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Modulation of magnetic properties of bilayer SnSe with transition-metals doping in the interlayer
Physica E 75 (2016) 106–111
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Modulation of magnetic properties of bilayer SnSe with transition-metals doping in the interlayer Xiao-Long Wang a, Wei Li a, Tian-Xing Wang a, Xian-Qi Dai a,b,n a b
College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, China School of Physics and Electronic Engineering, Zhengzhou Normal University, Zhengzhou, Henan 450044, China
H I G H L I G H T S
Charge transfer from transition metal atom to SnSe sheet decreases gradually with increasing atomic number. Two factors result in magnetic reduction for transition metal atom doped in bilayer SnSe. A complete spin-polarization at Ef is obtained in Mn (Co) doped bilayer SnSe system.
ar t ic l e i nf o
a b s t r a c t
Article history: Received 26 July 2015 Received in revised form 6 September 2015 Accepted 12 September 2015 Available online 15 September 2015
We investigate the spin-polarized electronic and magnetic properties of bilayer SnSe with transitionmetal (TM) atoms doped in the interlayer by using a first-principles method. It shows that Ni dopant cannot induce the magnetism in the doped SnSe sheet, while the ground state of V, Cr, Mn, Fe and Co doped systems are magnetic and the magnetic moment mainly originates from 3d TM atom. Two types of factors, which reduce the magnetic moment of TM atoms doped in bilayer SnSe, are identified as spin-up channel of the 3d orbital loses electrons to SnSe sheet and spin-down channel of the 3d orbital gains electrons from 4s orbital. The spin polarization is found to be 100% at Fermi level for the Mn and Co atoms doped system, while the Ni-doped system is still a semiconductor with a gap of 0.26 eV. These results are potentially useful for development of spintronic devices. & 2015 Elsevier B.V. All rights reserved.
Keywords: First-principles Bilayer SnSe Interlayer Electronic structure Magnetic property
1. Introduction Two-dimensional (2D) nano-materials such as graphene [1], BN [2], MoS2 [3] and black phosphorus [4] have shown novel properties due to the low dimensionality and quantum confinement effect. Recently, considerable attention has been paid to magnetic properties of 2D material for the spintronic applications [5–7]. As is known, substitutional doping and adsorptive doping are the efficient approaches to tune the properties of the materials and have been widely used to induce the magnetic ground state of the nonmagnetic semiconducting materials. For instant, Cu dopant induces the unexpected strong magnetism into monolayer MoS2 [8]. Yang et al. reported that TM atom can profoundly modulate the magnetism of monolayer ZrS2 [9]. Moreover, Sahin et al. investigated the electronic band dispersion and magnetic moment of monolayer silicone with TM adatom [10]. n Corresponding author at: College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, China. E-mail address: [email protected] (X.-Q. Dai).
http://dx.doi.org/10.1016/j.physe.2015.09.020 1386-9477/& 2015 Elsevier B.V. All rights reserved.
As a typical IV–VI group semiconductor, SnSe belongs to the orthorhombic crystal system. Similar to SnS, GeS, and GeSe, the bonding of SnSe interacts covalently within the layer, which comprises zig-zag double layer planes of the tin and selenium atoms and is separated by a weak van der Waals force between the layers [11–13]. The bulk SnSe production process is very mature [14–17] and its direct band gap (1.30 eV) [18] has attracted widespread interest in the research of its potential applications for optical devices [11,19]. Moreover, the monolayer SnSe has been prepared by pulse electrodeposited on tin oxide coated glass substrates at different duty cycles [20]. Recently, Huang et al. reported that the strain can not induce the magnetism in the SnSe nanoribbons, though the strain can induce the semiconductor– metal or metal–semiconductor transition [21]. The effects of TM atom doped in bilayer SnSe, which, to our knowledge, have not been reported as yet. The TM atoms doped in the interlayer could be hard to oxidize, which is beneficial in maintaining a stable performance in bilayer SnSe devices. To fully understand and control the magnetic property of the bilayer SnSe with TM impurity, a systematic theoretical study is required. In this work, we investigate the electronic and magnetic property of SnSe sheet
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Fig. 1. The relaxed structures of V (a), Cr (b), Mn (c), Fe (d), Co (e) and Ni (f) incorporated into bilayer SnSe.
doped with TM (V, Cr, Mn, Fe, Co and Ni) atoms. The TM (except Ni) atoms can induce magnetic moment to the SnSe sheet and the maximum magnetic moment is found in V-doped system. The high spin polarization at Ef is found in the bilayer SnSe doped with Mn and Co atoms. The rest of the paper is organized as follows. In Section 2, we summarize the computational details. The results and discussions will been present in Section 3. Finally, the conclusions are given in Section 4.
2. Calculational details For an ab initio description of the transition-metal atoms doped in SnSe we performed DFT [22,23] calculations on 3 3 bilayer SnSe supercell containing one adatom using the Vienna ab initio simulation package (VASP) [24,25] with the projector augmented wave (PAW) [26,27]. To treat electron exchange and correlation, we choose the Perdew–Burke–Ernzerhof (PBE) [28,29] generalized-gradient approximation (GGA) which can yield the correct groundstate structure of the combined systems. The cutoff energy of 400 eV and a precise 5 5 1 k-point sampling grid are used. The 15 Å vacuum region is adopted to decouple the adjacent atomic slabs between the supercells. The structural optimization is continued until the residual forces have converged to less than 1 10 2 eV/Å and the total energy to less than 1 10 5 eV.
Table 1 The calculated average distance (dTM-Se/dTM-Sn) of TM to its bonding Se/Sn atoms. The formation energy (Eform). ΔQ (the “ ” Denotes Losing Electrons) denotes the charge transfer. The magnetic moment (μ0) of the freestanding TM atoms, the local magnetic moment (μTM) of the TM dopants in the optimized systems and the total magnetic moment (μtot) of doped systems.
dTM-Se(Å) dTM-Sn(Å) Eform (eV) ΔQ (e) μ0 (μB) μTM (μB) μtot (μB)
V
Cr
Mn
Fe
Co
Ni
2.50 2.90 3.02 0.86 5.00 2.87 3.61
2.52 2.78 1.01 0.61 6.00 3.59 3.97
2.51 2.62 1.10 0.41 5.00 2.97 3.01
2.52 2.59 2.72 0.14 4.00 1.93 2.00
2.54 2.58 3.83 0.04 3.00 0.91 1.00
2.39 2.54 4.12 0.01 2.00 0.00 0.00
3. Results and discussion Recently, Eknapakul at al. [30] have successfully intercalated potassium in the interlayer of MoS2, which provides a promising method to dope the TM atom into the interlayer van der Waals gap. It is to be expected that SnSe sheet can be doped by the same method. To investigate the bilayer SnSe doped by TM atoms, we adopt the experimental parameters (a ¼b ¼4.29 Å)[20] to constructed a 3 3 SnSe sheet model, which contains 36 Sn atoms and 36 Se atoms as well as one TM atom doped in the interlayer. The distance between the two adjacent TM atoms is set at 12.87 Å,
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Fig. 2. (a), (b), (c), (d), (e) and (f) plot the charge density difference for the V, Cr, Mn, Fe, Co and Ni-doped SnSe sheet, respectively. Yellow regions represent the charge accumulation, blue regions represent the charge consumption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
which avoids any influence from TM atom coupling. To search for the most stable configuration, different interlayer doped positions have been investigated and the most stable doped structures are found by DFT total energy optimization. The formation energies of Fe, Co and Ni atoms are almost independent of the adsorption sites. This phenomenon is also observed in Fe and Co atom adsorbed on BN (8, 0) tube and BN sheet [31,32]. In this study, we only investigate the most stable doped configuration for every TMdoped system, as shown in Fig. 1. The TM atom interacts both with Se and Sn atoms and the most stable structures of Mn, Fe and Codoped systems are similar. The calculated average distance dTM-Se and dTM-Sn are given in Table 1. It is noted that dTM-Sn is decreasing with the increasing of TM atomic number, while the dTM-Se keeps almost unchanged except for the Ni-doped system. To evaluate the stability, the formation energies of TM-doped bilayer SnSe are calculated based on the following formula:
Eform = ESnSe−E TM − SnSe + nE TM, where ESnSe and ETM-SnSe are the total energies of undoped bilayer SnSe and the doped bilayer SnSe systems, respectively. ETM is the total energy of isolated TM atom. The n indicates the number of TM atom doped in the SnSe sheet. The positive (negative) value indicates the process is exothermic (endothermic). The formation energies of all configurations are presented in Table 1. Clearly, the
V, Fe, Co and Ni doped systems are more energetically favorable compared to Cr and Mn doped systems. In order to visualize the tendencies of the charge redistribution, we calculate charge density difference
Δρ = ρ TM − SnSe −ρSnSe −ρ TM , where ρTM-SnSe, ρSnSe and ρTM are the total charge density of TM-doped SnSe sheet, undoped SnSe sheet and TM atom, respectively. The Charge density differences distributions of TM doped-SnSe sheet are displayed in Fig. 2. It is clearly shown that V and Cr atom lose electrons to SnSe, while it is difficult to analyze the charge redistribution for Mn, Fe, Co and Ni doped systems (Fig. 2(c)–(f)). Furthermore, we found the TM atoms lose electrons by using Bader charge analysis, as listed in Table 1. With the increasing of atomic number, the charge transfer decreases gradually. In addition, the maximum electro-negative difference is 0.41/ 1.00 between Mn and Sn/Se atom, which is much smaller than 1.7, indicating that the regions between TM atoms and Sn/Se atoms behave a covalent-bonding character. The similar phenomenon can also be found in 3d TM-(Cr, Mn, Fe, Co- and Ni) doped doped bilayer MoS2 [33]. Mao et al. [34] studied the bonding and diffusion of Mn doped bilayer graphene and pointed out the mobility of TM atom is a key factor to determine the property of doped system. To investigate the thermal stability of the TM doped bilayer
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Fig. 3. The spin density distribution of TM-doped SnSe sheet. (a) V, (b) Cr, (c) Mn, (d) Fe, (e) Co, and (f) Ni. The yellow and blue colors represent spin-up and spin-down, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
SnSe, we make the TM atoms moved along the x-, y-, z-axis from its equilibrium position. When the moving distance are 1 Å (x-, zaxis) and 0.858 Å (y-axis), the doped systems are unstable (i.e. Eform o0), which indicates the doped TM atom tightly confined in a small region. To visualize the detailed distribution of magnetic moments of the 3d TM doped bilayer SnSe, we plot the spin density distributions as shown in Fig. 3. V, Cr, Mn and Fe atoms induce antiferromagnetic interactions and ferromagnetic interactions in SnSe sheet. For instant, Se (Se1, Se2 and Se3) atoms are antiferromagnetically coupled to the V atom, while the Sn (Sn1, Sn2 and Sn4) atoms are ferromangnetically coupled to V atom. However, Co atom only induces ferromagnetic interaction in SnSe sheet. The total magnetic moments mainly originate from the doped TM impurities except for Ni-doped system which is zero magnetic moment. The magnetic moment (μ0) of the freestanding TM atoms, the local magnetic moment (μTM) of the TM dopants and the total magnetic moment (μtot) of doped system are summarized in Table 1. The relatively large μtot are found in V, Cr and Mn doped systems. The μTM of V and Cr atom are reduced, respectively, by 2.13 μB and 2.41 μB in comparison with their freestanding states. For Mn (Fe and Co) doped system, the magnetic moment is almost reduced by 2 μB compared to its freestanding state. This is similar to the case of Cr-, Mn-, Fe-, Co-, and Ni-doped bilayer MoS2 carried out by Huang et al. [33].
To get a detail description of magnetic evolution, the partial density of states (PDOS) are investigated. As shown in Fig. 4, the black line and blue line represent the 4s orbital and 3d orbital of the freestanding TM atoms, respectively. The green line and red line represent, respectively, the 4s orbital and 3d orbital of the TM impurities in the bilayer SnSe. In all TM doped systems, the 4s orbital loses electrons to 3d orbital, this is similar to the case of BN sheets with absorbed TM atoms [32] and the case of Mn doped bilayer graphene [35]. Also, the issue has been systematically studied for the case of graphene [36]. In Fig. 4(a) and (b), there are more empty states above Fermi level (Ef) for the spin-up channel of the 3d orbital in comparison with that of freestanding TM (V, Cr) atom, while very few occupied states emerge the region below Ef for spin-down channel of the 3d orbital. This shows that the 3d orbital loses electrons to bilayer SnSe, which is responsible for magnetic reduction for V and Cr atoms doped in bilayer SnSe. In Fig. 4(c), the spin-up channel of the 3d orbital loses electrons and the spin-down channel of the 3d orbital gains electrons. Hence, two factors are responsible for magnetic reduction for Mn doped in bilayer SnSe. There are more occupied states below Ef for the spin-down channel of the 3d orbital after TM (Fe, Co, Ni) atom doped in SnSe sheet (see Fig. 4(d), (e) and (f)), while very few empty states emerged above Ef for spin-up channel of the 3d orbital. The calculations indicate that TM (Fe, Co and Ni) atoms lose very little electron to the bilayer SnSe and TM-4s orbital lose
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Fig. 4. The partial density of states of TM atom. The Fermi level is set to zero and marked by a dotted line.
electrons to the spin-down channel of the TM-3d orbital. The results of charge transfer are consistent with the previous Bader charge analysis. Therefore, the magnetic reduction for Fe, Co and Ni doped in bilayer SnSe, especially Ni doped in bilayer SnSe, can be ascribed to the charge transfer from 4s orbital to spin-down channel of the 3d orbital. To investigate the interaction between TM atoms and the SnSe in detail, the density of states is calculated. For the TM doped system, the TM 3d orbital can effectively modulate the electronic property of substrate. As shown in Fig. 5, the total density of states (TDOS) and PDOS of TM, Sn and Se atoms are represented by different color lines. Fig. 5 shows that the TM 3d orbital overlap with Sn 5s, 5p orbitals and Se 4p orbital below Ef and the TM 3d orbital dominates the magnetic property and the electronic structure around the Ef. For instant, the spin-up channel of V-3d orbital (see Fig. 5(a)) is extended from 5 eV to Ef and two sharp peaks locate in the region around Ef. Sn 5s, 5p orbitals and Se 4p orbital across the Ef and the spin polarization is easily found in the region adjoined the Ef due to their interaction with V-3d orbital. In addition, Mn/Co-doped SnSe sheet is a semi-metal with a gap of 0.12/0.27 eV for the spin-up channel. For Ni-doped system (see Fig. 5(f)), the spin-up and spin-down channel of every DOS are symmetrical (no spin polarization), which indicate that Ni-doped SnSe sheet is still a nonmagnetic semiconductor with a gap of 0.26 eV. We calculate the spin polarization P(Ef) at the Ef by
P (Ef ) = D(Ef ↑) − D(Ef ↓) /[D(Ef ↑) + D(Ef ↓)], where D(Ef↑) and D(Ef↓) are represent for the density of states of spin-up and spin-down states at Ef, respectively. The P(Ef) is 20%, 82%, 100%, 88% and 100% for V, Cr, Mn, Fe, and Co-doped system, respectively. It is well known that most electronic transport occurs
at the Ef. Therefore, the high spin polarization at Ef for bilayer SnSe doped with Mn and Co atoms ensure the high degree of passage of the preferred spin, which is crucial for the spintronics.
4. Conclusion Based on first-principles calculations we studied the stable configurations, electronic structures and magnetic behaviors of bilayer SnSe with V, Cr, Mn, Fe, Co and Ni dopants in the interlayer. The difference of formation energy is very small for Fe, Co and Ni atom at different adsorption site. The charge density distribution shows that the covalent-bonding features are found between TM and Sn, Se atoms. The magnetic moment of doped SnSe sheet mainly derives from TM impurities for V, Cr, Mn, Fe and Co doped system, while the Ni-doped system is still a nonmagnetic semiconductor with a gap of 0.26 eV. For V and Cr doped systems, the spin-up channel of 3d orbital loses electrons to SnSe, which result in magnetic reduction for V and Cr atom doped in bilayered SnSe. On the contrary, the spin-down channel of the 3d orbital gains electrons from 4s orbital, which result in magnetic reduction for Fe, Co and Ni atoms doped in bilayered SnSe. Moreover, both the two factors are responsible for magnetic reduction for Mn doped in bilayer SnSe. The 3d orbital of TM atom effectively modulate the band gap of SnSe sheet and a spin polarization of 100% at Ef is gained by doping Mn and Co atoms in SnSe sheet, which ensures a selective passage for the preferred spin. Our results indicate that the bilayer SnSe with TM (V, Cr, Mn, Fe, Co) atoms doped in the interlayer maybe have potential application in spintronics.
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Fig. 5. Density of states of TM doped SnSe sheet. The red line, green line, blue line and purple line represent the V-d orbital, Se-p orbital, Sn-s orbital and Sn-p orbital, respectively. The black line denotes the total density of states. (The scales of TDOS are narrowed down 20 times.) The Fermi level is set to zero and marked by a dotted line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments This work is supported by a Grant from the National Natural Science Foundation of China (NSFC) under Grant nos. U1304518 and U1404109.
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Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Modulation of magnetic properties of bilayer SnSe with transition-metals doping in the interlayer Xiao-Long Wang a, Wei Li a, Tian-Xing Wang a, Xian-Qi Dai a,b,n a b
College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, China School of Physics and Electronic Engineering, Zhengzhou Normal University, Zhengzhou, Henan 450044, China
H I G H L I G H T S
Charge transfer from transition metal atom to SnSe sheet decreases gradually with increasing atomic number. Two factors result in magnetic reduction for transition metal atom doped in bilayer SnSe. A complete spin-polarization at Ef is obtained in Mn (Co) doped bilayer SnSe system.
ar t ic l e i nf o
a b s t r a c t
Article history: Received 26 July 2015 Received in revised form 6 September 2015 Accepted 12 September 2015 Available online 15 September 2015
We investigate the spin-polarized electronic and magnetic properties of bilayer SnSe with transitionmetal (TM) atoms doped in the interlayer by using a first-principles method. It shows that Ni dopant cannot induce the magnetism in the doped SnSe sheet, while the ground state of V, Cr, Mn, Fe and Co doped systems are magnetic and the magnetic moment mainly originates from 3d TM atom. Two types of factors, which reduce the magnetic moment of TM atoms doped in bilayer SnSe, are identified as spin-up channel of the 3d orbital loses electrons to SnSe sheet and spin-down channel of the 3d orbital gains electrons from 4s orbital. The spin polarization is found to be 100% at Fermi level for the Mn and Co atoms doped system, while the Ni-doped system is still a semiconductor with a gap of 0.26 eV. These results are potentially useful for development of spintronic devices. & 2015 Elsevier B.V. All rights reserved.
Keywords: First-principles Bilayer SnSe Interlayer Electronic structure Magnetic property
1. Introduction Two-dimensional (2D) nano-materials such as graphene [1], BN [2], MoS2 [3] and black phosphorus [4] have shown novel properties due to the low dimensionality and quantum confinement effect. Recently, considerable attention has been paid to magnetic properties of 2D material for the spintronic applications [5–7]. As is known, substitutional doping and adsorptive doping are the efficient approaches to tune the properties of the materials and have been widely used to induce the magnetic ground state of the nonmagnetic semiconducting materials. For instant, Cu dopant induces the unexpected strong magnetism into monolayer MoS2 [8]. Yang et al. reported that TM atom can profoundly modulate the magnetism of monolayer ZrS2 [9]. Moreover, Sahin et al. investigated the electronic band dispersion and magnetic moment of monolayer silicone with TM adatom [10]. n Corresponding author at: College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, China. E-mail address: [email protected] (X.-Q. Dai).
http://dx.doi.org/10.1016/j.physe.2015.09.020 1386-9477/& 2015 Elsevier B.V. All rights reserved.
As a typical IV–VI group semiconductor, SnSe belongs to the orthorhombic crystal system. Similar to SnS, GeS, and GeSe, the bonding of SnSe interacts covalently within the layer, which comprises zig-zag double layer planes of the tin and selenium atoms and is separated by a weak van der Waals force between the layers [11–13]. The bulk SnSe production process is very mature [14–17] and its direct band gap (1.30 eV) [18] has attracted widespread interest in the research of its potential applications for optical devices [11,19]. Moreover, the monolayer SnSe has been prepared by pulse electrodeposited on tin oxide coated glass substrates at different duty cycles [20]. Recently, Huang et al. reported that the strain can not induce the magnetism in the SnSe nanoribbons, though the strain can induce the semiconductor– metal or metal–semiconductor transition [21]. The effects of TM atom doped in bilayer SnSe, which, to our knowledge, have not been reported as yet. The TM atoms doped in the interlayer could be hard to oxidize, which is beneficial in maintaining a stable performance in bilayer SnSe devices. To fully understand and control the magnetic property of the bilayer SnSe with TM impurity, a systematic theoretical study is required. In this work, we investigate the electronic and magnetic property of SnSe sheet
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Fig. 1. The relaxed structures of V (a), Cr (b), Mn (c), Fe (d), Co (e) and Ni (f) incorporated into bilayer SnSe.
doped with TM (V, Cr, Mn, Fe, Co and Ni) atoms. The TM (except Ni) atoms can induce magnetic moment to the SnSe sheet and the maximum magnetic moment is found in V-doped system. The high spin polarization at Ef is found in the bilayer SnSe doped with Mn and Co atoms. The rest of the paper is organized as follows. In Section 2, we summarize the computational details. The results and discussions will been present in Section 3. Finally, the conclusions are given in Section 4.
2. Calculational details For an ab initio description of the transition-metal atoms doped in SnSe we performed DFT [22,23] calculations on 3 3 bilayer SnSe supercell containing one adatom using the Vienna ab initio simulation package (VASP) [24,25] with the projector augmented wave (PAW) [26,27]. To treat electron exchange and correlation, we choose the Perdew–Burke–Ernzerhof (PBE) [28,29] generalized-gradient approximation (GGA) which can yield the correct groundstate structure of the combined systems. The cutoff energy of 400 eV and a precise 5 5 1 k-point sampling grid are used. The 15 Å vacuum region is adopted to decouple the adjacent atomic slabs between the supercells. The structural optimization is continued until the residual forces have converged to less than 1 10 2 eV/Å and the total energy to less than 1 10 5 eV.
Table 1 The calculated average distance (dTM-Se/dTM-Sn) of TM to its bonding Se/Sn atoms. The formation energy (Eform). ΔQ (the “ ” Denotes Losing Electrons) denotes the charge transfer. The magnetic moment (μ0) of the freestanding TM atoms, the local magnetic moment (μTM) of the TM dopants in the optimized systems and the total magnetic moment (μtot) of doped systems.
dTM-Se(Å) dTM-Sn(Å) Eform (eV) ΔQ (e) μ0 (μB) μTM (μB) μtot (μB)
V
Cr
Mn
Fe
Co
Ni
2.50 2.90 3.02 0.86 5.00 2.87 3.61
2.52 2.78 1.01 0.61 6.00 3.59 3.97
2.51 2.62 1.10 0.41 5.00 2.97 3.01
2.52 2.59 2.72 0.14 4.00 1.93 2.00
2.54 2.58 3.83 0.04 3.00 0.91 1.00
2.39 2.54 4.12 0.01 2.00 0.00 0.00
3. Results and discussion Recently, Eknapakul at al. [30] have successfully intercalated potassium in the interlayer of MoS2, which provides a promising method to dope the TM atom into the interlayer van der Waals gap. It is to be expected that SnSe sheet can be doped by the same method. To investigate the bilayer SnSe doped by TM atoms, we adopt the experimental parameters (a ¼b ¼4.29 Å)[20] to constructed a 3 3 SnSe sheet model, which contains 36 Sn atoms and 36 Se atoms as well as one TM atom doped in the interlayer. The distance between the two adjacent TM atoms is set at 12.87 Å,
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Fig. 2. (a), (b), (c), (d), (e) and (f) plot the charge density difference for the V, Cr, Mn, Fe, Co and Ni-doped SnSe sheet, respectively. Yellow regions represent the charge accumulation, blue regions represent the charge consumption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
which avoids any influence from TM atom coupling. To search for the most stable configuration, different interlayer doped positions have been investigated and the most stable doped structures are found by DFT total energy optimization. The formation energies of Fe, Co and Ni atoms are almost independent of the adsorption sites. This phenomenon is also observed in Fe and Co atom adsorbed on BN (8, 0) tube and BN sheet [31,32]. In this study, we only investigate the most stable doped configuration for every TMdoped system, as shown in Fig. 1. The TM atom interacts both with Se and Sn atoms and the most stable structures of Mn, Fe and Codoped systems are similar. The calculated average distance dTM-Se and dTM-Sn are given in Table 1. It is noted that dTM-Sn is decreasing with the increasing of TM atomic number, while the dTM-Se keeps almost unchanged except for the Ni-doped system. To evaluate the stability, the formation energies of TM-doped bilayer SnSe are calculated based on the following formula:
Eform = ESnSe−E TM − SnSe + nE TM, where ESnSe and ETM-SnSe are the total energies of undoped bilayer SnSe and the doped bilayer SnSe systems, respectively. ETM is the total energy of isolated TM atom. The n indicates the number of TM atom doped in the SnSe sheet. The positive (negative) value indicates the process is exothermic (endothermic). The formation energies of all configurations are presented in Table 1. Clearly, the
V, Fe, Co and Ni doped systems are more energetically favorable compared to Cr and Mn doped systems. In order to visualize the tendencies of the charge redistribution, we calculate charge density difference
Δρ = ρ TM − SnSe −ρSnSe −ρ TM , where ρTM-SnSe, ρSnSe and ρTM are the total charge density of TM-doped SnSe sheet, undoped SnSe sheet and TM atom, respectively. The Charge density differences distributions of TM doped-SnSe sheet are displayed in Fig. 2. It is clearly shown that V and Cr atom lose electrons to SnSe, while it is difficult to analyze the charge redistribution for Mn, Fe, Co and Ni doped systems (Fig. 2(c)–(f)). Furthermore, we found the TM atoms lose electrons by using Bader charge analysis, as listed in Table 1. With the increasing of atomic number, the charge transfer decreases gradually. In addition, the maximum electro-negative difference is 0.41/ 1.00 between Mn and Sn/Se atom, which is much smaller than 1.7, indicating that the regions between TM atoms and Sn/Se atoms behave a covalent-bonding character. The similar phenomenon can also be found in 3d TM-(Cr, Mn, Fe, Co- and Ni) doped doped bilayer MoS2 [33]. Mao et al. [34] studied the bonding and diffusion of Mn doped bilayer graphene and pointed out the mobility of TM atom is a key factor to determine the property of doped system. To investigate the thermal stability of the TM doped bilayer
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Fig. 3. The spin density distribution of TM-doped SnSe sheet. (a) V, (b) Cr, (c) Mn, (d) Fe, (e) Co, and (f) Ni. The yellow and blue colors represent spin-up and spin-down, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
SnSe, we make the TM atoms moved along the x-, y-, z-axis from its equilibrium position. When the moving distance are 1 Å (x-, zaxis) and 0.858 Å (y-axis), the doped systems are unstable (i.e. Eform o0), which indicates the doped TM atom tightly confined in a small region. To visualize the detailed distribution of magnetic moments of the 3d TM doped bilayer SnSe, we plot the spin density distributions as shown in Fig. 3. V, Cr, Mn and Fe atoms induce antiferromagnetic interactions and ferromagnetic interactions in SnSe sheet. For instant, Se (Se1, Se2 and Se3) atoms are antiferromagnetically coupled to the V atom, while the Sn (Sn1, Sn2 and Sn4) atoms are ferromangnetically coupled to V atom. However, Co atom only induces ferromagnetic interaction in SnSe sheet. The total magnetic moments mainly originate from the doped TM impurities except for Ni-doped system which is zero magnetic moment. The magnetic moment (μ0) of the freestanding TM atoms, the local magnetic moment (μTM) of the TM dopants and the total magnetic moment (μtot) of doped system are summarized in Table 1. The relatively large μtot are found in V, Cr and Mn doped systems. The μTM of V and Cr atom are reduced, respectively, by 2.13 μB and 2.41 μB in comparison with their freestanding states. For Mn (Fe and Co) doped system, the magnetic moment is almost reduced by 2 μB compared to its freestanding state. This is similar to the case of Cr-, Mn-, Fe-, Co-, and Ni-doped bilayer MoS2 carried out by Huang et al. [33].
To get a detail description of magnetic evolution, the partial density of states (PDOS) are investigated. As shown in Fig. 4, the black line and blue line represent the 4s orbital and 3d orbital of the freestanding TM atoms, respectively. The green line and red line represent, respectively, the 4s orbital and 3d orbital of the TM impurities in the bilayer SnSe. In all TM doped systems, the 4s orbital loses electrons to 3d orbital, this is similar to the case of BN sheets with absorbed TM atoms [32] and the case of Mn doped bilayer graphene [35]. Also, the issue has been systematically studied for the case of graphene [36]. In Fig. 4(a) and (b), there are more empty states above Fermi level (Ef) for the spin-up channel of the 3d orbital in comparison with that of freestanding TM (V, Cr) atom, while very few occupied states emerge the region below Ef for spin-down channel of the 3d orbital. This shows that the 3d orbital loses electrons to bilayer SnSe, which is responsible for magnetic reduction for V and Cr atoms doped in bilayer SnSe. In Fig. 4(c), the spin-up channel of the 3d orbital loses electrons and the spin-down channel of the 3d orbital gains electrons. Hence, two factors are responsible for magnetic reduction for Mn doped in bilayer SnSe. There are more occupied states below Ef for the spin-down channel of the 3d orbital after TM (Fe, Co, Ni) atom doped in SnSe sheet (see Fig. 4(d), (e) and (f)), while very few empty states emerged above Ef for spin-up channel of the 3d orbital. The calculations indicate that TM (Fe, Co and Ni) atoms lose very little electron to the bilayer SnSe and TM-4s orbital lose
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Fig. 4. The partial density of states of TM atom. The Fermi level is set to zero and marked by a dotted line.
electrons to the spin-down channel of the TM-3d orbital. The results of charge transfer are consistent with the previous Bader charge analysis. Therefore, the magnetic reduction for Fe, Co and Ni doped in bilayer SnSe, especially Ni doped in bilayer SnSe, can be ascribed to the charge transfer from 4s orbital to spin-down channel of the 3d orbital. To investigate the interaction between TM atoms and the SnSe in detail, the density of states is calculated. For the TM doped system, the TM 3d orbital can effectively modulate the electronic property of substrate. As shown in Fig. 5, the total density of states (TDOS) and PDOS of TM, Sn and Se atoms are represented by different color lines. Fig. 5 shows that the TM 3d orbital overlap with Sn 5s, 5p orbitals and Se 4p orbital below Ef and the TM 3d orbital dominates the magnetic property and the electronic structure around the Ef. For instant, the spin-up channel of V-3d orbital (see Fig. 5(a)) is extended from 5 eV to Ef and two sharp peaks locate in the region around Ef. Sn 5s, 5p orbitals and Se 4p orbital across the Ef and the spin polarization is easily found in the region adjoined the Ef due to their interaction with V-3d orbital. In addition, Mn/Co-doped SnSe sheet is a semi-metal with a gap of 0.12/0.27 eV for the spin-up channel. For Ni-doped system (see Fig. 5(f)), the spin-up and spin-down channel of every DOS are symmetrical (no spin polarization), which indicate that Ni-doped SnSe sheet is still a nonmagnetic semiconductor with a gap of 0.26 eV. We calculate the spin polarization P(Ef) at the Ef by
P (Ef ) = D(Ef ↑) − D(Ef ↓) /[D(Ef ↑) + D(Ef ↓)], where D(Ef↑) and D(Ef↓) are represent for the density of states of spin-up and spin-down states at Ef, respectively. The P(Ef) is 20%, 82%, 100%, 88% and 100% for V, Cr, Mn, Fe, and Co-doped system, respectively. It is well known that most electronic transport occurs
at the Ef. Therefore, the high spin polarization at Ef for bilayer SnSe doped with Mn and Co atoms ensure the high degree of passage of the preferred spin, which is crucial for the spintronics.
4. Conclusion Based on first-principles calculations we studied the stable configurations, electronic structures and magnetic behaviors of bilayer SnSe with V, Cr, Mn, Fe, Co and Ni dopants in the interlayer. The difference of formation energy is very small for Fe, Co and Ni atom at different adsorption site. The charge density distribution shows that the covalent-bonding features are found between TM and Sn, Se atoms. The magnetic moment of doped SnSe sheet mainly derives from TM impurities for V, Cr, Mn, Fe and Co doped system, while the Ni-doped system is still a nonmagnetic semiconductor with a gap of 0.26 eV. For V and Cr doped systems, the spin-up channel of 3d orbital loses electrons to SnSe, which result in magnetic reduction for V and Cr atom doped in bilayered SnSe. On the contrary, the spin-down channel of the 3d orbital gains electrons from 4s orbital, which result in magnetic reduction for Fe, Co and Ni atoms doped in bilayered SnSe. Moreover, both the two factors are responsible for magnetic reduction for Mn doped in bilayer SnSe. The 3d orbital of TM atom effectively modulate the band gap of SnSe sheet and a spin polarization of 100% at Ef is gained by doping Mn and Co atoms in SnSe sheet, which ensures a selective passage for the preferred spin. Our results indicate that the bilayer SnSe with TM (V, Cr, Mn, Fe, Co) atoms doped in the interlayer maybe have potential application in spintronics.
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Fig. 5. Density of states of TM doped SnSe sheet. The red line, green line, blue line and purple line represent the V-d orbital, Se-p orbital, Sn-s orbital and Sn-p orbital, respectively. The black line denotes the total density of states. (The scales of TDOS are narrowed down 20 times.) The Fermi level is set to zero and marked by a dotted line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments This work is supported by a Grant from the National Natural Science Foundation of China (NSFC) under Grant nos. U1304518 and U1404109.
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