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First-principles studies on substitutional doping by group IV and VI atoms in the two-dimensional arsenene
Accepted Manuscript Title: First-principles studies on substitutional doping by group IV and VI atoms in the two-dimensional arsenene Author: Juan Du Congxin Xia Tianxing Wang Xu Zhao Xiaoming Tan Shuyi Wei PII: DOI: Reference:
S0169-4332(16)30491-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.03.055 APSUSC 32821
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Please cite this article as: Juan Du, Congxin Xia, Tianxing Wang, Xu Zhao, Xiaoming Tan, Shuyi Wei, First-principles studies on substitutional doping by group IV and VI atoms in the two-dimensional arsenene, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.03.055 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.
First-principles studies on substitutional doping by group IV and VI atoms in the two-dimensional arsenene Juan Dua, Congxin Xiaa* [email protected], Tianxing Wanga, Xu Zhaoa, Xiaoming Tanb, Shuyi Weia a
Department of Physics, Henan Normal University, Xinxiang, Henan 453600, China
b
School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China
*
Corresponding author. Tel.: +86 373 3326151.
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Highlights ● Effects of group IV and VI dopants are obvious on electronic structures in the arsenene nanosheets. ●C, Si, Ge and O substituting As induce deeper impurity states and with with total magnetic moment 1µB. ●Te substituting As atom is the most possible n-type doping due to the shallow transition level.
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Abstract The electronic characteristics of group IV and VI atoms-doped arsenene are investigated by means of first-principles methods. The results show that the influences of group IV and VI impurities are obvious on electronic structures in the arsenene. The spin-up and spin-down states induced by C, Si, Ge and O substituting As atoms lie on the both sides of Fermi level in the arsenene, and induce deeper impurity states with total magnetic moment 1µB. However, Te substituting As atom is the most possible n-type doping due to the shallowest transition level. These results are useful to further investigate experimentally the electronic structures and magnetic properties of group IV and VI atoms-doped arsenene nanosheets.
Keywords: arsenene; electronic structures; first-principle methods ____________
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1．Introduction Since two-dimensional (2D) graphene was fabricated successfully [1], many kinds of graphene-like 2D materials, such as BN [2,3], transition metal dichalcogenides [4-8] and GaS [9-11], have attracted extensive attentions due to their extraordinary physical properties and promising applications. Among them, one particular topic of interest is exploring the characteristics of group V elements-based 2D materials. Recently, phosphorene has attracted lots of interest from physicists, chemists and material scientists due to its direct band gap, high carrier mobility (103-104 cm2 V-1s-1 ) [12,13] and high on/off ratio up to 104 [13,14]. In addition, as a neighbor of P in the group V of periodic table, 2D As-based nanosheets are also attaching increasing attentions [15-18]. Bulk arsenic has many allotropes, such as gray-As, back-As and yellow-As. Among them, gray arsenic (α-As) is the most stable and most common form [15], which was reported to be a layered structure with the space group R3m, and the interlayer is weakly coupled by the van der Waals interactions [16]. The typical layer structure is helpful to fabricate few-layer and monolayer structures [13,14]. More recently, first-principles predication show that although gray arsenic is typically semimetal in its bulk phase, it is transformed into an indirect semiconductor when thinned to one atomic layer (called arsenene) [17]. Interestingly, under biaxial strain, the 2D arsenene nanosheets materials are transformed from indirect to direct band-gap semiconductors [16,19], which indicate that this novel 2D semiconductor can be
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anticipated to function as mechanical sensors. Moreover, studies also show that the band gap value of free-standing arsenene is about 2.49 eV, which is larger than that of other explored 2D materials (MoS2, Graphene) and will broad the development of 2D semiconductor based optoelectronic devices with response to photons with wavelengths of less than 620 nm, such as blue- and UV-light emitting diodes (LEDs) and photodetectors [17]. Zhu et al’s studies show that electronic structures of few-layer arsenic nanosheets depend sensitively on layer number, strain, layer stacking and interlayer spacing [19]. The studies of Zhang et al’s show the hydrogenated arsenenes are reported to be planar magnet and 2D Dirac materials based on comprehensive first-principles calculations [20]. Moreover, few-layer arsenic is also found to have considerable carrier mobility (~103cm2V-1s-1) and on/off ratio of ~104 at room temperature [21]. In addition, Wang et al’s studies show that arsenene armchair and zigzag nanoribbons still preserve semiconducting properties [18]. All these studies show that arsenene can be a new promising 2D semiconductor materials for the application potential in the LED, laser, and even can be used for force - optical sensor. As is well known, impurity states play an important role in the semiconducting optoelectronic devices. However, to our knowledge, there are no studies involved in the n-and p-type doping in the arsenene nanosheet. In addition, Kamal et al’s studies show that arsenene presents the buckled and puckered honeycomb structures, and buckled arsenene is slightly more stable than puckered arsenene [22]. Thus, in this work, we will focus on the structural, electronic and magnetic properties in the group IV(C, Si, Ge, Sn) and VI (O, S, Se, Te) atoms-doped buckled arsenene by using 5
first-principles methods. 2. Computational methods In this work, all calculations are carried out by using spin-polarized density functional theory (DFT) as implemented in the Vienna Ab-initio simulation package [23,24]. The exchange-correlation functional is treated within the generalized gradient approximation (GGA) and parameterized by Perdew-Burke-Ernzerhofer (PBE) formula [25]. The electron-ion potential is modeled with projected augmented wave (PAW) potential [26] Moreover, a kinetic energy cutoff of 500 eV is selected for the plane wave expansion. The Monkhorst-Pack scheme of k-point sampling is used for integration over the first Brillouin-zone [27]. The Brillouin zone (BZ) is sampled using a 5×5×1 gamma-centered Monkhorst-Pack grid in the calculations. The geometry structures are relaxed until the force on each atom is less than 0.01eV/Å and the total energy convergence criteria is chosen as 10-5 eV. 3. Results and discussions 3.1 . Electronic structures of pristine and doped arsenene We start briefly our calculations on the structural parameters and electronic structures of the unit cell of arsenene, which contains two arsenic atoms. The modeled arsenene monolayer has a buckled structure proposed by previous publications [22,28], as shown in Fig.1. After the total energy relaxation, the lattice constant of arsenene unit cell is 3.608Å, the As-As bond length is 2.509Å, the bond angle is 91.972°and the buckle height is 1.398 Å, which agree well with recent theoretical results [17,22]. In
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addition, the calculated electronic band structures and partial density of states (PDOS) of the arsenene unit cell are also presented in Fig.2. It can be seen from Fig.2(a) that the valence band maximum (VBM) lies at Γ point, while the conduction band minimum (CBM) lies between Γ and M points(0.0 0.5 0.0), which indicates that pristine arsenene is an indirect semiconductor with the band gap value of 1.616 eV. The calculated results also agree well with the reported gap value 1.635eV [22]. In addition, Fig. 2(b) further shows that the PDOS can be divided into three regions: in the energy region(-9.5 ~ -7) eV, denoted as region A, the states are mainly contributed by the s orbitals of As atoms, while the energy region (-4.4 ~ 0 ) eV and the energy region(1.5 ~ 4.5) eV, denoted as region B and C, respectively, are dominated by the p orbitals of As atoms. These results are also in agreement with previous theoretical studies[29]. In order to investigate the influences of group IV and VI impurities on electronic structures of arsenene, we use one impurity atom X (X=C, Si, Ge, Sn, O, S, Se and Te) to substitute one arsenic atom in the 6×6×1 arsenene supercell, in which the percentage of doping is about 1.39%. Note that the size of the arsenene 6×6×1 suspercell is 21.651Å, and the distance between the dopant atoms is equal to the size of the suspercell, thus this distance is large enough to avoid the exchange interactions between impurity in the studied system. Firstly, the calculated structural parameters of X-doped arsenene are listed in Table 1. It can be seen that after the structure relaxation, the C, Si, Ge, O and S-doped arsenene systems show a shrinkage in As–X bond length with respect to the original 7
As–As bond, while the Sn, Se, and Te atoms move outward from their initial substitution sites and hence the As–X bonds become longer. The increasing percentage of As-X bonds compared to the As-As bond are 5.78%, 1.95% and 8.25%, respectively. The bond length show a growing trend with the atomic number of doping atoms in the same column. We attribute the changes of bonds to the difference of ion radius and electronegativity of the considered cases. This behavior of structure change is similar as that of the 2D MoS2 [30]. In addition, we also calculate the charge transfer between the dopant X and As atoms using Bader charge. Numerical results show that for C, Si, Ge, Sn, O, S, Se and Te substituting As atom, the charger transfers are -3.35, 1.50, 0.60, 2.73, -1.13, -0.58,-0.32 and 0.22, respectively, which display that there are obvious electrons transfer between doping atoms and As sheets, indicating strong coupling between impurity atom and As atom. We turn to focus on the studies of electronic structures in the group VI atoms-doped arsenene. In Fig. 3, we present the characteristics of total density of states (TDOS) and PDOS of the group VI (O, S, Se and Te) atoms-doped arsenene. The results show that compared with pristine arsenene, the Fermi level(the energy zero in the graph) shifts gradually towards the conduction band edge with the increasing group atom number in the periodic table, which indicates a transition occurs from p-type to n-type. The reason is that group VI atoms have more valence electrons than As atom. As a consequence, these doping atoms induce donor defect states below the CBM and lead to n-type doping. In addition, Fig. 3 also shows that as the atom radius of dopant increase, the donor states become nearer to the CBM. Moreover, we can see from 8
Fig.3 that for O-doped case, the spin-up and spin-down TDOS distribute on the both sides of Fermi level and lie inside the gap, which show that O doped arsenene exhibit the magnetic ground states and O impurity is a deeper donor state. However, for S, Se and Te-doped case, the spin-up and spin-down TDOS of group VI atom-doped arsenene are symmetric, which indicates they posses the non-magnetic ground states. These results about magnetism are in good agreement with the calculated total magnetic moment 1µB in the O-doped arsenene syetem and 0 µB in the other VI-group atoms-doped arsenene systems, respectively (see Table1). The behaviors of different magnetic moments may be due to different electronegatives for different VI atoms. When arsenic atoms are substituted by different VI atoms, different exchange interactions occur between arsenic and impurity atoms and thus have different magnetism. Moreover, we would also like to point out that although they have one additional electron with respect to As, there is not necessarily to display magnetism. These behaviors are similar as other 2D materials, such as MoS2 [31] and phosphorene [32]. In addition, Fig. 3 (b) also shows that the CBM mainly consists of the p states of X atom states. Then, we focus on the studies of electronic structures in the group IV atoms-doped arsenene. In Fig.4, we present the characteristics of TDOS and PDOS of the group IV atoms-doped arsenene. Numerical results show that different from O, S, Se and Te substituting As atom, the doping atoms C, Si, Ge and Sn, which have less valence electrons than As atom, create acceptor defect states above the the valence band edge, thus resulting in p-type doping of arsenene. Meanwhile, we can see from Fig.4 that as 9
the atom radius of dopant increase, the doping states move nearer to the
valence
band edge. In addition, Fig. 4(a) also shows that the spin-up and spin-down doping states of group IV atoms doped arsenene are not symmetric and distribute on the both sides of Fermi level (except Sn case), which indicates that group IV atom-doped arsenene systems exhibit the magnetic ground states. The results are also in agreement with the calculation of magnetic moment 1µB (see Table 1). Moreover, one can see from Fig. 4 (b) that the defect states located above the top of valence band edge consist of p states of X atoms. In addition, we can see from Figs.3 and 4 that for group VI atoms-doped arsenene cases, the impurity states across the Fermi level and thus exhibit the metallic properties. However, for group IV atoms-doped arsenene systems, the impurity states didn’t across the Fermi level. Moreover, defect states are induced in the band gap, the occupied and unoccupied states belongs to different spins states, which indicate that group IV atoms-doped arsenene systems present the characteristics of semiconductor. In order to further investigate the electronic properties of group IV and VI atoms doped two-dimensional arsenene, we plot their band structures in Fig.5, in which we can see clearly that group IV atoms doped two-dimensional arsenene exhibit the properties of p-type semiconductor, while group IV atoms doped two-dimensional arsenene displayed the n-type metallic or semimetallic properties. These results are consistent with the analysis of Fig.3 ang Fig.4. In order to understand the nature of magnetism in the group IV and O-doped arsenene systems, in Fig.6, the spin densities are plotted. Numerical results show that 10
for group IV and O atoms-doped arsenene systems, spin density distributes mainly around the impurity, which indicates the nature of magnetic moments is localized. Moreover, Fig.6 also shows that compared with group IV-doped arsenene systems, the spin density of O-doped can spread over further atoms around the impurity. 3.2 Formation energy of doped arsenene In order to study whether group IV and VI atoms substituting As are energetically favorable, we calculate the formation energy of the X-atoms doped arsenene according to the following equation [32] E f ( As : X q ) E[ As : X q ] E[ As ] ni i q[ EF EV V ]
(1)
where X represents the doping atom (C, Si, Ge, Sn, O, S, Se and Te), q denotes the different charge states, E[ As : X q ] and E[ As] are the total energies of the X-doped and pristine arsenene with the same size supercell, respectively.∑(sigma) is a summation operator. The upper limit is the definition of i, which is determined by the atomic species. The ni (i is for the atomic species As and X) indicates the number of atoms that has been added to ( ni >0) or removed from ( ni <0) the supercell. i is the corresponding chemical potential of constituent referenced to elemental solid/gas with energy.
q[ E F EV V ] is
a function of EF. Moreover, EF is the Fermi level referenced
to the energy position of the VBM of pristine arsenene and varies from 0 to the calculated band gap Eg of pristine aresenene. EV represents the energy level of the VBM in pristine arsenene.The correction term V is used to align the reference potential in the doped supercell with that in the pristine with the same size. The value of V is determined by the difference of 1s energy levels between the atom farthest 11
from the impurity in the group V atoms-doped arsenene supercell and the corresponding atom in pristine arsenene supercell. Moreover, the thermodynamic transition energy level which determines the ionization energy of a given doping, D (q / q' ) , is equal to the Fermi-level for which charge states q and q ' have equal energy, where q and q' are the different charge states of the doping system. D (q / q' ) is the transition energy level between different charge states. D ( q / q' )
E ( X q ) E ( X q' ) q' q
(2)
In Table 1, we list the calculated neutral formation energies. It can be seen from the results that the formation energies are low (especially O), which suggest that the group VI and IV atoms substituting As atoms are energetically favorable. In order to understand further the characteristics of the formation energy and transition level in the X-doped arsenene, the formation energy as a function of the Fermi level referenced to the VBM is presented in Fig. 7, where the abscissa position of links corresponds the transition level between different charge states of the X-doped arsenene systems. Thus, when the abscissa position of the point decreases or increases from the constant value, the number result can induce that the transition energy level decreases or increases. Moreover, we would also like to point out that for group IV and VI atoms substituting As atom in arsenene, the calculated transition levels of C, Si and O-doped arsenene are for 1052.01meV, 723.57meV and 795.49meV, respectively, which is too large to tune effectively the carrier of arsenene nanosheets. Thus, in the following, we present only the formation energy as a function of the Fermi level in the Ge, Sn, S, Se, Te-doped 12
arsenene. For the p-type doping case, one can see from Fig. 7 that the links of the formation energy curves move towards the VBM as the atomic number of impurity increases, indicating that the transition energy level between different charge states decreases with the increasing atomic number in the periodic table. The calculated (0/-) transition energy levels are 453.56meV and 317.8meV above the VBM for Ge and Sn-doped arsenene, respectively. These transition energy level values show that group IV doping are deep acceptor impurities, which also indicate that group IV atom substituting As atoms are not suitable for achieving p-type conductivity in the arsenene. These results are in agreement with the localization of defect states inside the band gap when X (X=C, Si, Ge and Sn) atom substitutes As atom in arsenene (see Fig.4). For the n-type doping case, Fig.7 also shows that the transition energy level decreases monotonously with the increasing group VI atomic number in the periodic table. In addition, we can also see from Fig.7 that for the S, Se and Te-doped arsenene, the (+/0) transition levels occur at 281.2meV, 255.57meV and 186.18meV below the edge of conduction band, respectively. Thus, Te-doped arsenene has the shallowest transition level among the group VI atom-doped arsenene, which indicate that Te substituting As atom is the most possible n-type impurity in the arsenene. 4. Conclusions In conclusion, based on the first-principles calculation, we have investigated the influences of group IV and VI dopants on electronic structures in the new 2D arsenene. The results show that for group IV atom (C, Si, Ge and Sn)-doped arsenene nanosheets, 13
they all exhibit the magnetic ground states. Group IV doping are deep acceptor impurities, which imply that group IV atoms substituting As atom are not suitable for achieving p-type conductivity in the arsenene. However, for the group VI atoms(O, S, Se and Te)-doped arsenene, they all exhibit the non-magnetic ground states (except O); defect states of O impurity lie in the middle of the band gap, which indicates that O impurity can not induce effective n-type carriers in the arsenene. Moreover, the results also show that Te atom doping is the most effective n-type doping due to the shallowest transition energy level in the arsenene. In addition, the formation energy and transition energy levels depend highly on the atomic size of impurity atom for all the doped systems.This dependence indicates that the suitable atomic size plays an important role on the feasibility of doping in the arsenene in experiments. We expect that our calculations are useful to carry out the further experimental investigations of group IV and VI atoms-doped arsenene nanosheets.
Acknowledgments This research was supported by the National Natural Science Foundation of China under Grant No.U1304518 and 11204121. The calculations are also supported by The High Performance Computing Center of Henan Normal University.
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Figure Captions Fig. 1 The top view (a) and side view (b) of the buckled honeycomb structure of a 6×6×1 supercell of gray arsenic monolayer doping with atoms. Atoms at the top and bottom of the nonplanar layers are distinguished by red and purple. Doping atom X is presented by yellow. The bond length (2.509Å) of As-As bond is also displayed in the graph. The rectangle is for the unit cell. Fig. 2 The band structures (a) and partial density of states (PDOS) (b) of the pristine arsenene. The energy zero is taken as the Fermi level. The shadowed area represents the band gap. Fig. 3 (a) The total density of states (TDOS) and (b) partial density of states (PDOS) for the group VI atoms-doped arsenene (X=O, S, Se and Te). The blue dashed line is for the Fermi energy of the doped systems. The energy zero (green solid line) is set at the top of valence band of undoped arsenene as a reference. Fig. 4 (a) The total density of states (TDOS) and (b) partial density of states (PDOS) for the group IV atom-doped arsenene (X=C, Si, Ge and Sn). The blue dashed line is for the Fermi energy of the doped systems. The energy zero (green solid line) is set at the top of valence band of undoped arsenene as a reference. Fig. 5 The band structures of (a) C (b) Si (c) Ge (d) Sn (e) O (f) S (g) Se (h) Te-doped arsenene. The spin-up and spin-down states are denoted by red and black, respectively. The energy zero of every doping system is taken as their Fermi level respectively.
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Fig. 6 The spin densities of (a) C, (b) Si, (c) Ge, (d) Sn and (e)O-doped arsenene. The isosurface level is taken as 0.005 e/Å3. Fig. 7 The formation energies as a function of the Fermi level from 0 to Eg in the X-doped arsenene systems (X=Ge, Sn, S, Se and Te), where As:X represents the X doped arsenene; 0, +1 and -1 denote different charge states, respectively.
21
Fig. 1
22
Fig. 2
23
Fig. 3
24
Fig. 4
25
Fig. 5
26
Fig. 6
27
Fig. 7
28
Table Table 1.The calculated bond length dX-As, magnetic moment Mtot and formation energy Ef for X-doped arsenene.
dX-As (Å)
Mtot (μB)
Ef(eV)
X= C
1.987
1
2.814
X=Si
2.405
1
0.978
X= Ge
2.490
1
0.871
X= Sn
2.654
1
0.941
X=O
2.090
1
0.012
X= S
2.444
0
0.806
X= Se
2.558
0
0.815
X= Te
2.716
0
1.095
X-doped As
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S0169-4332(16)30491-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.03.055 APSUSC 32821
To appear in:
APSUSC
Received date: Revised date: Accepted date:
6-10-2015 25-2-2016 5-3-2016
Please cite this article as: Juan Du, Congxin Xia, Tianxing Wang, Xu Zhao, Xiaoming Tan, Shuyi Wei, First-principles studies on substitutional doping by group IV and VI atoms in the two-dimensional arsenene, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.03.055 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.
First-principles studies on substitutional doping by group IV and VI atoms in the two-dimensional arsenene Juan Dua, Congxin Xiaa* [email protected], Tianxing Wanga, Xu Zhaoa, Xiaoming Tanb, Shuyi Weia a
Department of Physics, Henan Normal University, Xinxiang, Henan 453600, China
b
School of Physics and Optoelectronic Engineering, Ludong University, Yantai 264025, China
*
Corresponding author. Tel.: +86 373 3326151.
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Highlights ● Effects of group IV and VI dopants are obvious on electronic structures in the arsenene nanosheets. ●C, Si, Ge and O substituting As induce deeper impurity states and with with total magnetic moment 1µB. ●Te substituting As atom is the most possible n-type doping due to the shallow transition level.
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Abstract The electronic characteristics of group IV and VI atoms-doped arsenene are investigated by means of first-principles methods. The results show that the influences of group IV and VI impurities are obvious on electronic structures in the arsenene. The spin-up and spin-down states induced by C, Si, Ge and O substituting As atoms lie on the both sides of Fermi level in the arsenene, and induce deeper impurity states with total magnetic moment 1µB. However, Te substituting As atom is the most possible n-type doping due to the shallowest transition level. These results are useful to further investigate experimentally the electronic structures and magnetic properties of group IV and VI atoms-doped arsenene nanosheets.
Keywords: arsenene; electronic structures; first-principle methods ____________
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1．Introduction Since two-dimensional (2D) graphene was fabricated successfully [1], many kinds of graphene-like 2D materials, such as BN [2,3], transition metal dichalcogenides [4-8] and GaS [9-11], have attracted extensive attentions due to their extraordinary physical properties and promising applications. Among them, one particular topic of interest is exploring the characteristics of group V elements-based 2D materials. Recently, phosphorene has attracted lots of interest from physicists, chemists and material scientists due to its direct band gap, high carrier mobility (103-104 cm2 V-1s-1 ) [12,13] and high on/off ratio up to 104 [13,14]. In addition, as a neighbor of P in the group V of periodic table, 2D As-based nanosheets are also attaching increasing attentions [15-18]. Bulk arsenic has many allotropes, such as gray-As, back-As and yellow-As. Among them, gray arsenic (α-As) is the most stable and most common form [15], which was reported to be a layered structure with the space group R3m, and the interlayer is weakly coupled by the van der Waals interactions [16]. The typical layer structure is helpful to fabricate few-layer and monolayer structures [13,14]. More recently, first-principles predication show that although gray arsenic is typically semimetal in its bulk phase, it is transformed into an indirect semiconductor when thinned to one atomic layer (called arsenene) [17]. Interestingly, under biaxial strain, the 2D arsenene nanosheets materials are transformed from indirect to direct band-gap semiconductors [16,19], which indicate that this novel 2D semiconductor can be
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anticipated to function as mechanical sensors. Moreover, studies also show that the band gap value of free-standing arsenene is about 2.49 eV, which is larger than that of other explored 2D materials (MoS2, Graphene) and will broad the development of 2D semiconductor based optoelectronic devices with response to photons with wavelengths of less than 620 nm, such as blue- and UV-light emitting diodes (LEDs) and photodetectors [17]. Zhu et al’s studies show that electronic structures of few-layer arsenic nanosheets depend sensitively on layer number, strain, layer stacking and interlayer spacing [19]. The studies of Zhang et al’s show the hydrogenated arsenenes are reported to be planar magnet and 2D Dirac materials based on comprehensive first-principles calculations [20]. Moreover, few-layer arsenic is also found to have considerable carrier mobility (~103cm2V-1s-1) and on/off ratio of ~104 at room temperature [21]. In addition, Wang et al’s studies show that arsenene armchair and zigzag nanoribbons still preserve semiconducting properties [18]. All these studies show that arsenene can be a new promising 2D semiconductor materials for the application potential in the LED, laser, and even can be used for force - optical sensor. As is well known, impurity states play an important role in the semiconducting optoelectronic devices. However, to our knowledge, there are no studies involved in the n-and p-type doping in the arsenene nanosheet. In addition, Kamal et al’s studies show that arsenene presents the buckled and puckered honeycomb structures, and buckled arsenene is slightly more stable than puckered arsenene [22]. Thus, in this work, we will focus on the structural, electronic and magnetic properties in the group IV(C, Si, Ge, Sn) and VI (O, S, Se, Te) atoms-doped buckled arsenene by using 5
first-principles methods. 2. Computational methods In this work, all calculations are carried out by using spin-polarized density functional theory (DFT) as implemented in the Vienna Ab-initio simulation package [23,24]. The exchange-correlation functional is treated within the generalized gradient approximation (GGA) and parameterized by Perdew-Burke-Ernzerhofer (PBE) formula [25]. The electron-ion potential is modeled with projected augmented wave (PAW) potential [26] Moreover, a kinetic energy cutoff of 500 eV is selected for the plane wave expansion. The Monkhorst-Pack scheme of k-point sampling is used for integration over the first Brillouin-zone [27]. The Brillouin zone (BZ) is sampled using a 5×5×1 gamma-centered Monkhorst-Pack grid in the calculations. The geometry structures are relaxed until the force on each atom is less than 0.01eV/Å and the total energy convergence criteria is chosen as 10-5 eV. 3. Results and discussions 3.1 . Electronic structures of pristine and doped arsenene We start briefly our calculations on the structural parameters and electronic structures of the unit cell of arsenene, which contains two arsenic atoms. The modeled arsenene monolayer has a buckled structure proposed by previous publications [22,28], as shown in Fig.1. After the total energy relaxation, the lattice constant of arsenene unit cell is 3.608Å, the As-As bond length is 2.509Å, the bond angle is 91.972°and the buckle height is 1.398 Å, which agree well with recent theoretical results [17,22]. In
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addition, the calculated electronic band structures and partial density of states (PDOS) of the arsenene unit cell are also presented in Fig.2. It can be seen from Fig.2(a) that the valence band maximum (VBM) lies at Γ point, while the conduction band minimum (CBM) lies between Γ and M points(0.0 0.5 0.0), which indicates that pristine arsenene is an indirect semiconductor with the band gap value of 1.616 eV. The calculated results also agree well with the reported gap value 1.635eV [22]. In addition, Fig. 2(b) further shows that the PDOS can be divided into three regions: in the energy region(-9.5 ~ -7) eV, denoted as region A, the states are mainly contributed by the s orbitals of As atoms, while the energy region (-4.4 ~ 0 ) eV and the energy region(1.5 ~ 4.5) eV, denoted as region B and C, respectively, are dominated by the p orbitals of As atoms. These results are also in agreement with previous theoretical studies[29]. In order to investigate the influences of group IV and VI impurities on electronic structures of arsenene, we use one impurity atom X (X=C, Si, Ge, Sn, O, S, Se and Te) to substitute one arsenic atom in the 6×6×1 arsenene supercell, in which the percentage of doping is about 1.39%. Note that the size of the arsenene 6×6×1 suspercell is 21.651Å, and the distance between the dopant atoms is equal to the size of the suspercell, thus this distance is large enough to avoid the exchange interactions between impurity in the studied system. Firstly, the calculated structural parameters of X-doped arsenene are listed in Table 1. It can be seen that after the structure relaxation, the C, Si, Ge, O and S-doped arsenene systems show a shrinkage in As–X bond length with respect to the original 7
As–As bond, while the Sn, Se, and Te atoms move outward from their initial substitution sites and hence the As–X bonds become longer. The increasing percentage of As-X bonds compared to the As-As bond are 5.78%, 1.95% and 8.25%, respectively. The bond length show a growing trend with the atomic number of doping atoms in the same column. We attribute the changes of bonds to the difference of ion radius and electronegativity of the considered cases. This behavior of structure change is similar as that of the 2D MoS2 [30]. In addition, we also calculate the charge transfer between the dopant X and As atoms using Bader charge. Numerical results show that for C, Si, Ge, Sn, O, S, Se and Te substituting As atom, the charger transfers are -3.35, 1.50, 0.60, 2.73, -1.13, -0.58,-0.32 and 0.22, respectively, which display that there are obvious electrons transfer between doping atoms and As sheets, indicating strong coupling between impurity atom and As atom. We turn to focus on the studies of electronic structures in the group VI atoms-doped arsenene. In Fig. 3, we present the characteristics of total density of states (TDOS) and PDOS of the group VI (O, S, Se and Te) atoms-doped arsenene. The results show that compared with pristine arsenene, the Fermi level(the energy zero in the graph) shifts gradually towards the conduction band edge with the increasing group atom number in the periodic table, which indicates a transition occurs from p-type to n-type. The reason is that group VI atoms have more valence electrons than As atom. As a consequence, these doping atoms induce donor defect states below the CBM and lead to n-type doping. In addition, Fig. 3 also shows that as the atom radius of dopant increase, the donor states become nearer to the CBM. Moreover, we can see from 8
Fig.3 that for O-doped case, the spin-up and spin-down TDOS distribute on the both sides of Fermi level and lie inside the gap, which show that O doped arsenene exhibit the magnetic ground states and O impurity is a deeper donor state. However, for S, Se and Te-doped case, the spin-up and spin-down TDOS of group VI atom-doped arsenene are symmetric, which indicates they posses the non-magnetic ground states. These results about magnetism are in good agreement with the calculated total magnetic moment 1µB in the O-doped arsenene syetem and 0 µB in the other VI-group atoms-doped arsenene systems, respectively (see Table1). The behaviors of different magnetic moments may be due to different electronegatives for different VI atoms. When arsenic atoms are substituted by different VI atoms, different exchange interactions occur between arsenic and impurity atoms and thus have different magnetism. Moreover, we would also like to point out that although they have one additional electron with respect to As, there is not necessarily to display magnetism. These behaviors are similar as other 2D materials, such as MoS2 [31] and phosphorene [32]. In addition, Fig. 3 (b) also shows that the CBM mainly consists of the p states of X atom states. Then, we focus on the studies of electronic structures in the group IV atoms-doped arsenene. In Fig.4, we present the characteristics of TDOS and PDOS of the group IV atoms-doped arsenene. Numerical results show that different from O, S, Se and Te substituting As atom, the doping atoms C, Si, Ge and Sn, which have less valence electrons than As atom, create acceptor defect states above the the valence band edge, thus resulting in p-type doping of arsenene. Meanwhile, we can see from Fig.4 that as 9
the atom radius of dopant increase, the doping states move nearer to the
valence
band edge. In addition, Fig. 4(a) also shows that the spin-up and spin-down doping states of group IV atoms doped arsenene are not symmetric and distribute on the both sides of Fermi level (except Sn case), which indicates that group IV atom-doped arsenene systems exhibit the magnetic ground states. The results are also in agreement with the calculation of magnetic moment 1µB (see Table 1). Moreover, one can see from Fig. 4 (b) that the defect states located above the top of valence band edge consist of p states of X atoms. In addition, we can see from Figs.3 and 4 that for group VI atoms-doped arsenene cases, the impurity states across the Fermi level and thus exhibit the metallic properties. However, for group IV atoms-doped arsenene systems, the impurity states didn’t across the Fermi level. Moreover, defect states are induced in the band gap, the occupied and unoccupied states belongs to different spins states, which indicate that group IV atoms-doped arsenene systems present the characteristics of semiconductor. In order to further investigate the electronic properties of group IV and VI atoms doped two-dimensional arsenene, we plot their band structures in Fig.5, in which we can see clearly that group IV atoms doped two-dimensional arsenene exhibit the properties of p-type semiconductor, while group IV atoms doped two-dimensional arsenene displayed the n-type metallic or semimetallic properties. These results are consistent with the analysis of Fig.3 ang Fig.4. In order to understand the nature of magnetism in the group IV and O-doped arsenene systems, in Fig.6, the spin densities are plotted. Numerical results show that 10
for group IV and O atoms-doped arsenene systems, spin density distributes mainly around the impurity, which indicates the nature of magnetic moments is localized. Moreover, Fig.6 also shows that compared with group IV-doped arsenene systems, the spin density of O-doped can spread over further atoms around the impurity. 3.2 Formation energy of doped arsenene In order to study whether group IV and VI atoms substituting As are energetically favorable, we calculate the formation energy of the X-atoms doped arsenene according to the following equation [32] E f ( As : X q ) E[ As : X q ] E[ As ] ni i q[ EF EV V ]
(1)
where X represents the doping atom (C, Si, Ge, Sn, O, S, Se and Te), q denotes the different charge states, E[ As : X q ] and E[ As] are the total energies of the X-doped and pristine arsenene with the same size supercell, respectively.∑(sigma) is a summation operator. The upper limit is the definition of i, which is determined by the atomic species. The ni (i is for the atomic species As and X) indicates the number of atoms that has been added to ( ni >0) or removed from ( ni <0) the supercell. i is the corresponding chemical potential of constituent referenced to elemental solid/gas with energy.
q[ E F EV V ] is
a function of EF. Moreover, EF is the Fermi level referenced
to the energy position of the VBM of pristine arsenene and varies from 0 to the calculated band gap Eg of pristine aresenene. EV represents the energy level of the VBM in pristine arsenene.The correction term V is used to align the reference potential in the doped supercell with that in the pristine with the same size. The value of V is determined by the difference of 1s energy levels between the atom farthest 11
from the impurity in the group V atoms-doped arsenene supercell and the corresponding atom in pristine arsenene supercell. Moreover, the thermodynamic transition energy level which determines the ionization energy of a given doping, D (q / q' ) , is equal to the Fermi-level for which charge states q and q ' have equal energy, where q and q' are the different charge states of the doping system. D (q / q' ) is the transition energy level between different charge states. D ( q / q' )
E ( X q ) E ( X q' ) q' q
(2)
In Table 1, we list the calculated neutral formation energies. It can be seen from the results that the formation energies are low (especially O), which suggest that the group VI and IV atoms substituting As atoms are energetically favorable. In order to understand further the characteristics of the formation energy and transition level in the X-doped arsenene, the formation energy as a function of the Fermi level referenced to the VBM is presented in Fig. 7, where the abscissa position of links corresponds the transition level between different charge states of the X-doped arsenene systems. Thus, when the abscissa position of the point decreases or increases from the constant value, the number result can induce that the transition energy level decreases or increases. Moreover, we would also like to point out that for group IV and VI atoms substituting As atom in arsenene, the calculated transition levels of C, Si and O-doped arsenene are for 1052.01meV, 723.57meV and 795.49meV, respectively, which is too large to tune effectively the carrier of arsenene nanosheets. Thus, in the following, we present only the formation energy as a function of the Fermi level in the Ge, Sn, S, Se, Te-doped 12
arsenene. For the p-type doping case, one can see from Fig. 7 that the links of the formation energy curves move towards the VBM as the atomic number of impurity increases, indicating that the transition energy level between different charge states decreases with the increasing atomic number in the periodic table. The calculated (0/-) transition energy levels are 453.56meV and 317.8meV above the VBM for Ge and Sn-doped arsenene, respectively. These transition energy level values show that group IV doping are deep acceptor impurities, which also indicate that group IV atom substituting As atoms are not suitable for achieving p-type conductivity in the arsenene. These results are in agreement with the localization of defect states inside the band gap when X (X=C, Si, Ge and Sn) atom substitutes As atom in arsenene (see Fig.4). For the n-type doping case, Fig.7 also shows that the transition energy level decreases monotonously with the increasing group VI atomic number in the periodic table. In addition, we can also see from Fig.7 that for the S, Se and Te-doped arsenene, the (+/0) transition levels occur at 281.2meV, 255.57meV and 186.18meV below the edge of conduction band, respectively. Thus, Te-doped arsenene has the shallowest transition level among the group VI atom-doped arsenene, which indicate that Te substituting As atom is the most possible n-type impurity in the arsenene. 4. Conclusions In conclusion, based on the first-principles calculation, we have investigated the influences of group IV and VI dopants on electronic structures in the new 2D arsenene. The results show that for group IV atom (C, Si, Ge and Sn)-doped arsenene nanosheets, 13
they all exhibit the magnetic ground states. Group IV doping are deep acceptor impurities, which imply that group IV atoms substituting As atom are not suitable for achieving p-type conductivity in the arsenene. However, for the group VI atoms(O, S, Se and Te)-doped arsenene, they all exhibit the non-magnetic ground states (except O); defect states of O impurity lie in the middle of the band gap, which indicates that O impurity can not induce effective n-type carriers in the arsenene. Moreover, the results also show that Te atom doping is the most effective n-type doping due to the shallowest transition energy level in the arsenene. In addition, the formation energy and transition energy levels depend highly on the atomic size of impurity atom for all the doped systems.This dependence indicates that the suitable atomic size plays an important role on the feasibility of doping in the arsenene in experiments. We expect that our calculations are useful to carry out the further experimental investigations of group IV and VI atoms-doped arsenene nanosheets.
Acknowledgments This research was supported by the National Natural Science Foundation of China under Grant No.U1304518 and 11204121. The calculations are also supported by The High Performance Computing Center of Henan Normal University.
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Figure Captions Fig. 1 The top view (a) and side view (b) of the buckled honeycomb structure of a 6×6×1 supercell of gray arsenic monolayer doping with atoms. Atoms at the top and bottom of the nonplanar layers are distinguished by red and purple. Doping atom X is presented by yellow. The bond length (2.509Å) of As-As bond is also displayed in the graph. The rectangle is for the unit cell. Fig. 2 The band structures (a) and partial density of states (PDOS) (b) of the pristine arsenene. The energy zero is taken as the Fermi level. The shadowed area represents the band gap. Fig. 3 (a) The total density of states (TDOS) and (b) partial density of states (PDOS) for the group VI atoms-doped arsenene (X=O, S, Se and Te). The blue dashed line is for the Fermi energy of the doped systems. The energy zero (green solid line) is set at the top of valence band of undoped arsenene as a reference. Fig. 4 (a) The total density of states (TDOS) and (b) partial density of states (PDOS) for the group IV atom-doped arsenene (X=C, Si, Ge and Sn). The blue dashed line is for the Fermi energy of the doped systems. The energy zero (green solid line) is set at the top of valence band of undoped arsenene as a reference. Fig. 5 The band structures of (a) C (b) Si (c) Ge (d) Sn (e) O (f) S (g) Se (h) Te-doped arsenene. The spin-up and spin-down states are denoted by red and black, respectively. The energy zero of every doping system is taken as their Fermi level respectively.
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Fig. 6 The spin densities of (a) C, (b) Si, (c) Ge, (d) Sn and (e)O-doped arsenene. The isosurface level is taken as 0.005 e/Å3. Fig. 7 The formation energies as a function of the Fermi level from 0 to Eg in the X-doped arsenene systems (X=Ge, Sn, S, Se and Te), where As:X represents the X doped arsenene; 0, +1 and -1 denote different charge states, respectively.
21
Fig. 1
22
Fig. 2
23
Fig. 3
24
Fig. 4
25
Fig. 5
26
Fig. 6
27
Fig. 7
28
Table Table 1.The calculated bond length dX-As, magnetic moment Mtot and formation energy Ef for X-doped arsenene.
dX-As (Å)
Mtot (μB)
Ef(eV)
X= C
1.987
1
2.814
X=Si
2.405
1
0.978
X= Ge
2.490
1
0.871
X= Sn
2.654
1
0.941
X=O
2.090
1
0.012
X= S
2.444
0
0.806
X= Se
2.558
0
0.815
X= Te
2.716
0
1.095
X-doped As
29