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The electronic and magnetic properties of B-doping Stone–Wales defected graphene decorated with transition-metal atoms
Physica E 73 (2015) 257–261
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
The electronic and magnetic properties of B-doping Stone–Wales defected graphene decorated with transition-metal atoms Qingxiao Zhou a,n, Zhibing Fu b, Chaoyang Wang b, Yongjian Tang b, Hong Zhang c, Lei Yuan b, Xi Yang b a
College of Physical Engineering, Henan University of Science and Technology, Luoyang 471023, People's Republic of China Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, People's Republic of China c College of Physical Science and Technology, Sichuan University, Chengdu 610065, People's Republic of China b
H I G H L I G H T S
G R A P H I C
A B S T R A C T
The presence of Stone–Wales defect enhanced the adsorption of TM atoms. The introducing of B-dopant improved the activity of SW-defected graphene. The magnetic properties of the adsorption systems were mainly contributed by the 3d orbitals of TMadatoms.
The geometry, electronic, and magnetic properties of transition-metal (TM) atoms adsorbed on the Stone-Wales (SW) defected graphene with or without B-doped were investigated by the first-principles density functional theory (DFT) method.
art ic l e i nf o
a b s t r a c t
Article history: Received 30 October 2014 Received in revised form 27 May 2015 Accepted 28 May 2015 Available online 3 June 2015
The geometry, electronic, and magnetic properties of transition-metal (TM) atoms adsorbed on the Stone–Wales (SW) defected graphene with or without B-doped were investigated by the first-principles density functional theory (DFT), aiming to study the effect of a combination of B-dopant and SW-defect on the adsorption of TM-adatoms on graphene. It was found that the introducing of B-dopant enhanced the adsorption of TM-adatoms, while it hardly affected the electronic structure of defected graphene. Meanwhile, the magnetic properties of the adsorption systems were mainly contributed by the 3d orbitals of TM-adatoms. We hope our results will be useful for applications in the designing of devices based on graphene. & 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a flat monolayer of sp2-bonded carbon atoms arranged in a hexagonal structure, has attracted extensive attention due to its novel electronic, optical and mechanical properties [1– 5], since its experiment fabrication successfully obtained in 2004 [6]. However, structural defects, such as vacancies [7], impurities [8], Stone–Wales (SW) defects [9–11], and pentagonal-octagon [12] have been widely observed in graphene. The appearance of n
Corresponding author. E-mail address: [email protected] (Q. Zhou).
http://dx.doi.org/10.1016/j.physe.2015.05.033 1386-9477/& 2015 Elsevier B.V. All rights reserved.
various defects can alter the mechanical and electronic properties of graphene dramatically [7,13–15]. Stone–Wales defect was a typical topological defect in graphene or CNTs which was consisted of two pairs of pentagon-heptagonal rings. The presence of SWdefect reduces the band gap in the large band gap CNTs [16] and generates quasibond states in metallic CNTs [17]. Furthermore, the graphene with SW-defect is more sensitive than that of perfect graphene for absorbing ozone [18], alkanethiol molecules [19], and formaldehyde [20,21]. In another aspect, abundant theoretical and experimental investigations demonstrated that doping is a promising method to manipulate the electronic structure of graphene. Especially, Boron is a typical dopant acing as hole doping which
258
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makes the Dirac point move below the Fermi level [22]. Also, transition-metal absorbed on the carbon-based materials has been of great interest because of the decoration by TM-adatoms can give rise to novel physical and chemical properties [23–26]. In our paper, we have carried out the first-principles calculations based on DFT to investigate the effect of a combination of SW-defect and TM-adatoms (Ti, V, Cr and Mn) on the electronic and magnetic properties of graphene with or without B-doped. The corresponding parameters detailly describing the geometry and electronic structures were performed.
2. Computational method All the calculations were performed within the framework of first-principles DFT, implemented in the Dmol3 code [27]. We used the generalized gradient approximation (GGA) for exchange-correlation functional, as described by Perdew–Burke–Ernzerhof (PBE) [28]. We selected DFT semicore pseudopotential (DSSP) to replace core electrons as a single effective potential [29].A double numerical plus polarization (DNP) was employed as the basis set. The DNP basis set corresponds to a double-ζ quality basis set with a p-type polarization functions to hydrogen and d-type polarization functions added to heavier atoms, which is comparable with the Gaussian 6-31G (d, p) basis set and owns a better accuracy [30]. In our work, we used 5 5 1 supercell with periodic boundary condition on the x and y axes to model the infinite graphene sheet. The vacuum space of 20 Å was set in the direction normal to the sheets to avoid the interactions between periodic images. A 5 5 1 mesh of k-point and the global orbital cutoff of 5.0 Å were set in the spin-unrestricted calculations. All atoms were allowed to relax. Convergence in energy, force, and displacement was set at 2 10 5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively. The adsorption energy Eads of the TM-adatoms on the graphene is defined as
Eads (TM) = E T [TM/G] − E T [G] − E T [TM]
(1)
where ET[TM/G] is the total energy of TM-adatoms adsorbed on the modified graphene, ET[G] is the total energy of the modified graphene, ET[TM] is the total energy of a free TM atom.
3. Results and discussion 3.1. B-doping Stone–Wales defected graphene with dopants In this section, we first discussed the structures of Stone–Wales defected graphene (SWG) and B-doped SW-defected graphene (BSWG). The optimized atomic structures of SWG and BSWG were depicted in Fig. 1. As shown in Fig. 1a, the Stone–Wales defect was created by rotating a C–C bond by 90°and then formed pairs of 5-
and 7-atom rings. The formation energy (Ef) was defined as
Ef = EPG –ESWG
(2)
where EPG is the total ground state energy of the perfect graphene (PG) and ESWG is the total energy of the Stone–Wales defected graphene (SWG). In our results, the Ef was 3.78 eV, which was consistent with previous investigations [31,32]. After optimized, the rotated C–C bond (B3) was compressed from 1.420 Å (PG) to 1.318 Å, and the bond lengths of B1 and B2 were 1.459 and 1.460 Å. Especially, the carbon atoms consisting of the SW-defect moved out of the plane to allow the compressed carbon bonds to expand. For B-doping, five doping sits were examined in the structure of SWG, as marked in Fig. 1a. The calculated results suggested that the 1 site exhibited the highest stability with the lowest energy. After relaxation, the bond lengths of B–C bonds (B1, B2, and B3) changed to be 1.517 Å, 1.517 Å, and 1.373 Å, respectively. To deeply understand the effect of SW-defect and B-dopant on the electronic and magnetic properties of graphene, we performed the total density of states (TDOS) and partial density of states (PDOS) of SWG and BSWG in Fig. 2. The PDOS of C atoms provided in the figure were the adjacent C toms to the C1. As depicted in Fig. 2, the appearance of SW-defect produced energy gap open. According to the previous investigations, the perfect graphene exhibited zero band gap, whereas the SWG owned 1.088 eV. This phenomenon was consistent with the results reported previously [32]. After substituting one carbon atom at C1 site by B resulted in the formation of accepter states along with electron transfer around the defect. Furthermore, the empty pz state of B-dopant interact the sp2 bonds of the C atoms, then producing the unpaired electron. From Fig. 2a, it was found that the orbitals of B-2p and C-2p hybridized significantly below and above the Fermi level. Furthermore, the DOS intensity of BSWG exhibited higher activity than SWG around the Fermi level. Therefore, we expected the B-dopant defected graphene would offer reactive provision to stabilize the TM adatoms, and the corresponding interaction will further model the electronic structure of graphene substrate. 3.2. Adsorption of TM-adatoms 3.2.1. Adsorption of TM-adatoms on Stone–Wales defected graphene Firstly, we discussed the adsorption of TM-adatoms (Ti, V, Cr and Mn) on the SW-defected graphene. As marked in Fig. 1a, we considered three types of adsorption sites, namely, bridge site (B), hollow site (H), and top sites (T), which were the top of the bond of C–C, the center of carbon ring, and the top of C atom, respectively. In consideration of the symmetry of the structure, the hollow sites included the centers of pentagon ring (H1), hexagon ring (H2), and heptagon ring (H3). The calculated results indicated that the TM-adatoms absorbed on the bridge sites were the most stable. Furthermore, we summarized the parameters of adsorption energy and the smallest adatom-carbon distance in the Table 1. As
Fig. 1. The optimized atomic structures of the (a) SWG and (b) BSWG, the gray and pink spheres denotes the carbon and boron atoms, respectively.
Q. Zhou et al. / Physica E 73 (2015) 257–261
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Fig. 2. The electronic density of states (DOS) and partial density of states (PDOS) of (a) SWG and (b) BSWG. Table 1 Summary of results for transition metal atoms adsorbed on Stone–Wales defected graphene (SWG). The properties listed are adsorption energy (Eads) and the smallest adatom-carbon distance (dAC). System
Ti/SWG
V/SWG
Cr/SWG
Mn/SWG
Eads (eV) dAC (Å)
4.202 2.133
2.726 2.081
2.143 2.514
1.764 2.300
Table 2 Summary of results for transition metal atoms adsorbed on B-doped SW-defect graphene (BSWG). The properties listed are the adsorption energy (Eads), the adatom-boron distance (dAB), the nearest adatom-carbon distance (dAC), magnetic moment of TM atom before absorbed (miso), magnetic moment of TM atom after absorbed (madatom), magnetic moment of per supercell (mTM-G) and charge transfer of TM atoms (Q). System
Ti/BSWG V/BSWG Cr/BSWG Mn/BSWG
Eads (eV) dAB (Å) dAC (Å) miso (mB) madatom (mB) 5.460 4.428 2.514 1.996
2.114 2.039 2.031 2.003
2.239 2.205 2.190 2.084
4.000 5.000 6.000 5.000
1.011 0 2.355 0
mTM-G (mB) Q (e)
0.998 0 0.263 0
0.536 0.371 0.391 0.133
listed in Table 1, it was found that the Eads varied from 0.764 eV to 4.202 eV, and the Ti-adatom owned the highest value of 4.202 eV, which was much larger than that absorbed on perfect graphene with the value of 1.301 eV 2.220 eV [33–35]. The large Eads was an indication that the presence of SW-defect affected the adsorption process and enhanced the interaction between the TM-adatoms and the graphene substrate. 3.2.2. Adsorption of TM-adatoms on B-doped SW-defected graphene In this part, we mainly focused on the adsorption of TM-adatoms on the B-doped defected graphene. For comparison, we also consider the TM-adatoms absorbed on the bridge site of B–C bond, and a series of parameters detaily describing the adsorption complexes were performed in Table 2. We found that the value of adsorption energy ranged from 1.996 eV to 5.406 eV, which were larger than that of SWG (Table 1). Therefore, the introducing of B-dopant increased the activity of SWG and stabilized the adsorption of TM-adatoms. Meanwhile, for the same TM atom, the adatom located closer to the B atom than to C atom, which suggested the B-dopant acted an important role in the adsorption process. In addition, the charge transfer was investigated by Mulliken population analysis. Due to the unpaired electrons of TM3d orbital, the TM-adatoms transferred electron to the graphene
Fig. 3. (a) The band structure of B-doped Stone–Wales defected graphene with Ti-adatom. (b) PDOS of Ti-adatom, B, and C atoms forming bands with B-dopant.
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Fig. 4. (a) The band structure of B-doped Stone–Wales defected graphene with V-adatom. (b) PDOS of V-adatom, B, and C atoms forming bands with B-dopant.
Fig. 5. (a) The band structure of B-doped Stone–Wales defected graphene with Cr-adatom. (b) PDOS of Cr-adatom, B, and C atoms forming bands with B-dopant.
Fig. 6. (a) The band structure of B-doped Stone–Wales defected graphene with Mn-adatom. (b) PDOS of Mn-adatom, B, and C atoms forming bands with B-dopant.
Q. Zhou et al. / Physica E 73 (2015) 257–261
substrate. The Q of Ti was the largest owning the value of 0.536e, which was consistent with its highest adsorption energy. In addition, we also listed the calculated spin-polarized results in Table 2. According to the results, it was learned that the all the TM atoms have a magnetic ground state, whereas the BSWG was nonmagnetic. For TM-adatoms, the magnetic properties changed obviously after absorbed on the BSWG. For example, V and Mn adatoms exhibited no magnetic moments (0mB), while the two free TM atoms present the magnetic moments of 5mB. As for other TMadatoms (Ti and Cr), the magnetic moment decreased by the approximate value of 3mB. The changes of magnetic property were resulted from the orbital interaction which changed the structure of unpaired electrons of TM-adatoms. To deeply understand the effect of the combination of defect and B-dopant on the electronic properties, the band structures of adsorption complexes were illustrated in Figs. 3a–6a corresponding to different TM-adatoms (Ti, V, Cr and Mn). The spin-up bands (majority) and the spin-down bands (minority) were depicted in each case separately. For the cases of V- and Mn-adsorbed on the B-doped defected graphene, the majority and minority bands were found to be symmetrical, which indicated that their total magnetic moment was zero with the valence electrons arranged in pairs. In addition, the two systems exhibited metallicity with the conduction band through the Fermi level. As for the Ti-adatom adsorption structure, the spin-up bands presented metallicity with the Fermi level located at the impurity band induced by Ti-adatom, whereas the spin-down bands exhibited semicoductivity owning the band gap of 0.980 eV. Namely, the presence of half-metallic behavior was observed. In our previous study, we explored the adsorption of TM-adatoms on the center of heptagon carbon-ring of SW-defected graphene [36]. The band gaps of Ti-adsorbed on the BSWG decreased by approximately 0.25 eV, which was due to the introducing of B-dopant. The instance of Cr-adatom system, it was similar with that of Ti-adatom complex. In another words, the band structure also presented half-metallicity. The difference between the two instances was the spin-down bands located through the Fermi level exhibiting metallicity in the structure of Cr/BSWG. According to the above analysis, in comparsion with the instances of TM-adatoms on SWG without B-doping [36], the TMadatom certainly affected the electronic structure of the graphene significantly, while the B-dopant did not. The PDOS of TM-adatom, B, and C atoms forming bands with B-dopant were illustrated in Figs. 3b–6b. It was found that the spin-up and spin-down PDOS in the two cases, V- and Mn-adatoms absorbed on the SWG, were completely identical, which agreed our previous analysis indicating the interaction between the TM-adatom and the graphene substrate affected the unpaired valence electrons. Furthermore, the overlaps between the 3d orbital of V (Mn) and 2p orbital of B was observed below and above the Fermi level with several sharp peaks located at 3.45 eV, 2.14 eV, 0.86 eV, and 2.66 eV ( 3.57 eV, 1.95 eV and 0.75 eV), respectively. The hybridization of B and TM-adatoms was an indication that the B-dopant played an important role in the adsorption process, which was consistent with previous results that the introducing of B enhanced the interaction between the TMadatoms and the graphene substrate. As for the Ti and Cr-absorbed structures, the majority and minority PDOS of C and B atoms were almost symmetric, while the 3d-orbital of Ti and Cr were mismatched. Accordingly, the unpaired electrons produced the net magnetic moment to the systems. Based on above analysis, the magnetic properties of adsorption complexes were mainly attributed to the TM-3d orbitals, and the intense hybridization between B-2p and 3d orbitals of TM-adatoms increased the stability of structures.
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4. Conclusions In summary, the geometry, energetic, electronic and magnetic properties of TM-adatoms (Ti, V, Cr and Mn) on Stone–Wales defected graphene with or without B-doping were investigated by DFT calculations. It was found that the introducing of B-dopant increased the activity of the SWG and enhanced the adsorption of TM-adatoms on graphene substrate. Furthermore, the TM atoms characterized by their magnetic behavior endowed the adsorption complexes with different magnetic properties. According to the results of PDOS, the net magnetic moments of systems were mainly contributed by the 3d orbital of TM-adatoms. More interestingly, the Ti and Cr absorbed on the BSWG exhibited half-metallicity, while the cases of V and Mn presented metallicity.
Acknowledgments This work was supported by the National Natural Science Foundation of China (11074176, 10976019 and 51101141), the Research Fund for the Doctoral Program of Higher Education of China (20100181110080), and the Research Fund from Science and Technology on Plasma Physics Laboratory (9140C680501120C68254).
References [1] K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich, S. Morozov, A. Geim, Proc. Natl. Acad. Sci. 102 (2005) 10451. [2] K. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M.K.I. Grigorieva, S. Dubonos, A. Firsov, Nature 438 (2005) 197. [3] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [4] A.C. Neto, F. Guinea, N. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81 (2009) 109. [5] Y. Zhang, Y.-W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201. [6] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Science 306 (2004) 666. [7] G.-D. Lee, C. Wang, E. Yoon, N.-M. Hwang, D.-Y. Kim, K. Ho, Phys. Rev. Lett. 95 (2005) 205501. [8] M. Sternberg, L. Curtiss, D. Gruen, G. Kedziora, D. Horner, P. Redfern, P. Zapol, Phys. Rev. Lett. 96 (2006) 075506. [9] F. Ding, Phys. Rev. B 72 (2005) 245409. [10] F.J. Ribeiro, P. Tangney, S.G. Louie, M.L. Cohen, Phys. Rev. B 72 (2005) 214109. [11] E.J. Duplock, M. Scheffler, P.J. Lindan, Phys. Rev. Lett. 92 (2004) 225502. [12] P. Lehtinen, A. Foster, A. Ayuela, A. Krasheninnikov, K. Nordlund, R. Nieminen, Phys. Rev. Lett. 91 (2003) 017202. [13] F. Banhart, J. Kotakoski, A.V. Krasheninnikov, ACS Nano 5 (2011) 6. [14] M.H. Gass, U. Bangert, A.L. Bleloch, P. Wang, R.R. Nair, A. Geim, Nat. Nanotechnol. 3 (2008) 676. [15] L. Tapaszto, G. Dobrik, P. Nemes-Incze, G. Vertesy, P. Lambin, L.P. Biró, Phys. Rev. B 78 (2008) 233407. [16] V.H. Crespi, M.L. Cohen, A. Rubio, Phys. Rev. Lett. 79 (1997) 2093. [17] H.J. Choi, J. Ihm, S.G. Louie, M.L. Cohen, Phys. Rev. Lett. 84 (2000) 2917. [18] B. Akdim, T. Kar, X. Duan, R. Pachter, Chem. Phys. Lett. 445 (2007) 281. [19] S. Roh, J. Oh, Y. Choi, D. Sohn, W. Kim, C. Cho, W. Yi, J. Yoo, C. Lee, J. Kim, J. Vac. Sci. Technol. B Microelectron. Nanomater. Struct. 22 (2004) 1411. [20] X. Qin, Q. Meng, W. Zhao, Surf. Sci. 605 (2011) 930. [21] Q. Zhou, C. Wang, Z. Fu, Y. Tang, H. Zhang, Comp. Mater. Sci. 83 (2014) 398. [22] Q. Zhou, Z. Fu, Y. Tang, H. Zhang, C. Wang, Physica E 60 (2014) 133. [23] Y. Yagi, T.M. Briere, M.H.F. Sluiter, V. Kumar, A.A. Farajian, Y. Kawazoe, Phys. Rev. B 69 (2004) 075414. [24] K.T. Chan, J.B. Neaton, M.L. Cohen, Phys. Rev. B 77 (2008). [25] Y. Mao, J. Yuan, J. Zhong, J. Phys. Condens. Matter 20 (2008). [26] R.C. Longo, J. Carrete, J. Ferrer, L.J. Gallego, Phys. Rev. B 81 (2010). [27] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 4. [28] B. Delley, J. Chem. Phys. 113 (2000) 9. [29] Z.M. Ao, J. Yang, S. Li, Q. Jiang, Chem. Phys. Lett. 461 (2008) 4. [30] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5. [31] L. Chen, Y. Ouyang, Y.L. Wang, Y.Y. Sun, H.Z. Pan, Physica E 43 (2010) 5. [32] S. Azadi, R. Moradian, A.M. Shafaee, Comp. Mater. Sci. 49 (2010) 5. [33] Q. Pang, Y. Zhang, J.M. Zhang, K.W. Xu, Comp. Mater. Sci. 50 (2011) 8. [34] H. Sevincli, M. Topsakal, E. Durgun, S. Ciraci, Phys. Rev. B 77 (2008) 7. [35] K.T. Chan, J.B. Neaton, M.L. Cohen, Phys. Rev. B 77 (2008) 12. [36] Q.X. Zhou, Y.J. Tang, C.Y. Wang, Z.B. Fu, H. Zhang, Comp. Mater. Sci. 81 (2014) 5.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
The electronic and magnetic properties of B-doping Stone–Wales defected graphene decorated with transition-metal atoms Qingxiao Zhou a,n, Zhibing Fu b, Chaoyang Wang b, Yongjian Tang b, Hong Zhang c, Lei Yuan b, Xi Yang b a
College of Physical Engineering, Henan University of Science and Technology, Luoyang 471023, People's Republic of China Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, People's Republic of China c College of Physical Science and Technology, Sichuan University, Chengdu 610065, People's Republic of China b
H I G H L I G H T S
G R A P H I C
A B S T R A C T
The presence of Stone–Wales defect enhanced the adsorption of TM atoms. The introducing of B-dopant improved the activity of SW-defected graphene. The magnetic properties of the adsorption systems were mainly contributed by the 3d orbitals of TMadatoms.
The geometry, electronic, and magnetic properties of transition-metal (TM) atoms adsorbed on the Stone-Wales (SW) defected graphene with or without B-doped were investigated by the first-principles density functional theory (DFT) method.
art ic l e i nf o
a b s t r a c t
Article history: Received 30 October 2014 Received in revised form 27 May 2015 Accepted 28 May 2015 Available online 3 June 2015
The geometry, electronic, and magnetic properties of transition-metal (TM) atoms adsorbed on the Stone–Wales (SW) defected graphene with or without B-doped were investigated by the first-principles density functional theory (DFT), aiming to study the effect of a combination of B-dopant and SW-defect on the adsorption of TM-adatoms on graphene. It was found that the introducing of B-dopant enhanced the adsorption of TM-adatoms, while it hardly affected the electronic structure of defected graphene. Meanwhile, the magnetic properties of the adsorption systems were mainly contributed by the 3d orbitals of TM-adatoms. We hope our results will be useful for applications in the designing of devices based on graphene. & 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a flat monolayer of sp2-bonded carbon atoms arranged in a hexagonal structure, has attracted extensive attention due to its novel electronic, optical and mechanical properties [1– 5], since its experiment fabrication successfully obtained in 2004 [6]. However, structural defects, such as vacancies [7], impurities [8], Stone–Wales (SW) defects [9–11], and pentagonal-octagon [12] have been widely observed in graphene. The appearance of n
Corresponding author. E-mail address: [email protected] (Q. Zhou).
http://dx.doi.org/10.1016/j.physe.2015.05.033 1386-9477/& 2015 Elsevier B.V. All rights reserved.
various defects can alter the mechanical and electronic properties of graphene dramatically [7,13–15]. Stone–Wales defect was a typical topological defect in graphene or CNTs which was consisted of two pairs of pentagon-heptagonal rings. The presence of SWdefect reduces the band gap in the large band gap CNTs [16] and generates quasibond states in metallic CNTs [17]. Furthermore, the graphene with SW-defect is more sensitive than that of perfect graphene for absorbing ozone [18], alkanethiol molecules [19], and formaldehyde [20,21]. In another aspect, abundant theoretical and experimental investigations demonstrated that doping is a promising method to manipulate the electronic structure of graphene. Especially, Boron is a typical dopant acing as hole doping which
258
Q. Zhou et al. / Physica E 73 (2015) 257–261
makes the Dirac point move below the Fermi level [22]. Also, transition-metal absorbed on the carbon-based materials has been of great interest because of the decoration by TM-adatoms can give rise to novel physical and chemical properties [23–26]. In our paper, we have carried out the first-principles calculations based on DFT to investigate the effect of a combination of SW-defect and TM-adatoms (Ti, V, Cr and Mn) on the electronic and magnetic properties of graphene with or without B-doped. The corresponding parameters detailly describing the geometry and electronic structures were performed.
2. Computational method All the calculations were performed within the framework of first-principles DFT, implemented in the Dmol3 code [27]. We used the generalized gradient approximation (GGA) for exchange-correlation functional, as described by Perdew–Burke–Ernzerhof (PBE) [28]. We selected DFT semicore pseudopotential (DSSP) to replace core electrons as a single effective potential [29].A double numerical plus polarization (DNP) was employed as the basis set. The DNP basis set corresponds to a double-ζ quality basis set with a p-type polarization functions to hydrogen and d-type polarization functions added to heavier atoms, which is comparable with the Gaussian 6-31G (d, p) basis set and owns a better accuracy [30]. In our work, we used 5 5 1 supercell with periodic boundary condition on the x and y axes to model the infinite graphene sheet. The vacuum space of 20 Å was set in the direction normal to the sheets to avoid the interactions between periodic images. A 5 5 1 mesh of k-point and the global orbital cutoff of 5.0 Å were set in the spin-unrestricted calculations. All atoms were allowed to relax. Convergence in energy, force, and displacement was set at 2 10 5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively. The adsorption energy Eads of the TM-adatoms on the graphene is defined as
Eads (TM) = E T [TM/G] − E T [G] − E T [TM]
(1)
where ET[TM/G] is the total energy of TM-adatoms adsorbed on the modified graphene, ET[G] is the total energy of the modified graphene, ET[TM] is the total energy of a free TM atom.
3. Results and discussion 3.1. B-doping Stone–Wales defected graphene with dopants In this section, we first discussed the structures of Stone–Wales defected graphene (SWG) and B-doped SW-defected graphene (BSWG). The optimized atomic structures of SWG and BSWG were depicted in Fig. 1. As shown in Fig. 1a, the Stone–Wales defect was created by rotating a C–C bond by 90°and then formed pairs of 5-
and 7-atom rings. The formation energy (Ef) was defined as
Ef = EPG –ESWG
(2)
where EPG is the total ground state energy of the perfect graphene (PG) and ESWG is the total energy of the Stone–Wales defected graphene (SWG). In our results, the Ef was 3.78 eV, which was consistent with previous investigations [31,32]. After optimized, the rotated C–C bond (B3) was compressed from 1.420 Å (PG) to 1.318 Å, and the bond lengths of B1 and B2 were 1.459 and 1.460 Å. Especially, the carbon atoms consisting of the SW-defect moved out of the plane to allow the compressed carbon bonds to expand. For B-doping, five doping sits were examined in the structure of SWG, as marked in Fig. 1a. The calculated results suggested that the 1 site exhibited the highest stability with the lowest energy. After relaxation, the bond lengths of B–C bonds (B1, B2, and B3) changed to be 1.517 Å, 1.517 Å, and 1.373 Å, respectively. To deeply understand the effect of SW-defect and B-dopant on the electronic and magnetic properties of graphene, we performed the total density of states (TDOS) and partial density of states (PDOS) of SWG and BSWG in Fig. 2. The PDOS of C atoms provided in the figure were the adjacent C toms to the C1. As depicted in Fig. 2, the appearance of SW-defect produced energy gap open. According to the previous investigations, the perfect graphene exhibited zero band gap, whereas the SWG owned 1.088 eV. This phenomenon was consistent with the results reported previously [32]. After substituting one carbon atom at C1 site by B resulted in the formation of accepter states along with electron transfer around the defect. Furthermore, the empty pz state of B-dopant interact the sp2 bonds of the C atoms, then producing the unpaired electron. From Fig. 2a, it was found that the orbitals of B-2p and C-2p hybridized significantly below and above the Fermi level. Furthermore, the DOS intensity of BSWG exhibited higher activity than SWG around the Fermi level. Therefore, we expected the B-dopant defected graphene would offer reactive provision to stabilize the TM adatoms, and the corresponding interaction will further model the electronic structure of graphene substrate. 3.2. Adsorption of TM-adatoms 3.2.1. Adsorption of TM-adatoms on Stone–Wales defected graphene Firstly, we discussed the adsorption of TM-adatoms (Ti, V, Cr and Mn) on the SW-defected graphene. As marked in Fig. 1a, we considered three types of adsorption sites, namely, bridge site (B), hollow site (H), and top sites (T), which were the top of the bond of C–C, the center of carbon ring, and the top of C atom, respectively. In consideration of the symmetry of the structure, the hollow sites included the centers of pentagon ring (H1), hexagon ring (H2), and heptagon ring (H3). The calculated results indicated that the TM-adatoms absorbed on the bridge sites were the most stable. Furthermore, we summarized the parameters of adsorption energy and the smallest adatom-carbon distance in the Table 1. As
Fig. 1. The optimized atomic structures of the (a) SWG and (b) BSWG, the gray and pink spheres denotes the carbon and boron atoms, respectively.
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Fig. 2. The electronic density of states (DOS) and partial density of states (PDOS) of (a) SWG and (b) BSWG. Table 1 Summary of results for transition metal atoms adsorbed on Stone–Wales defected graphene (SWG). The properties listed are adsorption energy (Eads) and the smallest adatom-carbon distance (dAC). System
Ti/SWG
V/SWG
Cr/SWG
Mn/SWG
Eads (eV) dAC (Å)
4.202 2.133
2.726 2.081
2.143 2.514
1.764 2.300
Table 2 Summary of results for transition metal atoms adsorbed on B-doped SW-defect graphene (BSWG). The properties listed are the adsorption energy (Eads), the adatom-boron distance (dAB), the nearest adatom-carbon distance (dAC), magnetic moment of TM atom before absorbed (miso), magnetic moment of TM atom after absorbed (madatom), magnetic moment of per supercell (mTM-G) and charge transfer of TM atoms (Q). System
Ti/BSWG V/BSWG Cr/BSWG Mn/BSWG
Eads (eV) dAB (Å) dAC (Å) miso (mB) madatom (mB) 5.460 4.428 2.514 1.996
2.114 2.039 2.031 2.003
2.239 2.205 2.190 2.084
4.000 5.000 6.000 5.000
1.011 0 2.355 0
mTM-G (mB) Q (e)
0.998 0 0.263 0
0.536 0.371 0.391 0.133
listed in Table 1, it was found that the Eads varied from 0.764 eV to 4.202 eV, and the Ti-adatom owned the highest value of 4.202 eV, which was much larger than that absorbed on perfect graphene with the value of 1.301 eV 2.220 eV [33–35]. The large Eads was an indication that the presence of SW-defect affected the adsorption process and enhanced the interaction between the TM-adatoms and the graphene substrate. 3.2.2. Adsorption of TM-adatoms on B-doped SW-defected graphene In this part, we mainly focused on the adsorption of TM-adatoms on the B-doped defected graphene. For comparison, we also consider the TM-adatoms absorbed on the bridge site of B–C bond, and a series of parameters detaily describing the adsorption complexes were performed in Table 2. We found that the value of adsorption energy ranged from 1.996 eV to 5.406 eV, which were larger than that of SWG (Table 1). Therefore, the introducing of B-dopant increased the activity of SWG and stabilized the adsorption of TM-adatoms. Meanwhile, for the same TM atom, the adatom located closer to the B atom than to C atom, which suggested the B-dopant acted an important role in the adsorption process. In addition, the charge transfer was investigated by Mulliken population analysis. Due to the unpaired electrons of TM3d orbital, the TM-adatoms transferred electron to the graphene
Fig. 3. (a) The band structure of B-doped Stone–Wales defected graphene with Ti-adatom. (b) PDOS of Ti-adatom, B, and C atoms forming bands with B-dopant.
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Fig. 4. (a) The band structure of B-doped Stone–Wales defected graphene with V-adatom. (b) PDOS of V-adatom, B, and C atoms forming bands with B-dopant.
Fig. 5. (a) The band structure of B-doped Stone–Wales defected graphene with Cr-adatom. (b) PDOS of Cr-adatom, B, and C atoms forming bands with B-dopant.
Fig. 6. (a) The band structure of B-doped Stone–Wales defected graphene with Mn-adatom. (b) PDOS of Mn-adatom, B, and C atoms forming bands with B-dopant.
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substrate. The Q of Ti was the largest owning the value of 0.536e, which was consistent with its highest adsorption energy. In addition, we also listed the calculated spin-polarized results in Table 2. According to the results, it was learned that the all the TM atoms have a magnetic ground state, whereas the BSWG was nonmagnetic. For TM-adatoms, the magnetic properties changed obviously after absorbed on the BSWG. For example, V and Mn adatoms exhibited no magnetic moments (0mB), while the two free TM atoms present the magnetic moments of 5mB. As for other TMadatoms (Ti and Cr), the magnetic moment decreased by the approximate value of 3mB. The changes of magnetic property were resulted from the orbital interaction which changed the structure of unpaired electrons of TM-adatoms. To deeply understand the effect of the combination of defect and B-dopant on the electronic properties, the band structures of adsorption complexes were illustrated in Figs. 3a–6a corresponding to different TM-adatoms (Ti, V, Cr and Mn). The spin-up bands (majority) and the spin-down bands (minority) were depicted in each case separately. For the cases of V- and Mn-adsorbed on the B-doped defected graphene, the majority and minority bands were found to be symmetrical, which indicated that their total magnetic moment was zero with the valence electrons arranged in pairs. In addition, the two systems exhibited metallicity with the conduction band through the Fermi level. As for the Ti-adatom adsorption structure, the spin-up bands presented metallicity with the Fermi level located at the impurity band induced by Ti-adatom, whereas the spin-down bands exhibited semicoductivity owning the band gap of 0.980 eV. Namely, the presence of half-metallic behavior was observed. In our previous study, we explored the adsorption of TM-adatoms on the center of heptagon carbon-ring of SW-defected graphene [36]. The band gaps of Ti-adsorbed on the BSWG decreased by approximately 0.25 eV, which was due to the introducing of B-dopant. The instance of Cr-adatom system, it was similar with that of Ti-adatom complex. In another words, the band structure also presented half-metallicity. The difference between the two instances was the spin-down bands located through the Fermi level exhibiting metallicity in the structure of Cr/BSWG. According to the above analysis, in comparsion with the instances of TM-adatoms on SWG without B-doping [36], the TMadatom certainly affected the electronic structure of the graphene significantly, while the B-dopant did not. The PDOS of TM-adatom, B, and C atoms forming bands with B-dopant were illustrated in Figs. 3b–6b. It was found that the spin-up and spin-down PDOS in the two cases, V- and Mn-adatoms absorbed on the SWG, were completely identical, which agreed our previous analysis indicating the interaction between the TM-adatom and the graphene substrate affected the unpaired valence electrons. Furthermore, the overlaps between the 3d orbital of V (Mn) and 2p orbital of B was observed below and above the Fermi level with several sharp peaks located at 3.45 eV, 2.14 eV, 0.86 eV, and 2.66 eV ( 3.57 eV, 1.95 eV and 0.75 eV), respectively. The hybridization of B and TM-adatoms was an indication that the B-dopant played an important role in the adsorption process, which was consistent with previous results that the introducing of B enhanced the interaction between the TMadatoms and the graphene substrate. As for the Ti and Cr-absorbed structures, the majority and minority PDOS of C and B atoms were almost symmetric, while the 3d-orbital of Ti and Cr were mismatched. Accordingly, the unpaired electrons produced the net magnetic moment to the systems. Based on above analysis, the magnetic properties of adsorption complexes were mainly attributed to the TM-3d orbitals, and the intense hybridization between B-2p and 3d orbitals of TM-adatoms increased the stability of structures.
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4. Conclusions In summary, the geometry, energetic, electronic and magnetic properties of TM-adatoms (Ti, V, Cr and Mn) on Stone–Wales defected graphene with or without B-doping were investigated by DFT calculations. It was found that the introducing of B-dopant increased the activity of the SWG and enhanced the adsorption of TM-adatoms on graphene substrate. Furthermore, the TM atoms characterized by their magnetic behavior endowed the adsorption complexes with different magnetic properties. According to the results of PDOS, the net magnetic moments of systems were mainly contributed by the 3d orbital of TM-adatoms. More interestingly, the Ti and Cr absorbed on the BSWG exhibited half-metallicity, while the cases of V and Mn presented metallicity.
Acknowledgments This work was supported by the National Natural Science Foundation of China (11074176, 10976019 and 51101141), the Research Fund for the Doctoral Program of Higher Education of China (20100181110080), and the Research Fund from Science and Technology on Plasma Physics Laboratory (9140C680501120C68254).
References [1] K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich, S. Morozov, A. Geim, Proc. Natl. Acad. Sci. 102 (2005) 10451. [2] K. Novoselov, A.K. Geim, S. Morozov, D. Jiang, M.K.I. Grigorieva, S. Dubonos, A. Firsov, Nature 438 (2005) 197. [3] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [4] A.C. Neto, F. Guinea, N. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81 (2009) 109. [5] Y. Zhang, Y.-W. Tan, H.L. Stormer, P. Kim, Nature 438 (2005) 201. [6] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Science 306 (2004) 666. [7] G.-D. Lee, C. Wang, E. Yoon, N.-M. Hwang, D.-Y. Kim, K. Ho, Phys. Rev. Lett. 95 (2005) 205501. [8] M. Sternberg, L. Curtiss, D. Gruen, G. Kedziora, D. Horner, P. Redfern, P. Zapol, Phys. Rev. Lett. 96 (2006) 075506. [9] F. Ding, Phys. Rev. B 72 (2005) 245409. [10] F.J. Ribeiro, P. Tangney, S.G. Louie, M.L. Cohen, Phys. Rev. B 72 (2005) 214109. [11] E.J. Duplock, M. Scheffler, P.J. Lindan, Phys. Rev. Lett. 92 (2004) 225502. [12] P. Lehtinen, A. Foster, A. Ayuela, A. Krasheninnikov, K. Nordlund, R. Nieminen, Phys. Rev. Lett. 91 (2003) 017202. [13] F. Banhart, J. Kotakoski, A.V. Krasheninnikov, ACS Nano 5 (2011) 6. [14] M.H. Gass, U. Bangert, A.L. Bleloch, P. Wang, R.R. Nair, A. Geim, Nat. Nanotechnol. 3 (2008) 676. [15] L. Tapaszto, G. Dobrik, P. Nemes-Incze, G. Vertesy, P. Lambin, L.P. Biró, Phys. Rev. B 78 (2008) 233407. [16] V.H. Crespi, M.L. Cohen, A. Rubio, Phys. Rev. Lett. 79 (1997) 2093. [17] H.J. Choi, J. Ihm, S.G. Louie, M.L. Cohen, Phys. Rev. Lett. 84 (2000) 2917. [18] B. Akdim, T. Kar, X. Duan, R. Pachter, Chem. Phys. Lett. 445 (2007) 281. [19] S. Roh, J. Oh, Y. Choi, D. Sohn, W. Kim, C. Cho, W. Yi, J. Yoo, C. Lee, J. Kim, J. Vac. Sci. Technol. B Microelectron. Nanomater. Struct. 22 (2004) 1411. [20] X. Qin, Q. Meng, W. Zhao, Surf. Sci. 605 (2011) 930. [21] Q. Zhou, C. Wang, Z. Fu, Y. Tang, H. Zhang, Comp. Mater. Sci. 83 (2014) 398. [22] Q. Zhou, Z. Fu, Y. Tang, H. Zhang, C. Wang, Physica E 60 (2014) 133. [23] Y. Yagi, T.M. Briere, M.H.F. Sluiter, V. Kumar, A.A. Farajian, Y. Kawazoe, Phys. Rev. B 69 (2004) 075414. [24] K.T. Chan, J.B. Neaton, M.L. Cohen, Phys. Rev. B 77 (2008). [25] Y. Mao, J. Yuan, J. Zhong, J. Phys. Condens. Matter 20 (2008). [26] R.C. Longo, J. Carrete, J. Ferrer, L.J. Gallego, Phys. Rev. B 81 (2010). [27] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 4. [28] B. Delley, J. Chem. Phys. 113 (2000) 9. [29] Z.M. Ao, J. Yang, S. Li, Q. Jiang, Chem. Phys. Lett. 461 (2008) 4. [30] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5. [31] L. Chen, Y. Ouyang, Y.L. Wang, Y.Y. Sun, H.Z. Pan, Physica E 43 (2010) 5. [32] S. Azadi, R. Moradian, A.M. Shafaee, Comp. Mater. Sci. 49 (2010) 5. [33] Q. Pang, Y. Zhang, J.M. Zhang, K.W. Xu, Comp. Mater. Sci. 50 (2011) 8. [34] H. Sevincli, M. Topsakal, E. Durgun, S. Ciraci, Phys. Rev. B 77 (2008) 7. [35] K.T. Chan, J.B. Neaton, M.L. Cohen, Phys. Rev. B 77 (2008) 12. [36] Q.X. Zhou, Y.J. Tang, C.Y. Wang, Z.B. Fu, H. Zhang, Comp. Mater. Sci. 81 (2014) 5.