The electronic and magnetic properties of functionalized silicene: a first-principles study
© Zheng and Zhang; licensee Springer. 2012
Received: 15 June 2012
Accepted: 16 July 2012
Published: 28 July 2012
Based on first-principles calculations, we study the structural, electronic, and magnetic properties of two-dimensional silicene saturated with hydrogen and bromine atoms. It is found that the fully saturated silicene exhibits nonmagnetic semiconducting behavior, while half-saturation on only one side with hydrogen or bromine results in the localized and unpaired electrons of the unsaturated Si atoms, showing ferromagnetic semiconducting or half-metallic properties, respectively. Total energy calculations show that the half-hydrogenated silicene exhibits a ferromagnetic order, while the half-brominated one exhibits an antiferromagnetic behavior.
Recently, low-dimensional honeycomb graphene has attracted much interest because of its unique electronic properties as well as its potential applications in future nanoelectronics, and therefore is one of the most investigated materials in physics and nanoscience . Nevertheless, graphene is facing many challenges in its growth over large areas and, importantly, incompatibility with current silicon-based electronic technology. As the counterpart of graphene, the two-dimensional (2D) hexagonal silicene  recently is chemically exfoliated from calcium disilicide (CaSi2). In the more recent works, Si nanoribbons are fabricated by deposition on a silver substrate [3, 4]. The synthesis of silicon-based nanomaterials opens the way for studying their physical and chemical properties, with the added advantage of being compatible with existing semiconductor devices.
The chemical functionalization is generally an efficient way to tune the electronic and magnetic properties in 2D structures, such as graphene, BN, AlN, and CdS sheets [5–8]. Especially, on-plane chemical modification with hydrogen has been reported to induce long-range ferromagnetic order without 3d or 4f element doping in such 2D carbon-based materials [9, 10], not suffering from problems related to precipitates or secondary phase formation in 3d- or 4f-element-doped materials, which are undesirable for practical applications. For Si-based nanostructures, Jose and Datta  reported the structures and electronic properties of silicene clusters and Si-substituted benzenes, suggesting that silicene clusters may be a promising material for FET and hydrogen storage. Since silicene has only recently been realized [4, 5], the effects of adsorption of foreign atoms on the surface of silicene on magnetism have not been thoroughly explored. In the present letter, based on first-principles calculations, we focus on the possibility of realizing ferromagnetism in silicene with adsorption of hydrogen and the halogen element bromine (Br). It can be seen that the electronic properties of silicene can be tuned, and especially, the ferromagnetic order or half-metallicity is achieved upon adsorption of H and Br atoms, which may open a new route to design the silicon-based nanostructures in spintronics.
All the predictions have been performed using the Vienna Ab initio Simulation Package and density functional theory . The generalized gradient approximation  and a 450-eV cutoff energy for the plane-wave basis set were used. Pseudopotentials with 3s23p2, 1 s1, and 4s24p5 valence electron configurations for Si, H, and Br atoms were used, respectively. Following the Monkhorst-Pack scheme , Brillouin-zone integration was carried out at 9 × 9 × 1 k-points, and 15 × 15 × 1 k-points were used to obtain the electronic properties. The symmetry-unrestricted optimizations for geometry were performed using the conjugate gradient scheme until the largest Hellmann-Feynman force is smaller than 0.01 eV/Å.
Results and discussion
In the case of half-brominated silicene (Br@Si1), Bader analysis shows that it is spin-polarized with a local magnetic moment of 1.0 μB per unit cell, similar with that of H@Si1. More interestingly, the energy bands close to the Fermi level show a metallic spin-down channel and a semiconducting spin-up one with a 1.73-eV bandgap, and thus a half-metallic behavior with 100% spin-polarized current is obtained, suggesting a feasible way of building spin devices based on silicene. To determine the magnetic stability of Br-induced half-metallicity in Br@Si1, the total energy differences of ferromagnetic, antiferromagnetic, and nonmagnetic orders are calculated. We find that the antiferromagnetic state lies 0.17 and 0.51 eV lower per unit cell in energy than ferromagnetic and nonmagnetic states, respectively, indicating that Br@Si1exhibits an antiferromagnetic behavior.
In summary, based on first-principles calculations, we study the electronic structure and magnetic properties of 2D hexagonal silicene adsorbed with H and Br atoms. We find that the fully saturated silicene on both sides exhibits nonmagnetic semiconducting behaviors. For half-saturation on only one side of silicene, H@Si1 exhibits a ferromagnetic behavior, while Br@Si1 shows a half-metallic property due to the localized and unpaired electrons of unsaturated Si2 atoms. Calculations of total energies show that Br@Si1 exhibits an antiferromagnetic behavior, while H@Si1 shows a long-range ferromagnetic order with a Curie temperature at about room temperature. Once combined with advanced Si nanotechnology, these predicted properties may be very useful as a promising nanoscale technological application in spintronics. Therefore, our work suggests that it may be possible to realize long-range room-temperature ferromagnetism in silicene sheets and may motivate potential applications of Si-based nanostructures in spintronics.
FBZ is a graduate student and CWZ is a professor in the School of Physics and Technology, University of Jinan, Shandong, People's Republic of China.
This work was supported by the National Natural Science Foundation of China (grant no. 61076088), Foundation for Young Scientist in Shandong Province (grant no. BS2009BR012), and Technological Development Program in Shandong Education Department (grant no. J10LA16).
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