Electronic Structures of Silicene Nanoribbons: Two-Edge-Chemistry Modification and First-Principles Study
© The Author(s) 2016
Received: 17 May 2016
Accepted: 12 August 2016
Published: 22 August 2016
In this paper, we investigate the structural and electronic properties of zigzag silicene nanoribbons (ZSiNRs) with edge-chemistry modified by H, F, OH, and O, using the ab initio density functional theory method and local spin-density approximation. Three kinds of spin polarized configurations are considered: nonspin polarization (NM), ferromagnetic spin coupling for all electrons (FM), ferromagnetic ordering along each edge, and antiparallel spin orientation between the two edges (AFM). The H, F, and OH groups modified 8-ZSiNRs have the AFM ground state. The directly edge oxidized (O1) ZSiNRs yield the same energy and band structure for NM, FM, and AFM configurations, owning to the same s p 2 hybridization. And replacing the Si atoms on the two edges with O atoms (O2) yields FM ground state. The edge-chemistry-modified ZSiNRs all exhibit metallic band structures. And the modifications introduce special edge state strongly localized at the Si atoms in the edge, except for the O1 form. The modification of the zigzag edges of silicene nanoribbons is a key issue to apply the silicene into the field effect transistors (FETs) and gives more necessity to better understand the experimental findings.
KeywordsSilicene Electronic structure Localized edge states Density functional theory
As the silicon counterpart of graphene , silicene [2–5] becomes a hot spot in low-dimension material application after being recently grown on different metallic surfaces [6–12], which consists of a single layer of Si atoms arranged in a hexagonal network as well as low-buckled geometry. The buckled distance is 0.4 Å between the layers, and this low-buckled structure is attributed to the pseudo-Jahn-Teller distortion [13–15]. Theoretical calculations have revealed that s p 3 hybridization and s p 2 hybridization in a low-buckled silicene structure is different from the s p 2 hybridization of planer geometry in graphene [9, 16–18]. Tao et al. have successfully fabricated the first silicene-based field-effect transistors (FETs) operating at room temperature  and make a breakthrough for applications of silicene.
Because of its extraordinary electronic properties, silicene nanoribbons (SiNRs)[20–22] also attracts lots of attentions. The electronic properties of silicene nanoribbons are dependent on the structural size and chirality. Via assistances of recent investigations [23–26], the ZSiNRs exhibit rich electronic transport, magnetic properties, and may be applied in spintronic nanodevices potentially [27–32]. In particular, electronic transport properties of the SiNRs are vital in electronic industry [33–36]. Zigzag edges have a localized edge state at the Fermi level with semimetal properties [35–37]. Therefore, modifications of the ZSiNRs with chemical elements is important. Silicon atoms in silicene tend to adopt s p 3 hybridization over s p 2, which makes it extremely reactive towards O2 and H 2O and also has a tendency to self-aggregate. This can be suitably be avoided by using Ca intercalation . The effect of zigzag edges chemistry of graphene has been studied with an edge-modified form C–H (s p 2 σ-bonded edge C) and represents ferromagnetic edge magnetization . The similar significant properties may exist in the edge-chemistry-modified silicene nanoribbons, so we choose H, F, OH, and O atoms or atomic groups for investigation.
In this paper, we report the simulations of the geometry structures and electronic properties of the 8-ZSiNRs (with eight zigzag armchair chains) with edge-chemistry modifications by H, F, OH, and O atoms or atomic groups. We calculate three kinds of spin-polarized SiNRs: nonspin polarization (NM), ferromagnetic spin coupling for all electrons (FM), and anti-parallel spin orientation between the two edges (AFM). The electronic band structures of ZSiNRs with H, F, and OH edges have similar profile due to their same s p 2 hybridization. The band gap opening of these edge modifications are no larger than 0.14 eV, which is very small, indicating the exhibition of metallic properties. Except the directly oxidized (O1) ZSiNRs, other modifications all have special edge state strongly localized at the Si atoms in the edge. The H, F, and OH groups’ modified 8-ZSiNRs have the AFM ground state. The directly edge oxidized (O1) ZSiNRs yield the same energy and band structure for NM, FM, and AFM configurations. And replacing the Si atoms on the two edges with O atoms (O2) yield FM ground state. The modification of the zigzag edges of silicene nanoribbons is a key issue in applying the silicene into the FETs and gives more necessary to better understand the experimental findings.
The electronic properties are studied through the DFT calculations with the local density approximation (LDA)  and the projector-augmented wave (PAW) [41, 42] potentials, implemented as the Vienna ab initio simulation package (VASP) [43, 44]. A plane-wave basis set with kinetic energy cutoff of 400 eV has been used. In the structure optimizations and relaxations, a mesh of 13×1×1 Monkhorst-Pack  special k points is chosen for the Brillouin zone integrations. The vacuum separation between the nanoribbons in the adjacent unit cells is chosen to be 15 Å. All positions of the atoms and the lattice constant are optimized by minimization of the total energy and atomic forces. The convergence for energy is chosen as 10−6 eV between two steps. For the density of states (DOS) calculations, we used the tetrahedron integration method with a Monkhorst pack special 19×1×1k grid followed by the broadening with the Gaussian width of 0.05 eV. All of the ZSiNRs are studied with the width of eight zigzag chains (N Z =8,L=2.97 nm) with 16 Si atoms for the bare part. We consider four kinds of chemical edge modification (H, F, OH, O). Three kinds of spin-polarized calculation are performed for all of the ZSiNRs: NM, FM, and AFM.
Energy of spin-polarized configurations. The relative energy (in unit of meV) of ferromagnetic spin coupling for all electrons (FM) and anti-parallel spin orientation between the two edges (AFM) configurations to the no spin polarization (NM) configurations
Relative energy (meV)
E FM−E NM
E AFM−E NM
Edge Fluoridation and Hydroxylation
The ZSiNRs modified with oxygen (O) atoms have two kinds of structures. One includes oxidation directly, which can be called as O1 (SiO–SiO). And the other has the structure with the O atoms replacing the Si atom in the two edges and is referred to as O2 (Si 2O–Si 2O). The geometric constructions are clearly shown in Fig. 1 d, e. Notably, replacing the Si atoms on the edge with O atoms can be one method to change the buckling structure of silicene.
Oxidation Directly (O1)
Replacing the Si Atom with O Atom (O2)
We have calculated the band structures and the pDOS of the ZSiNRs with the two-edge-chemistry modified by the various functional groups: H, F, OH, and O. Three types of spin-polarized configurations (NM, FM, AFM) are considered. It is found that the electronic band structures of ZSiNRs oxidized by the F and OH groups have very similar profile with that of H passivation, due to the same s p 2 hybridization for each edge Si atom. It also results in the same effect of spin polarization, that is, the H, F, and OH groups’ modified ZSiNRs all have the AFM ground state. There are special edge states that strongly localized at the Si atoms in the edge, which especially originates from their p orbital electrons. The ZSiNR edge modified by O atoms directly (SiO–SiO) yields the same band structure for the three types of spin polarization (NM, FM, AFM), which means it is non-magnetic. Since the different spin configurations do not shift the electronic energy level, the calculated total energies are almost the same, and their spin magnetic moments are zero. One of the two singly occupied 2p orbitals of the O atom forms one σ bond with the edge Si atom. The other one left occupies the π-top (π∗-bottom) state. For replacing the Si atoms on the two edges with O atoms (O2), the two singly occupied 2p orbitals of the O atom forms two σ bonds with the inner edge Si atoms, and the edge states are highly localized at the two inner edge Si atoms. This modification yields FM ground state. The ZSiNRs edge-chemistry modified by H, F, OH, and O all exhibit metallic band structure. It is quite important to control the ZSiNR edge-chemistry modification for future applications of SiNRs.
The work is supported by the National Natural Science Foundation of China (No. 61575033) and the Fundamental Research Funds for the Central Universities (No. 106112016CDJXY30001).
YY participated in the numerical calculation, and drafted the manuscript. AL conceived of the study and participated in its design and coordination. JB performed the statistical analysis. XZ participated in the numerical calculation. RW participated in the design of the study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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