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Fluorine edge decoration on zigzag silicene nanoribbons
Superlattices and Microstructures 139 (2020) 106394
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Fluorine edge decoration on zigzag silicene nanoribbons Gang Guo a, *, Zhongxiang Xie a, Jie Tang a, Yong Zhang a, **, Guobao Xu b a
Department of Mathematics and Physics, Hunan Institute of Technology, Hengyang, 421002, China National-Provincial Laboratory of Special Function Thin Film Materials, School of Materials Science and Engineering, Xiangtan University, Hunan, 411105, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Edge fluorination Electronic and magnetic properties Zigzag silicene nanoribbon
We used the density functional theory (DFT) to explore the effects of fluorine (F) edge decoration on zigzag silicene nanoribbons (ZSiNR). Due to the strong electron affinity of fluorine atoms, a strong chemical bond between Si atoms and F atoms is formed. The formation energy also indicated that the fluorination configurations had higher stability than for the ZSiNR. The fluo rination sites at the edges efficiently modulate the electronic and magnetic properties of the ZSiNR. In particular, we found a significant change from a metallic to a semiconductor charac teristic in asymmetric edge fluorinated ZSiNR, in a mono-fluorinated system with one single edge, which was attributed to the weakening of the σ-π mixing effect. In contrast, a half-metallic character with a 100% spin polarization was induced in another other two full asymmetric fluorination systems due to the contribution from the F-Py, F-Pz, and Si-Py orbitals. All asym metric fluorinations maintained a ferromagnetic character. Symmetric edge fluorinated ZSiNR maintained a ferromagnetic metallic characteristic or an antiferromagnetic semi-conductor characteristic due to the Coulomb repulsion from both edges. The interesting and useful phys ical properties in ZSiNR with edge fluorination including the half-metallic character and the spin polarization implied that edge-fluorinated ZSiNR can be effectively used in spintronic devices.
1. Introduction Two-dimensional (2D) materials [1] have recently received extensive attention due to their remarkable physical and chemical properties. In particular, great progress on graphene has been made both theoretically and experimentally in recent years [2–4]. Nowadays, graphene is used in many fields including in solar cells [5], as high-performance metal-ion battery anodes [6], in com posites [7], as sensing materials [8], and for energy storage [9]. Along with the development of the experimental methods, graphene nanoribbons (GNRs) have also been prepared in recent years. Physical properties of the graphene nanoribbons such as its electronic and magnetic properties [10], thermoelectric properties [11], and transport properties [12] have been theoretically studied to promote the application of GNRs nanodevices. Motivated by these significant results on graphene and GNRs, a graphene-like analogue, silicene [13], has received much attention as well. The biggest difference between graphene and silicene lies in the different geometric structure. Due to its buckling geometric structure, silicene has novel physical properties [14]. For instance, silicene nanosheets have been grown on metal surfaces [15,16]. Silicene [17] has a perfect honeycomb structure and has outstanding electronic properties. In recent years, the growth of silicene * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Guo), [email protected] (Y. Zhang). https://doi.org/10.1016/j.spmi.2020.106394 Received 3 November 2019; Received in revised form 18 December 2019; Accepted 6 January 2020 Available online 10 January 2020 0749-6036/© 2020 Elsevier Ltd. All rights reserved.
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nanoribbons (SiNRs) on metal surfaces [18–20] was also extensively studied. SiNRs grown on Ag (110) substrate have a sp2-like �vila et al. reported [20] that SiNRs grown on an Ag (110) surface can be perfectly terminated by H atoms and hybridization [18]. Da have a stable structure as well as excellent electronic properties. Many theoretical studies have been performed to determine the physical properties of silicene nanoribbons (SiNRs). The presence of dangling bonds [21] or defects [24], the strain [22], and the electrical field [23] effectively tune the electronic and magnetic properties of SiNRs. A first-principles study [25] reported
Fig. 1. The relaxed structures of 6ZSiNR with adsorbed by F atoms. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, (c) nF2-6ZSiNR-nF1, (d) nF1-6ZSiNR-nF1, and (d) nF2-6ZSiNR-nF2 (n ¼ 1–3). The blue balls, green balls represent for Si atoms, F atoms, respectively. 2
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electron-phonon coupling effects in SiNRs. Gao et al. [26] discovered an interesting spin Seebeck effect and thermal spin transport properties in Christmas-tree shaped SiNRs. Many methods can be employed to tune the SiNRs physical properties. Many studies emphasize the doping effect on SiNRs. For instance, doping with Al modulates the magnetic and electronic properties of armchair SiNRs (ASiNRs) [27]. Zigzag SiNRs (ZSiNRs) produce spin-polarized currents when doped with Fe, Co, and Ni [28]. In addition to metallic elements, non-metallic elements such as B [29] and N [30] also yield efficient doping. Aadsorption is another effective way to modify the physical properties of the nanoribbons. Yang et al. [31] reported that atomic adsorption of Si on ZSiNRs induced a large spin thermo-power. Li et al. [32] proved that the energy gap of SiNRs was sensitive to the adsorption of O atoms. ZSiNR adsorbed with Cu atoms had a significant current-voltage characteristic [33] that can be used for nano-electronic devices. Molecular adsorption on SiNRs was also predicted for promising molecular sensing. Walia et al. [34] reported that defective ASiNRs were an ideal sensor for NO and NH3. More interestingly, a first-principle calculation [35] investigated organic molecules adsorbed on SiNR and indicated that SiNRs were a suitable for DNA sequencing. Some physical properties of the SiNR can be effectively tuned by edge decoration. Zhang et al. [36] found excellent spin-dependent electronic transport properties for ZSiNRs obtained by asymmetric edge hydrogenation. They also demonstrated that hydrogen edge modification induced a perfect spin-filtering effect in ZSiNR [37]. Our previous study [38] predicted that the existing edge dangling bonds in ZSiNR create an opportunity to form chemical hydrides. The maximum density of H adsorption is 5.56 wt% [38], which means that ZSiNRs can be used as a promising chemical hydrogen storage material due to its lower formation energy. Ding et al. [39] also reported that ZSiNRs with asymmetric sp2-sp3 edges were bipolar magnetic semiconductors due to the incorporation of Klein and zigzag edge states. This proved that the adsorption of nonmetallic elements is suitable way to tune the physical properties of ZSiNRs. However, the electron affinity of fluorine (F) atoms is much stronger than for H atoms. Therefore, significant chemical bonds between the Si and F atoms are formed to obtain extremely diversified physical properties. For example, silicene with fluorination [40] is used for sensor applications whereas SiNRs fully F-terminated at both edges always show a semi-conductor character [41] with the increase of the ribbon width. From these significant findings, it appears that fluorine atoms help modify the electronic structure and the magnetic behavior of the nanoribbons. So far, the electronic and magnetic properties of symmetric and asymmetric edge fluorinated ZSiNR have not been systematically studied. Even though hydrogenated ZSiNRs were reported in previous studies [38,39], it is not clear whether other interesting physical properties can be obtained in ZSiNRs with edge fluorination. Therefore, we focused on exploring interesting and useful physical properties of ZSiNRs with edge fluorination including the half-metallic character and the spin polarization behavior using first-principle calculations and considering the variation of the edge fluorination concentration and the number of sites for the effective application in spintronic devices. 2. Computational methods and models In this work, we used the Vienna ab initio simulation package (VASP) [42] with first-principle calculations based on the DFT to study the effects of fluorine edge decoration on the electronic and magnetic properties of ZSiNR. The projector augmented wave (PAW) method [43] and the Perdew Burke Ernzerhof (PBE) [44] exchange correlation function in the generalized gradient approximation (GGA) [45] were used to perform all calculations. We made the calculations with a cutoff energy of 450 eV, which was suitable for our systems. A k-point mesh of 13 � 1 � 1was used for structural optimization. For the calculation of the density of state (DOS), we used a k-point mesh of 39 � 1 � 1. The vacuum region of 15 Å was set in the super-cell. The criterion for the maximum force in the relaxed calculation is smaller than 0.01 eV/Å. van der Waals (VDW) interactions [46] were also considered in our calculations. This has been Table 1 Calculated results of pristine 6ZSiNR and its adsorption configurations. The formation energy of the stable magnetic state and the bond distance of Si–F bond are indicated by Ef and D, respectively. Δrepresents for the energy gap of band structure, where M represents for metal property. The spin polarization at the Fermi level is indicated by P. Mtot is the total magnetic moment in the stable magnetic state. The amount of charge obtained by the F atom is represented by Q , in which the bracket indicates the number of F atoms with same amount of charge obtained from Si atom. Structure
6ZSiNR 6ZSiNR-1F1 6ZSiNR-2F1 6ZSiNR-3F1 6ZSiNR-1F2 6ZSiNR-2F2 6ZSiNR-3F2 1F2-6ZSiNR-1F1 2F2-6ZSiNR-2F1 3F2-6ZSiNR-3F1 1F1-6ZSiNR-1F1 2F1-6ZSiNR-2F1 3F1-6ZSiNR-3F1 1F2-6ZSiNR-1F2 2F2-6ZSiNR-2F2 3F2-6ZSiNR-3F2
Ef ðeVÞ
3.784 3.888 3.949 3.989 3.936 4.035 4.115 3.994 4.138 4.247 3.951 4.064 4.133 4.036 4.207 4.335
D (Å) (Si–F)
Δ(e V)
L
R
up
down
/ / / / / / / 1.618 1.619 1.618 1.624 1.624 1.624 1.618 1.619 1.618
/ 1.624 1.624 1.624 1.618 1.619 1.618 1.624 1.624 1.624 1.624 1.624 1.624 1.619 1.619 1.618
M M 0.05 0.32 M 0.25 M M M M 0.11 0.08 0.32 M 0.09 M
M M 0.30 0.45 M 0.26 0.24 M M 0.27 0.11 0.08 0.32 M 0.09 M
3
P (%)
Mtot (μB)
70 75 / / 80 / 100 2.8 0.01 100 / / / 42 / 20
3.81 3.00 2.00 3.00 3.00 2.00 2.00 2.00 1.00 1.00 0.00 0.00 0.00 1.81 0.00 2.00
Q (e) (Si to F) / 0.81(1) 0.81(2) 0.81(3) 0.81(2) 0.81(4) 0.81(6) 0.81(3) 0.81(6) 0.81(9) 0.81(2) 0.81(4) 0.81(6) 0.81(4) 0.81(8) 0.81(12)
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tested that for adsorption in nanoribbons [38] and was demonstrated to be accurate. Song et al. [47] proved that pristine ZSiNRs always showed a metallic character regardless of the ribbon width. Moreover, ZSiNR with a width of 6 that named 6ZSiNR was as a suitable substrate material in a previous study [48]. We therefore chose 6ZSiNR as a typical ZSiNR. In 6ZSiNR edge-adsorbed with F atoms, the relaxed adsorption models shown in Fig. 1 can be divided into five forms according to the fluorination concentrations and sites: mono-fluorination at one single edge (6ZSiNR-nF1), bi-fluorination at one single edge (6ZSiNR-nF2), mono-fluorination at one edge and mono-fluorination at the other edge (nF2-6ZSiNR-nF1), mono-fluorination a both edges (nF1-6ZSiNR-nF1), and bi-fluorination at both edges (nF2-6ZSiNR-nF2). The number of adsorption sites (n) changed from 1 to 3 along with the fluorination concentration. To discuss the structural stability of the calculated configurations, we calculated the formation energies of ferromagnetic (FM), antiferromagnetic (AFM), and nonmagnetic (NM) states. Here, the formation energy was calculated using equation (1) Ef ¼
1 ðE6ZSiNR N
F
NSi ESi
(1)
NF EF Þ
Where N is the total number of the system, NSi and NF are the number of Si atoms and F atoms in the system, respectively. The total energy of fluorination of 6ZSiNR is indicated by E6ZSiNR-F. EF and ESi are the total energy of a single F atom and Si atom, respectively. The calculated formation energies listed in Table 1 of the supplementary material showed that all cases with asymmetric fluorination of 6ZSiNRs maintained a stable FM state, whereas the cases with symmetric fluorination had stable AFM or FM state. All calculations listed in Table 1 are based on the stable ground state. 3. Results and discussion 3.1. Asymmetric edge fluorination First, pristine 6ZSiNR has a stable FM metallic character according to the verification shown in Fig. S1 of the supplementary material. This result is in agreement with a previous study [47]. Fig. 1 indicates the fifteen relaxed structures of 6ZSiNR adsorbed with F atoms. The adsorbed configurations maintained a perfect buckling structure without obvious deformation. Table 1 shows that all adsorbed configurations had a lower formation energy than pristine 6ZSiNR. In order to analyze the structural information of the asymmetric edge fluorination configurations, we presented the optimized structure of 3F2-6ZSiNR-3F1 and its corresponding charge density as well as the charge density difference as the typical representative of the asymmetric fluorination configurations in Fig. 2. Fig. 2 (a)–(b) shows that the Si–F bond length at the edges was about 1.62 Å, which is similar to a previous study [41]. At the edge of 6ZSiNR with mono-fluorination, the bond angle of F–Si–Si was about 115� , which was close to a sp2 hybridization. In contrast, the bond angle of F–Si–Si at the edge of bi-fluorination was 105� , which was on the verge of a sp3 hybridization. From the charge density and the charge density difference shown in the right panel of Fig. 2 (a) and (b), the charge density is distributed between two Si atoms in the Si–Si bond. However, the density near the F atom was higher than that near the Si atom in the Si–F bond. This was mainly due to the electron gain by the F atom and the electron loss in the Si atom given the difference in Pauling electronegativity between F (4.0) and Si (1.9). Besides, the Bader analysis showed that each F atom gained 0.81 electrons (e) from the Si atom. This significant charge transfer was responsible for the stability of the adsorbed configurations. In addition to the case of 3F2-6ZSiNR-3F1, the structural information and charge transfer values of fourteen other adsorbed configurations are listed in Table 1. Although the adsorption of F atoms at the edges was different, the bond length of F–Si and the charge transfer values evolved similarly. Fig. 3 shows the band structures of 6ZSiNR with asymmetric edge fluorination. Fig. 3 (a) shows that the band structure of 6ZSiNR1F1 with mono-fluorination at one single edge had a metallic character due to the two sub-bands crossing the Fermi Level. However, when the number of adsorption sites (n) increased, 6ZSiNR-2F1 and 6ZSiNR-3F1 both had a semiconducting character. In particular, a band gap of 0.05 eV for the up-spin and a band gap of 0.30 eV for the down-spin were found for 6ZSiNR-2F1. Similarly, 6ZSiNR-3F1 had
Fig. 2. The optimized structure of 3F2-6ZSiNR-3F1 and its corresponding charge density as well as charge density difference. (a) The top view of 3F2-6ZSiNR-3F1 (left panel) and the charge density (right panel), (b) the side view (left panel) and the charge density difference (right panel). Some structural information such as bond length is indicated. The blue balls, green balls represent for Si atoms, and F atoms, respectively. 4
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Fig. 3. The spin-polarized band structures of asymmetric adsorption configurations. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, and (c) nF2-6ZSiNR-nF1 (n ¼ 1–3). The Fermi level is denoted by the dotted line.
a band gap of 0.32 eV in the up-spin and a band gap of 0.45 eV in the down-spin. It implied that mono-fluorination can tune the metallic character of 6ZSiNR-nF1 (n ¼ 1–3) to a semi-conductor character. When considering bi-fluorination at one single edge in 6ZSiNR-nF2 (n ¼ 1–3), a change from metal to semi-conductor and half-metal occurs when the number of adsorption sites increases, as shown in Fig. 3 (b). 6ZSiNR-1F2 and 6ZSiNR-2F2 had a metallic character and a semi-conductor character, respectively. However, 6ZSiNR-3F2 had a metallic character in the up-spin and a semi-conductor character with a band gap of 0.24 eV in the down-spin, resulting in a halfmetal character that is useful for applications in spin electronic devices [49]. When considering the mono-fluorination at one edge and the mono-fluorination at the other edge (nF2-6ZSiNR-nF1), as shown in Fig. 3 (c), a similar change from metal to half-metal appeared when the number of adsorption sites increased. From the above analysis, 6ZSiNR with asymmetric edge fluorination had a metallic, semi-conductor, and half-metallic character. Therefore, we calculated the partial density of states (PDOS) of three typical cases that had a metallic character (1F2-6ZSiNR-1F1), a semi-conductor character (6ZSiNR-3F1), and a half-metallic character (3F2-6ZSiNR-3F1) to find out the mechanism behind it. The metallic character of pristine 6ZSiNR originated from the Si-py (σ state) and Si-pz (π state) orbitals. Similarly, Fig. 4 (a) shows that 1F26ZSiNR-1F1 retained a metallic character. The contributions of the F-py, Si-py (σ state) and Si-pz (π state) orbitals at the Fermi Level 5
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played a major role for conduction over the Fermi Level, which meant that the σ-π mixing effect [41] was strengthened, leading to a metallic character. However, when the number of adsorption sites changed, there were no DOS peaks for the F-py orbitals at the Fermi Level in 6ZSiNR-3F1, as shown in Fig. 4 (b). Due to the increase in the sp2 hybridization bond, the σ-π mixing effect was weakened. The DOS peaks were pushed away from the Fermi Level, which led to the opening of the band gap. A similar explanation for the semi-conductor character was also reported in a study on F-adsorbed GNRs [49]. In 3F2-6ZSiNR-3F1, as shown in Fig. 4 (c), the peaks for the F-py, F-pz, and Si-py orbitals in the up-spin constituted the main contribution to the Fermi Level, whereas a band gap occurred in down spin, which made the system acquire a half-metallic character. We present the spin density in Fig. 5 to study the magnetic property of the cases with asymmetric edge fluorination. The calculated formation energies are listed in Table 1 of the supplementary material and show that asymmetric fluorination produced a stable FM state, which was the same as the FM state of pristine 6ZSiNR with a total magnetic moment of 3.81 μB. For the mono-fluorination at one single edge of 6ZSiNR, the total magnetic moments of 6ZSiNR-nF1 (n ¼ 1–3) listed in Table 1 changed from 2.00 μB to 3.00 μB. Fig. 5 (a) shows that the local magnetic moments of the Si atoms connected to F atoms were completely suppressed in 6ZSiNR-1F1 and 6ZSiNR-2F1. In contrast, the local magnetic moments of the Si atoms connected to F atoms in 6ZSiNR-3F1 were only suppressed to some extent. This was the main reason for the decrease of the total magnetic moment of 6ZSiNR-nF1 (n ¼ 1–3). The bi-fluorination at one single edge of 6ZSiNR caused the total magnetic moment of 6ZSiNR-nF1 to decrease from 3.00 μB to 2.00 μB when the number of adsorption sites varied from 1 to 2 and 3 due to the strong sp3 hybridization between the Si atoms and connected to F atoms, as shown in Fig. 5 (b). The spin density distribution in nF2-6ZSiNR-nF1 (mono-fluorination at one edge and mono-fluorination at the other edge) was similar to that in 6ZSiNR-nF1 and 6ZSiNR-nF2 (n ¼ 1–3), as shown in Fig. 5 (c). As a result, the total magnetic moment was reduced from 2.00 μB to 1 μB when n increased from 1 to 2 and 3. Our results showed that the magnetic behavior of 6ZSiNR can be effectively tuned by asymmetric fluorination. The adjustable magnetic moment makes ZSiNRs suitable for spintronic devices. To further study the spin polarization behavior and the magnetism of 6ZSiNR with asymmetric edge fluorination, we determined the total density of states (TDOS) in Fig. 6. The spin polarization (SP) was calculated using equation (2) SP ¼ jðN↑
(2)
N↓ Þ = ðN↑ þ N↓ Þj
WhereN↑ indicates the DOS of the up-spin at the Fermi level and N↓ indicates the DOS of the down-spin at the Fermi level. The same
Fig. 4. Partial density of states (PDOS). (a) 1F2-6ZSiNR-1F1, (b) 6ZSiNR-3F1, and (c) 3F2-6ZSiNR-3F1. Fermi level is denoted by dotted line. 6
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Fig. 5. Spin density distributions of asymmetric adsorption configurations. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, and (c) nF2-6ZSiNR-nF1 (n ¼ 1–3). The yellow and blue represent for the electron states of up spin and down spin, respectively. The blue balls, green balls represent for Si atoms and F atoms, respectively. The isosurface value of charge density is set to 4 � 10 4e/Å3.
method was used to study armchair GNRs adsorbed with Fe atoms [50]. The calculation results shown in Fig. S1 of the supplementary material and in Table 1 indicate that pristine 6ZSiNR was a FM metal with a 70% spin polarization. Similarly, 6ZSiNR-1F1 had a 75% spin polarization because of the presence of DOS in the up-spin and the down-spin at the Fermi Level, as shown in the left panel of Fig. 6 (a). In contrast, 6ZSiNR-2F1 and 6ZSiNR-3F1 had no spin polarization at the Fermi Level due to the absence of DOS in the up-spin and the down-spin at the Fermi Level. Similarly, 6ZSiNR-1F2 and 6ZSiNR2F2 had a 80% and no spin polarization, respectively (Fig. 6 (b)). However, DOS of the up-spin appeared at the Fermi Level in 6ZSiNR3F2, whereas the DOS of the down-spin at the Fermi Level had no distribution, leading to a 100% spin polarization. This corresponds to a half-metal character, which was reported in a previous study as well [50]. Fig. 6 (c) shows that the spin polarization in 1F2-6ZSiNR-1F1 and 2F2-6ZSiNR-2F1 can be neglected due to the presence of almost the same DOS of up-spin and down spin at the Fermi Level. As for 6ZSiNR-3F2, 3F2-6ZSiNR-3F1 also had a 100% spin polarization. The common feature was an asymmetric DOS of up-spin and down-spin, which led to an FM character. 3.2. Symmetric edge fluorination We discuss here the electronic and magnetic properties of 6ZSiNR with a symmetric edge fluorination. Fig. 7 presents the spinpolarized band structures of the corresponding configurations. For the mono-fluorination at both edges, the band structures in the up-spin and the down spin had a symmetric distribution and a semi-conductor character in all cases, as shown in Fig. 7 (a). For the bifluorination at both edges, 2F2-6ZSiNR-2F2 (Fig. 7 (b)) retained a semi-conductor property, whereas 1F2-6ZSiNR-1F2 and 3F2-6ZSiNR3F2 had a metallic character. According to the calculated formation energies shown in Table 1 of the supplementary material, the cases with a semi-conductor property had a stable AFM state. The semi-conductor character is mainly created by the coulomb repulsion interaction [51]. However, 1F2-6ZSiNR-1F2 and 3F2-6ZSiNR-3F2 had a stable FM state since their metallic character was determined by the σ-π mixing. Fig. 8 indicates the spin density of 6ZSiNR with a symmetric edge fluorination. Fig. 8 (a) and 8 (b) show that nF1-6ZSiNR-nF1 with n 7
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Fig. 6. Total density of states (DOS) of asymmetric adsorption configurations. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, and (c) nF2-6ZSiNR-nF1 (n ¼ 1–3). The Fermi level is denoted by the dotted line. The blue and red indicate the up spin and down spin, respectively.
¼ 1–3 (mono-fluorination at both edges) and 2F2-6ZSiNR-2F2 have the same total magnetic moment of 0 μB. In particular, 1F1-6ZSiNR1F1 and 3F1-6ZSiNR-3F1 had the magnetic distribution of an AFM state, whereas of 2F1-6ZSiNR-2F1 and 2F2-6ZSiNR-2F2 had no spin density distribution, indicating a non-magnetic character. Fig. S2 of the supplementary material shows that their corresponding DOS in the up-spin and the down-spin had a symmetric distribution, which proved their AFM property. Besides, they also showed no spin polarization at the Fermi Level due to their semi-conductor character in the up-spin and the down-spin. However, 1F2-6ZSiNR-1F2 and 3F2-6ZSiNR-3F2 were both FM metals with a 42% and a 20% spin polarization at the Fermi Level, respectively. 4. Conclusion We explored the effects of the fluorine edge decoration on zigzag silicene nanoribbons (ZSiNR) using first-principle calculations. The strong bond between the F atoms and the Si atoms at the edges gives ZSiNR a stable edge state. All adsorbed configurations had a high stability because of their low formation energy. In particular, the asymmetric edge-fluorinated 6ZSiNR-nF1 (n ¼ 1–3) was tuned from a metal to a semi-conductor when the number of fluorination sites increased, which originated from the weakening of the σ-π 8
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Fig. 7. The spin-polarized band structures of symmetric adsorption configurations. (a) nF1-6ZSiNR-nF1 and (b) nF2-6ZSiNR-nF2 (n ¼ 1–3). The Fermi level is denoted by the dotted line.
Fig. 8. Spin density distributions of symmetric adsorption configurations. (a) nF1-6ZSiNR-nF1 and (b) nF2-6ZSiNR-nF2 (n ¼ 1–3). The yellow and blue represent for the electron states of up spin and down spin, respectively. The blue balls, green balls represent for Si atoms and F atoms, respectively. The isosurface value of charge density is set to 4 � 10 4e/Å3.
9
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mixing. 6ZSiNR-nF2 and nF2-6ZSiNR-nF1 (n ¼ 1–3) changed from a metal to a half-metal when n increased from 1 to 3, which is attributed to the F-Py, F-Pz, and Si-Py orbitals. Besides, all cases of asymmetric fluorination retained a FM character. The total magnetic moment can be effectively controlled by varying the fluorination sites at the edges. For symmetric edge fluorinated ZSiNR, nF16ZSiNR-nF1 (n ¼ 1–3) and 2F2-6ZSiNR-2F2 always had a semi-conductor character due to the Coulomb repulsion from both edges and formed stable AFM states. However, nF2-6ZSiNR-nF2 (n ¼ 1 or 3) with different fluorination sites retained their FM metallic character. The interesting physical properties of ZSiNR obtained with edge fluorination, including the half-metallic character and the spin po larization, suggested that edge-fluorinated ZSiNR can be effectively used in spintronic devices. Declaration of competing interests The authors have no competing interests to declare. Acknowledgments This work is supported by the Scientific Research Foundation for Talented Scholars of Hunan Institute of Technology (HQ19011), by the National Natural Science Foundation of China (Grant No. 11704112), by the Program of Hunan Provincial Education Department of China (Grant Nos. 17B066 and Grant No18A427), by College Students’ Innovation and Entrepreneurship Training Program of Hunan Province (S201911528030), and the “Double Top Construction” Major Cultivation Project and physics discipline of Hunan Institute of Technology, and the aid program of science and technology innovative research team of Hunan institute of technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2020.106394. Credit author statement All authors contribute equally to this paper.
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Fluorine edge decoration on zigzag silicene nanoribbons Gang Guo a, *, Zhongxiang Xie a, Jie Tang a, Yong Zhang a, **, Guobao Xu b a
Department of Mathematics and Physics, Hunan Institute of Technology, Hengyang, 421002, China National-Provincial Laboratory of Special Function Thin Film Materials, School of Materials Science and Engineering, Xiangtan University, Hunan, 411105, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Edge fluorination Electronic and magnetic properties Zigzag silicene nanoribbon
We used the density functional theory (DFT) to explore the effects of fluorine (F) edge decoration on zigzag silicene nanoribbons (ZSiNR). Due to the strong electron affinity of fluorine atoms, a strong chemical bond between Si atoms and F atoms is formed. The formation energy also indicated that the fluorination configurations had higher stability than for the ZSiNR. The fluo rination sites at the edges efficiently modulate the electronic and magnetic properties of the ZSiNR. In particular, we found a significant change from a metallic to a semiconductor charac teristic in asymmetric edge fluorinated ZSiNR, in a mono-fluorinated system with one single edge, which was attributed to the weakening of the σ-π mixing effect. In contrast, a half-metallic character with a 100% spin polarization was induced in another other two full asymmetric fluorination systems due to the contribution from the F-Py, F-Pz, and Si-Py orbitals. All asym metric fluorinations maintained a ferromagnetic character. Symmetric edge fluorinated ZSiNR maintained a ferromagnetic metallic characteristic or an antiferromagnetic semi-conductor characteristic due to the Coulomb repulsion from both edges. The interesting and useful phys ical properties in ZSiNR with edge fluorination including the half-metallic character and the spin polarization implied that edge-fluorinated ZSiNR can be effectively used in spintronic devices.
1. Introduction Two-dimensional (2D) materials [1] have recently received extensive attention due to their remarkable physical and chemical properties. In particular, great progress on graphene has been made both theoretically and experimentally in recent years [2–4]. Nowadays, graphene is used in many fields including in solar cells [5], as high-performance metal-ion battery anodes [6], in com posites [7], as sensing materials [8], and for energy storage [9]. Along with the development of the experimental methods, graphene nanoribbons (GNRs) have also been prepared in recent years. Physical properties of the graphene nanoribbons such as its electronic and magnetic properties [10], thermoelectric properties [11], and transport properties [12] have been theoretically studied to promote the application of GNRs nanodevices. Motivated by these significant results on graphene and GNRs, a graphene-like analogue, silicene [13], has received much attention as well. The biggest difference between graphene and silicene lies in the different geometric structure. Due to its buckling geometric structure, silicene has novel physical properties [14]. For instance, silicene nanosheets have been grown on metal surfaces [15,16]. Silicene [17] has a perfect honeycomb structure and has outstanding electronic properties. In recent years, the growth of silicene * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Guo), [email protected] (Y. Zhang). https://doi.org/10.1016/j.spmi.2020.106394 Received 3 November 2019; Received in revised form 18 December 2019; Accepted 6 January 2020 Available online 10 January 2020 0749-6036/© 2020 Elsevier Ltd. All rights reserved.
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nanoribbons (SiNRs) on metal surfaces [18–20] was also extensively studied. SiNRs grown on Ag (110) substrate have a sp2-like �vila et al. reported [20] that SiNRs grown on an Ag (110) surface can be perfectly terminated by H atoms and hybridization [18]. Da have a stable structure as well as excellent electronic properties. Many theoretical studies have been performed to determine the physical properties of silicene nanoribbons (SiNRs). The presence of dangling bonds [21] or defects [24], the strain [22], and the electrical field [23] effectively tune the electronic and magnetic properties of SiNRs. A first-principles study [25] reported
Fig. 1. The relaxed structures of 6ZSiNR with adsorbed by F atoms. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, (c) nF2-6ZSiNR-nF1, (d) nF1-6ZSiNR-nF1, and (d) nF2-6ZSiNR-nF2 (n ¼ 1–3). The blue balls, green balls represent for Si atoms, F atoms, respectively. 2
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electron-phonon coupling effects in SiNRs. Gao et al. [26] discovered an interesting spin Seebeck effect and thermal spin transport properties in Christmas-tree shaped SiNRs. Many methods can be employed to tune the SiNRs physical properties. Many studies emphasize the doping effect on SiNRs. For instance, doping with Al modulates the magnetic and electronic properties of armchair SiNRs (ASiNRs) [27]. Zigzag SiNRs (ZSiNRs) produce spin-polarized currents when doped with Fe, Co, and Ni [28]. In addition to metallic elements, non-metallic elements such as B [29] and N [30] also yield efficient doping. Aadsorption is another effective way to modify the physical properties of the nanoribbons. Yang et al. [31] reported that atomic adsorption of Si on ZSiNRs induced a large spin thermo-power. Li et al. [32] proved that the energy gap of SiNRs was sensitive to the adsorption of O atoms. ZSiNR adsorbed with Cu atoms had a significant current-voltage characteristic [33] that can be used for nano-electronic devices. Molecular adsorption on SiNRs was also predicted for promising molecular sensing. Walia et al. [34] reported that defective ASiNRs were an ideal sensor for NO and NH3. More interestingly, a first-principle calculation [35] investigated organic molecules adsorbed on SiNR and indicated that SiNRs were a suitable for DNA sequencing. Some physical properties of the SiNR can be effectively tuned by edge decoration. Zhang et al. [36] found excellent spin-dependent electronic transport properties for ZSiNRs obtained by asymmetric edge hydrogenation. They also demonstrated that hydrogen edge modification induced a perfect spin-filtering effect in ZSiNR [37]. Our previous study [38] predicted that the existing edge dangling bonds in ZSiNR create an opportunity to form chemical hydrides. The maximum density of H adsorption is 5.56 wt% [38], which means that ZSiNRs can be used as a promising chemical hydrogen storage material due to its lower formation energy. Ding et al. [39] also reported that ZSiNRs with asymmetric sp2-sp3 edges were bipolar magnetic semiconductors due to the incorporation of Klein and zigzag edge states. This proved that the adsorption of nonmetallic elements is suitable way to tune the physical properties of ZSiNRs. However, the electron affinity of fluorine (F) atoms is much stronger than for H atoms. Therefore, significant chemical bonds between the Si and F atoms are formed to obtain extremely diversified physical properties. For example, silicene with fluorination [40] is used for sensor applications whereas SiNRs fully F-terminated at both edges always show a semi-conductor character [41] with the increase of the ribbon width. From these significant findings, it appears that fluorine atoms help modify the electronic structure and the magnetic behavior of the nanoribbons. So far, the electronic and magnetic properties of symmetric and asymmetric edge fluorinated ZSiNR have not been systematically studied. Even though hydrogenated ZSiNRs were reported in previous studies [38,39], it is not clear whether other interesting physical properties can be obtained in ZSiNRs with edge fluorination. Therefore, we focused on exploring interesting and useful physical properties of ZSiNRs with edge fluorination including the half-metallic character and the spin polarization behavior using first-principle calculations and considering the variation of the edge fluorination concentration and the number of sites for the effective application in spintronic devices. 2. Computational methods and models In this work, we used the Vienna ab initio simulation package (VASP) [42] with first-principle calculations based on the DFT to study the effects of fluorine edge decoration on the electronic and magnetic properties of ZSiNR. The projector augmented wave (PAW) method [43] and the Perdew Burke Ernzerhof (PBE) [44] exchange correlation function in the generalized gradient approximation (GGA) [45] were used to perform all calculations. We made the calculations with a cutoff energy of 450 eV, which was suitable for our systems. A k-point mesh of 13 � 1 � 1was used for structural optimization. For the calculation of the density of state (DOS), we used a k-point mesh of 39 � 1 � 1. The vacuum region of 15 Å was set in the super-cell. The criterion for the maximum force in the relaxed calculation is smaller than 0.01 eV/Å. van der Waals (VDW) interactions [46] were also considered in our calculations. This has been Table 1 Calculated results of pristine 6ZSiNR and its adsorption configurations. The formation energy of the stable magnetic state and the bond distance of Si–F bond are indicated by Ef and D, respectively. Δrepresents for the energy gap of band structure, where M represents for metal property. The spin polarization at the Fermi level is indicated by P. Mtot is the total magnetic moment in the stable magnetic state. The amount of charge obtained by the F atom is represented by Q , in which the bracket indicates the number of F atoms with same amount of charge obtained from Si atom. Structure
6ZSiNR 6ZSiNR-1F1 6ZSiNR-2F1 6ZSiNR-3F1 6ZSiNR-1F2 6ZSiNR-2F2 6ZSiNR-3F2 1F2-6ZSiNR-1F1 2F2-6ZSiNR-2F1 3F2-6ZSiNR-3F1 1F1-6ZSiNR-1F1 2F1-6ZSiNR-2F1 3F1-6ZSiNR-3F1 1F2-6ZSiNR-1F2 2F2-6ZSiNR-2F2 3F2-6ZSiNR-3F2
Ef ðeVÞ
3.784 3.888 3.949 3.989 3.936 4.035 4.115 3.994 4.138 4.247 3.951 4.064 4.133 4.036 4.207 4.335
D (Å) (Si–F)
Δ(e V)
L
R
up
down
/ / / / / / / 1.618 1.619 1.618 1.624 1.624 1.624 1.618 1.619 1.618
/ 1.624 1.624 1.624 1.618 1.619 1.618 1.624 1.624 1.624 1.624 1.624 1.624 1.619 1.619 1.618
M M 0.05 0.32 M 0.25 M M M M 0.11 0.08 0.32 M 0.09 M
M M 0.30 0.45 M 0.26 0.24 M M 0.27 0.11 0.08 0.32 M 0.09 M
3
P (%)
Mtot (μB)
70 75 / / 80 / 100 2.8 0.01 100 / / / 42 / 20
3.81 3.00 2.00 3.00 3.00 2.00 2.00 2.00 1.00 1.00 0.00 0.00 0.00 1.81 0.00 2.00
Q (e) (Si to F) / 0.81(1) 0.81(2) 0.81(3) 0.81(2) 0.81(4) 0.81(6) 0.81(3) 0.81(6) 0.81(9) 0.81(2) 0.81(4) 0.81(6) 0.81(4) 0.81(8) 0.81(12)
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tested that for adsorption in nanoribbons [38] and was demonstrated to be accurate. Song et al. [47] proved that pristine ZSiNRs always showed a metallic character regardless of the ribbon width. Moreover, ZSiNR with a width of 6 that named 6ZSiNR was as a suitable substrate material in a previous study [48]. We therefore chose 6ZSiNR as a typical ZSiNR. In 6ZSiNR edge-adsorbed with F atoms, the relaxed adsorption models shown in Fig. 1 can be divided into five forms according to the fluorination concentrations and sites: mono-fluorination at one single edge (6ZSiNR-nF1), bi-fluorination at one single edge (6ZSiNR-nF2), mono-fluorination at one edge and mono-fluorination at the other edge (nF2-6ZSiNR-nF1), mono-fluorination a both edges (nF1-6ZSiNR-nF1), and bi-fluorination at both edges (nF2-6ZSiNR-nF2). The number of adsorption sites (n) changed from 1 to 3 along with the fluorination concentration. To discuss the structural stability of the calculated configurations, we calculated the formation energies of ferromagnetic (FM), antiferromagnetic (AFM), and nonmagnetic (NM) states. Here, the formation energy was calculated using equation (1) Ef ¼
1 ðE6ZSiNR N
F
NSi ESi
(1)
NF EF Þ
Where N is the total number of the system, NSi and NF are the number of Si atoms and F atoms in the system, respectively. The total energy of fluorination of 6ZSiNR is indicated by E6ZSiNR-F. EF and ESi are the total energy of a single F atom and Si atom, respectively. The calculated formation energies listed in Table 1 of the supplementary material showed that all cases with asymmetric fluorination of 6ZSiNRs maintained a stable FM state, whereas the cases with symmetric fluorination had stable AFM or FM state. All calculations listed in Table 1 are based on the stable ground state. 3. Results and discussion 3.1. Asymmetric edge fluorination First, pristine 6ZSiNR has a stable FM metallic character according to the verification shown in Fig. S1 of the supplementary material. This result is in agreement with a previous study [47]. Fig. 1 indicates the fifteen relaxed structures of 6ZSiNR adsorbed with F atoms. The adsorbed configurations maintained a perfect buckling structure without obvious deformation. Table 1 shows that all adsorbed configurations had a lower formation energy than pristine 6ZSiNR. In order to analyze the structural information of the asymmetric edge fluorination configurations, we presented the optimized structure of 3F2-6ZSiNR-3F1 and its corresponding charge density as well as the charge density difference as the typical representative of the asymmetric fluorination configurations in Fig. 2. Fig. 2 (a)–(b) shows that the Si–F bond length at the edges was about 1.62 Å, which is similar to a previous study [41]. At the edge of 6ZSiNR with mono-fluorination, the bond angle of F–Si–Si was about 115� , which was close to a sp2 hybridization. In contrast, the bond angle of F–Si–Si at the edge of bi-fluorination was 105� , which was on the verge of a sp3 hybridization. From the charge density and the charge density difference shown in the right panel of Fig. 2 (a) and (b), the charge density is distributed between two Si atoms in the Si–Si bond. However, the density near the F atom was higher than that near the Si atom in the Si–F bond. This was mainly due to the electron gain by the F atom and the electron loss in the Si atom given the difference in Pauling electronegativity between F (4.0) and Si (1.9). Besides, the Bader analysis showed that each F atom gained 0.81 electrons (e) from the Si atom. This significant charge transfer was responsible for the stability of the adsorbed configurations. In addition to the case of 3F2-6ZSiNR-3F1, the structural information and charge transfer values of fourteen other adsorbed configurations are listed in Table 1. Although the adsorption of F atoms at the edges was different, the bond length of F–Si and the charge transfer values evolved similarly. Fig. 3 shows the band structures of 6ZSiNR with asymmetric edge fluorination. Fig. 3 (a) shows that the band structure of 6ZSiNR1F1 with mono-fluorination at one single edge had a metallic character due to the two sub-bands crossing the Fermi Level. However, when the number of adsorption sites (n) increased, 6ZSiNR-2F1 and 6ZSiNR-3F1 both had a semiconducting character. In particular, a band gap of 0.05 eV for the up-spin and a band gap of 0.30 eV for the down-spin were found for 6ZSiNR-2F1. Similarly, 6ZSiNR-3F1 had
Fig. 2. The optimized structure of 3F2-6ZSiNR-3F1 and its corresponding charge density as well as charge density difference. (a) The top view of 3F2-6ZSiNR-3F1 (left panel) and the charge density (right panel), (b) the side view (left panel) and the charge density difference (right panel). Some structural information such as bond length is indicated. The blue balls, green balls represent for Si atoms, and F atoms, respectively. 4
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Fig. 3. The spin-polarized band structures of asymmetric adsorption configurations. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, and (c) nF2-6ZSiNR-nF1 (n ¼ 1–3). The Fermi level is denoted by the dotted line.
a band gap of 0.32 eV in the up-spin and a band gap of 0.45 eV in the down-spin. It implied that mono-fluorination can tune the metallic character of 6ZSiNR-nF1 (n ¼ 1–3) to a semi-conductor character. When considering bi-fluorination at one single edge in 6ZSiNR-nF2 (n ¼ 1–3), a change from metal to semi-conductor and half-metal occurs when the number of adsorption sites increases, as shown in Fig. 3 (b). 6ZSiNR-1F2 and 6ZSiNR-2F2 had a metallic character and a semi-conductor character, respectively. However, 6ZSiNR-3F2 had a metallic character in the up-spin and a semi-conductor character with a band gap of 0.24 eV in the down-spin, resulting in a halfmetal character that is useful for applications in spin electronic devices [49]. When considering the mono-fluorination at one edge and the mono-fluorination at the other edge (nF2-6ZSiNR-nF1), as shown in Fig. 3 (c), a similar change from metal to half-metal appeared when the number of adsorption sites increased. From the above analysis, 6ZSiNR with asymmetric edge fluorination had a metallic, semi-conductor, and half-metallic character. Therefore, we calculated the partial density of states (PDOS) of three typical cases that had a metallic character (1F2-6ZSiNR-1F1), a semi-conductor character (6ZSiNR-3F1), and a half-metallic character (3F2-6ZSiNR-3F1) to find out the mechanism behind it. The metallic character of pristine 6ZSiNR originated from the Si-py (σ state) and Si-pz (π state) orbitals. Similarly, Fig. 4 (a) shows that 1F26ZSiNR-1F1 retained a metallic character. The contributions of the F-py, Si-py (σ state) and Si-pz (π state) orbitals at the Fermi Level 5
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played a major role for conduction over the Fermi Level, which meant that the σ-π mixing effect [41] was strengthened, leading to a metallic character. However, when the number of adsorption sites changed, there were no DOS peaks for the F-py orbitals at the Fermi Level in 6ZSiNR-3F1, as shown in Fig. 4 (b). Due to the increase in the sp2 hybridization bond, the σ-π mixing effect was weakened. The DOS peaks were pushed away from the Fermi Level, which led to the opening of the band gap. A similar explanation for the semi-conductor character was also reported in a study on F-adsorbed GNRs [49]. In 3F2-6ZSiNR-3F1, as shown in Fig. 4 (c), the peaks for the F-py, F-pz, and Si-py orbitals in the up-spin constituted the main contribution to the Fermi Level, whereas a band gap occurred in down spin, which made the system acquire a half-metallic character. We present the spin density in Fig. 5 to study the magnetic property of the cases with asymmetric edge fluorination. The calculated formation energies are listed in Table 1 of the supplementary material and show that asymmetric fluorination produced a stable FM state, which was the same as the FM state of pristine 6ZSiNR with a total magnetic moment of 3.81 μB. For the mono-fluorination at one single edge of 6ZSiNR, the total magnetic moments of 6ZSiNR-nF1 (n ¼ 1–3) listed in Table 1 changed from 2.00 μB to 3.00 μB. Fig. 5 (a) shows that the local magnetic moments of the Si atoms connected to F atoms were completely suppressed in 6ZSiNR-1F1 and 6ZSiNR-2F1. In contrast, the local magnetic moments of the Si atoms connected to F atoms in 6ZSiNR-3F1 were only suppressed to some extent. This was the main reason for the decrease of the total magnetic moment of 6ZSiNR-nF1 (n ¼ 1–3). The bi-fluorination at one single edge of 6ZSiNR caused the total magnetic moment of 6ZSiNR-nF1 to decrease from 3.00 μB to 2.00 μB when the number of adsorption sites varied from 1 to 2 and 3 due to the strong sp3 hybridization between the Si atoms and connected to F atoms, as shown in Fig. 5 (b). The spin density distribution in nF2-6ZSiNR-nF1 (mono-fluorination at one edge and mono-fluorination at the other edge) was similar to that in 6ZSiNR-nF1 and 6ZSiNR-nF2 (n ¼ 1–3), as shown in Fig. 5 (c). As a result, the total magnetic moment was reduced from 2.00 μB to 1 μB when n increased from 1 to 2 and 3. Our results showed that the magnetic behavior of 6ZSiNR can be effectively tuned by asymmetric fluorination. The adjustable magnetic moment makes ZSiNRs suitable for spintronic devices. To further study the spin polarization behavior and the magnetism of 6ZSiNR with asymmetric edge fluorination, we determined the total density of states (TDOS) in Fig. 6. The spin polarization (SP) was calculated using equation (2) SP ¼ jðN↑
(2)
N↓ Þ = ðN↑ þ N↓ Þj
WhereN↑ indicates the DOS of the up-spin at the Fermi level and N↓ indicates the DOS of the down-spin at the Fermi level. The same
Fig. 4. Partial density of states (PDOS). (a) 1F2-6ZSiNR-1F1, (b) 6ZSiNR-3F1, and (c) 3F2-6ZSiNR-3F1. Fermi level is denoted by dotted line. 6
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Fig. 5. Spin density distributions of asymmetric adsorption configurations. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, and (c) nF2-6ZSiNR-nF1 (n ¼ 1–3). The yellow and blue represent for the electron states of up spin and down spin, respectively. The blue balls, green balls represent for Si atoms and F atoms, respectively. The isosurface value of charge density is set to 4 � 10 4e/Å3.
method was used to study armchair GNRs adsorbed with Fe atoms [50]. The calculation results shown in Fig. S1 of the supplementary material and in Table 1 indicate that pristine 6ZSiNR was a FM metal with a 70% spin polarization. Similarly, 6ZSiNR-1F1 had a 75% spin polarization because of the presence of DOS in the up-spin and the down-spin at the Fermi Level, as shown in the left panel of Fig. 6 (a). In contrast, 6ZSiNR-2F1 and 6ZSiNR-3F1 had no spin polarization at the Fermi Level due to the absence of DOS in the up-spin and the down-spin at the Fermi Level. Similarly, 6ZSiNR-1F2 and 6ZSiNR2F2 had a 80% and no spin polarization, respectively (Fig. 6 (b)). However, DOS of the up-spin appeared at the Fermi Level in 6ZSiNR3F2, whereas the DOS of the down-spin at the Fermi Level had no distribution, leading to a 100% spin polarization. This corresponds to a half-metal character, which was reported in a previous study as well [50]. Fig. 6 (c) shows that the spin polarization in 1F2-6ZSiNR-1F1 and 2F2-6ZSiNR-2F1 can be neglected due to the presence of almost the same DOS of up-spin and down spin at the Fermi Level. As for 6ZSiNR-3F2, 3F2-6ZSiNR-3F1 also had a 100% spin polarization. The common feature was an asymmetric DOS of up-spin and down-spin, which led to an FM character. 3.2. Symmetric edge fluorination We discuss here the electronic and magnetic properties of 6ZSiNR with a symmetric edge fluorination. Fig. 7 presents the spinpolarized band structures of the corresponding configurations. For the mono-fluorination at both edges, the band structures in the up-spin and the down spin had a symmetric distribution and a semi-conductor character in all cases, as shown in Fig. 7 (a). For the bifluorination at both edges, 2F2-6ZSiNR-2F2 (Fig. 7 (b)) retained a semi-conductor property, whereas 1F2-6ZSiNR-1F2 and 3F2-6ZSiNR3F2 had a metallic character. According to the calculated formation energies shown in Table 1 of the supplementary material, the cases with a semi-conductor property had a stable AFM state. The semi-conductor character is mainly created by the coulomb repulsion interaction [51]. However, 1F2-6ZSiNR-1F2 and 3F2-6ZSiNR-3F2 had a stable FM state since their metallic character was determined by the σ-π mixing. Fig. 8 indicates the spin density of 6ZSiNR with a symmetric edge fluorination. Fig. 8 (a) and 8 (b) show that nF1-6ZSiNR-nF1 with n 7
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Fig. 6. Total density of states (DOS) of asymmetric adsorption configurations. (a) 6ZSiNR-nF1, (b) 6ZSiNR-nF2, and (c) nF2-6ZSiNR-nF1 (n ¼ 1–3). The Fermi level is denoted by the dotted line. The blue and red indicate the up spin and down spin, respectively.
¼ 1–3 (mono-fluorination at both edges) and 2F2-6ZSiNR-2F2 have the same total magnetic moment of 0 μB. In particular, 1F1-6ZSiNR1F1 and 3F1-6ZSiNR-3F1 had the magnetic distribution of an AFM state, whereas of 2F1-6ZSiNR-2F1 and 2F2-6ZSiNR-2F2 had no spin density distribution, indicating a non-magnetic character. Fig. S2 of the supplementary material shows that their corresponding DOS in the up-spin and the down-spin had a symmetric distribution, which proved their AFM property. Besides, they also showed no spin polarization at the Fermi Level due to their semi-conductor character in the up-spin and the down-spin. However, 1F2-6ZSiNR-1F2 and 3F2-6ZSiNR-3F2 were both FM metals with a 42% and a 20% spin polarization at the Fermi Level, respectively. 4. Conclusion We explored the effects of the fluorine edge decoration on zigzag silicene nanoribbons (ZSiNR) using first-principle calculations. The strong bond between the F atoms and the Si atoms at the edges gives ZSiNR a stable edge state. All adsorbed configurations had a high stability because of their low formation energy. In particular, the asymmetric edge-fluorinated 6ZSiNR-nF1 (n ¼ 1–3) was tuned from a metal to a semi-conductor when the number of fluorination sites increased, which originated from the weakening of the σ-π 8
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Fig. 7. The spin-polarized band structures of symmetric adsorption configurations. (a) nF1-6ZSiNR-nF1 and (b) nF2-6ZSiNR-nF2 (n ¼ 1–3). The Fermi level is denoted by the dotted line.
Fig. 8. Spin density distributions of symmetric adsorption configurations. (a) nF1-6ZSiNR-nF1 and (b) nF2-6ZSiNR-nF2 (n ¼ 1–3). The yellow and blue represent for the electron states of up spin and down spin, respectively. The blue balls, green balls represent for Si atoms and F atoms, respectively. The isosurface value of charge density is set to 4 � 10 4e/Å3.
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mixing. 6ZSiNR-nF2 and nF2-6ZSiNR-nF1 (n ¼ 1–3) changed from a metal to a half-metal when n increased from 1 to 3, which is attributed to the F-Py, F-Pz, and Si-Py orbitals. Besides, all cases of asymmetric fluorination retained a FM character. The total magnetic moment can be effectively controlled by varying the fluorination sites at the edges. For symmetric edge fluorinated ZSiNR, nF16ZSiNR-nF1 (n ¼ 1–3) and 2F2-6ZSiNR-2F2 always had a semi-conductor character due to the Coulomb repulsion from both edges and formed stable AFM states. However, nF2-6ZSiNR-nF2 (n ¼ 1 or 3) with different fluorination sites retained their FM metallic character. The interesting physical properties of ZSiNR obtained with edge fluorination, including the half-metallic character and the spin po larization, suggested that edge-fluorinated ZSiNR can be effectively used in spintronic devices. Declaration of competing interests The authors have no competing interests to declare. Acknowledgments This work is supported by the Scientific Research Foundation for Talented Scholars of Hunan Institute of Technology (HQ19011), by the National Natural Science Foundation of China (Grant No. 11704112), by the Program of Hunan Provincial Education Department of China (Grant Nos. 17B066 and Grant No18A427), by College Students’ Innovation and Entrepreneurship Training Program of Hunan Province (S201911528030), and the “Double Top Construction” Major Cultivation Project and physics discipline of Hunan Institute of Technology, and the aid program of science and technology innovative research team of Hunan institute of technology. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2020.106394. Credit author statement All authors contribute equally to this paper.
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