Mono-Elemental Properties of 2D Black Phosphorus EnsureExtended Charge Carrier Lifetimes under Oxidation: Time-DomainAb Initio AnalysisLili Zhang,†,‡ Andrey S. Vasenko,§ Jin Zhao,*,†,∥ and Oleg V. Prezhdo*,‡

†ICQD/Hefei National Laboratory for Physical Sciences at Microscale, and Key Laboratory of Strongly-Coupled Quantum MatterPhysics, Chinese Academy of Sciences, and Department of Physics, University of Science and Technology of China, Hefei, Anhui230026, China‡Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States§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

*S Supporting Information

ABSTRACT: An attractive two-dimensional semiconductor with tunabledirect bandgap and high carrier mobility, black phosphorus (BP), is used inbatteries, solar cells, photocatalysis, plasmonics, and optoelectronics. BP issensitive to ambient conditions, with oxygen playing a critical role instructure degradation. Our simulations show that BP oxidation slows downcharge recombination. This is unexpected, since typically charges aretrapped and lost on defects. First, BP has no ionic character. It interactswith oxygen and water weakly, experiencing little perturbation to electronicstructure. Second, phosphorus supports different oxidation states and bindsextraneous atoms avoiding deep defect levels. Third, soft BP structure canaccommodate foreign species without disrupting periodic geometry.Finally, BP phonon scattering on defects shortens quantum coherenceand suppresses recombination. Thus, oxidation can be regarded as production of a self-protective layer that improves BPproperties. These BP features should be common to other monoelemental 2D materials, stimulating energy and electronicsapplications.

Black phosphorus (BP) is a new functional two-dimen-sional (2D) material that has attracted intense atten-

tion1−10 owing to its tunable thickness-dependent direct bandgap, which varies from 0.3 eV for the bulk to 1.5−2.0 eV forthe 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 isa layered material, in which interlayers are stacked togetherwith van der Waals (vdW) interactions and intralayerphosphorus atoms covalently bond with three nearestneighbors through sp3 hybridization to form a puckeredhoneycomb structure.12 Furthermore, BP is a p-type semi-conductor whose band gap is sensitive to external electric fieldand strain.13−15 BP shows peculiar in-plane anisotropicproperties that can distinguish itself from other 2Dmaterials.9,11,16 These outstanding properties make BP apromising 2D material candidate for wide applications inbatteries,17,18 optoelectronics,19−21 photocatalysis,22,23 photo-voltaics,24 and plasmonics.25−27

However, BP is reported unstable and easy to degrade inambient conditions,28,29 which motivates a number of studiesaimed at elucidating the underlying chemical mechanisms ofthe degradation process.30−37 Oxygen, water, and light are

important factors for the photodegradation in air.30 Earlystudies observed degradation of BP surface after exposure toair even without photo assistance, supporting the combinedroles of oxygen and water in the chemical breakdown of BP.34

Recently, many more experiments and calculations verified thatoxygen plays a crucial role in BP’s degradation in ambientconditions.35−41 For example, Huang etc. reported jointexperimental and theoretical results that the reaction withoxygen is primarily responsible for chemical modifications andchanges in the electronic properties of BP.36 In contrast, BP isstable in contact with deoxygenated water, and the carriermobility of field-effect transistors made with BP increasessignificantly due to efficient dielectric screening of water.36 Theresults suggest that the oxidation happens first during thechemical degradation of BP and then it is followed byexothermic reaction with water, because BP surface becomesmore hydrophilic after oxidation.36,37

Received: January 7, 2019Accepted: February 19, 2019Published: February 19, 2019

Letter

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Since BP can be easily oxidized in ambient conditions, itcannot be fabricated by chemical vapor deposition (CVD), andlarge-scale efficient production of high-quality BP is stillchallenging. Currently, there are three common BP fabricationmethods: (1) pressure-induced phase transformation, whichneeds a complex apparatus, is time-consuming, and can onlyproduce small BP nanoparticles;42,43 (2) mechanical exfolia-tion, which is the original method of fabrication of layers of 2Dmaterials such as graphene, and which often suffers from lowyield 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 BPnanosheet surface that are difficult to remove.46−48 Recently,Kang etc. presented a high-yield fabrication method based onsurfactant-assisted exfoliation and postprocessing in deoxy-genated water to get stable and high-performance BP flakesbased on the positive effects of water.49 There existcontroversial opinions for the effects of oxygen. On the onehand, many effective encapsulation and passivation methodsare adopted to protect BP from air contact, employing AlOx,

28

graphene, and h-BN.29 On the other hand, there areexperiments showing the surface oxygen functional groupsbehaving as a self-protective layer.50 The oxidation andinteraction with water play important roles during all thedifferent fabrication methods and applications of BP. There-fore, it is important and urgent to understand how the excitedcharge carrier dynamics in BP depend on interactions withoxygen and H2O, for both fundamental science andapplications in optoelectronics, photocatalysis, and photo-voltaics.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 andH2O. Our results show that the oxidation slows down the e−hrecombination. Rather than creating e−h recombinationcenters, the oxidation modifies the phonon modes that coupleto the electronic subsystem, slowing down the inelasticscattering 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 thee−h recombination process in pristine BP, are scattered by Oatoms, generating softer phonons with frequencies below 200cm−1. As a result, the nonadiabatic coupling (NAC) betweenthe valence band maximum (VBM), the conduction bandminimum (CBM) and the defect states decreases, and the e−hrecombination rate drops. In addition, the phonon scatteringaccelerates decoherence between these states, which furthersuppresses the e−h recombination. Moreover, when theoxidized BP interacts with H2O, the deceleration of the e−hrecombination is retained. Our study reveals the positive rolesof oxygen and H2O in passivating BP and provides theoreticalguidance for its multiple functional applications.Our calculations employ the quantum-classical decoherence

induced surface hopping (DISH) technique implementedwithin the time-dependent Kohn−Sham theory51−53 under theclassical path approximation.54 The geometry optimization,electronic structure calculations, and adiabatic moleculardynamic are performed using the Vienna Ab initio SimulationPackage (VASP),55 the projector augmented wave (PAW)56

method to describe electron−ion interaction, and the Perdew−Burke−Ernzerhof (PBE) functional under the generalizedgradient approximation (GGA)57 to treat electron exchange−correction interactions. The van der Waals corrected functionalwith Becke86 optimization (optB86-vdW)58 is used fortreating dispersive interaction to get accurate lattice geometry.The NAMD simulation for e−h recombination is carried outby the PYXAID code.54,59 This method has been provenreliable in a variety of materials such as BP,60 MoS2,

61,62

graphene/TiO2 interface,63 and many other systems.64−69

The current method focuses on electron−phonon inter-actions, treating phonons at the fully atomistic ab initio level,e.g., without the harmonic approximation. The NAMDcalculations require thousands of electronic structure calcu-lations along the MD trajectories, and due to computationaldifficulties, excitonic effects cannot be included. Note that

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.

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excitonic interactions between electrons and holes bring themcloser together in space and lower the excited state energy. Ingeneral, one expects these factors to favor faster recombination.However, defects already create localized states, and excitoniceffects on defect wave functions and energy levels should beless significant than on free charges. Therefore, if chargerecombination proceeds faster in defect-free than defectedsystems without excitonic effects, this conclusion should beeven 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 eliminatespurious 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 thefirst Brillouin zone is sampled at the Γ point only, whichcontains the fundamental band gap. All the structures are fullyrelaxed until the total energy and atomic forces are smallerthan 10−5 eV and 0.01 eV/Å, respectively. After geometryoptimization, the systems are heated to room temperature for 5ps by velocity rescaling. Then, 6 ps adiabatic MD trajectoriesare obtained in the microcanonical ensemble with a 1 fs atomictime step. To simulate the charge carrier recombinationdynamics over a nanosecond time scale, the 6 ps nonadiabatic

Figure 2. Total and partial DOSs of (a) pristine BP monolayer, (b) BP-O, (c) BP-2O, (d) BP-Osub, (e) BP-O-H2O, and (f) BP-2O-H2O. Theblack, red, and blue lines represent contributions from P atoms, the O atom, and the water molecule, respectively. The partial DOSs of O and H2Oare multiplied by a factor of 30 to make it visible.

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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Hamiltonians are iterated 200 times. The simulation results arebased on averaging 50 random initial configurations and 2000surface hopping trajectories for each initial structure.We investigate six systems including the pristine BP

monolayer and five oxidized layers, Figure 1. Because oxidationtends to happen in the outermost layer of a multilayer BP, thecurrent systems can be viewed also as models for oxidizedmultilayer BP. We consider three different oxidation structures:(i) a single O atom adsorbed on BP (BP-O) forming adangling O; (ii) two O atoms adsorbed separately on top andbottom P atoms (BP-2O), forming one dangling O and onebridging O; (iii) a P atom substituted by an O atom (BP-Osub).These structures are in agreement with the previousinvestigations.36,70,71 According to ref 36, the structuresshown in Figure 1b,c are the most stable structures of oxidizedBP. Oxygen substitution is also observed to exist inexperiment.71 These oxidation structures are consistent withtwo kinds of bonds, OPO and POP, formed duringdegradation of the BP structure.70,72 DFT calculations predictthe adsorption energies per O atom for the BP-O and BP-2Ostructures to be 2.85 and 2.86 eV respectively, indicating thattheir stabilities are very similar. The most stable O substitutionstructure contains an O atom substituting a P atom andbonded with two other P atoms to form a POP bond, asshown in Figure 1d. The formation energy of BP-Osub is 1.34eV. The relatively small value suggests that the O substitutioncan happen quite easily in the experiments. Two more systemsare investigated, involving the BP-O and BP-2O structuresinteracting with an H2O molecule. These structures (labeled asBP-O-H2O and BP-2O-H2O) are shown in Figure 1e,f. H2Oadsorbs on the oxidized BP layer by forming a hydrogen bond(Hb) with the dangling O atoms in BP-O and BP-2O. TheH2O adsorption energies are 0.38 and 0.47 eV for BP-O andBP-2O, respectively. The energy is larger for BP-2O since theinteraction of H2O with two oxygens is stronger than with oneoxygen. The adsorbed O atom and H2O molecule have little

influence on the original BP structure, except that the bridge Oatom upshifts the bonded P atom by 0.62 Å.It is important to investigate the BP electronic structure

prior to considering the charge−phonon dynamics. Figure 2shows the densities of states (DOSs) of the six differentsystems. The calculated band gap of the pristine BP monolayeris 0.85 eV, which is underestimated10 due to the well-knownself-interaction error of DFT. The band gap increases to 0.90eV upon adsorption of O and H2O. The band gap increases to1.0 eV in the BP-Osub structure, and a deep isolated defect levelappears 0.3 eV above the VBM because of the additionalelectron introduced by O relative to P. The charge densities ofthe VBM and CBM of all systems and the defect state in BP-Osub are shown in Figure 3. VBM of the pristine BP monolayeris derived from the bonding orbitals between P atoms indifferent sublayers and the antibonding orbitals between Patoms in the same sublayer. The adsorption of single or doubleO eliminates electron density of both VBM and CBM aroundthe adsorption sites because of the O−P bond formation, asshown in Figure 3b,c. For BP-Osub, the substitution of an Oatom introduces one excess electron, which localizes on the Patoms around the O atom as shown in Figure 3d. Adsorptionof H2O on BP-O and BP-2O does not change the DOSs andorbital distributions of VBM and CBM significantly.The time-dependent e−h recombination dynamics in the

different BP systems are shown in Figure 4. For the pristineBP, the e−h recombination time scale is 285 ps, which isconsistent with the previous theoretical report,60 and theexperimental time scales range from 100 ps9 to 1 ns.73 Afterthe single and double oxygen adsorption, the e−h recombi-nation is significantly decelerated. As shown in Figure 4b,c, thee−h recombination time scales are increased to 468 and 1080ps in BP-O and BP-2O, respectively. They are longer than inthe pristine BP by factors of 1.6 and 3.8. The charge relaxationdynamics in the BP-Osub system (Figure 4d) start by holetrapping on a 115 ps time scale. After that, the hole recombineswith the CBM electron on a 575 ps time scale. The ground

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 givespopulation of the hole trap state.

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state recovery takes 724 ps, which is about 2.5 times longerthan in the pristine BP. The singly occupied trap level of theBP-Osub system can act as an electron trap as well. The electrontrapping takes longer than hole trapping, because the trap stateis farther in energy from CBM than VBM, Figure 2d. Theelectron trapping processes is considered in SupportingInformation. With H2O adsorption, the e−h recombinationtime scales for BP-O-H2O and BP-2O-H2O are 540 and 644ps, respectively, which are still significantly slower than in thepristine BP. In order to establish that the carrier recombinationis limited by nonradiative rather than radiative decay, wecalculated the radiative lifetimes for the pristine and oxidizedBP monolayers, as described in Supporting Information. Theradiative decay times are significantly longer than thenonradiative electron−hole recombination times, indicatingthat nonradiative relaxation is the main process responsible forthe carrier decay.The PBE functional used in the calculations is computa-

tionally efficient, however, it is known to underestimate energygaps. As an additional test, we used the hybrid HSE06functional74,75 to computed the DOSs and charge densities forthe optimized geometries of the six systems. Further, we scaledthe PBE energy gaps to the corresponding HSE06 values andrepeated the NAMD calculations. The data shown inSupporting Information confirm the original conclusions.The DOSs and charge densities of the key levels are verysimilar in the PBE and HSE06 calculations, apart from thelarger HSE06 energy gaps, Figures S1 and S2. The chargerecombination is still slowest in the pristine system, Figure S3.There are two main factors that affect the e−h

recombination time scale in the present calculation. Theseare the NAC, which governs inelastic charge−phononscattering, and the pure-dephasing time, which controls lossof coherence between the electronic states. Larger NAC andlonger coherence typically lead to faster dynamics. In Table 1

we list the NAC and the pure-dephasing times between theVBM, CBM, and defect states. All the five systems with defectshave smaller NAC and shorter pure-dephasing time comparedwith the pristine BP. The underlying physics can be revealedby the following analysis of the NAC and pure-dephasing time.First, consider the NAC that can be written as

dt

HRjk j k

j R k

k jφ φ

φ φ= ∂

∂=

⟨ |∇ | ⟩

ϵ − ϵ

(1)

where H is the Kohn−Sham Hamiltonian, φk, φj, εk, and εj arethe wave functions and eigenvalues for electronic states k and j,and R is velocity of the nuclei.76 Thus, the NAC depends on

the energy difference εk − εj, the electron−phonon couplingmatrix element ⟨φj|∇RH|φk⟩, and the nuclear velocity R. Thedominant phonon modes contributing to the charge−phononcoupling can be characterized by Fourier transforms (FT) ofthe evolution of the energies of the VBM, CBM, and defectstates in different systems (Figure 5). The intensity of the FTpeaks is determined by the amplitude of phonon-inducedfluctuations in the corresponding energies and characterizesthe electron−phonon coupling strength. Since NAC is also ameasure of electron−phonon coupling, the NAC values andFT intensities are related. As shown in Figure 5a for thepristine BP layer, the CBM and VBM couple with the out-of-plane Ag

1 mode and in-plane B2g and Ag2 modes along the

zigzag and armchair directions at around 400 cm−1.77,78 VBMalso 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 dominantpeaks around 400 cm−1 are replaced by many small peaks overa wide range, indicating that the BP modes are scatteredstrongly by the interaction between the P and O atoms.Significant contributions to the charge−phonon coupling arisefrom phonon modes below 200 cm−1. The low phononfrequencies lead to small nuclear velocities, and hence, smallNAC, eq 1 and Table 1. The interaction with H2O reducesslightly the phonon scattering, especially for BP-2O-H2O, asshown in Figure 5f. As a result, the e−h recombinationaccelerates from 1080 to 644 ps. But due to the significantcontribution of low frequency phonons in the range of 100−200 cm−1, the e−h recombination time scale of BP-2O-H2O isstill 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 couplingmatrix element ⟨φj|∇RH|φk⟩, eq 1. The oxygen adsorptioneliminates charge densities around the adsorption sites (Figure3b,c and Figure 3e,f). The oxygen substitution induces alocalized defect state (Figure 3d). The CBM and VBM orbitalredistribution in BP-O/BP-O-H2O and BP-2O/BP-2O-H2O,and the localization of the defect state in BP-Osub, reduceoverlap between the electron and hole orbitals states anddecrease the ⟨φj|∇RH|φk⟩ term, and hence, the NAC.Second, the nonradiative charge carrier relaxation dynamics

depends on the coherence (pure-dephasing) time between theelectronic states. Loss of coherence within the electronicsubsystem is induced by elastic electron−phonon scattering.The pure-dephasing time between a pair of states is calculatedusing the optical response theory from the autocorrelationfunction of evolution of their energy difference.53 Listed inTable 1, the time of coherence between the VBM and theCBM in the pristine system is 82.4 fs. This value correspondsto the period of the dominant optical phonon modes around400 cm−1, which couple with both the VBM and the CBM.After O adsorption, the VBM−CBM pure-dephasing time isdecreased to 35.2 and 22.9 fs in BP-O and BP-2O, respectively.The pure-dephasing times between the VBM, CBM, and defectstates in BP-Osub are also shorter than the VBM−CBM time inthe pristine system. They are 55.4, 18.3, and 22.1 fs, Table 1.After oxygen adsorption or substitution, the dominant phononpeaks around 400 cm−1 are changed into multiple peaks over awide frequency range due to scattering of the original BPphonons by the oxygen atoms. Such scattering enhancesdecoherence between the electronic states. The VBM−CBMpure-dephasing time in BP-O-H2O is similar to that in BP-O.For BP-2O-H2O, the interaction with H2O decreases the

Table 1. NAC and Pure-Dephasing Times for the State PairsInvolved in the e−h Recombination Dynamics

NAC (meV) pure-dephasing time (fs)

VBM−CBM

VBM−defect

CBM−defect

VBM−CBM

VBM−defect

CBM−-defect

pristine 1.30 82.4BP-O 1.05 35.2BP-2O 0.68 22.9BP-Osub 0.43 1.29 0.81 55.4 18.3 22.1BP-O-H2O

0.96 33.5

BP-2O-H2O

0.84 66.2

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phonon scattering slightly and the pure-dephasing time isincreased to 66.2 fs. The accelerated loss of coherence withinthe electronic subsystem due to scattering of the BP phononsby the O atoms provides another mechanism of suppression ofthe e−h recombination dynamics in BP interacting withoxygen and H2O.To summarize, we have investigated how BP oxidation and

interaction with water influence the nonradiative charge carrierrecombination. We have found that both the oxidation and theinteraction with water suppress the e−h recombination, incontrast to the a priori expectation that defects and fast polarmolecules should trap charges, enhance charge−phononcoupling, and accelerate charge losses. Adsorption of thedouble oxygen in particular can decelerate the e−hrecombination by a factor of 4. The e−h recombination isslowed down mainly because of scattering of the higherfrequency phonons of the pristine BP induced by oxygenadsorption or substitution. On the one hand, the scattering ofthe BP phonons on the oxygen defects leads to coupling ofcharges with soft and slow phonon modes at frequencies below200 cm−1, decreasing the NAC matrix elements. On the otherhand, the phonon scattering by the oxygen atoms acceleratesdecoherence between the electronic states. Both the decreasedNAC and the accelerated decoherence suppress the e−hrecombination. The effect is maintained when the oxidizedspecies interact with H2O.The arguments presented in the current work apply to

monoelemental 2D materials that contain no ionic or polarbonds and are flexible. The arguments can be extended tosimilar materials, such as graphene, graphene quantum dots

and ribbons, graphyne, borophene, germanene, silicone,stanene, bithmuthene, etc. Previous calculations demonstratethat in stiff materials, such as carbon nanotubes, large amountsof water can accelerate charge recombination by decreasing theenergy gap.80 Exposure to large amounts of water acceleratecharge recombination in halide perovskites by coupling thecharges to high frequency water motions, while small amountsof water slow down the recombination by facilitating chargeseparation.81 In the absence of ionic character, monoelementalmaterials interact with impurities weakly, experiencing littleperturbation to its geometric and electronic structure.Elements such as phosphorus can support a variety ofoxidation states and accommodate extraneous atoms, avoidingdeep trap states. 2D materials are soft and flexible and canincorporate foreign species, avoiding significant disruption totheir periodic structure. The enhancement of the charge carrierlifetimes in the presence of oxygen and water can beinterpreted as arising from a self-protection oxide layer createdduring material fabrication. The reported results providevaluable insights into the roles of oxygen and water in BPpassivation and generate important theoretical guidance forfuture energy and electronics applications of BP and other 2Dmaterials.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jp-clett.9b00042.

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.

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Densities of states, charge densities, and recombinationdynamics obtained with the help of the HSE06functional, analysis of the electron trapping pathway inthe BP-Osub system, and radiative decay times (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: zhaojin@ustc.edu.cn.*E-mail: prezhdo@usc.edu.ORCIDJin Zhao: 0000-0003-1346-5280Oleg V. Prezhdo: 0000-0002-5140-7500NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSL.Z. gratefully acknowledges the financial support provided byChina Scholarship Council (CSC) (No. 201706340139)during the one-year study at the University of SouthernCalifornia. A.S.V. acknowledges support of the joint Russian−Greek Projects RFMEFI61717X0001 and T4ΔPΩ-00031“Experimental and theoretical studies of physical propertiesof low dimensional quantum nanoelectronic systems”. J.Z.acknowledges the support of National Key Foundation ofChina, Department of Science and Technology, grant nos.2017YFA0204904 and 2016YFA0200604; National NaturalScience Foundation of China (NSFC), grant no.11620101003; the Fundamental Research Funds for theCentral Universities of China (WK3510000005). O.V.P.acknowledges the funding from the U.S. Department ofEnergy, grant no. DE-SC0014429. Calculations were per-formed at the Environmental Molecular Sciences Laboratory atthe PNNL, a user facility sponsored by the DOE Office ofBiological and Environmental Research, and at the Super-computing Center at USTC.

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