Effects of Cr,Sn,Co Doping on Electronic and Optical Properties of Layered Two-Dimensional Material MoSi2N4

LIANG Qian LUO Xiang⁃Yan WANG Yi⁃Xin LIANG Yong⁃Chao XIE Quan

(College of Big Data and Information Engineering,Institute of New Optoelectronic Materials and Technology,Guizhou University,Guiyang 550025,China)

Abstract:Based on the newly synthesized two⁃dimensional material MoSi2N4(MSN),we developed a series of doped models of MSN for first⁃principles calculations.Firstly,we calculated the electronic properties of intrinsic MSN,including its band structure and density of states.Then we investigated the effects of Cr,Sn,and Co⁃doping on the electronic and optical properties of MSN.Our work demonstrates that the Co⁃doped system exhibits the lowest forma⁃tion energy among the three doped systems,which indicates that the Co⁃doped system is the most stable one.The calculations of band gaps show that although all three doped models decrease the band gap of intrinsic MSN,three doped systems exhibit three different electronic properties.The densities of state diagrams also show that the Cr⁃doped system and the Co⁃doped system both produce local spikes near conduction band minimum(CBM)and valence band maximum(VBM).Furthermore,the optical properties of the MSN have also been improved a lot after doping.

Keywords:MoSi2N4;first⁃principles calculation;electronic property;optical property

Since the first mechanical exfoliation of single⁃layered graphene[1],two⁃dimensional(2D)materials have aroused wide concern among the general public.2D materials exhibit excellent mechanical[2],optical[3],electronic[4],piezoelectric[5],ferroelectric[6]properties,and so on,some of which don′t exist in the 3D bulk materials.Some interesting phenomena also have been found in 2D materials,such as super⁃long spin relax⁃ation time in graphene[7]and the spin valley locking ef⁃fect in some transition metal dichalcogenides[8],which shed new light on the unknown 2D materials.

In recent years,an emerging layered 2D material MoSi2N4(MSN)was synthesized by using the chemical vapor deposition(CVD)method by Hong et al.[9]MA2Z4family was also predicted to be dynamically stable in the air,where M denotes a transition metal(Mo,W,V,Nb,Ta,Ti,Zr,Hf,or Cr),A is Si or Ge,and Z repre⁃sents N,P,or As.Tremendous works have been done around MSN and its derivatives,for example,Wang et al.[10]investigated the electronic properties of MA2Z4,they found that different structures with a different total number of valence electrons tend to show different electronic properties.The structures with 32 and 34 valence electrons are mostly semiconductors,while those with 33 valence electrons can be nonmagnetic metals or ferromagnetic semiconductors.Yu et al.[11]investigated the lattice thermal conductivity of mono⁃layer MSN and found that its intrinsic lattice thermal conductivity is much higher than most semiconductors.Bafekry et al.[12]studied the outstanding mechanical,thermal,electronic,and optical properties of intrinsic MSN.Li et al.[13]showed that MSN,WSi2N4,and MoSi2As4are semiconductors with a valley degree of freedom and these valleys are of Dirac type and found that moderate strain to be added can change the band gaps of all three materials.Moreover,MSN and other 2D materials can be combined to form van der Waals heterostructures(VDWHS),MSN/NbS2contact shows ultralow Schottky barrier height,which was studied by Cao et al.[14]MSN has shown significant application potential in electronics and optoelectronic devices.

From a fundamental viewpoint,doping,adsorp⁃tion,and vacancy are usually used as common methods for tunable electronic and optical properties.Ray et al.[15]studied the defective properties of MSN and the results show that N and Si vacancies of MSN can introduce magnetism in MSN.In particular,the work function of MSN can be modulated by controlling the vacancy con⁃centrations.Three organic molecules were also used as dopants to enhance the electronic properties of MSN,where tetracyanoquinodimethane(TCNQ)and tetracya⁃noethylene(TCNE)acted as electron acceptors and tet⁃rathiafulvalene(TTF)as an electron donor,paving the way for the design of MSN based nano⁃electronic devic⁃es[16].Bafekry et al.[17]investigated systematically the ef⁃fect of different types of point defects using the ab ini‑tio method.After the doping of non⁃metallic elements As,P,F,and O into the MSN,only the As⁃doped sys⁃tem exhibited magnetic properties.They also revealed that the substitutional defects in the Si site are more stable structures.To our knowledge,few theoretical data have been reported on the electronic structure and optical properties of metal⁃doped MSN.For this reason,in our present work,three metal elements with similar electronegativity to Si,including two transition metals,Co and Cr,and one Si homologous metal,Sn,were selected as dopants.We investigated the effects of Cr,Sn,and Co doping on electronic and optical properties of MSN,providing a theoretical basis for the applica⁃tions of MSN in future optoelectronic devices.

1 Ideal model and computational methods

The intrinsic structure of MSN is shown in Fig.1a and it can be seen from the top view that the Mo atoms are located at the center of the hexatomic rings which are composed of Si and N atoms.Meanwhile,the N atoms are located at the center of the hexatomic rings which are composed of Si and Mo atoms.From the side view,we can conclude that monolayer MSN is com⁃posed of septuple atomic layers of N⁃Si⁃N⁃Mo⁃N⁃Si⁃N,which can be seen as stacked by a single layer of 2H⁃MoS2⁃like MoN2and α⁃InSe⁃like two layers of Si⁃N(Fig.1c).After well structural relaxation,the lattice con⁃stants are a=b=0.291 nm,which are in good agreement with previous experiments[18⁃19].Other relevant parame⁃ters will be given in the third part of this paper.

Fig.1 (a)Top and side views of MSN(the red lines represent the primitive cell);(b)Structure parameters of MSN;(c)Schematic illustration of the intercalation method that uses the structures of a 2H⁃MoS2⁃like monolayer and those of an α⁃InSe⁃like monolayer to construct the structure of an MSN monolayer

The calculated results of this paper were obtained through the Vienna ab initio simulation package(VASP)[20⁃21]using the projector augmented wave(PAW)[22]pseudopotential with the generalized gradient approximation(GGA)with Perdew ⁃Burke⁃Ernzerho(PBE)[23]based on density functional theory(DFT).The cut⁃off energy was set to 500 eV and a 15×15×1 Monkhorst⁃Pack K⁃points grid was applied for geome⁃try optimization.The convergence criterion for electron⁃ic self⁃consistency was set to 1×10-6eV to obtain more precise results.We established a 2×2×1 supercell for calculation(4 Mo atoms,8 Si atoms,and 16 N atoms)and used Cr,Sn,and Co three elements to replace the position of the top Si element of MSN,respectively.To eliminate the influence of close atoms on its surface,we added a vacuum layer of 3 nm along the z direction.Valence electrons involved in this calculation include 4d and 5s of Mo atoms,3s and 3p of Si atoms,2s and 2p of N atoms,3d and 4s of Cr atom,5s and 5p of Sn atom,3d and 4s of Co atom.

The cohesive energy(Ecoh)was also mentioned as follows:

Where Etotis the total energy of MSN structure,EMo,ESi,ENrepresent the ground⁃state total energy of a single Mo,Si,and N atom,respectively.In this equation,N is equal to 7,indicating the total number of atoms.The value of the cohesion energy obtained was about-8.44 eV·atom-1.

Phonon spectrum was considered to confirm the dynamic stability and the ab initio molecular dynamics(AIMD)method using canonical ensemble(NVT)was used to optimize MSN at room temperature(300 K)to examine the thermal stability of MSN.The total steps for performing AIMD were set to 3 000 steps,3 fs per step,9 000 fs in total.Fig.2a clearly shows that there is no imaginary phonon frequency in the phonon spec⁃trum,which means that the structure of MSN is dynami⁃cally stable.Energy and temperature fluctuation curves over time are shown in Fig.2b,demonstrating the ther⁃mal stability of MSN.

Fig.2 (a)Phonon spectrum of intrinsic MSN;(b)Energy and temperature fluctuation curves about time at 300 K

2 Results and discussion

2.1 Optimized structural parameters

Table 1 shows the optimized structural parameters of intrinsic MSN and Cr,Sn,Co single⁃doped of MSN.The lattice constants of optimized MSN area=b=0.291 nm and the thickness of optimized MSN is 0.701 nm.We can easily see from Table 1 that after the lattice relaxation,lattice parametersaandbbecame slightly larger than 0.291 nm of the original lattice parameters.The bond angles of Si—N—X(X=Cr,Sn,and Co)(θ1)and N—Mo—N(θ4)became larger than before,respec⁃tively.Reversely,the bond angles of Si—N—X(X=Cr,Sn,and Co)(θ2)and N—Mo—N(θ3)became smaller than before.The bond lengths of X—N(d2)and Mo—N(d3)became larger and Si—N(d1)became smaller than before.The thickness also slightly decreased.Further⁃more,we also found that the bandgap values tended to decrease in both the up and down spin energy band diagrams.

Table 1 Lattice constant(a)and(b),d1,d2,θ1,θ2,θ3,θ4,thickness layer(t),and up spin band gap Eg(↑)and down spin band gap Eg(↓)of samples

2.2 Electronic properties of intrinsic MSN

The band structure of intrinsic MSN is shown in Fig.3a and the band gap we calculated was about 1.80 eV,which is highly consistent with the experimental results(1.94 eV)[9].The wide bandgap of 1.80 eV makes single⁃layered intrinsic MSN have application potential in transistors,light⁃emitting diodes,and solar cells.The conduction band minimum(CBM)is located at the high symmetry K point and the valence band maximum(VBM)is located at the high symmetry Γ point.The CBM and VBM are located at different points of the Brillouin zone,indicating that the intrin⁃sic MSN is an indirect bandgap semiconductor.

Fig.3 (a)Band structure and(b)DOS of intrinsic MSN

To see the distribution of electrons in each orbit of intrinsic MSN more clearly,we plotted the density of states(DOS)diagrams in Fig.3b.As can be seen from it,the VBM is mainly contributed by the dz2,dx2-y2orbitals of Mo atoms and px,pzorbitals of N atoms,but the orbital contribution of Si atoms is very small.The CBM is mainly contributed by the dxz,dx2-y2orbitals of Mo atoms and pxorbitals of N atoms.The contribution of Si atoms is very low in both VBM and CBM.Because up and down spin DOS diagrams show complete consis⁃tency,for the sake of simplicity,we only plotted the up spin DOS diagram.This is also consistent with the previously calculated conclusions[12].

2.3 Electronic properties of the doped MSN

To further understand the doping effects on elec⁃tronic and optical properties of MSN,Cr,Sn,and Co three elements(X)were used to replace the top position of Si of the MSN supercell.The defect formation energy(Ef)can tell us the stability of defects and provide us with theoretical guidance on experiments.

It can be defined as:

Where Edoped,Epristine,μSi,and μXrefer to the total energy of MSN with X dopant,pristine structure,the chemical potential of the substituted Si host atom,and the chemi⁃cal potential of the substitutional atom X,respectively.For substituted Si host atom and substitutional atom X,the reference phases used are their bulk structures.After calculations,we obtained the formation energies of the three systems(Cr⁃MSN,Sn⁃MSN,Co⁃MSN):-3.34,-3.48,-5.20 eV.All three negative formation energies indicate that all structures after doping are stable,and a larger negative value of formation energy means a more stable structure.We found that the most stable one is Co⁃MSN,and Sn⁃MSN and Cr⁃MSN sys⁃tems are relatively stable.The phonon calculation of three structures also shows that all three structures are dynamically stable(Fig.4b,4d,and 4f).

Fig.4a,4c,and 4e show the band structures of X⁃MSN(X=Cr,Sn,and Co).As shown in Fig.4a,several impurity energy levels appeared near the Fermi energy level in the up spin energy band diagram with a bandgap of 0.69 eV.In the down spin energy band diagram,we found a bandgap of 1.62 eV which was not equal to the bandgap value in the up spin.Two differ⁃ent band gap values and the inconsistency shown in the up and down spin energy band diagrams indicated that doping with the Cr element caused the transition from non⁃magnetic to a magnetic system.However,bandgaps still existed in both up and down spin energy band dia⁃grams,demonstrating that the Cr⁃doped system still maintained the original semiconductor properties.

Fig.4c shows the band structure of the Sn⁃doped system with a band gap of 1.43 eV in both up and down spin energy band diagrams.The up and down spin energy band diagrams show perfect similarity and con⁃sistency,which implies that the Sn⁃doped system is a non⁃magnetic system as before.The biggest difference between the Sn⁃doped system and the undoped system is that doping with the Sn element decreases the bandgap(about 0.37 eV).

Compared to Fig.4a and 4c,Fig.4e exhibits com⁃pletely different energy band properties.Fig.4e shows that after doping with the Co element,impurity energy levels appeared near the Fermi energy level with the impurity energy level crossing the Fermi energy level in the up spin energy band diagram.While the band gap(0.38 eV)was preserved in the down spin energy band diagram.The inconsistency between up spin and down spin energy band diagrams indicates that the Co⁃doped system undergoes a transition from non⁃magnetic to magnetic as the Cr⁃doped system.But unlike the Cr⁃doped system and the Sn⁃doped system,spin splitting was observed in the Co⁃doped system which exhibits typically half⁃metallic properties.When compared with Co⁃doped MoS2and Cr⁃doped MoS2systems[24⁃25],both MSN and MoS2systems change from non⁃magnetic systems to magnetic systems after doping with Co and Cr elements.Both the doping of Co and Cr elements introduces magnetic properties to MSN and MoS2.However,unlike the doped MSN system,the Cr⁃doped MSN system maintains the semiconductor properties,while the Cr⁃doped MoS2system becomes a half⁃metallic system,and the Co⁃doped systems behave oppositely.Co⁃element doping allows MSN better used for emerging spintronic and nanoelectronic devices.

Fig.4 Band structures of(a)Cr⁃MSN,(c)Sn⁃MSN,and(e)Co⁃MSN;Phonon spectra of(b)Cr⁃MSN,(d)Sn⁃MSN,and(f)Co⁃MSN

It can be concluded that after doping,impurity energy levels appear near the Fermi energy level in all three doped systems but the three doped systems exhibit different properties.All three doped systems show reduced energy band gaps compared with the undoped system,which makes the electrons in the valence band more easily excited to the conduction band and enhances the conductivity of the undoped system.What′s more,the magnetic moments of the two doped systems obtained are 2.0μB(Cr⁃MSN),1.0μB(Co⁃MSN).The magnetic moments are mainly derived from the contri⁃butions of doped atoms.

The DOS of X⁃MSN(X=Cr,Sn,and Co)are shown in Fig.5.From Fig.5a and 5c,we can easily see that up and down spin DOS diagrams show inconsistency,which indicates that the systems change from original non⁃magnetic to magnetic after doping.Spin splitting occurred near the Fermi level when doping with Cr and Co elements and the contributions of Cr and Co two ele⁃ments mainly dominated near the VBM and CBM.Asymmetric local spikes were also generated near CBM and VBM in the total density of states(TDOS)with the largest contribution from the dx2-y2orbitals of Cr and Co elements,which are also the main cause of the magnetic moments.While Sn⁃doped system exhibit⁃ed complete symmetry between the up and down spin state densities,indicating that non⁃magnetic properties are retained.Fig.5 also shows that VBM and CBM are still dominated by the same states like intrinsic MSN because of the very small contribution of the X(X=Cr,Sn,and Co)states in the energy range considered.

Fig.5 DOS of(a)Cr⁃MSN,(b)Sn⁃MSN,and(c)Co⁃MSN

2.4 Optical properties

Then we calculated the optical properties of MSN and three other doped models.Optical properties are one of the most important properties of materials.The optical properties of materials are usually described by some physical quantities called optical constants and the optical properties of materials usually can be changed by doping elements.In our work,we mainly calculated three optical quantities:dielectric function,optical absorption,and transmission.

The dielectric function can be described as:

Where ε1(ω)denotes the real part of the dielectric func⁃tion and ε2(ω)is the imaginary part of the dielectric function.Fig.6a shows the real part of the dielectric function.When the photon energy is zero,the corre⁃sponding value of the real part is the static dielectric constant.As we can see from Fig.6a that the static dielectric constant of intrinsic MSN was about 3.7.When doping with Cr,Sn,and Co three elements,the values of static dielectric constant increased with the most obvious change being in the Co⁃doped system and producing an obvious spike in the low energy region.And the corresponding values for the three systems were 4.2,4.0,7.7,respectively.Two peaks appeared at around 2.0 and 5.0 eV.At around 2.0 eV,after doping with the Cr and Sn two elements,the maximum peak slightly increased.While the maximum peak decreased when the Co element was doped.Doping with the Sn element also increased the maximum peak at around 5.0 eV.But the other two doped systems were just the opposite.

Fig.6b depicts the curves of the imaginary part of the dielectric function as a function of photon energy.In the low energy region(0⁃4 eV),the value of the imag⁃inary part became larger as the photon energy increased then decreased and hit the bottom at about 4.7 eV.When doped with the Sn element,the maxi⁃mum peak showed a red⁃shift phenomenon near 4.7 eV.However,the maximum peak showed a blue⁃shift phenomenon when doped with Cr and Co elements.All three doped systems reduce the maximum peak near 4.7 eV.What is particularly intriguing is the appear⁃ance of a sharp peak in both the real and imaginary images of the Co⁃doped system.The imaginary part ε2(ω)is related to the conductivity,the larger the imagi⁃nary part is,the larger the free⁃electron conductivity is.Therefore,the conductivity of the intrinsic MSN is greatly improved by the doping of Co elements,which can be better used in some electronic and optical devices.

Fig.6 Dielectric function spectra of MSN and X⁃MSN(X=Cr,Sn,and Co):(a)real part and(b)imaginary part of the dielectric function

Optical absorption describes the phenomenon that luminous intensity weakens as the propagation dis⁃tance increases.The optical absorption coefficient α is defined as α =4πk/λ =2ωk/c.Among them,k denotes the extinction coefficient,λ denotes the wavelength,and ω denotes the frequency,respectively.

Fig.7 illustrates the absorption spectra of X⁃MSN(X=Cr,Sn,and Co).The black line shows the absorp⁃tion spectrum of intrinsic MSN and we can see that the absorption maximum peak reached 2.7×105cm-1at 9.5 eV.After doping,the absorption maximum peak decreased,but the maximum peaks still appeared around 9.5 eV.The diagrams also showed five obvious valley bottoms around 4.8,7.2,8.5,12.0,and 15.0 eV,respectively.The valley bottoms first appeared at around 4.8 eV and the absorption increased obviously at the first valley bottoms.We found the same phenom⁃enon in the second valley bottoms.At around 8.5 and 12.0 eV,the absorption decreased obviously compared to the intrinsic MSN.The absorption was basically the same as before at 12.0 eV.

Fig.7 Absorption spectra of MSN and X⁃MSN(X=Cr,Sn,and Co)

The transmission coefficient refers to the ratio of the transmitted luminous flux to the incident luminous flux.It can be defined as:

Where θ1,θ2denote the angles of incidence and angle of refraction,respectively.n1and n2denote the refrac⁃tive indexes of different media.I1and I2represent the intensities of the incident and refracted waves.While A1and A2represent the amplitudes of the incident and refracted waves.

Finally,we plotted the transmission spectra of MSN and X⁃MSN(X=Cr,Sn,and Co),which are shown in Fig.8.The curves started to fall from 0 eV and hit the bottom at 9.5 eV and then the curves rose again rapidly.We can smoothly draw the conclusion that after doping the valley bottoms at 9.5 eV became larger than before and the maximum peaks at 4.5 eV became lower than before.These changes in optical properties can be better applied to some optical devices and pro⁃vide theoretical guidance for laboratory modulation of optical properties.

Fig.8 Transmission spectra of MSN and X⁃MSN(X=Cr,Sn,and Co)

3 Conclusions

Inspired by the latest significant breakthrough that MSN was synthesized successfully,in our work,based on DFT,we investigated the doping effects on the electronic and optical properties of MSN.Our work suggests that the Co doped system is the most stable one among the three doped systems.After doping,the band gaps of all three doped systems become smaller than before and three doped systems exhibit three dif⁃ferent electronic properties,respectively.The Cr⁃doped system exhibits magnetic semiconductor properties and the Sn⁃doped system exhibits non⁃magnetic semicon⁃ductor properties.Unlike both Cr and Sn doped systems,the Co⁃doped system shows magnetic half⁃metallic properties.The DOS diagrams also show that both Co and Cr doped systems produce local spikes near the CBM and VBM,which are also considered to be the main reason for the generation of magnetic properties.Furthermore,the Co⁃doped system produces a spike in both real and imaginary low⁃energy regions of the dielectric function with increased static dielectric con⁃stant and conductivity.The absorption maximum peak of intrinsic MSN appeared at 9.5 eV and it decreased when doping with Cr,Sn,and Co.The valley bottom of transmission of intrinsic MSN also appeared at around 9.5 eV and it increased when doping with Cr,Sn,and Co elements.Our work may guide the application of MSN in optical and semiconductor applications.

Acknowledgment:The work was supported by the Industry and Education Combination Innovation Platform of Intelligent Manufacturing and Graduate Joint Training Base at Guizhou University(Grant No.2020⁃520000⁃83⁃01⁃324061),the National Natural Science Foundation of China(Grant No.61264004),and the High⁃Level Creative Talent Training Program in Guizhou Province of China(Grant No.(2015)4015).