ARTICLE pubs.acs.org/JPCA
Ferromagnetism Induced by Intrinsic Defects and Boron Substitution in Single-Wall SiC Nanotubes Yongjia Zhang, Hongwei Qin, Ensi Cao, Feng Gao, Hua Liu, and Jifan Hu* School of Physics, State Key Laboratory for Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China ABSTRACT: On the basis of density functional theory (DFT) methods, we study the magnetic properties and electronic structures of the armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes with various vacancies and boron substitution. The calculation results indicate that a Si vacancy could induce the magnetic moments in both armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes, which mainly arise from the p orbital of C atoms surrounding Si vacancy, leading to the ferromagnetic coupling. However, a C vacancy could only bring about the magnetic moment in armchair (4, 4) single-wall SiC nanotube, which mainly originates from the polarization of Si p electrons, leading to the antiferromagnetic coupling. In addition, for both kinds of single-wall SiC nanotubes, magnetic moments can be induced by a boron atom substituting for C atom. When two boron atoms locate nearest neighbored, both kinds of single-wall Si(C, B) nanotubes exhibit antiferromagnetic coupling.
1. INTRODUCTION In the past 2 decades, nanoscience and nanotechnologies have been dominated by the honeycomb structured C-based materials in different dimensionalities, such as carbon grapheme, ribbons, and nanotubes. In particular, a nanotube with one-dimensional honeycomb structure is one of the most intriguing carbon-based materials, which has been studied intensively since the first successful preparation by Iijima in 1991.1 Because of the unique structures, single-wall carbon nanotubes provide some unique properties for novel one-dimensional systems. For example, the electronic structure of carbon nanotubes can be either metallic or semiconducting depending on their chiral vectors (n, m).2 4 Carbon nanotubes exhibit interesting optical, electronic, superconducting, and magnetic properties. The magnetism of carbon nanotubes has great potential applications in electronics5 and spintronics.6 8 It has been theoretically predicted that magnetism can be induced in C-based materials by the introduction of defects9,10 or the adsorption of carbon11,12 or hydrogen13 atoms. Ma et al. have performed ab initio calculations to study the magnetic properties of nitrogen impurities in single-wall C nanotubes and found that the N adsorption atom has a magnetic moment of 0.6 μB.14 It has been indicated that the magnetism of various carbon systems can be tuned by controlled defect and doping.15 In addition, for boron nitride (BN) nanotubes, the magnetization could be introduced by vacancy,16 18 doping,19 hydrogenation,20 carbon adatom,21 or fluorination.22,23 Silicon carbide (SiC) is a wide band gap semiconductor, and the honeycomb SiC is a biocompatible material already successfully used in bone implants. On the basis of the structure of carbon nanotubes, SiC nantubes were first synthesized in 2001.24 The structure and stability of single-wall SiC nanotubes have been investigated by ab initio calculations,25 28 which indicate that the SiC nanotubes with a Si:C ratio of 1:1 are more stable, r 2011 American Chemical Society
and the energetically favorable configuration has been predicted to consist of alternating Si and C atoms.26 Among the various semiconducting nanotubes, SiC nanotubes exhibit unique physical and electronic properties, such as high thermal stability, which makes it a promising candidate for high-power, hightemperature electronics or biological sensors.29 In addition, the SiC nanotube has been shown to store more hydrogen in a given volume than carbon nanoubes.30 It has been found that SiC nanotubes have a highly reactive exterior surface, and the electronic properties of SiC nanotubes can be changed by the adsorption of H and N atoms.31,32 Gali and Baierle have explored the interaction between an individual H atom and the defective SiC nanotubes.33,34 Zhao et al. have investigated the magnetism of the first-row and different transition metal adsorption atoms on single-wall SiC nanotubes using density functional theory (DFT) methods.35,36 However, most of the above studies are focused on the electronic properties of the SiC nanotubes. Compared with that of C or BN nanotubes, the magnetism of SiC nanotubes has never been studied or understood. The reports on the magnetism of C and BN nanotubes with the vacancy and doping elements encourage us to investigate the magnetic properties of SiC nanotubes. In this article, we first investigate the magnetic properties of single-wall SiC nanotubes in both armchair (4, 4) and zigzag (8, 0) single-wall nanotubes with Si vacancy, C vacancy, and B substitution, respectively, using the DFT method. The results demonstrate that Si vacancy could induce the magnetic moments in both armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes, leading to the ferromagnetic coupling. However, C vacancy can only bring about the magnetic moments in an armchair Received: October 2, 2010 Revised: July 10, 2011 Published: July 29, 2011 9987
dx.doi.org/10.1021/jp109470r | J. Phys. Chem. A 2011, 115, 9987–9992
The Journal of Physical Chemistry A
ARTICLE
(4, 4) single-wall SiC nanotube, leading to the antiferromagnetic coupling. In addition, for both kinds of single-wall SiC nanotubes, magnetic moments can be induced when one C atom is substituted by a boron atom but cannot be induced when one Si atom is substituted by a boron atom. When two boron atoms substituting for C atoms locate nearest neighbored, both kinds of single-wall Si(C, B) nanotubes exhibit antiferromagnetic coupling.
2. CALCULATION METHODS Following the work of Zhao and co-workers,35,36 we investigated the magnetism of single-wall SiC nanotubes with vacancy and boron doping, respectively. We selected armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes. Ab initio calculations were carried out using the plane-wave pseudopotential method in the Vienna ab initio simulation package (VASP).37,38 GGA (PW91) as well as LDA (PBE) schemes were used to describe the exchange correlation energy.39,40 The periodic boundary condition along the tube axis was employed for SiC nanotubes with a vacuum region (10 Å) between tubes to make sure that there is no interaction between SiC nanotubes. We relaxed all the atomic coordinates together with the supercells using a conjugate gradient (CG) algorithm, until each component of the stress tensor was below 0.5 GPa and the atomic forces
