Article pubs.acs.org/JPCC
First-Principles Study of Ferromagnetism in Two-Dimensional Silicene with Hydrogenation Chang-wen Zhang*,† and Shi-shen Yan‡ †
School of Physics and Technology, University of Jinan, Jinan, Shandong, 250022, People’s Republic of China School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong, 250100, People’s Republic of China
‡
ABSTRACT: We performed first-principles simulation on the electronic structure and magnetic properties of two-dimensional hexagonal silicene, which was recently synthesized. The results show that the weak overlapping between 3pz orbitals of neighbor Si atoms leads to a very reactive surface, resulting in a more energetically stable semiconducting surface upon being fully hydrogenated. Half-hydrogenation breaks the extended π-bonding network of silicene, leaving the electrons in the unsaturated Si atoms localized and unpaired, and thus it exhibits ferromagnetic semiconducting behavior with a band gap of 0.95 eV. The long-range ferromagnetic coupling between Si atoms was also predicted, with a Curie temperature of about 300 K. These results demonstrated that hydrogenation is an efficient route to tune the electronic properties of silicene sheets.
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the substitutional atoms or the adatoms on the surface can induce magnetic states in these 2D nanostructures, which should be more fruitful than corresponding bulk structures. Especially, on-plane chemical modification with hydrogen has been reported to achieve the long-range ferromagnetism without transition-metal doping in such 2D-based materials.14,15 Because silicene sheets have only recently been realized, the effects and characteristics of hydrogen adsorption on ferromagnetism have yet to be thoroughly explored. In the present paper, we investigate by means of ab initio calculation on the electronic structures of a silicene with or without chemical modification. Our results show that the band gap of silicene can be tuned when the sheet is modified with hydrogen. Of particular interest is the realization of the longrange ferromagnetism in such a 2D structure upon hydrogenation, which may open a new route to these silicon-based nanostructures in spintronics applications. All the calculations were carried out using the Vienna Ab initio Simulation Package (VASP) and density functional theory (DFT).16 Throughout the computations, the generalized gradient approximation (GGA)17 and a 450 eV cutoff enrgy for the plane-wave basis set were used. Pseudopotentials with 3p23s2 and 1s1 valence electron configurations for Si and hydrogen atoms were used, respectively. Following the
raphene, a two-dimensional (2D) honeycomb network of carbon atoms, is currently a material of much interest because of its unique electronic properties as well as its potential applications in future nanoelectronics1,2 These features convey to graphene some remarkable properties, such as displaying Klein tunneling, the anomalous integer quantum Hall effect for relatively low magnetic fields at room temperature, and a finite minimum conductivity.3 More recently, hydrogenation has been achieved,4 leading not only to a material known as graphane but also to the prospect for application in spintronics due to its ferromagnetism induced by hydrogen adsorption.5 Nevertheless, graphene is facing many challenges, such as toxicity, difficulty in processing, and incompatibility with current silicon-based electronic technology, with the growth of graphene over large areas. As the counterpart of graphene, the 2D hexagonal lattices of Si, socalled silicene,6,7 recently were chemically exfoliated from calcium disilicide (CaSi2). In the more recent works, the Si nanoribbons were fabricated by deposition on a silver substrate.8,9 Similar to graphene, there is interest in modifying a silicene sheet in order to tailor its properties. For example, Takeda et al.10 first investigated the geometry of silicene and found that it would rather be buckled than flat, reflecting the reduced tendency of the group IV elements other than carbon to form sp2 bonding. Hydrogenation of silicene can also open a gap, and the resulting compound should be related to polysilane.11 For graphene, BN and AlN sheets,12−14 previous study showed that © 2012 American Chemical Society
Received: October 30, 2011 Revised: December 21, 2011 Published: January 24, 2012 4163
dx.doi.org/10.1021/jp2104177 | J. Phys. Chem. C 2012, 116, 4163−4166
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Monkhorst−Pack scheme,18 Brillouin zone integration was carried out at 9 × 9 × 1 k-points, and 15 × 15 × 1 k-points were used to obtain the density of states (DOS). The symmetry-unrestricted optimizations for geometry were performed using the conjugate gradient scheme until the maximum H−F force is smaller than 0.01 eV/Å. The computed lattice parameter a of relaxed silicene equals 3.85 Å, consistent with previous results (a = 3.86 Å),19 as shown in Figure 1a. The Si−Si bond length is calculated to be
Figure 2. Optimized geometric structures of the Si@H sheet from the top view (a) and from the side view (b). The band structure calculated after structural relaxation along high symmetry point (c).
more bulked than bare silicene (h = 0.45 Å). The relaxed bond lengths of Si−Si and Si−H are d1 = 2.34 Å and d2 = 1.51 Å, respectively. This Si−Si bond length lies between that in bare silicene and H@Si12@H structures, while all the H atoms are adsorbed on Si1 atoms with Si−H bonds arranged normal to the silicene plane. Similar to graphene, the H atoms form strong σ bonds with Si1 atoms, resulting in an sp3 hybridization of the Si atoms, while the Si2 atoms remain sp2-hybridized. Next, we discuss the magnetic properties of the Si1@H sheet. It is known that, in silicene, the p orbitals perpendicular to the plane of the Si ring system combine to form a weakly and extensive π-bonding network. The resulting delocalized π electrons lead to a metallic and nonmagnetic (NM) sheet. When half of the Si atoms are hydrogenated, strong σ bonds are formed between Si1 and H atoms and the π-bonding network is broken, leaving the electrons in the unsaturated Si atoms localized and unpaired, and thus resulting in Si2 being spinpolarized with a magnetic moment of 1.0 μB. As shown in Figure 3, we consider three magnetic configurations in a 4 × 4 supercell: (i) FM coupling; (ii) antiferromagnetic (AFM) coupling, and (iii) NM states. Note that the FM state lies 0.068 and 0.97 eV lower in energy than AFM and NM states, respectively, indicating that half-hydrogenation on the Si atom leads to an FM state. Adopting the mean-field approximation (MFA),22 the Curie temperature was estimated to be about 300 K. That is, the Si@H structure is predicted to be a roomtemperature FM material. In Figure 4a, we present the spatial spin-density distribution of the Si1@H sheet. It indicates that the hydrogen-induced magnetism is mainly localized around the unsaturated Si2 atoms (0.27 μB), while the hydrogenated Si1 and H atoms carried very small magnetic moments (less than 0.05 μB), indicating that the unpaired 3p electrons in the unsaturated Si2 sites contribute to magnetism. As shown in Figure 4b, because the valence electrons in 3p states on Si2 are more delocalized than those in d or f states, the larger spatial extension promotes long-range exchange FM coupling, due to the extended p−p interactions. In fact, the extended tails of wave functions have also been proposed to mediate long-range ferromagnetism between defect-induced moments in BN.21,23 To further illustrate the origin behind ferromagnetism in the Si1@H sheet, the band structure and partial density of states (DOS) are presented in Figures 2c and 4c. We can see that most of the states near the Fermi level are mainly contributed by the 3p electrons of Si2 atoms. Unlike the pristine silicene, which is semimetallic, the Si1@H sheet is an indirect band-gap semiconductor with the valence band maximum (VBM) located
Figure 1. Optimized geometric structures of silicene from the top view (a) and from the side view (b). The band structure calculated after structural relaxation along high symmetry directions (c).
d1 = 2.27 Å, corresponding to a contraction of the bond by about 0.11 Å compared with bulk Si. A similar contraction of the C−C bond length (0.12 Å) is observed when comparing diamond (sp3 hybridization) and graphene (sp2 hybridization).20 Compared with graphene, the larger Si−Si interatomic distance weakens the π−π overlaps, and thus it cannot maintain the planar structure, which results in a lowbuckled structure with h = 0.45 Å (Figure 1b). Such a bond contraction is characteristic of a transition from sp3 to sp2 hybridization in carbon, corresponding to the formation of π bonds between neighbor C atoms. Consistent with recent results,21 silicene is found to be a gapless semiconductor, the bonding π and antibonding π* bands crossing only at K points in the hexagonal Brillouin zone (Figure 1c). It points out that the π bonds in silicene result from the overlapping of 3pz orbitals of the Si atoms. However, this overlapping is relatively weak compared to the overlapping of 2pz orbitals in graphene,20 since the strength of the π bond is related to this atomic orbital overlapping. On the basis of total energy calculations, silicene is predicted to be 1.56 eV/atom less stable than bulk Si. This means that the growth of silicene necessitates the use of deposition techniques, which enable the control of a 2D material to avoid the formation of 3D islands. As an example, 2D silicene has been successfully deposited by MBE on Ag(110) or (100) substrates at temperatures below 250 °C,7 demonstrating the feasibility of growing this 2D nanostructures. When the silicene is exposed to gaseous hydrogen, the halfhydrogenated silicene (defined as Si1@H), in which Si1 atoms are hydrogenated and Si2 atoms remain unsaturated, in Figure 2, is easily formed in experiments. In the fully hydrogenated silicene (defined as H@Si12@H), all the Si atoms form a zigzag configuration similar to the sp3-hybridized form. The Si atoms in the Si1@H sheet become more planar-like with h = 0.61 Å (Figure 2b) as compared to H@Si12@H (h = 0.78 Å), but it is 4164
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Figure 3. Different magnetic configurations of the Si@H sheet, including the magnetic moments and their relative energies with respect to FM states for (a) FM, (b) AFM, and (c) NM states.
Figure 4. Spatial spin-density distribution and DOS in the Si1@H sheet. Top view and side view are presented in (a) and (b), respectively. The total and partial DOS are shown in (c). The vertical line is defined by the Fermi level.
orbitals form extended π-bonding, which would quench magnetism. Thus, we can safely confirm that only halfhydrogenation on Si1 atoms can introduce magnetism. Controlling the coverage of hydrogenation and the geometry is the key to observe the predicted ferromagnetism. The relative stability of the FM properties in the Si1@H sheet is important for the practical application in Si-based
at the K point similar to that in graphene,20 and the conduction band minimum (CBM) at the Γ point. This is caused by breaking of the π-orbital network due to partial hydrogenation of Si2 atoms. The band gap of Si1@H is found to be 0.95 eV, which is significantly smaller than that of bulk silicon. If we further remove H atoms adsorbed on Si1, the two unsaturated Si atoms alternatively form nearest neighbors, and thus their pz 4165
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spintronics devices. Therefore, we perform an ab initio molecular dynamics simulation on a 2 × 2 Si1@H supercell at room temperature (T = 300 K) with a time step of 1 fs. After running 1300 steps, the geometry structure of the Si1@H sheet is kept and the stable structure is still FM. Therefore, the FM properties of the half-hydrogenated silicene sheet will be preserved at room temperature. In conclusion, on the basis of first-principles calculations, we find that it is possible to introduce ferromagnetism into a 2D silicene sheet by surface modification with hydrogenation. These findings provide the following advantages over the previous FM materials: (i) The long-range ferromagnetism in Si1@H enriches the FM materials observed in previous works. Once combined with advanced Si nanotechnology, these predicted properties may be very useful as a promising nanoscale technological application in spintronics. (ii) The sp/d0 ferromagnetism induced upon hydrogenation in silicene is homogeneously distributed, not suffering from the problems related to precipitates or secondary phase formation in transition-metal-doped materials, which are undesirable for practical applications. (iii) It is unnecessary to cut the 2D sheets into finite systems with zigzag or armchair edges like 1D nanoribbons or 0D quantum dots, where magnetism appears at the edge or interior vacancies.24,25 Therefore, our work offers an efficient route toward long-range room-temperature ferromagnetism in silicene sheets and may motivate potential applications of Si-based nanostructures in nanoelectronics.
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AUTHOR INFORMATION
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 61076088), the Foundation for Young Scientist in Shandong Province (Grant No. BS2009CL012), and the Technological Development Program in Shandong Education Department (Grant No. J10LA16).
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