A first-principles study of light non-metallic atom substituted blue phosphorene

Applied Surface Science 356 (2015) 110–114

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A first-principles study of light non-metallic atom substituted blue phosphorene Minglei Sun a , Wencheng Tang a,∗ , Qingqiang Ren b , Sa-ke Wang c , Jin Yu d,e , Yanhui Du a a

School of Mechanical Engineering, Southeast University, Nanjing 211189, Jiangsu, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Hunan, China Department of Physics, Southeast University, Nanjing 210096, Jiangsu, China d School of Materials Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, China e Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, Jiangsu, China b c

a r t i c l e

i n f o

Article history: Received 27 June 2015 Received in revised form 2 August 2015 Accepted 3 August 2015 Available online 5 August 2015 Keywords: First-principles Light non-metallic atom Blue phosphorene Doping Spintronics

a b s t r a c t First-principles calculations are implemented to study the geometric, electronic and magnetic properties of light non-metallic atom (B, C, N, O and F) substituted blue phosphorene. All the substituted systems are highly stable. The B-substituted system is a direct bandgap semiconductor with a bandgap size about 1.5 eV. The C, O-substituted systems are promising systems to explore two-dimensional diluted magnetic semiconductors. Magnetism is observed for C and O substitution, while for the other impurities no magnetic moment is detected. Our works paves a new route at nanoscale for novel functionalities of optical and spintronics devices. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Recently, a new material named blue phosphorene, which has been predicted through using ab initio density functional theory by Zhu et al. [1]. The main advantage of the blue phosphorene is its sizeable fundamental band gap in excess of 2 eV. Moreover, it shares high stability with the black phosphorus allotrope [2]. Still, the author predicted it should exfoliate easily to form quasitwo-dimensional structures suitable for electronic applications [1]. These merits show that blue phosphorene is a promising 2D material. It is well known that chemical doping [3] is an effective and widely used method of tailoring the electronic and magnetic properties of 2D materials. Doped 2D materials are, therefore, useful for various applications including energy storage [4–8], energy conversion [9,10], and sensing [11–14]. Also, there are abundant works [15–26] focusing on the domain of nanoelectronics. For instance, monolayer MoS2 doped by transition metals can be used as a two-dimensional diluted magnetic semiconductor [25,26]. Spin polarized semiconducting state is realized in phosphorene by

∗ Corresponding author. Tel.: +86 02552090508. E-mail address: [email protected] (W. Tang). http://dx.doi.org/10.1016/j.apsusc.2015.08.009 0169-4332/© 2015 Elsevier B.V. All rights reserved.

substitutional doping of Ti, Cr and Mn, while a half-metallic state is obtained by V and Fe doping [23]. Recently, several theoretical studies exist on the geometric and electronic properties of blue phosphorene [1,27] and their nanoribbons form [28]. More recently, Ding et al. [29] investigate the adsorption characteristics of adatoms on blue phosphorene. However, a systematic study of the substitution of light non-metallic atoms on a blue phosphorene is still lack. Therefore, we study the geometric, electronic and magnetic properties of B, C, N, O and F substituted blue phosphorene. First, we investigate the geometry and magnetic properties after the substitutional doping of blue phosphorene by light non-metallic atoms. Second, we investigate the electronic properties of light non-metallic atoms substituted blue phosphorene. All these will be shown in our following works.

2. Method and calculation details The spin-polarized DFT is employed in our calculations using the VASP code [30–32]. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional [33] is adopted. The projector augmented wave (PAW) potentials [34] are used with a kinetic energy cutoff of 550 eV. PAW–PBE method has been shown to be very effective for cluster [35,36] and surface [37–40] calculations. The spin-polarization

M. Sun et al. / Applied Surface Science 356 (2015) 110–114

Fig. 1. The typical geometry of non-metallic atoms substituted blue phosphorene. The cyan, blue and green spheres, respectively, represent phosphorus atoms in the upper layer, lower layer and impurity atom (B, C, N, O and F). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. (a) Schematic representations of the electronic structure of N-substituted system. (b) The evolution of the nonbonding states in B, C, N, O and F-substituted systems.

approach was implemented in all calculations. A set of 9 × 9 × 1 Monkhorst–Pack grid [41] is used for Brillouin-zone (BZ) integration. The convergence criterion for the change in energy is 10−5 eV per cell. All the systems are fully relaxed until the Hellmann–Feynman forces are smaller than 0.02 eV/Å. The substitutional system is modeled in a 3 × 3 × 1 supercell with periodic boundary conditions as shown in Fig. 1 The vacuums in the z direction are larger than 20 A˚ through the calculations. 3. Results and discussion Table 1 summarizes the calculated structure parameters such as the binding energies (Eb ), bond lengths, charge transfer via Bader analysis [42–44] and magnetic moments on the B, C, N, O and Fsubstituted systems. The binding energy (Eb ) in our calculations is defined as follows: Eb = (EP + Ei ) − ET

(1)

111

In formula (1), Eb is the binding energy; EG is the total energy of defective blue phosphorene; Ei is the energy of an isolated impurity atom; ET is the total energy of the system. Based on this scheme, larger binding energy indicates a more stable structure. The calculated values of Eb is in the range of 4.649–7.803 eV. On the other hand, the Eb decreases gradually from B to F. It is evident that all atoms considered, form covalent bonds with the undercoordinated P atoms at the vacancy site. Furthermore, the bond lengths suggested that the substituted systems could be divided into two categories. The first category included elements from B to N, they form covalent bonds with three P atoms and with equal values of I-P1, I-P2 and I-P3. The second category included O and F, they form covalent bonds with two and one P atoms, respectively. Thus, the values of I-P1, I-P2 and I-P3 are not identical. We have also performed the Bader charge analysis which shows that electrons always transfer from the P atoms to the impurity atoms except B atom. This is because P atoms have a lower electronegativity than B atom and a higher electronegativity than C, N, O and F atoms [45]. Interestingly, the magnetic moment oscillates depending on even and odd numbers of valence electrons in impurity atoms. Impurity atoms with even number of valence electrons induce 1 B magnetic moment while an odd number induces non-magnetic behavior (Table 1). To better understand observed phenomenon, we developed a simple model, which can qualitatively explain the trends in magnetic properties. It is instructive to start the analysis with N-substituted system. The N atom has five valence electrons. Three of them form the N P covalent bonds with three undercoordinated P atoms, as depicted with black dots at the boundary of the circles in Fig. 2(a), while the other two valence electrons (depicted as the black dots in the middle of the circle) replace the two nonbonding electrons of the missing P atom. All the five valence electrons are saturated thus the magnetic moment is zero. After form three ␴ bonds with P atoms, B, C, O and F have 0, 1, 2 and 3 remaining electrons, respectively, which fill into nonbonding states and give rise to magnetic moments of 0, 1, 1 and 0 B , as shown in Fig. 2(b). More detailed analysis with respect to magnetic properties will be discussed in the following chapter. Fig. 3(a)–(e) represents the band structures of B, C, N, O and F-substituted systems, respectively. The red solid lines denote the spin-up bands while the blue dotted lines are for spin-down bands. In general, all the systems were semiconductors. For B-substituted system, the substitution of B atom shift the valence band maximum (VBM) to the  high-symmetry point (Fig. 3(a)). Therefore, the Bsubstituted system is a direct bandgap semiconductor with a wide bandgap. The gap size of the system is about 1.5 eV. It is well known that PBE considerably underestimate the bandgap. The real gap size may be larger. Moreover, the direct bandgap structure endow the B-substituted system with a clear advantage for its application in

Fig. 3. Band structure of B (a), C (b), N (c), O (d) and F (e)-substituted system. The Fermi level has been set to zero and indicated by cyan dashed line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Calculated bond lengths, bond angles, charge transfer and magnetic moments for 3 × 3 × 1 supercell of non-metallic atoms (B, C, N, O and F) substituted blue phosphorene. Impurity atom

Eb (eV)

d (Å)

Charge transfer (e)

Magnetic moment (B )

B

7.803

1.916 (I-P1)

−0.29 0.16 (P1)

0

C

7.795

1.834 (I-P1)

1.40 −0.45 (P1)

1

N

6.005

1.827 (I-P1)

1.66 −0.56 (P1)

0

O

5.899

1.727 (I-P1) 2.932 (I-P2)

1.37 −0.68 (P1) −0.04 (P2)

1

F

4.649

1.655 (I-P1) 3.336 (I-P2) 3.329 (I-P3)

0.77 −0.73 (P1) −0.03 (P2) 0 (P3)

0

optical devices [46]. In C and O-substituted systems, we observe narrow band gaps. Meanwhile, the spin-up and spin-down bands are split due to the introduction of magnetization. Consequently, two diluted magnetic semiconductors are found. In addition, we obtain the indirect band gap of approximately 15 meV and 20 meV in C and O substituted systems, respectively. These band gaps are slightly small for spintronics applications. However, as mentioned above, due to the DFT considerably underestimate the bandgap, the real size of the gap is at least larger than these values. Overall, our

calculated results suggest that C and O-substituted systems can be used in potential spintronics applications. To further study the electronic and magnetic structure properties of the non-metallic atom substituted systems, we performed the density of states (DOS) and projected density of states (PDOS) calculations (Fig. 4(a)–(e)). The PDOS curves of B atom and the three under-coordinated P atoms show that the contribution to the two degenerate (at  ) levels mainly come from the hybridization between the B pz and P px py orbitals. Nonetheless, the

Fig. 4. Spin-polarized DOS and PDOS of B (a), C (b), N (c), O (d) and F (e)-substituted system. The Fermi level has been set to zero and indicated by vertical cyan dashed line.

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Fig. 5. Spin density of the C (a) and O (b)-substituted system. A three-dimensional (3D) plot of the spin-polarized charge density with a charge density iso-surface value of 0.004 e/Å3 is shown, in which yellow and cyan region correspond to up and down spins, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

delocalized level about 1 eV above the Fermi level is dominated by B pz orbitals. For the DOS of C-substituted system (Fig. 4(b)) we found that the majority and minority density of states are asymmetrical for C-substituted system, indicating a magnetic character (Table 1). The PDOS analysis show that both VBM and CBM are mainly contributed by the hybridization of the C pz and P pz orbitals. The major contribution to the magnetic moment stems from the C pz orbitals. The PDOS of N-substituted system (Fig. 4(c)) exhibits interesting characteristics. The hybridization between the N pz and P pz orbitals contribute primarily to the VBM. However, the CBM is mainly origin from the N s orbitals. In addition, the N pz orbitals strongly hybridize with P pz orbitals over a wide range from about −0.2 eV to −0.8 eV above the Fermi level. This feature denotes a strong interaction between the N atom and defective blue phosphorene. In the PDOS of O-substituted system, two interesting feature can be founded. One feature is the VBM and CBM is mainly contributed by the P2 p orbitals. It seems that the O p orbitals also share some contribution to the VBM and CBM. However, compared with the contribution of P2 p orbitals, it is rather small and negligible. Hence, the O-substituted system is indeed a two-coordinated configuration. At least, it is a quasi-two-coordinated configuration. The other is the magnetism for the O-substituted system is somewhat different from the case of C. Both these two systems have 1 B magnetic moments. However, the origin of the magnetic moment in these systems is very different. The major source of spin polarization of O-substituted system originated from P2 p orbitals. In order to be more vivid, wepresent the spin-polarized charge density spin−up − spin−down of C and O substituted systems in Fig. 5. The spin charge for C-substituted system (Fig. 5(a)) chiefly arises from the C atom and formed by C pz shapes. However, it is obvious that the magnetic moment in O-substituted system (Fig. 5(b)) predominantly comes from the P2 atom. Only very few of total magnetic moments result from the polarized impurity atom. These images are consistent with the detailed analysis in PDOS (Fig. 4(b) and (d)). Finally, from the PDOS of F-substituted system (Fig. 4(e)) we find that the hybridization between the F pz and P1 s p orbitals gives rise to a peak at about −0.4 eV below the Fermi level and form the VBM. Furthermore, there is no evidence for hybridization between the F and P2 atoms, which in fact reflects the nonbonding picture between the F and P2 atoms. This detailed phenomenon is linked with the one-coordinated structure of the F-substituted system. 4. Conclusions We have performed first-principles calculations of light nonmetallic atoms (B, C, N, O and F) substituted blue phosphorene in order to study geometric structures, energetics, electronic and magnetic properties. All the impurities are covalently bonded to blue phosphorene (with a single vacancy) thus form the stable substitutional systems. All the substituted systems are semiconductors. Specially, B-substituted system exhibits direct bandgap

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