Exotic Physics and Chemistry of Two-Dimensional Phosphorus

Jun 9, 2017 - ... research scholar in the Department of Spectroscopy, IACS, Kolkata. ... Shining Light on New-Generation Two-Dimensional Materials fro...
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Exotic Physics and Chemistry of Two Dimensional Phosphorus – Phosphorene Chandra Chowdhury, and Ayan Datta J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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Exotic Physics and Chemistry of Two Dimensional Phosphorus – Phosphorene Chandra Chowdhury, Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur700032, West Bengal, India. Email: [email protected]

Abstract: Phosphorene, the monolayer form of black phosphorus is the most recent addition to graphene-like van der Waals 2D systems. Due to its immense interesting properties namely, its tunable direct band gap, high carrier mobility and unique in-plane anisotropy, it emerged as a promising candidate for electronic and optoelectronic devices. Phosphorene reveals a much richer phase diagram than graphene and it comprises the two forms namely the stapler-clip like (black Pn, α form) and chairlike (blue Pn, β form) structures. Regardless of its favorable properties, black Pn mainly suffers from the instability in oxygen and water which limits its successful applications in electronic devices. In this perspective we discuss the cause of structural diversity of Pn which leads to different properties of both black and blue Pn. We provide possible solutions for protecting phosphorene from chemical degradation and its applications in the field of energy storage namely for Li and Na ion batteries. TOC GRAPHIC

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Owing to their diverse physical, chemical and optical properties and high surface to volume ratio two dimensional (2D) materials have emerged as one of the most promising materials for different kind of applications in the materials science community. Among them, graphene, the elemental form of carbon allotrope and the most well-known 2D material lacks a finite band gap which limits its successful applications in the field of electronic and optoelectronic devices.1 Among the post graphene 2D materials, hexagonal boron nitride (h-BN) has created an attraction but due to its large band gap (5.9 eV) it becomes an insulator which prohibits its applications in electronic devices.2 The 2D congeners of graphene namely silicene and germanene share similar hexagonal lattice with a buckled structure that arises mainly due to pseudo-Jahn-Teller effects.3-5 However both of them are not synthesized as a freestanding monolayer and they require metal surfaces as support. This results in an interaction with the atomic monolayer to the metal surface which forms covalent bonds, localizing the π-electrons and consequently reducing the electronhole conductivity.6 Both of them have zero band gaps as that of graphene. Besides graphene and graphene congeners there are other 2D materials like transition metal dichalcogenides (like MoS2, WS2, VS2), functionalized silicane and germanane, single layer group IV monochalcogenides, various transition metal carbides which have been applied successfully in the different field of nanotechnology.7-10 The main drawback of such hetero-elemental materials like transition metal dichalcogenides is that there arises deep defect states formed mainly due to chemical disorder of the homo-elemental bonds which are not present in the perfect lattice and can cause degradation of this type of materials.11 This type of bond formation may be avoided in mono-elemental materials. Phosphorus, the most abundant pnictogen in earth’s crust, located in the third period and groupVof the periodic table. Phosphorus is unique among other pnictogens due to its superconducting 2

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behavior at high pressure arising out of its richer phase diagram.12 There are several allotropic forms for phosphorus namely white (comprises with monomeric P4 units), red (with extended chain like amorphous structure), violet or Hittorf’s phosphorus (extended chain like crystalline structure), A7 phase (layered structure similar to that of silicene) and black phosphorus (hexagonal graphite like structure in which layers are interconnected by van der Waals interactions) and each exhibits extremely different properties.13 Figure 1 shows the various allotropic forms of phosphorus.

Figure 1: Allotropic forms of Phosphorus.

Among these morphologies, white and red phosphorus are known to be easily ignitable and hence, air unstable. Black phosphorus is the thermodynamically most stable form out of all phases. In the course of investigation of the effect of high pressure on white phosphorus, P.W. Bridgman discovered a new phase of phosphorus (black-Pn), namely black phosphorus from around 100 years ago.14 Not only black-Pn is chemically inert but also it has a typical layered 3

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van der Waals type structure. Interest in black phosphorus was renewed when phosphorene, which is a one atom thick layer of black phosphorus was synthesized as the second real elemental 2D material stable in the free-standing form.15 Black phosphorus generated technological interests when a field effect transistor (FET) with a carrier mobility of 1000cm2V1 -1

s was prepared by Zhang and co-workers in 2014 for a 10 nm thick multilayer of black-Pn.15,16

Scheme 1: Exfoliation of Pn monolayers from bulk phosphorus by different physical and chemical technique has been shown schematically.

Few layer Pn was exfoliated by Castellanos-Gomez et al. through mechanical exfoliation where an intermediate viscoelastic surface was used to increase yield and reduce the possibility of contamination.17 O’Brien and co-workers exfoliated few layer Pn by using a chemical route which used a solution of N-methyl-2-pyrrolidone.18This is an emerging field as materials scientists are working to exfoliate ultrapure Pn layer by different techniques and the literature is expanding day by day.19,20 The main advantage of black-Pn is its tunable direct band gap which varies from 0.3 eV to 2.0 eV from bulk to monolayer and hence, it acts as a bridge between graphene (zero band gap) and hexagonal boron nitride (insulator).16 4

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Besides that of black-Pn, other forms of monolayer-P has been reported.21-24 Among them the main three are β, 𝛾 and 𝜹 forms. Figure 2 depicts the structure of the four forms from both top and side views.

Figure 2: Structure of (a) an α-P (black), (b) β-P (blue), (c) γ-P, and (d) δ-P monolayer in both top and side views. Top and bottom atom layers are characterized by color and shading. Shaded region represents the Wigner-Seitz cell. Reprinted figure 2 with permission from Ref. 21 Copyright 2014 by the American Physical Society.

The cohesive energies of all the four phases are within 0.1 eV and have been predicted to be stable confirmed by ab initio molecular dynamics simulations.21 α and 𝜹 form exhibit direct band gap semiconductor with a gap of 0.90eV and 0.45 eV, respectively whereas β and 𝛾 forms are indirect band gap semiconductor with a gap of 1.98 eV and 0.50 eV, respectively. Bulk blue-Pn is a layered material with a …ABAB… type of packing and the interlayer distance = 3.48 Å. Since, delta and gamma form of Pn arise from the layered forms of black and blue Pn, we focus our discussion mainly on these two.

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In this perspective we have discussed how structural diversity leads to the difference in their physical properties and suggest possibilities to improve their chemical and electronic properties for energy applications.

Properties of Phosphorene: Due to its unique puckered structures Pn has many fascinating properties which lead to the applications of Pn not only in device fabrication but also as a catalyst. On applying strain normal to the plane of Pn, it changes from semiconductor to metal and its change in structural and electronic properties in Pn has been of tremendous interest for fundamental physics studies.25 Application of in-plane strain and out of plane electric field on monolayer and few layer of Pn, modulated the band gap to convert it into an effective material for solar cell and optoelectronic devices.26 Pn shows a unique thermoelectric properties in which the armchair Pn nanoribbons (semiconducting in nature) show a large Seebeck coefficients.27 Wei and co-workers showed that Pn can sustain strain upto 27-30% in both directions which is indicative of its superior mechanical flexibility than that of other 2D materials like graphene.28 Due to its anisotropic and exceptionally high electron and hole mobility, it is a promising candidate for transistors. For example, black Pn/monolayer MoS2 heterojunction is fabricated as a p-n diode material and Pn/TMDs heterojunctions have been predicted to be an efficient exitonic solar cell materials.29,30 Recently Sun et al showed that strain engineering on Pn can act as a good photocatalyst.31 Due to its semiconducting nature, it is also predicted to be good energy storage material and recent studies have predicted Pn to be excellent anode materials for both Li and Na ion batteries with high specific capacities.32,33 Using first principle calculations, Liu et al

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showed that application of an electric field in the stacking direction leads to normal insulator → topological insulator transition in black-Pn.34 Doping is a chemical means to amplify its electronic properties as the surface states can be effectively manipulated. Using first principles calculations, it was shown that by adsorption of different metal atoms namely alkaline (Na, K, Li), transition (Sc, Ti, V, Cr, Mn, Fe, Co, Ni) and noble metals (Cu, Ag, Au, Pd, Pt), the surface properties of Pn changes and there is transition from semiconductor → metal and nonmagnetic → magnetic state which can have applications in dilute magnetic semiconductors.35 Besides adatom doping, doping with charge transfer molecules

like

tetracyanoquinodimethane

(TCNQ),

tetrathiafulvalene

(TTF)

and

tetracyanoethylene (TCNE), mid gap states can be created which decreases the band gap.36,37

One very distinguishing feature of Pn is its puckered structure which ultimately controls its properties discussed above. We now discuss how planar Pn gets distorted along e1g and b2g modes leading to C2h and D3d puckered form of α and β phases, respectively.38 Hexagonal P66ring is the building unit of Pn. Since, P66- ring contains six formed negative charges, it is difficult to converge its geometry and hence, the hydrogen terminated analog of this ring namely, P6H6 is considered. The model unit P6H6 shows an excellent structural resemblance with both black and blue Pn. For example, chair structure of P6H6 in C2h symmetry has two d(P−P) = 2.23 and 2.25 Å which converge well with that of black Pn which shows two bond distance of d(P-P) = 2.22 and 2.25 Å, respectively. P6H6, C2h Black Pn P6H6, D3d

dP-P (Å) 2.23 2.22

2.25 2.25

θPPP (°) 98.7 96.4

2.27

100.9 101.2

95.5

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Blue Pn

2.26

93.2

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86.7

Table 1: Comparison of bond length (Å), bond angle (°) and dihedral angle (°) of C2h and D3d symmetry of P6H6 with that of black and blue Pn. Reprinted with permission from Ref. 38 Copyright 2016 by American Chemical Society.

Similarly, for the other stable form of P6H6 in D3d symmetry, a bond length of d(P-P) = 2.27 Å, compares well with bond length of blue Pn d(P-P) = 2.26 Å. All the other structural features of C2h and D3d form of P6H6 and that of black and blue Pn are shown in Table 1. Figure 3 shows the molecular form of P6H6 structures in both C2h and D3dand how they transform into the extended layers preparing their basic unit intact.

Figure 3: Correlation between (a) molecular form of P6H6 with C2h symmetry to black Pn and (b) P6H6 with D3d symmetry to blue Pn. The shaded region corresponds to the molecular form of P66- ring and relates its connection with the extended form. Reprinted with permission from Ref. 38 Copyright 2016 by American Chemical Society.

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Pseudo-Jahn−Teller Instability of Planar Phosphorene: The planar form of P6H6 has a hexagonal geometry with D6h point group symmetry. This form is a saddle point producing 11 unstable normal modes namely, 4E2u + 2B2g +2E1g +1B1u + 2E2g. Among these modes, relaxation only along e1g and b2g modes produces the vibrationally stable forms with C2h and D3d symmetry, respectively and the others formed the structures which are not stable. In Figure4, the animation of the normal modes frequencies along e1g and b2g mode are shown. The energy difference between these two structures is very small and it was found that C2h form is more stable than the D3d form by -4.3 kcal/mol. This is consistent with the previous studies of PPh6 which shows that it is more stable in its chair form than that of its other forms.39 From Figure 4, it is seen that both the C2h and D3d forms exist in chair like structures and it may be possible to extend these two structures into black and blue Pn with a small energy penalty. This is in agreement with the small energy difference of 0.01 eV/atom between the two phases.21 For understanding the stability of black and blue Pn in C2h and D3d forms in the P6H6 geometry, distortion of planar P6H6 along the vibrational symmetry mode e1g and b2gwere undertaken to generate the adiabatic potential energy surfaces (APES). In Figure 4, the APES for both e1g and b2g were plotted for ground state and few relevant excited states. From the calculation, it is clear that for both e1g and b2g mode, 20-25 excited states lie within the energy range 4 eV which implies that several excited states might be approachable by low energy excitation for both case. The favorable condition for effective non-adiabatic coupling between ground states and several excited electronic states is that the states should lie within a reasonable energy range which is satisfied for both E1g and B2g electronic states in which the relevant excited states are at 2.99, 3.06 and 4.01 eV with respect to ground state. There are several examples where due to the highly separated electronic states, symmetry lowering does not occur by means of PJT distortion

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like that for graphene, highly electronegative element (F/O) substituted dithine rings or Li(I)/Ca(II) doped silicene.40-42 From Figure 4, it is clear that the ground state APES gets stabilized which creates a typical double-well potential and the energy barrier for this is 1.4 kcal/mol at q(e1g)/q(b2g) = 0.

Figure 4: Animation of distortion in P6H6 along (a) e1g and (b) b2g modes. The APES for (c) e1g and (d) b2g for ground state and several excited states. Numerical fitting of the ground state APES to the lowest root of the 4 × 4 secular determinant for PJT along (e) e1g mode and (f) along b2g mode. Reprinted with permission from Ref. 38 Copyright 2016 by American Chemical Society.

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Two PJT problems were set up for the distortion of P6H6 molecule along e1g and b2g modes by coupling of three excited states of E1g and B2g symmetry and the PJT problem resembles like (1Ag+1E1g+2E1g+3E1g) × e1g and (1Ag+1B2g+2B2g+3B2g) × b2g for e1g and b2g, respectively. This problem results a 4 × 4 secular determinant. Solution of this determinant gives equations were fitted with the ground states APES of e1g and b2g mode and shown in Figure 4. From the result of this fitting of the APES, the relevant vibronic couplings, primary force constants including the second order perturbation correction and effective ground state force constants are obtained. The general condition for satisfying the PJT instability is,

! !!!! ! ∆ !!

> 𝑘! − 𝑝! where 𝐹!! is the coupling

constant, ∆!! is the energy difference between the ground state and the excited state and (𝑘! − 𝑝! ) is ground state force constant and for both e1g and b2g the instability condition is satisfied.43 So, it is clear that the reason for the puckered structure of planar P6H6 ring is the spontaneous symmetry breaking arising due to PJT. From this discussion it can be concluded that planar Pn can not be stable and it may spontaneously breaks down to form either buckled black or puckered blue Pn to avoid PJT instability in its planar form. Planar P6 molecule in D6h symmetry is not stable and it spontaneously undergoes symmetry lowering (D2) by the (HOMO-1) and LUMO vibronic mixing along the puckering mode (e2u).44 Instability arising out of pseudo-JT distortions account for the black and blue phases of Pn which though energetically almost degenerate possess very different chemical and physical properties. Blue Pn is significantly less undulated than black Pn and should therefore possess more exposed area to interact with planar molecular surfaces. Hence, the extent of charge transfer (CT) to and from black and blue Pn would be different, which must affect their electronic properties. Major Problem and Possible Remedies: Though Pn has a large number of applications due to its outstanding properties,a significant bottleneck comes due to its instability in air. Previous 11

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studies show that Pn is unstable at ambient conditions and it gets oxidized to form phosphorene oxide and other oxide fragments of low molecular weights.45-48

Figure 5: Schematic representation of fabrication of air stable mono to few layerPn. Figure 5 reproduced from Ref. 48 (http://creativecommons.org/licenses/by/4.0/).

Besides this, it is revealed from both experimental and theoretical calculations that though pristine Pn weakly interacts with water however, in case Pn is oxidized then this oxidized surface reacts rapidly with water molecule (exothermic reaction) which appears to be the main reason for chemical degradation of Pn based electronic devices.46 Island and co-workers showed that in a 5 days period, black Pn flakes with a maximum of 30 nm thickness is totally covered with water resulting in large convex meniscus.47 To prevent this degradation, Pn should be encapsulated. Several recent studies have shown that the stability of Pn flakes can be improved using atomic layer deposition (ALD) of a dielectric layer like alumina, solvent exfoliation with anhydrous

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organic solvent like N-methylpyrrolidone (NMP) or N-cyclohexyl-2-pyrrolidone etc.18 In Figure 5, it is shown schematically how oxygen plasma etching can react with the top Pn layer producing PxOy which effectively protects Pn from degradation and this is further improved by passivation with Al2O3 by ALD.48 Another promising way to protect reactive Pn is capping with naturally stable 2D materials like graphene or BN. The Pn/graphene or Pn/h-BN heterostructure preserves the main characteristics of Pn which are the main reason for device applications.49 Pn/h-BN Heterostructure and Its Potential as LIBs: In terms of energy density, chemical energy is the most appropriate source. For the next generation energy storage devices, Li ion battery (LIB) and Na ion battery (SIB) have gained the prime interest among the other available energy storage devices. LIBs are attractive as anode materials in batteries due to their high energy density, good cycle life and enhanced rate capabilities.50 Pn monolayers and bilayers have also been explored as anodes for LIBs and SIBs with high theoretical capacity.25,32 But as discussed above, due to its high reactivity towards atmospheric oxidation, the electronic properties alter significantly and hence impeding its applications in such advanced electronic devices. Structural and chemical degradation of Pn might be avoided by capping it with graphene or hexagonal boron nitride (h-BN), so Pn capped with graphene or h-BN can act as good electrode materials. Pn/graphene heterostructures have been demonstrated to be good electrode materials for both LIBs and SIBs in both experimental and theoretical studies.51,52 Recently, Pn capped with h-BN has been demonstrated to act as a good anode material for both Li and Na ion batteries.53

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Figure 6: Schematic representation of the diffusion path of alkali (a) along the intercalated region between the h-BN and Pn layers and (b) above the surface of the Pn layer, (c) the diffusion barrier for Li along the armchair and zigzag pathways in the intercalated region, (d) the diffusion barrier for Na along the armchair and zigzag pathways in the intercalated region, (e) the diffusion barrier for Li along the armchair and zigzag pathway over the Pn surface, and (f) the diffusion barrier for Na along the armchair and zigzag pathways over the Pn surface. Reprinted with permission from Ref. 53 Copyright 2016 by American Chemical Society.

Such Pn/h-BN heterostructure exhibits very small diffusion barriers of 0.09 and 0.06 eV for Li and Na, respectively, and this advocates an excellent diffusion mobility and high charge−discharge rate for Li and Na atoms in the LIB and SIB. It was further shown that capping Pn by h-BN not only prevents chemical degradation of Pn but also improves its performance without perturbing its intrinsic in-plane high specific capacity. 14

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Summary and Outlook: Due to its suitable physical and chemical properties, Pn has tremendous applications in the field of electronic and optoelectronic devices which has become an attractive area.Motivated by the desire to develop next generation electronic and optoelectronic devices, plethora of research have been carried out after initial experiments showed that few layer and monolayer BP flakes can be exfoliated like graphene. Besides its usefulness as electronic device and energy storage applications, it can also be used in other emerging areas as well in material science. We list few of them as; 1) Over the past decade, two dimensional materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to provide excellent catalytic support. Due to its layered van der Waals type unique structure, Pn can be an attractive two-dimensional support for heterogeneous catalysts. Besides the pristine form, doped Pn with electronegative groups as for example frustrated Lewis pair based Pn can also be an attractive 2D catalyst.54 Possibility of exploiting doped Pn surfaces in heterogeneous catalysis namely, single atom catalyst (SAC) is anticipated in near future. 2) Phosphorus is an essential element in the cell. Hence, one might been design biomedical applications for Pn and bio-molecular interactions, cell imaging, drug delivery and toxicity for Pn based materials need to be explored. Besides this,Pn can also be used as a biosensor namely, DNA detection based on π-π stacking and hydrophobic interactions. 3) Molecular self-assembly on the two-dimensional surfaces is emerging as an interesting area of research. Two dimensional black Pn can be an excellent surface for molecular self-assembly and hence, utilized for the epitaxial growth of organic thin films for various 15

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optoelectronic devices namely organic light emitting diodes (OLEDs), organic photovoltaic (OPVs) and organic field effect transistors (OFETs).55The dispersion interactions between the hexagonal P6 units and aromatic/aliphatic molecules can lead to interesting nano-patterns.

AUTHOR INFORMATION Corresponding Author: [email protected] Biographies: Chandra Chowdhury is currently a senior research scholar in the Department of Spectroscopy, IACS, Kolkata. Her research interest is computational materials science focused mainly towards energy storage and environmental issues. Dr. Ayan Datta is currently an Associate Professor in the Department of Spectroscopy, IACS, Kolkata. His research interests span across various aspects of theoretical and computational chemistry. Special emphasis is given to provide qualitative models to understand and predict structure, reactivity and physical properties of molecules and materials at various length and time scales. ACKNOWLEDGMENT C.C. thanks CSIR India for SRF. AD thanks INSA, DST and BRNS for partial funding.

References: 16

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