2D Monoelements E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
2D Monoelements: Properties and Applications explores the challenges, research progress and future developments of the basic idea of two-dimensional monoelements, classifications, and application in field-effect transistors for sensing and biosensing. The thematic topics include investigations such as: * Recent advances in phosphorene * The diverse properties of two-dimensional antimonene, of graphene and its derivatives * The molecular docking simulation study used to analyze the binding mechanisms of graphene oxide as a cancer drug carrier * Metal-organic frameworks (MOFs)-derived carbon (graphene and carbon nanotubes) and MOF-carbon composite materials, with a special emphasis on the use of these nanostructures for energy storage devices (supercapacitors) * Two-dimensional monoelements classification like graphene application in field-effect transistors for sensing and biosensing * Graphene-based ternary materials as a supercapacitor electrode * Rise of silicene and its applications in gas sensing
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Seitenzahl: 583
Veröffentlichungsjahr: 2020
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Table of Contents
Cover
Title page
Copyright
Preface
1 Phosphorene: A 2D New Derivative of Black Phosphorous
1.1 Introduction
1.2 Pristine 2D BP
1.3 Phosphorene Oxides
1.4 Conclusion
Acknowledgment
References
2 Antimonene: A Potential 2D Material
2.1 Introduction
2.2 Fundamental Characteristics
2.3 Experimental Preparation
2.4 Applications of Antimonene
2.5 Conclusion and Outlook
References
3 Synthesis and Properties of Graphene-Based Materials
3.1 Introduction
3.2 Applications
3.3 Structure
3.4 Physical Properties
3.5 Conclusions
References
4 Theoretical Study on Graphene Oxide as a Cancer Drug Carrier
4.1 Introduction
4.2 Molecular Interaction of Biomolecules and Graphene Oxide
4.3 Computational Method
4.4 Results and Discussion
4.5 Conclusion
References
5 High-Quality Carbon Nanotubes and Graphene Produced from MOFs and Their Supercapacitor Application
5.1 Introduction
5.2 Carbonization of MOFs
5.3 Effect of MOF Pyrolysis Temperature on Porosity and Pore Size Distribution
5.4 MOF Derived Carbon as Supercapacitor Electrodes
5.5 Conclusions and Perspectives
Acknowledgement
References
6 Application of Two-Dimensional Monoelements-Based Material in Field-Effect Transistor for Sensing and Biosensing
6.1 Introduction
6.2 Field-Effect Transistor
6.3 Application of 2D Monoelements in Field-Effect Transistor for Sensing and Biosensing
6.4 Conclusions and Perspectives
References
7 Supercapacitor Electrodes Utilizing Graphene-Based Ternary Composite Materials
7.1 Introduction
7.2 Charge Storage Mechanism of a Supercapacitor Device
7.3 Graphene and its Functionalized Forms
7.4 Varieties of Graphene-Based Ternary Composite
7.5 Conclusion and Future Perspectives
References
8 Graphene: An Insight Into Electrochemical Sensing Technology
8.1 Introduction
8.2 Electronic Band Structure of Graphene
8.3 Electrochemical Influence of the Graphene Due to Doping Effect
8.4 Exfoliation of Graphite: Chemistry Behind Scientific Approach
8.5 Electrochemical Reduction of Oxidized Graphene
8.6 Spectroscopic Study of Graphene
8.7 Biotechnical Functionalization of Graphene
8.8 Graphene Technology in Sensors
8.9 Conclusion
Acknowledgements
References
9 Germanene
9.1 Introduction
9.2 Structural Arrangements
9.3 Fundamental Properties of Germanene
9.4 Applications of Germanene
9.5 Conclusions
References
10 2D Graphene Nanostructures for Biomedical Applications
10.1 Introduction
10.2 Applications of Graphene in Biomedicine
10.3 Conclusion
References
11 Graphene and Graphene-Integrated Materials for Energy Device Applications
11.1 Introduction
11.2 Graphene-Integrated Electrodes for Lithium-Ion Batteries (LIBs)
11.3 Graphene-Integrated Nanocomposites for Supercapacitors (SCs)
11.4 Conclusion
References
Index
End User License Agreement
List of Tables
Chapter 4
Table 4.1 Binding and dominant mechanisms of graphene oxide with various cancer ...
Chapter 5
Table 5.1 Quantitative and qualitative aspects of the pore features for the prev...
Table 5.2 Comparative table listing specific capacitance obtained for various MO...
Chapter 6
Table 6.1 Below table summarizes various elements in the periodic table which ca...
Table 6.2 Different types of two-dimensional monoelements (Xenes) and their vari...
Chapter 8
Table 8.1 Advantages and disadvantages of the graphene preparative methods.
Table 8.2 Experimental parameters in the graphene sensing technology.
Table 8.3 Graphene based assay for glucose analyte.
Table 8.4 Fluoroscence sensing technology in nucleic acid/aptamer assay.
Table 8.5 Bond order and bond length of gas molecules.
Table 8.6 Sensitivity of the drug and antioxidant sensors.
Chapter 9
Table 9.1 Structural and electronic parameters of elemental structures. Electron...
Table 9.2 Structural and electronic parameters of hydrogenated elemental structu...
Chapter 11
Table 11.1 Comparison of the different types of capacitors [107].
List of Illustrations
Chapter 1
Figure 1.1 Optimized crystallographic structure of (a) 3D BP and (b) 2D BP.
Figure 1.2 Graph of electronic features corresponding to 2D BP. (a) the band str...
Figure 1.3 Absorption spectra for undeformed monolayer phosphorene with 0% strai...
Figure 1.4 (a) and (b) dielectric function, (c) absorption coefficient, (d) refl...
Figure 1.5 Polar plot of (a) Young modulus in J/m
2
and (b) positive and negative...
Figure 1.6 Monolayer phosphorene under different values of in-plane compressive ...
Figure 1.7 The three possible adsorption sites.
Figure 1.8 DOS of spin up and down of adatoms.
Figure 1.9 Configurations of 50% oxidized phosphorene: (a) dangling structures a...
Figure 1.10 GGA and GW band structures of half-oxidized structures.
Figure 1.11 Absorption coefficient of dangling structures (on the left) and brid...
Figure 1.12 Excitons wave functions. Black balls represent the holes.
Figure 1.13 Part of polar plots of (a) Young modulus, (b) Poisson ratios.
Figure 1.14 (a) Top and (b) side views of phosphorene oxides PO.
Figure 1.15 Phosphorene oxide (a) band structure and density of states, (b) phon...
Figure 1.16 Absorption spectrum and exciton wavefunction for the first transitio...
Figure 1.17 Mode-dependent anharmonic phonon relaxation time for acoustic modes.
Chapter 2
Figure 2.1 (a) Top views of the relaxed antimonene monolayer allotropic forms wi...
Figure 2.2 HSE06 calculated electronic band structures of trilayer, bilayer, and...
Figure 2.3 (a) Diagram of the steps involved in the sophisticated version of mec...
Figure 2.4 (a) Optical image of a dispersion of exfoliated few-layer antimonene....
Figure 2.5 (a) Schematic diagram of the synthesis process of antimonene by the v...
Figure 2.6 (a) Schematic of growth process of monolayer antimonene on 2D PdTe
2
s...
Figure 2.7 (a) Phase shift of the antimonene-based AOM as a function of pump pow...
Figure 2.8 (a) Current-density-voltage (J-V) curves of devices without (Device 1...
Figure 2.9 (a) LSV curves of bulk Sb- and SbNSs-modified glassy carbon electrode...
Figure 2.10 (a) The first and two charge/discharge cycles of the SbNS-G film at ...
Figure 2.11 (a) Time-dependent photothermal heating curves of PEG-modified AMQDs...
Chapter 3
Figure 3.1 The graphene transformation as layered graphene, carbon nanotube, and...
Figure 3.2 Chemical structure.
Figure 3.3 Orbital structure.
Figure 3.4 Synthesis methods of graphene.
Figure 3.5 Organic structure of graphene.
Chapter 4
Figure 4.1 Schematic representation of (a) graphene and (b) graphene oxide.
Figure 4.2 Schematic representation of interaction between graphene oxide with s...
Figure 4.3 Schematic representation of binding coordinate between graphene oxide...
Figure 4.4 Schematic representation of binding coordinate between graphene oxide...
Chapter 5
Figure 5.1 (a) Schematic diagram showing various types of porous materials [23–2...
Figure 5.2 (a) Schematic representation of the structure of metal organic framew...
Figure 5.3 MOFs used as templates and/or precursors for the fabrication of porou...
Figure 5.4 (a) Nitrogen-enriched carbon tubes (NCNTs) derived from ZIF-67 MOF @ ...
Figure 5.5 (a) N
2
adsorption-desorption isotherms of the N-doped porous carbon (...
Figure 5.6 Charge storage mechanism in supercapacitors (SC). (a) Electric double...
Figure 5.7 Schematic diagram representing the difference between the charge stor...
Chapter 6
Figure 6.1 Application of 2D monoelements in field-effect transistor for sensing...
Figure 6.2 Schematic figure of field-effect transistor.
Chapter 7
Figure 7.1 Charge storage mechanism in EDLC.
Figure 7.2 Charge storage mechanism via redox reactions-based pseudocapacitance.
Figure 7.3 TEM images of reduced graphene oxide prepared by modified Hummers met...
Chapter 8
Figure 8.1 Band structure of graphene. The conductance band touches the valence ...
Figure 8.2 The hall mark of massless Dirac fermions is QHE plateau in σ
xy
at hal...
Figure 8.3 Doping graphene. Position of the Dirac point and the Fermi level of p...
Figure 8.4 Structural model for (a) as-prepared GO; (b) GO after treatment at 10...
Figure 8.5 Optimized structure of 10-AGNRs with gas molecule adsorption: (a) CO,...
Chapter 9
Figure 9.1 Ball and stick model of germanene. The honeycomb lattice is composed ...
Figure 9.2 Schematic diagrams for the different types of buckled structures: Fla...
Figure 9.3 Various geometrical configurations of decorated, 2D elemental sheets....
Figure 9.4 Typical configurations of composite structures for silicene, germanen...
Figure 9.5 Quasi-particle band structures of group 14 elemental structures along...
Figure 9.6 (a) The atomic structure of germanene. The band structures of silicen...
Figure 9.7 Photocatalytic H
2
evolution of GeH under different conditions: GeH/Pt...
Figure 9.8 Irradiation-time dependence of the relative concentration C/C
0
of the...
Figure 9.9 The reaction steps (a) and the corresponding energy profile (b) of CO...
Chapter 10
Figure 10.1 The different carbon-based nanostructures are originated from the gr...
Figure 10.2 (a) The different synthesis methods of graphene and (b) illustration...
Figure 10.3 The conversion process of graphene oxide and reduced graphene oxide ...
Figure 10.4 The basic components of tissue engineering [19].
Figure 10.5 Image of a healthy knee [22].
Figure 10.6 Schematic representation of the application of 2D graphene for vario...
Figure 10.7 Representing key milestones of development of graphene [84].
Figure 10.8 Showing a synthesis method of GO/FHA nanocomposite [87].
Chapter 11
Figure 11.1 Graphene as the major constituent in various energy devices [11].
Figure 11.2 Graphene and graphene oxide derived from graphite from conventional ...
Figure 11.3 Schematic diagram explaining working principle of LCO-graphite batte...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Monoelements
Properties and Applications
Edited by
Inamuddin, Rajender Boddula
Mohd Imran Ahamed
Abdullah M. Asiri
This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA
© 2020 Scrivener Publishing LLC
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-65525-1
Cover image: Pixabay.Com
Cover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Preface
The development of new two-dimensional (2D) monoelements-based semiconductor materials, such as phosphorene, graphene, antimonene, etc., has attracted many researchers due to their wide range of applications in diverse sectors along with their promotion of novel innovations in the field of science. Due to their impressive physical, chemical, electronic, and optical properties these 2D monoelements have been identified as potential agents for a variety of applications such as electronics, theranostics, therapeutic delivery, bioimaging, sensors, field-effect transistors, the environment, energy conversion, storage, etc.
This edition of Monoelements: Properties and Applications explores the basic idea of 2D monoelements, classifications, and application in field-effect transistors for sensing and biosensing. Finally, various challenges, future developments, and research progress are also discussed. This book will be useful for beginners and experts—from undergraduate students to industrial engineers—working in the area of semiconductors, materials science, and engineering. Based on thematic topics, this edition contains the following eleven chapters:
Chapter 1 investigates recent advances in phosphorene. In particular, the effects of its strong anisotropy and its high reactivity on the physical properties of pure and functionalized phosphorene are discussed in detail. This largely distinguishes phosphorene from other 2D materials and makes it the ideal platform for emerging devices.
Chapter 2 provides insights into the recent exploration of the diverse properties of 2D antimonene, continuous updating of preparation methods, as well as further development of potential applications, and then looks ahead to the opportunities and challenges facing antimonene in the future.
Chapter 3 presents a brief review of graphene and its derivatives. Applications of graphene and graphene oxide in the electronic and biomedical fields are discussed, and their physical properties like thermal and electrical properties are outlined.
Chapter 4 discusses the molecular docking simulation study used to analyze the binding mechanisms of graphene oxide as a cancer drug carrier.
Chapter 5 aims to group the significant work reported in the last few years on metal-organic frameworks (MOFs)-derived carbon (graphene and carbon nanotubes) and MOF-carbon composite materials, with a special emphasis on the use of these nanostructures for energy storage devices (supercapacitors).
Chapter 6 summarizes the basic idea of 2D classification like graphene application in field-effect transistors for sensing and biosensing. These 2D monoelements have been identified as potential agents for a variety of applications such as theranostics, therapeutic delivery, bioimaging, gas sensors, biosensors, field-effect transistors, semiconductors, etc.
Chapter 7 describes various graphene-based ternary materials as a super-capacitor electrode. Different forms of functionalized graphene are discussed, including graphene oxide and reduced graphene oxide and its characteristic properties. A main focus of this chapter is the synthesis and electrochemical features of various graphene-based ternary composites with conducting polymer, metal oxide and other carbon-based materials.
Chapter 8 explains the physico-chemical properties such as electronic band structure, electrochemical influence of graphene doping, and chemistry behind graphite exfoliation. Furthermore, it extends to electrochemical sensing of analytes such as glucose, DNA and aptamer, pollutants, gas, pharmaceutics, and antioxidants.
Chapter 9 describes the structures, fundamental properties, and applications of germanene and aims to draw the attention of researchers towards the possibilities offered by the use of germanene-based materials for improving their working characteristics and for replacing rather expensive traditional materials used in energy storage devices.
Chapter 10 discusses the fundamental milestones of graphene origin, synthesis routes, advantages, and various literature cites toxicity studies. The role of 2D graphene in gene-delivery, tissue-engineering, prosthetic-implants, cancer-therapy, and biosensing fields are explored as well. The major arena of focus is ascribed to in vivo and in vitro studies of graphene for biomedical use.
Chapter 11 describes graphene and graphene-based materials for energy devices. The chapter broadly discusses the role of graphene as electroactive anode and cathode materials with major focus on lithium ion batteries and supercapacitors. The chapter also includes basic synthesis and characterization techniques.
EditorsInamuddinRajender Boddula Mohd Imran Ahamed Abdullah M. AsiriSeptember 2020
1Phosphorene: A 2D New Derivative of Black Phosphorous
Lalla Btissam Drissi1,2,3*, Siham Sadki1 and El Hassan Saidi1,2,3
1Faculté des Sciences, Université Mohammed V de Rabat, Rabat, Morocco
2Academie Hassan II des Sciences et Techniques, Rabat, Morocco
3CPM, Centre of Physics and Mathematics, Faculty of Science, Mohammed V University, Rabat, Morocco
Abstract
Phosphorene is a stable 2D elemental material obtained via the exfoliation of 3D layered phosphorus. Phosphorene exhibits several interesting features, including its unique highly buckled structural characteristics which lead to strong anisotropies in the transport, electronic, optical, mechanical, and thermal properties of this material along its two directions: zigzag and armchair. These excellent properties render phosphorene an ideal platform for various optoelectronic devices. However, under atmospheric conditions, for example, in the presence of oxygen, water, and light, phosphorene is very reactive due to the free non-bonding electrons existing on its surface. Consequently, the O-concentrations effect on the optoelectronic response, the elastic parameters, and thermal conductivity of phosphorene is significant and indicates interesting results.
Keywords: Phosphorene, synthesis, chemical functionalization, optoelectronic properties, mechanical response, thermal conductivity
1.1 Introduction
The 3D phosphorus is a very abundant element existing in several polymorphous forms. Among the allotropes of phosphorus, namely, white, red, violet, black, and blue; the black one (BP) constitutes the most thermodynamically stable phase under ambient conditions. This layered allotrope was discovered for the first time more than a century ago through the high-pressure [1]. Recently, 3D BP was synthesized from red phosphorus using the new sonochemical method [2]. In bulk BP, the layers are weakly stacked together via VDW interactions [3]. In each layer, the P atoms are connected to their three nearest neighbors by covalent bonds that form a rippled honeycomb structure [4]. BP is a semi-conductor with a direct-gap, a strong in-plane anisotropy and a density greater than 2.5 g/cm3 [5, 6].
Like its counterpart graphene, stable 2D phosphorene can be mechanically extracted from 3D BP. In 2014, phosphorene was synthesized, for the first time, using a scotch tape based microcleavage method [7–9]. The phosphorene’s unit cell is composed of four P atoms and appears highly buckled in the armchair (AC) axis [10]. Because of its geometric characteristics, phosphorene exhibits highly anisotropic physical properties along its AC with respect to its zigzag one [11, 12]. Phosphorene is a p-type semiconductor [13–15] that shows a high flexibility, an important specific capacity and discharge potential that are very required for advanced battery applications [16–18]. In addition, it exhibits a strong excitonic effect [19], an optical gap located at 1.2 eV and its absorbs infrared to near ultraviolet radiation [20]. This new hexagonal material has great potential applications in optoelectronics and photovoltaic devices [21].
Furthermore, the puckered structure of phosphorene attributes its interesting elastic properties such as great structural flexibility and a resistance to 27% and 30% deformations along the zigzag and armchair directions, respectively [22, 23], which makes this material very suitable for wearable optoelectronic devices. Furthermore, the Young’s modulus and Poisson ratio exhibit their maximum values along ZZ-axis indicating how it is difficult to strain it. Consequently, phosphorene is super flexible along the armchair axis [23]. It is also well to mention that phosphorene is an auxetic material [24, 25] and that its non-centrosymmetric point group leads to a large piezoelectric response [23] showing that phosphorene can convert mechanical energy into electrical one [26].
Despite all the exceptional properties of phosphorene, it is very reactive with oxygen due to the non-bonding pairs present at its surface [27]. This fact limits its applications in optoelectronics, sensors, energy conversion, photocatalytic, and so on. To overcome this obstacle, many different techniques have been used to fabricate air-stable phosphorene. The passivated phosphorene by graphene, h-BN, Al2O3, and the polymeric material is a promising technique to avoid chemical debasement and to modulate its features [28]. The measurements shown smaller degradation when phosphorene only exposes to O2 or H2O [29]. Phosphorene with different oxygen concentrations confers excellent new properties in these 2D materials [30, 31]. At high concentration, oxidation leads to a new family, namely, 2D planar and 1D tubular forms, with a transition in the band gap from semiconductors to insulators [32].
In this chapter, we first present pure phosphorene starting from its crystalline structures, its fabrication methods, its physical properties, and ending with certain applications. Secondly, we will investigate how the oxidation’s arrangement and concentrations influence the electronic, elastic, and optical characteristics of phosphorene oxides.
1.2 Pristine 2D BP
Owing to its great buckle height, phosphorene has fascinating properties such as anisotropic optoelectronic and mechanical features which make it very attractive for nanodevices.
1.2.1 Synthesis and Characterization
Similar to graphene, 2D BP can be exfoliated from buckled material trough the top down method. The bottom-up method is still not promising for phosphorene CVD growth since most of the phosphorus precursors used in thermal depositions show a high amount of toxicity and cannot be adapted for CVD manufactures [33, 34]. It follows that the large-scale bottom-up method requires more effort from experimental scientists.
1.2.1.1 Top-Down Approaches
The mechanical exfoliation is an effective widely used method for cleaving 3D materials from mutilayers to some layers and then to isolate a single layer [34]. Graphene monolayer, for example, has been isolated from graphite simply by using adhesive tape [35, 36].
Monolayer, bi- and tri-BP sheets were successfully exfoliated using micromechanical cleavage of 3D BP with PDMS in 2014. This method was carried out using an adhesive tape in three steps.
First, the exfoliated phosphorene layers were transferred to PMMA/PVA (polymethyl methacrylate/Polyvinyl Alcohol) composites, and then, the resulting layers with the composites were moved to a SiN substrate with a thickness of 200 nm. Several chemicals are used to separate the obtained specimens from the PMMA/PVA composites and to ensure that no more scotch tapes was left [37]. Despite the success of the mechanical exfoliation process, it was found that it was not scalable and hence limited to be used in academic laboratories for fundamental studies. Thus, to obtain a phosphorene sheet, a more efficient manufacturing process has been introduced. In particular, an Ar+ plasma was used to produce monolayer phosphorene through thermal ablation. This process provides an improved means of controlling the phosphorene thickness, unlike it is still challenging for mass production [38, 39].
The interesting technique to fabricate large quantities of exfoliated phosphorene is the liquid phase preparation. The solution-based phosphorene synthesis is placed into the BP interlayers which enlarge the distance and allows the exfoliation. This approach is widely used to manufacture several 2D and 3D materials that have shown good performance in dispositive [40].
1.2.1.2 Bottom-Up Methods
Advanced chemical techniques were used intensively to fabricate large quantities of innovative devices based on new 2D sheets like graphene, germanene, borophene, silicene, and stanene [38]. For other synthesized 2D materials, this new processing route based on the deposition via thermal evaporation of their elemental forms is done on available suitable substrates/surfaces like Ag(111), Au(111), Pt(111), and Al(111) [34]. In parallel, other means, such as the successful epitaxial growth of graphene and TMDCs on insulating substrates made of sapphire or 300 nm of SiO2 on Si (SiO2/Si) [41] open up also the way to a possible phosphorene. These bottom-up methods are very used for materials stable under moisturizing conditions and at high temperature. In contrast, large-scale phosphorene CVD and epitaxial growth are still incubating and breakthroughs due to various reasons, such as lack of suitable substrate, high toxicity of phosphorus, as well as instability of phosphorene in the presence of moisture under high pressure [38, 42].
1.2.1.3 Geometric Structure and Raman Spectroscopy
Crystallographic data and elemental details of phosphorene were gained both theoretically and also experimentally using different techniques such as X-ray cristallography, high performance spectrometers, SEM microscope, and EDX analysis. Phosphorene has been shown to be a nonplanar lattice along and seems to be a bilayer material in the zigzag direction as displayed in Figure 1.1a.
Figure 1.1 Optimized crystallographic structure of (a) 3D BP and (b) 2D BP.
Measurements made by means of preliminary X-ray investigations indicate lattice constants of 3.31 Å and 4.38 Å in ZZ- and AC-axes, respectively, with four atoms forming the unit cell of phosphorene [43, 44]. The experimental result concorde with the theoretical values obtained using ab initio DFT calculations [23, 45].
In phosphorene monolayer, each phosphorus atom is linked to first three nearest neighbor atoms to constitute an sp3 hybridization in a covalent bound [10]. The non-planar geometry leads to two types of bonding, namely, the in-plane bond length d1 is about 2.224 Å , and the out-of-plane bond length d2 that is 2.244 Å, as illustrated in Figure 1.1b. The binding angles y and x are 96.3° and 102.095°, respectively. The height difference between the two half-layers is dz = 2.10 Å .
1.2.2 Physical Properties
1.2.2.1 Anisotropic Eectronic Behavior
Pristine phosphorene is a p-type semiconductor with a direct band gap [13, 46–48]. By using polarization-resolved photoluminescence excitation spectroscopy at room temperature, the quasi-particle band gap of phosphorene is measured to be 2.2 eV [49]. The same value is observed with the typical tunneling spectra of U-shaped electronic spectra [48].
Pure phosphorene has no spin polarization, which is confirmed by symmetrical density of spin-up and -down states displayed in Figure 1.2b. Meanwhile, Figure 1.2a shows that the band dispersion is highly anisotropic around the electronic gap. Indeed, one can observe a much greater dispersion along the Γ-X direction for CBM and VBM with respect to the vertical bands in Γ-Y region. The partial density of states (p-DOS) plots clearly show that px orbital contribute mainly in the states of the unfilled C-band, while the pz orbital of phosphorus dominates the valence band states [50]. The number of layers mainly affects the gap energy [24]. For instance, it takes the values of 1.51, 0.59, and 0.3 eV for the monolayer, the five layers, and the bulk black phosphorus [51]. Furthermore, the gap energy decreases with increasing the magnitude of external electrical field, which breaks the out-of-plane symmetry. Under biaxial strain and the two possible uniaxial strains, the deformed phosphorene shows a transition from the semiconducting to metallic phase [23].
Figure 1.2 Graph of electronic features corresponding to 2D BP. (a) the band structure, (b) represents the total and partial density of states.
1.2.2.2 Optical Properties
The puckered structure of phosphorene attributes it interesting optical properties. Phosphorene absorbs transverse radiation along its AC-axis, while it highly transmitted light along ZZ-axis [19, 52]. The photoluminescence excitation spectroscopy (PLE) measures an optical band gap of 1.31 eV owing to the exciton binding energy as discussed in [19, 49] and measured in [53]. Notice that the theoretical values are larger than the measured amounts because of the increased screening from the dielectric substrate that reduces the quasi-particle band gap and consequently the exciton binding energy [54]. Furthermore, phosphorene can absorbs the visible light since its optical absorption peak is located at 1.6 eV. All these features suggest phosphorene as a promising optoelectronic device for future applications.
The absorption peak can be tunable via strain as displayed in Figure 1.3. For deformed phosphorene, the absorption peak ranges from 0.38 to 2.07 eV under compressive and tensile strain revealing that the material absorbs both infrared and visible light. For the electric field vector E⊥, the graph displaying the imaginary part of the dielectric function E2(ω) shows a considerable shift of the first peak towards high energies when including quasi-particle corrections. However, the shape of E2(ω) spectrum changes considerably when taking into account the electron-hole correlations (BSE) [19]. The exciton binding energy is 0.818 eV for the first active one and 0.66 eV for the first dark one. As displayed in Figures 1.4a and b, the dielectric screening enlarges both the gap and binding energy at ω = 0. Furthermore, a large excitonic wave function distribution is observed for the first bright exciton in Figure 1.4c whose peak emerges in the IR part as shown in Figure 1.4d. The maximum reflectivity Rmax(ω) of 38% occurs in the IR range while it did not exceed 22% for the visible light (see Figure 1.4e). The electron energy loss spectra in Figure 1.4f reveal that the first plasmon peaks in phosphorene sheet has a height of 11.003 dispersed in the IR range of the spectrum.
Figure 1.3 Absorption spectra for undeformed monolayer phosphorene with 0% strain and deformed mono-layer under compressive –8%, –6%, and –4% and tensile strain of 4% and 5.5%. The curves are obtained through the GW+BSE method.
Figure 1.4 (a) and (b) dielectric function, (c) absorption coefficient, (d) reflectivity function, and (e) EELs function, obtained by using three approximations GW-BSE, GW-RPA, and GGA-RPA. (f) Represents the wave function of electron-hole.
In phosphorene multi-layers, the photoluminescence depends significantly on N. The first absorption peak is shifting to lower energies with increasing N. Consequently, the optical absorption coefficient ranges in the interval [0.3–1.2] eV [19], which means that the absorption radiation spectrum include IR and part of visible [52].
1.2.2.3 Elastic Parameters
The elastic properties of phosphorene are also very anisotropic since the Young values in AC-axis (x-direction) is four times lower than the one along the zigzag axis (y-axis), as indicated by the polar diagrams of Υ(θ) illustrated in Figure 1.5a [23, 55]. Notice that the weakest P-P bond strength is the main cause of these small values of the Young parameter [22], compared to 1 TPa and 270 GPa reported for graphene and MoS2, respectively. Furthermore, the Poisson’s ratio, namely, 0.73 and 0.165 in the AC and ZZ directions, respectively, confirms also the high anisotropy in phosphorene [56]. However, the negative value observed in the small interval [5π, 10/3π] as displayed in Figure 1.5b reveals that the material is auxetic. More precisely, for some particular stretch, phosphorene shows a lateral extension instead of longitudinal elongation as it is the case for conventional materials [57, 58]. Besides this, both the shear and compressional acoustic waves propagate more rapidly in the ZZ-axis as it is clearly deduced from the polar plot of the speed of sound in Figure 1.5c. Same anisotropic behavior is found for Debey temperature that is half times lower (see Figure 1.5d).
Besides the strong anisotropy of the elastic parameters, phosphorene exhibits a high elasticity compared with other monolayer materials such silicene, borophene MoS2, and graphene [59]. Indeed, without breaking phosphorene can withstand large tensile strains along its two possible directions [23]. Whereas, MoS2, for example, can only withstand deformation up to 13%. At 300 K and under a small magnitude of strain (E), Figure 1.6 depicts small ripples on the flat surface of phosphorene. When the compressive E grows, the buckling parameter increases. Interestingly, phosphorene maintains its structural stability in AC-axis at large compressive force up to 80%, but it breaks along the ZZ direction for a 17% deformation, which reveals the super flexible character of this material [60].
Figure 1.5 Polar plot of (a) Young modulus in J/m2 and (b) positive and negative values of Poisson ratio, (c) speed of sound in km/s of pure phosphorene.
Figure 1.6 Monolayer phosphorene under different values of in-plane compressive strain at 300 K in the two directions.
1.2.3 Applications
The remarkable properties and the strong anisotropy observed in phosphorene make it an ideal candidate for photodetectors, modulators, and sensors. Below, we will report some applications.
1.2.3.1 Gas Sensors
In 2D materials, the large ratio of surface area to volume renders them promising for gas sensors. In addition, the unique supplementary advantages of phosphorene, like its in-plane anisotropy, structural stability, and high chemical reactivity with molecules, make it highly desirable as a superior gas sensor [61]. Indeed, under gases exposition, phosphorene undergoes multiple modifications [25]. For example, the gas molecules adsorption on phosphorene leads to a reduction or an increase of the resistance that is very required for the markers in sensing applications. Furthermore, phosphorene depends mainly on certain toxic gases because of the high binding strength gas molecules. As a result, the selective behavior of the phosphorene in adsorbing gases influences significantly its transport properties along the two axis directions [61].
1.2.3.2 Battery Applications
According to [16, 17], phosphorene exhibits a specific capacity of 2,596 mAh/g, which is larger than the ones of sulfur and graphene. In addition, the discharge potential in phosphorene, that ranges in the interval [0.4–1.2] V, is smaller than 2.1 V obtained for lithium/sulfur battery [18]. It follows that 2D BP is a potential candidate for new generation of battery [10]. Compared to 2D anode materials, such as graphene and MoS2, phosphorene exhibits an ultrahigh diffusivity, that is 102–104 times faster. This is owing to its strong anisotropic diffusion barrier that is 0.68 eV, with respect to the small value of 0.08 eV found in the ZZ-axis. Furthermore, phosphorene-based Li battery shows a voltage of 2.9 V which is larger than the one of other 2D layered materials, making this kind of battery of potential use as a rechargeable battery for different electronic and energetic devices.
1.2.3.3 FETs
Another significant application of phosphorene is the fabrication of field-effect transistors (FETs) [38]. Phosphorene devices can offer many advantages over graphene transistors due to the good saturation of the current and their band gap [62]. Phosphorenes have attractive characteristics that are critical for advanced circuits and sophisticated amplifiers [10]. In particular, phosphorene exhibits drain current modulation of 105, high flexibility, and high carrier mobilities of about 1,000 cm2V−1s−1 which is larger than the other flexible transistors based on 2D monolayers like WSe2 and MoS2. Furthermore, when the length of channel is 300 nm, the measurements show that phosphorene exhibits a cutoff frequency of 12 GHz for the short-circuit current while frequency oscillation reaches the value of 30 Hz.
Beyond the multi-GHz frequency, phosphorene constitutes one of the best candidates for future generations of ultrathin layer transistors [10]. Moreover, phosphorene is not only used in field-effect transistor applications, but also in other electronic devices based on semiconductor materials due to its electronic properties and its charge mobility.
1.3 Phosphorene Oxides
The non-bonding pairs of electrons present on the surface of phosphorene leads to degradation of this 2D monolayer under ambient conditions, namely, oxygen, water, and light. This impedes phosphorene from some of its potential applications. To overcome this obstacle, phosphorene oxides with different O-concentration were investigated. It follows that phosphorene is stable at low O-concentrations. More precisely, half-oxidation is the best concentration to construct a stable material.
1.3.1 Challenges: Degradation of Phosphorene
In contrast to the unique properties and great potential of phosphorene, which distinguish it from other 2D materials, phosphorene remains unstable under atmospheric conditions, for example, in the presence of oxygen, water, and light, due to the non-bonding pairs of electrons present at its surface [27, 63]. The unprotected surface of phosphorene develops significant roughness, causing important changes and consequent degradation in the compositional and physical features of the material. In some cases, the degradation poses a serious performance problem of phosphorene-based devices [64].
1.3.1.1 Light Exposure
In phosphorene, the ambient degradation in the atmosphere is divided into three stages. In the first stage, the reaction induced by the ambient light O2 leads to the formation of oxygen. In that case, the reaction expressing the transfer if charge is given by: where P corresponds to phosphorene and h+ is a hole with positive charge. In the second stage, the oxygen molecule is separated at the surface leading to the following: . Finally, in the last step that is a hydrogen-bond interaction, the P atom is removed from the surface and the bonded O is absorbed by water molecules. It follows that the top layer of phosphorene is broken and excitons can be produced under ambient light.
To evaluate the evolution of BP degradation for various light’s wavelengths and at different time scales, six representative BP flakes were studied individually. Using atomic force microscopy (AFM), the exposures were evaluated in a dark room at six values of wavelengths ranging from 280 to 1,050 nm and imaged before and after identical exposure durations varying from 30 to 120 min with a step of 30 min [65]. The maximum degradation was observed for the UV light (280 nm), then for the blue one (455 nm). In contrast, phosphorene does not show any degradation for green, red, and infrared light, namely, 565, 660, 850, and 1,050 nm. Consequently, the UV light is the predominant contributor to the degradation of BP.
In addition, the engineering of the phosphorene’s band gap renders this material a good candidate for a photodetector, with a large spectral response ranging from the UV towards IR region. For instance, phosphorene photon detector shows a very fast response of 1.82 A/W in the presence of visible light irradiation of 550 nm. With photon energy and a bias of 0.1 V, the photoresponsivity attains the value of 175 A/W in the NIR regime, and at a higher bias of 3 V, it reaches 9×104 A/W offering phosphorene potential as a UV detector [66].
1.3.1.2 Phosphorene vs Air
When phosphorene is exposed directly to air, its reaction causes rapid degradation of phosphorene-based devices. Moreover, the exothermic process reveals that H2O will react with oxidized phosphorene. Both theoretical calculations and experiments have shown that at room temperature, phosphorene undergoes a spontaneous oxidation when it is exposed to O2. The oxidation pathway leads to the formation of phosphoric acid and defective phosphorene [29]. Besides, humidity (the presence of H2O) is very important to determine the stability of phosphorene in air. To avoid surface degradation of phosphorene and overcoming the oxidation barrier, some passivation techniques must be introduced. For example, graphene, h-BN, AlOx, Al2O3, PxOy, and polymeric materials are used to protect it from mechanical and chemical degradation [28]. Under low oxidation, phosphorene is stable and tend to be less stable when increasing the oxygen concentrations [25]. Consequently, 50% oxidation of phosphorene is the best amount to stabilize phosphorene after a two-day exposure to the atmosphere [67].
1.3.1.3 Functionalized Phosphorene
Non-metallic adatoms can also be strongly bound to phosphorene due to the lone electrons pair. Functionalization of phosphorene by adsorption of non-metallic atoms with a [He] core electronic, namely, B, C, N, F, and Al showed different site preferences as schematically illustrated in Figure 1.7. Indeed, while the adatoms B, C, and Al prefer to adsorb to the hollow (H) site, F adatom is adsorbed at the top site and N adatom prefers the bridge one [68]. Furthermore, the chemical functionalization of these non-metallic adatoms exists in three classes [69]. In the first one, the C and B adatoms get located at the interstitial site after breaking the P-P bonds. However in the second group, the N and F atoms remain on the surface of the P atoms and preserve the lattice structure of phosphorene. The last group is formed by the Al impurity, located at the top of the centre of the hexagon. The interatomic distances show that the smaller the adatom, the closer it is to the P monolayer, which implies a higher binding energy compared to the larger ones. This result is confirmed by the calculations of the binding energy (Eb).
Figure 1.7 The three possible adsorption sites.
Figure 1.8 DOS of spin up and down of adatoms.
For B, C, N, F, and Al adatoms on phosphorene, Eb is −5.08, −5.16, −2.98, −2.30, and −3.18 eV, respectively [68, 70]. The adsorption process is more stable in phosphorene since the values of Eb are much greater than the case of adsorbed graphene [71–73]. The higher values of Eb are mainly deserved to the buckled sp3 configuration of the reactive material as reported in [68]. Mid-gap states are observed in the spin-polarized density of states plotted in Figure 1.8 with 1 μB for B, N, and F systems. However, the curves for the C and Al impurities reveal the same number of electrons having up-spin and down-spin, indicating the absence of magnetic order in these configurations [70].
Moving to 3D transition metal (TM), such as Cu, Ti, V, Ni, Cr, and Fe adsorbed at the H site in phosphorene. According to [69], TM adatoms induce a magnetic moment ranging from 1.00 to 4.93 μB. In particular, the Ti adatom states contribute in the midgap and the conduction band (CB), which reduces the band gap to 0.41 eV in the presence of a magnetic order of 1.87 μB [70]. A magnetic of 2.00 μB is observed for Fe adatom systems. In the case of Cr and V adatoms, the spin-down is observed in CB. However, the spin-up of V states dominates the Fermi level and splits into two peaks for Cr adatom. The situation is different for the Ni and Cu adatoms which exhibit no spin-polarization.
1.3.2 Half-Oxided Phosphorene
Similar to graphene oxide, oxygen adsorption on phosphorene can be used efficiently to tune the optoelectronic properties as well as the protective layer of phosphorene. The absorption of a single oxygen atom on phosphorene can occupy numerous positions like interstitial, horizontal, diagonal ones [74]. As mentioned previously, phosphorene is stable at low oxygen concentrations.
1.3.2.1 Electronic Structure
An oxidation with a degree of 50% generates nine possible configurations among which only six are stable. As illustrated in Figure 1.9, the unit cell comprises four P-atoms assigned as P1, P2, P3, and P4 and two O-atoms, namely, OA and OB. Oxygen atoms bind to two P-atoms on the same side (more precisely, they are attached either to the up-side or to the down-side of the surface and or they are in opposite sides, namely . The index U and D referred to the up and the down side of the P-atoms. For example, to get , one should place the OA-atom up on P1 and the OB down on P2 (OB is in the opposite side of OA) forming a fashion on either side of the plane. The resulting new derivatives can be divided onto three main groups. The first group contains the structures and that have only dangling bonds P = O. In three conformers constituting the second group, dangling oxygen motif and bridging bond alternate, respectively. The third class concerns the configurations exhibiting only bridging bonds which are energetically less favorable (see [56] for more details). All the half-oxidized conformers exhibit a high buckling parameter confirming the anisotropic behavior of their properties. It was found, using different methods [24, 56], that half-oxidation of phosphorene (P4O2) allows to build a stable material.
Half O-functionalization influences significantly the electronic structure of phosphorene mono-layer as depicted in Figure 1.10. The oxidation induces a band gap modulation with the highest value observed in the bridge structures [56]. In all structures the band gap ranges from 0.54 (1.19) to 1.57 (2.88), calculated by GGA (GW) approximation. Moreover, a half O-functionalization tunes the band gap from direct to indirect in all the conformers, except P2OD which presents a direct band gap.
Figure 1.9 Configurations of 50% oxidized phosphorene: (a) dangling structures and (b) bridge structures.
Figure 1.10 GGA and GW band structures of half-oxidized structures.
POs can provide oxygen in its solid-phase to valve regulated Li-O2 batteries. In addition, when the Li atom is absorbed at the surface of the POs, it binds strongly to the O atoms indicating a strong ionic characteristic of the bond between oxygen and lithium [75]. The absolute values of binding energies of the Li atom adsorbed on the PO surface are greater than those of the Li atom on pure phosphorene, MoS2 and graphene [76–78]. POs promise high diffusivity owing to the anisotropy of POs cathode barrier that is reduced by half with respect with the armchair axis for Li diffusion on POs. Besides, Li-PO structures with a number of Li atoms lower than O atoms show stable discharge products for PO cathodes [75].
1.3.2.2 Optical Response
The optical absorption spectra of half oxidized phosphorene in Figure 1.11 show maximum values for dangling structures observed at 1.46, 1.71, and 2.62 eV in , and , respectively.
This absorption behavior is required for photodetector with high efficiency. In contrast, pics of spectra describing the bridge structures coincide with the ultraviolet part and the visible light, since the they are located at 1.81, 2.03, and 3.18 eV for , and , respectively. One deduces that 50% oxidation is an effective manner to enlarge the absorption range of phosphorene along the light spectrum [20].
Figure 1.11 Absorption coefficient of dangling structures (on the left) and bridge structures (on the right) of half-oxidized phosphorene sheets obtained using the GW-BSE methods.
Half oxidation is also used to modify the reflectivity of phosphorene as illustrated in Figure 1.11. Indeed, its maximum value in the UV region is around 38%, 50%, and 34% located at 8.21, 8.04, and 7.06 eV in , , and , respectively. In the visible part, these structures show a reflectivity lower than 15% indicating their potential use for transparent electronics. The situation is different for the dangling configurations P2OU, P3OD, and P4OD structures that reflect the visible light with a maximum value of 42%, 38%, and 39% found for the energy 1.48, 2.58, and 1.68 eV, respectively [20].
In contrast to pure phosphorene, the first peak of the optical absorption in all P4O2 structures is characterized by a dark exciton with long-lifetime. This result makes these new systems promising candidates as molecular sensors or applications in on-chip communication. Studying the excitonic effects of half-oxidized phosphorene conformers reveals that the wave function in the dangling phosphorene extends along the armchair direction, which is similar to pure phosphorene (see Figure 1.12). These results indicate that six half-oxidized phosphorene conformers are potential candidates for electronic devices and photovoltaic applications [20].
1.3.2.3 Strain Effect
Besides the high flexibility and strong anisotropic elastic properties of phosphorene, oxidation is so important to tune its elastic properties and extending the corresponding applications. Phosphorene half-oxides can be stretched becoming ideal for devices requiring flexibility [79]. Moreover, the degree of oxidation influences significantly the elastic parameters [30–80]. The polar plot of Young modulus and Poisson ratios reveals that the maximal values are attempted for armchair-strain resulting super flexible structures. However, it is hard to implement zigzag deformation direction that shows minimal values of elastic parameters (see Figures 1.13a, b). Importantly, the Poisson ratios of and conformers take negative values for some ranges of angles, which lead to an auxetic behavior (see [79]). Moreover, in all the conformers, the Poisson ratio is lower than 0.5, which correspond to incompressible materials. Stress-strain responses, under the armchair and zigzag tensile strain, show that half oxidation leads to much higher critical points, compared to pure phosphorene. It is found that both the dangling and bridge structures resist to a large axial deformation up to 27%–39% along the AC-axis with respect to the ZZ one. More precisely, the maximum values coincide with the dangling structures P4OU, P2OD, and P3OU which can resist a tensile strain up to 30%, 33%, and 39%, respectively, showing a high flexibility in the armchair direction. This result is owing to the high buckled honeycomb structures that exhibit these configurations in this direction. The mechanical flexibility of half-oxidized phosphorene make these structures an ideal candidate for wearable optoelectronic devices.
Figure 1.12 Excitons wave functions. Black balls represent the holes.
Figure 1.13 Part of polar plots of (a) Young modulus, (b) Poisson ratios.
Under half oxidation, the Debye temperature of phosphorene increases, with a maximum value reached in the ZZ-axis relative to AC. The high Debye temperature values indicate an important thermal conductivity in these new derivatives lattice lattice [79]. Furthermore, the curves describing the normal electrical polarization of the PO configurations in terms of applied strain are linear. With respect to pure phosphorene, the piezoelectric stress parameters increase under 50% oxidation while the piezoelectric strain coefficients d11 are three times lower than 2D BP [80].
When axial deformation is implemented, the electronic features of the POs become modulated. The band gap of dangling and bridge structures increases with low tensile strain, then it reduces to achieve a metallic state for large deformations. Besides, both groups of POs can maintain the semi-conductor behavior along the armchair direction for a strain ranging from 20% to 40% [81]. One can deduce that the adjustment of the phosphorene oxides features makes this class of materials potential candidates for advanced devices.
1.3.3 Surface Oxidation on Phosphorene
In contrast to the functionals H, F, and −OH which work like scissors by breaking down phosphorene into nanoribbons [82], a complete oxidation maintains the initial phosphorene configuration and shifts the lattice constants without breaking the Phosphorous bonds connecting the two P-half-layers, namely, the upper and lower ones [30].
1.3.3.1 Optoelectronic Features
At high concentration, oxidation leads to new derivatives of oxided phosphorene. As shown in Figure 1.14, up to fully oxidation, the interatomic P-P lengths increase to 2.32 and 2.37 Å while the direct gap located at point Γ reduces with respect to pure phosphorene. VBM is characterized by py orbitals of P- and O-atoms, while the P-s and O-pz orbitals dominate the conduction band minimum (CBM) [30]. Both interstitial and dangling oxygen form no states in the middle of the gap, while the horizontal and diagonal oxygen introduce levels in the gap, which deals with a deep acceptor state at near the conduction band. Furthermore, the planar phosphorene oxides exhibit a monotonic increase to reach a maximum value with a deoxidation degree of 0.25, then start to decrease to attempt the value of 0.6 eV in a fully oxidized PO structure. For the tubular structure, the band gaps take the values from 0.4 (1.62) to 5.56 (7.78) eV at PBE (HSE level) [32]. Interestingly, the GW corrected band gap shows that the increasing oxygen coverage leads to an increase in the band energy from 4 eV to 10 eV, indicating that the VBM and CBM part become more localized [83].
Figure 1.14 (a) Top and (b) side views of phosphorene oxides PO.
The application of electric field reduces the gap energy of PO to a minimum of about 0.4 eV for a field E = 1.5 V/Å. The band gap fluctuates also from direct found for 100% to indirect for O-concentrations of 12.5%, 25%, and 50 %. Also, the work function in phosphorene increases linearly with the increased of the oxidation degree. The calculated values for PO0.125, PO0.25, and PO0.5, are 4.9, 5.2, and 5.8, respectively, compared to PO that has 7.2 eV [30].
Under ambient conditions, phosphorene oxide is a stable material that did not exhibit any negative frequencies in its phonon dispersion curve [30]. Moreover, the simulation indicates that oxided structure is still robust and intact at low temperature, confirming its stability, while the material cut for large temperature values [84]. Unlike pure phosphorene, the phonon dispersion of PO exhibits three main regions as displayed in Figure 1.15b, namely, (i) the acoustic region, (ii) the middle region, and (iii) the high frequency range. Moreover, in contrast to the electron effective mass, the effective masses of holes are anisotrope [30].
Besides the band structure modification, the oxidation tunes also the optical features of phosphorene. In PO systems, the absorption spectrum reveals that the 1st absorption peak is located at 2.7 eV, in P4O2, it is also found that both phosphorus and oxygen atoms contribute in the transition and extend the wavefunction along the AC-axis (see Figure 1.16a). Further, in the P4O10 system, the absorption peak moves to high energy with a peak located at 7.0 eV. The wavefunction only localized on oxygen atoms adsorbed at the surface (see Figure 1.16b). This changes the binding energy Eb from −1.4 eV to reach −3.0 eV for P4O2 and P4O10. The electronic and optical band gap as well as the binding energy of P4O2 and P4O10 are very close to those of benzene [83].
Figure 1.15 Phosphorene oxide (a) band structure and density of states, (b) phonon dispersion curves and density of states.
Figure 1.16 Absorption spectrum and exciton wavefunction for the first transition peak for (a) P4O2 and (b) P4O10 structures.
1.3.3.2 Stress vs Strain
When the surface is oxidized, the electrons get transferred forming ions in phosphorene which influences mainly the mechanical response of the material describe by its stiffness against externally applied strains. It results that the oxidation changes the elastic moduli leading to a higher flexible structure [31]. This is also the case for reduced concentrations. Indeed, phosphorene with an oxidation degree of 12.5% can resist to a deformation up to 32% and 35% in AC- and ZZ-axes, respectively, which are higher than that corresponding to pure phosphorene [31]. Moreover, with respect to the pure material, the ideal strength in phosphorene oxide is reduced owing to the enhancement of interatomic distance in the oxide lattice [30, 31] in good agreement with the process of hydrogenating single-layer h-BN [85]. Therefore, the oxidation causes a two times reductions in the value of ideal strength [31].
For a tensile strain varying from −8% to 8%, the band gap in PO increases under compressive strain and decreases with tensile strain, ranging from 0.85 to 0.1 eV for the strain values of [30]. The variation of the gap width results only from the CBM since the VBM is not influenced by the elastic strength. For small tensile strain interval of [−0.006, 0.006], the linear change of polarizations of PO reveals two values of stress piezoelectric responses, namely, e11= 20.13 10−10C/m and e31= 4.06 10−10C/m that correspond to the piezoelectric coefficients given in [86]. All these results indicate that phosphorene oxide are excellent candidates for potential applications requiring the conversion of energy [86].
1.3.3.3 Thermal Conductivity
Compared with pure phosphorene (P), phosphorene oxide (PO) exhibits a much lower thermal conductivity over the whole temperature range [87]. Indeed, the values of the thermal conductivityfor both P and PO along the armchair axis, namely and are 2.5 times smaller than and along the zigzag one. At room temperature T = 300 K, the takes the value of 2.42 W/mK, which is very small compared to 65 W/mK reported for pure phosphorene as well as that of other 2D materials such as silicene (26 W/mK) [88] and MoS2 (34.5 W/mK) [89]. Low thermal conductivity renders PO an advantageous novel low dimensional candidate for high-performance thermoelectric materials [87].
To highlight the main responsible of such a low lattice thermal conductivity in PO, one should examine the various phonon modes. The puckered structure of PO allows more phonon-phonon scattering of the ZA mode with a contribution of 15% to 17%, while the longitudinal and transverse acoustic modes are the most dominant ones. Furthermore, the lattice thermal conductivity in a material results on the use of different phonon scattering sources. For the case of PO, only the phonon-phonon scattering is considered, since the other sources, such as Umklapp scattering, phonon-electron scattering, impurity effect and boundary effect are so negligible. As shown in Figure 1.17, the anharmonic relaxation times of as a function of frequency indicates that the phonon lifetime corresponding to three acoustic modes (ZA, TA, and LA) and the other higher modes of PO is more lower than that of pure phosphorene. This reduction is mainly owing to dangling bond connecting oxygen atom to its phosphorous neighbor which allow to O not only in the optical P-O vibration, but also vibrate along the in-plane directions together with phosphorous atoms contributing to the acoustic modes. It follows that this contribution is responsible for the acoustic phonon softening which decreases the thermal conductivity of PO [87].
Figure 1.17 Mode-dependent anharmonic phonon relaxation time for acoustic modes.
1.4 Conclusion
In this chapter, we have presented an overview of pure phosphorene, its geometric structure, its physical properties, its fabrication methods, and several applications. We have also shown thatowing to its puckered structure and its strong anisotropic electronic, mechanical, magnetic, and optical properties, phosphorene constitutes an ideal candidate for potential applications, including gas sensor, field-effect transistor, and solar cell application. Unstable under atmospheric conditions, we have reported phosphorene oxides and demonstrated how O-functionalization is a promising technique to enhance the features of this novel material.
Acknowledgment
Lalla Btissam Drissi et al. thank “Académie Hassan II des Sciences et Techniques-Morocco” for financial support.
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