Ferromagnetism and semiconducting of boron nanowires
© Li et al.; licensee Springer. 2012
Received: 12 November 2012
Accepted: 10 December 2012
Published: 17 December 2012
More recently, motivated by extensively technical applications of carbon nanostructures, there is a growing interest in exploring novel non-carbon nanostructures. As the nearest neighbor of carbon in the periodic table, boron has exceptional properties of low volatility and high melting point and is stronger than steel, harder than corundum, and lighter than aluminum. Boron nanostructures thus are expected to have broad applications in various circumstances. In this contribution, we have performed a systematical study of the stability and electronic and magnetic properties of boron nanowires using the spin-polarized density functional calculations. Our calculations have revealed that there are six stable configurations of boron nanowires obtained by growing along different base vectors from the unit cell of the bulk α-rhombohedral boron (α-B) and β-rhombohedral boron (β-B). Well known, the boron bulk is usually metallic without magnetism. However, theoretical results about the magnetic and electronic properties showed that, whether for the α-B-based or the β-B-based nanowires, their magnetism is dependent on the growing direction. When the boron nanowires grow along the base vector , they exhibit ferromagnetism and have the magnetic moments of 1.98 and 2.62 μB, respectively, for the α-c  and β-c  directions. Electronically, when the boron nanowire grows along the α-c  direction, it shows semiconducting and has the direct bandgap of 0.19 eV. These results showed that boron nanowires possess the unique direction dependence of the magnetic and semiconducting behaviors, which are distinctly different from that of the bulk boron. Therefore, these theoretical findings would bring boron nanowires to have many promising applications that are novel for the boron bulk.
Boron is very special in the periodic table as the nearest neighbor of carbon and has exceptional properties of low volatility, high melting point, stronger than steel, harder than corundum, and lighter than aluminum. Hence, studies on boron nanostructures have become more and more attractive in the recent years[1–13]. Among them, boron one-dimensional nanostructures are expected to have broad applications for their high conductivity, high aspect ratios, and excellent performance in harsh conditions[14–20]. In the last several years, so many experimental studies have performed on the one-dimensional boron nanowires, and a great progress has been obtained up to now[21–27]. Just recently, the vertically aligned single-crystalline boron nanowire arrays have been especially prepared. Therefore, further explorations theoretically and experimentally on the one-dimensional boron nanostructures appear to be timely and desirable. However, the possible configurations and stability, as well as the electronic and magnetic properties of boron nanowires, which are important for the experimental preparation and technological applications, have not been reported so far. As a result of the well-aligned single-crystalline boron nanowires reported, in this contribution, we perform a theoretical study on the stability and magnetic and electronic properties of boron nanowires growing from the unit cells of stable B bulks.
Herein, we firstly get the different boron nanowires from the growth of the unit cell of the bulk boron, respectively, along different base vectors. Well known among the various boron allotropes, the most stable phases of the boron bulk are the α-rhombohedral (α-B) and β-rhombohedral (β-B) boron. The α-B is the simplest one that consists of a distorted B12 icosahedron per unit cell, forming an fcc-like structure. The β-B is the most commonly found modification and can be considered as an fcc-like structure consisting of the B84 quasi-spheres together with the B10-B-B10 chains located in the octahedral interstices formed by the B84 spheres. In the following study, we respectively attain three different boron nanowires from the growth of the unit cell of the ground states of α-B and β-B along different base vectors. We then carry out the first-principles investigation of the stability and electronic and magnetic behaviors of the considered boron nanowires. Additionally, the dependence of the electronic and magnetic properties on the growth direction of boron nanowires is discussed. These investigations are expected to provide valuable information for future applications of boron nanostructures.
We perform the first-principles spin-polarized density functional theory (DFT) using the SIESTA computation code[30–32], which is based on the standard Kohn-Sham self-consistent DFT. A flexible linear combination of numerical atomic-orbital basis sets is used for the description of valence electrons, and norm-conserving nonlocal pseudopotentials were adopted for the atomic cores. The pseudopotentials are constructed using the Trouiller-Martins scheme to describe the interaction of valence electrons with atomic cores. The nonlocal components of pseudopotential are expressed in the fully separable form of Kleiman and Bylander[34, 35]. The Perdew-Burkle-Ernzerhof form generalized gradient approximation corrections are adopted for the exchange-correction potential. The atomic orbital set employed throughout is a double-ζ plus polarization function. The numerical integrals are performed and projected on a real space grid with an equivalent cutoff of 120 Ry for calculating the self-consistent Hamiltonian matrix elements. For boron nanowires under study, periodic boundary condition along the wire axis is employed with a lateral vacuum region larger than 25 Å to avoid the image interactions. The supercell of boron nanowires respectively contains one unit cell of α-B and β-B as translational unit growing along different directions. To determine the equilibrium configurations of these boron nanowires, we relax all atomic coordinates involved using a conjugate gradient algorithm until the maximum atomic force of less than 0.02 eV/Å is achieved. In the calculations of the total energies and the energy band structures, we use four k sampling points along the tube axis according to the Monkhorst-Pack approximation. Cohesive energy (E c ) is calculated according to the expression, E c = (Etotal − n × E B ) / n, where Etotal is the total energy of the considered boron nanowire, n is the number of B atoms, and E B is the energy of an isolated B atom.
Results and discussion
Cohesive energies and total magnetic moments of considered boron nanowires and of bulk α-B and β-B
E c (eV/atom)
To lend further understanding of the nature of the boron nanowires considered above, we studied the electronic structures of all configurations using the spin-polarized calculations. The calculated total magnetic moments of the six nanowires are listed in the second column of Table1. It is obvious that for the three boron nanowires obtained from the unit cell of α-B, the nanowires α-a  and α-b  have the total magnetic moments of approximately equal to zero, while the nanowire α-c  has a distinctly different total magnetic moment of 1.98 μB. Moreover, for the three boron nanowires obtained from the unit cell of β-B, the same trend about the total magnetic moments occurs. The nanowires β-a  and α-b  both have the total magnetic moments also approximately equal to zero, and the nanowire β-c  has the total magnetic moments of 2.62 μB. Additionally, in Table1, we also presented the calculated total magnetic moments of bulk α-B and β-B. Thus, these results indicate that both of the two kinds of boron bulks have no magnetism, with the total magnetic moments equal to zero.
In summary, we have performed a systematic study of the stability and electronic and magnetic properties of boron nanowires using the spin-polarized density functional calculations and found that the considered boron nanowires possess the direction dependence of ferromagnetic and semiconducting behaviors, which are distinctly different from those of the boron bulk that is metallic and not magnetic. The physical origins of ferromagnetic and semiconducting properties of boron nanowires were pursued and attributed to the unique surface structures of boron nanowires. Thus, these theoretical findings seem to open a window toward the applications of boron nanowires in electronics, optoelectronics, and spin electronics.
This work was supported by NSFC (U0734004 and 11004254), China Postdoc. Sci. Fund (201003387), GDNSF (S2011040004850), and partially by Shanghai Supercomputer Center.
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