Mono-Elemental Properties of 2D Black Phosphorus Ensure Extended

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Mono-Elemental Properties of 2D Black Phosphorus Ensure Extended Charge Carrier Lifetimes Under Oxidation: Time-Domain Ab Initio Analysis Lili Zhang, Andrey S. Vasenko, Jin Zhao, and Oleg V. Prezhdo J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00042 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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The Journal of Physical Chemistry Letters

Mono-Elemental Properties of 2D Black Phosphorus Ensure Extended Charge Carrier Lifetimes under Oxidation: Time-Domain Ab initio Analysis Lili Zhang,1,2 Andrey S. Vasenko,3 Jin Zhao,1,4,* Oleg V. Prezhdo2,* 1

ICQD/Hefei National Laboratory for Physical Sciences at Microscale, and Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 2

Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA 3 4

National Research University Higher School of Economics, 101000 Moscow, Russia

Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

Abstract: An attractive two-dimensional semiconductor with tunable direct bandgap and high carrier mobility, black phosphorus (BP) is used in batteries, solar cells, photocatalysis, plasmonics and optoelectronics. BP is sensitive to ambient conditions, with oxygen playing a critical role in structure degradation. Our simulations show that BP oxidation slows down charge recombination. This is unexpected, since typically charges are trapped and lost on defects. First, BP has no ionic character. It interacts with oxygen and water weakly, experiencing little perturbation to electronic structure. Second, phosphorus supports different oxidation states and binds extraneous atoms avoiding deep defect levels. Third, soft BP structure can accommodate foreign species without disrupting periodic geometry. Finally, BP phonon scattering on defects shortens quantum coherence and suppresses recombination. Thus, oxidation can be regarded as production of a self-protective layer that improves BP properties. These BP features should be common to other mono-elemental 2D materials, stimulating energy and electronics applications.

Keywords: black phosphorus, charge recombination, oxidation, water, nonadiabatic molecular dynamics, time-dependent density functional theory *Corresponding authors. Email: [email protected], [email protected]

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Black phosphorus (BP) is a new functional two-dimensional (2D) material that has attracted intense attention1-10 owing to its tunable thickness-dependent direct band gap, which varies from 0.3 eV for the bulk to 1.5~2.0 eV for the monolayer,10, 11 high carrier mobility up to 1000 cm2/V.s and an on/off ratio up to 104-105.1,

4

Similar to graphite, BP is a layered material, in which

interlayers are stacked together with van der Waals (vdW) interactions, and intralayer phosphorus atoms covalently bond with three nearest neighbors through sp3 hybridization to form a puckered honeycomb structure.12 Furthermore, BP is a p-type semiconductor whose band gap is sensitive to external electric field and strain.13-15 BP shows peculiar in-plane anisotropic properties which can distinguish itself from other 2D materials.9,

11, 16

These outstanding properties make BP a

promising 2D material candidate for wide applications in batteries,17,

18

optoelectronics,19-21

photocatalysis,22, 23 photovoltaics24 and plasmonics.25-27 However, BP is reported unstable and easy to degrade in ambient conditions,28, 29 which motivates a number of studies aimed at elucidating the underlying chemical mechanisms of the degradation process.30-37 Oxygen, water and light are important factors for the photo-degradation in air.30 Early studies observed degradation of BP surface after exposure to air even without photo assistance, supporting the combined roles of oxygen and water in the chemical breakdown of BP.34 Recently, many more experiments and calculations verified that oxygen plays a crucial role in BP’s degradation in ambient conditions.35-41 For example, Huang etc. reported joint experimental and theoretical results that the reaction with oxygen is primarily responsible for chemical modifications and changes in the electronic properties of BP.36 In contrast, BP is stable in contact with deoxygenated water, and the carrier mobility of field-effect transistors made with BP increases significantly due to efficient dielectric screening of water.36 The results suggest that the oxidation happens first during the chemical degradation of BP, then it is followed by exothermic reaction with water, because BP surface becomes more hydrophilic after oxidation.36, 37

Since BP can be easily oxidized in ambient conditions, it cannot be fabricated by chemical vapor deposition (CVD), and large-scale efficient production of high-quality BP is still challenging. Currently, there are three common BP fabrication methods: (1) pressure-induced phase transformation, which needs a complex apparatus, is time-consuming, and can only produce small BP nanoparticles;42,

43

(2) mechanical exfoliation, which is the original method of

fabrication of layers of 2D materials such as graphene, and which often suffers from low yield and intrinsic defects;44,

45

(3) liquid phase exfoliation, which has been adopted for large-scale

deposition thin films, but which may introduce residual organic molecules at the BP nanosheet surface that are difficult to remove.46-48 Recently, Kang etc. presented a high-yield fabrication method based on surfactant-assisted exfoliation and post-processing in deoxygenated water to get stable and high-performance BP flakes based on the positive effects of water.49 There exist controversial opinions for the effects of oxygen. On the one hand, many effective encapsulation and passivation methods are adopted to protect BP from air contact, employing AlOx,28 graphene and h-BN29. On the other hand, there are experiments showing the surface oxygen functional groups behaving as a self-protective layer.50 The oxidation and interaction with water play important roles during all the different fabrication methods and applications of BP. Therefore, it is important and urgent to understand how the excited charge carrier dynamics in BP depend on


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interactions with oxygen and H2O, for both fundamental science and applications in optoelectronics, photocatalysis and photovoltaics. In this report, we use ab initio nonadiabatic molecular dynamics (NAMD) to study nonradiative electron-hole (e-h) recombination of BP under low coverage of oxygen and H2O. Our results show that the oxidation slows down the e-h recombination. Rather than creating e-h recombination centers, the oxidation modifies the phonon modes that couple to the electronic subsystem, slowing down the inelastic scattering and shortening quantum coherence time. In this sense, the oxidation can be understood as generation of a self-protective layer that enhances BP optoelectronic properties. The original phonon modes around 400 cm-1, governing the e-h recombination process in pristine BP, are scattered by O atoms, generating softer phonons with frequencies below 200 cm-1. As a result, the nonadiabatic coupling (NAC) between the valence band maximum (VBM), the conduction band minimum (CBM) and the defect states decreases, and the e-h recombination rate drops. In addition, the phonon scattering accelerates decoherence between these states, which further suppresses the e-h recombination. Moreover, when the oxidized BP interacts with H2O, the deceleration of the e-h recombination is retained. Our study reveals the positive roles of oxygen and H2O in passivating BP, and provides theoretical guidance for its multiple functional applications. Our calculations employ the quantum-classical decoherence induced surface hopping (DISH) technique implemented within the time-dependent Kohn-Sham theory 51-53 under the classical path approximation.54 The geometry optimization, electronic structure calculations, and adiabatic molecular dynamic are performed using the Vienna Ab initio Simulation Package (VASP),55 the projector augmented wave (PAW)56 method to describe electron-ion interaction, and the Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA)57 to treat electron exchange-correction interactions. The van der Walls corrected functional with Becke86 optimization (optB86-vdW)58 is used for treating dispersive interaction to get accurate lattice geometry. The NAMD simulation for e-h recombination is carried out by the PYXAID code.54, 59 This method has been proven reliable in a variety of materials such as BP,60 MoS2,61, 62 graphene/TiO2 interface63 and many other systems.64-69 The current method focuses on electron-phonon interactions, treating phonons at the fully atomistic ab initio level, e.g. without the harmonic approximation. The NAMD calculations require thousands of electronic structure calculations along the MD trajectories, and due to computational difficulties, excitonic effects cannot be included. Note that excitonic interactions between electrons and holes bring them closer together in space and lower the excited state energy. In general, one expects these factors to favor faster recombination. On the other hand, defects already create localized states, and excitonic effects on defect wavefunctions and energy levels should be less significant than on free charges. Therefore, if charge recombination proceeds faster in defect-free than defected systems without excitonic effects, this conclusion should be even stronger if excitonic effects were included. We use a 5×4×1 supercell to model the BP monolayer. A 22 Å vacuum layer in the z-direction is used to eliminate spurious interactions between the periodic images. The relaxed



lattice constants along the zigzag and armchair directions are a=3.31 Å and b=4.56 Å for the unit cell of the BP monolayer, respectively. We choose the energy cutoff of 450 eV and the first Brillouin zone is sampled at the Γ point only, which contains the fundamental band gap. All the structures are fully relaxed until the total energy and atomic forces are smaller than 10-5 eV and 0.01 eV/Å, respectively. After geometry optimization, the systems are heated to room temperature for 5 ps by velocity rescaling. Then, 6 ps adiabatic MD trajectories obtained in the microcanonical ensemble with 1 fs atomic time step. To simulate the charge carrier recombination dynamics over a nanosecond timescale, the 6 ps nonadiabatic Hamiltonians are iterated 200 times. The simulation results are based on averaging 50 random initial configurations and 2000 surface hopping trajectories for each initial structure.

Figure 1. Side and top views of the atomic structures of (a) pristine BP monolayer, (b) BP-O, (c) BP-2O, (d) BP-Osub, (e) BP-O-H2O, and (f) BP-2O-H2O. Purple, red and white balls represent P, O and H atoms, respectively. We investigate six systems including the pristine BP monolayer and five oxidized layers, Figure 1. Because oxidation tends to happen in the outermost layer of a multi-layer BP, the current systems can be viewed also as models for oxidized multi-layer BP. We consider three different oxidation structures: i) a single O atom adsorbed on BP (BP-O) forming a dangling O; ii) two O atoms adsorbed separately on top and bottom P atoms (BP-2O), forming one dangling O and one bridging O; iii) a P atom substituted by an O atom (BP-Osub). These structures are in agreement with the previous investigations.36, 70, 71 According to Ref.,36 the structures shown in Figure 1b-c are the most stable structures of oxidized BP. Oxygen substitution is also observed to exist in


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experiment.71 These oxidation structures are consistent with two kinds of bonds, O=P-O and P-O-P, formed during degradation of BP structure.70, 72 DFT calculations predict the adsorption energies per O atom for the BP-O and BP-2O structures to be 2.85 and 2.86 eV respectively, indicating that their stabilities are very similar. The most stable O substitution structure contains an O atom substituting a P atom and bonded with two other P atoms to form a P-O-P bond, as shown in Figure 1d. The formation energy of BP-Osub is 1.34 eV. The relatively small value suggests that the O substitution can happen quite easily in the experiments. Two more systems are investigated, involving the BP-O and BP-2O structures interacting with an H2O molecule. These structures (labeled as BP-O-H2O and BP-2O-H2O) are shown in Figure 1e-f. H2O adsorbs on the oxidized BP layer by forming a hydrogen bond (Hb) with the dangling O atoms in BP-O and BP-2O. The H2O adsorption energies are 0.38 and 0.47 eV for BP-O and BP-2O respectively. The energy is larger for BP-2O since the interaction of H2O with two oxygens is stronger than with one oxygen. The adsorbed O atom and H2O molecule have little influence on the original BP structure, except that the bridge O atom upshifts the bonded P atom by 0.62 Å.

Figure 2. Total and partial DOS of (a) pristine BP monolayer, (b) BP-O, (c) BP-2O, (d) BP-Osub, (e) BP-O-H2O, and (f) BP-2O-H2O. The black, red and blue lines represent contributions from P atoms, O atom and water molecule, respectively. The partial DOS of O and H2O are multiplied by a factor of 30 to make it visible. It is important to investigate the BP electronic structure prior to considering the charge-phonon dynamics. Figure 2 shows the density of states (DOS) of the six different systems. The calculated band gap of the pristine BP monolayer is 0.85 eV, which is underestimated

10

due

to the well-known self-interaction error of DFT. The band gap increases to 0.90 eV upon adsorption of O and H2O. The band gap increases to 1.0 eV in the BP-Osub structure, and a deep isolated defect level appears 0.3 eV above the VBM because of the additional electron introduced by O relative to P. The charge densities of the VBM, CBM of all systems and the defect state in



BP-Osub are shown in Figure 3. VBM of the pristine BP monolayer is derived from the bonding orbitals between P atoms in different sub-layers and the anti-bonding orbitals between P atoms in the same sub-layer. The adsorption of single or double O eliminates electron density of both VBM and CBM around the adsorption sites because of the O-P bond formation, as shown in Figure 3b-c. For BP-Osub, the substitution of O atom introduces one excess electron, which localizes on the P atoms around the O atom as shown in Figure 3d. Adsorption of H2O on BP-O and BP-2O does not change the DOS and orbital distributions of VBM and CBM significantly.

Figure 3. Side and top views of the charge densities of the VBM, CBM and defect state in (a) pristine BP monolayer, (b) BP-O, (c) BP-2O, (d) BP-Osub, (e) BP-O-H2O, and (f) BP-2O-H2O. The isosurface value is set to 0.0004 e/bohr3.


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Figure 4. E-h recombination dynamics in (a) pristine BP monolayer, (b) BP-O, (c) BP-2O, (d) BP-Osub, (e) BP-O-H2O, and (f) BP-2O-H2O. The black and blue lines show populations of the initially excited state (band gap excitation) and the ground state, respectively. The red line gives population of the hole trap state. The time-dependent e-h recombination dynamics in the different BP systems are shown in Figure 4. For the pristine BP, the e-h recombination timescale is 285 ps, which is consistent with the previous theoretical report 60 and the experimental timescales ranging from 100 ps 9 to 1 ns. 73 After the single and double oxygen adsorption, the e-h recombination is significantly decelerated. As shown in Figure 4b-c, the e-h recombination timescales are increased to 468 ps and 1080 ps in BP-O and BP-2O, respectively. They are longer than in the pristine BP by factors of 1.6 and 3.8. The charge relaxation dynamics in the BP-Osub system (Figure 4d) start by hole trapping on a 115 ps timescale. After that, the hole recombines with the CBM electron on a 575 ps timescale. The ground state recovery takes 724 ps, which is about 2.5 times longer than in the pristine BP. The singly occupied trap level of the BP-Osub system can act as an electron trap as well. The electron trapping takes longer than hole trapping, because the trap state is farther in energy from CBM than VBM, Figure 2d. The electron trapping processes in considered in Supporting Information. With H2O adsorption, the e-h recombination time scales for BP-O-H2O and BP-2O-H2O are 540 ps and 644 ps, respectively, which is still significantly slower than in the pristine BP. In order to establish that the carrier recombination is limited by nonradiative rather than radiative decay, we calculated the radiative lifetimes for the pristine and oxidized BP monolayers, as described in Supporting Information. The radiative decay times are significantly longer than the nonradiative electron-hole recombination times, indicating that nonradiative relaxation is the main process responsible for the carrier decay. The PBE functional used in the calculations is computationally efficient, however, it is known to underestimate energy gaps. As an additional test, we used the hybrid HSE06 functional 74, 75

to computed the DOS and charge densities for the optimized geometries of the six systems.



Further, we scaled the PBE energy gaps to the corresponding HSE06 values and repeated the NAMD calculations. The data shown in Supporting Information confirm the original conclusions. The DOS and charge densities of the key levels are very similar in the PBE and HSE06 calculations, apart from the larger HSE06 energy gaps, Figures S1 and S2. The charge recombination is still slowest in the pristine system, Figure S3. There are two main factors that affect the e-h recombination timescale in the present calculation. These are the NAC, which governs inelastic charge-phonon scattering, and the pure-dephasing time, which controls loss of coherence between the electronic states. Larger NAC and longer coherence typically lead to faster dynamics. In Table I we list the NAC and the pure-dephasing times between the VBM, CBM and defect states. All the five systems with defects have smaller NAC and shorter pure-dephasing time compared with the pristine BP. The underlying physics can be revealed by the following analysis of the NAC and pure-dephasing time. First, consider the NAC that can be written as: 𝑑!" = 𝜑!

𝜑! ∇! 𝐻 𝜑! 𝜕 𝜑! = 𝑅 𝜕𝑡 𝜖! − 𝜖!

(1)

where H is the Kohn-Sham Hamiltonian, φk, φj, εk and εj are the wave functions and eigenvalues for electronic states k and j, and Ṙ is velocity of the nuclei.76 Thus, the NAC depends on the energy difference εk - εj, the electron-phonon coupling matrix element 𝜑! ∇! 𝐻 𝜑! and the nuclear velocity Ṙ. The dominant phonon modes contributing to the charge-phonon coupling can be characterized by Fourier transforms (FT) of the evolution of the energies of the VBM, CBM and defect states in different systems (Figure 5). The intensity of the FT peaks is determined by

the amplitude of phonon-induced fluctuations in the corresponding energies, and characterizes the electron-phonon coupling strength. Since NAC is also a measure of electron-phonon coupling, the NAC values and FT intensities are related. As shown in Figure 5a for the pristine BP layer, the CBM and VBM couple with the out-of-plane Ag1 mode and in-plane B2g and Ag2 modes along the zigzag and armchair directions at around 400 cm-1.77, 78 VBM also couples with the interlayer ZA mode around 80 cm-1.79 After oxygen adsorption or substitution, especially for the BP-2O and BP-Osub systems (Figure 5c-d), the three dominant peaks around 400 cm-1 are replaced by many small peaks over a wide range, indicating that the BP modes are scattered strongly by the interaction between the P and O atoms. Significant contributions to the charge-phonon coupling arise from phonon modes below 200 cm-1. The low phonon frequencies lead to small nuclear velocities, and hence, small NAC, Eq. (1) and Table I. The interaction with H2O reduces slightly the phonon scattering, especially for BP-2O-H2O as shown in Figure 5f. As a result, the e-h recombination accelerates from 1080 ps to 644 ps. But due to the significant contribution of low frequency phonons in the range of 100-200 cm-1, the e-h recombination timescale of BP-2O-H2O is still 2.3 times longer than that in the pristine BP. In addition to the phonon frequencies, orbital distributions also affect the NAC through the charge-phonon coupling matrix element 𝜑! ∇! 𝐻 𝜑! , Eq. (1). The oxygen adsorption eliminates charge densities around the adsorption sites (Figure 3b-c and e-f). The oxygen substitution induces a localized defect state (Figure 3d). The CBM and VBM orbital redistribution in BP-O/BP-O-H2O


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and BP-2O/BP-2O-H2O, as well as the localization of the defect state in BP-Osub, reduce overlap between the electron and hole orbitals states, decrease the 𝜑! ∇! 𝐻 𝜑! term, and hence, the NAC. Second, the nonradiative charge carrier relaxation dynamics depends on the coherence (pure-dephasing) time between the electronic states. Loss of coherence within the electronic subsystem is induced by elastic electron-phonon scattering. The pure-dephasing time between a pair of states is calculated using the optical response theory from the autocorrelation function of evolution of their energy difference.53 Listed in Table I, the time of coherence between the VBM and the CBM in the pristine system is 82.4 fs. This value corresponds to the period of the dominant optical phonon modes around 400 cm-1, which couple with both the VBM and the CBM. After O adsorption, the VBM-CBM pure-dephasing time is decreased to 35.2 fs and 22.9 fs in BP-O and BP-2O, respectively. The pure-dephasing times between the VBM, CBM and defect states in BP-Osub are also shorter than the VBM-CBM time in the pristine system. They are 55.4fs, 18.3fs and 22.1 fs, Table I. After oxygen adsorption or substitution, the dominant phonon peaks around 400 cm-1 are changed into multiple peaks over a wide frequency range due to scattering of the original BP phonons by the oxygen atoms. Such scattering enhances decoherence between the electronic states. The VBM-CBM pure-dephasing time in BP-O-H2O is similar to that in BP-O. For BP-2O-H2O, the interaction with H2O decreases the phonon scattering slightly and the pure-dephasing time is increased to 66.2 fs. The accelerated loss of coherence within the electronic subsystem due to scattering of the BP phonons by the O atoms provides another mechanism of suppression of the e-h recombination dynamics in BP interacting with oxygen and H2O. Table I. NAC and pure-dephasing times for the state pairs involved in the e-h recombination dynamics. NAC (meV)

Pure-dephasing time (fs)

VBM-CB

VBM-defec

CBM-defec

VBM-CB

VBM-defec

CBM-defec

M

t

t

M

t

t

18.3

22.1

Pristine

1.30

82.4

BP-O

1.05

35.2

BP-2O

0.68

22.9

BP-Osub

0.43

BP-O-H2O

0.96

33.5

BP-2O-H2

0.84

66.2

1.29

0.81

55.4

O



Figure 5. FT of the phonon-induced fluctuations of the VBM, CBM and defect state energies in (a) the pristine BP monolayer, (b) BP-O, (c) BP-2O, (d) BP-Osub, (e) BP-O-H2O and (f) BP-2O-H2O. To summarize, we have investigated how BP oxidation and interaction with water influence the nonradiative charge carrier recombination. We have found that both the oxidation and the interaction with water suppress the e-h recombination, in contrast to the a priori expectation that defects and fast polar molecules should trap charges, enhance charge-phonon coupling and accelerate charge losses. Adsorption of the double oxygen in particular can decelerate the e-h recombination by a factor of 4. The e-h recombination is slowed down mainly because of scattering of the higher frequency phonons of the pristine BP induced by oxygen adsorption or substitution. On the one hand, the scattering of the BP phonons on the oxygen defects leads to coupling of charges with soft and slow phonon modes at frequencies below 200 cm-1, decreasing the NAC matrix elements. On the other hand, the phonon scattering by the oxygen atoms accelerates decoherence between the electronic states. Both the decreased NAC and the accelerated decoherence suppress the e-h recombination. The effect is maintained when the oxidized species interact with H2O. The arguments presented in the current work apply to monoelemental 2D materials that contain no ionic or polar bonds and are flexible. The arguments can be extended to similar materials, such as graphene, graphene quantum dots and ribbons, graphyne, borophene, germanene, silicone, stanene, bithmuthene, etc. Previous calculations demonstrate that in stiff materials, such as carbon nanotubes, large amounts of water can accelerate charge recombination


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by decreasing the energy gap.

80

Exposure to large amounts of water accelerate charge

recombination in halide perovskites by coupling the charges to high frequency water motions, while small amounts of water slow down the recombination by facilitating charge separation.81 In the absence of ionic character, mono-elemental materials interact with impurities weakly, experiencing little perturbation to its geometric and electronic structure. Elements such as phosphorus can support a variety of oxidation states and accommodate extraneous atoms avoiding deep trap states. 2D materials are soft and flexible, and can incorporate foreign species avoiding significant disruption to their periodic structure. The enhancement of the charge carrier lifetimes in the presence of oxygen and water can be interpreted as arising from a self-protection oxide layer created during material fabrication. The reported results provide valuable insights into the role of oxygen and water in BP passivation, and generate important theoretical guidance for future energy and electronics applications of BP and other 2D materials. Acknowledgments L. Z. gratefully acknowledges the financial support provided by China Scholarship Council (CSC) (No. 201706340139) during the one-year study at the University of Southern California. A.S.V. acknowledges support of the joint Russian−Greek Projects RFMEFI61717X0001 and T4ΔPΩ-00031 “Experimental and theoretical studies of physical properties of low dimensional quantum nanoelectronic systems”. J. Z. acknowledges the support of National Key Foundation of China,

Department

of

Science

and

Technology,

grant

nos.

2017YFA0204904,

and

2016YFA0200604; National Natural Science Foundation of China (NSFC), grant no 11620101003; the Fundamental Research Funds for the Central Universities of China (WK3510000005). O. V. P. acknowledges the funding from the U.S. Department of Energy, grant No. DE-SC0014429. Calculations were performed at the Environmental Molecular Sciences Laboratory at the PNNL, a user facility sponsored by the DOE Office of Biological and Environmental Research, and at the Supercomputing Center at USTC. Supporting Information. Densities of states, charge densities and recombination dynamics obtained with the help of the HSE06 functional, analysis of the electron trapping pathway in the BP-Osub system, and radiative decay times. References (1) Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorus Field-Effect Transistors. Nat. Nanotech. 2014, 9, 372-377. (2) Castellanos-Gomez, A. Black Phosphorus: Narrow Gap, Wide Applications. J. Phys. Chem. Lett. 2015, 6, 4280-4291. (3) Chowdhury, C.; Datta, A. Exotic Physics and Chemistry of Two-Dimensional Phosphorus: Phosphorene. J. Phys. Chem. Lett. 2017, 8, 2909-2916. (4) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tomanek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033-4041. (5) Reich, E. S. Phosphorene Excites Materials Scientists. Nature 2014, 506, 19. (6) Kou, L.; Chen, C.; Smith, S. C. Phosphorene: Fabrication, Properties, and Applications. J. Phys. Chem. Lett. 2015, 6, 2794-2805.



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