P3Cl2: A Unique Post-Phosphorene 2D Material with Superior

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

P3Cl2: A Unique Post-Phosphorene 2D Material with Superior Properties against Oxidization Ning Lu, Zhiwen Zhuo, Yi Wang, Hongyan Guo, Wei Fa, Xiaojun Wu, and Xiao Cheng Zeng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03136 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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The Journal of Physical Chemistry Letters

P3Cl2: A Unique Post-Phosphorene 2D material with Superior Properties against Oxidization Ning Lu,*,1, Zhiwen Zhuo,2,  Yi Wang,1 Hongyan Guo,*,1 Wei Fa,3 Xiaojun Wu,2 and Xiao Cheng Zeng*,4,5 1 Anhui

Province Key Laboratory of Optoelectric Materials Science and Technology, Department of Physics, Anhui Normal University, Wuhu, Anhui, 241000, China. 2 CAS Key Laboratory of Materials for Energy Conversion, School of Chemistry and Materials Sciences and CAS Center for Excellence in Nanoscience, and Hefei National Laboratory of Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China 3 National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China 4 Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA 5 Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230026, China

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ABSTRACT Herein, a unique class of post-phosphorene materials, namely, phosphorene halogenides (e.g., α-P3Cl2) with superior oxidation resistance and desirable bandgap characteristics are proposed. Our first-principles computations show that monolayer α-P3Cl2 is a direct semiconductor with a wide bandgap of 2.41 eV (HSE06) or 4.02 eV (G0W0), while the bandgap exhibits only slight reduction with increasing the number of layers. The monolayer α-P3Cl2 also possesses highly anisotropic carrier mobility, with both ultrahigh electron mobility (56,890 cm2 V-1 s-1) and hole mobility (26,450 cm2 V-1 s-1). Meanwhile, the outstanding optical properties and favorable band alignment of 2D P3Cl2 suggests its potential as a photocatalyst for the visible-light water splitting. 2D α-P3X2 (X=F, Br, I) also exhibit good oxidation resistance and possess wide direct bandgaps ranging from 2.16 to 2.43 eV (HSE06). These unique electronic and optical properties render 2D phosphorene halogenide as promising functional materials for broad applications in electronic and optoelectronic devices. TOC GRAPHICS

KEYWORDS phosphorene halogenides, high carrier mobility, 2D material, density functional theory; electronic properties


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Two-dimensional (2D) materials with atomic thickness have attracted intensive attention over the past decade, largely due to their novel properties that are not seen in their bulk counterparts. Unlike some prevailing 2D materials, such as graphene,1 silicene,2 or MoS2,3 phosphorene possesses unique and highly anisotropic structure,4-12 thereby exhibiting strongly anisotropic electro-optical and thermo-mechanical properties.13 Novel electronic properties of phosphorene include a direct bandgap around 2 eV and a very high anisotropic carrier mobility, which endow the phosphorene a promising 2D semiconductor for applications in the post-silicon era.11 Indeed, drain current modulation up to 105 and hole mobility up to 5200 cm2 V-1 s-1 are reported for the few-layer black phosphorus (BP) field-effect transistors.6,

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Furthermore, the measured optical properties of

phosphorene are also highly anisotropic. For example, few-layer phosphorene exhibits extraordinarily strong layer-dependent photoluminescence,7 a novel characteristic that has been exploited in high-performance photodetectors.15 However, some issues associated with the phosphorene are still of concern for its practical applications: (i) Few-layer phosphorene can be easily oxidized in open air, thereby resulting in serious degradation of BP-based electronic devices.16, 17(ii) Monolayer phosphorene exhibits the largest electronic bandgap of only ~2 eV,18 which may limit its application as wide bandgap semiconductors, and also optoelectronic device in the visible and UV range. Note that semiconductors with wide bandgap, for example GaN (3.39 eV) or SiC (3.2 eV),19 are known as the third-generation semiconductor materials, and can be used in devices that operate at much higher voltages, temperatures, and frequencies than conventional semiconductor Si or GaAs.19, 20 (iii) The performance of phosphorene hinges on the number of the layers. For example, the bandgap and carrier-mobility decreases quickly with increasing the number of phosphorene layers.7 In view of these issues of 2D phosphorene, it is of both fundamental and practical importance to seek a closely-related material of 2D phosphorene with a wide bandgap (notably larger than 2 eV), high carrier mobility, good oxidation resistance, and weak band-gap dependence on the number of layers. To improve the stability of few-layer phosphorene under ambient conditions, several stabilization strategies have been examined, including encapsulation of phosphorene by a protective layer of graphene,21 or AlOx,16 hexagonal BN,22 perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA),23 or by surface modification with F,24 aryl diazonium,25 Ti sulfonate ligand,26 or polycyclic aromatic compounds.27 Although these approaches can improve the


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chemical stability of phosphorene in open air,25 they often lead to reduced hole mobility or on-off ratio compared to pristine phosphorene. Also, the life time of passivated phosphorene in open air still does not meet the high standard for practical application yet.23, 28 Recently new allotropes of phosphorene29, 30 and 2D phosphides31-35 with superior properties have been investigated as supplemental materials for phosphorene.36, 37 For example, ultra-high mobility has been predicted for 2D hittorfene,30 GeP331 at 103 cm2 V-1 s-1 order of magnitude, 2D CaP333 at 104 cm2 V-1 s-1 order of magnitude, and 2D PN (with group symmetry of P21/c) up to 105 cm2 V-1 s-1 order of magnitude.35 2D InP3 is predicted to exhibit extraordinary sunlight absorbance and tunable magnetism.32 These 2D phosphides could be fabricated via exfoliation from their corresponding bulk materials. However, since these 2D materials have similar structure as phosphorene, their stability against oxidation is still an open question. Our testing simulation suggests that these 2D materials are still easily oxidized by near-by O2 (see Figure S1). In this work, we propose a new class of 2D post-phosphorene materials, namely, 2D phosphorene halogenide P3X2 (X=F, Cl, Br, I) that likely possess excellent oxidation resistance, in addition to desired electronic properties. In particular, the monolayer α-P3Cl2 is a direct-gap semiconductor with a bandgap of 2.41 eV (HSE06 computation) and 4.02 eV (G0W0 computation), while the bandgap of the multi-layered α-P3Cl2 decreases only slightly with increasing the number of layers (0.05 eV from monolayer to bilayer). Moreover, monolayer α-P3Cl2 exhibits highly anisotropic carrier mobility with the electron mobility being as high as 56,890 cm2 V-1 s-1 and the hole mobility as high as 26,450 cm2 V-1 s-1. The outstanding optical properties and band alignment of 2D P3Cl2 suggest its possible application as efficient photocatalysts for visible-light-driven water splitting. Also, we find that 2D P3X2 (X=F, Br, I) exhibit good oxidation resistance as well, all with direct bandgap ranging from 2.16 to 2.43 eV (HSE06). These desirable properties suggest that 2D P3X2 (X=F, Cl, Br, I) have great potential for nanoelectronic and optoelectronic applications. As the group-VII elements in periodic table with electronic configuration of ns2np7, halogen atoms (X=F, Cl, Br and I) exhibit strong oxidability to accept one electron and to achieve X- state so that it becomes inert to oxygen. Thus, introducing halogen to cover the surface of phosphorene can be an effective way to protect the highly active phosphorene. To obtain phosphorene halogenide crystalline, the base structure of MP3 (M=Ca, Sr, Ba) with similar puckered layered structure to phosphorene is selected as a template, in which the metal atoms are substituted by


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halogen atoms through substituent reaction. Among different halogen elements, Cl is considered first due to favorable combination of strong electronegativity and relatively large atomic radius to cover the surface of phosphorene framework. In particular, α-P3Cl2 is found to be the most stable structure (Figure1a, b) among various optimized configurations with Cl substituted M (see Figure S2 and Table S1). The formation energy of α-P3X2 (X=F, Cl, Br, I) is from -0.27 eV to -1.80 eV (Table1). α-P3F2 has the lowest formation energy, while α-P3I2 has the highest. Overall, phospherene halogenide is energetically favorable. It’s noted that the interlayer interaction will become much weaker after halogenation. For example, the binding energy of bulk CaP3 is -0.34 eV/atom, with the layer distance of 0.75 Å. However, the binding energy of bulk α-P3Cl2 is just 0.05 eV/atom with the interlayer distance up to 2.55 Å, which indicates that the structure will be much easier to exfoliate to monolayer after halogenation.

Figure 1. (a) Schematic of bulk MP3 (M=Ca, Sr, Ba) to monolayer P3X2 (X= F, Cl, Br, I). (b) Top view of optimized monolayer α-P3X2 (X= F, Cl, Br, I) structure. The unit cell is marked in red line. The purple, green, and blue spheres represent P, X, and M atoms, respectively. (c) Computed phonon dispersion curves of the α-P3Cl2 monolayer. The geometry structures of α-P3X2 appear to have similar puckered configuration like 2D CaP3 or phosphorene. Monolayer α-P3X2 exhibits the P2/C symmetry with similar lattice constant in direction a, but different one in direction b (see Table S1). The lattice constant of monolayer αP3Cl2 is a = 8.67 Å and b = 7.89 Å. The P−Cl bond length is ~2.11 Å. The P−P bond length is


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2.25−2.29 Å, slightly longer than that in the monolayer CaP3 (2.21−2.24 Å) but close to that in the phosphorene (2.24−2.28 Å). The P−P bond length of other α-P3X2 (X=F, Br, I) is in the range of 2.25−2.31 Å. These results indicate that introduction of halogen does not change the host phosphorene framework much. Bader charge analysis suggests that phosphorene framework transfers about 0.48 e to each Cl atom. The charge transfer from phosphorene framework to halogen atom for F, Br and I atom is 0.78, 0.31 and 0.15 e, respectively, consistent with the electronegativity trend among the four elements (F > Cl > Br > I). Table 1. The formation energy Ef (in eV) per P atom, the computed bandgaps Eg (in eV), the work function Ewf and the average charge transfer from P to X (X = F, Cl, Br, I, H) atom of α-P3Cl2 monolayer, bilayer (BL-AC-P3Cl2), trilayer (TL-ACA-P3Cl2), and α-P3X2 (X= F, Br, I, H) monolayer. Ef (eV)

Eg (PBE)

Eg (HSE06) Ewf(HSE06)

Charge (|e|)

α-P3Cl2

-0.62

1.52

2.41

6.57

0.48

BL-AC-P3Cl2

-0.66

1.47

2.36

6.55

0.48

TL-ACA-P3Cl2

-0.68

1.43

2.32

6.53

0.48

α-P3F2

-1.80

1.40

2.33

6.87

0.78

α-P3Br2

-0.45

1.57

2.43

6.22

0.31

α-P3I2

-0.27

1.32

2.16

5.90

0.13

α-P3H2

0.03

2.24

3.06

6.11

0.36

The lattice dynamic stability of α-P3Cl2 monolayer is confirmed via computing the phonon dispersion curves (Figure 1c), which shows no imaginary phonon modes. The highest vibration frequency of α-P3Cl2 is 458 cm-1 (13.74 THZ) , almost the same as that of phosphorene (450 cm1)38

and 2D CaP3 (453 cm-1),33 reflecting mechanical robustness of the covalent P−P bonds in


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phosphorene chloride. Mechanical stability of the α-P3Cl2 monolayer is verified via computing the elastic constants, and the computed values are C11= 20.87 J/m2, C22= 30.94 J/m2, C12=6.78 J/m2 and C44=5.64 J/m2. It can be seen that these values satisfy the known mechanical stability rules,39 i.e., C11C22 − C122 > 0 and C44 > 0. Lastly, thermal stability of α-P3Cl2 structures is examined by using BOMD simulations in which the temperature of the system is controlled at 300, 500, 700 and 1000 K, respectively. The simulation results indicate that the structure is intact after 5 ps run at all temperatures even up to 1000 K (see Figure S3). Particularly, all Cl atoms of α-P3Cl2 remain their relative orientation during the simulation at 300 K, while only a few Cl atoms deflect at 500 K and 700 K, suggesting that the α phase of P3Cl2 is quite stable even above the room temperature. Computed electronic structure of monolayer α-P3Cl2 is shown in Figure 2, where it shows a direct bandgap of 1.52 eV (at the PBE level). The more accurate HSE06 functional yields a bandgap of 2.41 eV. From the computed partial density of states (PDOS), one can see that the valence band maximum (VBM) is contributed by s, p orbitals of P and p orbitals of Cl, while the conduction band minimum (CBM) is mainly contributed by the p orbitals of P. For P3Cl2 with other different orientation of Cl atoms, the bandgap is still between 2.02 eV and 2.61 eV at the HSE06 level (Table S2, Figure S4). The electronic structures of α-P3X2 (X=F, Br, I) are also computed, and the bandgap ranges from 2.16 eV to 2.43 eV at the HSE06 level. The porosity of phosphorene framework, deep electronic state from CBM and VBM of halogen atoms, and the notable charge transfer from phosphorene framework to halogen atoms, are likely the main reasons responsible for the wider bandgap of phosphorene halogenides than those of phosphorene and CaP3. It’s noted that for the phosphorene, due to the exciton effect, the obtained experimental value of the optical gaps is smaller than the electronic bandgap.33 The HSE06 functional tends to underestimate the electronic band gap but fortuitously gives reasonable optical gap, very close to the experimental value (see Supporting Information), whereas the computationally much more demanding GW method gives an electronic band gap closer to the experiment.40, 41 We have also computed the G0W0 electronic bandgap of α-P3Cl2, and obtained a value of 4.02 eV.


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Figure 2. (a) Computed band structures, and total and partial density of states (DOS) of (a) α-P3F2, (b) α-P3Cl2, (c) α-P3Br2, and (d) α-P3I2 monolayer by using PBE and HSE06 functional, respectively. The electronic properties of α-P3Cl2 multilayers are also computed. For the α-P3Cl2 bilayer, several different stacking orders, named as AA, AB and AC stacking, are considered. The AC stacking is found to yield the lowest energy (Table S2 and Figure S5). The computed electronic structure is similar for different stacking order, and all stacking structures retain the direct bandgap feature with the value around 2.36 eV (HSE06, Table S2). The bandgap decrease slightly with the value around 0.05 eV compare to monolayer. For α-P3Cl2 trilayer, different stacking order of ABA, ABC and ACA are considered. The electronic structure is also insensitive to different stacking


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order. All possess direct bandgap with the value around 2.32 eV (HSE06, Table S2). The slight reduction of the bandgap with the increase of number of layers is contrary to most 2D layered materials with van der Waals inter-layer interaction.7, 33, 42 For example, the bandgap decreases 0.5 eV for phosphorene from monolayer to bilayer,7 and 0.24 eV for CaP3 from monolayer to bilayer.33 Moreover, since the Cl atom has strong electronegativity, the Cl atomic-coating layer on α-P3Cl2 will lead to strong repulsion between different α-P3Cl2 layers. The binding energy of α-P3Cl2 bilayer with AC stacking is about -0.025 eV/atom, much less than the -0.06 eV/atom of phosphorene bilayer,43 indicating that the interlayer coupling is much weaker, thereby the electronic structure is weakly dependent on the stacking order and the number of layers. These results suggest that the electronic structure of multi-layered α-P3Cl2 thin film is expected to be only slightly changed, compared to that of monolayer. Detailed band structures and PDOS of αP3Cl2 bilayer and trilayer are shown in Figure S6. It is known that most phosphorene related materials exhibit high carrier mobility. The computed carrier mobilities of 2D α-P3Cl2 are shown in Table 2. For the α-P3Cl2 monolayer, the elastic moduli are 20.87 J/m2 and 30.94 J/m2 along the a and b direction, respectively, which are less than those of phosphorene monolayer (28.94 J/m2 and 101.60 J/m2 along the a and b direction respectively),7 due to the formation of porous structure with P-P dimer vacancy in the phosphorene framework of α-P3Cl2. The effective mass of electron is close to each other along the a [0.63 me (the mass of free electron)] and b [0.48 me] directions. However, the effective mass of hole is anisotropic. The effective mass along the a direction (1.14 me) is notably higher than that along the b direction (0.75 me). Similar to phosphorene, the deformation potential E1 is anisotropic, which is much smaller along the b direction than that along the a direction for both electron and hole. This leads to relatively high mobilities along the b direction, i.e., up to 56.89×103 cm2 V-1 s1

for electron and 26.45×103 cm2 V-1 s-1 for hole, while small electron and hole mobilities, 0.74×103

cm2 V-1 s-1 and 0.70×103 cm2 V-1 s-1 along the a direction, respectively. The ultrahigh carrier mobilities for monolayer α-P3Cl2 are a consequence of the extremely small deformation potential E1 (0.21 for electron and 0.19 for hole) along b direction. As shown in Figure S8 (a and b), for the monolayer α-P3Cl2, both the VB and CB wave function shows striped shape, and mainly along a direction, making the E1 much smaller for both electron and hole along b direction. The VBM state is a bonding state between P2 and P3 atoms, while the CBM state


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corresponds to a strong overlap between P1 and P2 atoms. In Figure S8 (e), the distance of P1 and P2 atoms is sensitive with the strain (3.113 Å to 3.147 Å with the strain from -0.6% to 0.6% along a direction), while the bond length of P2 and P3 atoms (2.296 Å) are little affected by the strain, making the hole to have a smaller E1 (0.78) compared to electron (1.32). For bilayer α-P3Cl2, the decrease of the overlap of CB wavefunction of P2 and P3 atoms along the a direction (Figure S8d) results in decrease of the E1 of electron to 0.44. The stacking-induced inter-layer interaction increases the E1 of hole up to 0.71 along b direction (Figure S8c). It is noteworthy that monolayer α-P3Cl2 possesses both the high electron mobility and the high hole mobility at the same time (at the order of 104). Both values are an order of magnitude higher than those of conventional bulk wide bandgap semiconductors (at order of 103).19 Note also that, for phosphorene, the carrier mobility decreases substantially from its monolayer (at order of 104) to bilayer (at order of 103). In contrast, the α-P3Cl2 bilayer possesses even higher electron mobility, up to 157.86 × 103 cm2 V-1 s-1, along the b direction, suggesting that the high mobility can be maintained by a thicker α-P3Cl2 multilayer. Table 2. Carrier effective mass (m*), elastic modules (C), deformation potential constant (E1), and carrier mobility (μ) of the α-P3Cl2 monolayer (ML) and bilayer (BL) with AC stacking. m*/me

C (J m-2)

E1 (eV)

μ (103 cm2V-1 s-1)

ML-Electron (a) 0.63

20.87

1.32

0.74

ML-Electron (b) 0.48

30.94

0.21

56.89

ML-Hole (a)

1.14

20.87

0.78

0.70

ML-Hole (b)

0.75

30.94

0.19

26.45

BL-Electron (a) 0.58

42.21

0.44

15.58

BL-Electron (b) 0.46

63.26

0.19

157.86


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BL-Hole (a)

1.97

42.21

0.74

0.70

BL-Hole (b)

0.72

63.26

0.71

3.13

Optical properties of 2D α-P3Cl2 are also computed based on the HSE06 functional. The frequency-dependent dielectric function ɛ(ω) = ɛ1(ω) + iɛ2(ω) is related to the absorption coefficient through the expression44, 45 α(ω) =

2ω 2 c {[ɛ1(ω)

1 2

1 2

+ ɛ22(ω)] ― ɛ1(ω)} . The absorption

coefficient, with the electric field vector E being polarized in parallel to the plane (α//) or being perpendicular to the plane (α⊥), is computed. Clearly, the α-P3Cl2 monolayer exhibits fairly strong optical absorption over wide frequency range between 2 eV and 6 eV (Figure 3b). The in-plane absorption (α//) is always greater than out-of-plane absorption (α⊥) due to the larger cross section. The absorption coefficient is large (105 cm−1 ) and comparable to that of the CaP3,33 InP3,32 GeP331 and organic perovskite solar cells.46 Moreover, the bilayer of α-P3Cl2 exhibits stronger optical absorption than the monolayer. Our HSE06 calculations also indicate that the work function of 2D P3Cl2 varies from 6.0 to 6.61 eV, as summarized in Table 1. The energy position of VBM and CBM are also calculated at the HSE06 level, as shown in Figure 3a, along with the standard reduction potential of H+/H2 (−4.44 eV vs vacuum level (set as 0 eV)) and oxidation potential of O2/H2O (−5.67 eV versus vacuum level).29 The computed positions of CBM and VBM of all 2D P3Cl2 allotropes match the hydrogen reduction potential and oxygen oxidation potential for visible-light-driven water splitting. The outstanding optical properties and favorable band alignment suggest potential applications of 2D P3Cl2 as a photocatalyst.


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Figure 3. (a) Computed band alignment of 2D P3Cl2 based on the HSE06 method. The vacuum level is set as 0 eV. The chemical reaction potentials for H+/H2 and O2/H2O are plotted with dashed lines. (b) Calculated optical absorption spectra of α-P3Cl2 monolayer and bilayer at the HSE06 level. The capability against oxidation is a main focus of this work, since easy oxidation in open-air is a major issue for the phosphorene and many of its related materials for practical application. First, by using BOMD simulation, the chemical stability of different 2D materials exposed to gasphase O2 can be examined. To this end, the temperature is set as 300 K, and the simulation time is 5 ps. Eight O2 molecules (per supercell) are placed about 4 Å above the surface of 2D structure. Many phosphorene-related materials, including phosphorene, hittorfene, GeP3, CaP3, InP3, P21/c-


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PN, phospherene halogenides, and phosphorene hydride (α-P3H2, for comparison with phosphorene halogenide; see Figure S9 and Table 1) are examined. The simulation results indicate that most 2D phosphorene-related materials can easily react with O2 molecules within 5 ps simulation time, except 2D phosphorene halogenides. In our BOMD simulations for most phosphorene-related materials, gas-phase O2 molecules move close to the surface and many of them can dissociate into O atoms on the surface, even resulting in structure disruption. As a result, these 2D structures are destructed by O2 with the O atoms either embedded or adsorbed on the 2D structures (Figure 4a-b and Figure S1). These BOMD results indicate that phosphorene and many of its related materials can be easily attacked by O2 and then oxidized at ambient temperature. In contrast, all phosphorene halogenides remain intact (see Figure 4c and Figure S10), O2 molecules tend to either bounce back or move away from the surface, rather than dissociate into oxygen atoms. For α-P3Cl2, a very high temperature up to 1000 K (up to 10 ps) was used (Figure S10d), and the simulation shows that α-P3Cl2 can still survive without being oxidized within the simulation time, suggesting its high stability against O2 oxidization. The O2 collision and bounce back from the α-P3Cl2 surface can be directly seen in the Supporting Movies S1. Especially, a series of snapshots that highlight the collision and bounce back of O2 molecule directly to/from the region between Cl terminals can be seen in the Figure S11.


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Figure 4. Snapshots of after 5 ps BOMD simulations with temperature controlled at 300 K, revealing interaction of O2 with (a) phosphorene, (b) α-P3H2 and (c) α-P3Cl2 monolayer. To gain additional insight into high stability against O2 oxidation, we use the CI-NEB method to compute the energy barrier (𝐸𝑎) of O2 dissociation on different surface. For phosphorene, our calculation result (Figure S12) is Ea = 0.58 eV, consistent with a previous study.47 For α-P3H2, Ea = 0.37 eV. However, Ea increase to 0.94 eV for α-P3Cl2. So the activation energy for O2 oxidation in the case of α-P3Cl2 is much higher than that in the case of α-P3H2, rendering the chemical reaction with O2 much harder. This difference in activation energy is likely due to two aspects. First, the radius of Cl is much larger than H. The vertical and relatively large Cl atoms covered on the surface of α-P3Cl2 incur stronger steric effect than the near-parallel and relatively small H atoms attached to the surface of α-P3H2 (Figure S12d). For instance, at the initial state for the physical absorption of O2, the P-O distance in α-P3Cl2 is 4.41 Å, much longer than that in α-P3H2 (3.33 Å) and phosphorene (3.26 Å). Second, the electronegativity of Cl is much stronger than H,


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resulting in stronger electrostatic repulsion when O2 is negatively charged, especially for the transition state. In the initial state, Bader charge analysis shows that the O2 accepts about 0.01 e, 0.08 e, and 0.05 e from α-P3Cl2, α-P3H2 and phosphorene, respectively. In the transition state, the O2 accepts about 0.37 e, 0.15 e and 0.54 e from α-P3Cl2, α-P3H2 and phosphorene, respectively. As a consequence, the strong steric effect and electrostatic repulsion make the pass of O2 through α-P3Cl2 monolayer quite difficult, and the energy barrier amounts to Ea = 0.94 eV for O2 dissociation. 𝐸𝑎 𝑅𝑇

According to Arrhenius equation, the chemical reaction rate constant 𝑘 = 𝐴𝑒 , where A is the pre-exponential factor, T is the absolute temperature (in unit of K), 𝐸𝑎 is the activation energy for the reaction, and R is the universal gas constant. For the pre-exponential factor A, it is relevant to the effective collision for chemical reaction, which can be lowered by reducing the number of exposed active sites. In other words, phosphorene covered by Cl with inertness to oxygen can lower the pre-exponential factor A for the oxidization process. Further consideration of the vibration effect of Cl, the exposed area would even smaller. As result, the pre-exponential factor A for α-P3Cl2 is much smaller than that for bare phosphorene and α-P3H2. In sum, the strong electronegativity and relatively large radius of Cl can lead to: i) reduce the proportion of the exposed part of the surface, resulting in a small pre-exponential factor A; ii) repel the O2 away from approaching to the surface via electrostatic repulsion; and iii) have notable steric resistance. Among the three factors above, ii) and iii) result in higher dissociation energy barrier of O2 in the case of α-P3Cl2. Overall, the antioxidant cover formed by the Cl coating on the surface can protect the phosphorene framework from oxidation. In conclusion, a new 2D wide direct-gap semiconductor named α-P3Cl2 with bandgap of 2.41 eV (HSE06) and 4.02 eV (G0W0) are suggested. The bandgap of multi-layered α-P3Cl2 is quite insensitive to the increase of the number of layers. 2D α-P3Cl2 exhibits strongly anisotropic carrier mobility with both ultra-high electron mobility (56,890 cm2 V-1 s-1) and hole mobility (26,450 cm2 V-1 s-1), the highest, to our knowledge, among all phosphorene related materials reported to date. Phonon dispersion curves and BOMD simulation confirm dynamic and thermal stability of α-P3Cl2 monolayer. Most importantly, contrary to other phosphorene related materials, α-P3Cl2 monolayer exhibits excellent oxidation resistance. Hence, coating halogen atoms on phosphorene surface


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seems to be an effective way to resist O2 attack and to protect the phosphorene framework from oxidation. The outstanding optical properties and desirable band alignment of 2D P3Cl2 suggest its potential as a photocatalyst. The 2D semiconductors, α-P3X2 (X=F, Br, I), are also predicted to possess a direct bandgap, ranging from 2.16 to 2.43 eV, as well as high stability against oxidization. Overall, the combined superior stability against oxidation, novel electronic properties, high charge mobility, and desired optical properties render phosphorene chloride even phosphorene halogenide exciting post-phosphorene functional material for future nanoelectronic and optoelectronic applications.

Computational Methods All calculations are performed within the framework of density-functional theory (DFT), implemented in the Vienna ab initio simulation package (VASP 5.3).48, 49 The generalized gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) functional, and projector augmented wave (PAW) potentials are used.50-52 Dispersion-corrected DFT method (optB88-vdW functional) is used in all the calculation,53, 54 which has been proven reliable for phosphorene systems.7 The vacuum spacing in the supercell is larger than 15 Å so that interaction among periodic images can be neglected. An energy cutoff of 500 eV is adopted for the plane-wave expansion of the electronic wave function. During the DFT calculation, the k-point sampling is carefully examined to assure that the calculation results are converged. Geometry structures are relaxed until the force on each atom is less than 0.01 eV/Å, while the energy convergence criteria of 1 × 10-5 eV are met. Since DFT/GGA method tends to underestimate bandgap of semiconductors, the screen hybrid HSE06 method is also used to examine the band structures.55 For αP3Cl2, the G0W0 calculation are also performed and details can be found in SI. The minimum-energy pathway for the elementary reaction step is computed by using the climbing-image nudged elastic band (CI-NEB) method56 implemented in the VASP. The Bader's atom in molecule (AIM) method (based on charge density topological analysis) is used for the charge population calculation.57 The Born-Oppenheimer molecular dynamics (BOMD) simulations are performed with the supercell containing more than 100 atoms, in the constant-temperature and constantvolume (NVT) ensemble. The time step in the BOMD simulations is 1 fs, and each independent BOMD simulation lasts 5 ps. The phonon dispersion spectrum is computed by using phonopy, for which the DFPT method is selected.58 The formation energy is defined as Ef = (Esystem – NPμP – NXμX)/NP, where Esystem is the total energy of unit cell (X = H, F, Cl, Br or I), NP and NX are the number of P and X atoms, respectively. The chemical potential μP is chosen as the cohesive energy per atom of the black phosphorene, and μX is


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the energy per atom of H2, F2, Cl2 Br2 or I2 molecules. The binding energies of phosphorene halogenide multilayers are defined as Eb = (Emultilayer – nEmonolayer)/Natom, where Emultilayer is the total energy of the multilayers, Emonolayer is the energy of the monolayer, n is the number of layers and Natom is the number of atoms in the system.

ASSOCIATED CONTENT Supporting Information Available: Discussion of different computational methods and details of carrier mobility computation. Lattice constant, bandgap, band structure and PDOS of P3X2 (X=F, Cl, Br, I) and 2D layers. Snapshots of BOMD simulations of configurations revealing interaction of O2 with monolayer α−P3Cl2 and other materials. The partial charge density of the monolayer and bilayer α-P3Cl2. Movies of 10 ps MD simulation at 1000K, revealing interaction of O2 with α-P3Cl2 monolayer. AUTHOR INFORMATION Corresponding Authors *Email: luning@ ahnu.edu.cn (N.L.). *Email: hyguowd@ ahnu.edu.cn (H.G.). *Email: [email protected] (X.C.Z.).

ORCID Ning Lu: 0000-0003-2846-2496 Xiaojun Wu: 0000-0003-3606-1211 Xiao Cheng Zeng: 0000-0003-4672-8585

Author Contributions N.L.

and Z.Z. contribute equally to this work.


17


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Notes There are no conflicts to declare.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21503002, No. 21403005

and

No.

11474150);

and

Anhui

Provincial

Natural

Science

Foundation

(No.1608085QB40). X.C.Z. was supported by US NSF through the Nebraska Materials Research Science and Engineering Center (MRSEC) (grant No. DMR-1420645), and by a State Key R&D Fund of China (2016YFA0200604) to USTC for summer research, and UNL Holland Computing Center, Supercomputing Center of USTC.

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P3Cl2: A Unique Post-Phosphorene 2D Material with Superior

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