CARBON NANOTUBES AND GENERAL ELECTRO-OP TO-MECHANIC PROPERTIES

Since the chance discovery (or re-discovcry) of Iijima in 1991, carbon nanotubes have been investigated extensively. The results to date show that they arc truly remarkable materials. As has been known experimentally² and theoretically³, carbon nanotubes can be semiconducting with a bandgap anywhere between 20mcV to 2cV. They can be metallic, but capable of sustaining current density hundreds of times greater than a metal. They are many times stronger than steel but much lighter. They are thermally more conductive than all crystals and metals'¹. And, they may be superconducting . electrostrictive⁶, ...etc. A list of merits already long and still growing. A "one size fits all" magic material! No wonder, many think, and some stated, that carbon nanotubc is the greatest development in electronic materials.

All these wonderful properties come about not by coincidence. They are rooted in the fact that in this system the electronic degree freedom is strongly coupled to the mechanic degree of freedom, more so than in any other semiconductor or metal. As such, they offer an ideal platform for extending electronics and therefore information technology into the vast and relatively untapped areas of information acquisition and execution. It is in this broad context, nanotubes stand out as a likely most enabling new materials.

One of the areas in which new break through technologies might be enabled by nanotubes is sensors - electrical, mechanic, optical and magnetic etc. either distributed over large areas or highly localized to sizes smaller than ever possible before.

In the context of Sensing and Photonics for Space Environments - the theme of this SPIE Conference, a few additional real or potential attributes of the nanotubc technology are perhaps particularly worth citing:

• Radiation hardness -arising from the strong covalent bonding

• Defect localization - arising from the nanotubc to nanombe spatial and electrical separations

• Normal incidence detection in the ordered array setting - arising from the cylindrical symmetry and the nanometric diameter

• Reduced sensitivity to temperature - arising from the quasi 1 -D phonon density of states therefore greatly suppressed phonon coupling to the surrounding, and from the related high thermal conductivity

Although the details of the underlying physics of the remarkable clcctxo-mechanic-optic properties of nanotubes arc sufficiently complex and not fully understood, an intuitive understanding of why carbon nanotube, be it single-walled and or multi-walled, can be reached via first principle considerations.

Carbon is interesting and unique in that it can form either the semi-metallic graphite or the very hard and nearly insulating diamond. In grahite, the carbon bonds arc in the same plane, so called SP2 bonds, whereas in diamond the bonds arc in the three-dimensional space, referred to as SP3 bonding. Carbon nanotube is a third form of Carbon lattice structure. Intuitively, it can be viewed as graphitic sheets rolled up into single-walled or multi-walled tubes in which the originally planar SP2 bond vectors arc bent along with the graphite plane, thus projecting a component to the third dimension and picking up some of the characteristics and hence the properties of the SP3 bonds. It is therefore not surprising to find that carbon nanotubes exhibit mechanical and electronic properties in between graphite and diamond. The smaller the diameter, the more the SP2 bonds arc bent, and the more SP3 like characteristics they pick up, hence the wider the nanorube band gap.

We presented a more detailed and quantitative analysis earlier , which arrives at an inverse dependence of the bandgap on the local bending curvature (i.e., diameter and helicity). In the case of multi-walled nanotubes, the system is likely to favor a distribution of the helicity of each wall over a wide range to minimize the overall energy. As a result, the multi-walled nanotubes lack a well defined helicity. However, this should be viewed as more of an assertion than experimentally verified fact, the measured bandgaps of multi-welled nanotubes do confirm that the bandgap of multi-walled nanotubes follows a simple linear relation to the inverse of the diameter, with little indication of the presence of a helicity dependence.

Underlying this intuitive model of nanotube's general properties is the observation that carbon nanorube is a unique physical system in which the mechanic and electronic degrees of freedom are tightly coupled. To minimize its total energy, the system can respond to a change in electronic properties with one in mechanical properties, and vice L versa, giving rise to giant electro-mechanical effects. Further discussion along this line, however interesting, is beyond the scope of this review as it is unlikely to be relevant beyond the bandgap-diameter dependence to IR applications.

Date: 2015-01-29; view: 956