Chemical Functionalization of GaN Monolayer by Adatom Adsorption

Article pubs.acs.org/JPCC

Chemical Functionalization of GaN Monolayer by Adatom Adsorption Yuewen Mu*,†,‡ †

Key Laboratory of Materials for Energy Conversion and Storage of Shanxi Province, Institute of Molecular Science, Shanxi University, Taiyuan 030006, People’s Republic of China ‡ National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China Downloaded by UNIV OF PRINCE EDWARD ISLAND on August 27, 2015 | http://pubs.acs.org Publication Date (Web): August 27, 2015 | doi: 10.1021/acs.jpcc.5b04695

S Supporting Information *

ABSTRACT: The effect of adatom adsorption on the magnetism and band structure of graphene-like GaN monolayer at different coverages (1/8 and 1/2) was studied by spin-polarized density functional theory calculations. Generally speaking, though bare GaN monolayer is an indirect-band gap semiconductor, it could be turned to magnetic half-metal/metal by adsorbing F or N atoms at certain coverage. The Curie temperatures for half-metallic Fadsorbed and metallic N-adsorbed GaN monolayer are estimated to be about 480 and 420 K by Monte Carlo simulations, respectively. The induced magnetism mainly comes from the spin splitting of N 2p (2pz or 2px/2py) orbitals. In the meantime, the adsorption of O atoms reduces the gap of the system. These excellent electronic and magnetic properties indicate great potential application of 2D GaN-based materials in future spintronics and optoelectronics.

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scarce. Li et al.22 investigated the two-dimensional (2D) GaN sheets and GaN NRs theoretically, and predicted that Gadefective GaN sheet was magnetic and half-metallic. However, it is difficult to control the defect (doping or vacancy) sites while avoiding additional defects in reality.34 In contrast, adatom adsorption is more attractive for that the coverage (Θ) could be easily changed without any additional defect introduced into the host lattice. Fluoridized ZnO monolayer23 and oxygenated BN monolayer25 were predicted to be magnetic theoretically, which indicated that adsorption may be a good way for the chemical functionalization of graphene-like monolayers. In this study, we investigated the effect of adatom adsorption on the geometry, magnetism and electronic properties of GaN monolayer by density functional theory (DFT) calculations. Botello-Méndez et al.24 attributed the ferromagnetism and metallicity in ZnO zigzag nanoribbons (NRs) to the oxygen dangling bonds at the edge sites, therefore some strong nonmetallic elements (e.g., F, O, and N) seem to be good candidates. We found that nonmagnetic wide band gap GaN monolayer could be turned into magnetic half-metal, magnetic metal or narrow band gap semiconductor by adsorbing specific adatoms at certain coverage.

INTRODUCTION Graphene,1 a two-dimensional (2D) honeycomb structure of carbon, has become a very active field in the past few years, due to its unusual chemical and physical properties.2−6 Graphene and its nanoribbons could be easily functionalized by many different ways for different purposes, such as doping,7−9 defect formation,10,11 adatom adsorption12−14 and edge modification.15,16 These experimental and theoretical studies on graphene stimulate great interests in graphene-like materials, such as honeycomb structures consisted of other group-IV elements and compounds of III−V and II−VI group elements.17 Freestanding BN monolayer,18 supported Si monolayer19 and ZnO bilayer20 have been synthesized successfully one after another, and some other graphene-like monolayer (ML) structures (e.g., SiC, AlN, GaN) were also predicted to be stable by density functional theory (DFT) calculations.17 A lot of attention has also been paid to the functionalization of these graphene-like monolayer materials,21−26 which promotes the development of nanoscale spintronic and electronic devices. In the last two decades, GaN materials, from bulk to nanostructures, have been attracting a lot of attention due to their potential application in mesoscopic electronic and optoelectronic devices.27−29 Low dimensional GaN materials, such as nanotubes,30 nanoribbons (NRs)31,32 and nanosheets (6−10 layers),33 have been synthesized successfully, though the synthesis of GaN monolayer (ML) are still under exploration. On the theoretical side, to our knowledge, the theoretical research on GaN nanostructures, especially on monolayer, are © XXXX American Chemical Society

Received: May 17, 2015 Revised: August 1, 2015

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DOI: 10.1021/acs.jpcc.5b04695 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Downloaded by UNIV OF PRINCE EDWARD ISLAND on August 27, 2015 | http://pubs.acs.org Publication Date (Web): August 27, 2015 | doi: 10.1021/acs.jpcc.5b04695

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METHODS AND COMPUTATIONAL DETAILS All calculations were performed using Vienna ab initio simulation package (VASP)35,36 with the projector augmented wave (PAW)37,38 pseudopotential method. The spin-polarized generalized gradient approximation (GGA) with the Perdew− Burke−Ernzerhof (PBE)39 exchange-correlation function was chosen. All structures were fully relaxed using conjugate gradient method without symmetry constraints for both spinpolarized and spin-unpolarized cases, until the Hellmann− Feynman force acting on each atom was less than 0.01 eV/Å. The kinetic energy cutoff for plane-wave basis set was set to 500 eV. A large vacuum spacing (more than 15 Å) was taken to prevent mirror interactions. The Brillouin zones were sampled with 0.1 and 0.08 Å−1 spacing in reciprocal space by the Monkhorst−Pack scheme40 for geometry optimizations and further property calculations, respectively. They corresponded to 19 × 19 × 1 and 29 × 29 × 1 k-point meshes for (1 × 1) GaN ML unitcell. Given that besides ferromagnetic (FM) and nonmagnetic (NM) states, antiferromagnetic (AFM) states may also appear, we considered four atoms (F, O or N) adsorbed symmetrically on each (2 × 2) GaN ML supercell as the model for high coverage (Θ = 1/2) and (4 × 4) supercell for low coverage (Θ = 1 /8). The adatoms were initially placed at four possible adsorption sites (TN, TGa, H, bridge) at a height of about 2

Ea = (EGaN + 4EA − EGaN + 4A )/4

(1)

where EGaN was the total energy of bare 2D GaN supercell, EA was the energy of an isolated adatom in the vacuum, and EGaN+4A was the total energy of 2D GaN supercell with four adatoms. The interaction energy between adatoms were estimated by ′ + EA − EGaN + A ) EI = Ea − (EGaN

(2)

where E′GaN and EGaN+A were the total energies of 10 × 10 2D GaN supercell without and with an adatom at the same site, respectively.

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RESULTS AND DISCUSSION A planar 2D hexagonal honeycomb lattice with D3h symmetry was obtained after full optimization of a one-atom-thick graphene-like GaN monolayer. Spin-polarized calculations showed that the GaN ML was classified as a nonmagnetic semiconductor with an indirect band gap. The calculated band gap and intralayer Ga−N bond length were 1.95 eV and 1.88 Å, respectively, which were consistent with previous LDA calculations (2.0 eV and 1.93 Å;22 2.27 eV and 1.85 Å17). The intralayer Ga−N bonds had robust covalent character due to planar sp2 hybridization, and nearest-neighbor pz orbitals formed π bonding like graphene. The structures with bridge site adsorption were always unstable and the adsorbate was apt to move toward a nearest-neighbor nitrogen atom for all considered adatoms at different coverages, which may be due to the big difference between their electronegativities (1.81 for Ga and 3.04 for N). For different adatoms at different coverages, corresponding properties of most favorable energetically site model, such as adsorption energy, band structure and magnetic moment, were listed in Table 1 and discussed as follows. Adsorption of F Atoms. The top of gallium (TGa) site was predicted to be most favorable energetically for fluorine adatoms on GaN ML at the low coverage (Θ = 1/8). In this case, the adsorption site gallium atoms were pulled out of the base plane of GaN ML. The average adsorption energy and Ga−F bond length were about 3.09 eV and 1.84 Å, respectively, which indicated that fluorine atom adsorption was exothermic and the fluorine adatoms were chemically adsorbed on the ML. The adjacent Ga−N bonds were elongated to about 1.95 Å in comparison with its original value (about 1.88 Å) in the pristine GaN ML. Corresponding F−Ga−N angles were about 107°, displaying typical character of sp3 hybridization. As a result, about one electron transferred from the gallium atom to the

Figure 1. A (2 × 2) supercell of 2D GaN honeycomb structure, where yellow (blue) balls stand for Ga (N) atoms and red balls stand for adatoms. Four possible adsorption sites are displayed as above: Hollow site (H), top of Ga site (TGa), top of N site (TN), and bridge site.

Å from GaN plane (as displayed in Figure 1). The average adsorption energy was defined as follows:

Table 1. Most Favorable Energetically Sites for Different Adatoms at Different Coveragesa coverage 1

/8

adatom

F O N 1 /2 F O N GaN monolayer

position

H

q

Ea

EI

magnetic GS

M

band structure

Δ/Δ′

TGa TN TN H TN TN

2.40 1.06 0.70 1.22 1.09 0.81

0.67 0.83 1.03/0.73 0.66 0.74 0.73

3.09 2.93 1.83 2.92 2.76 1.55

0.01 −0.04 1.74 −0.16b −0.21 1.46

FM NM FM AFM NM FM NM

1.0 0.0 1.0 0.0 0.0 1.0 0.0

HM SC-direct HM SC-direct SC-indirect M SC-indirect

0.46 1.69 0.11 0.74 1.89  1.95

a

The smallest substrate−adatom distance (H, Å), charge transfer from GaN ML to adatom (q, e) from Bader charge analysis, average adsorption energy (Ea, eV), interaction energy (EI, eV) between adatoms, magnetic ground state (GS) and average magnetic moment induced by each adatom (M, μB) are listed. The band structures can be specified as metal (M), half metal (HM) or semiconductor (SC). The type of the band gap can be either direct or indirect for semiconductors. The band gap (Δ, eV) for a semiconductor or half-metal gap (Δ′, eV) for a half-metal is also given. bThe adsorption energy of F (TGa) on 10 × 10 supercell instead of unstable F (H) was deducted. B


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fluorine adatom, leaving one electron unpaired in the neighboring nitrogen atoms. Our DFT calculations showed that the GaN monolayer, with fluorine adatoms at TGa sites at the coverage of 1/8, was ferromagnetic ordered with a moment of 4 μB per supercell, that is, a moment of 1 μB was induced by each fluorine adatom on average. The interaction energy between fluorine adatoms was negligible (about 0.01 eV as displayed in Table 1), so the magnetism came from isolated fluorine adatoms rather than the interaction between them. The ferromagnetic state was energetically preferred over the antiferromagnetic state by 0.177 eV. Though the above-mentioned GaN monolayer showed ferromagnetism, how the magnetism changes with the temperature needs to be understood for future applications. Two-dimensional Ising model was employed to study this issue, and the Hamiltonian was given by the following equation:

Figure 2. (a) Average magnetic moment induced by each F adatom of GaN monolayer at the coverage of 1/8 vs the temperature in our Monte Carlo (MC) simulation and (b) spin density distribution of Fadsorbed GaN monolayer at the coverage of 1/8 with the isosurface level of 0.005 e/Å3. MFT stands for Curie temperature (TC) calculated by mean-filed theory.

H(m) = − ∑ Jmimj (3)

Here J was the exchange parameter, and mi and mj stood for magnetic moments at sites i and j, respectively. The exchange parameter was calculated from the energy difference between the ferromagnetic and antiferromagnetic states of a supercell with four adatoms.41 Mean-field theory (MFT) was applied to estimate the Curie temperature. The partition function can be expressed as follows: eγJm⟨m⟩ / kBT = 2 cosh(γJ ⟨m⟩/kBT )

∑

Z=

(4)

m = 1, −1

where γ = 6 is the number of nearest neighbors of each adatom, and ⟨m⟩ is the ensemble-average magnetic moment. As a result, the average spin of each site is ⟨m⟩ =

1 Z

∑ m = 1, −1

meγJm⟨m⟩ / kBT ) = tanh(γJ ⟨m⟩/kBT ) (5)

The static solution ⟨m⟩ varies with the change of γ J/kBT, and the critical point locates at γ J/kBT = 1. According to this relation, the Curie temperature (TC) was estimated to be about 770 K. However, due to the neglect of spin fluctuations, the MFT method tends to overestimate the Curie temperature. So Monte Carlo (MC) simulations were carried out to get more precise Curie temperature. In the MC simulations, a 300 × 300 supercell (including 90000 adatoms) was used, and 9 × 107 steps were performed at each temperature. The MC simulation temperature was from 10 to 700 K with an increment of 10 K. As shown in Figure 2a, the average magnetic moment induced by each adatom started decreasing gradually from 1 μB at about 300 K, and it dropped to 0 μB (paramagnetic state) at about 480 K, which was reduced by 38% relative to that estimated by MFT method. In order to explore the origin of the magnetism, the band structure and p-projected density of states (p-PDOS) of the low-coverage F-adsorbed GaN ML were presented in Figure 3a,b. The band structure showed that the down-spin channel was metallic while the up-spin channel was semiconducting with a large band gap (about 2.7 eV). In other words, the adsorption of fluorine adatoms turned the indirect band gap semiconductor GaN ML into a magnetic half-metal with a large half-metal gap of 0.46 eV (as shown in Table 1). On the other hand, the PDOS revealed that a large part of N 2pz orbitals were pushed above the Fermi level, which led to a large

Figure 3. (a) Band structures and (b) p-projected density of states (pPDOS) of F-adsorbed GaN monolayer at the coverage of 1/8.

splitting of the up-spin and down-spin states of N 2p orbits. It constituted a major contribution to the magnetic moment while the contribution of gallium and fluorine atoms was very minor. The spin density distribution (as displayed in Figure 2b) showed that the magnetic moments were mainly localized around nitrogen atoms close to adsorption sites, which resembled the spin density distribution of Ga-defective GaN ML.22 Similar ferromagnetic ordering was also reported for fluorinated ZnO monolayer23 and hydrogenated graphene.12 Instead of the TGa site, the hollow (H) site was preferred when the coverage of fluorine adatoms grew to 1/2. The optimized structure was a trilayer configuration consisting of a gallium plane sandwiched by fluorine and nitrogen planes. The distance between fluorine and gallium planes was 1.22 Å, while the corresponding value between gallium and nitrogen planes was 0.56 Å. The calculated results showed that the antiferromagnetic state had a little larger average adsorption energy (2.92 eV) than the ferromagnetic state (2.90 eV). It was different from ZnO monolayer with fluorine atoms at hollow sites at the same coverage, which was magnetic and halfmetallic.23 The magnetic moments were mainly localized around nitrogen atoms with antiferromagnetic order. This Fadsorbed GaN ML was a semiconductor with a direct band gap C


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sites were about 2.0−2.13 Å, which was much larger than its original value. The adsorbed nitrogen adatoms triggered magnetic ground states with a moment of 4μB per supercell, which was much different to the O-adsorbed systems. It may be due to that there was one electron unpaired near each adsorption site, in spite of the existence of covalent N−N bonds (as shown in Figure 5a) like the O-adsorbed systems.


of 0.74 eV, which was a little larger than that of bulk germanium (0.67 eV) and may have an important application in optoelectronics. Adsorption of O Atoms. The top of nitrogen (TN) site was always preferred when oxygen atoms were adsorbed on the GaN ML, no matter whether the coverage was high (Θ = 1/2) or low (Θ = 1/8). The oxygen adatoms pushed the adsorption site nitrogen atoms out of the base plane. As shown in Figure 4c, the electron localization function (ELF) of the O-adsorbed

Figure 5. (a) Electron localization function (ELF), (b) spin density distribution, and (c) the average magnetic moment induced by each N adatom vs the temperature in our Monte Carlo (MC) simulation for N-adsorbed GaN monolayer at the coverage of 1/2. The isovalues for ELF and spin density are 0.5 and 0.005 e/Å3, respectively. MFT stands for Curie temperature (TC) calculated by mean-filed theory.

The interaction energy between nitrogen adatoms at high coverage was 1.46 eV, and the corresponding value at low coverage was even larger due to some stronger N−N bonds with smaller length (about 1.4 Å). The ferromagnetic ground state of high-coverage N-adsorbed GaN ML had a larger binding energy than the antiferromagnetic state by 0.15 eV, while the corresponding value was 0.12 eV at the low coverage. Their Curie temperatures were estimated to be about 415 K (as shown in Figure 5c) and 325 K by MC simulations, which were overestimated by about 237 and 197 K by MFT method. The band structure and p-projected density of states (pPDOS) of N-adsorbed GaN ML at the high coverage (Θ = 1/2) were plotted in Figure 6a,b. As shown in Figure 6a, both the upspin channel and down-spin channel were metallic with the N 2p orbitals crossed by the Fermi level, which indicated that it

Figure 4. Band structures of O-adsorbed GaN monolayer at the coverage (Θ) of 1/8 (a) and 1/2 (b), and the electron localization function (ELF) for Θ = 1/8 (c) and Θ = 1/2 (d). The isovalue for ELF is 0.5.

GaN ML at Θ = 1/8 revealed that the oxygen adatoms formed covalent bonds with adsorption site nitrogen atoms. Hence, there were no electrons unpaired for these nitrogen atoms after deduction of three electrons bonding with adjacent gallium atoms, which gave rise to a nonmagnetic ground state. The large average adsorption energy for oxygen adatoms (2.93 or 2.76 eV) also indicated strong N−O bonding and chemisorption of oxygen atoms on the GaN ML. In the case of low coverage, the bond lengths between the adsorption site nitrogen atoms and its adjacent gallium atoms were elongated to about 1.99 Å, while other atoms generally maintain their original positions in the base plane. Different to low-coverage GaN ML, which maintained its original hexagonal lattice, the high-coverage O-adsorbed GaN ML was transformed to an orthorhombic cell (as shown in Figure 4d). The oxygen adatoms, which were not right above the adsorption site nitrogen atoms, also formed ionic bonds with neighboring gallium atoms with a length of 2.0 Å. The interaction energy between O adatoms was minus, which indicated that the interaction energy could not compensate the increase of the energy derived from distortion. The band structures of O-adsorbed GaN ML at the coverage of 1/8 and 1/2 were presented in Figure 4a,b. It showed that the band gaps decreased a little after oxygen adsorption. Lowcoverage oxygen adatoms turned the indirect-band gap GaN ML to direct-band gap semiconductor, while the gap became indirect again at high coverage. Adsorption of N atoms. As for nitrogen adsorption, the most favorable energetically sites were the same as that of oxygen adsorption, in other words, they were both the top of nitrogen (TN) sites. The nitrogen adatoms and adsorption site nitrogen atoms were distributed on both sides of Gallium plane symmetrically. The stretched Ga−N bonds near adsorption

Figure 6. (a) Band structures and (b) p-projected density of states (pPDOS) of N-adsorbed GaN monolayer at the coverage of 1/2. D


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The Journal of Physical Chemistry C was a typical magnetic metal. In the meanwhile, the p-PDOS revealed that, different to F-adsorbed GaN ML, the magnetic moment mainly came from the splitting of N 2px/2py orbitals (as seen in Figure 6b), which was supported by the spin density distribution in Figure 5b. When the coverage decreased to 1/8, the N-adsorbed GaN ML became a magnetic half-metal with a half-metal gap of 0.11 eV. As is well-known, N2 is one of the strongest molecules, so the N−N pair at the adsorption site may desorb as N2 molecule. The energy barrier for N−N pair desorbed as N2 molecule on 4 × 4 GaN supercell was investigated (as shown in Figure S1d). The length of N−N bond at adsorption site was 1.6 Å and the desorption barrier was about 1.1 eV. At the high coverage (Θ = 1 /2), the N−N bond length at adsorption site was 1.62 Å, so corresponding desorption barrier should be more or less 1.1 eV due to similar chemical environment. While in the case of low coverage, the N−N bond length at adsorption site was about 1.9 or 1.4 Å, so the desorption barrier may become smaller for a stronger N−N bond with smaller length and vice versa. Considering that adatoms may migrate and form molecules or clusters, their migration barriers were investigated (as shown in Figure S1a-c). The migration barrier for F (TGa) was much smaller (about 0.22 eV) than that for O (TN) and N (TN) adatoms (1.6 and 2.18 eV), which may be due to that F−Ga ionic bonding was much weaker than N−O and N−N covalent bonding. From this point of view, F adatoms was much easier to form molecules or clusters than O or N adatoms. These results indicate that the magnetism of GaN ML induced by adatoms not only depends on the adsorbate species but also on the adsorbate coverage. The magnetism and band structure of GaN ML could be tuned by adatom decoration.

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AUTHOR INFORMATION

Corresponding Author

*(Y.M.) E-mail: [email protected]. Telephone: +86 (0)351 7010699. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No.21373130, 11302121 and 21473106). Y.M. gratefully acknowledges the support of a start-up fund from Shanxi University. The calculations were performed using supercomputers at the Network Center and Institute of Molecular Science, Shanxi University.

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CONCLUSIONS The effect of adatom adsorption (i.e., F, O and N) on the magnetism and band structure of GaN monolayer was studied by spin-polarized density functional theory. Adsorption of fluorine adatoms at low coverage (Θ = 1/8) caused the GaN monolayer to transform into a magnetic half metal from a nonmagnetic indirect band gap semiconductor, whereas the monolayer turned back to a semiconductor at high coverage (Θ = 1/2). By Monte Carlo simulation based on Ising model, we got an estimation of the Curie temperature (about 480 K) for the low-coverage F-adsorbed GaN monolayer. The GaN monolayer would become magnetic metal when adsorbing nitrogen adatoms at high coverage, while it would become magnetic half metal at low coverage. Their Curie temperatures were estimated to be about 415 and 325 K. The induced magnetism mentioned above mainly came from the spin splitting of N 2p (2pz or 2px/2py) orbitals. In contrast, the adsorption of oxygen adatoms did not induce magnetism but reduced the band gap of GaN monolayer. These excellent electronic and magnetic properties indicate great potential application of 2D GaN monolayer based materials in future spintronics and optoelectronics.

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Migration barriers for F (TGa), O (TN), and N (TN) adatoms on 4 × 4 GaN supercells and N−N (TN) desorption barrier on 4 × 4 GaN supercell (PDF)

ASSOCIATED CONTENT

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This material is available free of charge via the Internet at http://pubs.acs. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b04695. E


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Chemical Functionalization of GaN Monolayer by Adatom Adsorption

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