Boron Nitride Nanotubes and Nanosheets Dmitri Golberg,* Yoshio Bando, Yang Huang, Takeshi Terao, Masanori Mitome, Chengchun Tang, and Chunyi Zhi International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 3050044, Japan O ne- and two-dimensional nano- structures defined as those hav- ing at least one dimension be- tween 1 and 100 nm have attracted a prime attention over the past two decades due to their peculiar and fascinating properties and a wide range of interesting potential applications which, in many cases, shall be more fruitful than for the corresponding bulky material counterparts. The ability to generate such minuscule structures is es- sential for many branches of the modern science and technology. The original find- ings of carbon nanotubes (CNTs) 1 in the mid-1970s followed by their complete iden- tification in 1991 2 stimulated intense theo- retical and experimental studies in regard to other nanostructures based on similar honeycomb-like networks. Layered boron nitride is a structural analogue of graphite in which alternating B and N atoms substitute for C atoms. 3,4 In general, a BN nanotube 5-9 can easily be imagined as a rolled graphite- like BN sheet, where C atoms are fully substi- tuted by N and B atoms in a fashion shown in Figure 1. It is seen that, structurally, it is a very close analogue of the CNT. Recent rise of a carbon monatomic sheet, named graphene, 10 has also brought to the research forefront a question of the existence and sta- bility of its BN counterpartOa monatomic graphene-like sheet solely made of B and N atoms (Figure 1). Undisputedly, compared to its sister sys- tem (C), both the BN nanotubes and nanosheets have remained much less ex- plored. In fact, the number of BN-related publications is notably smaller relative to the well-explored C system (Figure 2). Since the BN nanosystems have distinct differ- ences/advantages compared to those of C, for example, they are electrically insulating (a band gap of 5-6 eV), 11,12 have pro- found chemical and thermal stabilities, 13,14 but at the same time are equally thermally conductive and mechanically robust as their C counterparts, such a shortage of studies does not reflect a fact that the BN system has been ignored and/or underestimated relative to the C nanomaterials; rather, this is primarily due to the fact that the well- defined synthesis of BN-based nanostruc- tures is a much more challenging task com- pared to the case of C. All well-established synthetic routes of C nanotubes and graphene formation have not properly worked for the BN nanostructures. Boron Nitride Nanomorphologies. BNNTs may crystallize in single- and multiwalled struc- tures. The sheets may also be mono- or sev- eral layers thick. The single-walled BNNTs have been rather rarely observed and stud- ied 15 compared with popular single-walled CNTs. A single-walled structure is not typi- cal in the BNNT system due to peculiar B-N stacking characteristics. Also, in spite of a *Address correspondence to golberg.dmitri@nims.go.jp. Published online May 12, 2010. 10.1021/nn1006495 © 2010 American Chemical Society ABSTRACT Hexagonal boron nitride (h-BN) is a layered material with a graphite-like structure in which planar networks of BN hexagons are regularly stacked. As the structural analogue of a carbon nanotube (CNT), a BN nanotube (BNNT) was first predicted in 1994; since then, it has become one of the most intriguing non-carbon nanotubes. Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a band gap of ca. 5 eV, basically independent of tube geometry. In addition, BNNTs possess a high chemical stability, excellent mechanical properties, and high thermal conductivity. The same advantages are likely applicable to a graphene analogueOa monatomic layer of a hexagonal BN. Such unique properties make BN nanotubes and nanosheets a promising nanomaterial in a variety of potential fields such as optoelectronic nanodevices, functional composites, hydrogen accumulators, electrically insulating substrates perfectly matching the CNT, and graphene lattices. This review gives an introduction to the rich BN nanotube/nanosheet field, including the latest achievements in the synthesis, structural analyses, and property evaluations, and presents the purpose and significance of this direction in the light of the general nanotube/nanosheet developments. KEYWORDS: boron nitride · nanotubes · nanosheets · chemical vapor deposition · transmission electron microscopy · atomic force microcopy REVIEW www.acsnano.org VOL. 4 ▪ NO. 6 ▪ 2979–2993 ▪ 2010 2979