SEM image of WS2 nanotubes provided by Reshef Tenne of the Weizmann Institute.

The stability of the cylindrical form is not limited to carbon nanotubes. In 1992, scientists at the Weizmann Institute discovered crystalline molecular cylinders (nanotubes) composed of Tungsten and Sulfur [1] (figure). Thus began the subject of inorganic fullerene-like materials and nanotubes. Any of the numerous elements and compounds known to form stable two dimensional sheets (as well as several other elements) can be used for theoretical simulations. Predictions made by these simulations are continually being confirmed as nanotube engineers find new ways to roll those sheets into tubes.

Several methods exist for the synthesis of inorganic nanotubes. Each method may result in a different type, size and yield. The 'type' of tube refers to its atomic structure and chirality, 'Size' means the diameter and length of the tube and 'yield' refers to the purity of the product. A common method for synthesis of both inorganic and organic nanotubes is exposure to high temperatures by laser heating [2] or an arc discharge [3]. Another technique is to substitute the atoms in an already fabricated tube by a substitution reaction [4]. Template assisted synthesis is perhaps the most promising method, since it allows more precise control of the nanotube type [5].

As the diversity of available nanotubes increases, nanoscientists are gaining access to a wide variety of molecular columns, pipes, bearings and springs. The mechanical properties of these tubes are controllable by electronic means, making them ideal components for Nano-Electromechanical Systems, otherwise known as nanomachines. Two of the best understood inorganic nanotubes, Boron Nitride and Tungsten Disulfide, are described in more detail below.

• Boron Nitride (BN) Nanotubes
• Tungsten Disulfide (WS₂) Nanotubes

Acknowledgements: Thank you Reshef Tenne of the Materials Synthesis Group, Weizmann Institute for providing images and insightful comments.

References:

[1] R. Tenne, L. Margulis, M. Genut, and G. Hodes, 'Polyhedral and Cylindrical Structures of Tungsten Disulphide' (1992) Nature 360, 444.

[2] T. Laude, A. Marraud, Y. Matsui, and B. Jouffrey, 'Long ropes of boron nitride nanotubes grown by a continuous laser heating' (2000) Appl. Phys. Lett., 76, 3239. abstract

[3] J. Cumings, A. Zettl, 'Mass-production of boron nitride double-wall nanotubes and nanococoons' Chem. Phys. Let. (2000) 316, 211. abstract

[4] W.Hang, Y.Bando, K.Kurashima and T.Sato, 'Synthesis of boron nitride nanotubes from carbon nanotubes by a substitution reaction' Applied Physics Letters (1998) 73, 3085-3087. abstract