A first-principles study of group IV and VI atoms doped blue phosphorene

Accepted Manuscript A first-principles study of group IV and VI atoms doped blue phosphorene Ruimin Bai, Zheng Chen, Manman Gou, Yixin Zhang PII:

S0038-1098(17)30393-9

DOI:

10.1016/j.ssc.2017.11.020

Reference:

SSC 13333

To appear in:

Solid State Communications

Received Date: 16 October 2017 Accepted Date: 30 November 2017

Please cite this article as: R. Bai, Z. Chen, M. Gou, Y. Zhang, A first-principles study of group IV and VI atoms doped blue phosphorene, Solid State Communications (2018), doi: 10.1016/j.ssc.2017.11.020. 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 proof before it is published in its final 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.

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A first-principles study of group IV and VI atoms doped blue phosphorene Ruimin Bai*, Zheng Chen, Manman Gou, Yixin Zhang State Key Laboratory of Solidification Processing, School of Materials Science and Engineering,

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Northwestern Polytechnical University, Xi’an 710072, PR China

*Corresponding author. Tel.: 15309276686; E-mail address: [email protected]

Abstract

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Using first-principles calculations, we have systematically investigated the structural, electronic and magnetic properties of blue phosphorene doped by group IV

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and VI atoms, including C, Si, Ge, Sn, O, S, Se and Te. All the doped systems are energetically stable. Only C, Si, Ge and O-substituted systems show the characteristics of spin polarization and the magnetic moments are all 1.0 µB. Moreover, we found that C, Si, Ge and O doped systems are indirect bandgap semiconductors, while Sn, S, Se and Te doped systems present metallic property. These results show

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that blue phosphorene can be used prospectively in optoelectronic and spintronic

Keywords: A. blue phosphorene; D. magnetism; E. first-principles; E. doping

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1. Introduction

Tow-dimensional (2D) materials has attracted tremendous attention since the successful preparation of graphene[1]. Most of them have been carried out extensive

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studies, such as graphene[2], MoS2[3], boron nitride[4], silicene[5], etc. Recently, few layers black phosphorus[6-9], so-called phosphorene, has been successfully synthesized through mechanically exfoliating from the black phosphorus[6, 7, 9, 10], which draw researchers attention to 2D phosphorus structures. Contrast to graphene, phosphorene is a direct bandgap semiconductor with a band gap of 1.51 eV[11], and it still maintain a high carrier mobility[6, 7, 11]. Previous theoretical research finds there are more than one structure of two-dimensional P[12, 13]. Among these structures, blue phosphorene is the most stable structure and shows unique electronic properties [14].

ACCEPTED MANUSCRIPT Very recently, blue phosphorene has been practically prepared through the molecular beam expitaxial growth by Zhang et al[15, 16]. Blue phosphorene shares high stability with black phosphorene and has puckered honeycomb layered structure, which is more similar to graphene[14]. Furthermore, it exhibits a sizable fundamental

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band gap about 2 eV[7], which suggests that blue phosphorene has great potential in applications in nanoscale devices. However, the magnetism of blue phosphorene are still worth investigating. The substitutional doping is an effective and realizable approach to produce magnetism in the material of nonmagnetic ground state[17, 18].

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There are certain studies have induced magnetism in nonmagnetic 2D materials by substitutional doping atoms[19-21]. For example, it was reported that Ti, Cr, and Mn

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doped black phosphorene are all spin polarized semiconductor[19]. Besides, Zheng et al. demonstrated that the magnetism are observed in blue phosphprene doped by C, Si and O with a magnetic moments about 1.0 µB[20]. Moreover, Sun et al. proposed that dilute magnetic semiconductor can be realized in the blue phosphorene doped by Si atom[21].

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In this paper, we implement the calculation of the geometric structure, electronic and magnetic properties of blue phosphorene doped by group IV and VI atoms through first-principles methods. And we analyze the internal mechanisms that cause

2. Methods

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these attributes.

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First-principles calculations based on density functional theory (DFT) were implemented in our calculations by using the Vienna Ab-initio Stimulation Package (VASP)[22-24].The generalized gradient approximation (GGA) is adopted for exchange-correlation functional using the Perdew, Burke, and Ernzerhof (PBE) functional[25]. The projector augmented wave (PAW) potentials are used to describe the ion-electron interaction[26] with a cutoff energy of 400eV. The doped system is modeled in a 3×3×1 supercell (contains 18 atoms) as shown in Fig. 1, and the vacuum space is about 20 Å in the z direction. A set of 9×9×1 Monkhorst-Pack[27] k points were employed for the sample of Brillouin zone (BZ). The geometry was sufficiently

ACCEPTED MANUSCRIPT relaxed until the energy and force converge to 10-5 eV and 0.02 eV/Å, respectively. Our calculations were all carried out under the spin-polarization approach with an initial magnetic moment of 1.0 µB and the parameters we used have been seriously tested. In our calculations, the formation energy Ef of doped systems is defined as

Ef = Etot − Epri − ( µ X − µ P )

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follows[20]:

where Ef is the formation energy of impurity-doped phosphorene, Etot and Epri

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are the total energy of doped and pristine blue phosphorene, respectively. µ X and

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µ P are the chemical potentials of the dopant X ( C, Si, Ge, Sn, O, S, Se, Te ) and phosphorus atom, respectively. The energy of a single atom in blue phosphorene is used as µ P . And the energy of an atom in bulk X or diatomic X2 molecule is adopted

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as µ X .

Fig. 1. Top (a) and side (b) view of the structure of blue phosphorene. P1, P2, P3and 0 are the three nearest P atoms of dopant atoms and the doping position, respectively.

3. Results and discussion

ACCEPTED MANUSCRIPT We first studied the structures of pristine and doped blue phosphorene. Fig. 1 depicts the optimized structure of pristine blue phosphorene. As we can see, the pristine blue phosphorene shows the puckered honeycomb layered structure. The obtained lattice constant is 3.28 Å (a1=a2=3.28 Å). The buckling height (h) and bond

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length of blue phosphorene are 1.24 Å and 2.26 Å, respectively. These parameters are in good conformity with the previous works[28-32]. Moreover, the blue phosphorene is an indirect bandgap semiconductor with a wide band gap (1.93 eV), which the valence band maximum (VBM) is located at the middle region of M-K line and the

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conduction band minimum (CBM) is situated between the Γ and M points. The results we obtained are very consistent with the results reported earlier[14, 29, 32]. For doped

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systems, the P atom is substituted by the dopant X at site 0 in the supercell, which is represented as P17X (see in Fig. 1(a)), and P1, P2 and P3 are the three nearest P atoms of dopant atoms. To prove the stability of doped systems, the formation energy Ef was calculated and the results are listed in Table 1. Seeing from Table 1, the formation energy of doped systems varies from -2.35 eV to 2.35 eV, and all of the values are

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little positive or negative value, which indicates the doped systems are all energetically favorable. Similarly, the formation energy of nonmetal atoms doped phosphorene were also reported by Zheng et al[20]. Besides, we also calculated the

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total magnetic moment of the various doped systems (as shown in the last column of Table 1). It is clear that only the doping of C, Si, Ge and O atom lead to

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spin-polarization in corresponding doped system with a magnetic moment of 1.00 µB. Therefore, the magnetization in the nonmagnetic blue phosphorene was accomplished by the doping of C, Si, Ge and O atoms. Table 1

The total energy of pristine phosphorene and doped systems, the formation energy (Ef) of impurity-doped phosphorene, and the magnetic moment of all systems. System

Total energy (eV)

Ef (eV)

Magnetic moment (µB)

pristine phosphorene P17C P17Si

-96.54 -98.24 -95.59

— -2.35 -0.13

0.00 1.00 1.00

ACCEPTED MANUSCRIPT -94.57 -93.77 -96.71 -94.35 -93.53 -92.94

0.82 1.51 -0.55 1.17 1.87 2.35

1.00 0.00 1.00 0.00 0.00 0.00

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P17Ge P17Sn P17O P17S P17Se P17Te

Fig. 2. Differential charge density of C (a), Si (b), Ge (c), O (d) -substituted system and pristine blue phosphorene (e). The red and blue represent the electron accumulation and depletion,

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respectively. The units of color scale is e/ Å3. The blue balls indicate the doping site (0) and the three nearest P atoms of dopant atoms.

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Then we draw the differential charge density of doped systems to study the bond between atoms and the magnetic mechanism in C, Si, Ge and O doped blue phosphorene, as shown in Fig. 2. The charge density difference of pristine blue phosphorene (Fig. 2 (e)) shows that there are three covalent bonds around each P atom, which indicate that the valence electrons of every P atom are all saturated in pristine blue phosphorene. Therefore, the blue phosphorene is essentially free of magnetism. From Fig. 2 (a), (b) and (c), we can see that the doping of C (Si and Ge) didn’t break the 3-fold symmetry of blue phosphorene, and C (Si and Ge) all form three covalent bonds with their three nearest P atoms (P1, P2 and P3). Thus, all the five valence

ACCEPTED MANUSCRIPT electrons of P1, P2 and P3 are paired, while one residual electron of C (Si and Ge) are unpaired, which is the origin of magnetic moment in C (Si and Ge) -substituted system. Different from C, Si and Ge doped systems, O-substituted system break the symmetry of blue phosphorene because O atom in P17O system move closer to P1, P3

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atom and keep away from P2 atom, as shown in Fig. 2 (d). It is clear that the charge only accumulate between O and P1, P3, so that there are only two covalent bonds of O-P1 and O-P3 in P17O, which lead one valence electron of P2 unsaturation. Thus, O doped system shows magnetic property.

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Fig. 3(a)-(h) shows the band structures of group IV and VI atoms doped phosphorene, respectively. The spin-up bands and spin-down bands are represented by

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black solid lines and red dotted lines, respectively. For the group IV atoms doped systems (Fig. 3(a)-(d)), all the systems are indirect bandgap semiconductors except Sn-substituted system, which displayed metallic property.

Obviously, the band

structures of these systems (C, Si and Ge doped systems) show the split of the spin-up and spin-down bands, which indicate the ground states of these systems all possess

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magnetism. Simultaneously, the band gap we obtained are approximately 0.17 eV, 0.74 eV and 0.49 eV in P17C, P17Si and P17Ge, respectively. For the group VI atoms doped systems (Fig. 3(e)-(h)), there are some differences with the group IV atoms

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doped systems. Only O-substituted system is semiconductor with an indirect band gap of 0.24 eV. The band structure of O doped system (Fig. 3(e)) shows the split of the

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spin- up and spin-down bands because of the magnetization. Therefore, O-substituted system is also a magnetic structure. In addition, for the cases of S, Se and Te doped systems, we can see from Fig. 3(f)-(h) that impurity states across the Fermi level, which reveals these systems all present metallic property. Specially, the spin-up and spin-down bands of S doped system shows rather small separation, so that there are very slight magnetism in S doped system. In brief, our results manifest that P17C, P17Si, P17Ge and P17O are all magnetic semiconductors with an indirect band gap. This property enable them to have a widely application in potential spintronics. For Sn, S, Se and Te-substituted systems, they all present metallic property.

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Fig. 3. The band structures of C (a), Si (b), Ge (c), Sn (d), O (e), S (f), Se (g), Te (h)-substituted system. The spin-up bands and spin-down bands are represented by black solid lines and red dotted lines, respectively. The energy zero is taken as the Fermi level and indicated by gray dashed line.

ACCEPTED MANUSCRIPT Next, we implemented the density of states (DOS) and projected density of states (PDOS) calculations to analyze the electronic and magnetic properties of doped systems, as shown in Fig. 4(a)-(e). The TDOS of C, Si, Ge, O-substituted systems all show that the spin-up and spin-down density of states are asymmetric near the Fermi

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level, which manifests the C, Si, Ge and O-substituted systems all possess magnetism. The PDOS of C doped system (Fig. 4(a)) show that the unpaired electron of C atom occupy the pz orbitals of C and partly occupy the s and pz orbitals of P1, P2 and P3 atoms (as shown in Fig. 1). Therefore, the root of the magnetic moment in C doped

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system mainly induced by dopant C and partially come from the three nearest P atoms of C atom. Analogously, for Si and Ge doped systems, their PDOS analyses (Fig. 4(b)

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and Fig. 4(c)) show that the two occupied states of Si and Ge doped systems are formed primarily by the hybridization of Si (Ge)-s pz, P1, P2-px pz, and P3-py pz orbitals. This demonstrate that the magnetism of Si and Ge doped systems are induced by Si (Ge) atom and P1, P2, P3 atom near the dopant Si and Ge. Fig. 4(d) shows the PDOS of O-substituted system, it is clear that the main source of magnetism in O

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doped system date from px orbital of P2, and there are a little contribution comes from O-px and P2-py pz orbitals. Obviously, P2 atom is the primary origin of the magnetic moment in O doped system. Interestingly, the DOS of S-substituted system (Fig. 4(e))

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shows very small asymmetry, which indicate dopant S induced a very feeble magnetism in doped system although the magnetic moment is 0.0 µB (as shown in Table 1). These results is in good accordance with the results of band structures (Fig.

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3) and magnetic moment (Table 1).

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Fig. 4. The total density of states (TDOS) and projected density of states (PDOS) of C (a), Si (b), Ge (c), O (d) and S (e) doped system. The energy zero is set as Fermi level and indicated by gray dashed line.

Fig. 5. The spin density distribution of C (a), Si (b), Ge (c) and O (d) doped systems. The yellow and blue region represent spin-up and spin-down density, respectively. The value of isosurfaces is 0.01 Å.

ACCEPTED MANUSCRIPT In order to explain the intrinsic mechanism of the magnetic properties of P17C, P17Si, P17Ge and P17O vividly, we present the spin density distribution of these systems, as shown in Fig. 5 ((a)-(d)). It is obvious that the spin density of C, Si and Ge-substituted systems mainly distribute at the periphery of dopant atoms (see in Fig.

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5 ((a)-(c)), and constituted by the pz shapes of dopant. Additionally, the three nearest P atoms (P1, P2, P3, see in Fig. 1) have also certain contributions to the spin density. Compared with P17C, P17Si and P17Ge, the spin density distribution of P17O principally around the P2 atom, and very few of spin density arise from O atom, as

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shown in Fig. 5 (d). We can see that all P atoms have three bonds, but O is not bonded with the P2 atom. As there are only two valence electrons, so that O has only two

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covalent bonds of O-P1 and O-P2 bonds, which leads one unpaired valence electron of P2 are left. Therefore, the magnetism in O doped system are observed. All of the above results are consistent with the results of the projected density of states (PDOS) (see in Fig. 4). 4. Conclusions

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In summary, we have carried out the calculation of the geometric structures, electronic and magnetic properties of the pristine blue phosphorene and group IV and VI atoms (C, Si, Ge, Sn, O, S, Se, Te) doped phosphorene systems through

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first-principles calculations. The calculations of formation energy show that all the doped systems are stable. Only C, Si, Ge and O-substituted systems possess

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magnetism and the magnetic moments are all 1.0 µB, which are induced mainly by the dopant atoms (C, Si, Ge and O) and the three nearest P atoms around dopant atoms. Furthermore, according to the analyze of band structures, we found the four magnetic structures (C, Si, Ge and O-substituted systems) are indirect bandgap semiconductors, while others (Sn, S, Se and Te-substituted systems) present metallic property. Our results indicate that blue phosphorene can be used in optoelectronic and spintronic devices through tuning the magnetism. Acknowledgments This work was supported by the National Natural Science Foundation of China

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Highlights


The doping of C, Si, Ge and O can all induce a magnetic moment of 1.0 µB.




The doping of Sn, S, Se and Te change the essential properties of the blue phosphorene.

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The orbital hybridization is the main factor that causes the spin polarization of

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doped blue phosphorene.

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