The Structural and Electronic Properties of Silicene Oxide:A First-Principles Study?

CHEN Chu,ZHANG Jun

(College of Physics Science and Technology,Xinjiang University,Urumqi,Xinjiang 830046,China)

Abstract: Using density functional theory with the generalized gradient approximation,the structural and electronic properties of a two-dimensional oxidized silicene are investigated.Our results show that oxygen atoms prefer to form the ordered arrangement of-(Si2O2)x-bonds in silicene with the increase of oxygen content.Each-(Si2O2)-bond makes six neighbor silicon atoms of the two oxygen atoms in a plane and its four Si-O bonds constitute a square vertical to this plane.When all silicon atoms are transferred to sp3 hybridization with the increase of the oxygen content,the fully oxidized structure(x=0.50)is the most stable structure with a calculated band gap of 0.46 eV.The small band gaps(from 0.30 eV to 0.38 eV)of partly oxidized silicene(from x=0.06 to x=0.33)are consistent with the study of oxidized Si nanoribbons(its band gap is 0.35 eV)and the band gap is originated from the coexistence of these ionic Si-O bonds and covalent Si-Si bonds.

Key words:atomic oxygen;band gap;oxidized silicene

0 Introduction

Silicene,the counterpart of graphene for silicon,with slightly buckled honeycomb structure in the form of nanoribbon(NR)or nanosheet,has been synthesized through epitaxial growth on a silver substrate.[1～4]Although isolated silicene has not been reported yet,free standing silicon nanosheets with a thickness<2 nm,not as thin as silicene,have been synthesized by chemical vapor deposition.[5]Meanwhile,theoretical results based on the density functional theory(DFT)have proved that silicene is dynamically stable and can be transferred onto an insulating substrate.[6,7]Recent investigations have demonstrated many interesting properties of silicene,including quantum spin Hall effect,electrically tunable band gap and ferromagnetism under semi-hydrogenation.[8～10]

Although silicene,like graphene,is a zero-gap semiconductor,the generation of band gap is necessary to realize its potential electronic application.Hydrogenation(or fluorination)of silicene is an effective way to achieve a gap up to 2.2 eV.[11,12]However,such a value is too large for semiconductor electronic applications.It has been reported recentlythat a lithiated silicene(silicel)is as stable as bulk Si and it can increase band gap to 0.37 eV.[13]Lu etc.predicted that it is possible to increase band gap to 0.3 eV in semi-metallic low-buckled silicene monolayer via an external vertical electric field of 1.0 V/?.[9]Although these values of band gap are acceptable,the oxidization of metal atom and the difficulty in intercalation technology may limit the application of silicel in practice.

Considering that silicene is less stable than bulk silicon and unlikely to grow naturally like grephene from theoretical calculations,[6]silicene in present is synthesized on a metal substrate such as siliver.[14～16]The oxidation process of Si nanoribbon on a Ag substrate has been studied at the atomic scale both by STM and PES performed at room temperature with the exposure to molecular oxygen of the surface.[17,18]STM images clearly demonstrated that the oxidation process was initiated at the termination side of the ribbons and was then propagating along the[110]direction.The formation of a SiO2/Si interface with sub-oxides localized at the oxidation front was further confirmed by the de-convolution of the Si 2p core-level.TheI(V)spectra on the oxidized Si NRs evidenced a small gap of 0.35 eV,resulting in a semi-conducting behavior of this very thin(one layer)silicon oxide,although a thick SiO2film was an insulator with a band gap of～9 eV.There still remain unclear about the generation of small band gap in oxidized Si NRs.In our case,we tried to explore the cause of the small band gap and the nature of interaction between oxygen(O or O2)and silicene.The stability of silicene oxide was investigated via structure optimization based on DFT.Furthermore,according to recent report,graphene monoxide(GMO)is semiconducting with a relative much wide gap(about 0.9 eV)by heating graphene oxide in vacuum.[19]In present study,similar to GMO,we also tried to explore a way to create a stable semiconducting material based on silicene.

1 Computational details

All the calculations were calculated by Dmol3code based on DFT.The generalized gradient approximation(GGA)with Perdew and Wang 91(PW91)approximation was employed to the exchange-correlation functional with double numeric quality basis set with polarization functions(DNP).All structures were optimized without any symmetry restriction and spin-unrestricted self-consistent field calculation was done with a convergence criterion of 10-5 a.u..To improve the convergence,thermal smearing was set to 0.002 Hartree.The distance between two layers is set to 30 ? to avoid the interaction between them.Supercells include 32 silicon atoms with various levels of oxidization,and periodic boundary conditions were used for all energy calculations.As a test,our calculated bond length of Si-Si in silicene is 2.32 ?,which is in good agreement with the experimental results(from 2.2 ? to 2.4 ?).The band gap of GMO is 0.86 eV which is also in consistent with the calculated gap(～0.9 eV)in ref[14].

This binding(or adsorption)energy value determines the stability with the same O content.To assess overall stability,one needs to compare with Gibbs free energies(?G),which are defined on the basis of cohesive energies(Ec).The cohesive energies and Gibbs free energies are calculated by the formulae as:

where,μ (Si)(about 3.94 eV)is set to be the cohesive energy of silicene per silicon atom withμ(O)=1/2Ec(O2),Ec(O2)(about 6.59 eV)is the cohesive energy of an isolated triplet oxygen molecule.

2 Results and discussions

Due to the special structure,atomic oxygen adsorption of silicene is a barrierless process.The optimized structures of Si1?xOx(fromx=0.03 tox=0.50)are shown in Table.1.For the case of one oxygen atom(x=0.03),silicene can adsorb one oxygen atom on bridge site.This oxygen atom breaks one Si-Si bond to form a Si-O-Si bond,and the distance between these two silicon atoms is 2.93which is much longer than the typical Si-Si bond length(from 2.2 to 2.4)in silicene.The bond length of Si-O is 1.68corresponding to a typical Si-O covalent bond with a binding energy of 2.04 eV/O.

Fig1 Structures,band gaps(Eg)and relative energies corresponding to oxygen content(from x=0.03 to x=0.50)(a)is the deformation density of electron(isovalue=0.04)of Si32O2(x=0.06),where the green represents electron accumulation region and the yellow represents reduction region.(b)is the fragment of-(SiO)2-and its nearest four sp2 hybridized silicon atoms,in which the up is the top view and the down is the side view.(c)is the PDOS,where the black line represents the p electron of oxygen atoms,the blue one represents that of sp2 hybridized silicon atom and the red one represents that of sp3 hybridized silicon atom

When two oxygen atoms bind to two sides of silicene,where each oxygen atom binds to two silicon atoms shown in Fig.1(b),the lowest-energy structure of Si0.94O0.06illustrates an obvioussp3hybridization.According to the deformation density of electrons in Fig.1(a),there remains a reduction region of electrons corresponding to the unbinding state between the two silicon atoms although the distance between them is decreased to 2.41from 2.93And an accumulation region around the oxygen atom indicates that the Si-O polar covalent bond shows a tendency towards ionic bond.According the Mullikan charge analysis,the charge of each oxygen atom is-0.82|e|.The partial density of state(PDOS)of oxygen atom in Fig.1(c)shows an obvious peak far below the Fermi energy,which means there is a quasi-ionization of oxygen atoms in Si-O bond.Furthermore,the binding energy of oxygen is 3.03 eV/O,which is about 1.00 eV larger than that of Si0.97O0.03.The total energy of Si0.94O0.06is also 1.58 eV lower than that of O2-silicene in Fig.1(c).Therefore,in energy,oxygen atoms prefer to form-(Si2O2)-bonds and show the quasi-ionization on silicene.

Note that six nearest silicon atoms of the two oxygen atoms can form a local planar structure as shown in Fig.1(b).This plane-type structure is very different from those hydrogenated or lithiated silicene,where the doped atom always leads to an enhanced buckling of silicene,but similar to the Li+doped Si6H6in ref.[19],which illustrates Li+cations,like oxygen atoms in our case,can accept thepelectrons of silicon atoms so as to suppress the PJT distortions resultingin a planar structure.Besides the planar structure,another important structural feature is the square configuration of the-(SiO)2-bonds perpendicular to the plane consisted of these six silicon atoms mentioned above.Comparing with the H-O bond in water molecule,each Si-O bond possesses a larger bond length(1.70),which reduces the repulsion between Si-O bonds.Therefore,∠SiOSi is compressed to a right angle by the repulsion of these neighboring Si-Si bonds.

For the case of four oxygen atoms,these oxygen atoms in the lowest-energy structure is also in the form of-(Si2O2)2-.With the increase of oxygen atoms,the-(Si2O2)-bonds prefer to be orderly rearranged in silicene.Whenx=0.20,this orderly rearrangement was confirmed by our calculation of the energies for three configurations corresponding to the dispersed,the aggregated and the ordered structures.Although the aggregated structure is more stable than the dispersed one,the ordered structure is the lowest-energy structure.Just like those oxygen-contained groups in GO,the lowest-energy structure though the synthesis of the ordered GO is very difficult in experiment.The synthesis of the ordered partial oxidized silicene maybe also very difficult in reality.

When all silicon atoms are transformed intosp3hybridization,Si0.5O0.5is the most energetically favorable structure.All the silicon atoms are in a same plane without any buckling and two parallel planes consisted of oxygen atoms are located on two sides of Si plane.This fully oxidized structure could also be viewed as a largersp3fraction in a real slice of silicene oxide.Each-(Si2O2)-bond also shows a square in the vertical plane of Si layer with a Si-Si bond length of 2.41corresponding to a weaker covalent bond strength than that of pure silicene.The average binding energies of O atoms increase from 2.04 eV to 3.22 eV with the increase of O content.In view of energy,oxygen is distributed on silicene randomly when the O content is small,and prefers to form-(Si2O2)x-bonds and an ordered structure on silicene sheet with the increase of O content.

Fig2 Variation of Gibbs free energy(ΔG)vs the content of oxygen(x)

Considering possible practical application,Gibbs free energies,corresponding to each lowest-energy configuration,are calculated to assess the stability of partially oxidized and fully oxidized silicene sheets.In Fig.2,?Grepresents the energy of formation for each compound,with?G=0 for the cases of pure silicene Si and pure O2.A positive value indicates that the adsorption is endothermic and yields less stable structures,while a negative value indicates a more stable structure with an exothermic adsorption.It can be seen that the structure trends to more and more stable with the increase of oxygen content.Si0.5O0.5,the fully oxidized silicene,is the most stable compound among them.To further investigate the thermostability,a molecular dynamic(MD)simulation based on first principles was performed on the system of Si0.5O0.5.The MD simulations in the NVT ensemble were performed from 600 K to 1800 K with the N-H thermostat.There was no obvious structural change over a simulation time of 2 ps at 600 K,900 K,1200 K and 1500 K.The planar structure was destroyed at 1800 K by breaking the ionic Si-O bonds,which lead to a lower stability than the pure silicene.Therefore,Si0.5O0.5can keep thermostability at 1500 K.

Fig3 (a)is the band structure of Si32O32(x=0.50),where the Fermi energy is set to zero.(b)is the deformation density of electron(isovalue=0.04).(c)is the PDOS,where the red line represents the p electron of oxygen atom and the black line represents that of silicon atom

In order to investigate electronic properties of fully oxidized silicene(Si0.5O0.5),the band gap,PDOS and defor-mation density of electron were calculated shown in Fig3.It shows that the linear dispersion at K point is disappeared in Fig.3(a),and a direct band gap is opened atGpoint(Gamma point)corresponding a transformation from a zero-gap semiconductor to a semiconductor with a 0.46 eV band gap.It is well known that the bulk SiO2is an insulator due to the stronger Si-O covalent bonds.The difference between bulk SiO2and fully oxidized silicene comes from the existence of covalent Si-Si bonds and the ionic Si-O bonds according to the deformation density of electron shown in Fig.3(b)and the PDOS in Fig.3(c).In Fig.4(b),the δ state between two silicon atoms contributes to the valence band near the Fermi level,and their emptyporbital contributes to the conduction band via the electron transformation from silicon to oxygen whichporbitals,like that of an ion,are occupied and laid in deep under the Fermi energy in Fig.3(c).While for the case of SiO2bulk,the conduction band is consisted of the occupiedporbitals of oxygen atoms,and the stronger Si-O covalent bonds lead to a too wide band gap of about 7 eV.Meanwhile,according to Table.1,some band gaps are in the range between 0.30 eV and 0.38 eV,which is close to the experiment value(0.35 eV)of the burned Si NRs,when silicene is partly oxidized corresponding to Si1?xOx(fromx=0.06 tox=0.33).Therefore,although our calculated gap of Si0.5O0.5is higher than the experimental value of the burned Si nanoribbon,we speculate some ionic Si-O bonds and the covalent Si-Si bonds co-exist in the burned Si NRs and the remained unoxidized Si fragment makes the gap small.

3 Conclusion

In conclusion,the stability and electronic properties of silicene oxide were investigated using DFT calculations.Our results show Si0.5O0.5can be easily produced as a two-dimensional semi-conductor material by the oxidization of silicene.Oxygen atoms prefer to form the-(Si2O2)m-bonds in silicene.Each-(Si2O2)-bond makes six neighbor silicon atoms of the two oxygen atoms in a plane and four Si-O bonds constitute a square vertical to this plane.The square-(Si2O2)-bonds prefer to form a ordered structure of silicene oxide(Si1?xOx)when the oxygen content increases fromx=0.03 tox=0.50.When all silicon atoms are transferred tosp3hybridization with the increase of the oxygen content,the fully oxidized structure(x=0.50)is the most stable structure with a direct gap of 0.46 eV.The calculated gap also indicates a semi-conducting behavior of silicene oxide(or oxidized Si NRs)is originated from the coexistence of the ionic Si-O bonds and the covalent Si-Si bonds.