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Strong effect of compressive strain on Ni-doped monolayer WSe2
Author’s Accepted Manuscript Strong effect of compressive strain on Ni-doped monolayer WSe2 Xiaomeng Liu, Xu Zhao, Qianqian Xin, Ninghua Wu, Xu Ma, Tianxing Wang, Shuyi Wei www.elsevier.com/locate/physe
PII: DOI: Reference:
S1386-9477(17)30175-3 http://dx.doi.org/10.1016/j.physe.2017.03.013 PHYSE12755
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 2 February 2017 Revised date: 17 March 2017 Accepted date: 20 March 2017 Cite this article as: Xiaomeng Liu, Xu Zhao, Qianqian Xin, Ninghua Wu, Xu Ma, Tianxing Wang and Shuyi Wei, Strong effect of compressive strain on Nidoped monolayer WSe2, Physica E: Low-dimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2017.03.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strong effect of compressive strain on Ni-doped monolayer WSe2 Xiaomeng Liu, Xu Zhao*, Qianqian Xin, Ninghua Wu, Xu Ma, Tianxing Wang, Shuyi Wei* College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan 453007, China [email protected]
Abstract We study the effect of strain on electronic and magnetic properties of Ni-doped monolayer WSe2 by using a method of plane wave potential technique based on the density function theory. We find that Ni-doped monolayer WSe2 becomes a magnetic metal material, and the magnetic moment is 1.466 μB. When we apply different compressive strain, Ni-doped WSe2 shows half-metallic frommagnetic properties firstly. With increasing compressive strain, Ni-doped WSe2 shows magnetic metal properties from -8% to -10%. The magnetic moment also increases firstly and then decreases with increasing compressive strains, the biggest magnetic moment is 3.280 μB at -3% compressive strain and the minimum magnetic moment is 0.658μB at -10% compressive strain. While, the magnetic moment increases slowly with increasing tensile strain and comes to 1.812μB at 10% tensile strain. By comparison, we find that there is stronger effect of compressive strain on electronic and magnetic properties of this doped system than that of tensile strain. Keywords:
Transition-metal
dichalcogenides;
Electronic
band
structure;
First-principles I.
Introduction In recent years, transition-metal dichalcogenides (TMDs) show a wide variety of
interesting physical properties, especially groups of IVB, VB, and VIB have received considerable attention[1-13]. They display semiconducting, metallic, superconducting, and magnetic behavior. Nowadays the atomic and electronic structures of TMDs have
been the subject of many experimental and theoretical investigations [14-18]. In addition, the fabrication of TMDs also attracted great attention in the field of inorganic compound. Layers of TMDs were manufactured by mechanical and chemical exfoliation of their layered bulk counterparts, as well as by chemical vapor deposition [19-23]. The pronounced anisotropy of these compounds results from the formation of MX2 layers with strong covalent intralayer bonds, but only weak van der Waals coupling between adjacent layers. WSe2 is a member in the family of layered transition metal dichalcogenides (TMDs) which display a wide variety of interesting physical properties. It is known for its remarkable stability against photocorrosion[24]. WSe2 is a semiconductor with a measured indirect gap of 1.2eV thus making it useful for photovoltaic and optoelectronic applications[25]. And the electrochemical devices based on WSe2 have been reported to possess conversion efficiencies up to 17%[26]. In addition, WSe2 has recently been a subject of a number of photoemission studies[27-30]. Another puzzling property of WSe2 has recently been discovered by scanning tunneling microscopy (STM)[31,32]. A number of experimental and theoretical studies have confirmed that the substitution of TM atoms can induce magnetism in nonmagnetic nanomaterials[33,34]. As we all know pristine monolayer WSe2 is nonmagnetic, in this work, Ni atom substitute a W atom in monolayer WSe2, and the doping concentration is 6.25%. Then we study the electronic and magnetic properties of monolayer WSe2 with different strain. We find Ni dopant can induce the magnetic moment, moreover, strain can change the magnetic moment of Ni-doped WSe2 monolayer. With different strain we can obtain half-metallic material and magnetic metal, respectively, the result might have important potential application in spintronic devices. II.Theoretical models and methods Mechanical stripping method is used to obtain monolayer WSe2, the layered WSe2 consists of stacked Se–W–Se layers, upper and lower two layers of Se atoms, intermediate of W atoms, both W atom and Se atom are linked by covalent bonds, and these layers are held together by Van der Walls interaction.WSe2 layers have a
P63/mmc space group symmetry with the W atoms having a trigonal prismatic coordination with the Se atoms, which showed in Fig.1. At the same time, a Ni atom instead of a W atom in a 4×4×1 WSe2 monolayer supercell for 6.25% (one Ni atom, 15 W atoms and 32 Se atoms). Our calculations were performed within first-principles DFT using the projector augmented wave (PAW) method[35]. And electron exchange, correlation effects were described within the generalized gradient approximation (GGA)[36] respectively used within the Vienna ab initio simulation package (VASP)[37]. In all calculations, an energy cutoff of 500 eV for the plane-wave expansion of the wave functions was used. For the Brillouin zone integration, a 9×9×1 centered Monkhorst-pack-K-point mesh is used for WSe2 monolayer. And the atomic positions and cell parameters are optimized until the residual forces fall below 0.01 eV/Å. III. Results and discussion: In this work, we investigate the structural and electronic properties in Ni-doped WSe2 by applied strain from -10% to 10%. And we adopt first–principles calculations based on the density functional theory (DFT). In the work, we calculate the magnetic moment, Mtot, and half-metallic band gapΔEHM. All of them are showed in Table1. From Table 1, we can see that the magnetic moment is 1.466μB at 0% strain, and increases firstly and then decreases with increasing compressive strains, the biggest magnetic moment is 3.280 μB at -3% compressive strain and the minimum magnetic moment is 0.658μB at -10% compressive strain. While, the magnetic moment increases slowly with increasing tensile strain and comes to 1.812μB at 10% tensile strain. The doped system shows half-metallic properties under compressive strain from -2% to -7%. The biggest half-metallic gap is 89meV at -5% compressive strain. According to these calculation results, we can see that there is stronger effect of compressive strain on electronic and magnetic properties of this doped system than that of tensile strain. At first, we find that Ni doped WSe2 semiconductor transforms to magnetic metal, we give the band structure of Ni-doped system at Fig.2. From Fig. 2, we can see some impurity states within the band gap, and the spin-up and spin-down states
are not symmetric and both spin-up and spin-down crosses at the Fermi level in band structure at 0% strain. These results indicate that not only Ni-doped WSe2 show metallic feature but also Ni substituting W can induce to the magnetism in the WSe2 monolayer. In addition we also give band structures of -5% and -10% in Fig. 2, at -5% strain, a band crossing at the Fermi level occurs with little dispersion in spin-up band structures, whereas in spin-down band structures, a band gap of 0.268eV is found for the Ni-doped WSe2 monolayer, these results indicate that the Ni-doped WSe2 monolayer turn into half-metallic magnetism material. The half-metallic gap is about 89meV. At -10% strain, the band crossing at the Fermi level occurs with little dispersion in spin-up band structures and spin-down band structures, which shows the doped system turns to magnetic metal material. By comparison, we see that the band gap decreases with increasing compressive train. In order to understand the magnetic and electronic properties in more details, we investigated the projected density of states (PDOS) and total density of states(TDOS) of the Ni-doped WSe2 monolayer at 0% and -5% compressive strain, the TDOS and PDOS are further shown in Fig. 3. For the case of 0% strain, we can see the electronic structures of spin-up and spin-down channels are consisting of the hybridization of the W-5d, Se-4p and Ni-3d states near the Fermi level. Moreover, the polarized charges mainly arise from the localized 3d orbitals of the Ni atom, a little from the localized 5d orbitals of the W atom and hardly from the localized 4p orbitals of the Se atom near the Fermi level. Meanwhile, both the spin-up and spin-down states cross the Fermi level and indicates to the magnetic metal feature. For the case of -5% compressive strain, we find that the band gap of doped system decreases to 1.495eV and the impurity states move to the valence band slowly. The spin-up states keep crossing the Fermi level, while the spin-down states have a band gap near the Fermi level. Consequently, the doped system shows a half-metallic properties and the half-metallic gap is about 89meV. Fig.4 gives the total density of states (TDOS) of Ni-doped WSe2 with different strain. In the picture, we can see that the intrisic band gap decreases quickly with increasing compressive strain and there is a half-metallic gap from -2% to -7% compressive strain. The doped system turns to magnetic metal
from -8% to -10%. These results indicate that the effect of strain on the electronic structure is strong and it cannot be ignored. For understand clearly the mechanism of magnetism induced by doped atom in the Ni-doped WSe2 monolayer, Fig. 5 gives the spin density of Ni-doped WSe2. It is noted that without strain, both the first neighboring six Se atoms and second neighboring four W atoms display ferromagnetically coupled to the Ni atom. For the case of 2% tensile strain, both the first neighboring six Se atoms and second neighboring six W atoms display ferromagnetically coupled to the Ni atom. When the tensile strain comes to 10%, we only find that the second six W atoms are ferromagnetically coupled to the Ni atom, while the spin density from the nearest neighboring six Se atoms is very little. By contrast, for the case of -10% compressive strain, the couple between Ni atom and the nearest neighboring six Se atoms is obviously strong. The strong p–d hybridization was found between the 3d orbital of Ni and 4p orbital of Se. Consequently, the biaxial strain affects strongly the distribution of spin density and changes the magnetic moment of doped system. To probe the stability of the Ni-doped WSe2 monolayer, the formation energy Eform can be calculated according to the following formula [38-39] Eform = E(doped)− E(pure) + n(μW – μNi). Where E(doped) and E(pure)are the total energies of Ni-doped WSe2 monolayer with and without the Ni dopant. n is the number of W atoms replaced by Ni dopants. μW and μNi are the chemical potential for W host and Ni dopant atoms, respectively, which depends on the material growth conditions and satisfies the boundary conditions. We use the energy per atom of Ni metal as μNi. μW is defined within a range of values corresponding to W-rich or Se-rich growth conditions. Fig.6 gives the formation energy of different growth conditions, one can find easily the formation energy of Se-rich always lower than that of W-rich. Under the Se-rich growth condition, the smallest Eform of 1.714eV was found at 1% tensile strain, the largest Eform of 20.549eV was found at -10% compressive strain. Moreover, the formation energy is more than 10 eV from -6% to -10% compressive strain and from 10% tensile strain, which indicates that compressive strain has bigger effect than tensile strain in Ni-doped
monolayer WSe2. Consequently, the formation energy calculations also indicate that it is energy favorable and relatively easier to incorporate Ni atom into the WSe2 monolayer under Se-rich experimental conditions. And the formation energy of the doped system at 1% and 2% tensile strain is lower than 0% strain. IV. Conclusion In this paper, we study the electronic and magnetic properties of Ni-doped monolayer WSe2 by strain. The results show that Ni-doped WSe2 becomes magnetic metallic and magnetic moment is 1.466μB. Both tensile strain and compressive strain change the magnetic and electronic properties of Ni doped WSe2 monolayer, while there is stronger effect of compressive strain on electronic and magnetic properties of this doped system than that of tensile strain. We find that the maximum magnetic moment is 3.280μB at -3% compressive strain and minimum magnetic moment is 0.658μB at -10% strain. Ni-doped WSe2 transforms to half-metallic from magnetic metallic firstly. The biggest half-metallic gap is 89meV. With increasing compressive strain, Ni-doped WSe2 shows magnetic metal properties from -8% to -10%. The magnetic moment originates primarily from the localized 3d electrons of the Ni atom, a little from the localized 5d electrons of the W atom and hardly from the localized 4p electrons of the Se atom near the Fermi level. In addition, the formation calculation shows that the system easier to incorporate Ni atom into the WSe2 monolayer under Se-rich, and the lowest formation is found at 1% tensile strain indicating the most stable structure. These results suggest that Ni-doped WSe2 monolayer might have important potential application in spintronic devices.
This work is supported by a Grant from the National Natural Science Foundation of China (NSFC) under the Grant No. 11504092 and U1304518, National Undergraduate Training Programs for Innovation and Entrepreneurship (No. 201510476043), Science and technology research key project of education department of Henan province (No. 14A140012), and High Performance Computing Center of Henan Normal University.
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Fig. 1Top and side views of WSe2 monolayer, the green and gray balls represent Se and W atoms, respectively. Fig. 2 Spin-polarized band structure of Ni-doped WSe2 monolayer with 0%, -5% and -10%. The rad and black lines stand for the spin-up and spin-down components. The Fermi level is indicated by the solid line. Fig. 3 Total density of stats (TDOS) and partial density of states (PDOS) of Ni-doped WSe2 monolayer with isotropic strain 0% and -5%, respectively. Ni dopant atom in a 4×4×1 monolayer WSe2 supercell (6.25% Ni doping). Fig. 4 Total density of stats (TDOS) of Ni-doped WSe2 monolayer with different strain, the black and rad lines stand for the spin-up and spin-down respectively. Fig. 5 Spin density for a single Ni dopant atom in a 4×4×1 monolayer WSe2 supercell (6.25% Ni doping) When the strain is
0%, 2%, 10%, and -10%. Yellow and cyan
isosurfaces represent positive and negative spin densities (±0.001 e/Å3). Fig. 6 The Formation with different strain at W-rich and Se-rich.
Table 1. The calculated magnetic moment, Mtot, and the half-metallic gap, EHM gap in Ni-doped WSe2. Stra in (%) △EH
-1 0
-9
-8
---
---
---
1. 15 9
1. 59 4
-6
-4
-3
-2
0
1
2
4
6
8
10
0. 01 8 3. 23 7
0. 06 4 3. 28 0
0. 06 5 1. 66 4
---
---
---
---
---
---
---
1. 46 6
1. 66 9
1. 74 0
1. 75 4
1. 79 8
1. 77 9
1. 81 2
-5
M-gap
(eV) mag 0. neti 65 c 8 mo men t (μB)
-7
0. 04 0 1. 60 1
0. 07 7 1. 60 6
0. 08 9 1. 60 7
Highlights
Ni doping induces magnetism in the WSe2 monolayer. The doped system shows half-metal feature from -2% to -7% strain. The largest half-metallic gap is 89meV at -5% tensile strain. The biggest magnetic moment is 3.280 μBat-3% compressive strain.
PII: DOI: Reference:
S1386-9477(17)30175-3 http://dx.doi.org/10.1016/j.physe.2017.03.013 PHYSE12755
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 2 February 2017 Revised date: 17 March 2017 Accepted date: 20 March 2017 Cite this article as: Xiaomeng Liu, Xu Zhao, Qianqian Xin, Ninghua Wu, Xu Ma, Tianxing Wang and Shuyi Wei, Strong effect of compressive strain on Nidoped monolayer WSe2, Physica E: Low-dimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2017.03.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Strong effect of compressive strain on Ni-doped monolayer WSe2 Xiaomeng Liu, Xu Zhao*, Qianqian Xin, Ninghua Wu, Xu Ma, Tianxing Wang, Shuyi Wei* College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan 453007, China [email protected]
Abstract We study the effect of strain on electronic and magnetic properties of Ni-doped monolayer WSe2 by using a method of plane wave potential technique based on the density function theory. We find that Ni-doped monolayer WSe2 becomes a magnetic metal material, and the magnetic moment is 1.466 μB. When we apply different compressive strain, Ni-doped WSe2 shows half-metallic frommagnetic properties firstly. With increasing compressive strain, Ni-doped WSe2 shows magnetic metal properties from -8% to -10%. The magnetic moment also increases firstly and then decreases with increasing compressive strains, the biggest magnetic moment is 3.280 μB at -3% compressive strain and the minimum magnetic moment is 0.658μB at -10% compressive strain. While, the magnetic moment increases slowly with increasing tensile strain and comes to 1.812μB at 10% tensile strain. By comparison, we find that there is stronger effect of compressive strain on electronic and magnetic properties of this doped system than that of tensile strain. Keywords:
Transition-metal
dichalcogenides;
Electronic
band
structure;
First-principles I.
Introduction In recent years, transition-metal dichalcogenides (TMDs) show a wide variety of
interesting physical properties, especially groups of IVB, VB, and VIB have received considerable attention[1-13]. They display semiconducting, metallic, superconducting, and magnetic behavior. Nowadays the atomic and electronic structures of TMDs have
been the subject of many experimental and theoretical investigations [14-18]. In addition, the fabrication of TMDs also attracted great attention in the field of inorganic compound. Layers of TMDs were manufactured by mechanical and chemical exfoliation of their layered bulk counterparts, as well as by chemical vapor deposition [19-23]. The pronounced anisotropy of these compounds results from the formation of MX2 layers with strong covalent intralayer bonds, but only weak van der Waals coupling between adjacent layers. WSe2 is a member in the family of layered transition metal dichalcogenides (TMDs) which display a wide variety of interesting physical properties. It is known for its remarkable stability against photocorrosion[24]. WSe2 is a semiconductor with a measured indirect gap of 1.2eV thus making it useful for photovoltaic and optoelectronic applications[25]. And the electrochemical devices based on WSe2 have been reported to possess conversion efficiencies up to 17%[26]. In addition, WSe2 has recently been a subject of a number of photoemission studies[27-30]. Another puzzling property of WSe2 has recently been discovered by scanning tunneling microscopy (STM)[31,32]. A number of experimental and theoretical studies have confirmed that the substitution of TM atoms can induce magnetism in nonmagnetic nanomaterials[33,34]. As we all know pristine monolayer WSe2 is nonmagnetic, in this work, Ni atom substitute a W atom in monolayer WSe2, and the doping concentration is 6.25%. Then we study the electronic and magnetic properties of monolayer WSe2 with different strain. We find Ni dopant can induce the magnetic moment, moreover, strain can change the magnetic moment of Ni-doped WSe2 monolayer. With different strain we can obtain half-metallic material and magnetic metal, respectively, the result might have important potential application in spintronic devices. II.Theoretical models and methods Mechanical stripping method is used to obtain monolayer WSe2, the layered WSe2 consists of stacked Se–W–Se layers, upper and lower two layers of Se atoms, intermediate of W atoms, both W atom and Se atom are linked by covalent bonds, and these layers are held together by Van der Walls interaction.WSe2 layers have a
P63/mmc space group symmetry with the W atoms having a trigonal prismatic coordination with the Se atoms, which showed in Fig.1. At the same time, a Ni atom instead of a W atom in a 4×4×1 WSe2 monolayer supercell for 6.25% (one Ni atom, 15 W atoms and 32 Se atoms). Our calculations were performed within first-principles DFT using the projector augmented wave (PAW) method[35]. And electron exchange, correlation effects were described within the generalized gradient approximation (GGA)[36] respectively used within the Vienna ab initio simulation package (VASP)[37]. In all calculations, an energy cutoff of 500 eV for the plane-wave expansion of the wave functions was used. For the Brillouin zone integration, a 9×9×1 centered Monkhorst-pack-K-point mesh is used for WSe2 monolayer. And the atomic positions and cell parameters are optimized until the residual forces fall below 0.01 eV/Å. III. Results and discussion: In this work, we investigate the structural and electronic properties in Ni-doped WSe2 by applied strain from -10% to 10%. And we adopt first–principles calculations based on the density functional theory (DFT). In the work, we calculate the magnetic moment, Mtot, and half-metallic band gapΔEHM. All of them are showed in Table1. From Table 1, we can see that the magnetic moment is 1.466μB at 0% strain, and increases firstly and then decreases with increasing compressive strains, the biggest magnetic moment is 3.280 μB at -3% compressive strain and the minimum magnetic moment is 0.658μB at -10% compressive strain. While, the magnetic moment increases slowly with increasing tensile strain and comes to 1.812μB at 10% tensile strain. The doped system shows half-metallic properties under compressive strain from -2% to -7%. The biggest half-metallic gap is 89meV at -5% compressive strain. According to these calculation results, we can see that there is stronger effect of compressive strain on electronic and magnetic properties of this doped system than that of tensile strain. At first, we find that Ni doped WSe2 semiconductor transforms to magnetic metal, we give the band structure of Ni-doped system at Fig.2. From Fig. 2, we can see some impurity states within the band gap, and the spin-up and spin-down states
are not symmetric and both spin-up and spin-down crosses at the Fermi level in band structure at 0% strain. These results indicate that not only Ni-doped WSe2 show metallic feature but also Ni substituting W can induce to the magnetism in the WSe2 monolayer. In addition we also give band structures of -5% and -10% in Fig. 2, at -5% strain, a band crossing at the Fermi level occurs with little dispersion in spin-up band structures, whereas in spin-down band structures, a band gap of 0.268eV is found for the Ni-doped WSe2 monolayer, these results indicate that the Ni-doped WSe2 monolayer turn into half-metallic magnetism material. The half-metallic gap is about 89meV. At -10% strain, the band crossing at the Fermi level occurs with little dispersion in spin-up band structures and spin-down band structures, which shows the doped system turns to magnetic metal material. By comparison, we see that the band gap decreases with increasing compressive train. In order to understand the magnetic and electronic properties in more details, we investigated the projected density of states (PDOS) and total density of states(TDOS) of the Ni-doped WSe2 monolayer at 0% and -5% compressive strain, the TDOS and PDOS are further shown in Fig. 3. For the case of 0% strain, we can see the electronic structures of spin-up and spin-down channels are consisting of the hybridization of the W-5d, Se-4p and Ni-3d states near the Fermi level. Moreover, the polarized charges mainly arise from the localized 3d orbitals of the Ni atom, a little from the localized 5d orbitals of the W atom and hardly from the localized 4p orbitals of the Se atom near the Fermi level. Meanwhile, both the spin-up and spin-down states cross the Fermi level and indicates to the magnetic metal feature. For the case of -5% compressive strain, we find that the band gap of doped system decreases to 1.495eV and the impurity states move to the valence band slowly. The spin-up states keep crossing the Fermi level, while the spin-down states have a band gap near the Fermi level. Consequently, the doped system shows a half-metallic properties and the half-metallic gap is about 89meV. Fig.4 gives the total density of states (TDOS) of Ni-doped WSe2 with different strain. In the picture, we can see that the intrisic band gap decreases quickly with increasing compressive strain and there is a half-metallic gap from -2% to -7% compressive strain. The doped system turns to magnetic metal
from -8% to -10%. These results indicate that the effect of strain on the electronic structure is strong and it cannot be ignored. For understand clearly the mechanism of magnetism induced by doped atom in the Ni-doped WSe2 monolayer, Fig. 5 gives the spin density of Ni-doped WSe2. It is noted that without strain, both the first neighboring six Se atoms and second neighboring four W atoms display ferromagnetically coupled to the Ni atom. For the case of 2% tensile strain, both the first neighboring six Se atoms and second neighboring six W atoms display ferromagnetically coupled to the Ni atom. When the tensile strain comes to 10%, we only find that the second six W atoms are ferromagnetically coupled to the Ni atom, while the spin density from the nearest neighboring six Se atoms is very little. By contrast, for the case of -10% compressive strain, the couple between Ni atom and the nearest neighboring six Se atoms is obviously strong. The strong p–d hybridization was found between the 3d orbital of Ni and 4p orbital of Se. Consequently, the biaxial strain affects strongly the distribution of spin density and changes the magnetic moment of doped system. To probe the stability of the Ni-doped WSe2 monolayer, the formation energy Eform can be calculated according to the following formula [38-39] Eform = E(doped)− E(pure) + n(μW – μNi). Where E(doped) and E(pure)are the total energies of Ni-doped WSe2 monolayer with and without the Ni dopant. n is the number of W atoms replaced by Ni dopants. μW and μNi are the chemical potential for W host and Ni dopant atoms, respectively, which depends on the material growth conditions and satisfies the boundary conditions. We use the energy per atom of Ni metal as μNi. μW is defined within a range of values corresponding to W-rich or Se-rich growth conditions. Fig.6 gives the formation energy of different growth conditions, one can find easily the formation energy of Se-rich always lower than that of W-rich. Under the Se-rich growth condition, the smallest Eform of 1.714eV was found at 1% tensile strain, the largest Eform of 20.549eV was found at -10% compressive strain. Moreover, the formation energy is more than 10 eV from -6% to -10% compressive strain and from 10% tensile strain, which indicates that compressive strain has bigger effect than tensile strain in Ni-doped
monolayer WSe2. Consequently, the formation energy calculations also indicate that it is energy favorable and relatively easier to incorporate Ni atom into the WSe2 monolayer under Se-rich experimental conditions. And the formation energy of the doped system at 1% and 2% tensile strain is lower than 0% strain. IV. Conclusion In this paper, we study the electronic and magnetic properties of Ni-doped monolayer WSe2 by strain. The results show that Ni-doped WSe2 becomes magnetic metallic and magnetic moment is 1.466μB. Both tensile strain and compressive strain change the magnetic and electronic properties of Ni doped WSe2 monolayer, while there is stronger effect of compressive strain on electronic and magnetic properties of this doped system than that of tensile strain. We find that the maximum magnetic moment is 3.280μB at -3% compressive strain and minimum magnetic moment is 0.658μB at -10% strain. Ni-doped WSe2 transforms to half-metallic from magnetic metallic firstly. The biggest half-metallic gap is 89meV. With increasing compressive strain, Ni-doped WSe2 shows magnetic metal properties from -8% to -10%. The magnetic moment originates primarily from the localized 3d electrons of the Ni atom, a little from the localized 5d electrons of the W atom and hardly from the localized 4p electrons of the Se atom near the Fermi level. In addition, the formation calculation shows that the system easier to incorporate Ni atom into the WSe2 monolayer under Se-rich, and the lowest formation is found at 1% tensile strain indicating the most stable structure. These results suggest that Ni-doped WSe2 monolayer might have important potential application in spintronic devices.
This work is supported by a Grant from the National Natural Science Foundation of China (NSFC) under the Grant No. 11504092 and U1304518, National Undergraduate Training Programs for Innovation and Entrepreneurship (No. 201510476043), Science and technology research key project of education department of Henan province (No. 14A140012), and High Performance Computing Center of Henan Normal University.
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Fig. 1Top and side views of WSe2 monolayer, the green and gray balls represent Se and W atoms, respectively. Fig. 2 Spin-polarized band structure of Ni-doped WSe2 monolayer with 0%, -5% and -10%. The rad and black lines stand for the spin-up and spin-down components. The Fermi level is indicated by the solid line. Fig. 3 Total density of stats (TDOS) and partial density of states (PDOS) of Ni-doped WSe2 monolayer with isotropic strain 0% and -5%, respectively. Ni dopant atom in a 4×4×1 monolayer WSe2 supercell (6.25% Ni doping). Fig. 4 Total density of stats (TDOS) of Ni-doped WSe2 monolayer with different strain, the black and rad lines stand for the spin-up and spin-down respectively. Fig. 5 Spin density for a single Ni dopant atom in a 4×4×1 monolayer WSe2 supercell (6.25% Ni doping) When the strain is
0%, 2%, 10%, and -10%. Yellow and cyan
isosurfaces represent positive and negative spin densities (±0.001 e/Å3). Fig. 6 The Formation with different strain at W-rich and Se-rich.
Table 1. The calculated magnetic moment, Mtot, and the half-metallic gap, EHM gap in Ni-doped WSe2. Stra in (%) △EH
-1 0
-9
-8
---
---
---
1. 15 9
1. 59 4
-6
-4
-3
-2
0
1
2
4
6
8
10
0. 01 8 3. 23 7
0. 06 4 3. 28 0
0. 06 5 1. 66 4
---
---
---
---
---
---
---
1. 46 6
1. 66 9
1. 74 0
1. 75 4
1. 79 8
1. 77 9
1. 81 2
-5
M-gap
(eV) mag 0. neti 65 c 8 mo men t (μB)
-7
0. 04 0 1. 60 1
0. 07 7 1. 60 6
0. 08 9 1. 60 7
Highlights
Ni doping induces magnetism in the WSe2 monolayer. The doped system shows half-metal feature from -2% to -7% strain. The largest half-metallic gap is 89meV at -5% tensile strain. The biggest magnetic moment is 3.280 μBat-3% compressive strain.