Uniform B-C-N Ternary Monolayer from Non-Metal Filled g-C3N4Sheet

N Zhng,Xio-jun Wu,b,c∗

a.CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering,University of Science and Technology of China,Hefei 230026,China

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

c.School of Chemistry and Materials Science and Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China,Hefei 230026,China

Uniform B-C-N Ternary Monolayer from Non-Metal Filled g-C3N4Sheet

Na Zhanga,Xiao-jun Wua,b,c∗

a.CAS Key Laboratory of Materials for Energy Conversion and Department of Materials Science and Engineering,University of Science and Technology of China,Hefei 230026,China

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

c.School of Chemistry and Materials Science and Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China,Hefei 230026,China

By using f i rst principles calculations,four two-dimensional B-C-N ternary sheets with ordered and uniform element distribution are predicted based on the C,B,or N f i lled g-C3N4sheet.These B-C-N ternary sheets are metallic except for B4-C3N4monolayer,which is a semiconductor with an energy band gap of 1.18 eV.In particular,the B3C-C3N4is a ferromagnetic metal with a net magnetic moment of 0.57µB/cell,which can be used to develop metal-free spintronic device.The calculated formation energy indicates these B-C-N ternary sheets are highly thermal stable.It presents a new route to obtain uniform B-C-N ternary sheet for electronic and spintronic applications.

Density functional theory,B-C-N ternary monolayer,g-C3N4

I.INTRODUCTION

Two-dimensional(2D)materials with single atomic thickness have attracted extensive attention for their unique electronic,optical and planar properties with respect to the bulk materials[1,2].In particular,two analogs,i.e.graphene characterized as a semi-metal or zero gap semiconductor with unique electronic properties[3-5]and hexagonal boron nitride sheet(h-BN)as an insulator with a bandgap larger than 5 eV[6,7], are expected to play a key role in future nanotechnology as well as to provide potential applications in next generation electronics.

In recent years,the hybrid 2D materials by combining graphene and h-BN monolayer,i.e.2D B-C-N ternary sheets,have induced rising interest for their novel electronic and magnetic properties,remarkably different from two pristine hosts[8-16].For instance,introducing h-BN f l akes into graphene’s framework opens a band gap of graphene,induces magnetism or half-metallicity, makes them suitable for electronic and spintronic applications[11].Experimentally,there have been some successes in preparing 2D B-C-N compositions[8-10]. Ci et al.synthesized large-area atomic layers of B-CN nanosheets,consisting of hybridized,randomly distributed domains of h-BN and C phases with compositions ranging from pure BN to pure graphene[8]. Raidongia et al.have obtained B-C-N nanosheets by the reaction of high-surface-area activated charcoal with a mixture of boric acid and urea at 900°C[9],and Qin et al.synthesized B0.38C0.27N0.35compounds with few atomic layers by microwave plasma CVD(MPCVD) [10].Theoretically,a lot of other B-N-C ternary systems such as BC2N[12],BC4N[13],B3CN3[14],B3C2N3[15],have been proposed.However,it is found that most of these ternary systems have strong preferences for much more B-N bonds and C-C bonds compositions and the B-C-N ternary sheet prefers a hybridization of graphene and h-BN domains[8].How to obtain B-C-N ternary monolayer material with uniform or ordered distribution of B,C,and N is still a challenge.

Graphitic carbon nitride(g-C3N4)is a stacked twodimensional sheets of tris-triazine connected via tertiary amines,which is an analogue to graphite with nitrogen substitution.Due to its novel optical and chemical properties,g-C3N4has been demonstrated as a potential candidate for optoelectronic conversion and photocatalytic water splitting[16-23].Recently,ultrathin g-C3N4sheets with enhanced photoresponsivity and possible magnetism have been reported experimentally,implying the great potential applications of 2D g-C3N4sheet[19,20].Compared with graphene in structure, the g-C3N4monolayer is porous sheet with uniformdistributed vacancy at atomic level[24,25].Thus,it provides a great potential to f i lling the pores in g-C3N4sheet with B,C,or N atoms to prepare 2D B-C-N ternary materials with uniform and ordered distribution of B,C,and N elements.

In this work,we predicted four 2D B-C-N ternary materials by f i lling g-C3N4sheet with C,B,and N atoms. On the basis of f i rst principles calculations,the designedB-C-N sheets were either metallic or semiconductor. In particular,the B3C-C3N4sheet was ferromagnetic metal,which can be used for developing metal-free spintronic devices.

II.METHODS

All calculations are performed by using the density functional theory(DFT)method within the Perdew-Burke-Ernzerholf(PBE)generalized gradient approximation(GGA)implemented in Vienna ab initio Simulation Package(VASP)[26,27].The projector augmented wave(PAW)potential and the plane-wave cutof fenergy of 500 eV are used[28,29].The periodic boundary condition was applied and we used a vacuum space of 15˚A along the z direction to avoid interactions between two BxCyNz-C3N4images in the nearest-neighbor unit cells.When performing the geometric optimization,both the cell shapes and atomic positions are fully relaxed for all considered BxCyNz-C3N4frameworks.The Brillouin zone is represented through Monkhorst-Pack special k-point meshes[30] with 5×5×1 and 13×13×1 for the geometry optimization and calculating the density of states.More than 20 K points along each high-symmetry line in the Brillouin zone were used to obtain the accurate band structure.The convergence criteria for total energy and force are set to be 500 eV,1×10-4eV,and-0.01 eV/˚A, respectively.As PBE method always underestimates the band gap,the screened hybrid HSE06 functional method is used to evaluate the band gap of semiconductor systems.

III.RESULTS AND DISCUSSION

We f i rst studied the geometric and electronic properties of the pristine 2D g-C3N4framework.Our g-C3N4model is based on the experimentally well characterized structure as shown in Fig.1(a)[31].The optimized lattice constant of g-C3N4unitcell is calculated to be 7.14˚A,agreeing with previous studies[24,25].The energy band structures with PBE and HSE06 methods indicate that the pristine g-C3N4sheet is nonmagnetic semiconductor.The PBE method presents an underestimated band gap of about 1.25 eV,and this value is 2.71 eV with HSE06 method(Fig.1(b)),which is consistent well with the experimental value(2.7 eV)[31]. The orbital projected DOS analysis shows that the valence band edge is mainly contributed by N2p orbitals, whereas the conducting band edge is mainly contributed by C2p and N2p orbitals.

Next,boron,carbon,or nitrogen atoms are incorporated into the porous pristine g-C3N4sheet to get uniform 2D B-C-N ternary monolayer.Four models with the compositions of B3C-C3N4,B3N-C3N4,B3-C3N4, and B4-C3N4,are designed,as shown in Fig.2.A structure distortion can be found in B3C-C3N4,B3N-C3N4, and B4-C3N4,whereas B3-C3N4monolayer has an exactly planar structure.The optimized lattice constants are summarized in Table I.These values are slightly larger than that of pristine g-C3N4monolayer.

FIG.1 (a)Geometric structure and(b)band structure of the 2D g-C3N4framework calculated by HSE06.Gray and blue balls represent C and N atoms,respectively.

TABLE I Optimized lattice constant a,binding energy Eform,total magnetic moment per unit cell M,and band gap Egapof the uniform B-C-N frameworks.

To investigate the structure stability of proposed 2D B-C-N ternary monolayer,the average formation energies of pristine g-C3N4and four B-C-N ternary monolayer models are calculated.The formation is def i ned as

FIG.2 The top(upper)and front(lower)views of(a)B3C-C3N4,(b)B3N-C3N4,(c)B3-C3N4,and(d)B4-C3N4.Gray, beige,and blue balls represent C,B,and N atoms,respectively.

FIG.3 The calculated average formation energy of pristine g-C3N4and four B-C-N ternary monolayers.

where Etotare the total energies of pristine g-C3N4or B-C-N ternary monolayer.The nB,nC,nNare the total number of B,C,and N atoms.TheµB(-5.97 eV), µC(-9.22 eV),andµN(-8.32 eV)are the theoretical chemical potentials for B,C,and N atoms,which are obtained from diamond boron phase,graphene and α-N2phase of gaseous nitrogen,respectively.The calculated average formation energy of pristine g-C3N4and B-C-N ternary monolayer are summarized in Fig.3.It can be concluded that inserting B,C,or N atoms into the pores of g-C3N4reduces its formation energy and these 2D B-C-N ternary monolayers are thermally stable.The B3N-C3N4sheet presents the highest stability in the considering systems,since the B-N bond has much higher bonding energy than B-C bond,which is consistent with the previous reports[8,13].The B3-C3N4and B4-C3N4systems show relatively low stability due to small bonding energy of B-B bond.The high thermal stability of B3N-C3N4and B3C-C3N4implies that they are the most probable structures to be formed in a synthesis process.

The electronic band structures of B-C-N ternary sheets are calculated within PBE and HSE06 funtional schemes.The calculations indicate that B3N-C3N4,B3-C3N4,and B4-C3N4sheets are nonmagnetic.The calculated energy band gaps and magnetic moment are summarized in Table I.Figure 4 displays the electronic band structures of B3N-C3N4,B3-C3N4,and B4-C3N4sheets.Different from pristine g-C3N4,the B3N-C3N4and B3-C3N4monolayers are metallic.The B4-C3N4semiconducts with an energy band gap of 1.18 eV,which is much smaller than that of pristine g-C3N4.The reduced band gaps of B4-C3N4indicate its advantages in the photocatalytic reaction.

FIG.4 The band structures of B3N-C3N4,B3-C3N4,and B4-C3N4are calculated with HSE06 method.The Fermi energy is set to be zero.

However,the B3C-C3N4system is magnetic.In order to explore the magnetic ground state,ferromagnetic(FM),antiferromagnetic(AFM),and nonmagnetic(NM),con fi gurations have been considered using a(2×2)supercell.It is found that the FM state is the most energetically stable,with energy of 36.47 and 37.29 meV lower than that of the AFM and NM confi gurations,respectively.The calculated magnetic moment is 0.57µB/cell for a FM state.The calculated electronic band structure and spin charge density distribution pro fi les of B3C-C3N4system is displayed in Fig.5.

From Fig.5(a),it can be concluded that the magnetic moment of B3C-C3N4is mainly contributed by the carbon atoms in the system.When three boron atoms and one carbon atom are added into the porous of the pristine g-C3N4,three neighboring carbon atoms are spin polarized and the system becomes the metal-free magnetic material.Figure 5(a)also shows that the magnetic moment mainly comes from the 2pzorbitals of the three neighboring carbon atoms and other atoms are slightly spin polarized.The calculated electronic band structures and atom-projected DOS(PDOS)(as shown in Fig.6(d))analysis indicate there is strong spin-splitting for 2pzorbitals of C atoms.Only one band crosses theFermi level monolithically in spin up and down channels,corresponding to the C2pzorbitals.As mentioned above,for the pristine g-C3N4,the valence band edge is mainly contributed by N2p orbitals,whereas the conducting band edge is mainly contributed by C2p and N2p orbitals.However,for the B3C-C3N4,as can be seen,the N2p states overlap significantly with those of C2p states near the Fermi level,suggesting a strong interaction between them,which results in the spin splitting of C2pzorbitals.The magnetism characteristics of B3C-C3N4monolayer indicate its potential applications in spintronics devices.

FIG.5(a)The top and side view of spin charge density of B3C-C3N4.The isosurface is 0.002.(b)The electronic band structures are plotted for spin up and down channel with HSE06 method,respectively.The Fermi energy level is set to be zero.

For the nonmagnetic systems,Fig.6 displays the calculated PDOS with PBE method.The analysis of the PDOS demonstrates that,near the Fermi level,the most significant contribution for the charge transport in such systems are associated with the p orbitals of carbon and nitride while boron atoms make a little contribution due to only one electron in p orbital.

IV.CONCLUSION

FIG.6 The calculated DOS of(a)B3N-C3N4,(b)B3-C3N4, (c)B4-C3N4,and(d)B3C-C3N4.The Fermi energy level is set to be zero.

In summary,we have proposed four B-C-N ternary sheets with uniform element distribution.It is demonstrated that all of the predicted new uniform B-C-N ternary systems are metals except the B4-C3N4which is a semiconductor with a band gap of 1.18 eV.Interestingly,B3C-C3N4is a metal-free ferromagnetic material and holds a magnetic moment of 0.57µB/cell.The calculated formation energy indicates these B-C-N ternary sheets are highly thermal stable.Our studies present a method to design B-C-N ternary monolayer with uniform element distribution for electronic and spintronic applications.

V.ACKNOWLEDGMENTS

This work was supported by the National Key BasicResearchProgram(No.2012CB922001and No.2011CB921404),the National Natural Science FoundationofChina(No.21121003andNo.51172223), the Strategic Priority Research Program of CAS (No.XDB01020300),One Hundred Talent Program of CAS,the National Young Top Talent Program of Organization Department of China,the Fundamental Research Funds for the Central Universities(No.WK2060140014 and No.WK2060190025),and USTCSCC,SCCAS,Tianjin,and Shanghai Supercomputer Centers.

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ceived on April 2,2014;Accepted on April 28,2014)

∗Author to whom correspondence should be addressed.E-mail：xjwu@ustc.edu.cn