Article pubs.acs.org/JPCC
Tunable Magnetism in a Nonmetal-Substituted ZnO Monolayer: A First-Principles Study Hongyan Guo,† Yu Zhao,‡ Ning Lu,§ Erjun Kan,∥ Xiao Cheng Zeng,*,‡ Xiaojun Wu,*,†,§ and Jinlong Yang§ †
CAS Key Lab of Materials for Energy Conversion and Department of Material Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States § Hefei National Lab of Physical Science at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China ∥ Department of Applied Physics, Nanjing University of Science and Technology, Nanjing, Jiangsu 210037, China S Supporting Information *
ABSTRACT: We have studied structural, electronic, and magnetic properties of the graphene-like ZnO monolayer doped with nonmetal species using the first-principles calculations. Particular attention has been placed on the ZnO monolayer with one or two oxygen atoms per supercell substituted by carbon, boron, or nitrogen atoms. We find that the ZnO monolayer with one oxygen atom per supercell substituted by a carbon or boron atom is ferromagnetic (FM) half metal (HM), while that with a nitrogen atom per supercell is a FM semiconductor. Upon the ZnO monolayer with two oxygen atoms per supercell substituted by carbon or boron, the magnetic properties vary, depending on the distance between two impurities. Two neighboring carbon or boron atoms in the ZnO monolayer form dimer pairs, which convert the ZnO monolayer into an n-type semiconductor with a nonmagnetic (NM) ground state. As the distance between two carbon or boron atoms increases, the doped ZnO monolayer undergoes both NM− AFM−FM and semiconductor−HM transitions. However, the ZnO monolayer with two N atoms per supercell is a p-type semiconductor with the antiferromagnetic (AFM) ground state, regardless of the distance between N atoms. The negligible energy difference between AFM and FM states of the N-doped ZnO monolayer implies it exhibits paramagnetic behavior at room temperature. Our study demonstrates that nonmetal-doped ZnO monolayers possess tunable magnetic and electronic properties, suitable for applications in electronics and spintronics at nanoscale.
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INTRODUCTION Developing magnetic devices with one-atom-thick materials, such as a graphene and boron-nitride (BN) monolayer, has attracted wide research attention to meet the need of miniaturization of electronic devices.1−3 Intensive studies have predicted that graphene or BN monolayer based nanostructures, e.g., nanoribbons, nanoflakes, and their hybrid structures,4−12 may possess magnetic properties. On the other hand, nonmetal-doped metal oxide semiconductors represent a new class of magnetic materials with d0 ferromagnetism, which have the potential for future magnetic device applications.13−21 In this context, zinc oxide (ZnO) is especially promising since ZnO doped with nonmetal species, such as carbon and nitrogen, can exhibit ferromagnetism even above room temperature.17−22 ZnO is an important metal oxide with a wide band gap of about 3.3 eV23 and exhibits various morphologies at nanoscale, e.g., nanowires,24 nanoclusters,25 nanobelts,26 and nanotubes.27 Recently, first-principle calculations predict that ZnO film © 2012 American Chemical Society
prefers a graphitic-like structure when the number of ZnO(0001) layers is reduced due to the depolarization of the surfaces,28−30 and the graphene-like ZnO monolayer is a semiconductor with a wide band gap of 3.57 eV (considering the GW correction).31 Tushce et al. were the first to successfully synthesize two-monolayer-thick ZnO(0001) films deposited on a Ag(111) surface, where Zn and O atoms are arranged in planar sheet like in the hexagonal BN monolayer.32 Previous theoretical calculations have shown that ZnO nanoribbons with zigzag edges can exhibit ferromagnetic properties,33 and the edge passivation with hydrogen, sulfur, NH 2 , or CH 3 can enhance their half metal (HM) ferromagnetism.34,35 Also, two-dimensional ZnO monolayers containing Zn vacancies,36 doped with Co,37 semifluoriReceived: December 27, 2011 Revised: May 3, 2012 Published: May 3, 2012 11336
dx.doi.org/10.1021/jp2125069 | J. Phys. Chem. C 2012, 116, 11336−11342
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nated,38−40 or surface-passivated with H and NH241 may possess half-metallicity. In this work, we perform a comprehensive first-principles study of the structural, electronic, and magnetic properties of a nonmetal-doped two-dimensional (2D) ZnO monolayer with O atoms substituted by C, N, or B atoms. Our studies predict that both the carbon- and boron-doped ZnO monolayer can exhibit a “nonmagnetic−antiferromagnetic−ferromagnetic” (NM− AFM−FM) transition, accompanied by a “semiconductor− half metal (HM)” transition. The nitrogen-doped ZnO monolayer is a p-type semiconductor with the AFM ground state, independent of the distance between two N defects. The tunable electronic and magnetic properties of the ZnO monolayer doped with nonmetal species can be exploited for nanoelectronic and spintronic applications.
structure of the ZnO monolayer is cut from a bulk wurtzite ZnO with (0001) polar surface, consistent with the reported structure of the experimentally synthesized ZnO monolayer.32 The fully relaxed ZnO monolayer possesses a graphene-like plane, and the Zn−O bond length is 1.90 Å. The DFT calcualtion suggests that the pure ZnO monolayer is a NM semiconductor with a direct band gap of 1.66 eV at the Γ point, consistent with previous theoretical calculations.28−30 It is known that the DFT/PBE method usually underestimates the band gap of semiconductors due to the self-interaction error.47 A test calculation with the screened hybrid HSE06 functional suggests that the ZnO monolayer is a semiconductor with a direct band gap of 3.25 eV.48,49 C-Doped ZnO Monolayer. Next, we consider the C-doped ZnO monolayer with one oxygen atom substituted by a carbon atom per supercell (Zn36O35C), as shown in Figure 1(a). The calculated band structure indicates that the C-doped ZnO monolayer is a ferromagnetic half-metal (FM-HM) with spinpolarized bands crossing the Fermi level (Figure 1(c)), similar to previous results of bulk ZnO doped with low-concentration carbon atoms.17 The total magnetic moment per supercell of Zn36O35C is 2.00 μB, and the local magnetic moment of the C dopant is 0.59 μB, as shown in Figure 1(b). As shown in Figure 1(d), the partial density of states (PDOS) of C and Zn atoms implies that the magnetic moment mainly originates from the strong coupling between the 2p orbital of C and 3d orbitals of Zn around the C dopant. The s and p orbitals of Zn also have a comparable weak contribution to the DOS at the Fermi level (see Supporting Information Figure S1). The formation energy of the C dopant is 2.77 eV, calculated based on the definition Eform = E(C-doped ZnO) − E(ZnO) − E(C) + E(O), where E(C-doped ZnO) is the total energy per supercell of doped ZnO and E(C) and E(O) are the total energy of an isolated C and O atom, respectively. To investigate the magnetic coupling between C dopants in a ZnO monolayer, two O atoms per supercell are substituted by two C atoms (Zn36O34C2) with various C−C distances. The optimized distance between two C dopants is summarized in Table 1. When two neighboring O atoms are substituted by C atoms, the C dopants form a dimer pair with C−C distance of 1.27 Å (see Supporting Information Figure S2). The charge analysis suggests that each C dopant attains about 0.53 e charge. The calculated band structure (Figure 2(d)) indicates that the ZnO monolayer doped with a C−C pair is a NM n-type semiconductor with a direct band gap of 0.68 eV. Previously, Wu et al. predicted that the bulk ZnO doped with the C−C pair is a FM-HM, due to the partially occupied ppπ antibonding state of the charged C−C pair.22 A reason for the different behavior between the bulk and monolayer is that the charge of the C−C pair in the ZnO monolayer is smaller than that in the ZnO bulk, and the charged C−C dimer pair in the ZnO monolayer has fully occupied ppπ bonding and empty ppπ antibonding states (ppπ*), as shown in Figure 2(a). As two C dopants are further apart, C dopants will occupy the original lattice-site positions of substituted O atoms. As the C−C distance increases, the calculated band structures indicate that the C-doped ZnO monolayer undergoes a NM−AFM− FM transition, as well as a semiconductor−HM transition, as shown in Figure 2(e)−(i). When the distance between two C dopants is less than 6.46 Å, two C dopants prefer an AFM coupling with a local magnetic moment of 0.60 μB per C atom, and C-doped ZnO is a semiconductor with an indirect band gap for both spin-up and spin-down channels. The charge
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METHODS All calculations are performed within the framework of spinpolarized plane-wave density functional theory (DFT), implemented in the Vienna ab initio simulation package (VASP).42,43 The generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional and projector augmented wave (PAW) potentials is selected in the DFT calculations.44−46 The ZnO sheet is modeled with a periodic supercell of 72 atoms. The vacuum size is set as 10 Å. An energy cutoff of 400 eV is used for the plane-wave expansion of the electronic wave function. Geometry structures are relaxed until the force on each atom is less than 0.01 eV/Å, and the convergence criteria for energy of 1 × 10−5 eV is met. The first Brillouin zone is sampled with 3 × 3 × 1 k-points using Γ-center scheme. A test calculation with a larger energy cutoff of 700 eV and 8 × 8 × 1 k-points yields nearly the same results.
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RESULTS AND DISCUSSION Pure ZnO Monolayer. First, the structural and electronic properties of a pure ZnO monolayer are computed to examine the reliability of the DFT/PBE calculations. The initial
Figure 1. (a) Optimized structure, (b) the spin charge density distribution, (c) the electronic band structure, and (d) DOS projected on Zn and C atoms of Zn36O35C. The red, gray, and green balls represent O, Zn, and C atoms, respectively. The isosurface value of spin charge is 0.008 e/A3. The Fermi level is set to zero. The red and blue lines represent spin-up and spin-down channels, respectively. 11337
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Table 1. Relaxed C−C Distance (Rd) (Å), Energy Band Gap (Eg) (eV), Localized Magnetic Moment on Carbon (MC) (μB), the Energy Difference Per Supercell between FM and AFM States (Δ = EFM − EAFM) (meV), the Nature of the Ground State (G), the Average Charge Per Carbon Transferred from the ZnO Monolayer to Carbon (C), and the Formation Energy (Ef) (eV) of the Ground State of Zn36O34C2 Rd
Eg (spin-up, down)
1.27 5.59 6.46 8.62 9.87 11.40
0.68 Direct 0.33, 0.32 Indirect 0.24, 0.26 Indirect 1.53, 0.35 Direct, Indirect 1.48, 0 HM 1.56, 0 HM
MC (FM/AFM) 0.66, 0.60/0.59, −0.59 0.60, 0.61/0.57, −0.57 0.61, 0.63/0.62, −0.62 0.59, 0.61/0.58, −0.59 0.60, 0.59/0.59, −0.59
Δ
G
C
Ef
2.0 50.0 −9.0 −16.0 −62.0
NM AFM AFM FM FM FM
0.53 0.27 0.27 0.27 0.28 0.27
2.15 5.32 5.34 5.33 5.47 5.49
Figure 2. (a) Schematic energy level diagram of a C−C pair. (b) and (c) The spin charge density distribution of Zn36O34C2 with C−C distances of 5.59 and 8.62 Å, respectively. The isosurface value is 0.02 e/A3. (d)−(i) Calculated band structures of Zn36O34C2 with C−C distance varying from 1.27 to 11.40 Å. The Fermi level is marked by a dotted line. ppπ(ppσ) and ppπ*(ppσ*) represent ppπ(ppσ) bonding and antibonding states, respectively. The red and blue lines represent spin-up and spin-down channels, respectively.
distance between two C atoms increases to 8.62 Å, the doped ZnO sheet becomes a FM semiconductor with a direct band gap of 1.53 eV in the spin-up channel and an indirect band gap of 0.35 eV in the spin-down channel. The energy difference Table 2. Optimized B−B Distance (Rd) (Å), Energy Band Gap (Eg) (eV), Localized Magnetic Moment on Boron (MB) (μB), the Energy Difference per Supercell Between FM and AFM States (Δ = EFM − EAFM) (meV), the Nature of the Ground State (G), Average Charge Per Boron Transferred from the ZnO Monolayer to Boron (C), and the Formation Energy (Ef) (eV) of the Ground State of Zn36O34B2 Rd 1.55 5.62
Figure 3. (a) Optimized structure, (b) the spin charge density distribution, (c) the electronic band structures, and (d) DOS projected on Zn and B atoms of Zn36O35B. The red, gray, and pink balls represent O, Zn, and B atoms, respectively. The isosurface value of spin charge is 0.02 e/A3. The Fermi level is set to zero. The red and blue lines represent spin-up and spin-down channels, respectively.
6.42a 8.60 9.87b 11.40
analysis suggests that the average charge transferred from the ZnO monolayer to the C atom is about 0.27 e. When the
a
11338
Eg (spin-up, down) 0.40 Indirect 0.40, 0.37 Indirect 0.38, 0.37 Indirect 0.76, 0.55 Direct 0.77, 0.14 Direct 0.80, 0 HM
MB (FM/ AFM)
Δ
G
C
Ef
NM
0.84
3.60
0.23, 0.23/− 0.17, 0.14 −0.12, 0.12
167.0
AFM
0.65
7.84
/
AFM
0.67
7.90
0.23, 0.23/− 0.22, 0.23 0.23, 0.23
−5.0
FM
0.65
7.88
/
FM
0.68
8.16
0.23, 0.19/0.50, −0.17
−52.0
FM
0.67
8.25
The FM state is not obtained. bThe AFM state is not obtained. dx.doi.org/10.1021/jp2125069 | J. Phys. Chem. C 2012, 116, 11336−11342
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Figure 4. (a) Schematic energy level diagram of a B−B pair. (b) and (c) The spin charge density distribution of Zn36O34B2 with B−B distances of 5.62 and 8.60 Å, respectively. The isosurface value is 0.02 e/A3. (d)−(i) Calculated band structures of Zn36O34B2 with B−B distance varying from 1.55 to 11.40 Å. The Fermi level is plotted with a dotted line. ppπ(ppσ) and ppπ*(ppσ*) represent ppπ(ppσ) bonding and antibonding states, respectively. The red and blue lines represent spin-up and spin-down channels, respectively.
Table 3. Optimized N−N Distance (Rd) (Å), Energy Band Gap (Eg) (eV), Localized Magnetic Moment on Nitrogen (MB) (μB), the Energy Difference Per Supercell between FM and AFM States (Δ = EFM − EAFM) (meV), the Nature of the Ground State (G), the Average Charge Per Nitrogen Transferred from the ZnO Monolayer to Nitrogen (C), and the Formation Energy (Ef) (eV) of the Ground State of Zn36O34N2
Figure 5. (a) Optimized structure, (b) the spin charge density distribution, (c) the electronic band structures, and (d) DOS projected on Zn and N atoms of Zn36O35N. The red, gray, and blue balls represent O, Zn, and N atoms, respectively. The isosurface value of spin charge is 0.008 e/A3. The Fermi level is set to zero. The red and blue lines represent spin-up and spin-down channels, respectively.
Rd
Eg (spin-up, down)
3.34
0.41 Direct
5.70
0.46 Direct
6.61
0.47 Direct
8.71
0.48 Direct
9.87
0.44 Direct
11.40
0.43, 0.48 Direct
MN (FM/AFM)
Δ
G
C
Ef
0.53, 0.53/0.53, −0.53 0.54, 0.54/0.54, −0.54 0.54, 0.54/0.54, −0.54 0.54, 0.54/0.54, −0.54 0.54, 0.54/− 0.54, 0.54 0.54, 0.54/0.54, −0.54
24.0
AFM
0.55
2.29
2.0
AFM
0.55
2.87
