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Tuning electronic and magnetic properties of blue phosphorene by doping Al, Si, As and Sb atom: A DFT calculation
Solid State Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Tuning electronic and magnetic properties of blue phosphorene by doping Al, Si, As and Sb atom: A DFT calculation Minglei Sun a, Yitong Hao b, Qingqiang Ren c, Yiming Zhao d, Yanhui Du a, Wencheng Tang a,n a
School of Mechanical Engineering, Southeast University, Nanjing 211189, Jiangsu, China Beijing Key Laboratory of Process Fluid Filtration and Separation, University of Petroleum, Beijing 100083, China c State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Hunan, China d Laboratory for Laser Energetics, University of Rochester, 250 E. River Road, Rochester, NY 14623-1299, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 20 February 2016 Accepted 24 April 2016
Using density functional theory computations, we systematically investigated the structural, electronic and magnetic properties of Al, Si, As and Sb doped blue phosphorene. The electronic properties of blue phosphorene can be effectively turned by substitutional doping. Especially, Al and Sb lead to an indirectto-direct-gap transition. The interaction between the impurity and P atoms should be responsible for the transition. In addition, blue phosphorene can exhibit dilute magnetic semiconductor property with doping of Si impurity. The magnetic moment in Si-substituted blue phosphorene predominantly originates from the hybridization of Si-s pz and P-pz orbitals. These results provide many useful applications of blue phosphorene in electronics, optoelectronics and spintronics. & 2016 Published by Elsevier Ltd.
Keywords: Blue phosphorene Doping Electronic properties Dilute magnetic semiconductors
1. Introduction Graphene, the perfect two-dimensional (2D) layers of sp2 bonded carbon [1,2], has attracted tremendous attention due to its excellent properties [3–9]. Such planar structure is not limited to carbon, for example, phosphorene have been mechanically exfoliated [10] into monolayer and well investigated [11,12]. Unlike graphene, phosphorene has an direct bandgap (1.51 eV) [11] while still maintaining a high carrier mobility [10–12], which would facilitate the application in optoelectronics [13]. Interestingly, many different layered phosphorene allotropes [14–16] have been predicted by ab initio density functional calculations. They are nearly as stable as the related black phosphorene structure but exhibit very different electronic properties [16]. Blue phosphorene is one of the allotrope which shares layered structure and high stability with the black phosphorus [14]. Still, it displays a sizable fundamental indirect band gap. Because of the weak interlayer interaction, bulk blue phosphorus should exfoliate easily to form quasi-2D structures for potential electronic applications [14]. However, at present, some interesting issues still need to be addressed for this novel material. For example, a direct band gap of semiconductor is always vital for the optoelectronics n
Corresponding author. E-mail address: [email protected] (W. Tang).
applications. Can we effectively tailor the electronics properties of blue phosphorene by substitutional doping? Besides, Hashmi et al. [17] proposed that the transition metal doped phosphorene layer can have dilute magnetic semiconductor (DMS) properties. Sun et al. [18] demonstrated the C- and O-substituted blue phosphorene were promising systems to explore two-dimensional diluted magnetic semiconductors. More recently, Yang et al. [19] found black phosphorene exhibit DMS properties for O doping. Thus, could the blue phosphorene layer also present DMS properties with doping effect? Here we carried out Al, Si, As and Sb substitutional doping of blue phosphorene. The geometry, electronic structure and magnetic properties are given in details. And the underlying mechanism is analyzed.
2. Methods The calculations were performed by means of VASP (Vienna Ab initio Simulation Package) code [20–22]. The code has already been used successfully for cluster [23,24] and surface [25–29] systems. The ion–electron interaction is described with the projector augmented wave (PAW) method [30]. The exchange–correlation potential is approximated by the generalized gradient approximation (GGA) using the Perdew, Burke, and Ernzernhof (PBE) functional [31]. A kinetic energy cutoff of 400 eV was used for valence electron wave functions. A
http://dx.doi.org/10.1016/j.ssc.2016.04.019 0038-1098/& 2016 Published by Elsevier Ltd.
Please cite this article as: M. Sun, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.019i
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3 3 1 supercell has been used to mimic the isolated sheet and the vacuum spacing is larger than 20 Å along the perpendicular direction to avoid interlayer interactions. The Monkhorst–Pack scheme [32] of 9 9 1 k points were used for sampling the Brillouin zone (BZ). Geometry optimizations were performed by using the conjugated gradient method, and the atomic positions are fully relaxed until an energy convergence of 10 6 eV and a force convergence of 0.01 eV/Å is achieved. All calculations were performed in the spin unrestricted manner and all the parameters in the calculations are carefully tested.
3. Results and discussion We first studied the structural and electronic properties of pristine blue phosphorene. Akin to silicene [33–35] and germanene
Fig. 1. (Color online) Top (above panel) and side (down panel) views of pristine blue phosphorene.
[35,36], blue phosphorene favors the buckled honeycomb structure (See upper panel of Fig. 1). The obtained lattice constant is 3.28 Å. The buckling height (h) in blue phosphorene is 1.23 Å (See down panel of Fig. 1). These obtained structural parameters are in excellent agreement with previous results [14,37,38]. Fig. 2a presents the band structure of pristine blue phosphorene. Clearly, the pristine phosphorene is an indirect gap semiconductor with a band gap of 2 eV calculated using PBE, with valence band maximum (VBM) at the middle region along the M– K line and conduction band minimum (CBM) at the middle region along the Γ–M line, which is in good agreement previous results [14,37,38]. As shown in Fig. 2b, the obtained density of states (DOS) of blue phosphorene also agrees well with previous results [14,37,38]. Projected density of states (PDOS) reveals that the VBM originates from pz orbital while the CBM is mostly built from the s and p orbitals. For all impurity-substituted systems, we find the impurities prefer to occupy an almost perfect symmetric configuration with C3v symmetry after structural relaxation (See Fig. 3a and b). The length of Impurity-P bonds are generally range from 2.254 to 2.552 Å (See Fig. 3c). It can be deduced that all impurities form covalent bond with three nearest P atoms. The height of impurity atoms generally ranges from 0.017 to 1.722 Å, varying with different impurity atoms (See Fig. 3d). Next, we investigate stability of the substituional systems by calculated binding energy. The binding energies ðEb Þ, calculated as a difference between the energy of a blue phosphorene sheet with a single vacancy plus energy of the isolated atom and the substitutional system, are presented in Fig. 4. The typical value of Eb is approximately 5 eV. As a comparison, almost computed with same method, the typical values of Eb of transition-metal (TM) atoms embedded in single vacancy in a graphene sheet are about 7 eV [39–41]. Moreover, the TM atoms substituted graphene has been experimentally achieved by many groups [42–44]. Noted that the Eb in Al-, Si-, As- and Sb-substituted systems are almost equal to this value. These systems is rather stable and can be artificially fabricated.
Fig. 2. (Color online) (a) Band structure, (b) total density of states (TDOS) and projected density of states (PDOS) for the pristine blue phosphorene. The Fermi level has been set to zero and indicated by cyan dashed line.
Please cite this article as: M. Sun, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.019i
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Fig. 3. (Color online) The (local) optimized geometry of the Al-, Si-, As- and Sb-substituted blue phosphorene: (a) top view and (b) side view, (c) The length of Impurity P bond, (d) The height of impurity atoms.
Fig. 4. (Color online) Binding energies of the Al-, Si-, As- and Sb-substituted doped blue phosphorene.
The substitutional doping is a powerful tool to tailor the electronic structure of 2D materials. For example, by doping with nitrogen and boron atoms, a sizable gap can be opened in the band structure of graphene nanosheet [45–48]. TM-doped MoS2 has shown dilute magnetic semiconductor behaviors [49,50]. Can we tailor the electronic structure of blue phosphorene by this tool? Thus, we computed the band structure of Al-, Si-, As- and Sbsubstituted systems, as shown in Fig. 5. For the Al-substituted system (Fig. 5a), the doping of Al impurity leads to an indirect– direct band gap transition. The VBM and CBM all located at the Γ point in BZ. The value of the band gap is approximately 1.59 eV. For As-substituted system (Fig. 5c), the indirect band gap feature remains unchanged and the gap size is calculated to be approximately 1.85 eV, which is quite similar with the pristine blue phosphorene. However, for Sb-substituted system (Fig. 5d), the doping of Sb impurity leads to an indirect–direct band gap transition again like Al impurity. It is a direct semiconductor with a
bandgap of approximately 1.54 eV. The VBM and CBM all located at the Γ point in BZ. Our computations demonstrated that the substitutional doping of Al, Si, As and Sb has a significant impact on the electronic properties of blue phosphorene. With the doping of Al and Sb impurities, the band gap of blue phosphorene undergo indirect-todirect-gap transition. What's the underlying mechanism for this transition? We performed the DOS and PDOS calculations (Fig. 6a and b) to investigate the shift of VBM and CBM in Al- and Sbsubstituted systems. For Al-substituted system (Fig. 6a), the VBM is formed by the hybridization of Al-px py and P-s p orbitals, while the CBM is mainly formed by the Al-s pz. For Sb-substituted system (Fig. 6b), the VBM is mostly originates from the Sb-px py orbitals while the CBM built from Sb-px py and P-s p orbitals. As abovementioned, in pristine blue phosphorene, the VBM originates from pz orbital while the CBM is mostly built from the s and p orbitals. However, the components of VBM and CBM of Al- and Sbsubstituted blue phosphorene are very different from the pristine blue phosphorene. Therefore, we conclude that the interaction between the impurity and P atoms should be responsible for the indirect-to-direct-gap transition in Al and Sb doped blue phosphorene. In the case of Si-substituted system (Fig. 5b), the Si doping introduces two impurity states near the Fermi level. The spin-up and spin-down channels are not degenerated indicating a magnetic character. Indeed, a magnetic moment of 1 μB was founded in Si-substituted system, which is consistent with the previous studies [51]. This is because the Si atom has for valence electrons.
Please cite this article as: M. Sun, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.019i
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Fig. 5. (Color online) Band structure of (a) Al-, (b) Si-, (c) As- and (d) Sb-substituted system. The Fermi level has been set to zero and indicated by cyan dashed line.
Fig. 6. (Color online) Spin-polarized DOS and PDOS of (a) Al-, (b) Sb- and (c) Si-substituted system. The Fermi level has been set to zero and indicated by vertical cyan dashed line.
Three of them form the Si–P covalent bonds with three undercoordinated P atoms, while the other one valence electrons is unsaturated, inducing a magnetic moments of 1 μB. Moreover, we observe narrow band gaps about 0.69 eV. Consequently, Sisubstituted system is a diluted magnetic semiconductor and may have potential application in spintronics [52]. What is the underlying mechanism for these phenomenon? We also explore the two highly localized states near the Fermi level and the origin of the magnetic moment in the Si-substituted system. Fig. 6c shows the PDOS of Si-substituted system, we can clearly see that the two highly localized states (corresponding to the two peaks in DOS) mainly come from the hybridization of Si-s pz and P-pz orbitals. This is also the origin of the magnetic moment in the Sisubstituted system.
4. Conclusion To summarize, by means of DFT computations, we systematically investigated the structural, electronic and magnetic properties of Al, Si, As and Sb doped blue phosphorene. All the impurities are covalently bonded to single vacancy in a blue phosphorene sheet. We demonstrated for the first time an transition of the indirect-to-direct-gap in blue phosphorene by substitutional doping of Al and Sb. The interaction between the impurity and P atoms should responsible for the indirect-to-direct-gap transition. Besides, with the doping of Si, the blue phosphorene can exhibit dilute magnetic semiconductor property. Detailed analysis shows that the magnetic moment comes from the hybridization of Si-s pz and P-pz orbitals. Overall, our results have potential applications in electronics, optoelectronics and spintronics.
Please cite this article as: M. Sun, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.019i
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Acknowledgments The authors would like to thank for the continuous funding support of the National Science and Technology Major Project of the Ministry of Science and Technology of China (2012ZX04002012). Fig. 1 was generated using the VESTA program [53].
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Please cite this article as: M. Sun, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.019i
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Tuning electronic and magnetic properties of blue phosphorene by doping Al, Si, As and Sb atom: A DFT calculation Minglei Sun a, Yitong Hao b, Qingqiang Ren c, Yiming Zhao d, Yanhui Du a, Wencheng Tang a,n a
School of Mechanical Engineering, Southeast University, Nanjing 211189, Jiangsu, China Beijing Key Laboratory of Process Fluid Filtration and Separation, University of Petroleum, Beijing 100083, China c State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Hunan, China d Laboratory for Laser Energetics, University of Rochester, 250 E. River Road, Rochester, NY 14623-1299, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 August 2015 Received in revised form 20 February 2016 Accepted 24 April 2016
Using density functional theory computations, we systematically investigated the structural, electronic and magnetic properties of Al, Si, As and Sb doped blue phosphorene. The electronic properties of blue phosphorene can be effectively turned by substitutional doping. Especially, Al and Sb lead to an indirectto-direct-gap transition. The interaction between the impurity and P atoms should be responsible for the transition. In addition, blue phosphorene can exhibit dilute magnetic semiconductor property with doping of Si impurity. The magnetic moment in Si-substituted blue phosphorene predominantly originates from the hybridization of Si-s pz and P-pz orbitals. These results provide many useful applications of blue phosphorene in electronics, optoelectronics and spintronics. & 2016 Published by Elsevier Ltd.
Keywords: Blue phosphorene Doping Electronic properties Dilute magnetic semiconductors
1. Introduction Graphene, the perfect two-dimensional (2D) layers of sp2 bonded carbon [1,2], has attracted tremendous attention due to its excellent properties [3–9]. Such planar structure is not limited to carbon, for example, phosphorene have been mechanically exfoliated [10] into monolayer and well investigated [11,12]. Unlike graphene, phosphorene has an direct bandgap (1.51 eV) [11] while still maintaining a high carrier mobility [10–12], which would facilitate the application in optoelectronics [13]. Interestingly, many different layered phosphorene allotropes [14–16] have been predicted by ab initio density functional calculations. They are nearly as stable as the related black phosphorene structure but exhibit very different electronic properties [16]. Blue phosphorene is one of the allotrope which shares layered structure and high stability with the black phosphorus [14]. Still, it displays a sizable fundamental indirect band gap. Because of the weak interlayer interaction, bulk blue phosphorus should exfoliate easily to form quasi-2D structures for potential electronic applications [14]. However, at present, some interesting issues still need to be addressed for this novel material. For example, a direct band gap of semiconductor is always vital for the optoelectronics n
Corresponding author. E-mail address: [email protected] (W. Tang).
applications. Can we effectively tailor the electronics properties of blue phosphorene by substitutional doping? Besides, Hashmi et al. [17] proposed that the transition metal doped phosphorene layer can have dilute magnetic semiconductor (DMS) properties. Sun et al. [18] demonstrated the C- and O-substituted blue phosphorene were promising systems to explore two-dimensional diluted magnetic semiconductors. More recently, Yang et al. [19] found black phosphorene exhibit DMS properties for O doping. Thus, could the blue phosphorene layer also present DMS properties with doping effect? Here we carried out Al, Si, As and Sb substitutional doping of blue phosphorene. The geometry, electronic structure and magnetic properties are given in details. And the underlying mechanism is analyzed.
2. Methods The calculations were performed by means of VASP (Vienna Ab initio Simulation Package) code [20–22]. The code has already been used successfully for cluster [23,24] and surface [25–29] systems. The ion–electron interaction is described with the projector augmented wave (PAW) method [30]. The exchange–correlation potential is approximated by the generalized gradient approximation (GGA) using the Perdew, Burke, and Ernzernhof (PBE) functional [31]. A kinetic energy cutoff of 400 eV was used for valence electron wave functions. A
http://dx.doi.org/10.1016/j.ssc.2016.04.019 0038-1098/& 2016 Published by Elsevier Ltd.
Please cite this article as: M. Sun, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.019i
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3 3 1 supercell has been used to mimic the isolated sheet and the vacuum spacing is larger than 20 Å along the perpendicular direction to avoid interlayer interactions. The Monkhorst–Pack scheme [32] of 9 9 1 k points were used for sampling the Brillouin zone (BZ). Geometry optimizations were performed by using the conjugated gradient method, and the atomic positions are fully relaxed until an energy convergence of 10 6 eV and a force convergence of 0.01 eV/Å is achieved. All calculations were performed in the spin unrestricted manner and all the parameters in the calculations are carefully tested.
3. Results and discussion We first studied the structural and electronic properties of pristine blue phosphorene. Akin to silicene [33–35] and germanene
Fig. 1. (Color online) Top (above panel) and side (down panel) views of pristine blue phosphorene.
[35,36], blue phosphorene favors the buckled honeycomb structure (See upper panel of Fig. 1). The obtained lattice constant is 3.28 Å. The buckling height (h) in blue phosphorene is 1.23 Å (See down panel of Fig. 1). These obtained structural parameters are in excellent agreement with previous results [14,37,38]. Fig. 2a presents the band structure of pristine blue phosphorene. Clearly, the pristine phosphorene is an indirect gap semiconductor with a band gap of 2 eV calculated using PBE, with valence band maximum (VBM) at the middle region along the M– K line and conduction band minimum (CBM) at the middle region along the Γ–M line, which is in good agreement previous results [14,37,38]. As shown in Fig. 2b, the obtained density of states (DOS) of blue phosphorene also agrees well with previous results [14,37,38]. Projected density of states (PDOS) reveals that the VBM originates from pz orbital while the CBM is mostly built from the s and p orbitals. For all impurity-substituted systems, we find the impurities prefer to occupy an almost perfect symmetric configuration with C3v symmetry after structural relaxation (See Fig. 3a and b). The length of Impurity-P bonds are generally range from 2.254 to 2.552 Å (See Fig. 3c). It can be deduced that all impurities form covalent bond with three nearest P atoms. The height of impurity atoms generally ranges from 0.017 to 1.722 Å, varying with different impurity atoms (See Fig. 3d). Next, we investigate stability of the substituional systems by calculated binding energy. The binding energies ðEb Þ, calculated as a difference between the energy of a blue phosphorene sheet with a single vacancy plus energy of the isolated atom and the substitutional system, are presented in Fig. 4. The typical value of Eb is approximately 5 eV. As a comparison, almost computed with same method, the typical values of Eb of transition-metal (TM) atoms embedded in single vacancy in a graphene sheet are about 7 eV [39–41]. Moreover, the TM atoms substituted graphene has been experimentally achieved by many groups [42–44]. Noted that the Eb in Al-, Si-, As- and Sb-substituted systems are almost equal to this value. These systems is rather stable and can be artificially fabricated.
Fig. 2. (Color online) (a) Band structure, (b) total density of states (TDOS) and projected density of states (PDOS) for the pristine blue phosphorene. The Fermi level has been set to zero and indicated by cyan dashed line.
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Fig. 3. (Color online) The (local) optimized geometry of the Al-, Si-, As- and Sb-substituted blue phosphorene: (a) top view and (b) side view, (c) The length of Impurity P bond, (d) The height of impurity atoms.
Fig. 4. (Color online) Binding energies of the Al-, Si-, As- and Sb-substituted doped blue phosphorene.
The substitutional doping is a powerful tool to tailor the electronic structure of 2D materials. For example, by doping with nitrogen and boron atoms, a sizable gap can be opened in the band structure of graphene nanosheet [45–48]. TM-doped MoS2 has shown dilute magnetic semiconductor behaviors [49,50]. Can we tailor the electronic structure of blue phosphorene by this tool? Thus, we computed the band structure of Al-, Si-, As- and Sbsubstituted systems, as shown in Fig. 5. For the Al-substituted system (Fig. 5a), the doping of Al impurity leads to an indirect– direct band gap transition. The VBM and CBM all located at the Γ point in BZ. The value of the band gap is approximately 1.59 eV. For As-substituted system (Fig. 5c), the indirect band gap feature remains unchanged and the gap size is calculated to be approximately 1.85 eV, which is quite similar with the pristine blue phosphorene. However, for Sb-substituted system (Fig. 5d), the doping of Sb impurity leads to an indirect–direct band gap transition again like Al impurity. It is a direct semiconductor with a
bandgap of approximately 1.54 eV. The VBM and CBM all located at the Γ point in BZ. Our computations demonstrated that the substitutional doping of Al, Si, As and Sb has a significant impact on the electronic properties of blue phosphorene. With the doping of Al and Sb impurities, the band gap of blue phosphorene undergo indirect-todirect-gap transition. What's the underlying mechanism for this transition? We performed the DOS and PDOS calculations (Fig. 6a and b) to investigate the shift of VBM and CBM in Al- and Sbsubstituted systems. For Al-substituted system (Fig. 6a), the VBM is formed by the hybridization of Al-px py and P-s p orbitals, while the CBM is mainly formed by the Al-s pz. For Sb-substituted system (Fig. 6b), the VBM is mostly originates from the Sb-px py orbitals while the CBM built from Sb-px py and P-s p orbitals. As abovementioned, in pristine blue phosphorene, the VBM originates from pz orbital while the CBM is mostly built from the s and p orbitals. However, the components of VBM and CBM of Al- and Sbsubstituted blue phosphorene are very different from the pristine blue phosphorene. Therefore, we conclude that the interaction between the impurity and P atoms should be responsible for the indirect-to-direct-gap transition in Al and Sb doped blue phosphorene. In the case of Si-substituted system (Fig. 5b), the Si doping introduces two impurity states near the Fermi level. The spin-up and spin-down channels are not degenerated indicating a magnetic character. Indeed, a magnetic moment of 1 μB was founded in Si-substituted system, which is consistent with the previous studies [51]. This is because the Si atom has for valence electrons.
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Fig. 5. (Color online) Band structure of (a) Al-, (b) Si-, (c) As- and (d) Sb-substituted system. The Fermi level has been set to zero and indicated by cyan dashed line.
Fig. 6. (Color online) Spin-polarized DOS and PDOS of (a) Al-, (b) Sb- and (c) Si-substituted system. The Fermi level has been set to zero and indicated by vertical cyan dashed line.
Three of them form the Si–P covalent bonds with three undercoordinated P atoms, while the other one valence electrons is unsaturated, inducing a magnetic moments of 1 μB. Moreover, we observe narrow band gaps about 0.69 eV. Consequently, Sisubstituted system is a diluted magnetic semiconductor and may have potential application in spintronics [52]. What is the underlying mechanism for these phenomenon? We also explore the two highly localized states near the Fermi level and the origin of the magnetic moment in the Si-substituted system. Fig. 6c shows the PDOS of Si-substituted system, we can clearly see that the two highly localized states (corresponding to the two peaks in DOS) mainly come from the hybridization of Si-s pz and P-pz orbitals. This is also the origin of the magnetic moment in the Sisubstituted system.
4. Conclusion To summarize, by means of DFT computations, we systematically investigated the structural, electronic and magnetic properties of Al, Si, As and Sb doped blue phosphorene. All the impurities are covalently bonded to single vacancy in a blue phosphorene sheet. We demonstrated for the first time an transition of the indirect-to-direct-gap in blue phosphorene by substitutional doping of Al and Sb. The interaction between the impurity and P atoms should responsible for the indirect-to-direct-gap transition. Besides, with the doping of Si, the blue phosphorene can exhibit dilute magnetic semiconductor property. Detailed analysis shows that the magnetic moment comes from the hybridization of Si-s pz and P-pz orbitals. Overall, our results have potential applications in electronics, optoelectronics and spintronics.
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Acknowledgments The authors would like to thank for the continuous funding support of the National Science and Technology Major Project of the Ministry of Science and Technology of China (2012ZX04002012). Fig. 1 was generated using the VESTA program [53].
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