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Theoretical Studies of the Magnetism of the First-Row Adatom on the Silicon Carbide (SiC) Nanotube Jing-xiang Zhao,† Yi-hong Ding,‡,* Qing-hai Cai,† Xiao-guang Wang,† and Xuan-zhang Wang† College of Chemistry and Chemical Engineering and ProVincial Key Lab for AdVanced Functional Materials and Excite State, Harbin Normal UniVersity, Harbin 150080, People’s Republic of China, and State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China ReceiVed: NoVember 8, 2009; ReVised Manuscript ReceiVed: January 27, 2010
Recently, light-element magnets have received great interest because they can overcome the technological limitations that normal transition metal-based magnetic materials would encounter. In this article, through density functional theory (DFT) methods, we systematically explore the magnetic properties and electronic structures of the adsorbed silicon carbide (SiC) nanotubes by various first-row atoms (H, Li, Be, B, C, N, O, and F). It is found that the studied eight atoms can be effectively adsorbed to the SiC nanotube with the adsorption energy ranging from 1.36 to 5.59 eV. Furthermore, the adsorption of H, Li, B, N, and F atoms can induce magnetization, whereas no magnetism is observed when Be, C, and O atoms are adsorbed on the SiC tube. Additionally, we study whether the local magnetic moments can result in collective magnetism involving long-range magnetic coupling, which is a crucial issue for applications. The present work provides a theoretical guidance to tune the magnetic properties of the SiC nanotube via selective atomic adsorption, which is useful to design the magnetic semiconductors or build blocks for spintronic devices. 1. Introduction In recent years, magnetism in light-elements-based materials has attracted much attention due to its great potential for use in spintronics devices. Compared with the usual metal-based magnets with complicated d or f electrons, these light-element magnets only involve s or p electrons. In particular, recent studies on magnetic carbon, such as polymerized C601 and proton-irradiated graphite,2,3 have stimulated a rapid surge of interest in searching for suitable candidate materials for spintronic devices with similar magnetism. For example, using spinpolarized density functional theory (DFT), Lehtinen et al. have found that magnetism can be induced to single-walled carbon nanotubes (CNTs) by adsorption of the carbon4 or hydrogen5 atom, or by introducing defects.6 Ma et al. have performed ab initio calculations to study the magnetic properties of nitrogen impurities in graphite and CNTs. They found that the N adatom has a magnetic moment of 0.6 µB.7 Talapatra have indicated that the magnetism of various carbon systems can be tuned by controlled defect generation and doping.8 In addition, the magnetization might also be induced to the boron nitride (BN) systems,9–15 such as BN graphite and BN nanotubes by vacancy,9,10 doping,11 hydrogenation,12 carbon adatom,13 or fluorination.14,15 As a structural analogue of the CNT, silicon carbide (SiC) nanotube,16,17 however, is different from its carbon counterpart in several aspects.18 For example, the SiC nanotube exhibits uniform semiconductor behavior independently of its chirality,19,20 whereas its stability is determined by the tube size: the SiC nanotube is more stable than thinner faceted nanotubes and nanowires but less stable than thick ones.21 Because the SiC nanotube is energetically more favorable than the thinner faceted ones or nanowires, cylindric SiC tube can be realized experi† ‡
Harbin Normal University. Jilin University.
mentally.21 Moreover, the SiC nanotube has high thermal stability, which makes it a promising candidate for high-power, high-temperature electronics or biological sensors.18 In addition, the SiC nanotube has been shown to store more hydrogen in a given volume than CNT.22 An interesting point is that SiC nanotube has higher chemical reactivity than CNT or BN nanotube due to its great polarity,19,23–27 therefore, it may be used as gas sensors (such as CO, HCN,24 or CO225), and metalfree catalysts (such as N-H or O-H cleavage26). Compared with that of CNTs or BN nanotubes, the adsorption of the first-row atom on the SiC nanotubes has been much less studied and understood. Zhao et al. have shown that the electronic properties of SiC nanotubes can be changed by the adsorption of H28 or N29 atom. Moreover, Gali30 and Baierle31 have independently explored the interaction between an individual H atom and the defective SiC nanotube and elucidated the changes in electronic properties of the SiC nanotubes. Very recently, Gali and co-workers have studied the adsorption of O atom on the SiC nanotube surface.32 However, we should point out that these studies mainly focus on the effects of functionalization on the electronic properties of the SiC nanotube. The reports on the functionalization of CNTs or BN nanotubes with the first-row atoms to modify their magnetic properties encourage us to exploit similar adsorption of SiC nanotubes. In this article, by studying the adsorption of eight first-row atoms (H, Li, Be, B, C, N, O, and F) on the SiC nanotube, we evaluate the changes in the magnetic properties of the SiC nanotube through DFT calculations. Our results demonstrate that the SiC nanotubes can indeed be magnetized upon adsorption of certain first-row atoms, suggesting that the functionalized SiC nanotube might be used to fabricate high-temperature magnetic semiconductors or spintronic devices.
10.1021/jp910644x 2010 American Chemical Society Published on Web 02/17/2010
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Figure 1. Detailed description of different adsorption sites of various first-row atoms on an (8,0) SiCNT. C, carbon; Si, silicon; H, hexagon; BA, axial Si-C bond; BZ, zigzag Si-C bond.
2. Computational Details We select the zigzag (8, 0) SiC nanotube, which has been successfully used in previous studies,25,26,32 to study the adsorption of the eight first-row atoms on the SiC nanotube. The supercell with lattice constants of a ) b ) 30 Å, and c ) 10.62 Å for the (8, 0) SiC nanotube is adopted in this work, which includes 32 Si atoms and 32 C atoms. The spin-polarized DFT calculations are carried out through double numerical basis sets plus polarization functional (DNP basis set) implemented in the DMOL3 package.33 The generalized gradient approximation (GGA) with the Perdew Burke Ernzerhof (PBE)34 is used to study the interaction between the first-row atom and the SiC nanotube. The Brillouin zone is sampled by 1 × 1 × 5 special k-points according to the Monkhorst-Pack scheme.35 The adsorption energy Eads is obtained from the following expression:
Eads ) Etot[SiC] + Etot[adatom]-Etot[SiC + adatom] where Etot[SiC], Etot[adatom], and Etot[SiC + adatom] are the total energies of the SiC nanotube, the free adatom, the adsorbed SiC nanotube, respectively. A positive Eads denotes exothermic adsorption. 3. Results and Discussion 3.1. Adsorption of the First-Row Atom on an (8, 0) SiC Nanotube. First, we study the stable configurations of various first-row atoms on the (8, 0) SiC nanotube. As shown in Figure 1, we consider five different initial sites to adsorb the first-row atom, including (1) the on-top of the carbon atom (C); (2) the on-top of the silicon atom (Si); (3) the hexagon site (H); (4) the zigzag Si-C bond (Z), and (5) the axial Si-C bond (A). After full structural relaxations, the stable adsorption configurations, and the corresponding adsorption energies are displayed in Figure 2 and Table 1. Taking H-adsorption as an example, two stable configurations at the Si and C sites are obtained. The C-site adsorption is energetically more favorable (Eads ) 1.97 eV), whereas the H-adsorption on the Si site has a slightly smaller adsorption energy (Eads ) 1.94 eV). This can be expected because the highest occupied molecular levels (HOMOs) are centered on the Si sites and the lowest unoccupied molecular levels (LUMOs) are centered on the C sites. Hence, the HOMO of the H atom denotes electrons preferably to the LUMO centered on the C sites. Furthermore, the adsorbed C site by H atom is outward from the tube surface. The Si-C bond lengths in the
Figure 2. Optimized morphologies of (a) H, (b) F, (c) Li, (d) Be, (e) B, (f) C, (g) N, and (h) O adsorption on the SiC nanotubes in the adsorption regions. The bond distances and angles in angstroms and degrees, respectively.
SiC nanotube involving H-adsorption on C site are 1.873, 1.902, and 1.906 Å respectively, which are longer than those in the perfect SiC nanotube (1.790 and 1.800 Å). Such structural
Silicon Carbide (SiC) Nanotube
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TABLE 1: Optimized Adsorption Sites, and the Corresponding Adsorption Energies Eads, and Charge Transfer of First-Row Atoms on the (8,0) SiC Nanotube atom
adsorption site
Eads (eV)
Q (e)a
H
C Si H C H BA BZ BA BZ BAb BA BZ BA BZ C Si
1.97 1.94 1.37 1.26 1.26 1.19 1.35 2.67 3.05 4.08 5.31 5.58 4.41 4.61 1.70 4.54
0.05 -0.09 0.38 0.42 0.17 0.19 0.19 -0.05 -0.07 -0.16 -0.23 -0.25 -0.25 -0.23 -0.19 -0.22
Li Be B C N O F
a The positive value denotes the charge is transferred from the atoms to the tube. b For the adsorption configuration, as shown in part f of Figure 2.
deformation is attributed to the transformation from sp2 to sp3 hybrization on the C atom. For other first-row atoms on the SiC nanotube, we find that the most stable sites are different. The Li atom prefers to stay on the H site, whereas it is energetically favorable for the F atom to be adsorbed on the Si site. However, Be, B, N, and O atoms exhibit strong binding at Z site. This might have contribution from the localized natures of occupied and unoccupied states near the Fermi level of the SiC tubes and the electronegativity of the adatom with respect to that of Si and C atoms in tube. On the other hand, as seen from the Table 1, these adsorbed first-row atoms on the SiC nanotube have relatively high adsorption energies. Among these adsorption, the N-adsorption on the tube provides the largest adsorption energy (5.58 eV), which is supported by the most significant charge transfer from the SiC tube to the N atom (0.25 e). Meanwhile, the N-adsorption leads to the largest structural deformation: because the adsorbed Si-C bond is significantly increased from 1.800 to 2.645 Å. Contrarily, the adsorption energy of Be atom on the tube is the smallest (1.35 eV). In addition, structural deformation in various ways is also found for the adsorption of these atoms, as shown in parts a-h of Figure 2. 3.2. Effects of Adsorption on the Magnetic Properties of SiC Nanotube. Previous studies have shown that the adsorption of H, C, N, or F atom on CNT or BN nanotube can induce magnetism.9–15 Different from transition metals with high cohesive energy; these light elements do not tend to aggregate to form a cluster on the nanotube. This is crucial to tune the magnetism of the nanotube by the control of the number of adsorbed atom. Moreover, the high cost and toxic concern of the transition metal-based magnetic materials might also limit their application to some degree. That is why magnetism in light elements has become a hot topic in recent years. Next, we systematically study the changes in the magnetic properties of the SiC tube upon adsorption of these first-row atoms through Hirshfeld mechanism. Remarkably, the Hirshfeld method is based on the deformation electron density, which seems less sensitive to the selected basis sets than the Mulliken method, although the Hirshfeld charge analysis generally underestimates the atomic charges.36,37 We find that the adsorption of a single H, Li, B, N, and F atoms on the outer sidewall
of the SiC tube can induce magnetization in the SiC tube, whereas no magnetism is observed for the adsorption of Be, C, and O atoms. Interestingly, the magnetic contributions in these adsorbed SiC tubes are drastically different from each other. In detail, the H-adsorption on the C atom in the SiC nanotube gives rise to a donor state near the lowest unoccupied conduction band (LUCB), which is split into spin-up and spin-down branches (part a of Figure 3). This leads to a strong spontaneous magnetism in the H-SiC nanotube. The net magnetic moment is about 1 µB. Moreover, we plot the spin density of the H-SiC nanotube, which shows that the magnetization density of H-adsorption is mainly localized on the Si atoms (Si1, Si2, and Si3) near to the hydrogenated C atom as shown in part a of Figure 3. The total net magnetic moment in this tube mainly comes from the 3p orbitals of the three Si atoms, and the magnetic moments are 0.138, 0.129, and 0.097 µB for Si1, Si2, and Si3, respectively. Similarly, our electronic structure calculations indicate that Li-adsorption on the SiC tube also introduces a donor level nearby the conduction band minimum, which is split into spinup and spin-down branches. The induced magnetism is mainly originated from the Si atoms nearest to the Li atom as shown in part b of Figure 3. The calculated net magnetic moments for these Si atoms are 0.07, 0.07, 0.07, and 0.08 µB for Si1, Si2, Si3, and Si4, respectively. For the H- and Li-adsorption, the similarity in the magnetic distributions is understandable: a certain amount of electrons (0.05 and 0.38 e) is transferred from the two atoms to the SiC tube. On the contrary, in the case of B- and N-adsorption, an acceptor level appears within the band gap, which is close to the valence-band edge. Moreover, a split energy band structure is found as shown in parts c and d of Figure 3, indicating that a magnetization has been introduced to the SiC nanotube. The calculated total net magnetic moment in the two SiC nanotubes is 1 µB. We note that the magnetic moment mainly comes from the adatom (B and N atom) and its adjacent atoms (Si and C atom) as shown in parts c and d of Figure 3: for the B-adsorption. The magnetic moments of B atom and its neighboring four atoms (C1, C2, Si1, Si2) are 0.14, 0.14, 0.14, 0.08, and 0.08 µB, respectively. For the N-adsorption, the magnetic moments of the N atom and its neighboring two atoms (C and Si) are 0.28, 0.23, and 0.14 µB, respectively. In addition, about 0.07 and 0.25 electrons is transferred from the B and N atoms to the SiC nanotube. For the F-adsorption, the introduction of acceptor level also makes the whole SiC tube possess magnetization (1 µB), which is similar to that of B- or N-adsorption. However, the total net magnetic moment is mainly contributed by the three C atoms bonding to the fluorinated Si atom as displayed in part e of Figure 3: the magnetic moments of the C1, C2, and C3 are 0.09, 0.08, and 0.08 µB, respectively. The F atom and all of the Si atoms have little contribution to the moment. On the other hand, for the Be, C, and O adatom on the SiC nanotube, we find that the obtained most stable configurations have no magnetism. Taking C-adsorption as an example, we check the validity of the nonmagnetic structures of the adsorbed SiC nanotubes in the ground state by restricting the system to the ferromagnetic (S ) 1) state. The results show that the energy of the ferromagnetic state is 0.33 eV higher than that of the corresponding paramagnetic (S ) 0). This is dramatically different from the case of C atom on the CNT or BNNT, where spontaneous magnetism is induced: the magnetic moment is about 0.22-0.44 µB for the CNT4 and 2 µB for the BNNT,13 respectively. Because the SiC nanotube possesses much higher
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Zhao et al. To develop the practical SiC nanoube-based magnetic devices, the question of whether the local magnetic moments can lead to collective magnetism, which involves long-rang magnetic coupling, is a crucial issue. Therefore, we further explore the magnetic interactions between the adatom-induced magnetic moments on the SiC tube. For H-, B-, N-, and F-adsorbed (8, 0) SiC nanotube, we find that when two adatoms are initially placed in the same hexagonal ring of the SiC nanotube, the local magnetic moment induced by each adatom is completely quenched, making the hybrid system nonmagnetic. Subsequently, two adatoms are initially placed on two different SiC hexagonal rings along the tube axis. After full structural relaxation, we find that the antiferromagnetic state is the most stable for these systems. However, the antiferromagnetic and ferromagnetic configurations of the adsorbed SiC nanotube by H, Li, B, N, and F atoms might become nearly degenerate when the distance between the two adatoms is larger than 10.6 Å. These unique magnetic properties suggest that the SiC nanotube can be used to build high-temperature blocks for spintronic devices. In short, through the systematic analysis of magnetic properties of the decorated SiC nanotube by eight first-row atoms, we find that the adsorption of H, Li, B, N, and F atoms can induce magnetism to the SiC nanotube, whereas no magnetism is observed for Be, C, and O atoms’ adsorption. Thus, one can experimentally fabricate SiC nanotube-based magnetic materials, such as spin valve or spin-polarization switch, by controlling adsorption of different atoms. Conclusions Using spin-polarized DFT calculations, we evaluate the effects of the adsorption of various first-row atoms on the magnetic properties of the SiC nanotube. Our results show that the adsorption of H, Li, B, N, and F atoms can induce strong magnetism in the SiC nanotube, whereas no magnetism is found for the case of Be, C, and O atoms. In terms of the calculated adsorption energies and the changes of magnetic properties of SiC naotube, we believe that the SiC nanotue is a good candidate as the design of magnetic material applied at high temperature, high power, and in harsh environments. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 20103003, 20573046, 20773054), Doctor Foundation by the Ministry of Education (20070183028), Excellent Young Teacher Foundation of Ministry of Education of China, Excellent Young People Foundation of Jilin Province (20050103), Program for New Century Excellent Talents in University (NCET), and Committee of Education of Heilongjiang Province (11541095). The reviewers’ invaluable comments are greatly appreciated. References and Notes
Figure 3. Band structures and the isosurface of the difference spin density at the isovalue of 0.02 e/Å3 for (a) H, (b) Li, (c) B, (d) N, and (e) F adsorption on the (8,0) SiC nanotube.
reactivity toward foreign adsorbates than CNT or BNNT, the localized pz states of the C adatoms strongly couple to the π states of the tube, which makes the pz orbital be delocalized and thus have no contribution to the magnetism. This well explains why the C-adsorbed SiC nanotube has no magnetism.
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