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REVIEW ARTICLE OPEN Recent advances in synthesis, properties, and applications of

Meysam Akhtar1,2, George Anderson1, Rong Zhao1, Adel Alruqi1, Joanna E. Mroczkowska3, Gamini Sumanasekera1,2 and Jacek B. Jasinski2

Since its first fabrication by exfoliation in 2014, phosphorene has been the focus of rapidly expanding research activities. The number of phosphorene publications has been increasing at a rate exceeding that of other two-dimensional materials. This tremendous level of excitement arises from the unique properties of phosphorene, including its puckered layer structure. With its widely tunable , strong in-plane anisotropy, and high carrier mobility, phosphorene is at the center of numerous fundamental studies and applications spanning from electronic, optoelectronic, and spintronic devices to sensors, actuators, and thermoelectrics to energy conversion, and storage devices. Here, we review the most significant recent studies in the field of phosphorene research and technology. Our focus is on the synthesis and layer number determination, anisotropic properties, tuning of the band gap and related properties, strain engineering, and applications in electronics, thermoelectrics, and energy storage. The current needs and likely future research directions for phosphorene are also discussed. npj 2D Materials and Applications (2017) 1:5 ; doi:10.1038/s41699-017-0007-5

INTRODUCTION properties, including high carrier mobility, ultrahigh surface area, Since the discovery of in 2004 (ref. 1), there has been a excellent , and quantum confinement effect 9 quest for new two-dimensional (2D) materials aimed at fully have been well documented. However, the lack of band gap is a exploring new fundamental phenomena stemming from quantum serious limitation for the use of graphene in electronic devices. confinement and size effects. This quest has spurred new areas of Lately, another unique 2D material consisting of a monolayer, or research with rapid growth from both theoretical and experi- few layers, of black (BP) has been attracting attention. mental fronts aimed at technological advancements. Among BPs most remarkable properties include thickness-dependent band recently discovered 2D materials, phosphorene is one of the most gap, strong in-plane anisotropy, and high carrier mobility. Recent intriguing due to its exotic properties and numerous foreseeable reports have brought to light the highly encouraging prospects of using this novel 2D material in both nano- and optoelectronics. The applications.2 This review discusses recent advances in phosphor- key characteristics that make it so promising are high carrier ene research with special emphasis on: (i) fabrication and mobility, strong in-plane anisotropy, particularly the anisotropy of techniques for rapid identification of the number of layers; (ii) 10 electric conductance, and highly tunable band gap, which anisotropic behavior; (iii) band gap and property tuning; (iv) strain changes with doping, functionalization, and the number of layers engineering and mechanical properties; (v) devices and applica- from ~1.5 eV for a monolayer to 0.3 eV for bulk BP.11 The on/off tions; and (vi) future directions. ratio and the carrier mobility are also layer-dependent.12 Further- 2D materials composed of a single--thick or a single- 3–5 more, compatibility of phosphorene with other 2D materials in so- polyhedral-thick layer can be grouped into diverse categories. called van der Waals heterostructures can offer solutions for fi The rst is comprised of layered van der Waals solids such as important issues, such as surface degradation, doping, or control of fl atomically at graphene, h-BN, phosphorene, SiC, Si2BN, transition surface/interface electronic structure, and can also enable novel metal dichalcogenides (TMDs) (MX2 (X-M-X layer) where M=Ti, Zr, Hf, functionalities and devices with unique or unprecedented perfor- 6 V,Nb,Ta,Mo,W,andX=S,Se,Te), layered metal oxides such as mance. In the last 2 years, since the first experimental demonstra- 13 vanadium oxide and Sb2Te3, etc. The second category is comprised tion of phosphorene, research activities related to this material of layered ionic solids, which consist of a charged polyhedral layer have rapidly expanded. The timeline reflecting this progress and sandwiched between hydroxide or halide layers by electrostatic how it compares to other 2D materials is summarized in Fig. 1a. The forces such as La0.90Eu0.05Nb3O10,KLnNb2O7,Eu(OH)2.5(DS)0.5. Finally, rapid take off in research activities on phosphorene is evident by the third category includes surface-assisted nanolayered solids such the number of related publications that appeared in the last 2 years. as , , stanene, etc. In a different approach, Gibaja and co-worker recently isolated few-layered antimonene by both liquid exfoliation7 and mechanical cleavage8 methods. PHOSPHORENE BASIC PROPERTIES Graphene, the first demonstrated and the most explored 2D Phosphorene is a single atomic layer of BP, one of four known material, has a crystalline form of sp2 packed in a that shows semiconducting properties single-atom-thick planar honeycomb network. Its superior and was first discovered more than a century ago by Bridgman.14

1Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA; 2Conn Center for Renewable Energy, University of Louisville, Louisville, KY 40292, USA and 3Exscien Corporation, 1044 East Chestnut Street, Louisville, 40204 KY, USA Correspondence: Jacek B. Jasinski ([email protected]) Meysam Akhtar and George Anderson contributed equally to this work Received: 22 September 2016 Revised: 20 January 2017 Accepted: 6 February 2017

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Fig. 1 a Comparison of 2D materials in terms of number of published articles during a decade (The vertical axis is in log scale). KEY WORDS: "Graphene", "Hexagonal ", "Phosphorene", "Mxenes", "MoS2 or WS2 or MoSe2 or WSe2 or MoTe2", "Silicene or Germanene". SEARCH ENGINE: Google Scholar. b Crystal structure of phosphorene (side and top view). c Structures of three predicted polymorphs of phosphorene. d Crystal structures of monolayer of MoS2 (left) and graphene (right). Images c adapted with permission from ref. 26 [Nature Publishing Group], and ref. 34 [American Chemical Society]

BP, similar to and other layered materials commonly spectral dependence, is also observed for electron–photon and referred to as van der Waals materials, is comprised of 2D sheets electron– interactions.31 The anisotropy of phosphorene’s that are vertically stacked with respect to one another. However, magnetic properties has also been predicted. In particular, for unlike graphite, in-plane bonding in BP is due to sp3 hybridiza- edges terminated along the ZZ direction, a dissipative quantum tion.15 Each phosphorus atom is bonded to three adjacent Hall effect has been calculated.32 Figure 2 lists some of the most phosphorus atoms; therefore, each “p” orbital retains a lone pair significant anisotropic properties of phosphorene. Figure 2a, b of electrons. The crystalline structure of layered bulk BP is show effective masses of the electron and hole in 2D phosphor- orthorhombic with Cmca (#64) (ref. 16). Its structural ene as a function of strain applied along the zigzag (x) and and other properties have previously been discussed.11, 17 Due to armchair (y) directions. It is evident that effective masses of charge the sp3 hybridization, phosphorene does not form atomically flat carriers depend on the direction of charge transfer and also can sheets like graphene.15 Instead, they form a puckered change significantly by strain. However, the change of effective honeycomb-structured layers18 (Fig. 1b). These layers are stacked masses also depends on the direction of applied strain, which is a together by weak van der Waals forces.19 As a result of the further indication of the anisotropic nature of phosphorene. puckered structure, each single layer of the honeycomb network Moreover, as it can be seen, the sharp changes in the effective contains two atomic layers where the distance between the two masses in Fig. 2a, b occur around critical strains for the direct- nearest atoms (d1 = 2.224 Å) and the distance between top and indirect band gap transitions (e.g., the effective masses in the ZZ bottom atoms (d2 = 2.244 Å) are slightly different. The values of d1 direction in Fig. 2a have a sudden change around critical strain and d2 are very close to each other because of the covalent bonds εx~8%). Additionally, based on effective masses in relaxed between phosphorus 3p orbitals.20 The electronic band gap of phosphorene (zero applied strain), one can conclude that AM is phosphorene has been predicted to be 1.5 eV (ref. 11), which is preferred direction for charge carriers’ transport as the effective much higher than the 0.3 eV value of bulk BP.21, 22 Theoretical masses are significantly smaller in this direction than ZZ direction. calculations indicate that the change in band gap is due to a loss In Fig. 2c, Raman modes show significant shifts as a function of of interlayer hybridization in few layer systems.12 Main phonon uniaxial strain. However, the rate of frequency shift depends on fi 1 modes of phosphorene are de ned as Ag (out of plane), B2g and the direction of applied strained (i.e., ZZ and AM). The Atomic 2 23 Ag (in-plane). These modes have been predicted to be found at vibration of modes, orientation and amplitude of strain, and −1 23 around 368, 433, and 456 cm , respectively. consequently variations of bond lengths are parameters that can Due to its unique topological structure and differences between determine the frequency shift in Fig. 2c. the armchair (AM) and zigzag (ZZ) directions, phosphorene 1 In Fig. 2d, e, prominent peaks for relaxed phosphorene (Ag,B2g, 2 −1 displays strong in-plane anisotropy with many properties in these and Ag) are located at 368, 433, and 456 cm , which are in good two principal directions drastically different. Anisotropy is strongly agreement with experimental results. These two plots shows that reflected in effective masses24 with an eight times increase in hole relative Raman peak positions and spacing can give information effective mass along the ZZ direction compared to the AM about the strain condition (i.e., amplitude and direction). As shown direction.12, 25 Phosphorene also has strong anisotropic phonon in Fig. 2d, all Raman peaks shift monotonically as a function of 2 dispersions that to much larger group velocities along the ZZ strain along AM direction. B2g, and Ag show red shift under 26–28 1 direction than those along the AM direction. This is the reason stretching and blue shift under compression while Ag modes shift for a strong orientation-dependent shear modulus29 and thermal in the opposite direction in both stretching and compression. In 30 1 transport. Strong anisotropic behavior, showing thickness and other words, energy spacing between Ag and B2g reduces under

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Fig. 2 Anisotropic properties of phosphorene. a, b Effective masses of the electron and hole under uniaxial strain “ε” in both zigzag “x” and “ ” 1 2 armchair y directions, respectively. c Frequency of Raman modes of phosphorene (Ag, B2g, and Ag) as a function of uniaxial strain along zigzag and armchair directions. d, e Raman spectra of phosphorene as a function of uniaxial strain along zigzag and armchair directions, respectively. f, g Direction dependence of Young’s modulus and Poisson’s ratio of phosphorene. h of phosphorene along zigzag (μx) and armchair (μy) directions as a function of biaxial strain at room temperature. Images a–b, c–e, f, g, and h adapted with permission from ref. 79 [American Physical Society], ref. 23 [American Institute of Physics], ref. 33 [American Institute of Physics], ref. 72 [Royal Society of Chemistry], and ref. 128 [American Chemical Society], respectively compression and rises under stretching strain in the AM direction. due to the phosphorene puckered structure that makes it softer 2 In the case of strain along the ZZ direction (Fig. 2e) B2g and Ag along the AM than the ZZ direction. Calculated orientation modes shift monotonically, i.e., blue shift under compression and dependence of Young’s modulus is illustrated in Fig. 2f. The red shift under stretching, which is in contrast with AM strain. Young’s modulus varies between a maximum of 166 GPa and a 1 However, Ag modes exhibit redshift under both compression and minimum of 44 GPa, which is along the ZZ and AM directions, 1 stretching strain. Interestingly, energy spacing between Ag and respectively. Furthermore, the Young modulus has an average B2g shows clear potential to justify the direction of the strain. Their value of 94 GPa for all orientations. energy spacing reduces under stretching and increases under Figure 2g presents the direction dependence of the Poisson’s fi 2 compression strain. Additionally, a signi cant increase of B3g peak ratio of phosphorene as another strong indication of anisotropic intensity under stretching strain indicates the impact of ZZ strain nature of phosphorene. The Poisson’s ratio along ZZ and AM on the intensity of Raman modes. Comparing Fig. 2d, e, one can directions is 0.703 and 0.175, respectively. However, it has the see Raman modes are more sensitive to ZZ than AM strain. This is minimum value of 0.064 around θ = 47.5°. Figure 2h shows how

Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2017) 5 Recent advances in synthesis, properties, and applications M Akhtar et al. 4

Fig. 3 Determination of phosphorene thickness. a image of a monolayer phosphorene. b Theoretical and PL-based 1 phosphorene band gap as a function of the number of phosphorene layers. c Intensity ratio of Ag Raman peak of phosphorene and Raman peak as a function of thickness. d Phase-shifting interferometry (PSI) image of phosphorene flake to determine the number of layers. e – 2 Raman spectra of bulk black phosphorus and 2 5 layers thick of exfoliated phosphorene. f Ag and B2g Raman modes of phosphorene (right axis) and difference of them (left axis) as a function of number of layers of phosphorene. g Thickness dependence of intensity ratio of Raman 1 2 18 modes Ag and Ag (ref. 50). h Layering number dependence of electron diffraction intensity ratio of (101) and (200) planes. i Optical contrast of 1–5 layers of phosphorene. Images a, b, c, and h, d, and e–g and i adapted with permission from ref. 12 [American Chemical Society], ref. 59 [American Chemical Society], ref. 18 [IOP Publishing], ref. 61 [Nature Publishing Group], and ref. [50] Springer, respectively

the applied biaxial strain modifies the calculated electron mobility mobility of the relaxed state at room temperature estimates of phosphorene at room temperature (T = 300 K). Furthermore, around 2200 cm2 V-1 s-1, which is much smaller than that at T =4K under around 3% biaxial strain, the mobilities along ZZ and AM (1.6 × 105 cm2 V-1 s-1). However, smaller mobility at room tempera- directions undergo the critical transition and, after this transition ture is due to additional effects that we may not need to consider (switch), the ZZ mobility becomes larger than the AM mobility in at low temperature, such as electron-phonon scattering and band contrast with the unstrained state of phosphorene. The AM average.33

npj 2D Materials and Applications (2017) 5 Published in partnership with FCT NOVA with the support of E-MRS Recent advances in synthesis, properties, and applications M Akhtar et al. 5 Unique properties of phosphorene can be further enhanced in solvent, the sample remained intact under ambient conditions for allotrope structures. Based on computational work, a number of several days, as the solvation shell still encapsulates it. Therefore, new phosphorene polymorphs/allotrope with buckled honey- in the liquid exfoliation approach, selecting a solvent that forms comb structure or non-honeycomb structure has been predicted the solvation shell in addition to having sufficient surface energy (e.g., non-honeycomb: ζ-P and η-P; honeycomb: β-P that is called can enhance the quality of the production. blue phosphorene and γ-P).34 The prediction of several poly- morphs adds more variety to the family of phosphorene-based materials and enables possibilities of potential novel phenomena DETERMINATION OF THE NUMBER OF LAYERS and applications.34–36 Several methods have been proposed in the literature for determination of the number of layers in phosphorene and BP flakes. However, to prevent unwanted reactions and material PHOSPHORENE FABRICATION METHODS degradation, fast and accurate characterization techniques are Since bulk BP is classified as a van der Waals material, similar top- needed. Atomic force microscopy (AFM) is one of the primary down techniques as those employed to produce graphene using tools allowing measurements of the thickness down to one atomic graphite as the precursor can also be applied to make layer, which for phosphorene is ~0.9 nm (ref. 12). However, the phosphorene from BP. In such approach, bulk BP needs to be drawback of using AFM is that the analysis requires extended first obtained. Usual synthesis routes include high pressure,14, 37, 38 periods of time. An AFM image of a monolayer exfoliated 39 40, 41 high-speed ball milling, and short transport reactions. phosphorene on a SiO2-coated silicon wafer with a thickness of Polycrystalline BP can also be grown from red phosphorus under ~0.85 nm is shown in Fig. 3a. This measured thickness is slightly high pressure in an anvil press, as reported recently.42 The larger than the theoretical value of 0.6 nm for a monolayer of successful conversion of red phosphorus to BP has also been phosphorene. However, this slight discrepancy is acceptable as it 43 12 demonstrated through sonication alone, which further lowers is observed in other 2D materials with SiO2/Si substrate. the cost and simplifies the production process. Photoluminescence (PL) is another determination method59 The interlayer cohesive energy of BP has been predicted at-151 and is based on the fact that the band gap, which is represented meV per atom,44 which is somewhat larger than that of graphite at by the excitonic emission in the PL spectrum, increases with the about −35 meV per atom.45 However, mechanical13 and liquid decrease of the number of layers. However, recent PL excitation exfoliation46 of BP have already shown encouraging results. In spectroscopy suggests a quasiparticle band gap of 2.2 eV, 2014, two separate research teams reported the first successful suggesting an exciton binding energy of around as high as 0.9 isolations of a single atomic layer of BP using mechanical eV (ref. 60). exfoliation.12, 47 Samples of a few layer-thick phosphorene can Figure 3b illustrates the high dependence of phosphorene band also be produced using liquid exfoliation.46, 48 Other techniques gap on its layer number. Experimental band gap values in Fig. 3b such as pulsed laser deposition,49 and plasma thinning50 have also are based on the PL spectroscopy measurements with 532 nm been demonstrated to fabricate few-layer phosphorene using BP laser for excitation. Additionally, during the PL measurement, a as the precursor material. flow of gas was used to avoid degradation of few-layer One of the major issues hindering phosphorene application in phosphorene. PL peaks were detected at 1.29, 0.98, 0.88, and 0.80 modern day devices is its lack of stability under ambient eV for two to five layers, respectively. The band gap drops by an conditions. Due to the two uncoupled pairs of electrons in each increase in the number of layers, which is in agreement with the phosphorus atom (caused by sp3 hybridization) and the enor- trend predicted by theoretical values of the band gap (Fig. 3b). mously high surface to volume ratio, phosphorene is highly Several Raman spectroscopy-based calibration methods have reactive to combinations of oxygen, water, and light.51 Under such also been developed18, 50 as well as a method based on electron conditions, it undergoes degradation where oxidized phos- diffraction that uses a thickness dependent intensity ratio phorus52 and species are formed.51 The most between the (101) and (200) diffraction spots.18 Thickness of likely mechanism of the chemical degradation is oxidation few-layer phosphorene on silicone substrate can be determined followed by exothermic reaction with water.53 by the intensity ratio between the silicon Raman peak and 1 While this is still in an early development stage and more efforts phosphorene Ag peak as shown in Fig. 3c. are required, several approaches, especially of using protective Recently, a fast, low-intensity and less destructive LED light- capping layers,13 have already been evaluated for achieving based phase-shifting interferometry (PSI) method has also been higher stability phosphorene including encapsulating phosphor- developed.61 Measurement of the layer number of phosphorene ene by (ALD) of a dielectric layer (e.g., by the optical interferometry method is rapid, noninvasive, and alumina),52 oxidizing top layers of phosphorene by oxygen plasma accurate (Fig. 3d). In this approach, the optical path length (OPL) dry etching to protect remaining phosphorene along with ALD of can be measured by PSI and evaluate the digitized interference alumina,54 van der Waals passivation by 2D graphene and/or h- pattern. Ultimately, by knowing the OPL value for each layering BN,55 and solvent exfoliation with anhydrous organic solvents step change, the number of layers can be determined. A detailed such as N-methylpyrrolidone (NMP).56 When using NMP, Kang summary of layer number determination techniques is depicted in et al. realized that residual NMP protects the phosphorene against Fig. 3. Figure 3 represents the Raman spectra of a few different fast degradation and postulated that this is potentially caused by thicknesses of exfoliated phosphorene. Thickness dependence of 2 NMP encapsulation or intercalation. Very soon after this work, Guo Ag and B2g modes is shown in Fig. 3f. The frequency difference of et al.57 reported that using saturated NaOH NMP solution for liquid these modes varies between 27.7 and 31 cm−1 for bulk and exfoliation of bulk BP drastically increases the stability of monolayer, respectively. As illustrated in Fig. 3g, the intensity ratio − 1 2 phosphorene. They found that OH ions were adsorbed onto of Ag to Ag exhibits a monotonic behavior with respect to change phosphorene and prevent water from degrading the material. The in the number of layers, therefore, can be used as another means results of their characterization show a clear improvement in to determine the number of layers. As can be seen in Fig. 3i, the comparison to reports by Kang et al. work. The significant role of optical contrast of the phosphorene drops noticeably as the solvents to stabilize the liquid-exfoliated phosphorene have been number of layers decrease. This dependence of optical contrast to emphasized in other research by Hanlon et al.58 They also the number of layers of phosphorene offers a practical resource of suggested that some solvents (e.g., N-cyclohexyl-2-pyrrolidone) thickness measurement. can protect BP against oxidation via encapsulating the phosphor- Subsequently, each mentioned approach has merits and ene during the liquid exfoliation. Interestingly, after removing the weaknesses. In particular, AFM and Raman spectroscopy have

Published in partnership with FCT NOVA with the support of E-MRS npj 2D Materials and Applications (2017) 5 Recent advances in synthesis, properties, and applications M Akhtar et al. 6 been routinely used to reliably determine the sample thickness of heterostructures are promising systems since the anisotropy of TMD with monolayer precision. However, these the electric polarization of phosphorene has been calculated to two methods are not reliable for the identification of very-few- disappear in such structures.68 Of interest are also interfaces layer phosphorene (one or two layers) under ambient conditions. between phosphorene and high-k dielectric materials due to their The scanning rate of AFM is slow compared to the fast ability to improve the performance of scaled electronics. For degradation of very-few-layer phosphorene under ambient example, heterostructures of phosphorene and hafnium dioxide conditions. AFM can also introduce potential contaminants that (HfO2) have been found to induce topological changes in might affect further characterizations on the same sample. Unlike phosphorene at elevated temperatures, which could alter device in TMD semiconductors, where Raman mode frequency has a performance.69 Highly effective photovoltaic systems, with exciton monotonic dependence on the layer number, phosphorene has a recombination times up to 1.08 ps and over 98% quantum non-monotonic dependence owing to the complicated Davydov- efficiency, have also been predicted theoretically in heterostruc- related effects. Moreover, the relatively high-power laser used in tures of phosphorene and TiO2 (ref. 70). Raman spectroscopy can significantly damage the phosphorene samples. Due to the aforementioned challenges, optical inter- ferometry offers a rapid, noninvasive, and highly accurate EFFECTS OF STRESS AND MECHANICAL PROPERTIES approach to determine the layer number.61 Strain engineering or applications of pressure can also be used to modify properties of BP or phosphorene. At high pressures, BP undergoes crystal structure transitions, first from an orthorhombic PROPERTY TUNING: HETEROSTRUCTURES, DOPING, SURFACE to a rhombohedral structure at ~5 GPa and then to a simple cubic FUNCTIONALIZATION, AND COMPOSITES structure at ~10 GPa (ref. 71). Lower pressures or strains have also Figure 4 summarizes phosphorene band gap variation for various significant impact on electronic and other properties of phos- methods reported in the literature. As mentioned, the band gap phorene. For example, the application of uniaxial stress can break can be increased by reducing the number of layer from the bulk the symmetry and enhance in-plane anisotropy.72 Other effects value of 0.3 to ~1.5 eV for a monolayer. However, more research include electron–phonon coupling enhancement,73 superconduc- has recently been focused on tuning the band gap and also tivity,74 to metal transition,75 band gap modifica- altering other properties of phosphorene through functionaliza- tion,15 and direct-indirect band gap transition12 (Fig. 5). There tion and doping. It has been found, for example, that oxygen have been several theoretical studies on the effect of strain on the adsorbs strongly onto the reactive edges of phosphorene and can electronic structure of single- and/or few-layer phosphorene. increase its stability.62, 63 Similarly, covalent diazonium functiona- density functional theory (DFT) calculations by Ju et al.76 of few- lization of phosphorene suppresses its chemical degradation and layer (1–5 layers) BP have indicated that compressive strain can significantly improves its stability.64 Other studies have found that result in a semiconductor-metal transition (SMT), whereas the the addition of Cu adatoms65 or chemical doping with benzyl biaxial tensile strain only affects the band gaps and does not affect viologen66 can be used as an effective electron doping strategy to essentially the electronic properties of the system. Based on the obtain n-type conductivity. It has also been found that doping, band structure and charge density calculations, they have even with non-magnetic atoms such as oxygen, sulfur, or predicted that the compressive strain to the reduction of selenium, can induce magnetism in phosphorene.63 This is P–P bond length, downward movement of the conduction band, believed to be due to that fact that such dopant atoms only and increased delocalization of states around the Fermi level, bond to two phosphorus atoms leaving an unpaired electron. which at the critical compressive strain are essential reasons of Similar effects have been calculated for blue phosphorene.67 SMT. Indeed, the calculations have shown a reverse relation Another means of affecting properties of phosphorene is by between the critical compressive strain for the SMT and the applying external electric fields which, in some cases can even thickness of few-layer BP. In a similar DFT study of bilayer 75 lead to semiconductor-to-metallic transitions.68 Combining phos- phosphorene, Manjanath et al. found strong interlayer interac- phorene with other materials and forming heterostructures or tions between phosphorpus pz orbitals and a reversible SMT at interfaces has also being explored as means of altering material ~13% of normal compressive strain. They also observed a unique properties. For example, graphene/phosphorene stacked band gap modulation pattern leading to a direct-indirect band gap transition at ~3% of strain (Fig. 5c). Furthermore, the study has suggested high structural integrity at relatively high strains, as demonstrated by the absence of negative frequencies in phonon 1 2 spectra. Raman active modes (Ag,B2g,and Ag) have been found to increase linearly with normal compressive strain, with the highest 1 increase rate for the out-of-plane Ag mode due to strong interlayer interaction (Fig. 5a). In another study, Seixas et al.77 have analyzed the effects of uniaxial compression on the optical response of phosphorene and the exciton stability. They have found that under high pressure, the material gradually undergoes the transition into an indirect gap semiconductor and eventually into a semimetal. They have also shown that uniaxial strain affects effective masses and, in consequence, the exciton anisotropy and binding strength. They have calculated the exciton binding energy of 0.87 eV for 5% of strain, which is among the highest value ever reported for 2D materials. Additionally, they have suggested that due to its strong and linear strain dependence, luminescence peak can be used to probe directly the strain state of single layer phosphorene as an alternative to using Raman peaks, which are weakly affected by AM strain.23 However, the use of PL as an experimental probe to study strain in phosphorene may not be reliable due to the lack of proper understanding of the origin of Fig. 4 Energy band gap engineering of phosphorene129–133 the PL in the material. It is well known that PL response due to the

npj 2D Materials and Applications (2017) 5 Published in partnership with FCT NOVA with the support of E-MRS Recent advances in synthesis, properties, and applications M Akhtar et al. 7 excitons and trions strongly depend on the dielectric environ- mobility of up to 6000 cm2 V−1 s−1 and quantum Hall effect in ment, defects, surface states etc. Consequently, many important high-quality van der Walls heterostructures.82 Recent studies have fundamental properties, such as exciton and trion dynamics, also found that trions and excitons in phosphorene can be tuned remain underexplored.61 The origin of the reported emissions due by using a topography set-up.83 It is noted that trions to highly anisotropic bright excitons and quasi particle band gap can be used in spintronic-based devices due to their non-zero in phosphorene are a still topic of debate.60 This makes PL a less spin.83 By having more control over hole–electron interactions, reliable technique to directly probe the strain state of single-layers efficient optoelectronic and spin-based devices can be fabricated. of phosphorene. In high-quality phosphorene-based heterostructures with negative Phosphorene shows unique mechanical properties, which make compressibility, a many-body effects strong correlations lead to an it a promising novel material for nanoelectromechanical systems enhanced capacitance in 2D electronic systems, suggesting and devices.78 It can sustain tensile strains of up to 30% (refs 33, potential for low-power nanoelectronics and optoelectronics.82 79), which is a higher value of those reported for graphene or The anisotropy of transport in phosphorene can also be single layer MoS2 (ref. 33). Phosphorene has a negative Poisson’s incorporated into devices. FET device performance and speed ratio that is attributed to its unique puckered structure.80 It shows may be increased by orientating the conducting channel along higher ductility for the AM direction compared to the ZZ direction the AM direction.25 The AM direction has lower effective mass, and good mechanical flexibility, as indicated by its Young’s which increases ON current.25 based on heterostruc- modulus value lower that those reported for graphene and single tures with high-k dielectrics have shown ~8 times increase of the 84 layer MoS2 (ref. 81). ON current when utilizing phosphorene’s anisotropy. In addition Based on the remarkable mechanical properties, phosphorene to several phosphorene-based transistors based on in-plane has a potential as an efficient filler to reinforce and transport geometry, first high performance structures with a composites. For example, incorporation of phosphorene (0.3 vol%) vertical FET geometry, such as a graphene/phosphorene van der into polyvinylchloride (PVC) substantially enhanced its mechanical Walls heterostructure,85 have also been realized recently. Figure 6a, properties. The remarkable improvement of all mechanical illustrate the application of phosphorene in a tunable parameters PVC after phosphorene reinforcement such as Yang photoinduced-carrier-transported FET.86 modulus, composite strength, and tensile toughness make it a The essential factors of materials for transistor applications great competitor for graphene reinforcement.58 include high charge carrier mobility, high conductivity, high on/off ratio and low off-state conductance. Comparing all these 2D materials, the carrier mobility of graphene can reach to 200,000 APPLICATIONS OF PHOSPHORENE cm2 V−1 s−1 (ref. 87), but it difficult to switch on/off due to its zero Electronic devices and sensors band gap and semimetallic nature, which limits its use as a digital 88 Some of the first studies of phosphorene-based devices involved logic device. TMDs layers such as MoS2 usually have band gap studying the basic properties of field effect transistors (FETs).12 around 1.5 eV can provide high on/off ratio, the low carrier The hole mobility in phosphorene is over 1000 cm2 V−1 s−1 (ref. mobility (200 cm2 V−1 s−1, even lower) limits the performance on 10), whereas phosphorene 2D electron gas shows exceptional Hall transistor applications. Phosphorene seems to satisfy all the key

Fig. 5 a Raman frequencies as a function of normal compressive (NC) strain for three Raman active modes. b The stress dependence of strain for a monolayer phosphorene under isotropic strain. c Band gap change as a function of strain for Perdew Burke-Ernzerhof (PBE), HeydScuseria-Ernzerhof (HSE) and G0W0 methods. d Band gap of the monolayer phosphorene (MLP) as a function of the strain. Images a, b, c, and d adapted with permission from ref. 75 [IOP Publishing], ref. 134 [IOP Publishing], ref. 132 [American Physical Society], and ref. 135 [IOP Publishing], respectively

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Fig. 6 Thermoelectric (a–c), electronic (d–f), and sensor (g–i) applications of Phosphorene. a A schematic illustration of an electric-double- layer transistor (EDLT) for thermoelectric measurements. b Tunable thermoelectric power of a BP-EDLT as a function of gate voltage at T = 210 K(inset: the optical microscope image of the real thermoelectric device of a BP flake with a thickness of 40 nm). c Theoretical calculations of thermoelectric power Sx and Sy for single-layer, bilayer, and five-layer phosphorene and bulk BP (from top to bottom) as a function of volumetric carrier density dependence at T = 210 K. d top: Schematic image of a BP/TiOx transistor under illumination (λ = 365 nm, power = −2 3.5 × 10–4 μW μm ); bottom: Mechanism of photoinduced charge transfer at the TiOx/BP interface. e Evolution of conductivity vs. gate-voltage curves for the TiOx/BP FET with increased irradiation time. The inset is the one for a pristine BP under the same illumination condition. f Recovery process of photoinduced doping by applying a positive bias. Log-scale curve of the device after photodoping was given in the inset. g Black phosphorus nanosheets current–voltage (I–V) characteristics at different relative humidity (RH), h sensitivity vs. RH plot (inset top: schematic diagram of the typical transistor device; bottom: optical photograph of the real transistor device), i typical current-time plot for three cycles of humidity switching between 32 and 97%. Images a–c, d–f, and g–i adapted with permission from ref. 136 [American Chemical Society], ref. 86 [American Chemical Society], and ref. 137 [American Chemical Society], respectively

factors for a transistor material. The band gap of 0.3–2 eV ensures emission over the Schottky barrier determines charge transport a large on/off ratio and its carrier mobility has been found over at high temperatures, whereas hopping transport dominates only 1000 cm2 V−1 s−1 (ref. 47), which can produces reasonably fast at low temperatures.85 operations. Phosphorene-based device structures show high photo- 2D Mott’s variable range hopping is a dominant in-plane sensitivity and high-performance broad spectral range photo- electrical and thermoelectric transport mechanism.89 For the out- detectors have been demonstrated.90 Solar cells have also been of-plane electronic transport in hetereostructures, thermionic suggested, as energy conversion efficiencies as high as 20% have

npj 2D Materials and Applications (2017) 5 Published in partnership with FCT NOVA with the support of E-MRS Recent advances in synthesis, properties, and applications M Akhtar et al. 9 been predicted for heterojunctions involving phosphorene with possible to reach very large ZT values.101–104 Theoretical calcula- hydrogen and fluorine.91 Experimentaly, first in-plane p-p junc- tions indicate that photocurrent response can be used to measure tions have recently been fabricated using chemical doping and its thermoelectric properties of BP indirectly. As calculated by Hong considerable performance has been shown.66 et al.105, the generated electron–hole pairs contribute to photo- Due to the lone electron pair on each phosphorus atom in current, which leads to the voltage difference and the Seebeck phosphorene, it is also a very promising material for gas sensing coefficient difference between BP and the electrode. The study by applications.92 Organic such as tetracyano-p- Low et al.106 has demonstrated that the photocurrent is quinodimethane (TCNQ) have been predicted to act as acceptors dominated by thermally driven thermoelectric and bolometric once absorbed onto phosphorene.93 TCNQ provides shallow processes rather than the photovoltaic effect. Recently, Flores states near the valence band, which enhances p-type conductivity. et al.107 have been able to measure thermoelectric properties On the contrary, the doping with electron donor tetrathiafulvalene directly. Figure 6 shows three recent examples of phosphorene does not enhance n-type conductivity, as the introduced donor devices. states are too deep into the band gap. Only by applying out of fi plane electric elds or in-plane tensile strain n-type conductivity Energy storage can be enhanced in this case. − BP has a high theoretical Li-storage capacity of 2596 mAh g 1 (ref. First principal studies have found that for gas species containing 108, 109), but suffers from the large volume change of ~300% and nitrogen-based molecules, transport properties in phosphorene rapid capacity loss during charging-discharging cycling when may be altered by as few as one .94 Single molecule used as an anode material in Li-ion batteries.110 Phosphorene detection would render phosphorene as one of the most sensitive does not show such problems and can also provide additional gas sensors ever created. Charge transfer between physisorbed advantages thanks to its 2D structure properties. It shows the gas molecule and phosphorene can be enhanced by introducing − reversible capacity of ~433 mA h g 1, low open circuit voltage, structural ripples, as the surface curvature increases the interac- small volume change, and good electrical conductivity.111 The tion between the molecule and phosphorene atoms.95 The diffusion of Li in phosphorene is ultrafast and anisotropic112 (1.6 × introduction of dopants into phosphorene is another method of 109 times faster along the ZZ direction than along the AM increasing gas-sensing abilities. For example, it has been direction, and 260 times faster than in graphite). Energy barriers calculated that for Ca doped or Ca-decorated phosphorene, the for the Li diffusions are very low (~0.09 eV)112, 113 suggesting binding affinity towards gas molecules increases.96 Ultrahigh excellent rate capabilities. Due to significant energy barriers sensitivity and layer-dependent sensing performance of difference (Fig. 6a), Li diffusion takes place mostly between phosphorene-based gas sensors have been reported by perform- grooves.11 ing the testing in an air-tight chamber.97 But this poses many Relatively weak (~1.9 eV) binding energy of Li in phosphorene practical problems for it to be implemented a real gas sensor. can be significantly increased (even by ~1 eV) by the introduction However, few-layer BP nanosheets, produced by liquid phase of intrinsic point defects.11 However, defects can also cause a exfoliation under ambient conditions in solvents such as N- significant increase of Li diffusion barriers, like in the case of cyclohexyl-2-pyrrolidone (CHP) have been found to be surprisingly Stone-Wales defects, which increase the diffusion barrier for the stable probably due to the solvation shell protecting the ZZ direction from −0.17 to 0.49 eV. Defects, however, can also lead nanosheets from reacting with water or oxygen.58 The reactions to new diffusion channels and effectively enhance the Li diffusion. are believed to occur only at the nanosheet edge. Biological For example, recent DFT study has shown that vacancy can allow agents have also been shown to have an impact on the electrical for the Li atoms to diffuse between two adjacent grooves with a resistance of phosphorene.98 For example, a lower detection limit − low energy barrier of 0.13 eV (ref. 11). The binding energy of Li can of about ten ng ml 1 for human immunoglobulin G, which is one also be enhanced in heterostructured composites. DFT calcula- of the main antibodies found in humans, has been reported,98 tions for phosphorene/graphene composite indicate a significant suggesting that phosphorene-based sensors may be used to increase in binding energy to 2.6 eV without affecting the high detect immune system disorders in humans. mobility of Li within the layers.11 Electrochemical performance of phosphorene is predicted to be Thermoelectric applications further improved in nanoribbons especially, in ZZ nanoribbons, All electronic devices generate heat during operation. Therefore, it where a moderate working voltage (0.504–0.021 V), high capacity is advantageous to utilize the thermoelectric materials as the FET (541 mA h g−1), and fast charge/discharge rate is calculated.114 channel to find a high efficient chip-cooling method by Compared to phosphorene (1.98 eV), there is a moderate converting the waste dissipation heat to electrical energy and enhancement of the Li binding energy for AM (2.18–2.50 eV) also to protect the device against performance degradation and and ZZ (2.07–2.52 eV) phosphorene nanoribbons (PNRs) due to breakdown. It has been shown that direct conversion between the presence of unique edge states. However, a strong anisotropy thermal and electrical energy using thermoelectric properties can of the Li diffusion, with a much lower barrier for the diffusion play an important role to answer future energy and environmental within two neighboring grooves (0.09 eV) than across a grove concerns. (0.65 eV), is also calculated for the nanoribbon morphology.115, 116 Efficient conversion between thermal and electrical energy can In addition, similarly, to pristine phosphorene, room temperature be realized by low thermal and high electrical conductivity at the diffusion constants inside the groove in AM and ZZ nanoribbons same time. Subsequently, phosphorene, with much lower thermal are about 109–1011 times larger than between two adjacent conductivity than graphene, has great potential for novel grooves. thermoelectric applications.99 It also shows strong thermal In addition to LIBs, phosphorene has also been considered for anisotropy and the orientation-dependent figure of merit ZT other electrochemical energy storage applications, such as all- value that is larger along the AM than the ZZ direction. However, solid-state supercapacitors117 and Na ion batteries (NaIBs),118 the Seebeck coefficient is practically isotropic28, 100–102 (Fig. 6a). which are considered safer than LIBs and use more abundant Na Preferred electrical and thermal transport are orthogonal in ions. The promise of using phosphorene for NaIB anode materials phosphorene and large ratios of electrical over thermal con- comes from the fact that, unlike the traditional, carbon-based ductivity can be achieved.101 Several theoretical studies revealed anode materials for LIBs, it can accommodate Na ions, which are that strain engineering has significant influence on the ZT value, significantly larger than Li ions (2.04 vs. 1.52 Å). Because of this, BP therefore, optimized conditions and strain engineering make it with its larger interlayer channel size (3.08 vs. 1.86 Å for graphite)

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Fig. 7 a Energy barrier and diffusion paths of Li atom inside and between two adjacent grooves of monolayer phosphorene. b Diffusion paths and energy barriers for the Na diffusion on monolayer phosphorene. c Schematic of structural evolution of the sandwiched P/G structure during sodiation (top), TEM images and Raman spectra of P/G hybrid (bottom-left), and reversible desodiation capacities of various phosphorene–graphene electrodes with different carbon–phosphorus mole ratios (C/P) between 0.02 and 1.5 V at a current density of 0.05 A g−1. Images a, b, and c adapted with permission from ref. 11 [Royal Society of Chemistry], ref. 118 [Royal Society of Chemistry], and ref. 123 [Nature Publishing Group], respectively

is a highly promising NaIB anode material with a theoretical calculations,11, 111, 112, 122 some first experimental studies have specific capacity of ~2600 mAh g−1, which dwarfs that of any other also been reported,123, 124 including the work of Cui et al.97, on the present materials. Recent DFT study of the intercalation of Na in mechanism of sodiation in BP and hybrid material of few-layer BP119 and few-layer phosphorene118 has demonstrated a fast phosphorene sandwiched between graphene (P/G) investigated (energy barrier of 0.04 eV) and anisotropic (Fig. 6b) Na diffusion using in-situ TEM and ex-situ XRD methods.123 This study showed and, additionally, that phosphorene undergoes SMT at high Na a two-step mechanism of intercalation of sodium along the x-axis 118 intercalation concentration. The preferred Na ion transport followed by alloying and formation of Na3P. The strong along the ZZ edge compared to that along the AM edge has also anisotropic volume expansion (estimated at 0, 92, and ~160%, been shown recently by in-situ aberration-corrected TEM study.120 along the x, y, and z-axis, respectively) observed in few-layer Furthermore, high performance NaIB anode in the form of phosphorene samples was found to be suppressed in P/G BP/Ketjenblack-multiwalled carbon nanotubes composite has nanocoposite electrodes. This was due to graphene layers, which also been recently obtained using ball milling.121 While most accommodated the volumetric change and provided high of the reports on the use of phosphorene for electrochemical electrical conductivity pathways. This resulted in high specific energy storage applications are theoretical DFT-based capacity of ~2440 mAh g−1 (calculated using the mass of

npj 2D Materials and Applications (2017) 5 Published in partnership with FCT NOVA with the support of E-MRS Recent advances in synthesis, properties, and applications M Akhtar et al. 11 phosphorus only) at a current density of 0.05 A g−1 and good ACKNOWLEDGEMENTS capacity retention of 83% after 100 cycles between 0 and 1.5 V We would like to thank Dr Congyan Zhang for preparing the model of phosphorene (Fig. 6c). In another in-situ TEM study, Xu et al.124 investigated the crystal structure used in Fig. 1. thickness-dependent lithiation/delithiation mechanims and have fi found signi cant capacity and electrical conductivity losses in COMPETING INTERESTS thick multilayer phosphorene due to structural decomposition and The authors declare that they have no competing interests. irreversibility of lithiated Li3P phase during delithiation processes. However, few-layer phosphorene samples have shown much better lithiation/delithiation cycling performance. 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