ABSTRACT
Hexagonal boron nitride (h-BN) is a layered material analogous to graphite that exhibits a hexagonal crystal structure with the lattice parameters of a = 2.50 Å, c = 6.66 Å. In such an h-BN crystal, the hexagonal layers stack each other, and the B3N3 hexagons overlap and alternate with N3B3 hexagons (Figure 4.1). h-BN crystals are insulators with a band gap of ~5.8 eV. Individual BN hexagonal sheets roll ideally into nanotube (NT) (similar to carbon nanotubes [CNTs]), rstly proposed in 1994. BNNTs are believed to be a wide-gap semiconductor with a band gap independent of their morphologies and/or geometries (tube diameter, chirality, and number of the wall). Due to their excellent optical and mechanical properties, high thermal conductivity, good oxidation
4.1 Introduction 91 4.2 Boron Nitride Nanotubes 93
4.2.1 Fabrications 93 4.2.1.1 Arc Discharge 93 4.2.1.2 Laser Ablation 93 4.2.1.3 Chemical Vapor Deposition 93 4.2.1.4 Ball Milling 95 4.2.1.5 Other Chemical Methods 95
4.2.2 Doped Boron Nitride Nanotubes 96 4.3 Boron Nitride Nanowires 96 4.4 Boron Nitride Nanoribbons 98 4.5 Boron Nitride Nanohorns 100 4.6 Properties and Applications 100
4.6.1 Physical Properties 100 4.6.2 Cathodoluminescent Properties 102 4.6.3 Field Emitters 103
4.7 Perspectives and Conclusions 105 Acknowledgments 105 References 105
resistivity, and chemical inertness, BNNTs show great potential for applications as unique electromechanical and optoelectronic components for laser, light-emitting diode, and medical diagnosis. Inspired by BNNTs, various BN nanostructures, such as NTs, nanobamboos, nanohorns (NHs), nanowires (NWs), and yard-glass BNNTs, have been synthesized. A wide range of catalysts, such as Fe, Ni, Co, Mg, and metal oxides, have been used to synthesize BN nanostructures
Figure 4.1 (a) Molecular model depicting the structure of h-BN; note the three layers and the interlayer spacing of 0.33 nm; (b) top view of h-BN showing alternating B and N atoms; (c) calculated band structure and density of states for a single sheet of h-BN showing that this material is an insulator with a band gap of ∼4.5 eV (Ef located at zero) (note that this local density approximation calculation underestimates the band gap; other studies considering the GW approximation reveal a band gap of 5.4 eV for bulk h-BN and 6.0 eV for a single h-BN layer); and (d) molecular model of a BNNT exhibiting a (10,0) zigzag chirality (two views are depicted). (Reprinted from Mater. Today, 10(5), Terrones, M., Romo-Herrera, J.M., Cruz-Silva, E. et al., Pure and doped boron nitride nanotubes, 10 (5), 30-38, Copyright 2007, with permission from Elsevier.)
by dierent methods including arc discharge, laser heating/ablation, ball milling-annealing, CNT substitution, soft chemical method, and chemical vapor deposition (CVD).
