The stabilities and electronic structures of Al n Si 12-n N 12 ( n = 0, 1, 2, and 4)
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AlnSi12nN12 (n 5 0, 1, 2, and 4) are electron redundant systems. The calculations show that the stabilities of AlnSi12nN12 and Al12N12 are very close. One Si atom in each Si2N2 square protrudes obviously and the cages are distorted. The excess electrons reside at the outside of the protrudent Si atoms as lone pair electrons. They occupy antibonding orbitals and form the highest occupied band. The Si–N bonds are covalent bonds with strong polarity. The overlap integral is 0.38 per Si–N bond and is 17% stronger than the overlap in Al12N12. The atoms in molecule charge on the in-plane and protrudent Si atoms are 3.13e and 1.65e, respectively. The lone pair electrons form large local dipole moments enhance the electrostatic interaction between the protrudent Si and N atoms. The energy gaps of the electron redundant cages AlnSi12nN12 (n 5 0, 1, 2, and 4) are about 1 eV smaller than the gap of Al12N12. As the lone pair electrons are loosely bond, the SiN-based cages have large hyper-polarizabilities and so have potential applications, such as nonlinear optical materials.
I. INTRODUCTION
The discovery of carbon nanomaterials, such as fullerenes, nanotubes and graphene, triggered new research opportunities in science and technology. As isoelectronic counterparts to carbon analogs, the boron nitride systems have been extensively investigated due to their hightemperature stability, low dielectric constant, large thermal conductivity and oxidation resistance.1–9 Different AlN nanostructures have been synthesized and their applications are growing rapidly.10–22 Wu et al.10 synthesized faceted hexagonal aluminum nitride nanotubes with the length of a few micrometers and diameters from 30 to 80 nm. Liu et al.11 reported the vapor–solid growth of quasialigned AlN nanocones on catalyst-coated wafers via the reactions between AlCl3 vapor and NH3 gas. Lei et al.12 synthesized hexagonal AlN nanowires by the direct nitridization method at high temperatures. Lei et al.13 reported the vapor–solid growth of urchin-like nanostructures in DC arc plasma via the direct reaction between Al vapor and N2 gas. Investigations also indicate that B12N12 and Al12N12 cages have extraordinary stability and can be used as all-purpose building blocks for the synthesis of new cluster-assembled materials.3,14–17,23–25 The electronic structure and stability could be changed by introducing different dopants. Hybrid BCN nanomaterials have received considerable attention as the
Contributing Editor: Mauricio Terrones a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.390 J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
potential materials to tune the electronic behavior of pure C and BN systems. Some hybrid materials have been successfully synthesized.26–28 The effect of carbon incorporation on the structure and properties of BCN is quite sensitive to the C concentration. Wurtzite (BC2N)n (n 5 1, 2, 3, 4, and 6) clusters have been studied using different theoretical models.29–31 The BN-substituted fullerenes C602
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