has been extended to other layered materials. Effectiveness and quality production of few-layer materials make the mechanical method receive much attention in fabrication of 2D materials.[25,70] BP is layered material and its layers are held together in out of plane by weak vdW interactions which make it easy to obtained monolayer and few-layer phosphorene via the mechanical exfoliation. The mechanism process for this method is simplest, because it involves the application of adhesive scotch tape to the pellets of bulk BP and pressed with the standard force, as demonstrated in Figure 8. Exertion of standard force for several times leads to the production of thinner phosphorene. Through this method, control of exfoliated phosphorene layer with high quality is possible, and it enables the study of physical properties in phosphorene without inaccuracy from the edge and surface defects. However, this technique is time-consuming and labor intensive. Moreover, this technique is the best choice for phosphorene devices’ fabrication with high quality, but it cannot be produced on a large scale and hence confined to be used for laboratory research. \n\n \nFigure 7. Schematic illustration of wet chemical approach. a) Wet chemicalproduction process via chemical solvothermal reaction. b) Holey phosphorus-based composite nanosheet. The morphology evolution of the bulk red phosphorus at high-temperature solvothermal reaction at c) $2h$ , d) $12\\mathsf{h}$ , and e) $24\\ h$ , respectively. f) Illustration of the formation of the holey phosphorus composite nanosheets. g) Sublimation of the bulk red phosphorus. h) Formation of the phosphorus nanodomain in ethanol solution (ethanol/phosphorus vapor on the top) at the initial stage. i) Formation of phosphorus nanosheets in the ethanol (near supercritical fluid) via bottom-up assembly. j) Final product in the enthanol solution. Reproduced with permission.[68] Copyright 2016, Wiley-VCH.",
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"category": " Results and discussion"
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},
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{
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"id": 18,
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"chunk": "# 5. Ambient Instability and Improving the Stability of Phosphorene",
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"category": " Results and discussion"
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},
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{
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"id": 19,
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"chunk": "# 5.1. The Mechanism of Ambient Instability \n\nEven though bulk BP is supreme in terms of stability but phosphorene is unstable in humid air and light,[71] and this has been the limiting factor for its full actualization in practical applications. It has been reported widely that a bare phosphorene possesses a strong affinity for water and oxygen molecules in the presence of photoenergy (light), and this has hindered the utilization of phosphorene in electronics and optoelectronic devices. Also, the degradation process is a factor of layer thickness and this has been confirmed by van der Zant et al.,[10b] that flake thickness (from bulk to a limit of single layer) exhibits insightful trends in degradation rate. The origin of instability emanates from the structural nature of phosphorene where the energy gap is an aspect, which plays an important role in instigating the photo-oxidation process. Each atom is covalently bonded $(\\mathsf{P}{\\mathrm{-}}\\mathsf{P})$ with a lone pair of electron resonating within the bonds, hence exposing bonds prone to attack, thus instigates the process of phosphorene degradations.[72] \n\nThe chemical instability of phosphorene is influenced by the lone pair of electrons in phosphorus atoms, and this has demonstrated the limited ambient stability of phosphorene. The photoenergy hastens the degradation process of phosphorene to form $\\mathrm{P}_{x}\\mathrm{O}_{\\gamma}$ on the surface, and the limiting step in oxidation rate is influenced by the oxygen concentration, energy gap, and light intensity. Further exposure will lead to the formation of phosphoric acid. This has been demonstrated in the recent experiment[73] where phosphorene was found to be steady under de-aerated solutions for several days, and this shows that water and phosphorene surface interact weakly. Moreover, oxidized surface can effectively control the phosphorene inner layer from degrading[74] and this support the fact that bulk BP is stable in ambient conditions. \n\nRecently, Wang and co-workers discovered theoretically that the BP degradation process involves three steps: first, the formation of unstable oxide within visible illumination; second, detachment of unstable oxide; third, separation of unstable oxide under water activity.[75] The covalent bond $_{(\\mathrm{P-O-P})}$ on the surface of BP plays an important role in stabilization of BP structure, and this shows that the superficial layer formed after the full process of oxidation is a protective cover for preventing further degradation. \n\n \nFigure 8. Mechanical exfoliation procedures of obtaining few-layer phosphorene from bulk BP by using adhesive scotch tape. \n\nFew-layer phosphorene fabricated via mechanical exfoliation is prone to degradation in ambient environment due to the presence of humid air as investigated by Neto and coworkers[76] and Hersam and co-workers,[77] respectively, and this demonstrated that oxygen, water, and photon energy (light) are needed concurrently for the degradation process to take place on the surface of phosphorene. Edges of phosphorene flakes are also prone to degradation because of the edge-induced mechanism, which is instigated by moisture and the presence of impurity traces at the edges of the flake, and high affinity of moist air by dangling bonds on the edges leads to high degradation rate.[78] Moreover, poor coating at the edge of phosphorene contributes to high rate of degradation. Also, the internal key factors such as thickness and lateral dimensions influence the environmental instability of phosphorene[25] Earlier reports indicated that phosphorene degradation in ambient conditions is strongly due to moisture and hydrophilicity nature of the phosphorene.[10b,79] The degradation process is not well understood, and the systematic investigations of degradation process have been done by employing spectroscopic instruments such as atomic force microscopy (AFM), polarized Raman, and transmission electron microscope (TEM) combined with high-angle annular dark field (ADF) and hyperspectral electron energy loss spectroscopy[80] to fully comprehend the principles behind this phenomenon. Free water, air, and photolight are the main variable elements behind the phosphorene degradation process, which was confirmed by Wang et al.[81] through DFT and ab initio molecular simulations; thus, it provides an insightful information on how $\\mathrm{O}_{2}$ and water interact on the surface of phosphorene. They demonstrated through their findings that oxygen extemporaneously detaches on phosphorene surface at room temperature, while, on the other hand, water–pristine BP interaction is not strong. However, some reported controversial observation that phosphorene –water reaction is feasible without the presence of oxygen,[82] and this negates the earlier report that phosphorene–water reaction is not feasible without the presence of oxygen. Walia and his group confirmed that moisture alone does not cause degradation on the freshly exfoliated phosphorene,[83] and this confirms that the three essential variable elements needed to jump-start the degradation process are all interdependent and their trading-off is a subject of intense study. Other than water, oxygen, and light, temperature is also believed to influence the phosphorene degradation to an extent. The light-induced chemical reactions on phosphorene surface and edge are demonstrated in the following \n\n$$\n\\mathrm{monolayer}\\left(\\mathrm{few-layerBP}\\right)+h\\nu\\rightarrow\\mathrm{BP^{\\ast}}\n$$ \n\n$$\n\\mathrm{BP^{*}}+\\mathrm{O}_{2}\\rightarrow\\mathrm{O}_{2}^{*-}+\\mathrm{BP}+h\\nu\\rightarrow\\mathrm{PO}_{x}\n$$ \n\nIn Equation (1), the incident visible light with phonon energy surpasses the electronic bandgap of phosphorene, thus generates excitons, which leads to photoinduction of electron and hole pairs in phosphorene. In Equation (2), the adsorbed oxygen molecules trap the photogenerated electrons to produce the intermediate superoxide anions $(\\mathsf{O}_{2}^{\\bullet\\bullet})$ . The excess $\\mathbf{O}_{2}^{\\bullet}$ and photogenerated holes further induce the oxidation of phosphorene and phosphorus oxide species $(\\mathrm{PO}_{x})$ are formed. Effects of photo-oxidation on phosphorene fabricated devices are enormous and limit its full implementation. The outcome of photo-oxidation is that the insulating layer of oxide enhances the surface protection from further corrosion, and also the layer formed increases the Ohmic resistance.[5a] Ambient dilapidation on phosphorene also yields significant physical changes, thus causes increase in surface roughness, triggering impediment in carrier mobility.[84] Severe degradation changes phosphorene’s intrinsic electronic properties which can alter the threshold voltage of phosphorene-based transistors, thus reducing its optimum performance via lowering $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ ratio on carrier transport.[77]",
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"category": " Results and discussion"
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},
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{
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"id": 20,
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"chunk": "# 5.2. Ways of Improving the Stability \n\nThe freshly exfoliated layers of phosphorene are prone to the oxidation process, which began immediately upon exposure to favorable conditions for the process to take to place on the exposed surface.[10b] The presence of moist air, light, and the phosphorene flakes’ surface or edge are the fundamental variable parameters needed for the formation of $\\mathrm{PO}_{x}$ .[80] Ambient degradation limits the performance of phosphorenefabricated devices; thus, stabilizing phosphorene is crucial to its applications. Chemical stability of phosphorene needs to be well studied before selecting the field to be applied. The quality of phosphorene layer reduces against the size, and the degradation severity depends on the layer thickness because of the quantum-confinement effects.[79b,80] This is because reducing layer thickness enlarges the gap, hence, drift energy gap to higher energies and shift the valence band maximum (VBM) and conduction band minimum (CBM) to align with the one of oxygen.[75] Despite the fact that phosphorene is less reactive as compared to other elemental 2D materials like silicene, and can be studied under the normal conditions for a certain duration of time, and prolonging the handling time has been a topic of concern. Enhancing the stability will enable phosphorene to be a potential material suitable for electronic and optoelectronic applications. This observation has prompted several researches towards understanding the chemistry behind the degradation process and possible passivation techniques. Passivation strategies with effective protective layers and coating with other inactive layered materials have proved to be very effective methods to protect the phosphorene surface from degradation.",
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"category": " Results and discussion"
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},
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{
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"id": 21,
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"chunk": "# 5.2.1. Encapsulations \n\nEncapsulation is a physical method of passivation where the phosphorene layer(s) is covered with appropriate material before exposing to air–water–light condition, devoid of other substances like chemical supports as shown in Figure 9a. This method has been confirmed to be noncovalent functionalization using different capping layers such as $\\mathsf{A l O}_{x},$ $\\mathrm{SiO}_{2}$ , and polymers to enhance the air stability of phosphorene.[77,85] High charge carrier mobility exhibited by phosphorene semiconductor has made it to have a potential to replace silicone in the making of very fast response nanodevices that consume less energy. However, rapid oxidation has limited this ability to substitute the silicone. Successful physical passivation and enhancement of electrical properties of phosphorene via encapsulation with $\\mathrm{Al}_{2}\\mathrm{O}_{3}$ layers have been reported.[86] Ozyilmaz and co-workers demonstrated phosphorene surface protection by employing the graphene sheet and hexagonal boron nitride to sandwich the phosphorene, where the encapsulated phosphorene showed enhanced electron mobility, thus exhibiting a balanced n-type and p-type transconductance behavior.[87] h-BN exhibits smooth surface free of dangling bonds and low roughness, compared to $\\mathrm{SiO}_{2}^{88}$ and atomically good material for encapsulating the phosphorene as demonstrated by Doganov et al.,[89] thus, enhanced the phosphorene stability. Atomic layer deposition over layers has been shown to deprive the defects caused by surface reaction, hence boosting the on–off current ratio to ${\\approx}10^{3}$ and carrier transport to ${\\approx}100~\\mathrm{cm}^{2}~\\mathrm{V}^{-1}~\\mathrm{s}^{-1}$ for several days.[77] Combination of dielectric materials with polymer can be a synergy approach to prolong the ambient stability of phosphorene, where Kim et al. established that the double-layer capping strategy stabilized the ultrathin film phosphorene in a transistor for an infinite period of time,[78] and thus overcoming a significant material challenge for applied research and development. Recently, Lau and co-workers demonstrated that capping phosphorene with polymer improves the stability, durability, and regulates the Schottky barriers between phosphorene surface and metal. Furthermore, this strategy is nondestructive and effective technique to achieve double strategy.[90] ${\\mathbb{W}}{\\mathbb{u}}$ et al.[91] recently demonstrated a very simple and effective encapsulation method by using $\\mathrm{SnO}_{2}$ film to limit the action of water vapor and $\\mathrm{O}_{2}$ in air from eroding the phosphorene surface. In this approach, tin oxide film was formed via electron-beam evaporation of a tin film and, subsequently, exposed to natural oxidation in air. The fabricated back-gate FETs from the passivated phosphorene exhibit a typical p-type character and maintained a constant hole mobility of ${\\approx}200\\ c m^{2}\\ \\mathrm{V}^{-1}\\ \\mathrm{s}^{-1}$ and a high $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ ratio of ${\\approx}10^{4}$ for over 15 days, as shown in Figure $^{9\\mathrm{c},\\mathrm{d}}$ respectively. The advantage of this method is that it can keep the stability of ultrathin phosphorene films for a long time, and their properties are basically unaffected while the disadvantage is that this method cannot fundamentally solve the problem of instability of phosphorene. \n\n \nFigure 9. Structure, electrical characteristics, and ambient performance of BP–FETs with and without $\\mathsf{S n O}_{2}$ passivation: a) Schematic of BP–FET with $\\mathsf{S n O}_{2}$ passivation. b) Transfer curves of the passivated BP–FET at different time, i.e., as fabricated, $\\mathsf{l o h}$ and 15 days, $V_{\\mathrm{ds}}=-0.7$ V. c) Hole mobility (pea field-effect miobility at $V_{\\mathrm{ds}}=-0.1~\\mathrm{V})$ ). d) $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ ratio with change with time exposed in air. Reproduced with permission.[91] Copyright 2019, Elsevier.",
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"category": " Results and discussion"
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},
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{
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"id": 22,
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"chunk": "# 5.2.2. Surface Functionalization \n\nDue to the atomically thin nature of phosphorene, chemical doping based on surface modification with adlayer provides a strong and nonvolatile doping strategy on phosphorene with a very simple way of device fabrication. Chemical doping of phosphorene’s FETs has been carried out with metal oxides $\\ensuremath{\\left(\\mathrm{Cs}_{2}\\mathrm{CO}_{3}\\right.}$ and $\\mathsf{M o O}_{3})$ ,[92] where FET’s performance was recorded. Considering the orientations and the chemistry of the lone pairs existing on phosphorene surface, functional group reacts with the lone pair to form the PX bonds and further functionalized via suitable substitution reactions, thus ensuring that the formed surface is not prone to oxygen attack. The covalent functionalization in phosphorene via aryl diazonium proved to limit the chemical reaction, hence reduces the severity caused by the photo-oxidation process (carbon–phosphorus bond formation) in phosphorene-fabricated devices for even 3 weeks upon exposure to ambient conditions.[42] The covalent and noncovalent functionalization[42,93] strategies via coating with polymer films have proved to be very effective in improving phosphorene stability. Hirsch and co-workers[94] recently reported unprecedented top-down strategy for thinning BP flakes at will by controlling the oxidation process and removing the oxidized phosphorus species such as phosphoric acid by utilizing water solvent, and this oxidative process can be terminated via noncovalent functionalization with perylenediimide chromophores resulting into prevention of the photo-oxidation process on the surface of phosphorene flakes which they demonstrated via femtosecond transient spectroscopy, and the electronic properties of the flakes were not compromised.[102] Many organic compounds have been used not only to modulate the transport but also to enhance protection from ambient degradation.[95] Lei et al.[96] demonstrated adsorption of Ca, Sr, Ba, Cs, La, and Cl on the surface of monolayer BP and the results showed CBM shift below $\\mathrm{O}_{2}/\\mathrm{O}\\overline{{2}}$ redox potential; thus, it prohibits the oxidation to take place and, hence, enhances the ambient stability of few-layer phosphorene. \n\nMetal-ion modification has been confirmed by Guo et al.[97] as one of the effective strategies for surface functionalization, which enhances the stability of BP. They have demonstrated that silver ion $(\\mathrm{Ag^{+}})$ adsorbed into phosphorene surface by conjugating $\\pi$ -bonds to yield a ${\\mathrm{Ag}}^{+}$ -modified BP $(\\mathrm{BP_{Ag(+)}})$ can limit the lone pair of electrons from $\\mathrm{\\DeltaP}$ atoms by interacting with oxygen from ambient environment, which leads to a stable phosphorene. The mechanism behind the interaction between ${\\mathrm{Ag}}^{+}$ and BP sheet is that the lone pair of electrons on the phosphorene surface are evenly distributed forming conjugated $\\pi$ -bonds, thus providing a platform for the ${\\mathrm{Ag}}^{+}$ to strongly bond on it through the cation– $\\pi$ interaction, preventing the oxygen from reacting with the lone pair of electrons of the $\\mathrm{\\DeltaP}$ atoms, as shown in Figure 10a. To check on the ${\\mathrm{Ag}}^{+}$ -modified BP, the AFM test was carried out and found that $\\mathsf{B P}_{(\\mathrm{Ag}+)}$ sheet was stable for 5 days as shown in Figure 10d. \n\nCovalent functionalization has been demonstrated by Ryder et al. that it protects the phosphorene from degradation, thus stabilizes and improves the FET characteristics.[42] Also, decorated exfoliated phosphorene with nickel nanoparticles (Ni/phosphorene) exhibits enhanced stability compared with pristine phosphorene when both are kept under ambient conditions in the dark.[98] Also, covalent functionalization of 2D materials via chemical modification schemes not only manipulates the chemical,[99] optical,[100] and electronic[101] properties, but also enhances the stability as seen in stable $\\mathsf{P{\\mathrm{-}}C}$ bonds evolution across phosphorene–graphite,[102] and this provides the carbon-chemical-based passivation strategy suitable for stabilization of phosphorene surface. The fabricated polydoamine (PDA)-modified nanosheets were reported to show an improved stability as compared to bared phosphorene nanosheets,[103] and the coated phosphorene nanosheets with PDA also exhibited an enhanced stability.[104] Both encapsulation and surface coordination have proved to be effective strategies, synergy for enhancing the phosphorene stability, as recently demonstrated by Zhang et al.,[105] by designing a new janus nanoparticle based on BP quantum dots and tetrahydraxyanthraquinonemetal–organic particles to concurrently demonstrate an improved microenvironment stability of phosphorene and at the same time boost the photocatalytic action for applications in the treatment of cancerous cells. Therefore, the sandwich BP quantum dots (QDs) in janus particles isolate the water and air and trap the lone pair of electrons in BP resulting in improved BPQDs’ ambient stability. \n\nThis method starts from the source of the instability of phosphorene and stabilizes the phosphorene by functionalizing the surface of phosphorene and binding the lone pair of electrons on the surface of phosphorene. Although the present research results cannot stabilize phosphorene for infinite time, this idea is expected to be the fundamental method to solve the stability problem of phosphorene.",
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"category": " Results and discussion"
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},
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{
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"id": 23,
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"chunk": "# 5.2.3. Liquid-Phase Surface Passivation \n\nThe phosphorene degradation poses a big challenge, and remedy to this drawback in liquid-exfoliated phosphorene is surface modification via liquid-phase surface passivation. This method has been utilized in preventing degradation of phosphorene.[42,93,106] To achieve this, appropriate conductive polymer[107] and ionic liquid (ILs)[108] are needed. Recently, phosphorene suspension prepared from different ILs was confirmed to exhibit air stability for 1 month,[108] and also, surface coating with ILs proved to be an effective suppressor of the oxidation process in mechanically exfoliated phosphorene nanoflakes.[25,109] Zhang et al.[110] showed stabilization of phosphorene via a single-step ionic liquid–assisted exfoliation process and synchronous fluorination step whereby the fluorinated phosphorene exhibited enhanced air stability during 7 days of air exposure. Polymer ionic liquid (PIL) has been proved to be an effective approach in surface passivation of few-layer BP,[111] where the polymer ionic liquid–modified phosphorene showed enhance stability which goes for up to 100 days, with little degradations observed on the phosphorene surface. Besides stabilizing the few-layer phosphorene, PIL-modified phosphorene provides dependable flexible contact across the phosphorene and nanodevice constituents. Walia et al.[109] used imidazolium-based ionic liquids to suppress the reactive oxygen species, which is responsible for phosphorene degradation, and this demonstrated that phosphorene remained stable for over 13 weeks without alteration of its key electronic properties. Recently, Fan et al.[112] demonstrated the phosphorene modification via the wet chemistry approach where thinning and surface protection of phosphorene was achieved. Combination of two electron-deficient reagents and triphenyl carbernium tetrafluorobor was applied for thinning, while 2,2,6,6-tetramethyl piperidyl-N-oxyl was used for enhancing the stability for up to 4 months through surface coordination. Deoxygenated water has been used to prolong the photocatalytic activity of exfoliated few layers of phosphorene for 15 days,[113] and this has given insights into the chemistry of phosphorene degradation. \n\n \nFigure 10. a) Schematic illustration of ${\\sf A g}^{+}$ adsorption on BP. b) ${\\mathsf{B P}}_{\\mathsf{A g(+)}}$ in three different views. c–e) AFM images of pristine BP sheet exposed to air for c) 1 day, d) 3 days, and e) 5 days. f–h) AFM images of ${\\mathsf{B P}}_{\\mathsf{A g(+)}}$ sheet exposed to air for f) 1 day, g) 3 days, and h) 5 days. Reproduced with permission.[97] Copyright 2017, Wiley-VCH. \n\nPolymers have been used to stabilize the exfoliated phosphorene and recognized as an excellent material for layer intercalation in nanoparticles,[114] forming heterostructures with enhanced performance optimal for optoelectronic, sensors and nonlinear optics.[115] Polymer coating, like poly(methyl methacrylate) (PMMA), not only preserves mechanically exfoliated phosphorene flakes but also improves its stability.[116] Moreover, polymers play a significant role in semiconductors,[117] detection platforms in biomedical field,[118] pseudocapacitors,[119] and lasers.[120] Passagalia et al.[85a] demonstrated polymerbased phosphorene hybrid material in situ radical polymerization, which has been proved to be an effective tool to obtain stabilized phosphorene flakes, and this is a breakthrough in preventing the intrinsic instability of exfoliated phosphorene, where the moisture and air are eliminated. The advantage of this method is that the phosphorene can be stabilized for a long time, but the operation is complicated, and the controllability needs to be further improved.",
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"category": " Results and discussion"
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},
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{
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"id": 24,
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"chunk": "# 5.2.4. Doping \n\nDoping phosphorene with tellurium improves its stability, making it one of the strategies for stabilizing the phosphorene as was demonstrated recently.[121] The Te atom will induce the reduction of CBM of phosphorene to a position which is not aligned to $\\mathrm{O}_{2}$ oxidation energy. The phosphorene flakes were exposed to ambient environment for 1 month and no degradation was noted, exhibiting that doping with appropriate dopant enhances the stability of phosphorene. Theoretical investigation via density functional theory has shown that doping phosphorene with organic molecules adsorbed on the surface noncovalently reduces the bandgap,[122] limiting from attaining the oxidation energy of $\\mathrm{O}_{2}$ . \n\nRecently, the surface-electron withdrawing and donating strategy has been reported to suppress the agent causing instability in phosphorene.[123] Ruan and co-workers[124] reported that electron doping on BPQDs exhibits prolonged stable phosphorene nanoflakes for up to 6 months, and this enables further studies on the prepared phosphorene materials for a long time without any change in electrical properties. Neto and co-workers[125] showed electron doping in ultrathin phosphorene via Cu adatoms where threshold voltage was lowered without degrading the transport properties of the Cu-doped phosphorene. Zhang and co-workers[126] demonstrated a nonvolatile complementary metal-oxide-semiconductor (COMS) is well matched and very stable in the n-type doping method of few-layer phosphorene where the induced effect from K-center of silicon nitride, of which electron(n) doping exhibits air-stable transport properties for over a month. Recently, Liu et al.[127] demonstrated sulfur doping to be effective method where they modeled phosphorene–FET doped with sulfur which exhibited robust and stable with an on–off current ratio of ${\\approx}1000$ lasting for a period of 21 days. Li et al.[128] carried out DFT calculations on doped monolayer phosphorene, and found that the bi-doping with sulfur, silicon, and aluminum is more favorable stable than single doping. Uniform and highly crystalline Se-doped phosphorene has been reported to exhibit an enhanced electronic transport of ${\\approx}561~\\mathrm{cm}^{2}~\\mathrm{V}^{-1}~\\mathrm{s}^{-1}$ at ordinary conditions,[129] demonstrating that even crystal is free of surface defects, which is one of the predisposing factor in phosphorene degradations. The advantage of this method is that the obtained samples are stable for a long time while the disadvantage is that it is difficult to operate, and some properties of phosphorene will inevitably be affected.",
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"category": " Results and discussion"
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},
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"id": 25,
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"chunk": "# 6. Applications of BP with Enhanced Stability \n\nPhosphorene exhibits attractive properties suitable for applications in next-generation devices,[130] especially the physical features such as mobility and optical properties which offer the phosphorene material the advantage over other 2D materials. However, obtaining a stable mono- and few-layer will make these wonderful phosphorene features’ useful for practical utilization. The semiconducting nature associated with phosphorene nanoparticle offers opportunity for practical use in electronics and optoelectronics.[131–133] Research on phosphorene attracts great $\\mathrm{deal}^{[24]}$ of attention owing to numerous fascinating properties, of which a number of them are layered dependent. For example, phosphorene shows high carrier mobility $(\\mu)$ with remarkably high hole mobility and high anisotropy between holes and electrons along $x$ and $\\gamma$ directions,[5b,10a] anisotropic optical response,[6a] and phonon anisotropy.[14] Phosphorene also shows its potential and important applications in spin-related or spintronics devices. Avsar et al.[32] performed an experiment and discovered that a spin valve based on ultrathin $(\\approx5\\ \\mathrm{nm})$ 1 phosphorene spin channel exhibited a fundamental spin property that supports the electrical spin injection, transport procession, and detection up to room temperature. The essential spin–orbit and spin-relaxation properties of phosphorene exhibit an interesting interplay of large anisotropy for in-plane and out-of-plane spin orientations.[31] The capability of tuning the source–drain contact resistances in the phosphorene-based devices by gate voltage to an optimal range for injection and detection of spin-polarized hole enables phosphorene to be a potential candidate for efficient nanoelectronic and spintronic devices.[33c] Absorption of metals such as V/Mn/Fe on phosphorene can enhance and manipulate the local magnetic moment with large exchange-splitting and spin-flip energies which are of great advantage for the spintronic applications.[33a] The ambient degradation-induced spin paramagnetism in phosphorene can be tuned by changing one of the ambient factors such as ambient temperature, humidity, and light intensity.[21] Moreover, BP possesses thickness-dependent bandgap,[6a,134] and all these characteristics make phosphorene the most preferred nanomaterial for consideration in the following application thematic areas.",
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"category": " Results and discussion"
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{
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"id": 26,
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"chunk": "# 6.1. Energy Storage",
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"category": " Results and discussion"
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{
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"id": 27,
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"chunk": "# 6.1.1. Lithium-Ion Battery \n\nLithium-ion batteries (LIBs) play an important role in human civilization since it is a portable power source, hence, making the availability of energy source very convenience. Also, rechargeable lithium ions have stable cycling performance,[135] high storage capacity,[135c,136] and high energy density.[137] The setup of lithium-ion battery consists of anode, cathode, a separator, and an electrolyte. Anode and cathode act as the host of lithium ion with a separator membrane to prevent short-circuit and the electrolyte is the source of lithium ions. \n\nPhosphorene possesses structures suitable for application in lithium-ion batteries which can replace graphene in the near future. Formation of $\\mathrm{Li}_{n}\\mathrm{P}_{m}$ via insertion reaction exhibited better electrochemical activity, which is attributed to the stable formed structure exhibiting excellent anode performance. Electrochemical mechanism processes during discharge and charge[138] in BP are as follows \n\nDischarge: $\\mathrm{BP}\\rightarrow\\mathrm{Li}_{n}\\mathrm{P}\\rightarrow\\mathrm{LiP}\\rightarrow\\mathrm{Li}_{2}\\mathrm{P}\\rightarrow\\mathrm{Li}_{3}\\mathrm{P}$ \n\nThe lithium-ion battery’s performance is always determined by the nature of electrodes used. Recently, great performance from cathode electrode was achieved and gained recognition,[139] and focus now is on search for suitable anode. Graphite is the proposed material due to the good electroconductivity, low cost, and plenty in abundance,[140] and has attracted a lot of attention as the appropriate anode material, but it delivers an energy density of ${\\approx}200$ W $\\mathrm{~h~kg^{-1}~}$ , but still too low to meet the growing demand for high-energy storage nanosystems. Other materials explored for anode are Ge,[141] $\\mathrm{sn}$ ,[142] Si,[143] and $\\mathrm{SnO}_{2}$ .[144] Yet, phosphorene is the most celebrated elemental 2D material with potential characteristics suitable for anode in LIB, due to small barrier energy of diffusion $\\scriptstyle(\\approx0.08\\ \\mathrm{eV})$ and improved theoretical specific capacity $(2596\\mathrm{mA}$ h $\\mathrm{g}^{-1})^{[145]}$ which are based on the $\\mathrm{Li}_{3}\\mathrm{P}$",
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"category": " Results and discussion"
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"id": 28,
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"chunk": "# 6.1.2. Lithium–Sulfur Battery \n\nResearches focusing on Li–S battery have been carried out because of growing demand for high-energy storage systems. Li–S battery has been investigated and found to have great the theoretical energy density of about $2597~\\mathrm{\\textperthousand}$ h $\\mathrm{kg^{-1}}$ . However, Li–S battery’s application in actual practical is yet to achieved,[146] due to the following limitations. First, weak ionic and S conductivities which can result into overpotential and underused of active and participating species. Second, irreversible loss of active species, due to termination of intermediary polysulfides, leads to short cycle life. Third, large volumetric variation in S cathode electrode during charging and discharging cycle leads to loss of contact between conductive species and the current collector. Alleviating some of these limitations has been demonstrated through configuration of electrode composite and architecture in order to enhance electronic conductivity, thus impeding the polysulfide termination through chemically and physically immobilizing the S species. With extraordinary physical and chemical properties of phosphorene, it offers a competitive advantage as anode for Li–S ion battery because of its nature to trap and co-ordinate covalently,[147] high surface-to-volume ratio, and high carrier mobility,[148] appreciably low diffusion energy barrier toward $\\mathrm{Li^{+,149}}$ All these excellent properties make phosphorene a prospective candidate for electrode (anode) material in the Li–S battery. Moreover, for lithium-ion battery, phosphorene/red phosphorus shows enhanced discharging and charging of about 2449 and $491\\mathrm{\\mA}$ h $\\mathbf{g}^{-1}$ in 100 complete cycles,[150] showing that hybrid improves the performance of Li-ion battery.",
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"category": " Results and discussion"
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},
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{
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"id": 29,
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"chunk": "# 6.1.3. Magnesium-Ion Battery \n\nReusable Mg-ion battery (MIB) has gained a lot of interest over the last few years due to its enhanced electrochemical capacity of bivalent $\\mathbf{M}\\mathbf{g}$ ion,[151] which has yielded an improved volumetric capability $(3868\\mathrm{mAh}\\mathrm{g}^{-1})$ ) compared to KIBs $(609\\mathrm{\\mAh\\g^{-1})}$ and thus stands out to be feasible for practical due to its high safety characteristics, high gravimetric $(2205\\mathrm{~A~h~kg^{-1}})$ , low standard electrode potential $(\\mathrm{Mg}^{2}+2\\mathrm{e}^{-}\\rightarrow\\mathrm{Mg})$ , and readily available material.[152] Moreover, $\\mathrm{{Mg}}$ electrodeposition does not either form the dendrites or a thick complex solid electrolyte interphase which is a concern to safety of rechargeable batteries. Despite these outstanding characteristics, there are underlying setbacks which need to be address, i.e., anode–electrolyte incompatibility, small window for electrolytes, deficiency of high voltage/cathode capacity, and very low rate of diffusion of $\\mathbf{M}\\mathbf{g}$ ions around the anode due to polarization of the bivalent cation.[153] Another limiting factor is associated with polar electrolyte which tends to hinder the migration of $\\mathbf{\\mathrm{Mg}}$ ions and electrons, and therefore, with the emergence of phosphorene, work has been done theoretically, and the report indicates that loading energy, specific capacity, and diffusion barrier of $\\mathbf{M}\\mathbf{g}$ ion on phosphorene are $0.99\\mathrm{eV}$ along the zigzag direction $,865\\mathrm{\\mA}\\mathrm{h}\\mathrm{g}^{-1}$ and $0.833\\mathrm{~V},$ respectively,[154] The formation of stable $\\mathrm{Mg}_{0.5}\\mathrm{P}$ at $11\\%$ variation in capacity could offer covalent framework (COF) to enhance the $\\mathrm{Mg^{2+}}$ diffusion rate as an anode host,155 and these could make the phosphorene as the ideal anode for a magnesium-ion battery.",
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"category": " Results and discussion"
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},
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{
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"id": 30,
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"chunk": "# 6.1.4. Supercapacitors \n\nPhosphorene has a puckered honeycomb structure with weakly bonded layers of $\\mathrm{\\DeltaP}$ atoms in the out of plane through vdW interactions and has emerged as a material with potentials suitable for application in energy storage. These properties enable the phosphorene to have fast ion diffusivity, good electrical conductivity, and dynamic stability, hence presenting the phosphorene as a potential material for supercapacitors.[10b,156] Flexible solid-state supercapacitor is reported to have shown a good volumetric capacitance of $\\approx17.78\\ensuremath{\\mathrm{~F~}}\\ensuremath{\\mathrm{cm}}^{-3}$ $(59.3\\mathrm{~F~g}^{-1})$ at $0.1\\mathrm{~V~}\\mathrm{s}^{-1}$ and unprecedented capacitance rate with constant $1.43\\ \\mathrm{F\\cm^{-3}}$ $(4.8\\mathrm{~F~g^{-1}})$ of volumetric capacitance at $10\\mathrm{V}\\mathrm{s}^{-1}$ .[106] Phosphorene maintained high mechanical stability even after a long period (30 000 charging–discharging cycles). Recently, a mask-assisted interdigital electrode pattern was developed by the use of layer-by-layer stacking of phosphorene nanosheets and electrochemically exfoliated graphene nanosheets (GNs) in IL electrolyte.[157] Micro-supercapacitors offered an excellent electrochemical performance of around $9.8~\\mathrm{mF~cm^{-2}}$ and a volumetric capacitance of $\\approx37.5\\ \\mathrm{F\\cm^{-3}}$ at $5~\\mathrm{mV}~\\mathrm{s}^{-1}$ and operates at $94\\%$ of their original capacitance. The performance of micro-supercapacitors is ascribed to the strong combination of phosphorene–GNs which offers large space for ionic storage and high mobility routes. Also, another supercapacitor which consists of 2D single-walled carbon nanotube (SWCNT) electrodes and ion gel was reported to have exhibited an excellent device performance where it only declines in its performance at $30\\%$ strain.[158] Chen et al.[150] reported a phosphorene–RP composite with a large value of ${\\approx}60.1\\mathrm{~F~g^{-1}}$ and long cycling self-life with a capacity retention of ${\\approx}83.3\\%$ in 2000 cycles, demonstrating that phosphorene is suitable for application in supercapacitor. Monolayer or few-layer phosphorenes stand out to be the best material for solid-state based-stretchable supercapacitor for energy storage devices which may found utilization in paper like mobile phone (touch screen), electronic newspaper, power dressing, and flexible and wearable computers.",
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"category": " Results and discussion"
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},
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{
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"id": 31,
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"chunk": "# 6.2. Field Effect Transistors \n\nA significant component in electronic field is FET, and thus it plays a great role in revolutionizing the electronics. The physical features of the FET are the three terminals which have semiconducting channel electrodes in-between the terminalsource and terminal-drain. The quantity of charges between the source and the drain terminals can be regulated through application of gate voltage. Gate voltage causes a transverse electric field which will either deplete (off state) or enhance (on state), where the ratio is $(l_{\\mathrm{on}}/l_{\\mathrm{off}})>10^{4}$ . \n\nSince the discovery of graphene, the continuous improvement on FET is based on the stability of the material in question, and the intrinsic electronic structure and associated electronic properties have the topic of intense research. Lack of bandgap in graphene has made it difficult to be implemented in the logic transistors, because it needs large $l_{\\mathrm{on}}/l_{\\mathrm{off}}$ ratio, and presence of optimum energy gap. Despite graphene exhibiting a transport mobility of ${\\approx}20~000~\\mathrm{cm}^{2}~\\mathrm{V}^{-1}~\\mathrm{s}^{-1}$ at room-temperature, however, its ability to attained ballistic transport limits in FETs is unbearable.[159] The tunable bandgap and high carrier mobility characteristic exhibited by phosphorene offer a hunting ground for an excellent FET as depicted in Figure 11. \n\nGuo et al.[97] demonstrated that FET fabricated from a passivated BP with ${\\mathrm{Ag}}^{+}$ metal ion exhibited enhanced electronic transport properties, where the prototype ${\\mathrm{Ag}}^{+}$ -modified BP FET as shown in Figure 11a, showed an improved hole mobility from 796 to $1666\\ c m^{2}\\mathrm{V}^{-1}\\mathrm{s}^{-1}$ and $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ ratio from $5.9\\times10^{4}$ to $2.6\\times10^{6}$ as shown in Figure 11d; this clearly demonstrates that ${\\mathrm{Ag}}^{+}$ played an important role in enhancing ambient stability of BP–FET without changing the electronic intrinsic properties of BP as confirmed in Figure 11b. Also, they performed the room-temperature switching modes of ${\\mathrm{Ag}}^{+}$ -modified BP–FET before and after $\\mathbf{A}\\mathbf{g}\\mathbf{+}$ modification as presented in Figure 11c, and established that the initial ambipolar transport character of BP tends to change to p-type carrier as concentration of ${\\mathrm{Ag}}^{+}$ gradually increases on the BP surface, and this is demonstrated by the fact that ${\\mathrm{Ag}}^{+}$ modification enhances the initial hole carriers by lowering the off-state resulting into high $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ ratio. In addition, they perform a comparative test to established the effect of other metal ions like $\\mathrm{Fe}^{3+}$ , $\\mathrm{Mg^{2+}}$ , and $\\mathrm{Hg}^{2+}$ on BP stability and FET performance. All the three metal ions are formed, $\\mathrm{BP}_{\\mathrm{Fe}(3+)}$ , $\\mathrm{BP_{Mg(2+)}},$ and ${\\mathrm{BP}}_{\\mathrm{Hg}(2+)}$ , due to favorable energy of formations. Also the metal ion-BP modification was established to be stable. Their FET performance improved after metal ions modification because the three metal ions act as an electron-deficient medium in the BP–FETs. Among the three metal ions, only the $\\mathrm{Fe}^{3+}$ exhibits a change in ambipolar transport to character after $^{2\\mathrm{~h~}}$ of modification to p-type, because of more cations of $\\mathrm{Fe}^{3+}$ . However, the ${\\mathrm{Ag}}^{+}$ modification provides the best BP stability and FET performance. \n\nThe phosphorene-based FET has been fabricated on polyimide substrates sandwich between bi-layers of ${\\mathrm{Al}}_{2}{\\mathrm{O}}_{3}$ . Compared to TMD transistors,[160] phosphorene-based FET exhibited a higher carrier mobility of ${\\approx}310~\\mathrm{cm}^{2}~\\mathrm{V}^{-1}~\\mathrm{s}^{-1}.$ ,[161] which is much larger, fivefold the one shown by TMD. Single-layer $\\mathbf{MoS}_{2}$ exhibits a robust large bandgap of $2.84\\ \\mathrm{eV},$ an $l_{\\mathrm{on}}/l_{\\mathrm{off}}$ ratio of $10^{8}$ , and a low carrier mobility of $200\\ c m^{2}\\ \\mathrm{V}^{-1}\\ \\mathrm{s}^{-1}$ ,[162] and these have made it not suitable for fast nanoelectronics. For high performance, phosphorene transistors are suitable because of their sizeable and tunable bandgap with high hole mobility at room temperature.[10a] It is anticipated that phosphorene stands out to be appropriate material to bridge the gap in $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ and mobility among the graphene and TMDs.",
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"category": " Results and discussion"
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},
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{
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"id": 32,
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"chunk": "# 6.3. Biomedicine \n\nPhosphorene possesses environmental benign properties with low lethal, and these are some of the reasons behind its widely applications in the field of biomedicine.[163] First, clinical trials on biocompatibility and cytotoxicity of phosphorene nanoparticles have been investigated and found to be biocompatible and nontoxic[25,164] under physiological conditions. Phosphorene decomposed into nonlethal phosphite, phosphate and other $\\mathrm{P}_{x}\\mathrm{O}_{\\gamma}$ species.[75] With these two fundamental properties, i.e., low cytotoxicity and good biocompatibility pave way for utilizing phosphorene nanoparticles in biomedicine field.[164] Recently, Song et al. fabricated transient FET using few-layer phosphorene and found that its performance is comparable to channel’s material for transient devices. Also the biodegradability was tested via cytotoxicity assay and confirmed that the rate at which the phosphorene dissolves was within $36\\mathrm{~h~}$ and therefore, phosphorene-based transient FETs provide a viable platform toward human-implantable electronics nanodevices.[165] The large surface area-to-volume ratio of phosphorene nanoparticles has offered a platform for drug delivery with high drug loading efficiency. The drug-delivery system (DDS) using phosphorene nanoparticles as a biodegradable drug delivery system has been demonstrated to be an excellent therapeutic system because of its high surface-tovolume ratio and low toxicity to microenvironments. Recently, Zhang and co-workers demonstrated the near-infrared light induced decomposition of BP hydrogel for accurate release of drugs in tumor tissue to eradicate subcutaneous cancers without inflicting pain to the patient.[166] Zhang and coworkers[167] prepared BPQDs with excellent biocompatibility and low cytoxicity even at high concentration as $5\\ \\mathrm{\\mg\\mL^{-1}}$ and re-engineered via coating with polyelectrolyte polymer to offer a nanoplatform for drug delivery. Photodynamic therapy (PDT) is also a strategy for managing the cancer disease. For PDT to occur, three factors are needed, i.e., light source, tissue oxygen, and photosensitizer, and the combination of these produces toxic substance, which kills the diseased cells with less injuries to the patient.[168] The PDT cycle process encompasses the transmission of light energy from the photosensitizer to oxygen molecules found in the tissue to generate reactive oxygen species (ROS), and the ROS induces toxicity to the cellular.[169] Ultrathin phosphorene has been found to be an excellent photosensitizer with large quantum yield, thus enhances the ROS evolution rate[170] and is very versatile in the PDT application. \n\n \nFigure 11. a) AFM image (top) and schematic of a BP–FET device on silicon substrate with a $300{\\mathsf{n m}}{\\mathsf{S i O}}_{2}$ . b) Raman spectra of a BP sheet before and after ${\\sf A}{\\sf g}^{+}$ modification. c) Current to gate voltage curve obtained from the BP–FET at room temperature after ${\\mathsf{A}}{\\mathsf{g}}^{+}$ modification for 0, 0.5, 1, and $2\\ h$ . d) Hole mobility and $I_{\\mathrm{on}}/I_{\\mathrm{off}}$ ratio of the FET device as a function of ${\\mathsf{A}}{\\mathsf{g}}^{+}$ modification time. Reproduced with permission.[97] Copyright 2017, Wiley-VCH. \n\nPhotothermal therapy (PTT) has also gained a lot of attraction in treatment of cancer because of minimal invasion, hence, less pain infliction to the patient. The mechanism process of the PTT is triggered by the absorbed light energy, where the photothermal agent transforms the light to heat, leading to thermal ablation to the cancerous cells[171] as illustrated in Figure 12a. Phosphorene nanostructures possess high extinction coefficient, excellent photothermal conversion efficiency, good biocompatibility, and photostability,[172b] making it a promising material for PPT. The combination of PDT and PTT forms a good synergy for cancer therapy, as demonstrated in Figure 12b. The phosphorene nanoparticles have good interaction with infrared light to generate ROS under $660~\\mathrm{nm}$ wavelength for PDT and heat generation under $808\\ \\mathrm{nm}$ laser irradiation for PTT.[173] Combining these two methods gives a powerful double cancer therapy for maximum eradication of cancerous cells. \n\n \nFigure 12. a) The photothermal therapy (PTT). Reproduced with permission.[166] Copyright 2018, PNAS Early Edition. b) The combination of PDT and PTT form a good synergy for cancer therapy. Reproduced with permission.[172a] Copyright 2017, Wiley-VCH.",
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"category": " Results and discussion"
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},
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{
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"id": 33,
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"chunk": "# 6.4. Photocatalyst for Water Splitting \n\nThe exploration of sustainable renewable source of energy has been an important research topic over the last decade.[174] Because of this, development of novel technology regarding the capture of sunlight as a natural source of energy and utilized to benefit the human being has been received well by the researchers, where low-dimensional materials have been utilized in the fabrication of solar devices. The proper use of sunlight supplement in the fossil source energy thus meets the energy demand in the society. Employing a semiconductor for concurrent solar light absorption and transformation in the photocatalytic water splitting reaction is of great concern in the renewable energy field.[175] Efficient water splitting into hydrogen has attracted great attention in the research community with a view of setting a new industrial photosynthetic process which proved to be a clean source of energy.[176] Photocatalysis from semiconducting materials is projected to make a significant contribution in enabling both the energy transformation in hydrogen evolution and valued chemical feedstock upon interacting with photon energy. Through photon absorption by semiconductor material, evolution of effective charge generation is experienced and for effective transfer of charge pairs needs appropriate bandgap of photocatalyst and, therefore, band structure of photocatalytic material is very important. Phosphorene-based photocatalytic technology for splitting of water into oxygen and hydrogen phase has been on the rise,[177] and this is a suitable technology for mitigating emission. Phosphorene exhibits extraordinary charge carrier migration characteristic, which is faster hence facilitates charge separations, and fulfills the requirements of the photocatalyst for hydrogen evolution. The bandgap features of BP such as tunable bandgap have demonstrated well that it is suitable for water splitting since it absorbed light up to even near-infrared region and the carrier migration is very excellent as compare to existing 2D materials such as $\\mathbf{MoS}_{2}$ , graphene, and $\\mathrm{g}–\\mathrm{C}_{3}\\mathrm{N}_{4,}{}^{178}$ as shown in Figure 13. Adjustable bandgap and strong broadband optical absorption in phosphorene are highly considered as the sole characteristics useful for photocatalyst.[177b] \n\nThe photocatalytic cycle for water splitting composed of three stages: 1) semiconducting materials get excited when light is illuminated on it to produce electron and hole pairs in the CBM and VBM, respectively; 2) photogenerated electron migrates from bulk to edge surface of the semiconductor to reduce protons to $\\mathrm{H}_{2}$ ; and 3) the holes facilitates the oxidation half-reaction.[179] Under normal conditions, splitting of one molecule of water $\\mathrm{(H}_{2}\\mathrm{O})$ into $\\mathrm{H}_{2}$ and $\\mathrm{O}_{2}$ needs the standard Gibbs free energy $(\\Delta G\\approx237\\mathrm{\\kJ\\mol^{-1}}$ $(1.23\\ \\mathrm{eV})$ . Appropriate selection of photocatalyst is governed by their bandgap energy $(E_{\\mathrm{g}}>1.23\\ \\mathrm{eV})$ ) values with the suitable CBM edge energy ( $[E_{\\mathrm{cbm}}$ and VBM edge energy $(E_{\\mathrm{vbm}})$ matching with the electrochemical potentials of $E^{\\circ}$ $\\left(\\mathrm{H}^{+}/\\mathrm{H}_{2}\\right)$ and $E^{\\circ}(\\mathrm{O}_{2}/\\mathrm{H}_{2}\\mathrm{O})$ . \n\nHigh hole mobility, tunable bandgap, and strong optical absorption have made phosphorene the most preferred material for water splitting.[10a,180] The high carrier mobility and anisotropy help in the separation of photogenerated carriers while the tunable bandgap supports the wide absorption spectrum. The determining factor of a photocatalyst semiconductor is the position of VB edge and the band CB edge, and therefore, the inherent band edge position determined the probability of phosphorene as the photocathode for hydrogen evolution reaction (HER). Edge modification is another method for attaining full water splitting in phosphorene by pseudohalogen.[181] Yang and co-workers computed the bandgap edge positions on the phosphorene nanoribbon (PNR) via edge passivation using nitrile cyanate functional groups. The bandgap edge values in the zigzag edge-modified PNRs exhibit an abrupt decrease as compare to those in armchair orientations due to their variant edge-shaped structure. The trend shows that the edge-modified bandgap value decreases with the increase in width of nanoribbons. \n\n \nFigure 13. Schematic illustration of band edge positions of $\\mathsf{T i O}_{2},\\mathsf{g}{\\cdot}\\mathsf{C}_{3}\\mathsf{N}_{4}$ , ${\\mathsf{M o S}}_{2}$ , and phosphorene as photocatalysts for water splitting. The reduction potential of $\\mathsf{H}^{+}/\\mathsf{H}_{2}$ and the oxidation potential of $\\mathsf{O}_{2}/\\mathsf{H}_{2}\\mathsf{O}$ are shown by dotted red and blue lines, respectively. The positions of CBM and VBM band edges are shown by solid lines and their redox potential are shown against each edges, respectively. The bandgap energy of each semiconductor is shown by the black arrows and all the energy levels are referenced to the vacuum level, set to be zero. Reproduced with permission.[178] Copyright 2017, American Chemical Society.",
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"category": " Results and discussion"
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},
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{
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"id": 34,
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"chunk": "# 7. Conclusion and Perspectives \n\nBP, beyond graphene, is the most celebrated layered material, not only because of its inherent exceptional physical properties but also due to its peculiar chemical properties, and it has found a special interest from the materials scientists who have carried out prototype applications in the electronics, optoelectronics, energy storage, energy conversion, and biomedical fields. BP is regarded as a rising star, but not the brightest star, in condensed matter physics, only because of the limitations due to its degradation behavior, and overcoming these problems will place BP at the top of the hierarchical order of 2D materials. Stabilization of phosphorene still represents a challenging task, and the search for effective and efficient passivation techniques raises questions of fundamental interest and importance. In this review, we focused on recent progress on ineffective passivation techniques that render phosphorene more stable, and some of the phosphorene stabilization approaches have been summarized in detail along with developments in the implementation of the stabilized phosphorene in real-life situations. The ambient instability of phosphorene is almost solved, although some new methods are still needed to further improve its stability and to attain the real commercialization of phosphorene-based devices. This review might assist researchers in understanding the chemistry behind phosphorene degradation, in identifying the best strategy or synergistic approach to mitigate phosphorene degradation, and in the long run, in obtaining long-term solution to the instability of phosphorene in air. Therefore, obtaining a stable monolayer or few-layer phosphorene in ambient conditions is likely to bring revolutionary change in the landscape of the electronics, optoelectronics, energy conversion, and biomedical fields, and thus offers improvements in human civilization. Understanding the chemistry behind the phosphorene degradation is very significant toward the appropriate design of excellent passivation techniques to obtain long-term stability of phosphorene, for full implementation in practical applications. \n\nThe goal of this paper was to review the recent research developments on passivation techniques of phosphorene as a rising star-layered material. There is a great hope that core advances in passivation techniques for phosphorene will be attained in the near future, which will mitigate the disadvantages of BP nanoparticles. Much work still needs to be performed to understand the chemistry behind phosphorene degradation, such as the design of preparation methods that will not cause defects to the phosphorene surface and edges and the development of production methods that are environmentally friendly, cost effective, and efficient enough to meet the anticipated industrial demand. Breakthroughs in these aspects will make phosphorene a full-fledged star 2D material with impeccable properties.",
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"category": " Conclusions"
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},
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{
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"id": 35,
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"chunk": "# Acknowledgements \n\nD.K.S. and H.W. contributed equally to this work. Financial supports from the National Natural Science Foundation of China (Grant Nos. 61605131, 61435010, and 61875138), Natural Science Foundation of Guangdong Province for Distinguished Young Scholars (2018B030306038), and Science and Technology Innovation Commission of Shenzhen (Grant Nos. JCYJ20180507182047316, KQJSCX20180328095501798, KQTD2015032416270385, and JCYJ20150625103619275) are gratefully acknowledged.",
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"category": " References"
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},
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{
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"id": 36,
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"chunk": "# Conflict of Interest \n\nThe authors declare no conflict of interest.",
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"category": " Conclusions"
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},
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{
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"id": 37,
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