Journal of Materials Science & Technology  2019 , 35 (8): 1803-1808 https://doi.org/10.1016/j.jmst.2019.03.032

Orginal Article

Micromechanism study on electronic and magnetic properties of silicene regulated by oxygen

Li-Ping Dinga, Peng Shaoa*, Lin-Tai Yanga, Wei Guo Sunb, Fang-Hui Zhanga, Cheng Lubc**

a College of Elecrical & Information Engineering, Shaanxi University of Science & Technology, Xi'an, 710021, China;
b Department of Physics, Nanyang Normal University, Nanyang, 473061, China
c Department of Physics and High Pressure Science and Engineering Center, University of Nevada, Las Vegas, NV, 89154, United States

Corresponding authors:   *Corresponding author.**Corresponding author at: Department of Physics, Nanyang Normal University, Nanyang, 473061, China. E-mail addresses: scu_sp@163.com (P. Shao), lucheng@calypso.cn (C. Lu).*Corresponding author.**Corresponding author at: Department of Physics, Nanyang Normal University, Nanyang, 473061, China. E-mail addresses: scu_sp@163.com (P. Shao), lucheng@calypso.cn (C. Lu).

Received: 2018-07-1

Revised:  2018-10-16

Accepted:  2019-02-28

Online:  2019-08-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Silicene, a two-dimensional (2D) silicon counterpart of graphene with attractive electronic properties, has attracted increasing attention. Understanding of its interaction with oxygen is of fundamental importance for nano-electronics in silicon-based technology. Here, we have systematically studied the structural, electronic and magnetic properties of silicene with oxygen atoms adsorption by using an unbiased structure search method coupled with First-principles calculations. The results show that the most favorable oxygen adsorption site on silicene surface is bridge site and oxygen atoms tend to chemisorb on silicene. A detailed analysis of the electronic band structure and density of state (DOS) suggests that there is a band gap opening near Fermi level after oxygen adsorption, which lead to pristine silicene changing from a gapless semiconductor to a direct or indirect bandgap semiconductor. The important finding is that two and six oxygen atoms adsorbed silicene are more advantageous due to the relatively large direct band gaps at the K point. The calculated magnetic moments and spin density isosurfaces reveal that the total magnetic moments are mostly localized on silicene sheet. This finding provides new insights for further materials design based on two-dimensional silicon systems.

Keywords: Silicene ; CALYPSO method ; Band gap ; Oxygen adsorption

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Li-Ping Ding, Peng Shao, Lin-Tai Yang, Wei Guo Sun, Fang-Hui Zhang, Cheng Lu. Micromechanism study on electronic and magnetic properties of silicene regulated by oxygen[J]. Journal of Materials Science & Technology, 2019, 35(8): 1803-1808 https://doi.org/10.1016/j.jmst.2019.03.032

1. Introduction

Silicene [1,2] is a novel two-dimensional (2D) material [3], possessing graphene-like Dirac Cone electronic structure. In this case, most of the quantum effects which exist in graphene can also be found in silicene. On one hand, silicene possesses the unique linear Dirac Cone, the massless fermions and high carrier mobility, which resembles graphene. On the other hand, silicene has a buckled honeycomb structure formed by six-membered silicon rings, which is different from the flat honeycomb lattice of graphene. The buckled honeycomb atomic arrangement of sp2/sp3 hybridized Si atoms [4]. Due to the strong spin orbit interaction, silicene [5] has the potential to overcome limitations encountered in graphene. The topological insulating states and quantum Hall effects have also been found in silicene [6,7]. More importantly, it can be easily integrated into the currently well-developed Si-based semiconductor industry. Consequently, silicene is a promising material for electronic applications because it supplies an ideal interface with the existing Si devices.

Although the high carrier mobility of silicene guarantees the low power consumption and high working efficiency of electronic devices, the narrow band gap (about 1.55 meV) [5] severely hinders its application in nano-electronic devices [8]. Therefore, it is very meaningful to modulate the electronic structure of silicene in order to open its band gap and keep the high carrier mobility. Up to now, a number of investigations have been performed on modulating the band gap such as introduction of local defect or Si adatoms, [9] applying external electric fields [10] and adsorbing or doping foreign atoms [11]. Among them, adsorbing or doping foreign atoms not only can open band gap but also can functionalize silicene. Such as hydrogenation [12], halogenation [13], adsorbing alkali metal [14], 3d transition metal atoms [14,15], or some other light atoms [16]. The hydrogenation is the earliest method for functionalization of silicene. The fully hydrogenated silicene is called silane, and the band gap can be obtained. Lv and his co-workers [17] investigated the alkali metal Na atom adsorbed silicene based on density functional theory. The results showed that the band gap reaches the value of 0.5 eV by modulating the coverage of adsorbed Na atoms. Jiang et al. [18] functionalized silicene by a series of halogens and found that the silicene which is functionalized by F and I atoms has the potential to produce high efficiency field effect transistors. Zhang et al. [19] studied Br atoms adsorbed silicene. The results show that the band gap of silicene is opened and its magnetism can be controlled. The fully saturated silicene exhibits nonmagnetic semiconductor properties, whereas the half saturated silicene exhibits an anti-ferromagnetic half metal nature.

Inspired by extensive attempts to open the band gap in gapless silicene, we choose the oxygen atoms to modulate the electronic structure as well as the magnetism of silicene. In fact, the oxygen-functionalization has been employed in some novel two-dimensional materials, such as graphene [20], arsenene [21] and so on. As for silicene, Ciraci et al. [22] have investigated the local reconstructions of silicene induced by O adatom and O2 molecule. They pronounced that such adding oxygen to the silicene lattice gives rise to a stable single-layer silica [23]. Pi's group [24,25] have systemically investigated the silicene oxides, as well as the formation energies, electronic and magnetic properties of silicene with vacancies. Their work indicates that the oxidation of silicene should be exquisitely controlled to obtain specific silicene oxides with desired electronic properties.

However, the evolution of electronic structure and magnetism of silicen against the oxygen coverage are still unclear. In this paper, the silicene adsorbed by oxygen atoms up to eight have been investigaited by using an unbiased structure search method combined with density functional theory. We mainly address the following questions: Whether the band gap of silicene can be opened? How do the geometry and electronic structure of silicene change with the increasing of oxygen atoms? Can silicene be rendered magnetic? This research will provide theoretical support for application of silicene in nano electronic devices.

2. Computational details

To search for the lowest-energy structures of On-silicene (n = 0, 2-8), a two-step computation procedure is undertaken. Firstly, an unbiased structure search is performed using the CALYPSO method (Crystal structure Analysis by Particle Swarm Optimization) [[26], [27], [28]], which requires only the chemical compositions under the given conditions. This method has been successfully used in various systems [[29], [30], [31], [32], [33], [34], [35]]. For On-silicene (n = 2-8), the number of O atoms corresponds to the coverage of O on silicene. We followed 30 generations to achieve convergence. Each generation contains 50 structures, 60% of which are generated by particle swarm optimization (PSO), while the others are generated randomly. Next, among the 1000-1500 isomers, the top fifteen low-lying isomers are collected as candidates for lowest-energy structure. These isomers with energy difference from the low-lying isomers less than 5 eV are further reoptimized by B3LYP/6-311+G* [36,37] theoretical method. All the structural relaxations are carried out using density functional theory, as implemented in the Gaussian09 program package [38]. In the geometric optimization procedure, various possible spin multiplicities are considered to determine the preferred spin state. Meanwhile, the vibrational frequency calculations are performed to make sure the structures correspond to real local minima without imaginary frequency.

3. Results and discussion

To test our computational procedure, we begin with the calculations of pristine silicene, as is shown in Fig. 1. The lowest energy structure of pristine silicene, which includes thirty-two silicon atoms, is found to be a buckling structure with C2h symmetry and quintet spin multiplicity. The calculated average buckling, average Si-Si bond lengths and band gap are 0.47 Å, 2.27 Å and 1.46 meV, respectively. The results are in good agreement with those of previous studies [5,14,16,39,40]. The average buckling of silicene sheet upon surface decoration is calculated by the following formulas:

Δ=2(sumofheightoftopatoms-sumofheightofvalleyatoms)/totalnumberofatoms

Fig. 1.   (a) Top and (b) side views of the geometry of pristine silicene. The yellow and green ball represent the top and valley silicon atoms, respectively. (c) The corresponding electronic band structure.

3.1. Geometries

The adsorption of oxygen atoms (from two to eight) on silicene surface are considered by us. Due to the sp3-like lattice structure of silicene, we can expect more active sites compared with the completely flat surface of graphene. As a consequence of buckled hexagonal lattice structure, silicene has four possible adsorption sites: above the center of the hexagonal silicon ring (hollow site), on top of Si-Si bond (bridge site), on top of the upper silicon atoms (hill site) and on top of the lower silicon atoms (valley site).

Using an unbiased structure search method CALYPSO combined with density functional theory, we obtained lots of different oxygen absorbed silicene structures. Here, we only selected three low-lying structures for each type of On-silicene (n = 2-8). The results of O2-silicene and O6-silicene are shown in Fig. 2 together with their relative energies, in which the digits (2 and 6) stand for the number of oxygen atoms. The structures of On-silicene (n = 3-5, 7-8) are collected in Figs. S1 and S2 of supplementary information, respectively. In the following sections, we mainly focus on O2-silicene and O6-silicene to illustrate the structures, electronic and magnetic properties of silicene with oxygen atoms adsorption. The reason is that both of them have the relatively large direct band gaps and high stability among all the investigated compounds. The relative energies of the structures are determined from the difference energy which are defined as Ere = Ethe low-lying isomers - Ethe ground state isomer. In addition, the calculated average Si-Si bond lengths, the shortest and longest O-Si bond lengths, the average buckling parameter Δ, as well as the adsorption energy Ead are listed in Table 1. Owing to the fact that the electronic features are significantly correlated with the geometric structure of silicene, we mainly focus on the most stable configurations. As can be seen from Fig. 2 and S1-S2, we clearly find that the most favorable site for adsorbed oxygen is the bridge site irrespective of the number of oxygen atoms, which is in agreement with the previous results [41]. For O2-silicene and O3-silicene, oxygen atoms mainly adsorbed on bridge sites of peripheral Si-Si bonds. The structures with oxygen atoms adsorbed on bridge sites of internal Si-Si bonds are higher in energy. The average buckling are 0.53 and 0.49 Å, which are slightly larger than that of pristine silicene (0.47 Å). The average bond length of Si-Si (2.29 Å) is also slightly longer than that of the pristine sheet (2.27 Å). With the increasing of oxygen atoms, partial oxygen atoms adsorb on bridge sites of internal Si–Si bonds. This results in the considerably destroyed structure of silicene due to the chemical bonding between oxygen atoms and silicon atoms. Oxygen adsorbate is almost completely immersed into the silicene layer and pushes the underlying Si atom up from its original position. Therefore, the average buckling becomes quite complicated. In this case, the average buckling of On-silicene (n = 4-8) is not calculated.

Fig. 2.   Structures of silicene with two and six oxygen atoms adsorption together with their relative energies. 2 and 6 represent the numbers of adsorbed oxygen atoms.

Table 1   Average Si-Si bond lengths (in Å), the shortest and longest O-Si bond lengths (in Å), the average buckling parameter Δ (in Å), adsorption energy Ead (in a.u.), and band gap Eg (in meV) for the lowest-energy structures of On-silicene (n = 0, 2-8). The subscript from 2 to 8 stand for the number of oxygen atoms adsorbed on silicene. The superscript I and II stand for the shortest and longest O–Si bond lengths, respectively.

StructuresSi–SiO–SiIO–SiIIΔEadEg
Silicene2.27--0.47-1.46
O2-Silicene2.291.651.740.53-0.51612
O3-Silicene2.291.631.830.49-0.7630
O4-Silicene2.311.641.78--0.9613
O5-Silicene2.311.621.75--1.19157
O6-Silicene2.361.681.86--1.37244
O7-Silicene2.321.521.90--1.63175
O8-Silicene2.321.641.86--1.82144

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Moreover, as can be seen from Table 1, it is found that the O-Si bond lengths ranging from 1.52 to 1.90 Å with the increasing number of oxygen atoms from two to eight. The adsorption energy of the oxygen atoms on silicene is defined as Ead = Esystem - (Esilicene + Eadatom), where Esystem, Esilicene and Eadatom are the total energies for oxygen atoms adsorbed silicene, pristine silicene, and oxygen atoms, respectively. The calculated adsorption energies for On-silicene (n = 2-8) are -0.51, -0.76, -0.96, -1.19, -1.37, -1.63 and -1.82 a.u, respectively. According to the definition, negative adsorption energy means that the oxidation of silicene is exothermic. In general, adsorption behavior can be classified as physisorption and chemisorption. On one hand, the corresponding O-Si bond lengths range from 1.52 to 1.90 Å, which are much shorter than the Van der Waals distance (3.52 Å). This indicates that the oxygen atoms physisorption is not controlled by typical dispersion forces, but enhanced the chemical interaction. On the other hand, the adsorption energies are in the range of -0.51 ˜ -1.82 a.u. Those large adsorption energies also indicate the chemical adsorption between oxygen atoms and silicene. In this case, we first identified that no matter where the oxygen atoms are placed on silicene, the oxygen atoms are chemisorbed on silicene.

3.2. Electronic structures

Owing to the small band gap limits the application of silicene on electronic devices, the electronic properties of oxygen atoms adsorbed silicene were investigated in an attempt to open the band-gap of pristine silicene. As mentioned in introduction section, various effects, such as applying an external field or adsorbing foreign atoms on silicene, have been devoted in opening bandgap in the Dirac cone [10,[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]].

The calculated electronic band structures for O2-silicene and O6-silicene are shown in Fig. 3. The results of On-silicene (n = 3-5, 7-8) are listed in Fig. S3 of supplementary information. Furthermore, all the values of band gaps are listed in Table 1. When two and six oxygen atoms adsorbed silicene (Fig. 3), the conduction and valence bands of silicene move up and down, respectively. This clearly demonstrates that there is a band gap opening at Fermi level. The values of band-gap increases from 1.46 meV to 612 meV and 244 meV, which are relatively larger among our obtained values. As a result, the silicene changes from direct-gap semimetallic to semiconductor after adsorbing two oxygen atoms. For On-silicene (n = 3-5, 7-8), the band structures, in which a band-gap opening is introduced at Fermi level, are presented in Fig. 3. It is clearly observed that the silicene changes from direct-gap semiconductor to indirect-gap semiconductor. The band-gaps of On-silicene (n = 3-5, 7-8) are 30, 13, 157, 175 and 144 meV, respectively. From these data, we found that the band-gaps properties are strongly dependent on the amount of oxygen atoms, which may be related to the influence of oxygen atoms on pristine silicene structure. It is anticipated that two oxygen atoms adsorbed silicene is more advantageous among these investigated compounds, owing to the relatively large direct band gap (612 meV) at the K point. In addition, the Dirac cone around Fermi level, which emerges in pristine silicene, disappears in these band structures. Based on the electronic properties of various oxygen adsorbed silicene, we find that the adsorption configurations and amounts of oxygen adatoms on the silicene surface are critical for band gap engineering.

Fig. 3.   Electronic band structures and electronic density states (DOS) of O2-silicene (a) (c) and O6-silicene (b)(d).

In addition, we also calculated the total and partial density of states (TDOS and PDOS), the results of O2-silicene and O6-silicene are given in Fig. 3. The other results are listed in Fig. S4 of supplementary information. According to characteristics of DOS, the On-silicene (n = 2-8) monolayer is found to be metallic as the partially occupied bands. The metallicity of On-silicene is in accord with its high carrier mobility and good electrical conductivity. In addition, the typical feature of O2-silicene and O6-silicene is that there is the presence of pseudogap (a sharp valley around the Fermi energy) in the total DOS, which separate the bonding and antibonding states. The presence of pseudogap will surely implies the relative strong bonding between O and Si atoms in these two compounds. In other words, two and six oxygen atoms adsorbed silicene possess the higher structural stabilities among all the studied compounds. As for TDOS, the electronic states near the Fermi level of oxidized silicene are dominated by the Si-3p orbitals, which make a big contribution to the observed metallicity. The conduction bands are mainly contributed by the Si-3p orbitals, and the valence bands originate from the Si-3 s orbitals and O-2p orbitals. Moreover, the valence bands in the range of -23-17 eV are mainly dominated by the O-2 s orbital. The width of the band gap is predominantly influenced by the adsorption sites and dose of oxygen adatoms. Therefore, we conclude that the adsorption oxygen atoms could open the band gap of silicene, and then, adsorbed silicene transforms from a gapless semiconductor to a direct or indirect band-gap semiconductor.

3.3. Magnetic properties

Magnetism is the key property to the application of silicene on spintronic devices. However, the magnetic properties of silicene have not been systematically studied so far. Thus, we now study the effect of oxygen atoms adsorption on the spin polarization of the silicene. As is well known, the contribution of orbital magnetic moment is usually very small compared with spin magnetic moment. The spin moment is though to be reasonable estimate of the total magnetic moment for given cluster. That is to say, the spin magnetic moment is equal to spin multiplicity minus one [42]. For the isolated silicene, we found that the magnetic moment of silicene is 4.00 μB because of its spin multiplicity is quintet state based on the full geometry optimization. In Table 2, we summarized the total magnetic moments of oxygen atoms adsorbed silicene and the local magnetic moments on silicon atoms. Meanwhile, the corresponding α and β spin electron configurations of silicon atoms are also listed in Table 2. From Table 2, we note that oxygen atoms may decrease the magnetic moment of silicene. By observing the local magnetic moments of silicon atoms in On-silicene (n = 2-8), it is found that the total magnetic moments of oxygen atom adsorbed silicene mostly localized on silicene sheet and the main contributions come from 3p orbital of silicon atoms. The oxygen atoms are polarized ferromagnetically, but with very small magnetic moments (0.07-0.13μB). In conclusion, the silicene can maintain its magnetic moment if we properly choose the number of adsorbed oxygen atoms.

Table 2   Spin total magnetic moments of On-silicene (n = 0, 2-8), as well as the local magnetic moments of silicene sheet and the corresponding α and β spin electronic configurations on silicene sheet. The superscripts from 2 to 8 stand for the number of oxygen atoms adsorbed on silicene.

Structuresμ (μB)Natural electron configuration
TLαβ
Silicene44.343s23.343p40.483d0.304p0.223s22.73p36.943d0.304p0.06
O2-Silicene44.353s23.263p40.423d0.314p0.183s22.893p36.433d0.304p0.19
O3-Silicene44.023s23.333p40.213d0.314p0.205p0.013s22.933p36.713d0.314p0.085p0.01
O4-Silicene21.983s23.013p39.063d0.344p0.155p0.023s22.723p37.403d0.334p0.055p0.02
O5-Silicene21.883s23.043p38.223d0.344p0.145p0.023s22.853p36.543d0.344p0.135p0.02
O6-Silicene21.943s23.653p36.933d0.314p0.185p0.033s23.553p35.093d0.314p0.185p0.03
O7-Silicene43.923s23.343p37.663d0.344p0.135p0.043s22.943p34.263d0.304p0.075p0.02
O8-Silicene44.023s23.493p36.993d0.344p0.135p0.013s23.123p33.543d0.344p0.095p0.01

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To visualize this complicated spin polarization, the spin density isosurfaces of On-silicene (n = 2 and 6) are plotted in Fig. 4, and the others are listed in Fig. S5 of supplementary information. In those figures, the green and purple isosurfaces represent positive and negative spin density, respectively. By comparing the spin density isosurface of the pristine silicene with that of oxygen atoms adsorbed silicene, it is obviously found that the spin densities are mainly contributed by silicene sheet. Some oxygen atoms are spin polarization, but with very low spin density. This result has been confirmed by the calculated total and local spin magnetic moment, as is shown in Table 2.

Fig. 4.   Spin density isosurfaces of O2-silicene and O6-silicene.

4. Conclusion

In conclusion, we present a comprehensive study of the structural, electronic and magnetic properties of silicene with oxygen adsorption. Based on an unbiased structure search method combined with First-principles calculation, we identify the ground state structures for On-silicene (n = 2-8). The results show that the most favorable adsorption site of oxygen atoms on the silicene surface is the bridge site and the oxygen atoms are chemisorbed on silicene. By analyzing the electronic band structures and DOS, we find that there is a band gap opening at Fermi level after oxygen adsorption. In particular, two and six oxygen atoms adsorbed silicene are more advantageous due to the relatively large direct band gaps at the K point. Futhermore, the magnetic moments and spin density isosurfaces show that the total magnetism is mostly localized on silicene sheet which is mainly contributed by the 3p orbitals of silicon atoms. We hope that the results presented here will inspire new experiments to synthesize and design the potential silicene-based electronic devices.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Nos. 11604194, 11804212 and 21671114), The 973 Program of China (No. 2014CB660804), the Natural Science Foundations of Shaanxi Province (Nos. 2016JQ1028 and 2016JQ1003), the Shaanxi University of Science & Technology Key Research Grant (Nos. 2016BJ-01 and BJ15-07), and the Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 15HASTIT020).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.03.032.

The authors have declared that no competing interests exist.


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